Nonthermal Plasma Synthesis of Nanocrystals: Fundamental

Review pubs.acs.org/CR

Nonthermal Plasma Synthesis of Nanocrystals: Fundamental Principles, Materials, and Applications Uwe R. Kortshagen,*,† R. Mohan Sankaran,‡ Rui N. Pereira,§,∥ Steven L. Girshick,† Jeslin J. Wu,† and Eray S. Aydil⊥ †

Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, United States § Department of Physics and I3N, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal ∥ Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany ⊥ Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡

ABSTRACT: Nonthermal plasmas have emerged as a viable synthesis technique for nanocrystal materials. Inherently solvent and ligand-free, nonthermal plasmas offer the ability to synthesize high purity nanocrystals of materials that require high synthesis temperatures. The nonequilibrium environment in nonthermal plasmas has a number of attractive attributes: energetic surface reactions selectively heat the nanoparticles to temperatures that can strongly exceed the gas temperature; charging of nanoparticles through plasma electrons reduces or eliminates nanoparticle agglomeration; and the large difference between the chemical potentials of the gaseous growth species and the species bound to the nanoparticle surfaces facilitates nanocrystal doping. This paper reviews the state of the art in nonthermal plasma synthesis of nanocrystals. It discusses the fundamentals of nanocrystal formation in plasmas, reviews practical implementations of plasma reactors, surveys the materials that have been produced with nonthermal plasmas and surface chemistries that have been developed, and provides an overview of applications of plasma-synthesized nanocrystals.

CONTENTS 1. Introduction 2. Fundamentals of Nonthermal Plasmas 2.1. Nonequilibrium 2.2. Plasma Sheaths 2.3. Average Nanoparticle Charge 2.4. Nanoparticle Charge Distribution 2.5. Nanoparticle Generation and Growth 2.5.1. Nanoparticle Nucleation 2.5.2. Nanoparticle Coagulation 2.5.3. Nanoparticle Surface Growth 2.6. Nanoparticle Heating in Plasmas 3. Practical Implementation of Synthesis Reactors 3.1. Low Pressure Synthesis 3.1.1. Batch Reactors 3.1.2. Continuous Flow-Through Reactors 3.2. Atmospheric Pressure Synthesis 4. Synthesis of Elemental and Alloy Group IV Semiconductor Nanocrystals 4.1. Silicon 4.1.1. Nonquantum Confined Si Nanocrystals 4.1.2. Quantum Confined, Luminescent Si Nanocrystals 4.2. Germanium 4.3. Carbon 4.4. Group IV Alloys © 2016 American Chemical Society

5. Doped Group IV Semiconductor Nanocrystals 5.1. Phosphorus Doping of Silicon Nanocrystals 5.1.1. Direct Observation of Dopants 5.1.2. Phosphorus Incorporation Efficiency and Surface Segregation 5.1.3. Doping Compensation and Donor Electron Concentration 5.2. Boron Doping of Silicon Nanocrystals 5.3. Doping of Germanium and Silicon−Germanium Nanocrystals 5.4. Doping and Light Emission Properties 5.5. Electronic Properties of Dopants: Confinement and Isolation 5.5.1. Quantum and Dielectric Confinement 5.5.2. Dopant Isolation and Dopant−Dopant Interactions 5.6. Charge Transport 6. Synthesis of Compound Semiconductor Nanocrystals 6.1. Oxides 6.2. Nitrides 6.3. Phosphides

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Special Issue: Nanoparticle Chemistry Received: January 17, 2016 Published: August 23, 2016 11061

DOI: 10.1021/acs.chemrev.6b00039 Chem. Rev. 2016, 116, 11061−11127

Chemical Reviews 6.4. Sulfides 7. Synthesis of Metal Nanoparticles and Other Nanostructures 7.1. Homogeneous Nucleation from Metal−Organic Vapors 7.2. Homogeneous Nucleation from Solid Metal Targets 8. Surface Chemistry 8.1. Silylation and Germylation 8.1.1. Liquid Phase Reactions 8.1.2. Gas Phase Reactions 8.2. Organometallic Reactions with Si and Ge Surfaces 8.3. Silanization 8.4. Hypervalent Reactions with Solvents 8.5. Inorganic Surface Coatings 8.6. Polymer Surface Coatings 9. Applications 9.1. Semiconductor Nanocrystal Plasmonics 9.1.1. Impurity-Doped Nanocrystals 9.1.2. Self-Doped Semiconductor Nanocrystals 9.1.3. Surface-Doped Nanocrystals 9.2. Doping of Crystalline Bulk Semiconductors 9.3. Thermoelectrics 9.4. Electronic Devices 9.4.1. Diodes 9.4.2. Field-Effect Transistors 9.5. Light-Emitting Devices 9.6. Photovoltaics 10. Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

in review articles by other authors.11,12 Generally, gas-phase processes can operate under conditions that are close to thermal equilibrium or very far from it. Approaches in the former category include flame synthesis,13,14 thermal pyrolysis in furnaces,15,16 laser pyrolysis,17,18 and thermal plasmas.19,20 These synthesis methods rely on providing energy in some form to a volume of gas by heating the gas to high temperatures such that thermal decomposition of nanoparticle precursors becomes an important if not the predominant mechanism. A common attribute of these synthesis processes is that the nanoparticles are often not charged and are thus prone to agglomeration. As particles are also at high temperatures, they may rapidly fuse, which in turn may result in broad particle size distributions. Gas-phase approaches of this kind are outside the scope of this review, and the reader is referred to the appropriate literature. Nonthermal plasmas, the focus of this review, are at the opposite end of the spectrum of gas-phase synthesis approaches in that they are very far from thermal equilibrium. As we discuss in detail in the following sections, nonthermal plasmas feature widely different temperatures of their constituent species: the heavy gas species are often at temperatures very close to room temperature, while free plasma electrons can achieve temperatures between ∼10 000 and 50 000 K. Collisions between these hot electrons and molecules very effectively dissociate and ionize gaseous nanoparticle precursors, producing highly reactive radicals and ions. These radicals and ions react exothermically on the nanoparticle surfaces, which heats the nanoparticles to hundreds of Kelvin above the neutral gas temperature. The presence of such heating mechanisms is essential for forming nanocrystals. The plasma electrons also charge nanoparticles in the plasma negatively, reducing or eliminating agglomeration, in contrast to most other gas-phase processes where agglomeration is difficult to avoid. To summarize, the following list of attributes, taken in combination, sets nonthermal plasmas apart from liquid phase and other gas-phase synthesis approaches: • Nonthermal plasma synthesis is inherently solvent and ligand-free, which enables the synthesis of nanocrystals with high purity; • High melting point materials can be synthesized in crystalline form since nanoparticles are intensely heated by surface reactions; • Nanoparticles immersed in the plasma are primarily negatively charged, which prevents particle agglomeration, enabling narrow size distributions as well as the synthesis of very small nanocrystals with diameters between 2 and 10 nm; • The walls of the plasma synthesis reactors are also negatively charged, confining nanoparticles to the plasma and reducing diffusion losses; • Electron collisions with molecules create highly reactive growth precursors, making the nanoparticle growth largely irreversible and driving reactions at low temperatures, thus allowing materials to be grown far from their chemical equilibrium; • The irreversible nature of particle growth in plasmas enables the efficient inclusions of dopants during nanoparticle growth. In the following sections, we introduce the basic principles of nonthermal plasma synthesis and discuss practical implementations of nonthermal plasma synthesis reactors. We then review

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1. INTRODUCTION Ever since the discovery of size-dependent electronic properties in nanometer-sized crystals of semiconductors,1,2 their synthesis, characterization, and applications have intrigued a significant fraction of the scientific community. Nanocrystals have led to promising advances in the fields of third generation solar cells,3,4 quantum dot light emitting devices,5,6 and bioimaging,7 to name just a few. The majority of nanocrystal research is based on materials that are conveniently synthesized in the liquid phase, most prominently, group II−VI and IV−VI semiconductors. Numerous excellent reviews have been dedicated to these materials, e.g., refs 8−10. Several more articles in this thematic issue will review the state of the art in liquid phase synthesis of nanocrystals. The temperatures of liquid phase synthesis approaches, however, are limited by the boiling points of available solvents. Thus, the liquid phase synthesis of some materials is inherently more difficult as they usually require higher temperatures to produce in crystalline form. This is where gas phase approaches excel, as they do not require organic solvents during the synthesis process and are thus inherently capable of high process temperatures. A wide variety of gas-phase methods for the synthesis of nanomaterials is available and has been covered 11062


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accelerated and contribute to an exponential growth of the electron density in the reactor. This process is referred to as the breakdown. At steady state, the creation of ions and electrons by these reactions is balanced by diffusion losses to the reactor walls24 or by recombination within the plasma volume. For reasons that are discussed below, electrons and ions are lost from the plasma at equal rates. Hence, the plasma remains electrically neutral with the exception of small regions, called sheaths, in the vicinity of walls in contact with the plasma. This condition is known as plasma “quasi-neutrality”, ne ≈ ni, where ne is the density of electrons and ni is the density of positive ions. As discussed in any introductory textbook on plasma physics,21−23 even small deviations from electrical neutrality generate large electric fields that will immediately restore the quasineutrality condition. The electron-impact ionization of common plasma-forming gases requires electron energies on the order of 5−25 electron volts (eV).25 Electrons in nonthermal plasmas have an energy distribution that is often approximated as a Maxwellian.21 This implies that the average energy of the electron distribution function should be on the order of a few eV, so that there are adequate numbers of electrons in the high-energy tail of the distribution that can ionize gas atoms and molecules to sustain the plasma. While the electrons in the plasma are very “hot”, the heavy species in nonthermal plasmas (ions, atoms, and molecules) remain at very modest temperatures. This is easily understood by comparing the rates of energy absorption (i.e., power, P) from the applied field, E, by the electrons, Pe, and the heavy ions, Pi:

the literature on the synthesis of various nanocrystal materials as well as the doping of nanocrystals in plasmas. While nonthermal plasmas initially attracted attention for their ability to produce high quality nanocrystals of covalently bonded group IV materials, the library of nonthermal plasmasynthesized nanomaterials has recently expanded to include compound semiconductors and metals, which were previously the domain of other synthesis techniques. We complete the review by an overview of surface chemistry approaches for plasma-produced nanocrystals as well as emerging applications of the plasma-produced nanocrystal materials.

2. FUNDAMENTALS OF NONTHERMAL PLASMAS Plasmas used in materials processing are partially ionized gases in that only a small fraction of gas atoms, typically between 10−8−10−3, is ionized. There are numerous excellent books that introduce the reader to the topic of plasmas and their more established applications in thin film etching and deposition.21−23 The reader is referred to these resources for a more comprehensive discussion of the physics and chemistry of plasmas. Here, we give a rather simplified introduction to the properties of plasmas and their essential interactions with nanoparticles and nanocrystals suspended in them. 2.1. Nonequilibrium

Nonthermal plasmas are characterized by a lack of equilibrium between the different species in the plasma, which, among other things, manifests in strongly disparate temperatures of the different plasmas species. In essence a plasma may be viewed as interpenetrating fluids of electrons, ions and neutral molecules, each species with its own translational velocity distribution and therefore temperature. These differences in the translational energy distribution come about because of inefficient energy exchange between these species. In the following, we present a simple explanation for this nonequilibrium environment. As shown in Figure 1, a plasma is typically created by applying an electric field to a gas, which can be at atmospheric

Pe,i = σe,iE2

where σe,i = ene,iμe,i is the electrical conductivity of electrons (subscript e) or ions (subscript i), e is the elementary charge, ne,i is the concentration of electrons or ions, μe,i is the mobility of electrons or ions. Because of plasma quasineutrality, ne≈ ni, the ratio of the electrical powers absorbed by the electrons and the ions scales as μ Pe ≈ e Pi μi

(3)

The mobilities are given by μe,i = e/(me,iνe,i ), where me,i is the electron and ion mass, and νe,i stands for the collisions frequency of electrons and ions, respectively. At a given pressure of the plasma gas, the electron collision frequency is typically by 2−3 orders of magnitude larger than the ion collision frequency,25 but the mass of ions exceeds that of electrons by 4−5 orders of magnitude. Hence

Figure 1. Schematic of elementary processes and ranges of important parameters in nonthermal plasmas. λe is the mean free path of electrons, p the pressure, and E the electric field strength. The terms “O()” in the legend indicate the order of magnitude of the respective properties.

Pe ≈ 102 − 103 Pi

or subatmospheric pressure. The electric field accelerates free electrons, providing some of them with sufficient energy to ionize gas atoms or molecules, A, upon collision according to the reaction: A + efast → A+ + 2eslow

(2)

(4)

which implies that the vast majority of the electrical power provided to the plasma is absorbed by the electrons. Furthermore, the energy exchange between the electrons and heavier species is very poor: only an extremely small fraction of the very light electron’s momentum and kinetic energy can be transferred to heavy species because ΔEkin,e/Ekin,e = 2me/mh, where Ekin,e is the electron kinetic energy and mh is the heavy species mass.26 Accordingly, the electrons are by far the hottest species in a nonthermal plasma.

(1)

When the applied electric field is high enough, the few free electrons present in the gas are accelerated to high enough energies to start an electron avalanche via reaction 1. This avalanche produces more electrons, which in turn are 11063


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Typical temperatures and thermal speeds of important plasma species in a nonthermal plasma are summarized in Table 1.

je =

⎛ e|ϕ | ⎞ 1 1 vth,ene(ϕw ) = vth,ensexp⎜ − w ⎟ 4 4 ⎝ kBTe ⎠

(5)

where νth,e = (8kBTe/πme) is the thermal speed of the electrons, kB is the Boltzmann constant, Te is the electron temperature, ϕw is the wall potential (in volts) referenced to the potential of the plasma at the boundary of the electrically quasineutral plasma, referred to as the sheath boundary, and ns ≈ ne ≈ ni are the electron and ion densities at that position (and ne ≈ ni because of quasineutrality). The negative walls attract the positive ions and accelerate them. Bohm, in his famous 1949 article, showed that the ions reach a directed drift velocity at the sheath boundary νb = (kBTe/mi)1/2, which is significantly larger than their thermal velocity28 (see also the discussion in ref 27). Here, mi is the ion mass. Assuming no additional ions are generated within the sheath, the ion flux is given by 1/2

Table 1. Typical Temperatures and Speeds of Important Plasma Species species

typical temperature (eV)

typical temperature (K)

typical speed (m/s)

e Ar+ He+

1−5 0.025−0.1 0.025−0.1

∼11 000−55 000 300−1200 300−1200

6.6 × 105−1.5 × 106 ∼400−800 ∼1200−2400

2.2. Plasma Sheaths

The large difference in the thermal speeds of the plasma electrons and positive plasma ions has an important consequence that distinguishes the plasma environment from neutral gas phase environments: the formation of electrical “sheaths” in front of any wall bounding the plasma. This includes reactor walls, electrodes in contact with the plasma, and also the surfaces of nanoparticles immersed in the plasma (Figure 2).

ji = nsv b

(6)

Balancing the electron flux, eq 5, with the ion flux, eq 6, and solving for |ϕw| yields

|ϕw | =

kBTe mi ln 2e 2πme

(7)

For typical plasma-forming gases, the value of the logarithm varies from 5.7, for hydrogen ions, to 9.4, for argon ions. This means that the typical planar wall potential varies between |ϕw| ≈ 2.8 kBTe/e, for hydrogen ions, and 4.7 kBTe/e, for argon ions. Hence, all solid surfaces in contact with a nonthermal plasma will charge to negative potentials of several volts to a few tens of volts with respect to the plasma potential. A scaling analysis of Poisson’s equation with the assumptions used to derive eq 7 yields an approximate length scale for the width of a plasma sheath, the Debye length, which is given by27

Figure 2. Schematic of the charging of nanoparticles and reactor walls in a nonthermal plasma. The arrows exemplify velocity vectors. The velocity vectors of the ions are too short to be depicted. (Size of the nanoparticle is not to scale.)

⎛ ε k T ⎞1/2 λD,e = ⎜ 0 B2 e ⎟ ⎝ nee ⎠

(8)

where ε0 is the vacuum permittivity. The above analysis is based on a one-dimensional model, which is not applicable to nanoparticles in a plasma, as discussed in the next section. However, the basic fact that nanoparticles in the plasma are negatively charged due to the large thermal speed of electrons remains valid.

In a simple thought experiment, consider an initially neutral solid wall, which by some means is immersed and suspended in the plasma without any electrical connection to the outside. Immediately, a large electron current charges this wall negatively because electrons are the fastest species in the plasma. This charging continues for a while during which the electrical potential of the wall becomes more negative compared to the surrounding plasma. However, as the wall potential becomes more negative, the electron current starts to decrease because electrons have to overcome an increasingly negative potential to reach the wall (i.e., an electric field is created to repel and reduce the electron current flowing to the surface). At the same time, the negative potential at the wall attracts and accelerates positive ions toward the surface increasing their velocity significantly above their thermal speed. This process reaches a steady state when the wall potential has become sufficiently negative to balance the electron and ion currents flowing to the wall.27 The potential of a planar wall can be estimated from the balance of the electron and ion fluxes using a simple onedimensional model. As electrons experience a repulsive potential, their density decreases following a Boltzmann distribution when approaching the wall. At the wall, the thermal electron flux is

2.3. Average Nanoparticle Charge

Nanoparticle formation in plasmas involves the charging of particles that are often only a few nanometers in size and much smaller than the collision mean free paths of the electrons and ions. Hence, a one-dimensional analysis is not applicable. A more applicable model is based on the orbital motion limited (OML) theory, which posits that electrons and ions travel on collisionless trajectories in the electrostatic potential around the nanoparticle. This problem was first treated theoretically by Mott-Smith and Langmuir29 and later solved self-consistently by Allen, Boyd, and Reynolds30 and by Bernstein and Rabinovitz.31 The authors pointed out that, in the collisionless motion of positive ions around a spherical particle, the angular momentum needs to be taken into account as it adds a repulsive component to the attractive electrostatic potential. Due to the attractive potential experienced by the positive ions, the effective collection cross section of the spherical particle is increased beyond its geometric cross section by a factor of 11064


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Review

around the particle with radius R0 is trapped in the potential well and will eventually be collected by the particle and contribute to the ion current. By calculating the probability of an ion colliding within a capture sphere of radius R0, the authors derived an analytical model for the ion current in the weakly collisional regime, defined by λi ≳ R0. They found that the particle potential predicted by this model was about a factor of 2−3 smaller than the potential predicted by the OML theory and that it matched their experimental measurements on micron-sized particles quite well. This analytical model was also in good agreement with the analytical theory developed by Lampe et al.38,39 D’yachov et al. attempted to develop a model that bridged the range of collisionality from the collisionless OML limit to the highly collisional hydrodynamic limit.42 However, their model was based on the particle radius, a, as the scaling length and agreement with experimental data was limited. Gatti and Kortshagen43 developed a model that specifically described the charging of nanoparticles in the plasma. They adopted the concept of a capture radius and showed that for nanoparticles, the capture radius R0 can be approximated as

(9)

where ϕp is the particle potential, which is on the order of a few times Te. Since Te ≫ Ti, according to Table 1, the effective cross section of the particle for ion collection is larger than its geometric cross section by a factor that is on the order of 100. This enhances the ion current per unit area collected by a particle and, consequently, spherical particles are less negatively charged than predicted by the simple planar geometry analysis leading to eq 7. A detailed analysis shows that the particle potential ϕp according to the OML theory is ∼2.4 kBTe/e for argon ions, compared to ∼4.7 kBTe/e for the simple onedimensional analysis that yields eq 7. Detailed reviews of the OML theory can be found in the articles by Allen and coauthors.32,33 Daugherty et al. developed a sophisticated kinetic model to investigate the validity of the OML theory for very small particles in collisionless plasmas and found that the OML theory is indeed a good approximation.34 They showed that the potential around small particles (e.g., nanoparticles) is well described by the Debye−Hückel potential, with a linearized Debye length as the relevant decay length scale. This linearized Debye length is often much closer to the ion Debye length than to the much larger electron Debye length, eq 8. In recent years, a much improved understanding of the very important impact of collisions on the charging of nanoparticles has emerged. It is now understood that collisions of ions with neutral atoms, in particular, charge exchange collisions, according to + + A fast + A slow → A fast + A slow

R0 =

e|φp| E kin,i

a (11)

where Ekin,i is the kinetic energy of the ions. Their model described the ion current flowing to a nanoparticle as a sum of three contributions: (i) the collisionless OML current, (ii) the collision-enhanced current for the transitional regime, and (iii) the highly collisional hydrodynamic regime. The authors derived a general expression for the ion current to the nanoparticle by using weight factors, which turned these three components on or off, depending on the collisionality of the plasma. The authors also used a particle-in-cell Monte Carlo simulation to independently determine the particle charge and potential and found that their simple analytical model was in good agreement with the numerical simulation over a range of 6 orders of magnitude in pressure. Figure 3a shows results from their paper in terms of the normalized particle potential z = e|φp|/kBTe versus the Knudsen number KnR0 = λi/(2αR0), where 2R0 is the diameter of the capture sphere and α is a factor on the order of 1, arising from the assumption that the ion energy distribution is Maxwellian. Note that KnR0 ∝ 1/p, where p is the pressure, i.e., in Figure 3a, low pressures are toward the right and high pressures are toward the left. At large Knudsen numbers (KnR0 ≫ 1), the OML theory applies, and the particle potential, for the case of argon ions, is ϕP ≈ 2.4kBTe/e. In the transition regime (KnR0 ≈ 1) the ion current toward the particle is strongly enhanced by the impact of collisions, and the particle potential drops to ϕP ≈ 0.5kBTe/e, i.e., about a 5-fold reduction from the OML prediction. At smaller Knudsen numbers (KnR0 ≪ 1, i.e., large p), the ion transport toward the particle becomes highly collisional, which leads to a reduction of the ion current and more negative particle potentials. Figure 3b from another numerical study of the charging and ion impact energies on nanoparticles exemplifies the collisional enhancement of the ion current for a 500 nm particle in an argon plasma.44 Compared to the OML prediction at very low pressure, the ion current is enhanced by more than a factor of 3 as a result of collisions in the transition regime KnR0 ≈ 1.

