Review pubs.acs.org/cm
X‑ray Photoelectron Spectroscopy: A Unique Tool To Determine the Internal Heterostructure of Nanoparticles D. D. Sarma,*,§,∥,† Pralay K. Santra,§,⊥ Sumanta Mukherjee,§ and Angshuman Nag§,‡ §
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Council of Scientific and Industrial ResearchNetwork of Institutes for Solar Energy (CSIR-NISE), New Delhi, India ⊥ Radiation Laboratory, Departments of Chemistry & Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ∥
ABSTRACT: We review the existing literature on the application of X-ray photoelectron spectroscopy in the study of nanocrystals. The unique ability of this technique to provide quantitative and reliable descriptions of highly complex internal structures of a variety of nanocrystals has been discussed in detail. We show that an accurate description of the nanocrystal internal structure is crucial and a prerequisite to understand many different properties, particularly optical properties, of such nanocrystal systems. We also discuss limitations and future outlook of this technique. KEYWORDS: quantum dot, heterostructured nanocrystal, internal structure, X-ray photoelectron spectroscopy, gradient alloy, luminescence
■
INTRODUCTION Research in the field of nanocrystals (NCs) has reached a high degree of maturity, where a wide range of NCs with varying degrees of complexity are first conceptualized, keeping in mind different possible applications, and then realized through diverse synthetic strategies. Specific applications often require designing of complex systems, such as core/shell, core/shell/ shell, and coupled dots. While the overall shape and size of such NCs can be easily determined by transmission electron microscopy, the internal structure of such complex entities is not, in general, amenable to such microscopy studies due to various complicating factors, such as an absence of sufficient contrast between different components, and/or interdiffusion across material interfaces. In this article, we discuss how a solution to this problem can be found in the use of an improbable tool, namely photoelectron spectroscopy without any spatial resolution in the ordinary sense, which is, therefore, an unexpected route to investigate what essentially appears to be a problem for microscopy. At first, we give a short introduction to NCs, followed by a discussion on the need to have complex structures and the range of complex structures achieved so far. We follow this with a discussion of the usual tools to investigate structures of such nanoparticles, pointing out their limitations. Then we discuss in detail applications of photoelectron spectroscopic techniques in investigating internal structures of a variety of complex NCs. We end the article with a discussion on the limitations of this technique and future outlook. While we have discussed primarily the case of semiconductor NCs due to the prevalence of reported results on such systems, it is important to note that the described methodology to utilize photoelectron spectra for © 2013 American Chemical Society
quantitative analysis of the internal structure is perfectly general and can be used in all other cases, including metal NCs.
■
SEMICONDUCTOR NANOCRYSTAL (QUANTUM DOT) When an electron is excited from the valence band to the conduction band of a semiconductor, the coulomb attraction between the conduction band electron and the valence band hole may form a bound pair, known as an exciton. A single electron−hole pair system is described by hydrogen-like wave functions, with a characteristic length scale, just as in the case of the hydrogen atom, defining the spatial extension of the wave function. This characteristic length scale is termed the excitonic Bohr radius. The binding energy for an electron−hole pair is a material dependent quantity, but it is generally orders of magnitude smaller than that of the hydrogen atom. Correspondingly, the excitonic Bohr radius is considerably larger than the hydrogenic Bohr radius (0.53 Å). When the dimension of a semiconductor sample becomes comparable to or smaller than the excitonic Bohr radius, the spread of the excitonic wave function is further constrained by the boundaries of the sample. Therefore, the electronic and optical properties respond sensitively to the change in the crystal size in this size regime. This phenomenon, known as the quantum size ef fect,1 provides an unprecedented tunability of physical and Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: November 4, 2012 Revised: January 18, 2013 Published: February 14, 2013 1222
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
ing CdSe/ZnS/CdSe NCs, where the higher band-gap material is embedded between lower gap ones.21 Properties of a heterostructured NC are often governed by the nature of the interface between the constituent NCs. However, such interfacial compositions are rather poorly characterized, mainly because of the lack of any easy and generic experimental technique.
