Scanning Probe Characterization of Heterostructured Colloidal

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Scanning Probe Characterization of Heterostructured Colloidal Nanomaterials Sanjini U. Nanayakkara,* Jao van de Lagemaat, and Joseph M. Luther* National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States thin-film heterojunctions has resulted in the development of many advanced devices. Nanoscale and quantum-confined materials offer new physics for optical and optoelectronic devices and have inspired advanced synthetic methods to create multicomponent nanostructures containing material junctions by selectively arranging individual domains for complex functionalities. Some examples of heterostructuring in nanomaterials include photoluminescence manipulation in core−shell quantum dots (QDs),22−25 slowed cooling,26 reduced blinking,22,25,27 materiCONTENTS als with plasmon-assisted absorption enhancement28 and 1. Introduction A doping,29 charge-separating interfaces,30−32 energy funnel1.1. Scanning Tunneling Microscopy and Specing,33,34 and strain35 effects. Such nanoheterostructures offer troscopy B promising new materials for modernizing industries such as 1.2. Atomic Force Microscopy D biological sensing,36,37 photovoltaics,38−42 and photocataly2. Single-Component Colloidal Nanostructures E sis.30,43−45 2.1. Energy Levels and Density of States F Progress in the synthesis of nanocrystals and nanoheteros2.2. “Blinking” in Tunneling Current I tructures has led to the manipulation of particle size, shape, 2.3. Charge, Dipole Strength, and Polarizability I morphology, and surface chemistry.46−49 However, with such 2.4. Energy Levels in Doped Single Nanocrystals I small structures and domains, the ability to accurately 3. Heterostructured Colloidal Nanostructures J characterize many aspects of the individual components and 3.1. Charge Transport K their interfaces is greatly reduced compared to macroscopic 3.2. Energy Level or Band Alignment L bulk and thin film materials. Since the objects are so small (far 3.3. Wave Function Imaging N below diffraction limits), weak signals arise that are often at or 3.4. Energy Levels and Charge Separation O below the noise level of most instrumentation. Therefore, 3.5. Built-in Potential P averaging large numbers of nanocrystals becomes the logical 4. Outlook and Future Prospects Q way to achieve manageable signal-to-noise ratios for many Author Information R techniques. While such “ensemble”-type measurements provide Corresponding Authors R useful physical/chemical information about a collection of Notes R nanostructures, characterizing the difference between inhomoBiographies R geneous and homogeneous effects (such as size or shape Acknowledgments R dispersion vs intrinsic homogeneous broadened line widths) is Abbreviations R critical for developing optimal structures. Interfaces govern References S electronic properties of the overall heterostructure and affect operation; therefore, the ability to understand individual nanoheterostructures will greatly assist the design of novel complex materials. One unique instrument that is capable of measuring 1. INTRODUCTION nanoscale surface properties with high spatial resolution is the Material junctions form the basis of modern solid-state scanning probe microscope (SPM).50−53 Since the advent of 1−5 and are key technology by controlling current flow scanning tunneling microscopy (STM)54 and atomic force components in digital electronics, electronic switches, signal microscopy (AFM),55 our understanding of surface science and amplification and processing,6−9 sensing, light-emitting dinanotechnology has improved dramatically. These techniques odes,10−12 lasers,13,14 and photovoltaics.15−19 A heterojunction have helped researchers understand surface structure,56−59 arises when dissimilar semiconductors come into contact and adsorbed molecules,60−62 tethered metal clusters,63−68 semiform an abrupt interface. The junction physics are controlled by conductor nanocrystals (NCs),69−75 and biological molethe band gap, electron affinity, and chemical potential or Fermi cules76−79 with nanometer-scale resolution. Both techniques level of each material as well as how the energy levels or bands align across the material interface (which can be influenced by dipoles or interfacial defects/alloying).20,21 Over the years, Received: May 27, 2014 engineering the material interface and film characteristics of © XXXX American Chemical Society

A

DOI: 10.1021/cr500280t Chem. Rev. XXXX, XXX, XXX−XXX

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protruding valence states of the tip material also matters.84−87 Transition metals with protruding d valence states that are spatially sharper are better than alkali metals with s states, which are broader due to their spherical geometry.84,85 When an electrical bias is applied between the tip and the surface, electrons tunnel through the tip−sample gap and the STM electronics monitor this current in a feedback loop to maintain tip position over the sample surface. The polarity of the applied bias will determine the direction of the tunneling current. When the tip is negatively biased with respect to the surface (Figure 1B), tunneling electrons flow from the filled states of the tip to the empty states of the surface, and vice versa. An STM can be operated in either constant-height mode or constant-current mode.53,54,88 In constant-height mode, the net tip to substrate distance is not adjusted during scanning and changes in current are recorded, rendering a current map of the surface. Constant-height mode has been used to study real-time surface diffusion of atoms89 and in lateral manipulation90 of surface-bound molecules. The standard mode is constantcurrent mode, where the tip−sample distance is under feedback control in order to maintain a constant electrical current. When atomic-scale or nanoscale height variations on the surface cause changes to the measured tunneling current, the feedback loop compensates by changing the tip−sample distance and thus generating three-dimensional topographic images. The basic quantum tunneling equation that shows the relationship between tunneling current and tip−sample distance is shown in eq 1:91

have the same rudimentary design consisting of a tip (probe) attached to a piezoelectric tube or motor that permits precise control over the tip position, raster scanning across and interacting with a surface. However, the type of interaction between the tip and the surface that is monitored and used as feedback differentiates these two techniques. In this review, we will describe the two scanning probe techniques briefly and discuss experimental methods that helped elucidate electrical characterization of single-component and heterostructured nanomaterials. Typically, for scanning probe experiments, NCs are assembled onto conducting substrates by solutionphase casting techniques such as spin-, drop-, or dip-coating (outlined in section 2). Additionally, NCs could also be grown directly on a sample surface or tethered by use of bifunctionalized molecules. The density of particles on the surface allows one to study either individual single-particle physics or meso/macroscopic effects of electronic coupling80−83 when the nanomaterials are densely packed. Both SPM techniques will be crucial for advancing the development of heterostructured nanomaterials toward new technologies. We will review various effects arising from quantum confinement and nanoscopic interfaces that have been studied by both STM and AFM. 1.1. Scanning Tunneling Microscopy and Spectroscopy

Scanning tunneling microscopy and spectroscopy is a surface analytical technique that probes the local density of states (LDOS) with high spatial and energetic resolution.54 In STM, a sharp metallic tip (mechanically cut or chemically etched) is brought into close proximity (∼3−10 Å) to a conducting or semiconducting surface (Figure 1A). The most commonly used tip materials are platinum−iridium and tungsten, due to their chemical stability and mechanical strength. The shape of the

⎡ 2mϕ Ι ∝ exp⎢ −2 ⎢⎣ ℏ

⎤ d⎥ ⎥⎦

(1)

The current (I) depends exponentially on the tip−sample distance (d) as electrons tunnel through the tunneling barrier and m, ϕ, and ℏ are electron mass, energy barrier, and Planck’s constant, respectively.92−94 Based on eq 1, the current (I) changes by an order of magnitude for every 1 Å change in d, making atomic and molecular resolution feasible. It has been noted that the sensitivity of the distance-dependent tunneling magnitude allows tip−sample distance accuracy to within 1 pm.95 However, imaging with atomic resolution depends on the substrate and the sample preparation and experimental conditions as well as the quality (shape) and material of the tip. Often, in order to achieve atomic resolution on crystalline surfaces, the top atomic layer of the surface must be pristine and measured under ultrahigh vacuum (UHV) conditions. This minimizes mobile ambient contaminants such as moisture and oxygen, which affect the measurements by contamination and oxidation. In order to interpret STM topographic images, an understanding of how the tunneling current is influenced by the local density of states (LDOS) of the tip as well as that of the sample is needed. In 1983, Tersoff and Hamann94,96 developed a theory for the tunneling current, in which the tip wave function was simplified and assumed to be a spherically symmetric s-type wave function. This was further advanced by Wagner and others97,98 by using the Wentzel, Kramer, and Brillouin (WKB) approach. This approach leads to a general expression that is an integration over the local density of states in the tip, ρt, and the sample, ρs, and a transmission probability function, T, here given for a trapezoidal barrier:97−100

Figure 1. Basic operation of a scanning tunneling microscope. (A) Illustration of the STM tip and substrate separated by distance d. (B) Energy-level diagram of a one-dimensional tunnel junction. The Fermi levels of the tip and conducting substrate are offset by an applied bias, qVbias. In this schematic, the STM tip is negatively biased with respect to the substrate. B


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Figure 2. Fundamentals of scanning tunneling spectroscopy in a double-barrier tunnel junction (DBTJ) geometry. (A) Schematic of DBTJ formed when an STM tip addresses a single NC tethered to a conducting substrate by linker molecules. (B) This junction is represented by two RC circuits in series, where the tip, NC, and substrate are assumed to be conductive. (C) Energy-level diagram at zero applied bias, when the Fermi levels of both the tip and the substrate are aligned. (D, E) Schematic representation of energy-level shifts when a positive bias is applied to the surface, with respect to the tip, for (D) shell-tunneling and (E) shell-filling conditions. For illustrative purposes, the Fermi level of the tip is aligned with 1Se state of the NC, thus opening up a conduction pathway from the tip to the substrate. Electrons from the tip will tunnel to the substrate through the 1Se state, giving rise to a step in the I/V curve. (F) (Top) Scanning tunneling spectroscopy (I/V) over a single CdSe NC. (Inset) Topographic STM image and line profile illustrating the measured apparent height. (Bottom) Corresponding dI/dV curve. At positive (negative) sample bias, peaks associated with electron (hole) states of the CdSe NC are labeled. Panel F reprinted with permission from ref 118. Copyright 2006 American Physical Society.