(10)

enhance the positive ion current beyond the predictions of the OML theory. Goree showed that charge exchange collisions in the vicinity of a particle can lead to ions in trapped orbits.35 The author pointed out that the trapping of ions may play a role even at low pressures (low collision frequencies) because both the rates of ion trapping through charge exchange and detrapping through collisions are proportional to the pressure. In a later article, Zobnin et al. used a particle-in-cell simulation to study the effect of collisions.36 They reported that collisions can reduce the negative particle potential by at least a factor of 2 compared to the value predicted by the OML theory. Other authors later confirmed these results through numerical simulations.37 Lampe and co-workers deserve credit for developing an analytical theory for the effect of collisionenhanced ion collection.38,39 The overprediction of the negative particle potential by the OML theory was also confirmed in experiments on micron-sized particles that were injected into the positive column of a DC glow discharge.40,41 In understanding the impact of collisions on nanoparticle charging, it is important to consider the important scaling lengths. For a long time, it had been assumed that the ratio of the ion mean free path, λi, to the particle radius, a, would be the relevant scaling parameter for the validity of the OML theory. However, the papers discussed above suggested that the particle radius was not the appropriate scaling length. In 2005, Khrapak et al. introduced the concept of a capture radius, defined as the radius R0 at which the potential around the nanoparticle decays to a value of −kbTi/e with respect to the surrounding plasma.41 The physical relevance of the capture radius is that every ion produced through a charge exchange collision within a sphere 11065


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n i ≈ ne + n pQ p,ave

where np is the concentration of the nanoparticles, and Qp,ave is the average charge on the particles.45 In this case ni > ne and the negative particle potential will be smaller than those estimated from Table 2. It should be noted that other mechanisms have been proposed that may reduce the amount of negative charge carried by a nanoparticle,46 among these the relaxation of excited species at the nanoparticle surface and photodetachment via UV radiation.47,48 Moreover, the nanoparticle charge may be limited by field emission.49 However, little work has been done to date to assess the importance of these mechanisms. The central message of this section is that nanoparticles will be charged to negative potentials on the order of kBTe/2e, which for typical electron temperatures may imply negative particle potentials of a few volts. Potentials of such magnitude can be achieved by quite modest charges. For instance, a nanoparticle with a radius of a = 1 nm carrying a single electron will have a negative potential of ∼−1.4 V. It is important to note that nanoparticles of different sizes will charge to approximately the same steady state potential, which is required to establish a balance between electron and ion currents reaching the particles. As the capacitance of an isolated sphere is proportional to its radius, the charge of a nanoparticle to establish this steady state potential is also proportional to the particle radius. Hence a good rule of thumb for nanoparticle charging is that a nanoparticle will carry about one to two elementary charges per nanometer of its radius. Accordingly, small nanoparticles carry only very few elementary charges and the stochastic nature of the charging is important, which is discussed in the next section.

Figure 3. (a) Comparison of the results of the analytical model (lines) developed in ref 43 with results of molecular dynamics simulations (symbols). The figure depicts the normalized particle potential z = e|φp|/kBTe versus the ion capture radius Knudsen number KnR0 = λi/ (2αR0). Figure reproduced with permission from ref 43, in which a is used to denote the particle radius. Copyright 2008 American Physical Society. (b) Ion energies, particle potential and ion flux to a 500 nm particle. Reproduced with permission from ref 44. Copyright 2012 Institute of Physics.

2.4. Nanoparticle Charge Distribution

Cui and Goree investigated the stochastic charge fluctuations on particles in plasmas with a numerical Monte Carlo simulation in 1994.50 The authors found that the charge of a nanoparticle is not constant and fluctuates over time due to the stochastic capture of electrons and ions. They specified the root-mean-square fluctuation as 0.5 |N|1/2, where N is the average number of elementary charges. Matsoukas and co-workers51,52 developed an analytical model for the stationary particle charge distribution in a plasma. They developed a set of rate equations that described the time dependence of the particle charge, q, (in multiples of elementary charge ± e) through capture of ions (q → q + 1) and electrons (q → q − 1) with the capture frequencies given by the OML theory. As shown in Figure 4, for a Maxwellian energy distribution of the electrons, the nanoparticle charge distribution is described rather well by a Gaussian envelope. It should be noted that the average charge predicted by Matsoukas’s theory is consistent with the predictions of the OML theory. In ref 52 the authors also point out that the polarizability of the nanoparticles plays a role for very small particles and that the induced image charges lead to a more negative charge distribution. It should be noted that Figure 4 and previous analytical models of particle charge distributions do not account for the fact that the amount of negative charge a nanoparticle can hold is limited by various mechanisms. Recently the effect of singleparticle charge limits on charge distributions was analyzed, and an analytical expression for the charge distribution was derived that modifies the Matsoukas et al. expression.53

Table 2 summarizes results from the above discussion. It should be pointed out that, in each case, the plasma was Table 2. Charging of Large Objects (Planar Wall) and Nanoparticles in a Plasma Assuming Argon Ions as the Dominant Ions geometry

potential (V)

planar wall (ne = ni = ns)

4.7 Te(eV)

nanoparticle (ne = ni = ns): collisionless limit, OML, λi ≫ 2R0 transition regime, λi ≈ 2R0 collisional limit, λi ≪ 2R0

∼2.4 Te(eV) ∼0.5 Te(eV) >0.5 Te(eV)

(12)

assumed to be quasineutral at the plasma-sheath boundary. This may not be a good assumption during the plasma synthesis of nanoparticles because a significant fraction of the electrons may have been acquired by the nanoparticles, which lowers the concentration of free electrons in the plasma. In this case, the plasma is quasineutral in the sense that 11066


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(“agglomerates”) are formed. In the following discussion, we use the term coagulation, as the collision of clusters and small nanoparticles under high temperatures likely leads to their coalescence to form larger spherical particles.] 3. Nanoparticle Surface Growth: In this stage, nanoparticles are negatively charged and their growth continues through surface reactions with the remaining precursor or with precursor that is replenished in the reactor. This three-phase model was originally derived from studies on a plasma enhanced chemical vapor deposition reactor, where nanoparticles can get trapped in the plasma after their formation by electric fields and can grow to tens or even hundreds of nanometers as new precursor continues to enter the reactor.67−69 This is fundamentally different from plasma reactors designed for nanoparticle synthesis, in which particles are often generated in a flow-through configuration. Despite these fundamental differences, the general aspects of Bouchoule and Boufendi’s model are also expected to apply to nanoparticle synthesis reactors and are discussed in more detail below. 2.5.1. Nanoparticle Nucleation. The conventional scenario for homogeneous nucleation, defined as the growth of neutral clusters by condensation of a supersaturated vapor, is typically found in thermal plasmas,70 but is unlikely to be important in nonthermal plasmas. One reason for this is that many of the precursors used in plasma synthesis of nanoparticles such as silane or acetylene inherently lead to cluster growth by a sequence of chemical reactions rather than by physical condensation of monomers. Even when physical condensation of monomers dominates, as may be the case for nucleation of elemental metals, the abundance of ions of the nucleating species is likely to make ion-neutral clustering a more robust pathway than neutral−neutral clustering. This is partly because ion-neutral reactions are generally faster than their neutral−neutral counterparts, and partly because anions are typically confined in the bulk plasma (away from the walls and sheath) by an electric field, with the bounding walls at negative potential with respect to the plasma (section 2.2). Thus, the confined anions spend ample time in the plasma to contribute to nucleation, while neutral species and cations, which can diffuse out of the bulk plasma, may not spend enough time in the plasma to participate in processes that lead to nucleation before they are lost to the walls. Silane has been probably the most studied chemical system in nonthermal plasmas and is the most widely used feedstock gas for plasma synthesis of silicon nanocrystals. Mass spectrometry measurements by Hollenstein and co-workers in the early 1990s, shown in Figure 5, demonstrated that silicon hydride anion clusters can grow to large sizes (several tens of Si atoms) in radio frequency (RF) silane plasmas, while corresponding neutral and cation clusters were detected only at much smaller sizes.71−74 Measurements of radical concentrations by Watanabe et al., together with light scattering from particles, indicated a strong correlation between nucleation and the presence of neutral SiHm radicals with short-lifetimes.75,76 These experiments, together with modeling,77−80 suggest that the dominant mechanism for nucleation in these types of plasmas consists of reactions between silicon hydride anions and small neutral species containing a single Si atom. Perrin et al., based on a review of kinetic data for reactions expected to occur in silane plasmas, together with consideration of confinement of anions versus diffusion of non-negative species,

Figure 4. Typical charge distribution of nanoparticles with a radius of 1 nm in a nonthermal plasma. Reproduced with permission from ref 51. Copyright 1995 AIP Publishing LLC.

The charge distribution of nanoparticles is important for the understanding of the very early stages of nanoparticle growth in a plasma. While Coulomb repulsion prevents negatively charged nanoparticles from aggregating with each other, this does not apply to neutral particles. In particular, for particles 25 nm. Subsequently, Mangolini et al.96 improved the reactor design such that it was capable of synthesizing nanocrystals as small as 2 nm with good control over the nanoparticle size, a narrow size distribution, and control over the crystallinity. A more recent version of this reactor, shown in Figure 14, includes a nozzle at the exit to accelerate the nanocrystals toward a substrate and deposit them via inertial impaction: that is the particles impinge on the substrate surface with very high velocities and form a dense (up to ∼60% solid fraction) film comprising the nanocrystals.160 The reactor scheme typically uses two electrodes that surround a dielectric tube to capacitively couple the applied radiofrequency electric field to the plasma. The tube is usually made of quartz, glass, or alumina. The gases are introduced from one end of the tube and pumped out from the other end. As indicated in Figure 14, the plasma will often couple to one of the metal fittings that hold and seal the tube. This is important because the overall length of the plasma region is an important parameter in defining the residence time of the gas in the plasma zone. The residence time is the most important parameter that determines the nanocrystal size. It is a function of the gas flow rate, the plasma zone length, and the reactor tube inner diameter. For the synthesis of sub-10 nm nanocrystals, the residence time is usually on the order of several milliseconds.96 The plasma is typically maintained at 100−1000 Pa and radio frequency power ranges from several tens to a few hundreds of Watts. Over the past decade, this reactor scheme has proven very versatile for the synthesis of nanocrystals, including elemental 11073


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discussed above. The pressure during plasma operation was on the order of 666 Pa. Shiratani et al. also had a second multihollow cathode plasma next to the one that produced the silicon nanocrystals (Figure 15). This second plasma produced atomic nitrogen to nitride the silicon nanocrystal surfaces in situ. 3.2. Atmospheric Pressure Synthesis

Nonthermal plasmas are typically formed under vacuum (∼10 mTorr to 100 Torr) where ionization of the gas is made easier by longer electron mean free paths. Longer mean free paths give the electrons more time between collisions to be accelerated to energies necessary for ionizing the gas molecules. From a processing standpoint, higher pressures, up to or exceeding atmospheric pressure, are more attractive to avoid the operating and equipment costs associated with maintaining vacuum. This has been the main driving force behind interest in atmospheric-pressure plasmas for nanoparticle synthesis.174 There are several critical challenges facing the implementation of atmospheric-pressure plasmas. Because the mean free path is inversely proportional to pressure, gas breakdown is more difficult at high pressures. Plasmas tend to be more unstable at high pressures as well, either forming as filaments, instead of uniform and diffuse discharges, or thermalizing and transitioning to arcs. As discussed previously, at low pressures, the electron temperature Te and the gas temperature Tg are typically very disparate because the hot electrons ionize a very small fraction of the gas and most of the atoms and molecules remain cold (i.e., near room temperature). As Elenbaas showed in his classic measurements on mercury vapor discharges in the 1950s,175 the electron and neutral gas temperatures tend to approach each other as the pressure is increased. They eventually equilibrate because higher collision frequencies at higher pressures enhance the thermal coupling between the electrons and the gas. These plasmas, in which the electron and the neutral gas temperatures have equilibrated, are called thermal plasmas. Such thermal plasmas have been employed for nanoparticle synthesis.176,177 In particular their high gas temperatures, often in excess of thousands of Kelvin, can allow for nanoparticles to be synthesized from a solid source material. However, thermal plasmas suffer from several disadvantages. First, high temperatures may also melt and evaporate reactor components, introducing contaminants, as in the case of arc welding where nanoparticle formation from the metal parts is undesired for health reasons.178 Second, the high gas temperature inside the reactor can lead to loss of nanoparticles at the colder sections from thermophoresis.179 Third, the thermal equilibrium of plasma species could have several cumulative effects. As discussed previously, nonthermal plasmas heat and crystallize nanoparticles during synthesis through collisions with electrons and ions.112,116,117,120 In contrast, thermal plasmas heat and crystallize nanoparticles through collisions with hot neutral atoms and molecules (i.e., convective heat transfer). Thus, nanoparticle heating and crystallization may still be possible during high pressure synthesis. However, more critically, electron temperature is lower at high pressures than at low pressures. This lower electron temperature will reduce particle charging, as discussed in section 2.3, and particles will be more susceptible to agglomeration.180 Over the past decade, major strides have been made to develop atmospheric-pressure plasma sources that operate nonthermally. One approach that has been proven effective is

Figure 14. Capacitively coupled flow-through plasma reactor to produce sub-10 nm nanocrystals of various materials and deposition mechanism to produce dense nanocrystal films. Reproduced with permission from ref 160. Copyright 2010 Institute of Physics.

semiconductors, doped nanocrystals, and compound semiconductors, as discussed below. Nanocrystal heating in the plasma can be controlled through the applied plasma power, and the negative charge on the nanocrystals helps achieve narrow size distributions. Consequently, this reactor scheme is now used by a number of different research groups.161−170 More recently, Shiratani and co-workers proposed the capacitively coupled “multi-hollow plasma” discharge,171−173 shown in Figure 15. This approach in a sense is a parallelized version of the capacitively coupled plasma reactor discussed above. In Shiratani’s design, radio frequency power is applied to a set of 30 mm diameter disk electrodes which have 5 mm holes drilled through them. These holes are aligned in stacked plates to form discharge channels, each similar to the reactor tube

Figure 15. Schematic of a multi-hollow cathode discharge for the synthesis of silicon nanocrystals. Adopted with permission from ref 173. Copyright 2012 Japan Society of Applied Physics. 11074


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growth of carbon nanotubes from catalyst nanoparticles, which is briefly discussed in section 7.1. Both direct-current (DC) and high frequencies have been used to power microplasmas for nanoparticle synthesis. The DC configuration shown in Figure 17a was originally designed

the use of microplasma sources. Microplasmas are plasmas confined to sizes less than approximately 1 mm in at least one dimension. The small scale in one dimension helps produce large electric fields that help gas breakdown and stability at high pressures. Their small volume also allows efficient heat loss to maintain relatively low gas temperatures while still producing high-energy electrons (>10 eV) similar to low-pressure nonthermal plasmas. Using a simple volume averaged balance,174 it can be shown that the ratio of Te to Tg, which is a measure of the nonthermal characteristics, increases with decreasing plasma size, D, see Figure 16. The dependence of Te and Tg on power illustrates the importance of decreasing D especially at higher power densities.

Figure 17. (a) Illustration of electrode configuration for direct current (DC) microplasma reactor consisting of a stainless steel capillary tube and another stainless steel tube sealed inside a quartz tube. (b) Illustration of electrode configuration for pulsed (VHF or RF) microplasma reactor consisting of powered and grounded copper rings surrounding a quartz capillary tube. A trigger electrode is sometimes used to aid initial ignition of the discharge. (c) Illustration of electrode configuration for DBD microplasma reactor consisting of a powered Cu ring surrounding a quartz tube and a grounded stainless steel capillary and tungsten wire inside. Common to all reactor configurations is the continuous-flow, microreactor geometry, which enables atmospheric-pressure operation and nanoparticle formation.

Figure 16. Estimated ratio of Te to Tg as a function of plasma size, D, at atmospheric pressure, calculated from a 0D energy balance.174 Each curve represents a different power density, decreasing from left to right.

From the perspective of nanoparticle synthesis, microplasmas offer several additional key advantages over other plasma sources.181 The small dimensions combined with the ability to flow gas continuously through the plasma volume provide a microreactor geometry that has been shown to be important in the liquid phase to control reactor residence time distributions for nanoparticle formation.182 Although this has not been rigorously shown for gas-phase studies, the relatively small particle sizes and narrow size distributions that have been produced in microplasmas supports this idea. The continuous flow operability at atmospheric pressure is also valuable as it allows microplasmas to be combined with aerosol (ion) mobility measurements183,184 to detect and study nanoparticle formation online. Briefly, particles formed in the microplasma are charged by soft ion attachment in a radioactive beta emission source, size classified by their aerodynamic mobility, and counted, either by optical scattering after condensing vapor on their surface or by detecting their charge using a Faraday cup. This diagnostic allows particle size distributions to be obtained using a lower detection limit of ∼2.0 nm with commercial instruments, and has recently been pushed to subnanometer sizes with a modified high-sheath-flow setup. Although the measurements do not provide detailed information about the structure or morphology of the particles including distinction between single densified particles and aggregates, it can help rapidly optimize plasma conditions. In addition, kinetic studies of reactions involving nanoparticles are possible such as thermal oxidation of silicon nanoparticles,185 hydrogen desorption from silicon nanoparticle surfaces,186 and

by Sankaran and co-workers187 based on a microhollow cathode discharge,188 but with a stainless steel capillary tube as the hollow electrode to facilitate gas flow.189 Later studies by others have added a coaxial sheath gas flow to further confine the microplasma, cool the electrodes, and push synthesized nanoparticles out of the reactor.190 Compared to pulsed plasmas, DC power is much simpler and easier to implement, but requires conductive metal electrodes which could evaporate or sputter and contaminate the process.190 In addition, the reported production rate for these reactors has been low, on the order of several μg/h (compared to several mg/h or sometimes tens of mg/h in low pressure nonthermal plasmas), necessitating collection times of more than 24 h to obtain sufficient material quantities for characterization. Scale up is possible by fabricating arrays, but challenging because DC operation requires that the microplasma elements are individually ballasted.189 A significant amount of power is dissipated across the ballast, which also makes this approach very energy inefficient. Inspired by the work on low-pressure plasma systems, radio frequency (RF) has been employed to operate atmosphericpressure microplasmas as well. RF electronics are complex and need matching networks but are highly efficient and capacitively couple power to the microplasma with the electrodes on the outside of the reactor, eliminating any 11075


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nanocrystals from silane (SiH4) diluted in Ar background gas at a mixture ratio of 2:98. They maintained the plasma at 26.6 Pa using 400 W of power. Compared to many other studies, the silane flow rates were rather high, between 10 and 30 sccm, which likely is the reason for their ability to grow larger silicon nanocrystals at relatively short residence times of about 10 ms. The Si nanocrystals were 5−15 nm in diameter and mostly spherical and covered by an amorphous shell. The authors noted that at high plasma powers, the collection of particles decreased significantly and a film like deposit was formed. This may indicate that the density in the inductively coupled plasma was too high for nanocrystal nuclei to form in the plasma, as the intense surface heating in the plasma may have led to disintegration of clusters and nuclei. Oda and co-workers studied the formation of Si nanocrystals in the very high frequency plasma batch reactor, previously discussed in section 3.1.2 (Figure 9).124 The authors found that the use of undiluted SiH4 as a precursor yielded a very broad size distribution with particle sizes ranging from less than 5 to more than 30 nm. They attributed the broad size distribution to a lack of control of the residence time of silicon crystals in the plasma. By introducing gas pulses of H2, the authors succeeded in producing Si nanocrystals with significantly narrower size distributions with mean diameters around 8 nm and a narrow spread (±1 nm). By controlling the residence time between ∼0.4-3 s, they were able to adjust the particle size between 8 and 25 nm. The authors attributed this to better control of the nanocrystal growth time in their batch reactor and suggested that the nanocrystals would be ejected from the reactor by the periodic H2 pulses. In a later study195, Oda and co-workers observed that larger (∼20 nm diameter) silicon nanocrystals were faceted. Using the microwave reactor reviewed in section 3.1.2 (Figure 12), Wiggers and co-workers produced silicon nanocrystals from SiH4 diluted in argon and hydrogen.150,151 Typical gas pressures were in the range of 2000−5000 Pa and the silane concentration was on the order of 0.2−0.4%. At microwave powers ranging from 300 to 550 W, they observed Si nanocrystals with average sizes from 6 to 11 nm. In ref 196 the authors focused on producing luminescent Si nanocrystals, however, they also produced significantly larger nanoparticles. Interestingly, they also note some faceting among larger Si nanocrystals. Bapat et al. used a capacitively coupled plasma to synthesize silicon nanocrystals of 20−30 nm in diameter.159 The reactor consisted of a quartz tube with 47 mm inner diameter. The precursor was silane diluted in helium (5:95) and further diluted in argon, and plasma powers were ∼200 W. Particle residence times were as long as several seconds. The authors discovered that within a narrow window of synthesis conditions, the Si nanocrystals were strongly faceted, with cube-like morphologies as shown in Figure 18. The cube faces were aligned with Si [100] planes. The formation of cubic particles was found to be linked with the formation of a rotating plasma filament, which the authors associated with a thermal instability facilitated by using a large diameter quartz tube. Laser scattering studies indicated that particles were created upstream of the constricted plasma zone. In a later study,197 the authors identified two distinct plasma regions: a diffuse plasma zone for particle growth and a constricted plasma zone for particle annealing/sintering. Based on measurements of the plasma properties in the zone of the constricted plasma and on a simple model for particle heating, the authors suggested that

metal contamination. In general, the configuration for RFpowered microplasmas shown in Figure 17b consists of two metal (e.g., Cu) ring electrodes, one powered and one electrically grounded, surrounding a dielectric (e.g., quartz) capillary which now confines the plasma. A third metal ring is sometimes powered closer to the grounded electrode to initially trigger the ignition (breakdown) of the discharge. Nozaki and co-workers were the first to study Si nanoparticle synthesis using this approach at very-high radio frequencies (144 MHz).191 The nanoparticles were deposited onto a glass substrate 30 mm away from the end of the discharge as a film at relatively fast rates (3 μm/min). Mariotti and co-workers studied synthesis using the more typical and lower frequency power applied at 13.56 MHz.192 Nanoparticles were collected by flowing the reactor effluent into liquid ethanol. The estimated mass production rate was ∼100 μg/h, significantly higher than that achieved using a DC microplasma. Although the precursors were fed into the reactor at higher mass flow rates in this study, these results still support the higher efficiency of pulsed microplasmas which should dissociate a higher percentage of the precursor to form nanoparticles. Further increase in the production rates is possible by scaling up these reactors in the dimensions perpendicular to the flow direction.193 A special case of a pulsed plasma is the dielectric barrier discharge (DBD) which is perhaps the most common type of atmospheric-pressure plasma used in industry because it combines simplicity, not requiring matching networks, with the ability to scale up. Recently, the DBD configuration shown in Figure 17c was studied for nanoparticle synthesis. This configuration consisted of a powered Cu ring electrode surrounding a quartz tube.194 Nanoparticle synthesis was characterized both with a tungsten wire and a stainless steel capillary tube inserted coaxially within the quartz tube as the grounded electrodes. Interestingly, the presence of the metal capillary as the gas injector was found to produce much smaller and narrower size distributions of nanoparticles. The authors surmised that this arrangement increased the gas velocity through the microplasma and produced a sharper residence time distribution.