chemical properties of semiconductor materials that is not accessible with conventional bulk materials. Semiconductor NCs, in which charge carriers are confined in all three dimensions, are called quantum dots (QDs).2 Typically, a few hundreds to thousands of atoms constitute a QD, which is just big enough to hold crystallinity, but exhibit discrete atomic-like energy levels, in contrast to the continuous energy band of a bulk semiconductor. If the radius of a spherical QD is R, the band gap varies roughly as 1/Rα (α > 1),3 such that the smaller the radius, the larger is the band gap. Additionally, the excitonic oscillator strength, proportional to the absorption coefficient, increases with a decrease in QD size, because of the enhanced spatial confinement of electron and hole with a decreasing size of the sample.1c,4 All these interesting modulations of intrinsic properties with QD size open up an enormous scope of novel applications, including light emitting devices, photovoltaics, and sensors.5
■
CHARACTERIZATION OF HETEROSTRUCTURED NANOCRYSTALS Basic characterization techniques such as UV−visible absorption spectroscopy, photoluminescence (PL) spectroscopy, powder X-ray diffraction (XRD), and transmission electron microscopy (TEM) provide much useful information about a core/shell structure. Red-shifts in both absorption and emission along with an enhanced quantum yield are observed upon shell growth for a type-I core/shell NC, and such a red-shift is often used to ascertain the formation of a core/shell structure in qualitative terms. Type-II core/shell NCs, in contrast, show large stokes shift between absorption and emission. In order to grow the shell material epitaxially on the core, most of the time it is required that the core and the shell materials have the same structure and similar lattice parameters. This causes the XRD patterns of the two components to appear close together. Given the large width of the XRD peak of small NCs, it gives rise to just essentially a single pattern, with peak positions in-between those of the individual components with slightly larger widths, but the peak positions do not follow the Vegard’s law of alloy formation, if an average composition is determined based on the amount of core and shell materials present in the core/shell NCs. This is further confounded by the fact that the shell material in real systems is often strained in order to grow epitaxially on a lattice mismatched core. Thus, XRD cannot be used to determine the details of a core/shell NC; it cannot even distinguish between a core/shell structure and alloyed composition for the NC. If the core and shell materials have sufficient contrast in TEM and there is a sharp interface separating the two materials, the core/shell structure can be revealed directly by a high resolution transmission electron microscopy, as has been illustrated for a few systems, such as Au NC embedded in a SiO2 NC.22 However, very often one encounters situations where either the contrast is not sufficient to distinguish different components of a complex NC or the interface separating the various components is not atomically sharp. In such cases, one of the usual ways to estimate the average core size and shell thickness has been to measure the size distribution of the core material before growing the shell material and then again after the shell growth. This approach relies on the validity of the assumption that the preformed core size does not change when dispersed in the reaction medium for the growth of the shell and that the interface is a sharp one without any interdiffusion of the shell material into the core region and vice versa. Unfortunately, these are often invalid assumptions, as can be seen in later sections of this article. More advanced microscopic techniques such as scanning transmission electron microscopy (STEM) coupled with either electron energy loss spectroscopy (EELS) or energy-dispersive X-ray spectroscopy (EDX) can provide more useful information about core/shell interfaces.23 Spectroscopic techniques such as Raman spectroscopy have been used to study such core/shell interfaces.24 High frequency electron paramagnetic resonance spectroscopy (HF-EPR) has
■
HETEROSTRUCTURED NANOCRYSTALS Colloidal NCs can be treated as precursors for further reaction, similar to a chemical reagent. For example, synthesized NCs have often been used as a core to grow a shell of another material on its top, forming onion-like core/shell NCs, for example, CdSe/CdS NCs.6 Different functionalities can be achieved by proper choices of core and shell materials, for example, CdSe/ZnS (a semiconductor in a semiconductor),6a Au/PbS (a metal in a semiconductor),7 and FePt/PbS (a magnet in a semiconductor),8 etc. There are also possibilities of binary NCs such as CdSe exhibiting a Cd-rich (Cd1+mSe) surface in contrast to its stoichiometric (CdSe) core,9 and inhomogeneously doped NCs with dopant-rich NC surfaces,10 in the category of heterostructured NCs. Not only core/shell but also many other types of structures, including coupled QDs11 and NC-oligomers,12 can be achieved. Many of these NCs have interesting optical properties.13 Semiconductor core/shell NCs can be categorizes into different types in terms of band alignment at the core/shell interface. In a type-I structure (e.g., CdSe/ZnS NCs), both electrons and holes are confined to the core, leading to bright and stable luminescence.6,14 Similarly, in an inverted type-I structure (e.g., ZnSe/CdSe NCs), both the charge carriers reside in the shell.15 One can get a quasi type-II configuration (e.g., CdSe/CdS NCs with a thick shell), where one of the charge