I (d , V ) ≅

Aπeℏ3 2m2

∫0

cules,105,115,116 metal nanoparticles,63,65,67,68,117 and semiconducting nanocrystals.73,74,118,119 Inelastic tunneling spectroscopy (also known as STM-IETS) can reveal vibrational modes of adsorbed molecules as a result of energy transfer between electronic and vibration modes120 and is observed in the second derivative of the tunneling current (d2I/dV2).106−108 However, a recent study by Sun et al.121 on single CdSe NCs resolved vibronic modes by use of differential conductance. To perform STS, nanomaterials are generally immobilized on conducting substrates. When the STM tip is positioned over a single semiconducting NC, the STM tip−NC−substrate forms a double-barrier tunnel junction (DBTJ) as shown in Figure 2A,B. In the DBTJ, the tunneling barriers are between (1) the tip and the NC and (2) the NC and the substrate, and each junction can be modeled by an equivalent resistor−capacitor (RC) circuit diagram as shown in Figure 2B. Often the NCs are tethered to the substrate by use of a bifunctionalized selfassembled molecular monolayer (SAM), where the molecular length mainly determines the tunneling rate through the second barrier (Figure 2A). Schematic band diagrams, showing the mechanism by which electron and hole states in single NCs are accessed, are shown in Figure 2C,D. As shown in Figure 2C, when 0 V is applied across the DBTJ, the tip and substrate Fermi levels align. For the sake of simplicity, the schematic shows the aligned Fermi levels centered with respect to the band gap of the NC. This may not always be the case, and this will be discussed further in section 2.4 of this review. When a bias is applied to the STM

eV

T (d , V , E)ρs (E)ρt (E − eV ) dE (2)

The topographic images recorded in STM are therefore a convolution of surface geometric structure and local LDOS of the tip and the sample surface.94,96,101 The surface LDOS information can be separated from topography; this is achieved by either imaging in spectroscopic mode or recording spectroscopy point by point. Spatially resolved scanning tunneling spectroscopy (STS)102−105 can measure electronicstate positions of surfaces or surface-bound features such as molecular adsorbates or nanostructures, and in some instances it can resolve vibration modes as well.106−108 High-resolution STS is best performed under stable UHV and low-temperature (77 or 4 K) conditions to minimize the thermal spread of energy levels as well as thermal drift of components that hold the tip during measurements. Scanning tunneling spectroscopy is performed by suspending the tip over a surface feature at a specific height (momentarily disabling the feedback) and measuring the tunneling current (I) as a function of bias (V) applied between the tip and the substrate.109,110 Along with tunneling current (I/V),105,111,112 the first and second derivative of this signal can be recorded directly by use of a lock-in amplifier.113 Differential conductance (dI/dV), the first derivative of the tunneling current, resolves the electronic energy-level positions and LDOS of the probed surface species.114 Examples include surface adatoms and moleC


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Figure 3. Fundamentals of scanning Kelvin probe microscopy (SKPM) operation. (A) Basic instrumentation schematic for AFM-based SKPM. (B− D) Schematics of energy-band diagrams of a conducting AFM tip and sample. (B) The AFM tip and sample are made of two different materials and thus have different work functions. (C) When in contact, charges flow from the low work function material to the other until their Fermi levels are in equilibrium, resulting in a contact potential difference (CPD). This CPD results in an electrostatic force being exerted on the AFM tip. (D) The SKPM feedback circuit works to nullify these electrostatic forces by adding a dc bias to the AFM tip. (E) Measuring built-in potential across single heterostructured nanorods by use of SKPM. Panel E reprinted with permission from ref 31. Copyright 2013 American Chemical Society.

to measurements of electronic states and density of states.71,74,126 The electrons and holes are able to tunnel from tip to substrate, through the states (or shells) within the NC, and this is termed shell-tunneling.74 In contrast, by adjusting the rates such that Γ1/Γ2 ≫ 1, multiple electrons (or holes) can be on the NC at any time and Coulomb interactions between charges can be measured. Adjusting the width of each barrier by tuning either the tunneling current set point or the length of the tethering molecule, affords control over the tunneling rates, and shell-filling conditions can be reached.69,74,118 Under these conditions, charges accumulate in the NC states (or shells) leading to electron−electron and electron−hole interactions. A discussion of the crossover point between these two tunneling conditions and its limitations has been previously shown theoretically and experimentally.71,127

tip, the Fermi level shifts from that of the substrate (schematic in Figure 2D shows the Fermi level shift when a negative bias is applied to the tip). When in energetic resonance with an electronic state in the NC, electrons tunnel from the tip through that energy state to the substrate. The schematic in Figure 2D shows the tip Fermi level aligned with the 1Se state of a NC. The spectrum of energy-level positions is measured as the bias voltage is swept while current is recorded. Jdira et al.118 used low-temperature STM/STS to measure energy levels in CdSe NCs. STS spectra, both I/V and dI/dV, over a single CdSe NC tethered to a Au(111) substrate via hexanedithiol molecules, are shown in Figure 2F. In the I/V spectrum (top), discrete current steps are observed as the STM tip accesses the various charge states within the NC. These current steps translate into peaks in the dI/dV spectrum (bottom) and thus provide information about the energy-level positions of the electron and hole states. The relative intensities of the assigned s, p, d, and f electron states in the dI/dV spectrum are indicative of their relative density of states (DOS) in this single, 4 nm CdSe NC. This technique has revealed electronic states of single PbSe, 75 CdSe,118 and doped InAs NCs 122 and heterostructured nanorods123−125 (NRs), as will be discussed. Interpretation of the peaks in the dI/dV spectra depends on the relative tunneling rate between tip and NC (Γ1) and that between NC and substrate (Γ2), leading to two succinct measurement conditions defined by the ratio of tunneling rates Γ1/Γ2: shell-tunneling (Figure 2D) and shell-filling (Figure 2E). These conditions were first discussed by Bakkers et al. in 2001,69 and tuning Γ1/Γ2 allows one to study energy-level positions or splitting due to charging interactions.69,74,118 When Γ1/Γ2 ≪ 1, electrons tunnel out of the NC to the substrate at a faster rate than electrons tunneling in to the NC from the tip, preventing charge accumulation on the NC and hence leading

1.2. Atomic Force Microscopy

While STM relies on monitoring electrical current from a tip, AFM and its related techniques can provide complementary information by use of a vastly different tip−sample interaction scheme. The key features and components of AFM and STM are similar, except for the probe tip. In AFM,55 a force-sensing cantilever replaces the sharp metallic tip that detects current in STM. A tip with a diameter ranging from a few to tens of nanometers is attached at the end of the cantilever (Figure 3A). During scanning, the flexible cantilever contorts and deflects as a result of tip−sample interactions (such as van der Waals, electrostatic, magnetic, etc.), and its deflection is commonly measured by optical techniques. Generally, a laser impinges on the backside of the cantilever as shown in Figure 3A and reflects onto a four-quadrant photodiode detector to measure the deflection and movement of the cantilever as it interacts with the sample. The AFM can be operated in static or dynamic D


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In SKPM, the feedback loop works to nullify these electrostatic forces by the addition of a dc bias (Figure 3D) to the probe tip, thus generating a two-dimensional surface map that represents CPD between the tip and the surface. Generally, this is experimentally achieved by the addition of an ac bias modulation to the AFM tip to modulate the electrostatic forces (Figure 3A).128,139,140 By modeling the gap between the AFM probe tip and surface as a capacitor, the total electrostatic force (Ftotal) exerted on the cantilever can be derived and is shown in eqs 3a−3d:141,142