4. SYNTHESIS OF ELEMENTAL AND ALLOY GROUP IV SEMICONDUCTOR NANOCRYSTALS 4.1. Silicon

Among plasma-produced semiconductor nanocrystals, silicon has attracted by far the greatest attention to date. This is based on the fact that silicon is a material that is hard to synthesize in crystalline form in the liquid phase. Compared to many other semiconductor materials, it is also benign and abundant. Silicon nanocrystals have been studied for a wide range of applications, from building blocks of electronic films, electronic devices, thermoelectric materials, battery anodes, and even for its lightemissive properties. Applications other than light emission often benefit from larger nanocrystals, while nanocrystals smaller than the Bohr exciton radius (∼5 nm) are desired for applications that focus on light emission. 4.1.1. Nonquantum Confined Si Nanocrystals. The formation of silicon nanocrystals larger than 5 nm is usually achieved by using high precursor concentrations and/or long residence times of nanoparticles in the plasma. Gorla et al.157 used the inductively coupled plasma discussed in section 3.1.2 (Figure 13) at low pressure to synthesize Si 11076


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pressure plasmas though atmospheric pressure plasmas have also been used successfully to synthesize quantum dots. These studies rely on short nanoparticle residence times, usually on the order of several milliseconds and modest precursor concentrations. Mangolini et al. reported the synthesis of silicon nanocrystals with sizes tunable between 3 and 6 nm in a capacitively coupled plasma, Figure 19.96 The plasma reactor tube had an inner diameter of 6.3 mm and volumetric flow rates were between 40 and 60 sccm, resulting in residence times of the nanocrystals in the plasma zone of ∼2−7 ms. Typical pressures were between 186 and 1866 Pa and the precursor was SiH4 diluted in argon at a ratio of ∼1:100 to 1:40. The authors demonstrated luminescence of oxidized silicon nanocrystals with peak emission ranging from 700 to 850 nm. At the time, the synthetic yield of the process was quite high with up to 50 mg/ h. While the luminescence in this study was achieved by growing a native oxide on the silicon nanocrystal surfaces, the authors showed in subsequent studies that an even larger luminescent potential can be unlocked by organically passivating the Si nanocrystals. By attaching alkene organic ligands to the nanocrystals, photoluminescent quantum yields as high as ∼70% were achieved (Figure 20).201,202 Unfortunately, such efficient photoluminescence was only achieved in the near-infrared and red range of the spectrum, and quantum yields quickly dropped off toward the green and blue range of the spectrum. Anthony and Kortshagen later investigated the importance of the silicon nanoparticle crystallinity for the photoluminescence quantum efficiency.203 They found that crystalline silicon nanoparticles have a significantly higher quantum yield than amorphous silicon particles of the same size. Anthony et al. also pointed out that the injection of H2 downstream of the synthesis plasma is essential for achieving high photoluminescent quantum yields.204 The downstream hydrogen injection was important for achieving the best possible surface coverage of silicon nanocrystals with hydrogen, which proved beneficial for the photoluminescence even after surface coverage of the silicon crystals with organic ligands. Pereira et al. used electron paramagnetic resonance to show that downstream hydrogen injection led to silicon nanocrystals

Figure 18. Silicon nanocrystals with cubic shapes synthesized in a capacitively coupled plasma reactor operating in a constricted plasma mode. Reproduced with permission from ref 197. Copyright 2007 Institute of Physics.

the particle temperature exceeds the gas temperature by several 100 K, which enabled surface diffusion to attain nanocrystals with a shape close to their equilibrium shape. For hydrogen terminated silicon surfaces, Barnard and Zapol had predicted the cube shape to be the minimum energy shape for silicon nanocrystals.198 The silicon nanocubes were later used in several studies to demonstrate vertical surround-gate transistors.199 Yasar-Inceoglu et al. studied the formation of silicon nanocrystals from SiCl4 as a precursor diluted in argon.161 The authors found that admixtures of H2 are required to scavenge some of the chlorine and no nanocrystal formation is observed without H2 addition. Different from the synthesis with silane, nanocrystals produced from SiCl4 have a largely chlorine terminated surface. The authors pointed out that such nanocrystals are much more prone to oxidation compared to nanocrystals produced from silane. However, Wheeler et al. showed an interesting benefit of the presence of Si−Cl groups on the Si nanocrystal surfaces in that they engage in hypervalent interactions with hard donor molecules. The authors found that these interactions provide colloidal stability of the silicon nanocrystals without the use of additional surfactants.200 4.1.2. Quantum Confined, Luminescent Si Nanocrystals. One of the major attractions of nonthermal plasmas is their ability to produce quantum-confined, luminescent silicon nanocrystals that are smaller than ∼5 nm (also called quantum dots). A majority of the studies were conducted with low-

Figure 19. Silicon nanocrystals produced using a flow-through capacitively coupled radiofrequency reactor. Reproduced with permission from ref 96. Copyright 2005 American Chemical Society. 11077


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electron hole pairs are generated upon absorption of a single photon. The authors studied 9.5 nm Si crystals and found an MEG threshold of 2.4 × Eg, where Eg is the band gap energy. This was the first report of MEG in an indirect bandgap semiconductor. In a later study, Bergren et al.162 measured the time-resolved electrical properties of isolated Si nanocrystals. Upon femtosecond excitation at 400 nm, i.e., below the direct bandgap of Si, the authors observed hot carrier generation and cooling on a time scale of ∼2 ps, at which time excitons are formed. They measured a multiple exciton lifetime of ∼50 ps followed by a long-lived single exciton lifetime >7 ns. Sykora et al. investigated the size-dependence of the fluorescence lifetime of Si nanocrystals.209 While it was commonly believed that the efficient photoluminescence of Si nanocrystals is an indication of a direct bandgap character of very small Si crystals, the authors showed that even for very small silicon nanocrystals with diameters of ∼2 nm, the radiative lifetimes were still very long and >100 ns, indicative of a largely indirect bandgap behavior. Hannah et al.210 performed a pressure-dependent study of the photoluminescence and X-ray diffraction of alkaneterminated colloidal Si crystals. The authors observe several transitions of the crystalline phases at high pressures. Importantly, they found a systematic photoluminescence red shift that matches the X(conduction band)-to-Γ(valence band) transition of bulk crystalline silicon. These results suggest that photoluminescence originates from core states that remain indirect despite quantum confinement. However, Wen et al. suggest that this is only correct for particles that emit in the yellow-red range of the spectrum and that blue-green emission is linked to surface states rather than quantum-confined core emission.211 In a later study, Hannah et al.212 associated a fast photoluminescence component that is sometimes observed in time-resolved PL measurements of Si nanocrystals with an amorphous surface layer. A recent Raman spectroscopy study by Zhang et al. also supports the presence, at least under some conditions, of an amorphous shell surrounding Si nanocrystals synthesized in a plasma.213 In another Raman spectroscopy study, Sagar et al. found that the longitudinal optical (LO) phonon spectrum is asymmetrically broadened toward the low energy side. They also observe antiresonance on the highenergy side, which is a characteristic of a Fano line shape. The authors propose that this Fano line shape results from the interference of the optical phonon with photoexcited carriers.163 While the size distribution of plasma-produced Si nanocrystals is relatively narrow, improvements in optical properties can be achieved using size filtering. For example, Miller et al. narrowed the size distribution of plasma-produced colloidal Si nanocrystals using size separation via density gradient ultracentrifugation214 and achieved narrower PL line widths. They also reported significant PL enhancement in films assembled from these Si nanocrystals with narrow size distribution. They attributed this enhancement to Si nanocrystal interactions in ordered ensembles of monodisperse nanocrystals. In a later study, the authors also reported changes in the low-temperature photoluminescence of these Si nanocrystals215 as well as enhanced stability of the photoluminescence in aggregates.216 A method to link the PL emission line shape to the Si nanocrystal size distribution was recently reported in ref 217. Atmospheric-pressure plasmas have also been employed to synthesize Si nanoparticles. Sankaran first demonstrated Si nanoparticle synthesis from mixtures of Ar and SiH4 gas using a

Figure 20. Photoluminescence quantum yield of hydrosilylated plasma-produced silicon nanocrystals. Reproduced with permission from ref 202. Copyright 2006 Elsevier Limited.

with extremely low defect density, on the order of 2−5 EPR active defects per 1000 4 nm silicon crystals.205 If the goal is to synthesize very small nanocrystals 3 μm/min at >1%. Abstraction reactions between hydrogen species and SiCl4 to remove chlorine have been previously shown to be critical in the deposition of Si thin films220 and, as previously described in low pressure plasma synthesis, could have similar effects in Si nanoparticle formation. The H2 concentration also strongly influenced the optical properties, but in an unsystematic way. At very low concentrations of 400 m/s which allows for high-speed impaction on substrates that are sufficiently close to the nozzle. For Ge nanocrystals, the authors report film densities exceeding 50% of the solid state density, which is close to the limit for randomly packed spheres. An attractive feature of this method is that nanoparticles are deposited without the use of any solvents or ligands, which avoids the need to later remove those organic substances. Based on films prepared by this deposition technique, Holman and Kortshagen228 reported optical measurements of the absorption edge of films comprised of Ge nanocrystals. They found band gaps extracted from the absorption data that varied from the near-bulk value of 0.73 eV for 9.0 nm nanocrystals to over 1.0 eV for 4.7 nm nanocrystals. Interestingly, these values are comparable to those of isolated Ge nanocrystals, which suggests that Ge nanocrystals in a dense nanocrystal film are still essentially quantum confined. Holman and Kortshagen also studied methods to produce colloidal solutions of Ge nanocrystals. In ref 229 they showed that hydrogermylation is an effective method to graft alkene

Figure 24. Narrow, size-tunable bandgap emission from dodecylmagnesium bromide reacted chlorine-terminated Ge nanocrystals. Adapted with permission from ref 232. Copyright 2013 American Chemical Society.

produced Ge nanocrystals, with the error bars indicating the full width at half-maximum (fwhm) of the emission. The optical emission features are by at least a factor of 2 narrower than those reported for other synthesis methods, indicating that the plasma synthesis produces a narrower size distribution than other approaches. 11080


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4.3. Carbon

from selected area electron diffraction (Figure 25a). Energydispersive spectroscopy (EDS) confirmed that the material was

Carbon presents a unique material because of its diversity, particularly at the nanoscale, and ability to exhibit metallic or semiconducting properties. There have been numerous studies of carbon nanomaterial synthesis in plasmas. In fact, the discovery of fullerene was essentially based on a plasma formed by laser vaporization of graphite targets by Krato, Smalley, and co-workers,233 and fullerene has also been produced by arc discharges. Arc discharges were also behind the discovery of carbon nanotubes.234 However, the focus of this review is nonthermal plasmas, and we only review carbon nanomaterials synthesized by this class of plasmas. A benefit of employing nonthermal plasmas for the synthesis of carbon nanomaterials is the possibility of synthesizing diamond-phase carbon. Diamond has long been known as the metastable form of carbon at normal conditions. Traditionally, plasmas have been used to deposit diamond films (i.e., plasmaenhanced chemical vapor deposition) because of their ability to produce atomic hydrogen, which has an important role in suppressing the growth of nondiamond phases, stabilizing the diamond surface, and preferentially etching graphitic phases.235 A small number of studies dating back to at least 1979 by Fedoseev and co-workers have explored the possibility of homogeneously nucleating diamond in the gas phase using plasmas.236 However, much of this work was theoretical and only empirical observations were made with very little supporting materials characterization. More thorough investigations of this idea were carried out by Frenklach and coworkers using a low-pressure microwave plasma from chlorinated (e.g., dichloromethane)237 and combustible mixtures of acetylene and oxygen.238 The as-synthesized material was acid treated to remove the nondiamond fraction and relatively large diamond crystals ∼50 nm to 0.1 μm in size were recovered and characterized by electron microscopy. The selectivity for diamond versus nondiamond phases was later improved by seeding the growth with silane or diborane which nucleate small clusters and support the nucleation and growth of diamond.239 More recently, Rouzaud and co-workers demonstrated the synthesis of nanosized diamond particles 2 to 10 nm in size, known as nanodiamonds, in a microwave plasma from mixtures of either CO and H2 or CH4 and CO2.90,240 The gas-phase chemistry has been known to be key to promoting the growth of diamond, and in particular, the C− H−O ratio has been universally connected across different reactors and gas mixtures.241 Another underlying principle that may be relevant to diamond formation by homogeneous nucleation is the unique stability of this phase at the nanoscale. Theoretical calculations on clusters of carbon less than ∼3 nm in size with hydrogen surface terminations have shown that sp3 forms, the precursor to diamond, are energetically more stable than the sp2 forms, the precursor to graphite.242 This prediction has been supported by interstellar observations of diamond243 and recovery of nanodiamonds in meteoritic residues.244 More recently, Sankaran and co-workers have produced nanodiamonds with sizes commensurate with these theoretical predictions using a DC microplasma (Figure 17a).245 In their experiments, ethanol was dissociated, and particle nucleation was monitored by on line aerosol mobility measurements. The carbon product was collected by filtration and further characterized by materials analysis. Transmission electron microsopy (TEM) revealed the existence of nanodiamonds with 2−5 nm diameters, as confirmed by their lattice spacings

Figure 25. (a) TEM image of carbon nanoparticles synthesized from mixture of Ar and ethanol vapor in an atmospheric-pressure DC microplasma. Inset shows selected area electron diffraction which contains diffraction spacings corresponding to various structures of diamond. (b) EDS of carbon nanoparticles showing only the presence of carbon (Cu peaks are from the TEM grid). (c) UV micro Raman spectra (λex = 325 nm) of carbon nanoparticles synthesized from gas mixtures of Ar and ethanol vapor (black) and Ar, ethanol vapor, and H2 gas (red). Insets show the deconvolution and fitting of corresponding spectra. (d) XRD of carbon nanoparticles synthesized from gas mixtures of Ar and ethanol vapor (black) and Ar, ethanol vapor, and H2 gas (red). Peaks corresponding to crystalline planes of lonsdaleite (L) and cubic diamond (CD) phases are indicated. Modified with permission from ref 245. Copyright 2013 Nature Publishing Group.

carbon (the Cu lines were from the TEM grid; Figure 25b). To improve the purity of the diamond phase, H2 gas was added to the gas mixture. Micro Raman spectroscopy with UV excitation (λex = 325 nm) showed that the H2 caused a small shift in the band near 1400 cm−1 to lower wavenumbers. Deconvolution and fitting of the peaks indicated that the shift resulted from a new peak at ∼1307 cm−1 which was modeled and found to match up with the phonon-confined Raman scattering peak for nanodiamonds with a diameter of 2.5 nm (see insets of Figure 25c), consistent with TEM observations. The enhancement of nanodiamond formation in the presence of H2 was further shown by X-ray diffraction (XRD) characterization. In the case of only ethanol (no H2), no peaks were observed, indicating that the carbon nanoparticles were predominantly amorphous. Adding H2 resulted in clear diffraction peaks that could be indexed to the well-known structure of cubic diamond, as well as the less frequently observed lonsdaleite phase. Another important carbon nanomaterial that has been synthesized in the gas phase by a nonthermal plasma is graphene. Frenklach and co-workers showed that graphene flakes could be homogeneously nucleated from ethanol vapor in an atmospheric-pressure microwave plasma similar in setup to Figure 17c.246,247 Although graphene as large-area sheets is 11081


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often desired and this approach produced flakes with sizes on the order of only ∼100 nm, the large production rate of 2 mg/ min (Figure 26a) is attractive for applications such as

of a mixture of Si and Ge nanocrystals or nanocrystals with a Si core and Ge shell (or vice versa). Pi and Kortshagen showed that Si1−xGex alloy nanocrystals could indeed be synthesized from plasmas containing SiH4 and GeH4 diluted in Ar at 300− 500 Pa.250 They controlled x by varying the ratio of the SiH4 to GeH4 flow rates. XRD and Raman scattering characterization (Figure 28) showed that the lattice parameter of the

Figure 26. (a) A 6 cm tall vial containing a 100 mg sample of graphene. The atmospheric-pressure microwave plasma produces material at a rate of 2 mg/min. (b) A typical low magnification TEM image of synthesized graphene sheets. Scale bar = 100 nm. (c) A high-resolution image taken at 80 kV. The white arrow indicates the edge of the sheet. Scale bar = 4 Å. Reprinted with permission from ref 247. Copyright 2009 Royal Society of Chemistry.

lubrication.248 In addition, the quality of the graphene flakes was extremely high, perhaps only superseded by those obtained by mechanical exfoliation (Figure 26b,c). The pristine nature of the graphene produced by the plasma process has been demonstrated by their application as TEM supports.249 Figure 27a,b shows TEM images of citrate-capped

Figure 28. (a) Raman spectra and (b) XRD from Si1−xGex alloy nanocrystals synthesized from SiH4 and GeH4 containing Ar plasma. (c) Lattice parameter of the Si1−xGex nanocrystals calculated from the XRD patterns as a function of x. Reproduced with permission from ref 250. Copyright 2009 Institute of Physics.

Figure 27. (a) Enhanced-contrast filtered image of the citrate-capped gold nanoparticle. Inset shows graphene reflections subtracted in a digital diffractogram. (b) An enhanced-contrast filtered image of the citrate molecules. Inset shows the graphene and gold reflections masked in the digital diffractogram. Reprinted with permission from ref 249. Copyright 2009 American Chemical Society.

nanocrystals varied monotonically from 0.543 nm for Si (x = 0) to 0.566 nm for Ge (x = 1). This is a strong evidence that Si and Ge was alloyed and not segregated into separate domains. Moreover, Raman scattering showed peaks at 514, 390, and 290 cm−1, assigned to Si−Si, Si−Ge, and Ge−Ge bond vibrations, respectively. These values are very close to Si−Si, Si−Ge, and Ge−Ge bond vibrations in bulk Si−Ge alloys, at 520, 410, and 300 cm−1, respectively.251 The small shifts are consistent with and are expected when the phonons are confined in nanocrystals. Importantly, The Si−Ge vibrations indicate the unambiguous presence of Si−Ge bonds and alloying. Total flow rate was kept constant so that only a narrow range of particle sizes could be formed. The nanocrystals were approximately 3 nm in diameter. When the authors studied the uniformity of the nanocrystals by slowly etching them in HF and measuring their average composition as a function of etching time, they found no sign of Si or Ge segregation in the particles for x < 0.5. Thus, they ruled out the formation of core−shell type structures. For x > 0.5, the data were less conclusive and they observed some Si enrichment near the surface. However, they also observed that nanocrystals with x > 0.5 were more susceptible to oxidation and they suspected that Si oxidation at the surface drove Ge toward the center of the particles. Like Si and Ge nanocrystals, the surfaces of the Si1−xGex nanocrystals were passivated with H, though the H coverage

gold nanoparticles mixed with a suspension of graphene flakes and deposited onto a Cu TEM grid with a lacey carbon support. Images of regions where the nanoparticles were dispersed on the graphene support showed unprecedented resolution because of the atomic-scale thickness and highly ordered structure of the graphene background which made it nearly completely transparent to electrons. Importantly, the images revealed the interface between the hard Au nanoparticle core and the soft citrate interface, the first atomic-resolution image of its kind. This process has now been commercialized for the production of graphene-supported TEM grids by Electron Microscopy Sciences. 4.4. Group IV Alloys

A perhaps straightforward step was to expand the group IV nanocrystal synthesis to group IV alloys such as Si1−xGex (0 ≤ x ≤ 1) and SiC. It should be possible, for example, to synthesize Si1−xGex nanocrystals from SiH4 and GeH4 under conditions similar to those used for synthesizing Si and Ge nanocrystals and then adjusting the SiH4 to GeH4 ratio in the gas to adjust x. There is no guarantee, however, that Si1−xGex will form instead 11082


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appeared to decrease with increasing x. Nevertheless, these alloy crystals, like their Si and Ge counterparts could be hydrosilylated and dispersed in organic solvents. One of the most unexpected results from the study by Pi and Kortshagen was that the photoluminescence peak from the 3 nm diameter Si1−xGex nanocrystals did not shift with x. This is in stark contrast to bulk material where the bandgap shifts from that of Si to that of Ge, and the photoluminescence due to interband exciton recombination follows this shift. The authors suggested that their results were in fact predicted by Reboredo and Zunger, who showed that the bandgaps of Ge nanocrystals with diameters smaller than 5 nm are similar to those of Si nanocrystals.252 Trying to avoid pyrophoric compressed gases, Yasar-Inceoglu and Mangolini replaced SiH4 and GeH4 with SiCl4 and GeCl4, respectively, and synthesized Si1−xGex nanocrystals that were less than 10 nm in diameter.253 Their Raman spectra and XRD were also consistent with formation of an alloy rather than formation of a mixture of Si and Ge nanocrystals. Recently, as described in section 5.3, Rowe and Kortshagen showed that boron and phosphorus can be incorporated into Si1−xGex nanocrystals to dope them.254 Lin et al. used a low-pressure (3 Torr) microwave plasma reactor to synthesize 4−6 nm amorphous silicon carbide (SiC) nanoparticles.255 Tetramethylsilane (TMS) diluted in argon carrier gas served as both the silicon and the carbon source (TMS partial pressure was 0.001−0.02 Torr). The SiC nanoparticles were carbon-rich due to the incorporation of methyl and other hydrocarbon fragments produced in the plasma through dissociation of TMS. Presence of these impurities was confirmed by the presence of CC and C−H bonds in the infrared absorption spectra. Lin et al. added hydrogen gas to reduce the excess carbon in the nanoparticles. Indeed, addition of hydrogen increased the ratio of the Si 2p to the C 1s peak in the XPS spectra from 0.5 to 1.4, indicating reduction of excess carbon. In an interesting recent contribution, Coleman et al. synthesized hollow SiC nanoparticles via a two-step tandem process wherein silicon nanoparticles produced in a nonthermal plasma were carburized in a second nonthermal plasma.256 The authors ruled out the nanoscale Kirkendall effect as a possible mechanism for the formation of the hollow structure. Instead they suggested that volume expansion upon formation of silicon carbide in the periphery of the silicon particle forms the hollow structure.