carriers (electron or hole) is delocalized throughout the NC, but the other one is confined in the core only.16 In the type-II core/shell NC,17 the photogenerated exciton dissociates easily, and either the electron or the hole resides in the core and the other one (the hole or the electron) in the shell. Such excitonic dissociation is required for photovoltaic applications. One can design more complex core/shell systems comprising multiple shells, such as CdSe/ZnSe/ZnS core/shell/shell NCs.18 Introduction of a ZnSe intermediate shell, with its lattice parameters in between those of CdSe and ZnS, reduces the interfacial defects; thus, CdSe/ZnSe/ZnS NCs are even brighter than CdSe/ZnS NCs without compromising the stability. Interfacial lattice strain can be tuned continuously by making a gradient interface, such as CdSe/CdSeS/CdS core/ gradient-shell/shell NCs.19 Quantum-dot−quantum-well (QDQW) NCs (e.g., CdS/HgS/CdS or ZnS/CdSe/ZnS) constitute another interesting class of materials, where a lower band-gap material is embedded between a higher bandgap core and an outer shell, with type-I band alignment.20 An inverse QDQW structure has also been achieved by synthesiz1223
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
been recently used to extract structural information at the core/ shell interface by incorporating an EPR active dopant such as Mn2+ in the NC.25 HF-EPR can probe the local environment around a Mn2+ ion at the interface and, thus, showed the formation of a heterogeneous alloy at the CdSe(core)/ ZnS(arm) tetrapod interface. However, since doping is difficult in NCs 26 and the possibility of additional interface reconstruction as an effect of dopant inclusion27 cannot be excluded, this approach, though an interesting one, cannot be easily generalized. Radiative PL lifetime is inversely related to the spatial overlap of electron and hole wave functions and can sometimes provide a qualitative insight. For example, CdSe(core)/CdS(shell) NCs exhibit shorter PL lifetime than CdSeS(gradient-core)/CdS(shell) NCs,19 or conversion of ZnSe/CdSe inverted type-I to type-II with increasing shell thickness significantly increases the radiative lifetime.28 Thus, such results can at times be used as qualitative indicators of the internal structure; however, the very indirect nature of the relationship between such experimental probes and the internal structure of NCs makes even qualitative inferences somewhat uncertain. Elemental analysis using inductively coupled plasma atomic emission spectroscopy (ICP-AES) or other techniques can provide information about the internal structure within some limitations. For example, it has been observed29 that when Se and S precursors are added simultaneously to a solution of Cd precursor, the Se/(Se + S) ratio in CdSe1−xSx NCs decreases with the growth time. This suggests the formation of a CdSe rich core and CdS rich shell. Conversely, one may also perform a kinetic study, monitoring the change in concentrations of precursors, tri-octyl phosphine-selenium (TOPSe) and tri-octyl phosphine-sulfur (TOPS), using 31P NMR (nuclear magnetic resonance) spectroscopy.30 Faster consumption of one precursor (TOPSe) compared to the other (TOPS) demonstrates the higher reactivity of TOPSe with the cationic precursor (Cd2+ in this case), thus forming a CdSe-rich core and a CdS-rich shell instead of a homogeneous CdSe1−xSx alloy. While such kinetic studies can indicate selective formation of different alloyed and heterostructured NCs, all such information describes the internal structure of complex NCs only in qualitative terms and, therefore, is not very useful for an indepth understanding of NCs prepared or to model them for further theoretical studies. In the following sections, we will discuss how X-ray photoelectron spectroscopy (XPS) can be used in determining the internal structure of NCs in a quantitative manner that is inaccessible to all other techniques.
Figure 1. (a) Schematic diagram showing the photoelectric effect. Upon shining light of energy hν, photoelectrons are ejected from the material. (b) Energy level diagram of a metal showing electrons being photoejected from a core level to a state above the vacuum level with a finite kinetic energy.
energies. With all states below the Fermi level being occupied and those above it being unoccupied, the Fermi level provides a convenient energy reference to measure the BE’s of all electrons in the system. The vacuum level separates the bound and the unbound states of the system, thereby providing a natural reference to measure the kinetic energy of the photoejected electron. On absorbing a photon of sufficient energy, an electron from any of the occupied levels is excited above the vacuum level, in other words, photoionized and then detected. The intensity of such photoemitted electrons (or photoelectrons, in short) as a function of its kinetic energy constitutes the photoelectron spectrum. It is easy to see from Figure 1b that this kinetic energy, KE, is given by the energy conservation as
KE = hν − (BE + ϕ)
(1)