mode. In static mode, the AFM tip is in contact with the sample surface and the force between the tip and the surface is kept constant during scanning. In dynamic mode, the AFM tip is oscillated at or near its resonance frequency and can be operated in noncontact or intermittent-contact mode where the sinusoidal amplitude of the cantilever is fixed to a desired range of motion (typically about 100 nm of motion). During scanning, when the probe tip encounters varying surface morphologies, the resonance frequency, oscillation amplitude, or oscillation phase is affected. The AFM feedback monitors these changes with respect to a reference signal and provides information about the sample surface. During dynamic-mode operation, by varying the tip−sample distance, the feedback can maintain either a constant amplitude (amplitude-modulated AFM) or a constant frequency shift (frequency-modulated AFM) of the cantilever resonance frequency. Changes in the phase of the sinusoidal oscillation of the cantilever occur when the tip scans over regions of the surface with different material properties (adhesion, friction, etc.). This change in phase can also be monitored simultaneously with topography and is often used to gain a more comprehensive understanding of the surface and to distinguish different types of materials. Multiple surface properties can simultaneously affect the cantilever deflection and, in order to isolate and quantify each property, advanced experimental techniques use lock-in amplifiers to modulate and detect specific signals.50,128 The use of AFM-based techniques that study surface morphology and other physical phenomena span a wide range of materials ranging from biological systems to atomically pristine surfaces and are discussed in the noted references.79,129−133 Electric force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM, also called Kelvin probe force microscopy and scanning surface potential microscopy) constitute the two main advanced AFM techniques that have been used to probe nanoscale electrical properties.31,134−137 To measure surface electrical properties, the AFM probe tips are coated with a conducting material, typically Pt/Ir. Both of these techniques are based on the detection of electrostatic forces governed by Coulomb’s law between the probe tip and the sample. The schematic diagrams in Figure 3B−D illustrate the definition and formation of the contact potential difference (CPD). However, in reality, as the tip and the sample are connected through an external circuit, the energy-level diagram shown in Figure 3C is always realized. Charges flow between the tip and the sample until the two Fermi levels are in equilibrium, resulting in a CPD (Figure 3B−D). This results in an electrostatic force between tip and sample and the oscillation frequency/amplitude or phase change of the cantilever is detected and used as feedback to generate the potential image. In EFM, changes in the phase of oscillation frequency are used to measure the electrostatic force gradient (i.e., change in electrostatic force over distance) and thus can yield a higher spatial resolution compared to SKPM. However, unlike SKPM, EFM does not generate a direct measurement of the surface potential and therefore requires calibration of the phase signal.136,138 For example, Lei et al.138 calibrated the EFM phase signal to surfaces with known potentials and demonstrated its effectiveness by studying potential distributions across a metal−polymer−metal structure at various applied biases. Phase changes in EFM have also been used to generate built-in potential values across single heterostructured NRs and will be discussed further in section 3.5.

Ftotal = Fdc + Fω1 + Fω2

(3a)

Fdc =

⎤ 1 ∂C ⎡ 1 (Vtip − Vsurface)2 + Vac 2 ⎥ ⎢ ⎦ ⎣ 2 ∂z 2

(3b)

Fω1 =

∂C (Vtip − Vsurface)Vac sin(ωt ) ∂z

(3c)

Fω2 = −

1 ∂C 2 Vac cos(2ωt ) 4 ∂z

(3d)

These equations include forces when a sinusoidal ac bias of frequency ω is added to the cantilever. C, Vtip, Vsurface, and Vac are the capacitance between tip and sample, electrochemical potential of tip, electrochemical potential of surface, and amplitude of applied ac bias, respectively.142,143 Within the total force, Fdc is constant with time and consists of the dc components of all the forces acting on the AFM tip (van der Waals, electrostatic, etc.). Force Fω1 oscillates at the same frequency as the ac bias modulation and is proportional to Vtip − Vsurface, which allows for direct calculation of surface potential. Force Fω2 oscillates at the second harmonic frequency of the ac modulation and represents the capacitive portion of the total force. By monitoring Fω1, a feedback loop nullifies the electrostatic forces on the tip by dynamically adjusting Vtip to keep Vtip − Vsurface = 0. Thus, a direct measurement of the surface potential can be recorded simultaneously with surface topography. This method has been used to directly measure built-in potential along cross sections142 of solar cells and also individual heterostructured NRs (Figure 3E).31 Advanced AFM techniques have probed surface potential, charging, and light-induced electrical properties in nanoscale materials; however, the presence of the feedback laser can often induce unwanted optical excitation.142 While some systems have options for feedback lasers of various wavelengths (and position-sensitive photodiodes to match), optical excitation cannot be avoided for materials with broadband absorption. An AFM technique that avoids the use of cantilevers, and therefore the laser- and photodiode-based detection method altogether, is tuning fork AFM. In tuning fork AFM, the mechanical vibration of a quartz tuning fork is directly converted to an electrical signal by the piezoelectric effect of the quartz material.144 Probe tips are adhered to the quartz fork with well-characterized vibrational resonances for imaging, and the high stiffness of the tuning fork can achieve high-resolution imaging of surfaces under true dark conditions.145

2. SINGLE-COMPONENT COLLOIDAL NANOSTRUCTURES Quantum confinement leads to discrete molecular-like energy states in semiconductor NCs.114,146,147 Size-controlled tunability of these electronic states and the optical properties have E


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Figure 4. Sample preparation methodologies for STM/AFM studies. (A) Isolation and tethering of single nanocrystals by use of a hexanedithiol selfassembled monolayer on Au(111). (B) Isolation of individual nanorods by spin-coating a dilute solution of the nanomaterial on a highly oriented pyrolitic graphite (HOPG) surface. (C) The simplest way to make an array is by drop-casting a small volume of a very dilute solution on a solid substrate (e.g., HOPG) and allowing the solvent to dry. (D) Close-packed array of nanocrystals on a HOPG surface, formed by slow annealing subsequent to drop casting. (E) Coupled array of nanocrystals formed by chemical treatment of a drop-cast array.

Within these arrays, the native ligands on the NCs are unperturbed. Subsequently, the inter-NC interactions within this array can be varied by postdeposition techniques such as annealing or chemical treatments (Figure 4D,E). Annealing can lead to a reduction in the spacing between the NCs, leading to the formation of closed-packed arrays (Figure 4D). A chemical treatment can result in the replacement of the native ligand shell, thus forming a coupled-array of NCs (Figure 4E). In both these instances, the individual NCs within the array retain their quantum confinement properties.

made single-component, semiconductor NCs a platform for a variety of optoelectronic devices. A comprehensive understanding of the relationship between electronic states and optical characteristics is essential for the advancement of these new technologies. While optical spectroscopies provide important feedback for the design of new materials, these techniques are limited to the observation of allowed transitions between the valence and conduction energy levels in the NCs. In contrast, in STS, the valence and conduction energy levels are individually probed and the measurement technique is not hindered by selection rules. Therefore, STM/STS can be utilized to gain insight into optical transitions.73,75 Sample preparation is the crucial first step in being able to make these measurements. Figure 4 summarizes examples of methods that have been commonly used for scanning probe measurements. Five different methodologies are outlined: tethering via bifunctionalized molecules (Figure 4A); solution-phase casting techniques such as spin-, drop-, or dip-coating (Figure 4B,C); and subsequent treatments (Figure 4D,E). Each technique results in a unique NC or NR surface coverage, and can be utilized for specific experiments. Tethering NCs via bifunctional molecules can isolate individual nanomaterials that are robustly bound to the substrate. Arrays of NCs can be formed on a surface by simple drop- and dip-casting techniques (Figure 4C).

2.1. Energy Levels and Density of States

Due to strong quantum confinement and high static dielectric constant (ε = 210),148 PbSe NCs have fascinating properties for use in optoelectronic devices. Hence the electronic structure of PbSe NCs in this size regime has been of great interest. Experimentally, the size-dependent electronic structure of PbSe NCs has been characterized through optical absorbance, excitation, and luminescence spectroscopy.147 Absorbance measurements reveal transitions between the filled and empty energy levels in an ensemble of NCs, thereby elucidating the band gap (i.e., first optical transition), and other allowed transitions between states are also observed. This measurement provides an energy difference, not the actual energy states that F


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Figure 5. Energy levels in single PbSe NCs measured by scanning tunneling spectroscopy. (A) Low-temperature STM topographic image of individual, 4 nm PbSe NCs bound to a Au(111) substrate via hexanedithiol linker molecules (inset) and a representative dI/dV spectrum. (B) Simulation of bias voltage distribution across the DBTJ by solving the Poisson equation. The color scale illustrates the percentage drop in applied bias between the tip−NC and the NC−substrate gaps. (C) Comparison of STS measured band gaps for various PbSe NCs sizes with band gaps measured by optical measurements and tight-binding calculations. (D) Comparison of energy levels measured from STS with those of optical transitions. (Inset) Absorption spectrum of PbSe. Panel D reprinted with permission from ref 75. Copyright 2005 American Physical Society.