During integrated circuit manufacturing, silicon wafers are doped with boron and phosphorus by heating the wafers to temperatures above 900 °C in the presence of the dopant and by maintaining this elevated temperature for a few hours. This process achieves diffusion depths that are on the order of a micrometer. In contrast, during nanocrystal synthesis, the nanocrystals spend only a very small fraction of the residence time at high temperatures (10 s of μs).120 Moreover, the highest temperatures that the nanocrystals reach are on the order of 500 °C, high enough for crystallization but too low for dopant diffusion. Even at 900 °C and using the residence time (∼1 ms) as the characteristic time scale, the diffusion length for boron and phosphorus is less than 0.01 nm, much smaller than the nanocrystal size. Thus, assuming that the diffusion constants obtained for bulk diffusion are valid for nanocrystals, boron and phosphorus do not diffuse in silicon nanocrystals during the synthesis. The dopants are irreversibly incorporated into the nanocrystal during the growth as soon as they adsorb on the nanocrystal surface. Whether they are buried immediately and become active dopants in the bulk or rejected to the surface depends on the rate of growth versus rate of segregation to the surface. The latter is expected to depend on the difference in the chemical potential of the dopant at the surface compared to its chemical potential in the bulk. If this difference is small, dopants are expected to be uniformly incorporated into the growing nanocrystal with little segregation. If on the other hand this difference is large, the dopants can remain on the surface and segregate. In either case, the dopants are unlikely to return to the gas phase where their chemical potential is very high. This is in stark contrast to growth from solution where the dopants can easily dissolve back into the growth medium (see also the discussion in section 6.1 for a specific example). These processes are unique to the nonequilibrium environment of the plasma synthesis and enable nanocrystal doping. In this section, we will review investigations carried out with substitutional doping of plasma-grown nanocrystals that were doped in situ. Ex situ wet chemistry doping approaches have also been proposed.284,285 However, these do not preserve the nanocrystal’s morphology as they involve laser-assisted sintering. We will focus on the significant body of work on the doping of Si and Ge, which have received by far the most attention. Some of the more recent successes of either substitutional or vacancy doping of plasma-produced compound semiconductors will be reviewed in sections 6 and 9.3. The synthesis of doped semiconductor nanocrystals is presented with challenges in developing techniques that enable simultaneous control over doping levels and crystallite sizes. Nonthermal plasmas have proven to be effective in delivering a wide range of doping levels and nanocrystal sizes in group IV materials. This highly controllable, gas-phase technique has enabled systematic studies of nanocrystal doping and helped the community reach a deeper understanding of doping in nanomaterials, especially in nanocrystals made of silicon, which is the core material used in semiconductor devices, and has applications in the fields of thermoelectrics, electronic devices, and plasmonics (section 9).

5. DOPED GROUP IV SEMICONDUCTOR NANOCRYSTALS Doped semiconductor nanocrystals are expected to play a critical role in future technologies. 257 The intentional introduction of impurities, or dopants, into nanocrystals can drastically modify their electronic,258−261 optical,262−265 and magnetic266−270 properties. A substitutional impurity consisting of one or more valence electrons than the host material donates its electron(s) to the semiconductor (n-type doping) and an impurity with fewer valence electrons donates holes (p-type doping). For semiconductor nanocrystals, the free charge carriers derived from these donated electrons and holes allow the fabrication of electrically conductive films that are essential in many applications. To date, dopants have been incorporated in a wide range of nanomaterials, including nanocrystals of Si,258,271 SiGe,272,273 Ge,272 CdSe,259,274−277 ZnSe,278−280 PbSe,260,281 InP,282 and InAs.261,283

5.1. Phosphorus Doping of Silicon Nanocrystals

5.1.1. Direct Observation of Dopants. Stegner et al. reported the first demonstration of electronic impurity doping in a nonthermal plasma.258 The authors produced phosphorusdoped Si nanocrystals with mean diameters between 4 and 50 11083


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nm in a flow-through microwave plasma reactor at low pressure using argon and hydrogen gas and silane (SiH4) as a silicon precursor.258 Phosphorus doping was induced by adding phosphine (PH3) to the synthesis gases (Figure 29),152,258

doped crystalline bulk Si with doping concentrations above 1016 cm−3. This signal is attributed to electrons from substitutional P atoms that are sufficiently close to each other to interact via exchange coupling. In addition to this signal, large Si nanocrystals with an average diameter of ∼46 nm exhibited a pair of EPR lines with magnetic field splitting of 4.1 ± 0.1 mT (Figure 30), which in bulk crystalline Si is associated with isolated (noninteracting) substitutional P. This line pair, commonly denoted by hf(31P), stems from the hyperfine interaction between the donated electron spin (S = 1/2) and the spin of phosphorus dopant nucleus (I = 1/2 for 31P).152,258 Nakamine et al. used a very high frequency (VHF, 144 MHz) plasma deposition system to produce P-doped Si nanocrystals from SiH4 and PH3 (diluted in Ar) precursors.286 They used the flow rate of PH3/Ar to control the atomic P concentration in the Si nanocrystals,286 which was analyzed by energy dispersive spectroscopy (EDS) in a transmission electron microscope (TEM). The presence of substitutional P dopants was probed using electrically detected magnetic resonance (EDMR), an unconventional magnetic resonance technique which detects paramagnetic states through resonant changes of current passing through a sample. The hf(31P) signals in the EDMR spectra revealed the presence of substitutional P atoms in the nanocrystals.286 Further evidence of substitutional P was found in the thicker native oxide shells, grown upon air exposure around nanocrystals, which were observed for higher doping levels;286 this is consistent with first-principles studies on the oxidation of bulk Si(001) surfaces.287 5.1.2. Phosphorus Incorporation Efficiency and Surface Segregation. The efficiency of P dopant incorporation in Si nanocrystals grown by silane plasmas has also been studied quantitatively.271,288 Stegner et al. used secondary-ion mass spectroscopy (SIMS) combined with EPR to investigate the efficiency of P-doping Si nanocrystals that were synthesized in a microwave plasmas from SiH4 and PH3.288 They compared the effective P atom concentration in the nanocrystals, [P]eff, measured by SIMS with the nominal P atom concentration, [P]nom, defined as the fraction of PH3 flow over the total precursor gas flow (SiH4 and PH3) and multiplied by the Si atomic density (nSi = 5 × 1022 cm−3). For 3−30 nm Si nanocrystals with a native oxide shell and [P]nom between 1019 cm−3 and 5 × 1020 cm−3, they found that the effective and the nominal P atomic concentrations were approximately equal.288 This suggests that almost 100% of the precursor P atoms were incorporated into the nanocrystals during microwave plasma synthesis. Pi et al. found similar doping efficiencies for P-doped Si nanocrystals synthesized from SiH4 and PH3 in the flowthrough capacitively coupled radio frequency (RF) plasma reactor shown in Figure 31.271 Here, the elemental analysis was performed using an inductively coupled plasma atomic emission spectrometer (ICP-AES).271 Results showed almost 100% incorporation efficiency; the efficiency reduced to ∼63% at high nominal doping concentrations above 2 × 1021 cm−3.271 Stegner et al. also investigated the effect of oxidation on P doping for microwave plasma-synthesized Si nanocrystals. As mentioned above, the effective P concentration, measured by SIMS, of the surface-oxidized Si nanocrystals neared 100% of the nominal P concentration. Hydrofluoric acid (HF) etching of the native oxide shell led to a reduction in [P]eff by a factor of 20,288 which suggests that about 95% of the P atoms were contained in the outer native oxide shell surrounding the Si nanocrystals. This implies that the majority of P atoms segregated to the surface during the synthesis and that only

Figure 29. Schematic of microwave plasma system used to synthesize doped Si nanocrystals by Stegner et al.258 Dopant gases (PH3 for P doping and B2H6 for B doping) were added to the nanocrystal precursors (SiH4 and/or GeH4) and carrier gases for doping.

and the density of P atoms was simply controlled by varying the PH3 flow rate. The authors used electron paramagnetic resonance (EPR) to establish that the nanocrystals were ntype doped.152,258 The nanocrystals exhibited a signal at g = 1.998 (Figure 30) that is similar to the signal observed in P-

Figure 30. (a) EPR spectra of undoped and P-doped Si nanocrystals with an average diameter of ∼46 nm for different nominal doping concentrations. (b) EPR spectra of P-doped Si nanocrystals with average diameters of 4.3 and 11 nm. Adapted with permission from ref 258. Copyright 2008 American Physical Society. 11084


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to EPR. Figure 32b illustrates this doping compensation effect. This picture was confirmed through vacuum annealing and temperature-programmed desorption (TPD) in combination with EPR and Fourier transform infrared (FTIR) spectroscopy.152,290 TPD and FTIR indicated that annealing at 550 °C removed nearly all hydrogen that terminated the nanocrystal surface. The H desorption increased the Si-db density,290 which caused a drop in the EPR signal related to the donor electrons152 due to the charge compensation caused by the higher density of electron traps. Stegner et al. also studied the free electron density, [e], of Pdoped Si nanocrystals synthesized in a microwave plasma using EPR. They found that [e] was lower than the atomic P concentration, [P]eff, as measured by SIMS.288 The authors developed a statistical model based on a binomial probability distribution of P atoms and Si-db defects in a nanocrystal. They proposed that for nanocrystals larger than 15 nm in diameter, the difference between [e] and [P]eff can be solely explained by the aforementioned charge compensation by Si-db defects (Figure 32b).288 However, charge compensation alone cannot account for this disparity in smaller nanocrystals. The reason behind this remains unclear. One hypothesis is that the difference in smaller nanocrystals can be attributed to a higher number of interface sites, where P is not an active donor (i.e., the relative number of interface sites increases with decreasing dNC).

Figure 31. Efficiency of doping Si nanocrystals synthesized in a capacitively coupled RF plasmas. Shown are the atomic dopant concentrations obtained from ICP-AES vs the nominal dopant concentrations, calculated from the flow rates of Si and dopant precursor gases, i.e., SiH4 and PH3 (or B2H6), respectively. Adapted with permission from ref 271. Copyright 2008 AIP Publishing LLC.

5% of the P dopants were incorporated in the nanocrystal cores. This is illustrated in Figure 32a. The segregation of

5.2. Boron Doping of Silicon Nanocrystals

Besides phosphorus, Si nanocrystals were also successfully doped with boron (B) using nonthermal plasmas.271,291 Pi et al. employed diborane (B2H6) as a B doping precursor in a flowthrough RF plasma reactor.271 In comparing the effective atomic concentration of B atoms, [B]eff, with the nominal boron concentration, [B]nom (Figure 31), the authors discovered only 10% of B was incorporated in the nanocrystals271a clear contrast to the almost 100% incorporation efficiency measured for P impurities in Si nanocrystals.288 The authors attributed this difference to the larger formation energy for substitutional B than P (in the absence of lattice relaxation in the doped nanocrystals).292 Additionally, B doping in a RF plasma did not exhibit the surface segregation effects that were evident with P doping; the atomic B concentration of sequentially oxidized and HF etched nanocrystals was higher than that of as-grown nanocrystals.271 Interestingly, in microwave plasmas, almost 100% B atom incorporation was found, similar to P doping experiments.291 Segregation of B dopants to the Si nanocrystal surface was also not observed. Hence, it appears that, for B doping, the dopant atoms are primarily incorporated in the Si nanocrystal cores.271,291

Figure 32. (a) Schematic showing segregation of P dopants to (or close to) the surface, observed for P-doped Si nanocrystals grown by microwave plasma and RF plasma methods.271,288 (b) Energy diagrams of states induced by P donors and Si-dbs (e.g., Pb0 defects in the interface between the Si nanocrystals crystalline silicon (c-Si) core and the surface silicon oxide shell). Because the energy state of the negatively charged defect is below the P state, a donor electron from P is captured by the defect, leading to compensation of electronic doping.288

dopants to the surface was also observed in molecular-beam epitaxy (MBE) grown P-doped Si.289 In similar oxidationetching experiments, Pi et al. observed that 80% of the P dopants had segregated to the nanocrystal surface layer during RF plasma synthesis.271 5.1.3. Doping Compensation and Donor Electron Concentration. Several studies using EPR focused on the presence of silicon dangling bond (Si-db) defects (paramagnetic, S = 1/2) at (or close to) the surface of intrinsic and P-doped Si nanocrystals.152,205 Interestingly, Stegner et al. discovered that an increasing nominal P concentration, [P]nom, causes a decrease in the density of these paramagnetic defects.152 This suggests that electrons donated by P atoms were trapped by Si-db defects. These become negatively charged (diamagnetic, spin S = 0) and, hence, become invisible

5.3. Doping of Germanium and Silicon−Germanium Nanocrystals

Doped Ge and SiGe nanocrystals with narrow size distribution were also synthesized using plasma methods with silane (SiH4) and germane (GeH4) as silicon and germanium precursors, respectively.97,293 Stoib et al. reported the synthesis of 20−30 nm spherical P-doped Ge and SiGe nanocrystals with [P]nom of 5 × 1020 cm−3 in a microwave plasma employing phosphine as the dopant precursor.272 Employing the methodology developed for Si nanocrystals by Stegner et al.,288 [P]eff was obtained by SIMS before and after etching of the native oxide shell surrounding the nanocrystals.272 From these experiments, the authors established that P atoms also tend to segregate to (or close to) the Ge or SiGe nanocrystal surface. For the case of 11085


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diffraction (XRD) revealed smaller core sizes of doped nanocrystals compared to undoped nanocrystals produced under similar conditions.273 Rowe et al. suggested that this was caused by the strain induced by B atoms located in the nanocrystal cores. They did not observe a similar size reduction for P-doped SiGe nanocrystals, where the dopants were believed to be located close to the nanocrystal surface.

Si80Ge20 nanocrystals, 60% of the P atoms were incorporated at or close to the nanocrystal surface.272 Rowe et al. synthesized both B and P-doped SiGe nanocrystals using a RF plasma reactor depicted in Figure 33a.273 B- and P-doped SiGe nanocrystals were produced from

5.4. Doping and Light Emission Properties

Electronic doping plays a major role in the optical properties of nanocrystals. Pi et al. studied the effect of P and B doping on the photoluminescence (PL) of Si nanocrystals (dNC = 3.6 ± 0.8 nm).271 For B doping, they observed a PL reduction for [B]nom larger than 7.5 × 1019 cm−3 while no major change in PL was found for [B]nom below 2.5 × 1019 cm−3.271 The authors suggested that the PL reduction was caused by Auger recombination involving free holes or by nonradiative recombination at defect sites stemming from dopant-induced strain in the Si nanocrystals.271,292,297−299 Oxidation of the Bdoped Si nanocrystals and subsequent surface oxide removal led to an intensification and blue-shift of PL, which corroborated the conclusion that the B dopants were incorporated in the core of the nanocrystals.271 The optical properties of both RF plasma and VHF (144 MHz) plasma-grown, P-doped Si nanocrystals were studied.271,300 At low doping levels, the P-doped nanocrystals exhibited higher PL intensities than intrinsic nanocrystals.271 As the doping concentration increased, the PL intensities increased until a certain threshold level at which the PL starts to degrade (Figure 34).300 For RF plasma-grown nanocrystals, this Figure 33. (a) Schematic of a SiGe nanocrystal synthesis reactor and deposition chamber. (b) P atomic fraction and (c) B atomic fraction as measured via EDX vs the respective atomic fractions calculated from the PH3 and B2H6 flow rates. The SiGe nanocrystal diameters measured via XRD are indicated by the bubble sizes. The linear fits to the data yield the P and B incorporation efficiencies, ηP and ηB, respectively. Adapted with permission from ref 273. Copyright 2014 AIP Publishing LLC.

B2H6 or PH3, respectively, in an admixture of SiH4 and GeH4. Films were fabricated by impacting the nanocrystals directly onto substrates placed downstream of the plasma, Figure 33a.273 This impaction technique enables the fabrication of multilayered films of differently doped nanocrystals by simply alternating precursor gases and/or using multiple reactors. For example, p−n junctions can be formed with ease using alternate layers of B and P-doped nanocrystals, and multilayered structures can be produced for modulation doping of thermoelectric materials.294 Using energy-dispersive X-ray spectroscopy (EDX), the Bdoping efficiency was measured (Figure 33b,c).273 Approximately 16% of the B precursor was incorporated into the nanocrystals, comparable to the doping efficiency of Si nanocrystals produced using a similar RF reactor (see section 5.1).271 However, the P-doping efficiency was found to be only 29%, which is much lower than the nearly 100% P incorporation efficiency in Si nanocrystals obtained from both microwave and RF plasma reactors.271,288 Similar trends were found for SiGe films produced via chemical vapor deposition (CVD); with increasing Ge content P segregation increased due to the competition of Ge and P atoms for Si atomic sites.295,296 FTIR and EDX analyses of the plasma-synthesized SiGe nanocrystals also showed P atom segregation to the nanocrystal surface.273 In B-doped SiGe nanocrystals, X-ray

Figure 34. Photoluminescence spectra of P-doped Si nanocrystals. The curves correspond to different flow rates of the PH3/Ar precursor. The photoluminescence intensity and peak position as a function of the PH3/Ar flow rate are plotted in the inset. Adapted with permission from ref 300. Copyright 2012 The Japan Society of Applied Physics.

threshold was found to be at [P]nom of ∼2 × 1020 cm−3.271 The increase in PL at the low doping levels was attributed to passivation of surface defects by P atoms,271,300−302 which were more likely to be incorporated at the nanocrystal surface.271 Doping compensation may have also played a role in the PL intensification; upon capture of a free electron (from dopant), the Si-dbs become negatively charged and repel further electrons. The decrease in PL in heavily P-doped Si nanocrystals is commonly associated with Auger recombination involving donor electrons.271,300,302,303 Someno et al. observed 11086


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decreased PL lifetime at increased doping levels for their VHF plasma-grown P-doped Si nanocrystals.300 Furthermore, the authors found that the PL decay curve deviated further from a simple exponential curve with increasing P concentration, which may be due to P incorporation inducing increased structural disorder. The observation of PL from Si nanocrystals with [P]nom of 2 × 1020 cm−3 indicates the presence of inactive dopants.271 At ∼ 3.6 nm, this [P]nom would correspond to more than 1 electron per nanocrystal if the concentration of donor electrons, [e], equaled [P]nom. Hence, Auger recombination should quench the PL. However, as a result of P atom segregation to the surface and doping compensation effects, [e] can be lower than [P]nom by 2 orders of magnitude;288 thus, there may have been a considerable fraction of Si nanocrystals that do not contain free electrons and that show PL. The disparity between [e] and [P]nom can also be attributed to the high ionization energy of dopants in small nanocrystals; theoretical studies of P donors in Si nanocrystals indicated that the ionization energy is considerably larger than kT (k is the Boltzmann constant, and T is temperature).292 Pi et al. also observed a PL blueshift in both P- and B-doped Si nanocrystals after oxidation and HF etching of the oxide shell as a result of a reduction in nanocrystal core size.271 This blueshift was stronger for P-doped nanocrystals than for the intrinsic and B-doped nanocrystals, due to an enhanced oxidation of P-doped Si nanocrystals. The authors suggested that free electrons provided by P doping led to enhanced formation of adsorbed O2− species and accelerated the oxidation process.304

Figure 35. Illustration of the electron probability density |Ψ(r⃗)|2 (dashed blue lines) and potential energy V(r⃗) (solid red lines) for (a) hydrogen-like donor model in a bulk semiconductor, (b) quantum confinement model in a nanocrystal, and (c) dielectric confinement model in a nanocrystal. The blue and red dotted lines in (b) and (c) illustrate |Ψ(r⃗)|2 and V(r⃗) in the bulk situation, respectively. Adapted with permission from ref 305. Copyright 2009 American Physical Society.

(dielectric confinement, Figure 35c).305 The model predicted that quantum confinement dominates for small nanocrystal sizes (below 4 nm), while for large nanocrystal sizes (above 12 nm) the donor localization is dominated by the dielectric confinement effect. 5.5.2. Dopant Isolation and Dopant−Dopant Interactions. Plasma-synthesized doped nanocrystals have also been used to study the electronic structure of nanocrystals containing more than one dopant.311 Here, a central issue is the interdopant coupling in a single nanocrystal. Besides being confined in a small volume, dopants in a nanocrystal are also isolated from those in neighboring nanocrystals, which strongly affects the coupling between dopants. In bulk semiconductors, dopant−dopant interactions cause closely spaced impurity states that form an impurity sub-band near the corresponding intrinsic band edge (Figure 36a).312 For P-doped bulk silicon, this sub-band appears at concentrations larger than 3.7 × 1018 cm−3.313,314 However, in nanocrystals smaller than ∼10 nm such doping levels correspond to only a few dopants per nanocrystal (Figure 36b). The interdopant interactions in doped nanocrystals are limited to those between the few dopants contained in a nanocrystal (dopant isolation), because the interaction between dopants located in different nanocrystals is negligible in comparison. The electronic states that result from interactions between a few dopants are discrete; therefore, the bulk impurity sub-band model does not apply to nanocrystals.315 Only for extremely high doping concentrations in the alloy range (above 5%) were Mocatta et al. able to demonstrate the emergence of an impurity sub-band in individual nanocrystals.261 For doping concentrations below this threshold, the energies of the discrete, dopant-induced electronic levels depend on the number of dopants and the dopant configuration, i.e. the dopant position in the nanocrystal and their relative orientation

5.5. Electronic Properties of Dopants: Confinement and Isolation

5.5.1. Quantum and Dielectric Confinement. Plasma synthesis, based on its ability to tune particle size and doping level, has enabled detailed studies of confinement of dopant states in nanocrystals. Several groups have reported evidence of dopant localization/confinement using EDMR and EPR.286,303,305 Studies focused on the Fermi contact hyperfine splitting (hfs) A, which stems from the interaction between the donor electron spin and 31P nucleus spin and scales directly with the donor electron probability density at the P nucleus. For microwave305 and VHF plasma-grown286 P-doped Si nanocrystals, the hfs was observed to increase with decreasing nanocrystal size, consistent with confinement effects. Donors in bulk semiconductors are commonly described as hydrogenic impurities. In this model, the Coulomb potential of the impurity ion screened by the bulk dielectric constant, εbulk, determines the donor electron potential energy. As illustrated in Figure 35a for the case of P in bulk silicon, the donor electron ground-state is the 1s state of a hydrogen-like atom with an effective Bohr radius, abulk*.306 In a nanocrystal, the surface potential, V0, imposed on the donor electron is expected to enhance the donor electron probability density at the P nucleus (quantum confinement, Figure 35b). However, the variation of the hfs A with nanocrystal size observed by Pereira et al. could not be completely explained by quantum confinement effects.305 The variation was described by a model that considered both the surface confining potential, V0, and a previously predicted nanocrystal size dependent dielectric constant, εNC,307−310 which resulted in a size dependent screening (and electron localization) of the electron−nucleus Coulomb interaction 11087


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first-principles calculations. The study also revealed a strong dependence of the exchange energy on the dimer configuration; the authors found that the energy splitting between the singlet and triplet donor dimer states can differ by up to 3 orders of magnitude for randomly placed dimers in nanocrystals.311 5.6. Charge Transport

The charge transport properties of a nanocrystal thin film are dependent on the density of free carriers generated by dopant incorporation. Stegner et al. used films of P-doped Si nanocrystals to investigate this relationship. They discovered that surface defects, dopant percolation, and high ionization energies could hinder the effects of doping.152,258 In their early studies, Stegner et al. measured the electrical conductivity, and its temperature dependence, of thin films of H-terminated Si nanocrystals with different concentrations of P ([P]nom).258,316 The films were produced by spin-coating surface-oxidized Si nanocrystals dispersed in ethanol onto a substrate. The nanocrystal films were subsequently etched in diluted HF to remove the surface oxide and terminate the nanocrystal surfaces with Si−H bonds.258 For intrinsic Si nanocrystal films, electrical measurements revealed that the conductivity, σ, varied with temperature, T, following an Arrhenius dependence ⎡ ⎛ E ⎞⎤ σ ∝ exp⎢ −⎜ a ⎟⎥ ⎢⎣ ⎝ kBT ⎠⎥⎦

Figure 36. Illustration of (a) a doped bulk semiconductor and (b) an ensemble of doped nanocrystals (dopant isolation) with similar ensemble doping concentration as the bulk semiconductor. Dopant atoms are indicated as red dots. In (b) the lines represent the electronic structure of each individual nanocrystal, showing the expected discrete electronic states (represented in green) induced by the dopants and the conduction band (CB) and valence band (VB) edges (brown and blue, respectively). The bottom panels depict (a) the subband formed in a doped bulk semiconductor and (b) with the distribution of electronic states that appear in an ensemble of nanocrystals resulting from the combination of all doping related states from each nanocrystal of the ensemble.