where BE is the binding energy of the electron prior to the photoionization process in a specific core level and ϕ, shown in Figure 1b, is the work function. It is important to note here that the BE of the electron does not depend only on the element and the core level it comes from, but also on the chemical state of the element in the given compound. This dependence of BE on the chemical state of the element for any given core level is known as the chemical shift. The chemical shift plays an important role in determining the chemical or the valence or even the effective charge state of the element in the given compound. The intensity of the photoelectron signal is obviously proportional to the photoionization cross section of the specific level from which the photoionization takes place and is a function of the photon energy for any given level. The photoelectron signal is also a function of the path length that the photoionized electron has to traverse through the sample in order to reach the surface and, therefore, be ejected into the vacuum. This dependency on the path length of the electron in the material arises from the fact that the photoelectron has a finite probability to suffer collisions while traveling through the sample. It can suffer both elastic and inelastic collisions. In the case of inelastic collisions, the photoelectron loses its kinetic energy, thereby not contributing to the photoelectron signal and appearing as the broad background of the XPS spectrum. The probability of the inelastic scattering process can be conveniently described in terms of an average length, λ, between two such inelastic processes, defining a mean free path for the inelastic scattering. Then, it is easy to see that the
■
PRINCIPLE OF X-RAY PHOTOELECTRON SPECTROSCOPY AND ITS BASIC APPLICATION IN THE STUDY OF NANOCRYSTAL SYETEMS X-ray photoelectron spectroscopy (XPS), also known as X-ray photoemission spectroscopy, is a very powerful technique commonly used to elucidate electronic structure and elemental compositions, including the oxidation state of an element in a given material. In this technique, monochromatic photons with energy hν are incident on a sample, causing emission of photoelectrons from various energy levels of every elements present in the material, as shown schematically in Figure 1. A schematic of a typical energy level diagram for a metal is shown in Figure 1b with essentially localized core levels appearing at higher binding energies (BE’s), while more extended valence and conduction band states appear at relatively shallow 1224
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
Figure 2. (a) Photoelectrons generated at different distances, xA and xB, from the surface of the material show different intensities in the photoemission signal, with the intensity dropping off exponentially with x. (b) Schematic illustration of spectral dependence on photon energy for two NCs with different internal structures. One NC is chosen to represent a homogeneous alloy system of two elements (say X, indicated in blue color, and Y, indicated in red color), while the other one has a graded alloy structure with the surface rich in X and the concentration of Y progressively increasing deeper down. The graphs on the right side show schematically changes in the relative intensities with increasing photon energy. Horizontal slices from the top of the NCs are supposed to represent changing mean escape depth of the photoelectrons, λ, with a changing photon energy, where hν1 < hν2 < hν3, giving rise to λ1 < λ2 < λ3.
geometric structures of a variety of nanoscaled materials. One of the early investigations of metallic NCs was primarily focused on the electronic structure elucidation with a probable metal−insulator transition as a function of the cluster size.32 In the context of luminescent porous silicon nanostructures, photoemission spectra in conjunction with stepwise argon-ion etching of the surface layer were used to determine33 different compositions found at various depths from the surface. However, it is very difficult to quantify such results due to preferential etching of certain elements compared to others. In yet another study,34 a huge excess of I 3d signal compared to Pb 4f signal from PbI2 NCs was used to suggest stabilization of these NC in solution due to the presence of excess I− ions surrounding them, akin to a repulsive electrostatic double layer. XPS has also been used to study the environment dependent crystal structure tuning of CdSe NCs of different sizes.9a While these studies essentially talk about heterogeneous systems in describing the entirety of these nanomaterials, similar to a core/shell structure, an explicitly designed CdSe/ ZnSe core/shell structure was also investigate by XPS,35 establishing the ability of XPS to distinguish between a core/ shell structure and a homogeneous alloy structure. XPS study of TOPO passivated CdSe NCs of various diameters revealed the surface bonding of TOPO to the surface Cd, leaving surface Se essentially unpassivated; this was supported by observation of preferential oxidation of Se on prolonged exposure to air.36 XPS analysis of the CdSe/CdS core/shell was found to be consistent with the targeted core/shell structure and also with TEM results.6b In another study, XPS along with TEM and XRD was used to elucidate the formation of alloy InP/ZnS NCs.37 In one of the earliest uses of synchrotron radiation to carry out XPS studies, ref 38 identified several S-species in a CdS NC system stabilized by a thiol capping agent. As in the case of ref 36, this study38 was primarily focused on the NC/ capping agent interface. In fact, the existence of different S species in a CdS NC capped with thiol was published a little earlier39 using two laboratory sources for the X-ray, namely Al Kα and Mg Kα. However, ref 39 took the first step in analyzing photoelectron spectral intensity ratios at multiple photon
detected differential intensity, dI, of the photoelectron signal corresponding to an average photoionization signal I0 per unit volume, originating from a differential volume dv, and traveling a distance, x, within the bulk before emerging from the surface is given by ⎛ x⎞ dI = I0 exp⎜ − ⎟ dv ⎝ λ⎠
(2)