theory. As shown in Figure 5C, there is good agreement in the band gap derived by all three methods, thus highlighting the utility of STM/STS measurements. In Figure 5D, the second and third optical transition energies have been plotted as a function of the first transition energy, from both STS and optical absorbance measurements. From this plot, it is evident that by utilizing the STS data, the controversial second peak can be assigned to a transition between Pe and Ph states. The advantage of STM/STS goes beyond that of measuring individual energy levels in single NCs. Among the many other notable measurements are size- and shape-dependent electronic level changes in single CdSe118,150 and InAs71 NCs, singleelectron charging events in Si NCs,150 and impurity doping in NCs.122 For many colloidally synthesized NC materials, enhancing electronic communication between individual NCs through ligand exchange is required to form functional (conductive) thin films for optoelectronic applications. Solar cells made from films of PbSe and PbS QDs benefit from chemical treatments that lead to enhanced coupling, resulting in mobile charges in the films.49,147,151−157 Chemical treatments can form new energy levels within the treated arrays,81,158−160 but the resulting excitonic structure80 and the mechanism by which charge transport is enhanced161,162 is still being studied. Addressing individual NCs within a coupled thin film by STM/STS can directly examine such mesoscopic electronic states. Initial work by Steiner et al.83 and Liljeroth et al.82 demonstrated that energy levels of individual NCs within a twodimensional array are different from those of isolated, single NCs. In 2013, Diaconescu et al.73 used STM/STS to probe electrical properties of individual PbS NCs within a weakly coupled array. The PbS NCs were spin-cast on a glass substrate coated with indium tin oxide (ITO, 0.5−1 kΩ) and treated with

were involved in the transition. In the case of PbSe, assignment of the second optical transition in the absorbance spectra has been controversial.73,75 In 2005, Liljeroth et al.75 used lowtemperature STM/STS to determine the origin of the second optical transition in PbSe NCs. A representative STS spectrum is shown in Figure 5A, recorded over a PbSe NC shown in the STM topographic image (inset). Multiple electron and hole states of the 4 nm PbSe NC are resolved in the STS spectrum. To be able to assign the energetic position of the measured states, two factors need to be satisfied: STS must be recorded under shell-tunneling conditions, and the bias voltage distribution across the tip−NC−substrate junction needs to be evaluated. Shell-tunneling conditions can be adjusted by tuning the tip−sample distance via the tunneling current set point. By solving the Poisson equation for representative tip− NC and NC−substrate distances, the electrostatic potential distribution across the DBTJ can be evaluated (Figure 5B). In STM, the bias is applied between the tip and the conducting substrate, and η refers to the percentage drop in the applied bias between tip and NC. In this specific example, ∼70% (depicted in color scale) of the bias voltage drops between the tip and the NC (η = 0.7), and the energy scale (sample bias voltage) will need to be adjusted accordingly. Furthermore, when an electron is resident in the NC, one must account for an associated polarization or charging energy. This charging energy depends on the dielectric constant of the NC material and the surrounding environment and is estimated by calculating the electrostatic potential energy created by the presence of a single electron present in the NC.149 With all these factors taken into account, the relationship between the STS measured band gap and NC band gap is shown in the expression in Figure 5A. Liljeroth et al.75 also compared STSmeasured band gaps as a function of STM-measured NC diameters with those of optical measurements and tight-binding G


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Figure 6. Size-dependent energy-level changes measured by STM/STS. (A) Multiple electron and hole states are identified in three different PbS NCs by dI/dV spectra. Measured spectra (solid lines) are overlaid with fits (dashed lines) from Lorentzian analysis. The term midgap states/bands (MGB) is used to describe the asymmetric background observed within the band gap region in the STS spectra. (B) Varying energies for individual states within different PbS NCs, both measured (by STS) and calculated. Reprinted with permission from ref 73. Copyright 2013 American Physical Society.

Figure 7. Tuning fork AFM measurements of current intermittency in single PbS NCs. (A) Schematic representation of a tuning fork AFM; a metallic tip attached to a quartz tuning fork hovering over a single NC on a substrate, and its control electronics. (B) Current measured over a single PbS NC for a period of 30 s, showing several approximate discrete energy levels. (Bottom) Proposed mechanism for observed current intermittency. (C) Power law distribution of the two observed blinking patterns measured in c-AFM. Reprinted with permission from ref 145. Copyright 2013 American Chemical Society.

1,2-ethanedithiol (EDT), which leads to a coupled thin film.73 Such films of coupled NCs behave as macroscopic semiconductor films in solar cells, field effect transistors, and lightemitting devices, yet they exhibit properties (such as the band gap) resulting from quantum confinement of each NC making up the film. Differential conductance spectra of individual NCs of various sizes within the thin film are shown in Figure 6. For each NC (Figure 6A), discrete yet broadened electron and hole states were identified and fit by use of Lorentzian functions (bottom red spectrum). These STS spectra confirm that quantum confinement effects within the coupled PbS arrays are preserved. Interestingly, a broad asymmetric band was also observed within the band gap region of the NCs (Figure 6A, bottom blue dashed line). This broad band was assigned as a midgap band (MGB) resulting from improper passivation during removal of the native ligand by EDT treatment. The midgap states are described as a network of NC surface defects

propagating throughout the thin film, facilitating charge transport.73,159 Further analysis of the size-dependent STS spectra revealed that the energetic position of the midgap states is independent of the PbS NC size. A plot of NC band gap versus measured (STS) and calculated (solid line, kp calculations)163,164 electronic states is shown in Figure 6B. The midgap states are centered at ∼0.27 eV below the Fermi level position of the measured PbS NCs. Comparison of optical and STS measurements in PbS arrays indicates that, unlike the single PbSe NCs (Figure 5D), parity selection rules are relaxed in a weakly coupled array of PbS NCs. Through the use of STS data, here the second and fourth transitions in the optical absorption spectrum have been assigned to asymmetric transitions between states of different symmetries (i.e., 1Se−1Ph and 1Pe−1Dh respectively). An experimental observation of a localized, midgap state that is confined and localized to a single NC (i.e., due to a surface H


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fork AFM tip and the NC surface, equivalent to Coulomb blockade, which can lead to current intermittency. This measured electrical blinking is therefore analogous to optical blinking, as they both result from the presence of trap states. The proposed mechanism for electrical blinking in the solid state for this tuning fork AFM measurement is illustrated in Figure 7B (bottom).

state or an atomic vacancy) is unlikely to be measurable by STS due to the confined wave function and the low probability of transferring charge carriers through a DBTJ. However, in the STS measurements presented by Diaconescu et al.,73 the authors hypothesize the existence of a delocalized midgap state that encompasses the network of chemically treated NCs that is measurable by STS.73 Another very recent experimental observation of midgap states in a chemically treated NC film was made by Zhang et al.165 More measurements in this area are needed to firmly establish the existence of such states in NC arrays and their origin.

2.3. Charge, Dipole Strength, and Polarizability

In addition to the topic of blinking, another highly debated subject is the presence of a permanent dipole moment181−183 in colloidal NRs. Experimental184−186 and theoretical187,188 studies have been inconclusive in establishing its existence or origin. It is believed this dipole moment exists, due to the lack of inversion symmetry in perfect hexagonal wurtzite crystal structures, but such permanent dipoles could easily be overshadowed by randomly present surface charges.186 Krishnan et al.189 measured polarization surface charge density across single CdSe NRs using EFM, but they did not observe the presence of a dipole across the z-axis of the NR. By modeling the CdSe NR as a uniformly polarized (Figure 8A, solid line) or uniformly charged (Figure 8A, dashed line) dielectric cylinder, the authors simulated the EFM signal across the NR. The measured EFM signal profile over a single CdSe NR on HOPG is shown in Figure 8B (image in inset). The measurement did not correspond to the simulated results that would indicate the presence of a dipole. Furthermore, the CdSe NRs displayed varying contrast in the EFM charge image and were assigned as being either positively/negatively charged or charge-neutral (Figure 8C). The difference in contrast in the charge image suggests that NRs exist at different surface charge densities. The authors concluded that the varying surface charge was due to a lack of complete cylindrical symmetry along the long axis of the NR.189 Ben-Porat et al.190 demonstrated that the charge state of NCs can be determined by EFM and that NCs can display different charge states influenced by illumination or the underlying substrate. Figure 8D shows two rows of EFM images of PbSe NCs on n-type silicon; topography (left), charge (middle), and polarizabilty (right). Images in the top row were recorded with no external laser illumination of the NCs, and those in the bottom row were recorded after 25 min of illumination with a 442 nm wavelength external light source. The authors observed an increase in the amplitude of the charge image for some PbSe NCs. However, when a p-type silicon substrate was used, some PbSe NCs showed negative charging. The authors concluded that charge transfer between the NCs and the underlying conducting substrates could influence the charge state observed in NCs.