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which indicated thermally activated charge transport with an activation energy, Ea, of ∼500 meV. For films of doped Si nanocrystals, an increase of [P]nom led to an increase in film conductivity and a decrease in the T dependence of the conductivity.258 This demonstrated that electrons donated by P, as measured by EPR experiments,258,288 enhanced the charge transport in the Si nanocrystal films. Laser-annealed films of Pdoped Si nanocrystals, studied by Stoib et al., also exhibited a decrease in T dependence with increasing [P]nom. The authors used a power law, σ ∝ Tm, to fit the data and found that the exponent, m, decreased with increased doping, which they attributed to percolation effects.272 In an investigation of the conductivity of laser-annealed films of doped Si nanocrystal with a wide range of nominal doping concentrations, Lechner et al. found that the conductivity is independent of the doping level when the nominal doping of the P- or B-doped Si nanocrystals is below 1019 cm−3 or 1018 cm−3, respectively.291 Above these concentrations, the conductivity increases rapidly by up to 7 orders of magnitude for every order of magnitude increase in the doping level.291 The authors attributed this dramatic increase to doping compensation, as discussed above;288 the dopants in Si nanocrystals only become active when the effective dopant concentration overcomes that of electron traps. The different threshold concentrations for Pand B-doped Si nanocrystals are likely caused by the segregation of P dopants to the nanocrystal surfaces during plasma synthesis, as only the 5% of P atoms that are incorporated in the core can become active dopants.288,291 In another study that compared the charge transport properties of films of H-terminated and oxidized Si nanocrystals, it was also observed that for oxidized nanocrystals, doping did not yield a significant enhancement of conductivity even when nominal doping concentration was very high (5 × 1020 cm−3).317 From this, it was concluded that the charge transport in oxidized Si nanocrystals was not limited by the density of available charges, and electronic states provided by the oxide

to each other (Figure 36b). In a large nanocrystal ensemble, the electronic structure of the dopant states is a combination of the varying dopant electronic levels in each nanocrystal and, thus, should be characterized by a broad distribution. Although, the distribution of dopant-related states in a nanocrystal ensemble may resemble that of the impurity subband of a bulk semiconductor, knowledge about electronic doping in bulk materials cannot be directly applied to nanocrystals. Pereira et al. used phosphorus-doped Si nanocrystals synthesized in a microwave plasma to investigate, for the first time, inter−dopant interactions in nanocrystals.311 For an electron concentration, [e], of 1.4 × 1019 cm−3, which is higher than the threshold for forming an impurity sub-band in bulk Si,314 only one or two dopants reside in the 3.9 nm Si nanocrystals. In this case, the dopants induced discrete electronic states in the nanocrystal’s bandgap (Figure 36b).311 The authors measured the exchange coupling interaction between closely spaced donor pairs (a donor dimer) in nanocrystals by monitoring the temperature-induced transition between the singlet (ground) and triplet states (excited).311 The deviation of the triplet state magnetic resonance from Curie paramagnetism, monitored via temperature-dependent EPR, was described analytically by effective mass theory,311 taking into account dielectric confinement.305 This analytical method enabled the consideration of the wide range of possible donor dimer configurations315 that could not be achieved using 11088


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shells were involved in internanocrystal charge transfer; on the other hand, for films of H-terminated Si nanocrystals, direct internanocrystal charge transfer was likely the primary means of charge transport.317 Two studies used EDMR to investigate the role of doping and defect states in charge transport in Si nanocrystal films.258,316 EDMR spectra measured in the dark showed signals stemming from Si-dbs and isolated substitutional P atoms, which demonstrated that both play a role in charge transport.258 Moreover, the current enhancement found at EDMR resonances indicated that P donors and Si-dbs contribute to the dark conductivity via spin-dependent hopping. Under white light illumination (photoconduction), EDMR revealed signals from P donors (isolated and exchangecoupled) and from Si-dbs, demonstrating the impact of Si-dbs as centers for spin-dependent recombination of photogenerated charges. Interestingly, exchange-coupled dopants exhibited activity only under illumination, while isolated (noninteracting) P dopants were EDMR active both under illumination and in the dark. For the nanocrystals with diameters >15 nm studied here, the electrons in exchange-coupled donor states were delocalized, similar to those in metallic impurity sub-band gap states of heavily doped bulk semiconductors.258 These delocalized states should contribute less to spin-dependent hopping conduction than isolated P donors. However, these delocalized electrons can also recombine via Si-db defect states similar to electrons in isolated P donors. Plasma-produced P-doped Si nanocrystals recently played an important role in improving the understanding of carrier transport in highly doped nanocrystal films. In doped bulk semiconductors, the transition from insulating to metallic behavior, also known as metal−insulator transition, is described by the Mott criterion.318 This criterion describes the minimum free carrier density for which a semiconductor will start to behave as a metal. However, the criterion is not applicable to semiconductor nanocrystal films, as these films still behave semiconductingly at carrier densities much higher than the minimum density predicted by Mott’s criterion. Chen et al.319 developed a new theory for the metal−insulator transition in films of nanocrystals that touch at contact facets of a radius ρ. Their theory yielded an analog to the Mott criterion for doped nanocrystal films, kFρ ≈ 2, where kF is the Fermi vector and is a function of the free carrier density. The critical carrier densities for nanocrystal films predicted by this new criterion are about 2 orders of magnitude larger than those predicted by Mott’s criterion for bulk semiconductors. In associated experiments, Chen et al. studied the transport in films of plasma-produced Pdoped Si nanocrystals. They found that for all doping concentrations studied the transport is consistent with EfrosShklovskii variable range hoping.320 Based on their analysis of the temperature-dependence of the conductivity, the authors were able to calculate the electron localization length ξ. They found that for low doping concentrations, the electron localization length is about 1 nm, but it increased to ∼26 nm, more than three times the particle diameter, for the highest doping concentration of 20%, Figure 37. These experimental findings indicate the approach to the metal−insulator transitions at carrier densities that are by a factor of 2 smaller than the critical density predicted by the new criterion. The experimental data on P-doped Si nanocrystals thus support the new theory presented in ref 319.

Figure 37. Electron localization length ξ versus the free electron density in films of plasma-produced P-doped Si nanocrystals with an average diameter of 7.5 nm. XP,nom = [PH3]/([PH3] + [SiH4]) represents fractional PH3 flow rate. The average nanocrystal diameter is indicated by the horizontal dashed line. Reproduced with permission from ref 319. Copyright 2015 Nature Publishing Group.

6. SYNTHESIS OF COMPOUND SEMICONDUCTOR NANOCRYSTALS The synthesis of compound semiconductor nanocrystals presents an additional level of complexity because stoichiometry of the nanocrystals must be controlled. Compared to the synthesis of elemental semiconductors, well-controlled plasma synthesis of compound nanocrystals is in its infancy and many opportunities and challenges remain. Below we review the synthesis of nanocrystals of oxides, nitrides, phosphides, and sulfides. 6.1. Oxides

There are many reports of metal oxide (and in fact compound) nanocrystal synthesis using thermal atmospheric pressure plasmas typically operated using microwave power.321−325 Early research also explored the subatmospheric (10−100 Torr) pressure range as an alternative to thermal plasmas. In fact, many authors referred to this range between 10 Torr and atmospheric pressure as low pressure (compared to atmosphere). As early as the beginning of 1990s Vollath and Sickafus were reporting the synthesis of oxide and nitride nanocrystals using a low-pressure (77−100 mbar) microwave plasma.326−328 They used AlCl3, TiCl4, or ZrCl4 vapor produced outside the plasma, by evaporating metal halide salts, as the metal precursor and O2 and H2O as the oxygen sources to produce 5−30 nm diameter alumina, titania, or zirconia nanocrystals, respectively, from these metal halide vapors.326 Water vapor led to larger particle sizes (>20 nm) as compared to O2 alone (∼5 nm), though no explanation or hypothesis was offered for this observation. The authors recognized that the unique environment created by the plasma and small sizes of the particles may make it possible to synthesize nonequilibrium phases or solid solutions that are not observed in equilibrium bulk materials. However, the gas temperatures they reported downstream of the plasma (600−700 °C) were significantly higher than the gas temperatures in most nonthermal plasmas, indicating there was 11089


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this approach is usually touted as having low cost and high throughput compared to vacuum deposition methods, this comparison is usually with low-rate vacuum deposition methods such as sputtering. As demonstrated by Holman,160 an important advantage of low-pressure plasma synthesis is the ability to deposit nanocrystals directly onto substrates by supersonic expansion227 of the plasma effluent carrying the nanocrystals. Rates can be 1 to 2 orders of magnitude higher than vacuum deposition methods such as sputtering. Following Holman,160 Thimsen et al. eliminated the solvent and dispersion step by depositing thin films via supersonic expansion through a slit nozzle and subsequent impaction of the plasma-synthesized nanocrystals. The authors studied electronic conduction in these ZnO nanocrystal films and demonstrated transparent conducting films with high electron mobilities by removing OH groups from the nanocrystal surface using a post treatment. Specifically, atomic layer deposition (ALD) post treatment filled the voids between nanocrystals with Al2O3, ZnO, or Al-doped ZnO, forming solid transparent and conducting ZnO nanocrystal thin films. The infilled materials effectively removed the surface OH groups that acted as charge traps rendering the films insulating. Their results were consistent with observations by Felbier et al., who showed that photoluminescence associated with OH groups decreased in vacuum while band gap emission increased.330 Recently, Greenberg et al., using the same synthesis approach as Thimsen et al., produced Al-doped ZnO nanocrystals and showed that plasmon resonances exhibited by the nanocrystals could be tuned by adjusting the Al concentration and the size of the nanocrystals via quantum confinement.332 They doped the ZnO nanocrystals by introducing trimethylaluminum (TMA) along with DEZ into the plasma. Characterization supported the conclusion that the nanocrystals were doped uniformly sans undoped cores, which were shown to be prevalent in nanocrystals synthesized in colloidal solutions.333 This advantage of plasma synthesis over colloidal routes to produce uniformly doped nanocrystals was attributed to the high chemical potential of ions and radicals in the plasma and to irreversible chemical reactions that incorporate the dopants into the nanocrystals (Figure 40). In contrast, relatively small chemical potential differences between dopant and host atoms in colloidal solutions result in reversible adsorption−desorption of dopants. The chemical potentials of dopants in the nanocrystals are higher than in solution because dopants induce more lattice strain than host atoms. This difference can lead to dopant exclusion during colloidal synthesis, particularly during nucleation and early stages of growth. This may lead to nanocrystals with undoped cores.333 In contrast, in plasmas, dopants are incorporated essentially whenever they collide with the particles and incorporation is irreversible. Hence, it may be easier to dope nanocrystals during plasma synthesis than during colloidal synthesis because of the nonequilibrium environment created by the plasma.

significant energy exchange between the electrons, ions, and neutrals, and the system was in the transition region from a nonthermal to thermal plasma. Later, Vollath and Sickafus also synthesized 5−10 nm γ-Fe2O3 and Cr2O3, nanocrystals from FeCl3 and Cr(CO)6 vapors, respectively.329 Cr2O3 nanocrystals that were larger than 10 nm were elongated. Synthesis of oxides at even lower pressures is relatively recent. Felbier et al. used a radio frequency plasma maintained in the 0.5−1.3 Torr range to synthesize 2.1−3.4 nm diameter wurtzite ZnO nanocrystals from diethylzinc (DEZ) and O2 gases diluted in Ar.330 These ZnO nanocrystals are the smallest that have been synthesized in the plasma so far. Figure 38

Figure 38. XRD from an ensemble of ZnO nanocrystals consistent with wurtzite crystal structure whose expected powder diffraction pattern is shown with sticks at expected 2θ positions. The inset is a high resolution TEM image of a single ZnO nanocrystal. Reproduced with permission from ref 330. Copyright 2013 Wiley and Sons.

shows XRD from an ensemble of ZnO nanocrystals and a high resolution TEM image for a 3.4 nm ZnO nanocrystal. Photoluminescence from these ZnO nanocrystals was generally dominated by defect luminescence in the yellow-green range. A smaller band gap emission components were blue-shifted with respect to the bulk band gap emission, indicating quantum confinement (Figure 39).

Figure 39. Photoluminescence from ZnO quantum dots with average diameters between 2.1 and 3.4 nm showing the blue-shifted emission with decreasing size. Reproduced with permission from ref 330. Copyright 2013 Wiley and Sons.

6.2. Nitrides

Vollath and Sickafus extended their approach to synthesizing oxide nanocrystals to the synthesis of nitrides by replacing O2 and H2O with N2 and NH3.327 Specifically, they synthesized ∼4−8 nm diameter nominally ZrN nanocrystals. Curiously, the lattice constant of the nanocrystals was significantly larger (0.506 nm) than bulk ZrN (0.4578 nm). They attributed the 0.506 nm value to the formation of ZrH0.6N rather than ZrN. ZrH0.6N has been shown334 to form upon thermal disintegra-

Thimsen et al. also used DEZ and O2 diluted in Ar but in the 3−6 Torr range to synthesize 5−7 nm diameter ZnO nanocrystals in a radio frequency plasma.331 One unique aspect of this work was the fast deposition of films comprised of nanocrystals without exposing the nanocrystals to solvents or ambient air. The usual approach to making films comprised of nanocrystals is to first disperse them in a solvent and then coat surfaces with dispersions containing the nanocrystals. While 11090


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semiconductors such as CdSe. Shimada et al. used trimethylgallium (TMGa), a high pressure metalorganic precursor (114 Torr at 15 °C) routinely used in thermal chemical vapor deposition, and, like Li et al., mixtures of NH3 and N2 as the nitrogen source. The particle sizes ranged from 5 to 20 nm but the selected area electron diffraction patterns indicated that the nanoparticles were amorphous. Apparently, the nanoparticles did not reach high enough temperatures to crystallize. Given that 4 nm silicon nanocrystals crystallize at ∼700 K (>0.4Tm where Tm is the bulk melting temperature) before they can crystallize,120 similarly sized GaN nanoparticles would need to attain temperatures over 1100 K because the melting point of bulk GaN is significantly higher (2773 K). It appears that the temperatures in the DC arc approach used by Li et al. were significantly higher than the microwave flow system used by Shimada et al. Indeed, the same year, Anthony et al. reported the synthesis of GaN nanocrystals with diameters ranging from 15 to 60 nm (Figure 41).338 While both Shimada et al. and

Figure 40. Schematic illustration of the differences in aluminum doping of ZnO nanocrystals in a solvent (left) and in a plasma (right). Dark green, light green, and red spheres represent Zn, O, and Al atoms, respectively. Left side of panel (a) illustrates the small difference between the chemical potential of Zn or Al precursor in the solvent and the chemical potential of Zn or Al in the nanocrystal dispersed in that solvent. Right side of the panel (a) illustrates the large difference between the chemical potential of Zn or Al precursor in the plasma and the chemical potential of Zn or Al in the nanocrystal in the plasma. Small chemical potential differences in the solvent result in reversible atomic processes that may exclude dopants from the nanocrystals (panel b, left, i-iii). Large chemical potential differences in the plasma result in irreversible incorporation of dopant Al atoms (panel b, right, i-iii). Reproduced with permission from ref 332. Copyright 2015 American Physical Society.

Figure 41. (a) Transmission electron micrograph (TEM) of GaN particles synthesized in a microwave plasma from a mixture of NH3 (500 sccm), GaCl3 (0.54 sccm), and Ar (75 sccm) at 7 Torr. (b) Highresolution TEM of a single particle showing the lattice fringes. The scale bar in (a) is 100 nm, the small scale bar in b is 50 nm. The scale bar in the magnified image in (b) is 10 nm. Reproduced with permission from ref 338. Copyright 2011 Cambridge University Press.

tion of ZrN(NH2) which may have formed as an intermediate from ammonia fragments created in the plasma. One of the earliest reports of compound semiconductor nanocrystal synthesis was by Li et al., who synthesized wurtzite GaN nanocrystals using a direct current arc plasma sustained between a tungsten cathode and a copper crucible filled by solid gallium and acting as the anode.335 The working gas was N2, NH3, or a mixture of the two. This work was again in an arc maintained between 100 and 600 Torr, clearly a thermal plasma. The debris in the crucible contained Ga and GaN particles. While using N2 alone yielded very little GaN, using pure NH3 or a 15% N2−40% NH3 mixture maximized the amount of GaN nanocrystals produced. Nitrogen is a very stable molecule and is difficult to dissociate even in a plasma.336 Thus, NH3 is a better source for nitrogen. The average nanocrystal size was 50 nm, and electron diffraction indicated that the crystal structure was wurtzite. The nanocrystals were photoluminescent but the PL spectrum shown in this article was not quantitative, which makes assessment of the material quality difficult. Nearly a decade later, Li’s work inspired Shimada et al., who synthesized GaN nanoparticles using a subatmospheric pressure plasma maintained in a 2.45 GHz microwave resonant cavity.337 This work was motivated by the difficulty of synthesizing light-emitting group III nitrides (e.g., GaN, InN) using liquid-phase-based approaches which, by this time, was routinely producing very high quality (e.g., high emission with quantum yields approaching 100%) II−VI

Anthony et al. used 2.45 GHz microwaves to excite the plasma, there were four significant differences. First, Anthony et al. operated their plasma at 7−15 Torr, significantly lower than the previous work. Second Anthony et al. used GaCl3 instead of TMGa as the gallium source. Third, Ar was used in addition to NH3 to maintain the plasma. Finally, and most importantly, the power density in Anthony’s system (∼4400 W/cm2 based on inner tube diameter) was ∼7 times higher than that used by Shimada et al. (∼650 W/cm2). While nanoparticle heating in the plasma is a complex process and many processes contribute to the energy flux impinging on the nanoparticle, Kramer et al. have recently shown that rates of all processes increase with power, making it the single most important plasma operating parameter that influences the nanoparticle temperature and hence its crystallinity.120 While plasma synthesis of GaN nanocrystals has been demonstrated, there have not been any systematic studies of the dependence of the nanocrystal size and optical properties on plasma operating conditions. All GaN nanocrystals synthesized in the plasma exhibited photoluminescence, some ensembles even showing shift of the band gap due to quantum confinement. However, dependence of the quantum yield on the synthesis conditions and surface passivation has not been studied in detail yet. It would be interesting, for example, to produce colloidal dispersions of GaN nanocrystals similar to those produced using colloidal chemistry.339 With trimethylgallium and GaCl3, the overall reactions with NH3 are 11091


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and GaCl3(g ) + NH3(g ) → GaN(s) + 3HCl(g)

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respectively. Of course, the individual reactions that lead to nucleation and growth are complex and involve radicals, ions and molecular fragments produced in the plasma. The identities of these precursors and the elementary reactions are unknown and have not been studied. In both studies that yielded GaN, the nitrogen source was in excess and easily dissociated so that nominally stoichiometric GaN could be formed. In fact, in some cases nanocrystals were even N rich.338 Anthony et al. also used a radio frequency (rf) generated (13.56 MHz) plasma to produce 5−15 nm diameter GaN nanocrystals from GaCl3 and NH3 as precursors.338 RF plasma synthesis of GaN nanocrystals required less power (∼600− 1000 W/cm2 based on the inner tube diameter) than the microwave approach. In fact, the authors estimated even less power was coupled into the plasma. Radio frequency plasma appears to be more efficient at producing GaN nanocrystals than microwaves though this statement may not be general. 6.3. Phosphides

Gresback et al. used the same approach as Anthony et al. but replaced TMGa with trimethylindium (In(CH3)3 or TMIn) and NH3 with PH3 to synthesize InP nanocrystals in a 13.56 MHz rf plasma.340 The precursor gases were heavily diluted in Ar and the feed gas contained Ar, H2, PH3, and TMIn in the 90:17:3:1 ratio. Gresback et al. varied the residence time of the gases systematically between 2 and 10 ms by varying the gas flow rate and synthesized InP nanocrystals with diameters up to 4.3 ± 0.8 nm at 10 ms. XRD, Raman, and TEM established that the nanocrystals were zinc blende InP. Gresback et al. formed colloidal dispersions by attaching organic ligands (myristic acid, amines, phosphine oxides and fatty acids) to the surfaces of the InP nanocrystals. The InP nanocrystals capped with organic ligands exhibited no or very little photoluminescence. If the nanocrystals were allowed to oxidize, the quantum yields increased but did not exceed 1%. Apparently, dangling bonds on the surfaces of the as synthesized nanocrystals act as nonradiative recombination sites and suppress photoluminescence and organic ligands were not effective passivants. However, following colloidal synthesis techniques, Gresback et al. were able to grow ZnS shells on InP nanocrystals and increase the quantum yield to 10−15%. Figure 42 shows the optical absorption spectra from InP nanocrystals synthesized with different residence times. The shoulder immediately after the onset of absorption corresponds to the first exciton absorption peak. This peak is smeared and is not as clearly defined as in some solution-synthesized colloidal InP nanocrystals because the size dispersion of the plasma synthesized nanocrystals is still relatively high (∼20%). There is opportunity to explore the origins of this relatively wide size distribution and to improve the synthesis to narrow it. Figure 39 also displays the photoluminescence spectra from nanocrystals synthesized with different residence times. Both optical absorbance and photoluminescence spectra from the ZnS capped InP nanocrystals shifted to higher energies with decreasing size (decreasing residence time), making these experiments the first demonstration of quantum confinement in compound semiconductor nanocrystals produced via nonthermal plasma synthesis.

Figure 42. (a) Optical absorption spectra of InP nanocrystals synthesized using different residence times and capped with myristic acid. (b) PL spectra of InP nanocrystals synthesized using different residence times and covered with ZnS shell. (c) PL peak emission intensity as a function of residence time: nanocrystal size decreases with decreasing residence time. Inset shows the effect of ZnS shell thickness on the PL intensity. Reproduced with permission from ref 340. Copyright 2011 Springer.