where I0 itself is the product of the photoionization cross section of the specific electronic level, the number densities of the element in that volume element, and the number of photons. Since λ is usually much smaller than the typical attenuation length of high energy photons used in such experiments, it is customary to assume uniform photon flux throughout the relevant volume of the sample. The mean free path depends on the kinetic energy of the photoelectron, and it has been found to vary approximately as
λ=
1 KE 2
(3)
when KE is expressed in electronvolts (eV) and λ in angstroms (Å). This empirical relation is found to be valid for a wide range of the kinetic energy above ∼150 eV. There are other ways to determine the mean free path of the photoelectron, for example, by using the TPP and TPP-2 formulas.31 For a specific core level, the BE is constant and the KE of the photoelectron of these specific core levels can be changed by varying the photon energy (see eq 1). With the availability of synchrotron sources, it is possible to tune the photon energy over a wide range, thereby tuning KE of the electrons for a specific level, leading to a continuously tunable range of λ’s. In general, the λ can be easily tuned in the range of 0.5−4 nm. This being on the order of typical dimensions of nanoparticles that one is interested in, XPS provides a unique and ideal tool to determine the internal structures of nanoparticles, as will be shown in the next section. We point out here that the photoemission technique has been used sporadically to obtain many important, though at times qualitative, insights concerning the electronic and 1225
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
increasing depth with the increasing photon energy. Since the composition of the homogeneous alloy does not change with the probing depth, the intensity ratio of spectral features of elements X and Y will remain unchanged for this case, whereas the intensity ratio of X and Y will change systematically in the case of the graded alloy structure due to the dependence of I0’s on the concentration of the corresponding element, as shown in the right panel of Figure 2b, thereby providing information on the internal structure. In reality, the intensity ratio IX/IY will change also in the case of the homogeneous alloy, since the matrix element or the transition probability of X and Y, say σX and σY, contained in the corresponding I0’s, depends on the photon energy, hν. Thus, one needs to normalize the spectral intensity, IX and IY, with corresponding ionization probabilities σX and σY, while comparing the spectral intensity ratio as a function of the photon energy. So the quantity of interest here is (IXσY/IYσX) as a function of hν. From the illustrative example in Figure 2b, it is clear that a core/shell NC with a thick X shell, representing a discontinuous change in the composition, will show an even more rapid change in the spectral ratio with the photon energy compared to the graded alloy with a smoothly changing alloy composition. It is then clear that one may obtain detailed information on the elemental composition as a function of depth, defining the internal structure of any given NC by recording photoelectron spectra of its constituent elements as a function of the photon energy for several values of hν. It is necessary to keep in mind that the illustration in Figure 2b is only a schematic one. Specifically, the probing depth in XPS does not have a sharp cutoff, as would be suggested by flat planes, shown in Figure 2b. Instead there is an exponentially decaying contribution from deeper down, as shown by eq 4. At any given photon energy, the total intensities of the spectral features corresponding to elements X and Y are given by the integral of the differential contribution from each depth, typically in the form of I = ∫ ∫ ∫ I0(x) exp[−x/(λ cos α)] dv) over appropriate limits. Even in this more realistic description, the basic features discussed above in terms of sharp cut offs on the probing depths schematically shown in Figure 2b are retained quite accurately. The only difference between the realistic and the schematic descriptions of Figure 2b is in terms of quantitative values. It is interesting to contrast this method to the more conventional application of microscopic techniques to probe structures. Microscopic techniques, for example, TEM or EELS coupled with TEM, make use of a lateral spatial resolution to map out elements and structures in a direction perpendicular to the probing beam of electrons. In contrast, XPS probes the composition of the material, so to speak slice by slice, along the direction of the photoejected beam of electrons, replacing the lateral spatial resolution of microscopy with a vertical spatial resolution provided by the differential increase in the bulk compositional sensitivity tuned by an increasing electron kinetic energy achieved by a systematic change in the photon energy (see eq 1). If the NC has a spherical symmetry, it is more convenient to formulate the discussion in terms of spherical polar coordinates, as shown in Figure 3. If the photoelectron originates at a point (r, θ, ϕ) of a NC of radius, R, the depth of the origin of the photoelectron from the surface, x, in the vertical direction can be expressed in spherical coordinate as
energies in quantitative terms and provided the foundation to determine the internal structure of complex NC systems from such studies. We deal with such approaches in detail in the next section, wherein the quantitative analysis allows one to obtain considerably more detailed knowledge of the internal structure of any given NC system.