2.2. “Blinking” in Tunneling Current

Semiconducting NCs, along with virtually all other fluorophores, exhibit fluorescence intermittency or “blinking”. Blinking is the phenomenon of random switching between emissive (labeled as ON) and dark (labeled as OFF) states of luminescence under continuous irradiation.166−169 Currently, many researchers are exploring band structure and heterostructuring concepts to reduce blinking in NCs, which may be important for solid-state lighting, lasing, or optical communication technologies.25,170−176 Over the past decade, several models have been proposed to explain the mechanism behind blinking, but a universal model has not been found that can rationalize all observations.176 One proposed model states that optical blinking is caused by Auger recombination resulting from charged NCs,177 and thus far, research has been focused on using optical techniques to understand this phenomenon. Recently in 2013, Maturova et al.145 demonstrated that the conditions leading to blinking could also be observed electrically. “Current intermittency” was measured in single PbS NCs by a tuning fork-based AFM technique. A schematic of the tuning fork AFM tip is shown in Figure 7A, where an ultrasharp, conducting Pt/Ir tip is adhered to the quartz tuning fork.145 The metallic tips were first electrochemically etched and further sharpened by focused ion beam lithography. In this unconventional conducting atomic force microscopy (c-AFM) method, current is measured while the tip makes intermittent contact with the surface. In contrast, conventional c-AFM experiments are performed with the AFM tip in constant contact with the sample surface, which is not a suitable option for measuring NCs tethered to a surface, as they become attached to and thus contaminate the probe tip. Maturova et al.145 showed that in intermittent contact mode, when the tip is in contact or within close proximity to the surface, tunneling electrons can be measured. Monitoring tunneling current along with the tip motion led to timeresolved current measurements that displayed intermittency analogous to fluorescence blinking events. Tunneling current measured in a single PbS NC over a period of 30 s is shown in Figure 7B. Random and discrete current values were observed, where high current amplitudes were designated as ON states and low current amplitudes were designated as OFF states. Both ON and OFF states showed a time-dependent, power-law distribution (Figure 7C). Power-law distributions in photoluminescence blinking have been previously measured in singleparticle studies. In 2011, Galland et al.178 measured two types of blinking events for bright and dark states in single CdSe/CdS core−shell NCs using solution-phase spectrophotochemistry, and variation in trap state occupancy174,179,180 was proposed as a mechanism for the observed blinking patterns. Trapped charges (electrons and holes) can create an additional barrier to tunneling electrons between the tuning

2.4. Energy Levels in Doped Single Nanocrystals

Realizing controlled doping in NCs and NC films is a longstanding issue for NC-based optoelectronic devices. Incorporating dopant atoms into NCs is not as well developed as doping in bulk materials, and the resultant doped NCs often cannot be characterized by the same techniques (Figure 9A).49,191−194 Examples of recent synthetic advances to dope NCs include remote doping,195,196 where surface ligands donate carriers to the NC, and the introduction of dopant precursors during the synthesis process that have to survive “selfpurification” during growth.49,191−194,197,198 Mocatta et al.122 developed a method to dope InAs NCs with several different metal impurities at room temperature and used STM/STS along with absorption and emission spectroscopy to measure I


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bottom). However, significant broadening of the energy levels was seen in Au-doped NCs compared to undoped NCs, as a result of inclusion of the impurity atom into the pure InAs lattice. In contrast, for Cu- and Ag-doped NCs, evidence of doping was observed in the STS spectra (Figure 9D, top). Copper-doped InAs NCs showed evidence of being n-type, while Ag-doped NCs showed p-type character. The rationale for the STS observations is based on the oxidation state and dopant location within the InAs NC lattice. A schematic diagram of possible dopant locations within the InAs lattice structure is shown in Figure 9D.199 The small Cu ions are hypothesized to be located on interstitial sites, while the larger Ag and Au ions are expected to substitute cations. Interstitial copper, which can have a formal oxidation state of +1 or +2 and is compensated by conduction electrons in the NC, will result in n-type doping. However, upon replacing a lattice In ion with Ag, p-type character is expected because the lower formal charge of Ag ions is compensated by holes. The observed changes in the STS spectra indicate a change in the position of the Fermi level as expected for the type of doping (Figure 9C, top). The measurements shows that, between Ag and Cu dopants, the Fermi level shifts by nearly 0.5 V, as evident by the applied sample voltage required to access the electron or hole states.122 While intentionally adding dopant atoms can influence the energy levels of quantum dots, the passivating ligand shell can also have an effect through its electrical dipole or charge transfer, as was shown for silicon NCs by Wolf et al.200 In a series of STM/STS experiments on single silicon NCs, it was shown that ligands such as allylamine and NH4Br doped the NCs p-type, while dodecyl-capped NCs demonstrated an opposite effect by doping the NCs n-type.200

Figure 8. Charge and polarizability measurements in singlecomponent CdSe NRs and PbSe NCs by electric force microscopy. (A) Calculated EFM charge profile expected for a uniformly polarized (solid line) or uniformly negatively charged (dashed line) CdSe nanorod. (B) Measured EFM charge profile (inset: EFM topographic image) over a single CdSe nanorod. (C) Representative topographic (left), charge (middle), and polarizability (right) images of CdSe nanorods on a graphite surface. (D) Representative topographic (left), charge (middle), and polarizability (right) images of PbSe NCs on an n-type silicon substrate, under dark conditions (top row of images) and under illumination with 442 nm light (bottom row of images). Individual NCs are circled in the charge (middle) image. Panels A−C reprinted with permission from ref 189. Copyright 2004 American Physical Society. Panel D reprinted with permission from ref 190. Copyright 2004 American Chemical Society.

3. HETEROSTRUCTURED COLLOIDAL NANOSTRUCTURES Incomplete passivation of NC surfaces by the organic ligand shell leads to surface trap states, which can affect the carrier dynamics in NCs (e.g., reduce fluorescence quantum yield).201−203 Core−shell NCs often have enhanced photoluminescence quantum yields over conventional NCs due to reduced interaction between the light-emitting core and ligands by way of a shell semiconductor that better passivates and confines carriers to the core. The optical characteristics of core−shell NCs are tunable by tweaking core diameter, shell thickness, and material compositions and have led to a variety of composite NCs that have use in biological systems204−206 and devices.207,208 Recent advances in the synthesis of multicomponent nanomaterials now allow for a variety of shapes and structures. Figure 10 summarizes some of the various synthetic routes by which heterostructured nanomaterials can be made.46,47,209 The electrical properties of these heterostructured nanomaterials are governed by the relative alignment of electron and hole energy levels of the two components at the material interface. When two semiconductor materials form a heterojunction, three possible band alignments can occur: types I, II, and III. A schematic diagram for type I/II band offsets is shown in Figure 11A for a PbSe/CdSe core− shell NC. Type III represents the case when the conduction band of one material is deeper than the valence band of the other, such that the bands are completely offset. These naming conventions also apply to nanomaterials, where (as opposed to energy bands) the electron and hole states can also align in these three configurations. The alignment affects the spatial

the doping effect on the NC energy levels. Unlike bulk semiconductors, the energy levels in single NCs are discrete, and addition of a single impurity can lead to formation of individual dopant energy levels, as described by the hydrogenic model (Figure 9B). Further addition of impurity atoms can lead to dopant band formation and can potentially cause major changes to the NC through increased disorder. Mocatta et al.122 incorporated Cu, Ag, and Au dopant atoms into InAs NCs. Electrical characterization was performed on individual NCs by low-temperature STM/STS (Figure 9C). Doping characteristics were determined by shifts in the electron and hole states with respect to the Fermi level. The Fermi level position within the band gap is assumed when the sample bias is at 0 V; therefore, shifts in the electronic state positions with respect to 0 V bias indicate apparent shifts in the Fermi level within the band gap of the NC. In the case of undoped and Audoped InAs NCs, similar energy levels and energy-level spacings were observed in the STS spectra (Figure 9C, J


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Figure 9. Measuring energy levels in doped single InAs NCs. (A) Schematic band diagram illustrating the effect of n- and p-doping in a bulk semiconductor. Changes in the absorption (ABS), photoluminescence (PL), Fermi level (Ef), and band gap (Eg) are shown. (B) Schematic band diagram illustrating an n-doped NC, where both the NC states (red) and dopant states (green) are shown. (C) Measurement of energy-level shifts by STM/STS in Ag-doped (top, red), Cu-doped (top, blue), undoped (bottom, black) and Au-doped (bottom, green) InAs NCs. Doping with Ag and Cu atoms leads to p- and n-type NCs, respectively. (D) Schematic diagram illustrating each dopant atom location within the InAs lattice structure. Panels A−C reprinted with permission from ref 122. Copyright 2011 AAAS. Panel D reprinted with permission from ref 199. Copyright 2013 American Chemical Society.

Figure 10. Reaction scheme illustrating synthetic routes toward forming heterostructured nanomaterials. (A) Core−shell NCs formed by nucleation and overgrowth of a symmetric shell onto presynthesized NCs. (B) Seeded NRs, where the shell material is asymmetrically grown over the NC, forming a nanorod over a spherical seed crystal. (C) Two-stage growth of the second material onto the side or end of a NR. Growth often occurs preferentially at one or both ends due to higher reactivity of the end facets. (D) Material replacement by partial cation or anion exchange.

distribution of the electron and hole wave function within the composite nanostructure.124 Typically, the energy-level alignment in heterostructured nanomaterials is inferred by optical studies, and direct measurements of the energy levels remain challenging.210−213 Here we will highlight several studies on heterostructured NCs where scanning probe techniques were used to better understand their energetics.

deposited on HOPG is shown in Figure 11D. Individual NCs are clearly discernible within the close-packed array. To be able to identify spectral features intrinsic to the core−shell NC, experiments were also performed on single-component CdSe and PbSe NCs as well. The energy levels measured in PbSe/ CdSe were significantly different from those of singlecomponent PbSe and CdSe (Figure 11E).75,118,121 In PbSe/ CdSe NCs, multiple doublet peaks (where a doublet is defined as two peaks in close proximity) were observed. The energy spacing between each doublet remained constant, while the energy between consecutive doublets varied (Figure 11E, red). These energy gaps were measured over ∼20 NCs, and the results are shown in a histogram (Figure 11F). A common characteristic of STS measurements is charging.71,74 In the case of DBTJ-based measurements, shelltunneling conditions prevent charging, while shell-filling

3.1. Charge Transport

In 2010, Swart et al.214 used STM/STS to study electronic structure in isolated PbSe/CdSe core−shell NCs. For STS measurements, the NCs were held in a DBTJ geometry where energy levels are accessed by tunneling electrons/holes. As previously discussed in section 1.1, tunneling rates of electrons and holes into and out of the NC dominate the measurement (Figure 11C). An STM topographic image of PbSe/CdSe NCs K


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equation149,215−217 for single-electron transport to simulate the STS results over a range of electron- and hole-tunneling rates.214 By use of the simulations, the intradoublet spacing was ascribed to the electron−hole interaction energy. When a hole is trapped in the core, an electron channel becomes available at a lower bias, with the energy difference defined by the electron−hole interaction energy.214 In core−shell NCs, understanding the effect of varying shell thicknesses on the energy levels can give insight into orbital overlap and intermixing of states as a result of the material interface. Millo et al.218 measured STS on InAs NCs with different ZnSe shell thicknesses and compared the electrical measurements with photoluminescence excitation spectra (PLE). Tunneling conductance spectra recorded over singlecomponent InAs as well as InAs with two different ZnSe shell thicknesses (2 and 6 monolayers) are shown in Figure 12A.