InP is an important optoelectronic material with applications that rely on quantum yields and emission. While InP core ZnS shell nanocrystals exhibited 10−15% quantum yield this was still lower than the best (40%) InP nanocrystals synthesized using colloidal chemistry.341 Clearly, there is opportunity to increase the PL quantum yield and to improve the size dispersity of the plasma-synthesized InP nanocrystals. The former requires a deeper understanding and exploration of the state of the InP nanocrystal surface while the latter requires a 11092


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better understanding of the kinetics of elementary processes leading to InP nucleation and growth in the plasma. There is no fundamental barrier for the plasma-synthesized InP nanocrystals to perform as well as those synthesized using colloidal chemistry. While growing the ZnS shell using liquid-based approaches removes the advantages of plasma synthesis (no solvent, high throughput, etc.), this shell can also be grown using a plasma. For example, a tandem plasma configuration where the InP nanocrystals are synthesized in the first plasma and then fed into a second plasma that enables the growth of a ZnS shell on these InP nanocrystals is certainly a possibility though forming the ZnS on the InP without nucleating ZnS particles may be a challenge. There is in fact, a recent demonstration of core−shell nanocrystal synthesis in a nonthermal plasma using this tandem plasma approach. Chaukulkar et al. used a tandem plasma reactor to first synthesize Si nanocrystals using a SiH4-containing plasma and then coat the surfaces of these nanocrystals with carbon using a second C2H2-containing plasma inserted in series.342

Figure 43. Optical absorption spectra of Cu2S nanocrystals capped with oleylamine (OAm) immediately after synthesis (green curve) and two months after synthesis (blue curve). Optical absorption of uncapped Cu2S nanocrystals exhibit plasmon absorption two months after storage in air (black curve). Reproduced with permission from ref 345. Copyright 2014 Royal Chemical Society.

compares the UV−vis-NIR spectra of Cu2S nanocrystals that were capped with oleylamine and dispersed in toluene immediately after production with the UV−vis-NIR spectra of Cu2S nanocrystals that were stored in air without capping. Spectra for the oleylamine-capped nanocrystals are shown immediately after synthesis and 2 months after synthesis. A plasmon absorption is observed in the spectrum of the nanocrystals stored on the bench without capping but not in the spectra for oleylamine capped nanocrystals, indicating that the capped nanocrystals are stable. Consistent with the absorption, TEM studies confirmed the presence of an oxide shell around the uncapped nanocrystals but not around the capped nanocrystals. The copper−sulfur phase diagram is complex and copper can form a variety of sulfides with different Cu-to-S ratio. Thimsen et al. demonstrated that they could synthesize different copper sulfide nanocrystals, and even metallic Cu nanocrystals, by changing the ratio of (HFAC)Cu(VTMS) to sulfur flow rates. While, the chemistry in the plasma is complex, there was a monotonic relation between the fraction of (HFAC)Cu(VTMS) in the feed gas and the nanocrystal stoichiometry. Thimsen et al. quantified the amount of Cu in the feed gas by using the ratio XCu = FCu/(FCu + 8FS8) where Fi is the molar flow rate of species i (i = (HFAC)Cu(VTMS) or S8). For example, Figure 44 shows the copper atomic fraction in the nanocrystals as measured by EDS as a function of the fraction of Cu precursor in the feed gas. The overall composition of the

6.4. Sulfides

Recently, Thimsen et al. extended the nonthermal plasma synthesis to sulfides and demonstrated that synthesis of ZnS, Cu2S and SnS is also possible.343 They reacted volatile metalorganic precursors with sulfur vapor in a plasma that contained primarily Ar gas. Two possible sulfur sources for plasma synthesis of inorganic sulfide nanocrystals are sulfur vapor and H2S. The latter is toxic and raises safety concerns, while the former is solid with limited volatility. Sulfur melts at 115 °C and its vapor pressure reaches as high as ∼0.15 Torr at 150 °C. At these low temperatures, sulfur vaporizes mostly as S8 but as the temperature rises, its thermodynamic speciation gets more complicated.344 Keeping the sulfur source at 120− 130 °C and the chamber and tube walls at 150 °C avoids sulfur condensation and allows the delivery of S8 vapor into the plasma at reasonable rates. Thimsen et al. synthesized stoichiometric Cu2S nanocrystals using hexafluoroacetylacetonate Cu(I) vinyltrimethylsilane [(CF3CH2CF3OO)Cu(CH2CHSi(CH3)3) or (HFAC)Cu(VTMS)] and sulfur vapor in an Ar plasma at ∼2 Torr.345 The nanocrystal stoichiometry depended on the ratio of the (HFAC)Cu(VTMS)] and S8 flow rates. Interestingly, when the ratio of the molar flow rate of Cu (FCu) to the total molar flow rate of Cu and S atoms (8FS8) [i.e., (FCu/(FCu+8FS8)] was approximately 0.61 the nanocrystals were nominally stoichiometric (0.63 ± 0.6), within the error of the EDS measurements. The nanocrystals were approximately 3.5 nm in diameter. They could be easily capped with oleylamine after synthesis and dispersed in toluene, a common method to store Cu2S nanocrystal dispersions synthesized using colloidal chemistry. The most significant finding in this study was that these Cu2S nanocrystal dispersions in toluene showed no signs of oxidation after 2 months in the ambient. This is significant because surfaces of Cu2S nanocrystals synthesized using colloidal chemistry oxidize even when they are stored as dispersions in organic solvents. The oxidized nanocrystals exhibit a plasmonic absorption346−348 because, facile diffusion of Cu from Cu2S into the surface oxide introduces copper vacancies into the bulk Cu2S. Copper vacancies dope Cu2S nanocrystals p-type, eventually turning the nanocrystals into a degenerately doped hole conductor. This sequence of events can be conveniently followed optically by detecting the plasmonic absorption that appears when the nanocrystals become doped. Figure 43

Figure 44. Nanocrystal composition as a function of the fraction of Cu precursor in the feed gas, XCu = FCu/(FCu + 8FS8) where Fi is the molar flow rate of species i. Reproduced with permission from ref 343. Copyright 2015 Institute of Physics. 11093


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46).349,350 It appears that the excess sulfur in the powder was in the form of polysulfide molecules.

nanocrystals could be changed by changing XCu. XRD from nanocrystals at select XCu values showed that the phase of the copper sulfide nanocrystals could also be controlled. Figure 45

Figure 46. Optical absorption spectra of plasma-synthesized ZnS nanocrystals dispersed in a solution of oleylamine (0.09 M) and toluene. Copyright 2015 Institute of Physics.

Thimsen et al. recorded optical emission from the plasma during Cu2S, ZnS and SnS synthesis. Sulfur plasma without the metal organics appears whitish blue with broadband and molecular emissions spanning 250−650 nm. Thimsen et al. identified S 2 emissions, indicating that the gas-phase composition of the plasma is likely to be significantly different than a thermal sulfur vapor, which at low temperatures is dominated by S8. It is reasonable to hypothesize that the S8 rings are broken up by electron impact reactions to S2 dimers. Interestingly, when metal organic precursors were admitted into the plasma sulfur optical emission was quenched noticeably. Similarly, normalized (with Ar emission) optical emission from Zn was lower when sulfur was added to DEZ/Ar plasma. They interpreted this mutual decrease in emission of Zn and sulfur as a sign that these species reacted with each other in the gas phase to form ZnS. In contrast to optical emission from DEZ/Ar/sulfur mixtures, the normalized S2 emission (with respect to Ar emission) from plasmas maintained in mixtures of (HFAC)Cu(VTMS)/Ar/sulfur increased significantly when (HFAC)Cu(VTMS) was added to Ar/sulfur plasma. This is in spite of the fact that the broadband emission in the 250−650 nm decreased in both DEZ and (HFAC)Cu(VTMS) containing plasmas. The authors interpreted this as reactions of (HFAC)Cu(VTMS) with S8 favoring formation of S2. As (HFAC)Cu(VTMS) was increased, however, normalized S2 emission eventually decreased. This nonmonotonic behavior of emission underscores the complexity of plasma reactions. Clearly, one of the challenges in understanding and controlling the reactions in the plasma will be the reliable and quantitative detection of organometallic precursor fragments and Sn (1 ≤ n ≤ 8) molecules.

Figure 45. (a) XRD from nanocrystals synthesized with different fractional copper feed rates: (A) XCu = 0.1, (B) XCu = 0.6, and (C) XCu = 0.85. Panels b−d show the XRD patterns expected from Cu, Cu2S and CuS, respectively. The XRD in (a) are consistent with (A) CuS, (B) Cu2S, and (C) Cu metal. The XRD patterns in panel a are offset for clarity. Reproduced with permission from ref 343. Copyright 2015 Institute of Physics.

shows the XRD patterns collected form nanocrystals synthesized using different XCu. Low fractions of Cu precursor in the feed gas (e.g., XCu = 0.1) yielded CuS while high values (XCu ≥ 0.850) yielded metallic Cu. At intermediate values (e.g., XCu = 0.6), Cu2S nanocrystals are obtained. Thimsen et al. also synthesized ZnS and SnS by replacing (HFAC)Cu(VTMS) with diethyl zinc (DEZ) and tetrakis(dimethylamido) tin (TDMAT), respectively.343 They showed that the elemental composition of the ZnS could be controlled by changing the DEZ flow rate and made nominally stoichiometric as well as sulfur-rich ZnS nanocrystal powders. These powders could be dispersed in organic solvents containing oleylamine. Nominally stoichiometric ZnS nanocrystals formed visibly transparent and clear solutions with absorption increasing in the ultraviolet above the band gap of ZnS. Dispersions made from sulfur-rich nanocrystal powders, on the other hand, were yellow with absorption features indicating the presence of polysulfides in the dispersion (Figure

7. SYNTHESIS OF METAL NANOPARTICLES AND OTHER NANOSTRUCTURES There are two general strategies to synthesizing metal nanoparticles which parallel the deposition of metallic thin films. The first one relies on the dissociation of metal−organic vapors in the plasma, similar to metal−organic chemical vapor deposition (MOCVD) of thin films. In contrast to thin film deposition, which exploits heterogeneous nucleation on a substrate, homogeneous nucleation in the gas phase is promoted. In the second approach, similar to either physical or chemical sputtering of solid metal targets, a metal electrode 11094


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Figure 47. (a) Aerosol mobility measurements of Ni nanoparticles synthesized in a DC atmospheric-pressure microplasma from nickelocene vapor. (b) Histograms of Ni nanoparticle diameters from TEM analysis of electrostatically precipitated samples. (c−e) TEM images of Ni nanoparticles. Red = 15% v/v, green = 20% v/v, and blue = 40% v/v of nickelocene flow rate in Ar (v/v refers to volumetric fraction). Scale bars in TEM images are 5 nm and insets are 2 nm. Modified with permission from ref 352. Copyright 2008 American Chemical Society.

TEM analysis also revealed that the Ni nanoparticles were crystalline, exhibiting lattice spacings corresponding to111 crystalline plane of fcc Ni, the most thermodynamically stable plane and structure of bulk Ni (γ-Ni) at normal conditions (Figure 47c−e). The trends for nanoparticle size as a function of precursor vapor concentration suggest that smaller and smaller particles could be synthesized in the DC microplasma by continuing to lower the precursor concentration. Based on supersaturation theory, a limit should be reached where the vapor concentration is too low for particle nucleation. Atomic force microscopy (AFM) was used to address this question.353 AFM images of Ni nanoparticles shown in Figure 48 indicate that the reduction in particle size with vapor concentration is continuous, with clusters as small as 0.5 nm formed at the lowest concentrations studied. Since these clusters could be oxidized and their composition is not clear, these results were corroborated by aerosol mobility measurements. Supporting calculations of the collision cross sections measured by aerosol mobility and cluster structures showed that the smallest synthesized Ni clusters contained less than 100 atoms. More than one metal−organic precursor has also been combined to produce bimetallic nanoparticles.354,355 Thus, far, this has been demonstrated most completely with the NixFe1−x material system. Figure 49a,b show TEM images of Ni0.23Fe0.73 nanoparticles that were synthesized in a DC atmosphericpressure microplasma by mixing nickelocene and ferrocene vapor in a volumetric ratio of 27%:73%. The total metallocene vapor flow rate in Ar was varied from 9 sccm for the nanoparticles shown in Figure 49a to 15 sccm for the nanoparticles shown in Figure 49b which tuned the final particle size from a mean diameter of ∼2.3 to ∼3.7 nm, respectively, analogous to the case of pure Ni nanoparticles. The high-resolution images in the insets of Figure 49a,b reveal that the bimetallic nanoparticles are crystalline and exhibit a lattice spacing that is slightly larger than the111 crystalline plane

or target is sputtered in the plasma. Again, as opposed to deposition of the sputtered moieties on a substrate to heterogeneously nucleate a thin film, homogeneous nucleation in the gas phase is the goal. 7.1. Homogeneous Nucleation from Metal−Organic Vapors

To date, Sankaran and co-workers have done the most extensive studies of metal nanoparticle synthesis from metal− organic compounds. Most of their studies focused on Ni and Fe nanoparticles synthesized from their corresponding metallocenes, biscyclopentadienyl nickel (nickelocene) and biscyclopentaidenyl iron (ferrocene), using a DC atmospheric-pressure microplasma (Figure 17a).351 These precursors are both solid powders at room temperature, and vapors were introduced in the microplasma by sublimation in a flow of Ar gas. The vapor pressures at room temperature were found to be sufficiently high to produce nanoparticles. The final vapor concentration was controlled by diluting the sublimed flow with a flow of pure Ar gas and varying the ratio of the two flows. The synthesized nanoparticles were initially characterized by aerosol mobility measurements using a commercial instrument (TSI, Inc.). Figure 47a shows particle size distributions at three different steady-state concentrations of nickelocene vapor. The shift in the as-synthesized mean diameter while maintaining relatively narrow size distributions (σ < 20%) demonstrates the ability to control nanoparticle size.352 To verify the particle sizes and further characterize the shape and structure of the nanoparticles, the aerosol flow was electrostatically precipitated onto TEM grids. The histograms of the deposited Ni nanoparticle diameters from TEM analysis shown in Figure 47b agree very well with the aerosol mobility measurements both in terms of trends and the actual nanoparticle diameters; the slightly larger diameters observed in TEM could be the result of oxidation during transfer of the nanoparticles in room air. The close matching of the mobility diameters with the actual primary particle diameter indicates that the particles are relatively unagglomerated and can be size-tuned in the microplasma. 11095


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Figure 49. (a and b) TEM images of Ni0.27Fe0.73 nanoparticles synthesized from a 27% nickelocene and 73% ferrocene vapor mixture. The total metallocene flow rate in Ar was 9 sccm (a) and 15 sccm (b). Insets show high-resolution images of the nanoparticles which are crystalline and exhibit lattice spacings of 0.21 nm which are slightly larger than the (111) crystalline plane of fcc Ni. Scale bars in TEM images are 5 nm and insets are 2 nm. Reprinted with permission from ref 355. (c) EDS spectra of NixFe1−x nanoparticles synthesized at varying ratios of nickelocene and ferrocene vapor concentrations. The total metallocene vapor concentration in Ar was adjusted to obtain a constant mean particle diameter of 3.0 nm as measured by aerosol mobility. Reprinted with permission from ref 354. Copyright 2009 Nature Publishing Group. (d) Atomic percentage of Ni in NixFe1−x nanoparticles with respect to Fe measured by EDS analysis. Reprinted with permission from ref 355. Copyright 2008 John Wiley and Sons. (e) XRD spectra of NixFe1−x nanoparticles synthesized at varying ratios of nickelocene and ferrocene vapor concentrations. The total metallocene vapor concentration in Ar was also varied to obtain a constant mean particle diameter of 3.0 nm as measured by aerosol mobility. Reprinted with permission from ref 354. Copyright 2009 Nature Publishing Group. (f) Lattice parameters corresponding to fcc (afcc) and bcc (abcc) structures of NixFe1−x nanoparticles as a function of nickelocene concentration in microplasma. Literature values for bulk NiFe alloys are also shown for comparison (open symbols).357 Reprinted with permission from ref 354. Copyright 2009 Nature Publishing Group.

Figure 48. AFM images of Ni nanoclusters synthesized in a DC atmospheric-pressure microplasma at nickelocene flow rates of (a) 15 sccm, (b) 8 sccm, and (c) 5 sccm in Ar (total flow rate = 100 sccm). Below (b) and (c) are the height profiles obtained from the indicated lines. Reprinted with permission from ref 353. Copyright 2014 Institute of Physics.

of fcc Ni. This is an indication that Fe is incorporated into the lattice, since Fe has a larger atomic radius than Ni and should cause an expansion, to produce alloyed nanoparticles. To confirm, the synthesis was extended to other compositions and the nanoparticles were characterized by EDS and XRD. These techniques are independent and complementary for assessing bimetallic nanoparticles.356 EDS allows a very small sample size to be analyzed to ensure that compositional variations are not averaged, but gives no structural information. The crystal structure of nanoparticles is typically determined by XRD, but averaged over many nanoparticles contained in a thin film. Figure 49c shows EDS spectra of three samples of NixFe1−x nanoparticles synthesized with varying vapor concentration ratios of nickelocene to ferrocene. The total metallocene concentrations were adjusted to obtain the same mean particle diameter of 3.0 nm in each sample, as determined by aerosol mobility measurements. The indicated atomic fractions (x = Ni) were predicted based on the vapor concentrations. Lines corresponding to Ni and Fe in the spectra decrease and increase, respectively, as x decreases, confirming that the final Ni atomic fraction in the particles is controlled. Semi-

quantitative analysis was carried out by comparing the relative peak intensities of the EDS lines corrected by their respective k factors to the nickelocene vapor concentration in the microplasma reactor, which showed excellent correspondence (Figure 49d). XRD analysis of the same three samples is shown in Figure 49e. Peaks corresponding to bulk fcc Ni are marked and observed in all three samples. Samples containing Fe exhibit a shift that increases with Fe content of these peaks to 11096


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nanoparticles synthesized from nickelocene shown in Figure 50a indicates that only a negligible amount of carbon is present

smaller 2θ, indicating an expansion of the lattice, in agreement with TEM analysis. In the case of Ni0.27Fe0.73 particles, additional peaks are observed, which were assigned to the bcc structure of Fe, the thermodynamically favored structure of bulk Fe (α-Fe) at normal conditions. The lattice parameter calculations from XRD characterization are shown in Figure 49f and compare favorably with the thermodynamic phase equilibrium of bulk NiFe,357 including the coexistence of the fcc and bcc phases at high Fe concentrations. The slightly larger lattice parameter as compared to bulk alloys is related to the higher surface energy of nanoparticles which have been shown to cause significant compressive stresses.358 The ability to control the nanoparticle size and composition, and do so independently, has important implications. Size- and compositionally tuned metal and bimetallic nanoparticles could be used as model catalysts to systematically correlate activity and selectivity in chemical conversion. This has been thus far carried out in studies of carbon nanotube growth. Varying the Ni nanoparticle size has been shown to control the diameter of carbon nanotubes grown by the floating catalyst method.352 Kinetic studies were carried out to find that the composition of Ni1−xFex nanoparticles influences the activation energy for carbon nanotube growth with the following increasing order: Ni0.67Fe0.33 > Ni0.27Fe0.73 > Ni0.88Fe0.12 > Ni > Fe.354 These results led to the discovery of a minimum growth temperature of 300 °C for Ni0.67Fe0.33, one of the lowest temperatures ever reported for carbon nanotube growth in a thermal CVD process. The composition of NixFe1−x nanoparticles has also been found to affect the chirality of the as-grown single-walled carbon nanotubes.355 The chirality distribution narrows and shifts to smaller diameter tubes as the Fe content is increased, which leads to a higher percentage of semiconducting chiralities.359 Although not as extensively characterized, other bimetallic nanoparticles have been synthesized by this approach as well including NixCu1−x.360 In that case, because both Ni and Cu adopt fcc structures, the compositions could be calculated both from EDS and XRD, the latter using Vegard’s Law, and were found to agree well, but with slightly higher compositions from EDS analysis than predicted. EDS has been shown to be less accurate in determining metal content in bimetallic nanoparticles.356 The approach has also been extended to three metals, Ni, Fe, and Cu, with varying relative compositions, showing a general strategy to produce multimetallic nanoparticles.360 As the number of metals increase, predicting the composition of the nanoparticles becomes complex and ex situ characterization may be cumbersome. Assuming that the metals mix uniformly throughout the nanoparticle volume, an empirical approach to calculating the multimetallic composition based on a relationship of the measured mean mobility diameter and the precursor vapor concentration for the pure metals has been proposed.361 A critical question for synthesis of metal nanoparticles from metal−organic vapors is whether the organic components of the precursor, such as carbon, are incorporated into the nanoparticle material, either in the core or on the surface. This is particularly a concern in plasma processes where electrons carry sufficient energy to collisionally dissociate not only the metal−carbon bonds, but also the nonmetal bonds. The presence and nature of carbon in metal nanoparticles synthesized from metal−organics using a DC atmosphericpressure microplasma has been assessed by X-ray photoelectron spectroscopy (XPS).362 High-resolution C 1s spectra of Ni

Figure 50. High-resolution XPS spectra of C 1s region for (a) Ni nanoparticles synthesized from nickelocene and (b) Cu nanoparticles synthesized from copper acetylacetonate using an atmosphericpressure DC microplasma. Two different discharge currents are shown. Reprinted with permission from ref 362. Copyright 2012 John Wiley and Sons.

at low discharge currents of 4 mA, but is observed at higher discharge currents of 8 mA. Deconvolution of the spectra shows two peaks corresponding to C−C/CC (284.6 eV) and carbidic carbon, e.g., Ni−C (282.5 eV), which suggests that carbon is both on the particle surface and in the particle core. In comparison, high-resolution C 1s spectra for Cu nanoparticles synthesized from copper acetylatonate shown in Figure 50b show significant carbon at both low and high discharge currents. Fitting of the spectra indicate C−C/CC and carbidic carbon, with the latter increasing in relative amount at higher currents. Higher discharge currents could increase the electron density and/or temperature in the plasma, as well as gas temperature, which would increase the fragmentation of the precursor to produce carbon moieties and enhance carbon incorporation in the nanoparticles during their formation. The difference in carbon contamination for the two precursors suggests that the precursor bond configuration is also important. 7.2. Homogeneous Nucleation from Solid Metal Targets

There is a long history of plasmas used to evaporate or sputter metals. Most of these studies have been performed with thermal plasmas such as arcs or laser-induced plasmas, which is 11097


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not the focus of this review and will not be covered here. More recently, microplasmas have been used in different configurations to sputter solid metal sources. Because the processes appear to be nonthermal and the metal is sputtered rather than evaporated, these examples are introduced here. Shimizu and co-workers have done the most extensive investigations of metal nanoparticle synthesis by sputtering. The reactor configuration in their experiments is similar to the pulsed microplasma (Figure 17b) except that a metal wire to be sputtered is inserted inside the reactor tube. Au nanoparticles have been synthesized from Au wire with a diameter of 100 μm.363 The plasma was operated at 450 MHz as shown by the voltage waveform in Figure 51a. The relatively small power of

Figure 52. (a) Cross-sectional illustration of in situ microplasma cell for sputtering Au nanoparticles consisting of Si-supported SiNx membranes separated by 75 μm coated on their interior faces with 25−50 nm thick Au film. The cell was sealed with 760 Torr Ar. (b) Dependence of Au island growth rate on the power delivered to the microplasma. The solid line is a prediction based on ref 368. The dashed line is a fit to the experiment which is ∼3× that of the prediction. Modified with permission from ref 367. Copyright 2013 Nature Publishing Group.

with Au film served as both the TEM window and electrodes for the microplasma. Au was sputtered by ion bombardment at the cathode and deposited onto the anode. The deposition rate was compared to sputtering in low-pressure, parallel plate DC discharge368 and found to be three times higher (Figure 52b).