■
DETERMINATION OF THE INTERNAL STRUCTURE OF NANOCRYSTAL SYSTEMS The basic idea involved in probing the internal structure of nanoparticles using XPS is quite simple. Let us imagine two equal volume elements “A” and “B” at two different depths xA and xB from the surface, which contribute spectral intensities dIA and dIB, respectively, as shown schematically in Figure 2a. With no loss in generality, we assume that xB > xA, implying the volume element “A” to be closer to the surface compared to the volume element “B”. From eq 2, it is clear that the intensity ratio, dIA/dIB, is given by IA dIA = 0B exp[(x B − xA)/λ cos α] dIB I0
(4)
provided λ is the same for both the signals being considered. We note that (x/cos α) is the distance traveled by the electron emanating from a depth x and moving at an angle α to the surface normal, defined by the electron detection direction. Generalizing to the case of two distinct mean free paths, λA and λB, is quite straightforward. Increasing the photon energy systematically increases the λ, as shown in eq 3. Equation 4 shows that an increase in λ leads to a systematic increase in the relative contribution of the signal dIB from the volume element “B”, or in other words from deeper into the material. Thus, by increasing the photon energy, we probe progressively deeper into the sample, analyzing the elemental composition of the material as a function of the depth from the surface; this is exactly the information that one requires to reveal the internal structure of any NC. This is schematically illustrated with a hypothetical example in Figure 2b by contrasting the spectral dependence on photon energy for two different NCs with the same external size. For simplicity, we assume these NCs to have flat top surfaces, such that any constant depth from the surface is defined by a flat plane parallel to the surface, as shown in Figure 2b with three planes at three different depths. In general, the geometry of the plane at a constant distance from the surface will depend on the shape of the NC; later in the text, we shall discuss the case of spherical NCs in detail. Among the two cases schematically shown in Figure 2b, one NC is chosen to represent a homogeneous alloy system of two elements (say X and Y, indicated by blue and red, respectively), while the other one has a graded alloy structure with the surface reach in X and the concentration of Y progressively increasing deeper down. For low photon energy, say hν1, photoelectron spectra of both systems will be primarily contributed by a thin slice of the sample due to a small value of the corresponding mean free path of the electron, say λ, providing representative spectra for the average composition approximately over the slice of the sample shown, in spite of the photons illuminating throughout the NC bulk uniformly, as previously mentioned. With an increasing photon energy, hν2 and hν3 (hν3 > hν2 > hν1), corresponding λ’s, λ2, and λ3, will increase (λ3 > λ2 > λ1). Consequently, the photoemission will probe deeper into both samples and the corresponding spectra will average over an
x(r , θ ) = 1226
R2 − r 2 sin 2 θ − r cos θ
(5)
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
XPS with two different photon energies (1486.6 and 1253.4 eV). The XPS spectra of the S 2p core level region collected with Al Kα radiation (hν = 1486.6 eV) are shown in Figure 4 for bulk CdS along with two differently sized CdS NC samples. The S 2p spectrum of bulk CdS (Figure 4a) exhibits a doublet arising from the spin/orbit split 2p3/2 and 2p1/2 spectral features at 162.3 and 163.4 eV. This can be described with two Gaussians having the same full width at half maxima (fwhm) of 0.44 eV and separated by a spin orbit split of 1.1 eV, as shown by the calculated fit (thin line), which is the sum of the two Gaussians with an intensity ratio of 1:2, consistent with the degeneracy ratio of the two 2p components with J = 1/2 and 3 /2, respectively. The spectral features of S 2p from bulk CdS are, therefore, consistent with a single sulfur species being present in the bulk CdS, as expected. In contrast, S 2p spectra from both CdS nanoparticles (Figure 4b and c) show multiple features that are impossible to describe in terms of a single set of two Gaussians representing the spin−orbit splitting of a single S species. Three sets of doublet Gaussian functions (labeled 1, 2, and 3) with the 2p3/2 signal centered at 161.8, 162.9, and 163.9 eV, representing three distinct S species, were required for a proper description of the experimental spectrum; the same was found to be true for the S 2s spectrum as well. While the corresponding spectra recorded with Mg Kα radiation with hν = 1253.4 eV also required the same three component spectra for a reliable description, it was found that the relative intensities of these three components change between the spectra recorded with Al Kα radiation and those recorded with Mg Kα radiation. For example, it was found that the relative intensity of component 3 compared to that of component 1 was enhanced significantly when recorded with Mg Kα radiation in comparison to the relative intensity observed in the spectrum recorded with Al Kα radiation. Since Mg Kα radiation has a lower photon energy compared to Al Kα radiation, thereby rendering spectra more surface sensitive, the changes in the relative intensities allow one to ascribe component 3 to arising from a S species in the outermost layer of the NC, namely from the thiol group of the capping agent used to stabilize the NC. Similar considerations allowed
Figure 3. Representation of the origin of the photoelectron at a depth, x, from the surface of a spherical NC of radius R in spherical polar coordinate with the origin at the center of the NC.