Figure 12. Measurment of energy-level changes as a function of shell thickness. (A) S−P energy-level changes in InAs NCs as a function of varying thicknesses of the ZnSe shell. (B) Energy difference between s and p states as measured by (○) STS and (●) PLE. Reprinted with permission from ref 218. Copyright 2001 American Physical Society.

Figure 11. Measurement of electron−hole interaction energy in PbSe/ CdSe heterostructured NCs. (A) Schematic diagram of possible band alignments for the PbSe/CdSe core−shell NC depending on the size and thus band gap of the PbSe core. (B) TEM image of a single PbSe/ CdSe NC, showing the anisotropic thickness of the CdSe shell around the PbSe NC. (C) Band structure illustrating the electron and hole transport process across the core−shell NC in DBTJ geometry. (D) Low-temperature STM topographic image of a single layer of PbSe/ CdSe NCs on HOPG. (E) Low-temperature dI/dV spectra over PbSe/CdSe (red), a 4.3 nm PbSe NC (blue), and a 6.1 nm CdSe NC (black). The dI/dV spectrum over the PbSe/CdSe showed multiple doublet peaks; the spacings within each doublet (intradoublet spacing) and between each pair of doublets (interdoublet spacing) were measured, and a histogram of the results is shown in panel F. Reprinted with permission from ref 214. Copyright American Chemical Society.

While the STS-measured band gap remains constant, the spacing between 1Se and 1Pe states at positive sample bias decreases with growing shell thickness. It is expected that the 1Se state will be confined to the InAs core, while the 1Pe state spatially extends to the ZnSe shell and therefore red-shifts upon increasing shell thickness. Complementary information about this energy shift is extracted from PLE measurements.218 Even though PLE measurements reveal a similar red shift in the 1Pe state, the magnitude of the observed shift differed from that of the STS measurements (Figure 12B). This is because STS measurements can directly measure individual states, and therefore the energy difference between conduction band states is more easily determined.70 In contrast, PLE measurements detect optical transitions between states, and the energy differences of conduction band states are convoluted with changes in valence band levels.219−221 In addition to measuring energy levels, STM can also be used to image the spatial distribution of these states within the whole nanostructure, and this will be discussed in section 3.3.

conditions leads to measurements where electron−electron or electron−hole interactions can occur.71,74 For the PbSe/CdSe NCs, tunneling through two components needs to be considered, and this added complexity can lead to charging effects. Even under stringent synthetic control of the shell growth, there can be some variations as shown in the TEM image (Figure 11B), where an incomplete CdSe shell over a PbSe core is shown.214 First, the asymmetric shell growth and random orientations on the surface can lead to variable tunneling widths for the core material, PbSe. About 30% of the measured NCs showed the characteristic doublet features. Furthermore, the type II band alignment in PbSe/CdSe confines hole wave functions to the PbSe core, leading to a greater hole-tunneling barrier and thereby effectively lowering the hole-tunneling rates compared to that of the electrons. Therefore, shell-tunneling conditions needed for electrons may not hold true for hole-tunneling. The authors used the master

3.2. Energy Level or Band Alignment

In section 3.1, we discussed that the electrical properties of heterostructured nanomaterials are sensitive to the relative alignment of electron and hole states across the material interface of the heterostructure. While this offset can be inferred by optical studies and bulk material properties,211−213 direct measurements across nanoscale interfaces have been predominantly measured by scanning probe techniques. L


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In 2008, Steiner et al.124 used STM/STS to spatially resolve electronic states in individual seeded NRs, with an emphasis on how electronic tunneling through the seed area differs from the rest of the NR. The energy-level offsets were determined by comparing the STS-measured energy levels across the length of the NRs in two different types of NRs (CdSe/CdS and ZnSe/ CdS).124 On the basis of known properties of the bulk materials, the CdSe/CdS NR is expected to form a tunable type I band offset (Figure 13A), whereas ZnSe/CdS forms a type II structure (Figure 13B). In type I CdSe/CdS, both carriers are confined to the CdSe seed. However, quantum confinement of the 1Se level in the CdSe seed has been shown to promote a delocalized groundstate electron wave function over the entire length of the NR, which is termed as quasi-type I and is shown in Figure 13A.222 In contrast, a more formal type II ZnSe/CdS has favorable

energy alignment that separates a photogenerated electron− hole pair by transfer of one carrier across the interface such that the electron mainly resides in the CdS segment. In this case, a significant barrier exists that excludes the electron wave function from the ZnSe seed. In both cases, the ground-state hole wave function is localized to the seed.124 In the low-temperature STM/STS experiments, spectra were recorded at several positions along the entire length of the NR, including the area over which the seed is expected to be located.124 Even though seeded NRs do not have an open interface, tunneling through the different regions of the NR provides evidence for determining energy-level alignment across the interface. In CdSe/CdS, identical STS spectra were measured at the ends of the NR (Figure 13C, spectra 1 and 2). A band gap of 2.9 eV was measured at both ends, whereas a band gap of 2.3 eV was measured approximately in the center of the NR. The lower band gap in the center is due to the CdSe seed within the rod. Comparison of the first energy level at both bias polarities shows that, in the CdSe seed, both the electron and hole states are contained within the CdS band gap, indicative of a type I structure. Subsequently, the CB and VB energy-level offsets were extracted from the STS spectra. An interesting conclusion of this measurement is that, in the CdSe/CdS NRs measured here, the electron ground-state wave function is not necessarily delocalized over the entire composite structure but localized within the CdSe seed.124 Therefore, this STM/STS experiment allows one to directly measure the seed size at which such a structure transitions from true type I through quasi-type II and then to formal type II. Reported values for the CB energy-level offset in the literature vary significantly, with some reports suggesting a type I offset and others type II for CdSe/CdS NRs.203,223,224 These literature examples further strengthen the need to better understand energy levels of nanoscale composite materials. The authors performed a similar STS experiment and analysis over the ZnSe/CdS NR (Figure 13D). While STS spectra at the edges of the NR showed the expected CdS band gap, spectra over the location of the ZnSe seed did not show the optical band gap of the ZnSe seed. Instead, the type II band gap of the composite structure was measured, which is labeled as Eg in Figure 13B. In this scenario, during STS, it requires less energy for the electron to tunnel into the CB of the CdS and the hole to tunnel into the VB of the ZnSe, thus giving rise to a band gap of the composite structure and not simply the ZnSe seed. The calculated STS spectrum is shown in Figure 13D, supporting the measurement and type II designation.124 Measuring the energy-level alignment in heterostructured NRs with an axial interface gives direct access to both material components. In 2014, Bera et al.123 used STM/STS to measure the conduction and valence energy states across axial p−n junctions in individual CdS/Cu2S nanorods and elucidated the depletion region across the nanoscale material interface. The CdS/Cu2S NRs were synthesized by a cation-exchange method as discussed previously (Figure 10D)225,226 and deposited on HOPG by drop-casting. The STS measurements at 77 K were recorded along the long axis of the NR (Figure 14A). Measured I/V and corresponding dI/dV curves are shown in Figure 14panels B and C, respectively. Each point corresponds to an average of 50 curves. The dI/dV curves shown in Figure 14C show variations in the energetic position of the first valence (at negative sample bias) and conduction (at positive sample bias) states across the interface, indicative of a change in the Fermi level position within the band gap. Furthermore, the STS-

Figure 13. Measurement of quantized energy-level alignment in seeded NRs that have a “buried” interface by STM/STS. (A, B) Schematic illustrations of spatial distribution of electron and hole wave function within (A) CdSe/CdS seeded NR and (B) ZnSe/CdS seeded NR. (C, D) STM images and low-temperature dI/dV spectra over (C) a single CdSe/CdS NR and (D) a single ZnSe/CdS NR. Simulated spectra are shown in red, and STS measurements are shown in black. Reprinted with permission from ref 124. Copyright 2008 American Chemical Society. M