8. SURFACE CHEMISTRY The surface functionalization of nanocrystals is an important field of study. When the particle size is on the order of a nanometer, more than half of the atoms in the particle may lie at its surface;369 thus, the surface of the nanocrystal plays an important role in its properties, including its electrical and optical properties as well as its dispersibility and stability in colloidal solutions.370,371 In colloidal synthesis techniques, the formation of nanocrystals requires surfactants to tune the kinetics of nucleation and growth.372 Consequently, liquid-phase synthesized nanocrystals are typically terminated by long, insulating ligands, and steric hindrance prevents complete passivation of surface defects, which is detrimental to the nanocrystals’ electrical and optical properties.373 Nonthermal plasmas allow the synthesis of ligandless nanocrystals with extremely low defect densities on the order of one dangling bond per 200 nanocrystals,161,205,374 nanocrystals with a wide range of surface chemistries from hydrogen to alkyl termination have been demonstrated with both atmospheric and low-pressure nonthermal plasmas, with much of the research focused on group IV nanocrystals. The surface chemistries of the nanocrystals can be controlled simply through precursor selection. Hydrogen-terminated Si and Ge nanocrystals are typically produced with silane (SiH4) and germane (GeH4) precursors, respectively.375 Halides can also be grafted onto the nanocrystal surface. Chlorine-rich surfaces have been derived from precursors such as silicon and germanium tetrachloride (SiCl 4 and GeCl 4 , respectively),161,165,376 where the concentration of chlorine to hydrogen on the nanocrystals can be tuned with the injection of hydrogen gas into the plasma.191 Fluorine passivation has been accomplished using two-stage, flow-through plasma reactor, where silane-produced Si nanocrystals were etched/ passivated in a second plasma fed with carbon tetrafluoride (CF4) or sulfur hexafluoride (SF6).206,377 Hydroxyl (OH) groups are typically formed on metal oxide nanocrystals.330,378 The wide range of surface chemistries enables the study and

Figure 51. (a) Pulse-modulated UHF voltage waveform of microplasma. (b) Photo of microplasma demonstrating low temperature. (c) Optical image of Au nanoparticles deposited directly on paper. (d) Time-resolved emission spectrum of Hβ line. Reprinted with permission from ref 363. Copyright 2009 AIP Publishing LLC.

0.8 W and low duty cycle of 5% resulted in a room temperature plasma (Figure 51b). The Au nanoparticles could be deposited directly on paper as shown in Figure 51c. Curiously, H2 gas was found necessary in the Ar gas mixture to produce any Au nanoparticles, suggesting that the mechanism was reactive sputtering perhaps mediated by radicals such as atomic hydrogen which were detected by optical emission spectroscopy (Figure 51d). TEM characterization showed that the nanoparticles were relatively uniform with a mean diameter of ∼8 nm. Microplasma sputtering of wires has been extended to other metals including W, Fe, and Cu.364 Two nozzle sizes were studied, one with a diameter of 700 μm and the other 300 μm. The nanoparticles were deposited by flowing the effluent into room air, resulting in oxidation of the transition metals. The smaller nozzle was believed to reach supersonic conditions and led to a production rate that was ∼100× higher than the larger nozzle. Similar enhancements in deposition rates for nanostructured thin films by a supersonic microplasma have been recently reported.365 Mariotti and co-workers applied the same plasma system to the synthesis of Mo nanoparticles, but added O2 gas to the feed to produce metal oxides.366 Eden and co-workers integrated a planar DC microplasma device with TEM to sputter metal nanoparticles for in situ applications.367 As illustrated in Figure 52a, a cell sealed with 760 Torr Ar consisting of Si-supported SiNx membranes coated 11098


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Figure 53. Schematics of proposed radical-based thermal (a) hydrosilylation and (b) silylsilylation processes on nonthermal plasma-synthesized Si nanocrystals. Adapted with permission from ref 379. (Copyright 2002 American Chemical Society) and ref 164 (Copyright 2015 American Chemical Society).

RNO-H groups was observed, which suggests homolytic cleavage of surface Si−H bonds was an insignificant initiation reaction. It should be noted that the propagation reaction along the nanocrystal surface likely still proceeds through the abstraction of hydrogen atom from neighboring sites. In addition to imparting solubility, the functionalization reactions also passivate nonradiative defects such as dangling bonds that degrade the nanocrystals’ optical properties.380−383 The hydrosilylation of freestanding, low-pressure, RF plasmasynthesized nanocrystals has yielded the highest photoluminescence quantum yields (PLQYs) in Si quantum dots, likely due the initial surface chemistry consisting of the easily cleaved Si-SiH3 groups with the stronger silicon mono- and dihydrides. Mangolini et al. discovered that by injecting hydrogen gas into the expansion region of the nonthermal plasma reactor, post nanoparticle synthesis region, they were able to increase the surface hydrogen coverage and improve the reaction.202 Hydrosilylation of these Si nanocrystals with 1dodecene in an oxygen-free environment yielded size-tunable emission between 700 and 820 nm from 3 to 4.3 nm Si nanocrystals.202 Notably, the largest nanocrystals exhibited PLQYs nearing 70%.201 The alkyl ligands, however, do not inhibit oxidation on hydrosilylated Si nanocrystals. The Si nanocrystals displayed significant degradation in PLQY (over 20% decrease) and blueshift in emission wavelength, corresponding to reduction in core size, after several days of air exposure.201,384 This is not surprising as significant amount of hydrides remain on the surface post hydrosilylation; thus, oxidation can still proceed via the attack of oxygen/water molecules on surface hydrogen or on Si−Si bonds to form Si−O−Si moieties.385−387 The residual hydrides on the surface of the hydrosilylated Si nanocrystals also play an important role in the photostability of the nanocrystals. Wu and Kortshagen discovered that highenergy UV light was able to cleave weaker surface bonds, leaving dangling bonds on the nanocrystal surface.388 FTIR spectroscopy revealed almost 20% reduction in Si−Hx (x = 1, 2, 3) stretching signal after 4 h of UV irradiation at 4 mW/cm2, while the ligand coverage remained constant. Similar to Wheeler et al.’s findings,164 silicon trihydrides, molecule-like groups that are attached to the nanocrystal surface via a single Si−Si bond, were primarily desorbed from the surface. The consequent dangling bonds led to ∼20% (absolute) degradation in PLQY. Repassivation of these dangling bonds with dodecyl ligands resulted in the recovery of the nanocrystals’ optical properties and UV stability.

application of a myriad of surface functionalization techniques from simple air oxidation to the more complex Grignard reaction. Functionalization of nanocrystals with organic ligands is typically necessary for roll-to-roll device processing and for biological imaging. While ligandless nanocrystals can be stabilized in some solvents,187 surface ligands enable the nanocrystals to overcome interparticle van der Waals forces to achieve homogeneous colloidal solutions. Hydrosilylation, hydrogermylation, silanization, and Grignard reactions have been intensively studied on freestanding Si and Ge nanocrystals to attach organic molecules onto the nanocrystal surface.379 These powerful surface modification techniques enable the attachment of a wide range of organic molecules to achieve solubility in a variety of solvents from hexane to water. 8.1. Silylation and Germylation

8.1.1. Liquid Phase Reactions. Several groups have successfully produced homogeneous dispersions of hydrogenterminated, nonthermal plasma-produced Si and Ge nanocrystals in nonpolar solvents such as hexane and toluene via thermal hydrosilylation153,164,202 and hydrogermylation.229 On flat surfaces, the reaction is believed to be initiated via the desorption of surface hydrides at elevated temperatures (>150 °C) which creates surface dangling bonds that can react with the vinyl group of unsaturated organic molecules to produce Si−C (or Ge−C) bonds (and a carbon-based radical). The further addition of ligands is a result of a chain propagation reaction where the carbon-based radical extracts a hydrogen atom from a neighboring site to produce a second surface dangling bond, which can be passivated with additional ligands.379 The radical-based reaction is summarized in Figure 53a below. It has recently been discovered that the disordered surface of low-pressure, RF plasma-synthesized Si nanocrystals, consisting of mono-, di-, and trihydrides,169 results in a new functionalization path termed silylsilylation.164 Wheeler et al. discovered that the abstraction of trihydrides (SiH3) was the dominant initiation step in their silylation reactions (Figure 53b). The authors employed a free radical, 3-cyano-proxy (3-cyano2,2,5,5-tetramethyl-1-pyrrolidinyloxy), to trap the silicon hydride groups that were desorbed from the surface. The resulting compounds were then probed via proton nuclear magnetic resonance (1HNMR) and Fourier transform infrared (FTIR) spectroscopy, which both showed the presence of RNO-SiH3 groups from the reaction of surface desorbed silicon trihydrides with the introduced free radical. No evidence of 11099


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8.1.2. Gas Phase Reactions. Functionalization with short organic ligands is desirable in order to achieve improved electrical conductivities compared to the long, insulating hydrocarbon chains.399 In the liquid phase, the thermal input needed for the initiation of the hydrosilylation reactions renders it unfeasible to utilize the shorter ligands due to their low boiling points. Mangolini et al. devised a novel technique to functionalize the nanocrystals in the gas phase with a wider range of molecules,389,390 which further provides the benefit of removing the time demands of the liquid-phase functionalization processes and reducing waste generation. Si nanocrystals synthesized in a low-pressure, RF plasma were injected into a secondary plasma, where radicals of vaporized 1-dodecene were grafted onto the nanocrystal surface (Figure 54). 1-Hexyne, 1-

surface coverage. Functionalization with PA and acetylene yielded hydrocarbon coverage ∼58%, approaching the predicted theoretical limit of 60%.392 In comparison, only 46% surface coverage was attained when the nanocrystals were not exposed to acetylene. The remarkable surface coverage led to enhanced environmental stability, while Si nanocrystals with ∼30% ligand coverage resulted in almost 40% reduction in SiHx (x = 1, 2, 3) infrared absorbance signal with oxidation, less than 5% reduction was observed with coverage of ∼58%. 8.2. Organometallic Reactions with Si and Ge Surfaces

Organometallic reactions have also been successfully performed on reactive chlorine-terminated silicon and germanium surfaces to obtain singly dispersed nanocrystals with enhanced photoluminescence. In Grignard reactions, alkyl ligands can be grafted onto the surface of chlorinated nanocrystals by reacting the electrophilic surface groups with a nucleophilic Grignard reagent (Figure 55). Wheeler et al. reacted 4.8, 6.8, and 10.2

Figure 55. Schematic of Grignard reaction of chlorinated germanium surfaces. Adapted with permission from ref 379. Copyright 2002 American Chemical Society.

nm chlorinated Ge nanocrystals produced in a low-pressure RF plasma with methylmagnesium bromide and dodecylmagnesium bromide (in diethyl ether) at 80 °C in an oxygen-free environment.232 FTIR spectroscopy revealed reductions in both Ge−Cl and Ge−Hx signals, which suggests alkylization of germanium hydrides with the Grignard reagents. This effect has also been observed in silicon and is attributed to reactive alkyl radicals generated by alkyl halide impurities.393 In addition to improved solubility, the reaction with dodecyl magnesium bromide yielded size-tunable, excitation independent emission from 0.77 to 1.03 eV, which corresponds to germanium bandgap emission.232 Perhaps the most noteworthy of Wheeler et al.’s work was the narrow width of the Gaussian emission spectrum; the plasma-synthesized Ge nanocrystals possessed at least a factor of 2 narrower emission than nanocrystals synthesized from other routes.394−396

Figure 54. Schematic of a two-stage plasma reactor for the synthesis of silicon nanocrystals in the first stage and the gas phase hydrosilylation of their surfaces in the second stage. Reproduced with permission from ref 389. Copyright 2007 John Wiley and Sons.

hexene, hexyl-alcohol, and hexane molecules were also investigated. FTIR spectroscopy demonstrated successful attachment of the organic ligands onto the nanocrystal surface, with 1-hexyne molecules exhibiting the highest surface coverage, followed by 1-hexene. The plasma-aided alkylation allowed the dispersion of 1-dodecene functionalized nanocrystals into nonpolar solvents without any post processing. Unfortunately, these gas-phase functionalized nanocrystals exhibited weak photoluminescence with QYs under 10%. Their poor optical properties can be attributed to insufficient surface passivation; the plasma-grafted nanocrystals possessed a higher density of electron paramagnetic resonance (EPR) active defects in comparison to liquid-phase hydrosilylated nanocrystals.389 By making use of vaporized short- and long-chain alkynes, Jariwala et al. and Weeks et al. were able to reduce steric hindrance and dramatically improve the hydrocarbon coverage.391,392 The authors thermally hydrosilylated films of hydrogen-terminated, capacitively coupled plasma-produced Si nanocrystals using styrene or phenylacetylene (PA) and sequentially exposed the films to acetylene, followed by styrene or PA. Due to the fast reaction rate of the smaller acetylene molecule with the surface hydrides, the introduction of styrene or PA prior to acetylene was necessary to achieve the optimum

8.3. Silanization

Silanization is another important reaction, which has enabled the production hydrophilic nanocrystals. Hydroxyl groups (OH) on silica surfaces readily react with siloxane groups of silane coupling agents (SCAs) with various functional groups to yield both hydrophobic and hydrophilic nanoparticles.397 For the latter, Sehlleier et al. investigated the functionalization of microwave plasma-synthesized silica NPs with n-octyltriethoxysilane (OTES) SCA. In the reaction, the SCAs are first hydrolyzed in water to form silanol groups, which then react with the silanol groups on the nanoparticle surface in a condensation reaction (Figure 56a). Dynamic light scattering (DLS) revealed single particle dispersions of the OTESfunctionalized NPs in toluene. Colloidal dispersions of silica NPs in water were achieved by functionalizing the particles with 3-aminopropyltriethoxysilane (APTES). The amines on the surface led to improved water stability compared to the unfunctionalized NPs. Hydrophilicity was further improved by 11100


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8.4. Hypervalent Reactions with Solvents

The reduction in length or removal of surface ligands can be important to enhance charge transport in nanocrystal films.399 Recently, Holman et al. demonstrated the possibility of producing ligandless colloids with chlorine-passivated, lowpressure, RF plasma-synthesized Ge nanocrystals, 400 a technique that was proven to be transferable to chlorinepassivated Si nanocrystals.200 Wheeler et al. found that by tuning the acidity of the nanocrystal surface, the nanocrystal can hypervalently interact with specific (Lewis basic) solvent molecules, see Figure 57.200 Solvents with Gutmann donor number (DN) between 13 and 17 kcal mol−1, such as ketones/ aldehydes and nitriles, yielded a favorable interaction between the acidic Si nanocrystal surface and enabled nanocrystal solvation without ligands. These unbound solvent molecules were easily desorbed from the nanocrystal surface under vacuum, which is beneficial for device fabrication. The electron donating solvents can further act as surface dopants, improving the conductivity of nanocrystal films. Dark conductivities as high as 10−5 S cm−1 were measured. Similar hypervalent interactions were recently also observed for boron-doped silicon nanocrystals, in which surface boron appears to be the Lewis acidic surface group.401,402 8.5. Inorganic Surface Coatings

Nanocrystals passivated with a native oxide shell can feature enhanced photoluminescence compared to the as-synthesized counterparts. Anthony et al. and Liptak et al. reported over ten folds amplification of photoluminescence intensities, with QYs approaching 45%, simply through the oxidation of low-pressure, RF plasma-produced Si nanocrystals in ambient conditions.203,403 While the native shell formation required over 80 days for hydrogen-terminated Si nanocrystals, grafting of electronegative atoms such as chlorine and fluorine onto the nanocrystal surface significantly enhanced oxidation rate;161,191,403 Liptak et al. reported a complete shell was grown in less than 30 days.377 Furthermore, the counterpassivation etching of Si nanocrystals by chlorine and fluorine radicals in the plasma enabled the synthesis of smaller nanocrystals (150 °C activates additional dopants and causes a blueshift of the LSPR. Zhou et al.427 reported a comparative study of boron and phosphorus-doped Si nanocrystals. Different from Rowe et al., who studied phosphorus-doped Si nanocrystals while avoiding oxidation, this study used nanocrystals whose surfaces had been oxidized in air over a long period of time. The authors demonstrated that the Drude model applies to both types of 11103


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cost using colloidal or impaction techniques.432 The method has been tested using a 248 nm laser on both B- and P-doped Si nanocrystals synthesized in a hot wall reactor and a microwave plasma, respectively, with nominal doping concentrations ∼5 × 1020 cm−3 for both doping types. Meseth et al. observed a constant doping level of ∼1020 cm−3 below the surface of the silicon substrate (quantified by electrochemical capacitance voltage, or ECV) that decreased nearly exponentially down to the original substrate doping concentration deeper beneath the surface. The dopant penetration depth also increased with increasing laser power, with a maximum depth of 200 nm attained. The aforementioned laser-doped structures were used in diodes as a proof of concept. The doped Si nanocrystals were deposited on complementarily doped Si substrates (e.g., Bdoped nanocrystals on n-type substrate), HF etched, and subsequently laser annealed for device fabrication.431 Both n-p and p-n devices displayed similar current−voltage characteristics in the dark. Rectifying behavior with an on/off ratio of 227 at ±1 V was also observed. Light-beam-induced current (LBIC) measurements were used to probe the electrical homogeneity of the diodes; apart from edge effects, the short circuit current maps showed homogeneity on the micrometer scale, which indicates homogeneous doping distribution.431

both for Al-doped ZnO and self-doped ZnO nanocrystals. For self-doped ZnO the LSPR energy increases from 0.2 to 0.31 eV as the particle diameter is reduced from 12.6 to 4.1 nm. Similarly, for 1.3% Al doping the LSPR energy shifts from 0.35 to 0.5 eV for the particle size decreasing from 11.3 to 3.6 nm. A similar behavior had been observed previously by Schimpf et al. for photodoped zinc oxide.429 The authors interpreted this blueshift as a result of the quantum confinement of electrons in the semiconductor nanocrystals. In a subsequent theoretical study, Zhang et al.430 used time-dependent density functional theory (DFT) to study this phenomenon and, based on their DFT results, suggested a phenomenological formula to describe the size-dependent LSPR data. The results of Greenberg et al. agree well with this formula. 9.1.2. Self-Doped Semiconductor Nanocrystals. Many compound semiconductor nanocrystals are self-doped through vacancies.346,348 To date, plasmonic behavior in plasma synthesized nanocrystals has been observed in ZnO and Cu2S. Though plasmonics was not the primary topic of their investigation, Thimsen et al.331 observed plasmon resonances in films of plasma-produced ZnO nanocrystals. It is believed that oxygen vacancies in ZnO donate electrons to the material making it an n-type conductor. Nanocrystals in this study were about 7.7 nm in size. LSPRs were only observed after OH surface groups on the as-deposited nanocrystals had been removed by atomic layer deposition of Al2O3. The authors showed that the LSPR peak could be modeled assuming a free electron density of 5.5 × 1019 cm−3 and an electron mobility of 21 cm2 V−1 s−1. The carrier density was consistent with Hall measurements, however, the Hall mobility was lower than that derived from the LSPR. This is consistent with the LSPR response being indicative of the intrinsic electron mobility, which does not account for the effect of interfaces that is present in Hall effect measurements. The studies of the plasmonic response of plasma-produced Cu2S nanocrystals by Thimsen et al.345 are discussed in detail in section 6.4. Here it should be mentioned that different from ZnO, copper vacancies are believed to be the source of free holes, leading to p-type conductivity and the plasmonic response of the Cu2S crystals. 9.1.3. Surface-Doped Nanocrystals. As discussed in section 8.4, Wheeler et al. observed strong surface doping of chlorine terminated Si nanocrystals interacting with hard donors. The silicon crystals were not doped through any impurity dopant and as-produced did not exhibit an LSPR. Upon exposure to 2-butanone, a strong LSPR emerged, peaking at ∼1000 cm−1, as shown in Figure 57. The surface doping effect appeared to be reversible. The plasmonic response diminished but did not fully disappear after letting the solvent evaporate. The authors observed a redshift of the LSPR peak upon applying a vacuum to the silicon nanocrystals. FTIR measurements indicated that even after vacuum treatment there is still a certain amount of solvent molecules bound to the surface, which explains that the LSPR did not completely disappear.