The total photoelectron intensity can be expressed in spherical polar coordinates by integrating over the entire volume of the NC as39,40 I = I0
⎛
⎞
∫r ∫θ ∫ϕ exp⎜⎝− x(rλ, θ) ⎟⎠r 2 sin θ dr dθ dϕ
= 2πI0
∫r ∫θ
⎛ x(r , θ ) ⎞ 2 ⎟r sin θ dr dθ exp⎜ − ⎝ λ ⎠
(6)
Since x does not depend on ϕ, the integration of ϕ is trivial, giving a factor of 2π in every case of spherical NCs. The actual measured intensity at the spectrometer is proportional to I, with the proportionality constant being dependent on various instrumental factors, such as the instrument geometry of the electron lens and the analyzer, the detector efficiency, and the solid angle over which photoelectrons are collected by the lens. Equations 5 and 6 can be easily modified to suit different shaped NCs; for example, a cylindrical coordinate system may be used to describe nanorods and nanowires. The first ever quantitative analysis of photoelectron spectra from an NC system was presented in ref 39, soon followed by ref 40, establishing the efficacy of XPS in quantitatively establishing the internal structure, comprising the composition and sizes of various chemically distinct parts of the NC. In ref 39, 1-thioglycerol capped CdS nanoparticles of two different sizes, namely 2.5 and 4.5 nm diameter, were investigated using
Figure 4. XPS spectrum of S 2p core level for (A) bulk CdS, (B) 2.5 nm CdS NCs, and (C) 4.5 nm CdS NCs. The spectra were collected using Al Kα as photon source. The dots represent the experimental data, the solid line indicates the total fit, and dotted lines indicate different S components: (1) from the core region; (2) from the surface; (3) from the capping agent. The triangle represents the error in fitting. (Reprinted with permission from ref 39. Copyright 1999 American Physical Society.) 1227
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
substantial uncertainty in determining the parameter values, but use of two photon energies provides us with four unique intensity ratios, namely ICap/ICore and ISur/ICore for each photon energy, and requires five parameters, λ1 and λ2 for the two photon energies and R0, R1, and R2. In refs 39 and 40, the authors used two different photon energies, Al Kα at 1486.6 eV and Mg Kα at 1253.4, and two core level spectra, namely 2p and 2s, obtaining eight intensity ratios for the analysis. Fixing values of relevant λ’s for the 2p and 2s levels of S using Al Kα and Mg Kα radiations according to eq 3, R0, R1, and R2 were so chosen that one unique set of values provides the best description of all the eight experimentally determined ratios in the least-squared-error sense, thereby completely determining the internal structure of the nanoparticle within the model core/shell/shell description implicit in Figure 5. It is clear from the above discussion that it is advantageous to have experimental spectra recorded with several photon energies, thereby enhancing the reliability of the data analysis. This has led all subsequent applications of this approach to determine the internal structure of NCs to use synchrotron radiation, with its continuously tunable photon energy.19,20b,38,41 It is also to be noted here that the above approach is quite general and can be extended easily to many other geometries. For example, if the NCs would have cylindrical, platelet, or rodlike structures, it would be straightforward to express the intensities of various components with equations such as in eqs 7−9 in cylindrical polar coordinates instead of spherical polar coordinates. Similar analysis can also involve spectra of different elements, such as those of S and Se, from an NC system (e.g., CdS/Se NCs), allowing one to map out the compositional distributions of these elements in the NC. Additional parameters that enter the analysis of spectral intensities of different elements are the photon energy dependent matrix elements of different spectral components. This is not the case while analyzing a single spectral feature, namely S 2p, since the intensity ratios of the same spectral feature at different photon energies do not depend on the matrix element, as already discussed above. Since reliable estimates of photoionization cross section of different elements at various photon energies are already available in the literature,42 extension of the analysis to involve different spectral features19,20b,41 has become possible. Following such arguments, ref 41a analyzed ZnS passivated InP NCs capped with TOP/TOPO. The analysis was carried out by fixing the core radius to what was observed for the InP core prior to the shell formation by TEM, thereby obtaining the ZnS shell and the capping thickness. However, the validity of this approach is dependent on the validity of the two assumptions implicit in the analysis, namely the existence of a sharp interface between the core and the shell and the constancy of the core size during the reaction to form the shell. As we shall discuss a little later, these assumptions have been shown to be invalid in certain sulfide/selenide systems, though they may be valid in the case of InP/ZnS. In a subsequent publication,41b these authors applied essentially the same approach to more complex systems, such as CdS/HgS/CdS quantum dot/quantum well structure as well as CdS/HgS/ CdS/HgS/CdS double quantum well structure, concluding that a reliable analysis is possible only for the comparatively simpler CdS/HgS/CdS structure, provided the innermost core and outermost radius are fixed according to TEM observations. This implies that such analysis aims to determine the HgS layer thickness as the only unknown parameter. While it is
the authors to assign component 2 to S atoms at the surface of the NCs and component 1 to S atoms in the NCs that are fully coordinated. On the basis of these spectral decompositions, it is then possible to model a single nanoparticle as a core/shell/shell structure having an inner CdS core, a surface CdS layer, and finally, the capping layer, as shown schematically in Figure 5,
Figure 5. Schematic model of CdS nanoparticles as derived from XPS analysis showing a CdS core region till R0, followed by a thin shell of surface CdS layer of thickness (R1 − R0). Surface S atoms exhibit a different core level binding energy due to what is known as surface core level shift. Finally, there is a protective layer of capping agent (thiol) of thickness (R2 − R1) on the top. (Adapted with permission from ref 39. Copyright 1999 American Physical Society.)