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Figure 14. Measurement of offsets in bicomponent NRs with “open” interfaces by STM/STS. (A) STM topographic image of a single CdS/Cu2S NR on HOPG imaged at 77 K and (B) I/V curves recorded across the long axis of this NR. (C) Corresponding dI/dV curves, which show variations in the first valence and conduction states across the nanoscale material interface. Lines are drawn as a guide, to illustrate this variation. (D) From these STS measurements, the band diagram across the CdS/Cu2S NR is drawn, illustrating the depletion width and energy level bending across the CdS and Cu2S interface. (E) Schematic band diagram projecting the valence/conduction states before and after contact between the CdS and Cu2S material components. Reprinted with permission from ref 123. Copyright 2008 American Chemical Society.

current maps, the S state is spherical (Figure 15C) while the shape of the P state differed at the two different bias voltages (Figure 15D,E). Calculated isoprobability surfaces (or contour maps) of the S state and the in-plane and perpendicular components of the P state are shown in Figure 15F−H. While the measured shape of the S state in the current map is as expected, the different shapes of the P state (Figure 15D,E) imply varying contributions from in-plane and perpendicular components. Equal contributions from all components will lead to a spherically shaped P state (Figure 15E), while a larger contribution from the in-plane components (Px2 + Py2) can lead to a torus-like P state (Figure 15D). The advantages of pairing optical measurements with scanning probe techniques are evident. Recent instrumentation advances have enabled tremendous progress,230−234 thus making it possible to probe excited-state carrier wave functions in relation to structure/geometry of nanomaterials. Yoskovitz et al.235 combined apertureless near-field optical microscopy and AFM to spatially image exciton wave functions within individual seeded CdSe/CdS NRs (Figure 16A). In this dual technique, while the AFM tip scans over the NR, changes in fluorescence of the NR are measured as the tip−sample distance varies during intermittent contact with the surface (Figure 16A). The two measurements are synchronized and the recorded AFM topographic image and optical images are then overlaid (Figure 16B−D) to gain structural and optical characteristics. In this example, the location of the CdSe seed within the CdS NR was revealed.235

measured band gap at the ends of the NR agreed with the values for each individual material component: 2.8 eV for CdS (position 1) and 1.3 eV for Cu2S (position 6).123 Depiction of the band diagram (Figure 14D) from the STS measured conduction and valence state positions revealed a depletion region of 42 nm for a 70 nm CdS/Cu2S NR, with localized energetic states that mimic band bending in the range of 0.3− 0.4 eV (Figure 14E). Thus, the use of STM/STS can enable visualization of energetics within complex structures, which will become important for designing structures for many applications.227−229 3.3. Wave Function Imaging

In addition to direct measurements of energy levels, STM/STS is capable of imaging the shape and spatial distribution of each electronic state. In 2001, Millo et al.218 spatially mapped the S and P electron states in core−shell InAs/ZnSe NCs by STM/ STS. First, a typical STS spectrum was recorded and the bias voltage of the S and P states was determined (Figure 15A). In this example, the S state appeared at 0.9 eV and the P state was observed as a multiplet at 1.4−1.9 eV. By spatially mapping the current at each above-mentioned bias voltage, the shape of each electronic state was determined through current images (Figure 15C−E). Current images were recorded simultaneously with topography (Figure 15B), and significant differences were observed between the S and P states. The S state was localized to the InAs core (Figure 15C) in the center of the InAs/ZnSe core−shell NC, while the P state extended over the entire NC (Figure 15D,E). This is in agreement with STM/STS spectra of InAs/ZnSe core−shell as shown in Figure 12 and discussed in section 3.1. With regard to the shape of each state in the N


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generated electrons to the metal. Additionally, such structures allow enhanced electrical contact to semiconductor NCs.240 Steiner et al.125 used low-temperature STM/STS to elucidate energy-level changes across the metal−semiconductor junction in a Au−CdSe NR and mapped the spatial extent of the interface. Spatially resolved dI/dV spectra over the Au, the metal−semiconductor interface, and the CdSe NR were recorded and compared (Figure 17A). Tunneling spectroscopy over the Au component revealed multiple electronic states that resemble single-electron-tunneling steps previously observed in individual Au nanoparticles63,65−68 (Figure 17A, red). In the center of the CdSe NR, STS showed a well-resolved energy gap (Figure 17A, blue) showing negligible effects from the Au components. The influence of the Au tip was observed up to ∼5 nm into the CdSe portion of the nanostructure.125 While knowing the energy-level alignment across the semiconductor−metal interface is important, understanding the charge separation mechanism is also required for photocatalytic applications. The mechanism for charge separation under illumination has been addressed by Costi et al.134 using EFM (Figure 17B−G). For the EFM experiments, the authors coated an HOPG surface with poly(vinyl butyral) to isolate the NCs from the conducting substrate. Au−CdSe hybrid NRs, single-component CdSe NRs, and Au nanoparticles were deposited on polymer-coated HOPG substrates. Topographic and EFM charge images with and without illumination over Au−CdSe NRs (Figure 17B−D) and over single-component CdSe NRs (Figure 17E−G) are shown. The Au−CdSe NRs displayed negative charging upon illumination, while the single-component CdSe NRs and isolated Au nanoparticles (not shown) showed positive charges upon illumination.134 This observation was made on the basis of contrast in the EFM images. To further investigate this charging behavior, the authors changed the thickness of the insulating layer, as well as the carrier concentration of the silicon substrates in addition to HOPG, and observed variations in the magnitude of the charging.134 The authors deduced that charge transfer between the NR and the substrate resulted in a net

Figure 15. Use of STM/STS to spatially map energy levels. (A) STS spectrum over a single InAs/ZnSe core−shell NC. (B) STM topographic image. (C−E) Tunneling current images recorded at different biases to access specific S and P states. (F−H) Calculated isoprobability surfaces of S state and in-plane and perpendicular components of the P state illustrate the expected shape of each state. Reprinted with permission from ref 218. Copyright American Physical Society.

3.4. Energy Levels and Charge Separation

Hybrid NRs that consist of a metal−semiconductor junction are often studied for photocatalytic applications.236−239 Heterostructured NRs are designed to absorb light with the semiconductor portion, while the metal−semiconductor junction facilitates charge separation and supplies photo-

Figure 16. Combined AFM and optics for measurement of spatially resolved excited-state wave functions in NRs. (A) Schematic diagrams of instrumentation setup and data acquisition methodology. (B) Topographic AFM image over a single CdSe/CdS seeded NR and its height profile (inset). (C) Lifetime image over the same area, showing a ∼15 nm wide area with shortened lifetime. (D) Overlay of lifetime and topography image, where the hotspot indicates a region of shorter lifetime. Reprinted with permission from ref 235. Copyright 2010 American Chemical Society. O


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Figure 18. Extraction of built-in potential values from EFM phase shifts in individual CdS/Cu2S NRs. (A) Schematic diagram showing use of a positional marker to correlate TEM and EFM images. (B) Overlay of topographic AFM image (top) and TEM (bottom), enabling the resolution of single NRs. (C) For three individual CdS/ Cu2S NRs, TEM opacity (top), EFM phase shift (middle), and simulated potential (bottom) are shown. Reprinted with permission from ref 137. Copyright 2010 American Physical Society.

Figure 17. Probing metal−hybrid nanostructures. (A) Spatially resolved STM/STS across a CdSe NR with Au tips grown on both ends. Spectra 1−4 were recorded over several positions from the Au tip to the center of the CdS NR. (B−G) Series of EFM images of Au− CdSe and CdSe NRs on HOPG: (B) topography, (C) charge image, and (D) charge image under illumination of Au−CdSe NRs on HOPG and (E) topography, (E) charge image, and (G) charge image under illumination of single-component CdSe NRs on HOPG. Panel A reprinted with permission from ref 125. Copyright 2005 American Physical Society. Panels B−G reprinted with permission from ref 134. Copyright 2009 American Chemical Society.

potentials ranging from 100 to 920 mV for various CdS/Cu2S NRs.137 Recently, Nanayakkara et al.31 directly measured the built-in potential in individual CdS/PbS axial NRs using SKPM. Similar to the work of Zaniewski et al.,137 CdS/PbS NRs were synthesized by a cation-exchange process and deposited on HOPG by spin coating.225 A three-dimensional rendition of a potential and topographic image over a single CdS/PbS NR is shown in Figure 19A. As seen in the topographic image, the surface topography shows that the height of the NR is uniform, but the surface potential shows a pronounced potential change near the center of the NR. The line profiles over NRs (extracted from the images and overlaid in Figure 19B) show the magnitude (150 mV) and width (10 nm) of the potential change across the CdS/PbS junction. A high-resolution TEM image of a single CdS/PbS NR (Figure 19C) clearly shows the crystallographic interface between CdS and PbS regions. Since the PbS region is introduced postsynthetically, one or both end facets of the NR can react, sometimes leading to NRs with a segment exchanged to PbS at both ends, while the center of the NR remains as CdS. A topographic and potential image series at varying opacities highlights the positions where PbS replaced CdS (Figure 19D), and as expected, a varying distribution of exchange is observed in the potential images. Therefore, the regions with deeper potential relative to the tip (darker color scale) are assigned to PbS regions of the CdS/PbS NR.31

negative charge on the NR. The substrate plays an important role in charge equilibration, with and without illumination, and this has been observed in single NCs (section 2.3) as well as NRs. 3.5. Built-in Potential