9.3. Thermoelectrics

Conversion of heat to electricity via the Seebeck effect can be a fruitful way of recovering heat dissipated in many electrical and mechanical systems. In electronic circuits, semiconductor nanocrystal-based thermoelectric devices can play a central role in heat management. A successful thermoelectric device requires both low thermal conductivity and high electrical conductivity; therefore, highly doped semiconductors are promising candidates. Heavily doped nanocrystals possess low thermal conductivity due to impurity-triggered phonon scattering and high electrical conductivity from dopant-induced free charge carriers.433,434 Thermal conductivity can be further reduced by employing thin films of nanomaterials; grain boundaries and interfaces in these films can induce phonon scattering, reducing thermal conductivity and improving thermoelectric performance.435−437 Naturally, highly doped nanocrystals, especially those produced using nonthermal plasmas, are promising candidates as building blocks for thin film-based thermoelectric devices.294,438,439 Thermoelectric films have been fabricated with doped, microwave plasma-synthesized Si nanocrystals.440 Lechner et al. cast thin films of Si nanocrystals on flexible plastic substrates and laser annealed the films at energy densities exceeding 60 mJ cm−2 to form dense n- or p-type films.440 The annealing process plays a critical role for the film’s electrical conductivity; the conductivity increased by more than 2 orders of magnitude upon annealing.291 The authors found maximum Seebeck coefficients of 300 μV K−1 for films made of P-doped and Bdoped nanocrystals with nominal doping concentrations of 1019 cm−3; films of intrinsic Si nanocrystals exhibited no sizable thermopower (see Figure 60).440 For the best thermoelectric films, a thermoelectric figure of merit, ZT, of ∼10−3 was estimatedonly about a factor of 10 lower than that of similarly doped bulk Si. Thermoelectric films from doped Si nanocrystals can be also fabricated using field-assisted sintering at high temperatures.438,441 The more conventional sintering techniques such as compaction and hot-pressing at high temperatures

9.2. Doping of Crystalline Bulk Semiconductors

Meseth et al. proposed heavily doped Si nanocrystals as a dopant source to produce doped bulk crystalline silicon via the laser doping method.431 This process involves the laser annealing of films of doped nanocrystals on bulk Si with a pulsed excimer laser.431 The nanocrystal plasma synthesis can be scaled to industrial levels and films can be produced at low 11104


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Figure 60. Absolute thermopower of laser-sintered B-doped and Pdoped Si nanocrystal films as a function of their (nominal) doping concentration. The samples were laser-sintered at an energy density of 100 mJ/cm2. To measure the thermopower, a temperature difference of 320 K was applied under ambient conditions. The dotted line is to guide the eye. Adapted with permission from ref 440. Copyright 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

induce fast grain growth, which damages the nanogranular structure necessary for low thermal conductivity. Field-assisted sintering of electrically conducting nanocrystals can sinter nanocrystals into dense materials, while simultaneously maintaining the film’s nanostructural properties.442 Two studies reported field-effect sintering to produce thermoelectric bulk Si material from doped Si nanocrystals grown in microwave plasmas.438,441 The flow-through plasma reactors are highly suitable to supply sufficient quantities of nanocrystals that are needed to fabricate macroscopic samples and devices by field-assisted sintering. This technique has enabled the synthesis of Si pellets with densities up to 98% of bulk density using P-doped Si nanocrystals with nominal doping efficiency of 5 × 1020 cm−3.438 The resulting material also possessed larger crystallites than its source due to the variation in sintering rates of the differently sized nanocrystals.438,443 The sintering process also suffered from oxidation effects. Only a third of the P atoms were electrically active after sintering; the remaining P atoms were incorporated into oxide precipitates, as evident from EDX-coupled scanning electron microscopy (SEM) (Figure 61).444 This segregation of P atoms to oxide shells is in agreement with findings from previous studies on similar plasma-grown Si nanocrystals.271,288 Furthermore, due to the lower melting point of Si nanocrystals than bulk Si,445 the sintering activity was observed to start at about 350 K below the densification temperature of Si.438 The thin native oxide shells covering the nanocrystals appeared to facilitate the sintering.438 Seebeck coefficients and electrical conductivities of 100−150 μV K−1 and 200−900 S cm−1, respectively, were measured for the sintered pellets, which are similar to the reported values for bulk Si. The thermal conductivity was 1 order of magnitude lower (10−20 W m−1 K−1) than that of bulk Si. The lower thermal conductivity, coupled with the similar Seebeck coefficient and electrical conductivity, of the sintered P-doped Si nanocrystals yielded ZT values exceeding those of doped bulk Si (Figure 62). Schierning et al. used DC-current sintering of P-doped Si nanocrystals to obtain silicon pellets with densities 95−96%. Pdoped nanocrystals with nominal doping concentration of 5 × 1020 cm−3 were produced by microwave plasma. The authors examined the role of oxidation on the structure and charge transport properties of the resulting Si pellets.443 It was

Figure 61. SEM image of a sawed sample cross section of a fieldassisted sintered sample. The inset represents an SEM-EDX scan across the sample as indicated in the SEM image. The sulfur data represent the noise level (detection limit) of the EDX system. Adapted with permission from ref 444. Copyright 2012 Elsevier Limited.

discovered that native oxide would preferentially densify during sintering,443 in agreement with earlier studies.438 The oxidized Si nanocrystals also yielded lower thermal conductivities after sintering than the nonoxidized nanocrystals; consequently, the thermoelectric properties were improved. A best ZT of 0.5 (at 950 °C) was acquired from a sample formed by sintering 15 nm doped nanocrystals at the highest temperature studied of 1060 °C.443 At the higher temperatures, electrical conductivity also decreased (metallic-like behavior), and a carrier concentration of 2 × 1020 cm−3 was estimated. The difference between this estimated carrier concentration and the nominal doping concentration ([P]nom = 5 × 1020 cm−3) is likely attributed to the segregation of P atoms to the surface during nanocrystal synthesis.288 As the surface of these nanocrystals oxidized, the dopants were trapped in the oxide shell, and during sintering, the oxide shell of each nanocrystals aggregated in the form of oxide precipitates443 Stoib et al. used laser-sintering to produce films from doped SiGe nanocrystals and from mixtures of doped Si nanocrystals and doped Ge nanocrystals.272 Laser sintering of the nanocrystals was performed in vacuum instead of air. These lasersintered SiGe films were characterized by a network of meander-like structures that cover 60−80% of the substrate (Figure 63). The Seebeck coefficients of the films agreed well with those of films fabricated by current-assisted sintering of the same nanocrystals439 and showed a temperature dependence similar to that of a degenerately doped semiconductor. The latter films, produced via current-assisted annealing of Si0.8Ge0.2 nanocrystals with 4 × 1020 cm−3 nominal doping concentration, demonstrated ZT of 0.8 at 1000 °C.439 Possibly due to temperature-dependent activation energy, the electrical conductivity of the laser-annealed films exhibited a power law 11105


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Figure 63. (a) Top view SEM image of laser-sintered SiGe thin film showing the porosity of the meander structure typical of the samples produced by Stoib et al.272 (b) Side view SEM image of the film illustrating the partial substrate coverage. Adapted with permission from ref 272. Copyright 2012 AIP Publishing LLC. Figure 62. Comparison of the thermal conductivity and figure of merit ZT (open and solid red dots, respectively) obtained by Petermann et al. for their field-assisted nanocrystalline silicon processed from Si nanocrystals to literature data.438 Adapted with permission from ref 438. Copyright 2011 Institute of Physics.

with lower doping may yield significant improvements in device performance.444 9.4.2. Field-Effect Transistors. 9.4.2.1. Silicon Nanocrystals. Silicon nanocrystals, as candidates in thin film fieldeffect transistors (FETs), offer many benefits over the more commonly studied metal chalcogenide nanocrystals: CdX and PbX (X = S, Se, Te).281,447−452 One of the obvious advantages is the potential nontoxicity of the Si nanocrystals. Silicon, additionally, is naturally abundant, and benefits from the mature silicon-based microelectronics technology. The application of plasma-grown Si nanocrystals in FETs was reported for the first time by Dutta et al. The channel of the FET was composed of a chainlike structure of 2−3 surfaceoxidized Si nanocrystals.453 The devices, schematically shown in Figure 64, were fabricated using electron beam lithography associated with chemical etching,453 and Si nanocrystals were deposited directly onto the transistor substrate using a very high frequency plasma-enhanced chemical vapor deposition (CVD) system.454 The transistors were dominated by single electron transport. Current−voltage curves showed a nonlinear behavior with an onset of the current at 1.1−1.4 V.453 For gate voltages, Vg > 3 V, the plots of drain-source current, Ids, as a function of Vg showed Coulomb oscillations with equal peak positions for the temperature range of 20 K to room temperature. The authors explained their data based on the Coulomb charging energy and the quantized nature of the energy states in the Si nanocrystals.453 Zhou et al. also fabricated FETs with similar surface oxidized Si nanocrystals.455 The transistor characteristic curves showed an increase in Ids with increasing Vg, which indicates n-type behavior. However, the Ids showed a nonlinear behavior with the drain-source voltage, Vds, and Ids did not saturate for high Vds. This behavior was justified by internanocrystal tunneling of charge carriers through barriers represented by the oxide

dependence of σ ∝ T 1.2 .272 An effective free carrier concentration between 4 × 1019 and 9 × 1019 cm−3 was estimated for Si nanocrystals with nominal doping concentration of 5 × 1020 cm−3.272 The disparity between the effective and nominal doping concentrations is, again, likely caused by surface segregation of dopants during synthesis in the microwave plasma and doping compensation by defect traps.288,291 9.4. Electronic Devices

9.4.1. Diodes. P−n junctions have been successfully fabricated from plasma-grown Si nanocrystals. Becker et al. sintered films of microwave plasma-produced p- and n-type Si nanocrystals with doping concentrations of 5 × 1020 cm−3 and 2 × 1020 cm−3, respectively.446 They used field-assisted sintering which makes use of pressure and of Joule heating to sinter the nanocrystals.441,446 Using a Seebeck microscan technique, the authors observed a spatial spreading of the pand n-type dopants in the sintered p−n junction. They interpreted this observation as a mixing of p- and n-type nanocrystals during the sintering process.446 The consequent compensation of doping within the intermixing region reduced the charge carrier concentration at the interface. The diodes exhibited rectifying behavior and an on/off ratio of 3.5 measured at ±5 V.444 Leakage currents, possibly from defectassisted tunneling, degraded the performance of the devices. As the use of highly p- and n-type doped Si nanocrystals should yield a very narrow space-charge region, the use of nanocrystals 11106


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Figure 65. Drain-source current Id vs drain-source voltage Vd recorded with different gate voltages Vg for FETs made of RF plasma-grown Si nanocrystals. Adapted with permission from ref 459. Copyright 2010 American Chemical Society.

Figure 64. (a) Schematic diagram of the devices fabricated by Dutta et al.453 The electron transport channel contained only (b) three or (c) two Si nanocrystals. Adapted with permission from ref 453. Copyright 2000 The Japan Society of Applied Physics.

with the source and drain electrodes deposited directly on the silicon nitride gate dielectric that covered the highly p-doped silicon gate contact.461 The electrodes consisted of tens of interdigitated Au structures, and the Si nanocrystal films were spin-cast onto the substrate. The devices exhibited n-type behavior with linear Ids vs Vds at low Vds, and Ids saturation at high Vds. It was also shown that the conductivity of solutionprocessed nanocrystal FETs can be strongly enhanced via adsorption of specific molecules onto the surfaces of adjacent nanocrystals.461 The effect was demonstrated with a concentration study of tetrafluorotetracyanoquinodimethane (F4TCNQ).461 F4-TCNQ was added at varying concentrations to the nanocrystal solution used to fabricate the FETs. The electrical conductivity of the FETs increased by up to 2 orders of magnitude upon doping the Si nanocrystal films with F4TCNQ (Figure 66).461 These FET devices exhibited charge mobilities of up to 4 × 10−5 cm2 V−1 s−1, which is slightly higher than values previously reported by Holman et al.459 The authors performed density functional calculations to study the electrical activation and electronics of the F4-TCNQdoped nanocrystal films, using nanocrystal superlattice models.461 It was found that in the doped systems, hybrid

shells.455 At room temperature, a field-effect charge mobility of ∼2 × 10−4 cm2 V−1 s−1 was obtained from the transfer curves. The threshold voltage of the transconductance data increased with decreasing temperature, indicating that charge carriers were trapped in localized states.455 At lower temperatures, more carriers were trapped; therefore, a higher Vg was needed to balance the free charges in the FET channel. Annealing the nanocrystal films in H2 (450 °C, 1 h) led to a decrease in trap density, which suggests that these traps were associated with silicon dangling bonds (Si-dbs) that were passivated during the H2 annealing.456 The role of Si-db defects as trap states in Si nanocrystals was revealed in vacuum annealing experiments carried out on oxidized Si nanocrystals.457 A 10−40 fold increase in conductance was observed after vacuum annealing at 200 °C for 30 min. This increase was found via EPR to be an effect of a decrease in Si-db densities.457,458 Holman et al. reported the fabrication of the first FET devices made of plasma-grown Si nanocrystals with Ids vs Vds characteristics typical of thin-film FETs.459 The devices were fabricated with H-terminated Si nanocrystals, as opposed to devices reported earlier that were of surface-oxidized Si nanocrystals.453,455,460 The Si nanocrystals used by Holman et al. were 10−20 nm and were synthesized in a RF plasma reactor.459 The FETs displayed n-type behavior, with electron mobilities estimated between 10−5 and 10−6 cm2 V−1 s−1. The transistor characteristics displayed a linear behavior for low Vds and saturated at higher Vds, Figure 65. However, the devices exhibited significant hysteresis, most probably arising from the trapping of free charges in defects. Contrary to previous experiments, where up to ∼400 fold increase in conductance and photoconductance was found upon annealing at 200 °C in vacuum,457 Holman et al. did not observe major changes in performance when their FETs were annealed at temperatures below 300 °C in nitrogen.459 This difference is likely due to the different atmospheres in which the films in the two studies were annealed, which was found to have a strong influence.457 Pereira et al. investigated nanocrystal FETs made of Hterminated Si nanocrystals synthesized in a microwave plasma.461 The FETs were fabricated in the bottom-gate configuration, unlike the top-gate devices of Holman et al.,459

Figure 66. Evolution of the ratio between the current of doped films Idoped and the current of undoped films Iundoped (measured at 30 V) as a function of F4-TCNQ doping concentration for films of H-terminated Si nanocrystals with difference mean diameters: (i) 4.2, (ii) 15.7, and (iii) 17 nm. Adapted with permission from ref 461. Copyright 2014 American Chemical Society. 11107


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Interestingly, both P- and B-doped nanocrystal-containing FETs exhibited electron conduction. For high doping levels (nominal doping concentrations ∼5 × 1021 cm−3), no gating behavior was observed, consistent with semiconductor films in the degenerate doping regime. The highest nominal doping concentrations also resulted in reduced electron mobility; the highest mobility of 2 × 10−4 cm2 V−1 s−1 was found for Pdoped Si nanocrystals with the smallest nominal doping concentration (1 × 1020 cm−3).462 From transfer curves (Ids vs Vg), the authors found that a more positive (negative) threshold voltage VT resulted from a higher [B]nom ([P]nom). They used the gradual channel approximation for thin film FETs to analyze the expected dependence of VT on the doping concentration.222 These analyses showed that only a small fraction of the dopants, between 10−2 and 10−4 of the nominal B or P concentration, was electronically active.462 These efficiencies are similar to those obtained via EPR and SIMS of P-doped Si nanocrystals grown from microwave-induced decomposition of SiH4.288 The authors also compared the behavior of FETs containing small and large nanocrystals.462 For large nanocrystals, the nanocrystals gradually transitioned from semiconducting to metallic with increased doping; however, a sudden transition was observed with the small nanocrystals. As small nanocrystals may be statistically either intrinsic or doped, the films may be made of a mixture of conducting (doped) and insulating (intrinsic) nanocrystals, which would cause a sharp conductivity transition near the percolation threshold. In a separate study, FETs made of intrinsic, chlorine-terminated Si nanocrystals grown in a RF plasma165 also displayed no gating.462 This behavior is again indicative of degenerate doping of the Cl-terminated Si nanocrystals. This is consistent with the view that solvent molecules such as benzonitrile, which engage in Lewis acid− base interactions with the surface Si−Cl groups, can act as dopants, donating electrons to the nanocrystals.200 9.4.2.2. Germanium Nanocrystals. Holman et al. used Cland H-terminated Ge nanocrystals (Cl/H-terminated Ge nanocrystals), synthesized in a nonthermal plasma through decomposition of germanium tetrachloride (GeCl4) and H2, to fabricate thin-film nanocrystal FETs.459 The devices had a topgate configuration, with Ge nanocrystal films deposited by spincoating 1,2-dichlorobenzene solutions of as-grown nanocrystals onto highly doped silicon substrates covered with a SiO2 dielectric. The Ge nanocrystals formed stable, well-dispersed colloids, which resulted in films with low surface roughness (∼10 nm).459 While no gating behavior was observed for FETs made of as-grown Ge nanocrystals, devices annealed at 200− 400 °C in N2 or under vacuum displayed typical behaviors of thin-film FETs. n-type conduction was observed, and electron field-effect mobilities of 10−4 − 10−2 cm2 V−1 s−1 were measured. Upon annealing at 500 °C, the FETs became ambipolar, and p-type behavior was observed at 600−700 °C. Hole mobilities close to 10−2 cm2 V−1 s−1 were measured. The origin of the transition from n-type to ambipolar to p-type is unclear. H and Cl bonded to the surface desorb upon annealing, which leads to the formation of surface Ge dangling bonds; thus, the p-type behavior could originate from an effective p-type doping of the nanocrystals from the dangling bonds. Surface dangling bonds in Ge are expected to induce acceptor states that are close to the valence band (VB) edge (shallow acceptors).463−465

molecule/nanocrystal electronic states that are located close to the conduction band (CB) edge of the Si nanocrystals are formed (Figure 67). These states are electron acceptors and are

Figure 67. (a) Calculated (-/0) and (=/-) acceptor states (black horizontal lines) of F4-TCNQ molecules in different configurations (“Center”, “Face”, “Diag”, “Corner”, and “Bridge”) inside a Si nanocrystal superlattice. Arrows are labeled with energy differences with respect to the ionization potential IU and electron affinity AU energies of the undoped superlattice. Superlattice band energies are represented as blue (bottom) and red (top) horizontal lines. (b) Electron density isosurface (red) of the highest occupied molecular orbital of doubly negative F4-TCNQ molecule in the “Corner” configuration. Circles enclosing individual nanocrystals are for eye guidance purposes only. Adapted with permission from ref 461. Copyright 2014 American Chemical Society.

expected to provide enhanced electronic connectivity across the nanocrystal network as they reduce the energy barriers for internanocrystal charge transfer by inducing electronic states in the otherwise vacuum-filled interstitialcies on the films. Gresback et al. studied thin films of P- and B-doped Si nanocrystals as active layers in nanocrystal FETs. They used two sizes P- and B-doped Si nanocrystals: 8−15 nm (large nanocrystals) and 4−7 nm nanocrystals (small nanocrystals).462 The nanocrystal surfaces were H-terminated. The authors used spin coating to form nanocrystal films on highly doped silicon substrates with a silicon oxide layer as gate dielectric.462 Aluminum source and drain contacts were deposited on the top surface of the nanocrystal films. For nanocrystals with nominal doping of 5 × 1020 cm−3 and undoped nanocrystals, the Ids vs Vds curves exhibited typical thin-film FET behavior, with a linear regime at low Vds and a saturation region at higher Vds. 11108


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the efficient quantum dot photoluminescence.473 By utilizing wide band gap materials, poly(N,N0-bis(4-butylphenyl)-N, N0bis(phenyl)benzidine (poly-TPD) and tris(8-hydroxyquinolinato) aluminum (Alq3), as the hole and electron transport layers respectively (Figure 68a), Cheng et al. were able to

9.5. Light-Emitting Devices

The unique optical properties of semiconductor quantum dots have fueled widespread interest in their applications in optoelectronics.373 Their size-dependent bandgap allows for precise tuning of luminescent colors, and their spectrally narrow emission (compared to inorganic phosphors) establishes quantum dots as superior materials for expressing vivid, saturated colors. Furthermore, their colloidal stability is a benefit in producing low-cost displays. The solubility of ligandcapped nanocrystals enables the use of roll-to-roll processing techniques such as spin-coating, spray coating, and inkjet printing. Quantum dot-based LEDs (QLEDs) have improved in external quantum efficiencies from 0.01% to 20% in the last 20 years.6,466 Colvin et al., in 1994, pioneered the first hybrid organic/inorganic QLED, which utilized CdSe nanocrystals as the active layer.467 The simple device, consisting of a bilayer of semiconducting polymer and CdSe nanocrystals sandwiched between two electrodes (ITO and Mg), achieved external quantum efficiencies (EQEs) < 0.01% at brightness of 100 cd m−2. Improvements up to 0.22% at 600 cd m−2 was made by replacing the active layer with CdSe/CdS core/shell nanocrystals with enhanced PLQYs.468 Coe et al. significantly improved EQEs of QLEDs by decoupling the dual functions of the quantum dots, which performed as electron transport and emissive layers in the aforementioned LEDs. By employing a monolayer of CdSe/ ZnS nanocrystals at the interface of a bilayer of organic polymers, the authors successfully isolated the luminescence of the quantum dot layer from charge conduction, which led to EQEs exceeding 0.5%.469 The QLED efficiency was further enhanced (∼2%) with advanced phase-separation techniques for high-quality monolayer formation. A potential breakthrough in QLEDs was made by Mashford et al. EQEs as high as 18% were demonstrated by an inverted inorganic/organic hybrid LED with colloidal CdSe/CdS core/ shell quantum dots.470 This remarkable EQE was made possible through improved coupling between the quantum dots and charge transport layers that enabled efficient electron transfer and improved charge balance in the LED. Dai et al.6 further improved upon this record in 2014 with QLEDs with external quantum efficiencies up to 20.5% and with device lifetimes of 100 000 h at 100 cd m−2. This performance rivals that of vaporphase deposited OLED devices. The QLEDS discussed above all rely on heavy metalcontaining quantum that pose environmental and health concerns. The potential application of nontoxic Si quantum dots in LEDs emerged in the early 1990s when roomtemperature light emission was observed from porous silicon.471 A first QLED based on plasma-produced surface oxidized Si quantum dots was reported by Ligman et al.472 However, charge transport in these devices was poor and the external quantum efficiency was low. Since then, the nonthermal plasma technique has enabled the synthesis of highly luminescent Si quantum dots with PLQYs comparable to that of II−VI nanomaterials; Jurbergs et al. reported QYs exceeding 60% from Si nanocrystals produced in a low-pressure, nonthermal plasma.201 The first reported Si quantum dot LED was fabricated with a layer of nanocrystals sandwiched between a bilayer of polymers for hole injection and transport on one side and an electron transport layer on the other. Due to inefficient confinement of charge carriers in the quantum dot layer, the device only demonstrated EQEs around 0.6% despite

Figure 68. Si quantum dot inorganic−organic LED produced by Cheng et al. (a) Proposed band structure of the LEDs. (b) Photograph of LEDs with near-infrared (5 nm) and red-emitting (3 nm) Si nanocrystals. (c) External quantum efficiencies of the respective devices. Reprinted with permission from ref 473. Copyright 2010 American Chemical Society.

optimize charge carrier injection and confinement, and EQEs as high as 8.6% were attained (Figure 68c).473 Conceivably, the exploitation of even more effective charge transport layers, following the technique devised by Dai et al., can lead to higher EQEs. The nonthermal plasma technique also enables the fabrication of an entire LED in the gas phase to minimize solvent wastes. Anthony et al. synthesized, functionalized, and deposited Si quantum dots onto substrates in the gas phase (Figure 69).390 Vaporized 1-dodecene was injected into the expansion region of the nanocrystal synthesis plasma, where radicals of the organic molecules react with the nanocrystal surface. The functionalized nanocrystals were then accelerated through an orifice and impacted as thin (∼80 nm) and dense (∼40%) films onto ITO-coated substrates and coated with lithium fluoride (LiF) and aluminum (Al) cathode. The simple device, despite lacking in charge transport layers and comprised of quantum dots with low PLQY (

Nonthermal Plasma Synthesis of Nanocrystals: Fundamental

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