Heterostructured NRs with axial junctions (junction along the axis of the NR rather than the radius) provide components that are individually accessible by the SPM tip.46,47 Zaniewski et al.137 used EFM in conjunction with TEM to extract EFM phase changes across single CdS/Cu2S NRs. In this study, positional markers on a substrate enable the authors to spatially correlate the EFM and TEM images (Figure 18 A,B). They then extracted the change in phase signal of the EFM measurement across the long axis of individual NRs. TEM images and profiles of opacity changes (extracted from the TEM image) in three NRs are shown in Figure 18C, along with EFM phase shifts and simulated potential changes within each NR. By using a three-dimensional finite element electrostatic model in COMSOL, the electrostatic potential differences across the NRs were determined from the phase shift values of the EFM signal. These potential differences revealed built-in P


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Figure 19. Built-in potential measurements in single CdS/PbS NRs by SKPM. (A) Three-dimensional rendition of topographic and potential images of a single CdS/PbS NR. (B) Overlapped topography and potential line profiles over the NR shown in panel A, demonstrating the potential difference across the long axis of the NR. (C) Representative high-resolution TEM image of a single CdS/PbS NR, displaying the atomically sharp material interface. (D) Topographic and potential images overlaid with varying opacity to highlight the NR positions at which sharp potential changes were observed. Reprinted with permission from ref 31. Copyright 2013 American Chemical Society.

photovoltaics is tremendous; however, progress has been slow. One of the main challenges is that the synthesis of semiconductor nanomaterials is not a standardized procedure where precise products are formed every single time. A distribution in the size, varying shape, and elemental composition and different facet surface exposure of the final product can give rise to varying device performances. This brings forth the issue of characterization of an ensemble of products. Ensemble-type characterization methods average materials with varying composition and size and do not provide information about individual nanomaterials. In order for simple and complex nanostructures to reach their full potential for applications in optoelectronic devices, powerful and precise characterization tools of individual nanomaterials are needed to better understand the factors that affect their ultimate performance. In this review, we have shown specific examples of how SPM provides unique insight into energy-level alignment, doping, charge transport, density of states, charge, and potential in a variety of colloidally synthesized nanocrystal materials. Since the advent of the STM in 198154 and the AFM in 1986,55 both techniques have made tremendous advances in imaging a large variety of materials (metals, semiconductors, insulators, biomolecules, etc.) with unprecedented spatial resolution, as well as accessing complex material properties, moving beyond the realm of electrical characterization. A few prominent examples are (a) understanding spin dynamics by use of pump−probe STM,255 (b) understanding the dynamics of photoexcited charges by use of time-resolved EFM measurements, and (c) understanding thermoelectricity in molecular junctions by use of custom-built STM.256 A portion of future thrusts in SPM measurements will likely be geared toward acquiring atomic-scale to nanoscale information within materials scaled up to be utilized for specific applications. This is not a detour away from basic research but a move toward application-driven basic research. For this purpose, nanoscale local conductivity measurements can be made, especially conductivity across material interfaces, grain boundaries, and contacts. The recent development of multiprobe SPM,257 which is an SPM microscope with independently operable multiple probe tips (2−4), can be used for this purpose. More advances in the direction of multiprobe SPM measurements

By comparing potential measurements across singlecomponent CdS, heterostructured CdS/PbS, seeded NR CdSe/CdS, and sequentially grown CdS/PbSe, Nanayakkara et al.31 attributed the potential change across the NRs to the difference in work function of each material. In the nanoscale material interface with the NR, charge exchange occurs to equilibrate the Fermi level in each component, resulting in an electrical potential difference. COMSOL was used to simulate the spatial distribution of charges across the CdS/PbS NR, and a boundary element method241−243 was used to compute the attenuation factor for the SKPM scanning conditions. In order to reach equilibrium between the two ends of the CdS/PbS NR, electrons transfer from the n-type CdS244 to the p-type PbS,245 reaching built-in potential values between 375 and 510 mV. Simulations showed 10−15 charges transfer across the interface to reach the measured built-in potential values, and furthermore, the charges are homogeneously distributed across each portion of the NR.

4. OUTLOOK AND FUTURE PROSPECTS Recent strides in the synthesis of colloidal heterostructured and hybrid semiconductor nanomaterials have brought to light new physics promoting their application in optoelectronics, catalysis, and medicine.246−249 While other avenues of production of nanomaterials exist, such as gas-phase synthesis and deposition,250 due to the low cost and ease of processing materials, we have chosen to highlight advances in the characterization of colloidally synthesized nanomaterials in this review. Recently, colloidal nanomaterials have been commercialized in televisions and displays and have also made advances toward potential commercial products such as light-emitting diodes and photovoltaics. For example, lead chalcogenide-based NCs as absorber layers in third-generation solar cells have demonstrated 9.9% power conversion efficiency under standard solar illumination.251 The use of semiconductor NCs as the absorber material could exploit their advanced physics. Semonin et al.254 successfully demonstrated the phenomenon of multiple exciton generation252,253 in PbSe NC solar cells by extracting multiple electrons per high-energy photon with the first solar cell to exhibit a region of the solar spectrum with >100% external quantum efficiency. The potential for growth in the field of Q


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should be expected in the future, especially in the area of nanotechnology, nanophotonics, and renewable energy applications. While the use of SPM instruments can greatly aid in highresolution measurement of properties, it can also be utilized in the development of samples for scientific exploration. The accuracy and resolution to which probes can image samples will likely be further exploited to mechanically manipulate nanostructures or induce precise interfaces by assembling the structure from the ground up with the SPM tip (i.e., nanomanufacturing). Furthermore, use of the probe tip to induce effects in materials is an interesting future avenue of ultrafast spectroscopic research with STM and AFM measurements. When metallic tips are used, plasmonic enhancement can lead to very local excitation as well as very high detection efficiency. At the same time, one could locally perturb the system with electrical current and use lock-in detection for single-molecule detection of the optical signals and even to enable multidimensional spectroscopies. Such a combination could lead to the development of nanoscale transient techniques as well as vibrational spectroscopies localized at a tip, as has already begun in some groups.258 The vast capability afforded with STM and AFM techniques will continue to play a key role in the measurements and characterization of new materials and their applications.

Jao van de Lagemaat received his Ph.D. in 1998 from the University of Utrecht, working on exciton dynamics and charge transport properties of large band gap semiconductors with Daniel Vanmaekelbergh and John Kelly. In 1998 he joined NREL as a postdoctoral researcher. He is currently principal scientist and center director at NREL, working on single-particle phenomena, plasmon/exciton coupling, exciton dissociation, and other effects in solar energy conversion systems based on quantum dots, molecular semiconductors, and carbon nanostructures.

AUTHOR INFORMATION Corresponding Authors

*E-mail [email protected] (S.U.N.). *E-mail [email protected] (J.M.L.). Notes

The authors declare no competing financial interest. Biographies Joseph M. Luther obtained B.Sc. degrees in electrical and computer engineering from North Carolina State University in 2001, a M.Sc. in electrical engineering from the University of Colorado in 2004, and a Ph.D. in physics from Colorado School of Mines in 2007. His graduate work was performed under the direction of Arthur Nozik at NREL. As a postdoctoral fellow, he studied under Paul Alivisatos at the University of California and Lawrence Berkeley National Laboratory. In 2009, he rejoined NREL as a senior research scientist. His research interests lie in fundamental properties of growth, electrical coupling, and optical properties of colloidal nanocrystals and quantum dots and applications in thin film and next-generation photovoltaics.

ACKNOWLEDGMENTS We thanks Ryan Crisp and Al Hicks for help with the figures and Mathew Beard for helpful discussions. S.U.N. and J.M.L acknowledge the Energy Frontier Research Centers program within the Center for Advanced Solar Photophysics supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences. J.V.D.L. acknowledges support from the solar photochemistry program. DOE funding was provided to NREL through contract DE-AC36-08G028308.

Sanjini U. Nanayakkara received her B.Sc. (chemistry) from the University of Colombo, Sri Lanka, and a M.Sc. in chemistry from the University of Nevada, Las Vegas in 2001. In 2006, she received her Ph.D. in chemistry from the Pennsylvania State University, specializing in atomic-scale measurements using UHV-LT scanning tunneling microscopy and spectroscopy under the guidance of Professor Paul S. Weiss. Later, she moved on to a postdoctoral position at the University of Pennsylvania with Professor Dawn A. Bonnell. She is currently a scientist at NREL, working on understanding material interfaces within heterostructured nanomaterials and interfaces in cross sections of functional solar cells.

ABBREVIATIONS SPM scanning probe microscopy R


Chemical Reviews STM STS AFM NC NR LDOS UHV I/V dI/dV DBTJ SAM CPD SKPM EFM c-AFM ac dc EDT MGB HOPG PLE CB VB TEM

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