where R0 is the radius of the core, R1 is the radius including the surface layer, and R2 is the radius including the capping layer. With the help of eq 6, photoemission signal intensities from these different layers, apart from a constant multiplicative factor, can be expressed as Icore = I0CdS ISur = I0Sur
R0
∫0 ∫0 R1
∫R ∫0
π
0
ICap = I0Cap
R2
∫R ∫0 1
π
⎛ −x(r , θ ) ⎞ 2 ⎟r sin θ dθ dr exp⎜ ⎝ ⎠ λ
⎛ −x(r , θ ) ⎞ 2 ⎟r sin θ dθ dr exp⎜ ⎝ ⎠ λ π
⎛ −x(r , θ ) ⎞ 2 ⎟r sin θ dθ dr exp⎜ ⎝ ⎠ λ
(7)
(8)
(9)
where ICore, ISur, and ICap represent S core level X-ray photoelectron intensities from the core region, surface region, and capping agents, respectively. Assuming the same photoionization cross section for S in the core, the surface region, and the capping region at any given photon energy, it is only the Sur Cap number densities of S atoms that distinguish ICdS 0 , I0 , and I0 . The ratios of these quantities can be estimated from the respective molecular weights of the capping agent and CdS. Thus, the ratios of intensities from these three S species, namely ICap/ICore and ISur/ICore, are independent of photoionization cross sections but functions of the various dimensions, R0, R1, and R2 defining the internal structure of the NC and the mean free path, λ, relevant for specific photon energies used to record the spectrum. One important point to be noted here is that the specific value of λ to be used for the analysis depends on the photon energy, hν, used to record the spectrum, while the other three parameters defining the intensity ratios, namely R0, R1, and R2 naturally do not depend on hν. Thus, use of several photon energies is desirable to obtain reliable estimates of R0, R1, and R2. For example, the use of one photon energy provides us with only two unique intensity ratios, for example ICap/ICore and ISur/ICore, but requires four parameters (λ, R0, R1, and R2) for the analysis, leading to 1228
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
Figure 6. Schematic illustration of how to arrive at a unique solution for the internal structure from several possibilities by combining XPS measurements at different photon energies (in this case, hν1 and hν2) with other independent measurements, such as TEM and ICP-AES, providing the overall size of the NC and the average composition, respectively. The top two spectra are simulated spectra at two different photon energies for a pure core shell system of selenide core of 2.2 nm radius and sulfide shell of 0.8 nm radius. The table shows how the stepwise use of independent experimental results systematically leads to a unique solution. The numbers R1, R2, and R3 are radius in nanometers of different compatible structures (A−E).
size of the NC; (B) a graded alloy with linearly varying Se/S composition in a NC of radius 2.5 nm; (C) a different core shell alloy with a core Se radius of 3.6 nm with a sulfide thickness of 0.9 nm; (D) an alloy core of 3.2 nm with a pure sulfide shell of 0.6 nm; and (E) a pure selenide core/graded alloy shell/pure sulfide shell NC with a 4.55 nm core radius, 1.5 nm thick graded alloy with a 0.8 nm thick sulfide overcoating. This is the essential nonuniqueness problem encountered in the analysis of XPS data of NC systems. This nonuniqueness problem can be tackled in several ways. For example, the use of a large number of independent spectra in the analysis invariably improves the chances of arriving at a relatively unique solution. For example, the use of the second photon energy, hν2, at once eliminates the possibility of finding a homogeneous alloy NC as a solution to the problem in the example shown in Figure 6. In the original reports on CdS and ZnS,39,40 the interpretation was validated by comparing the overall size of the NC obtained from the analysis of the photoelectron spectral intensity with those obtained from TEM studies, besides using four independent spectra (S 2p and S 2s core levels, each recorded with two different photon energies) for each system. Since TEM provides a very reliable estimate of the overall size of the NC system, an agreement of the size determined from XPS with the one from TEM is considered to be a rigorous enough validation. The same information concerning the size of the NC from TEM may also be used in an equivalent, but alternate way, where the outer diameter (e.g. R2 in eq 9) is fixed to the value obtained from TEM, instead being treated as a parameter, thereby reducing the parameters to be obtained from the XPS analysis by one. Such a reduction in the number of parameters invariably reduces the chances of nonunique solutions.
reasonable to constrain the outermost radius, the overall particle size, from TEM observations, it is less obvious that the inner core size will not change in subsequent chemical processing to grow two more shells, as already pointed out. In the above discussion, it is evident that the intensity variation of X-ray photoelectron spectra of different component elements in a NC with the photon energy is a very powerful tool to qualitatively determine the formation of a homogeneous or an inhomogeneous composition of the NC system; however, a quantitative analysis requires several assumptions and approximations. For example, in all examples discussed so far, not only the shape of the NCs was assumed to be spherical, but also the compositional variation was assumed to have the same spherical symmetry. While these may indeed be reasonable assumptions, these also introduce a certain amount of uncertainties in the determination of the internal structure, requiring further validation of the deduced results from independent experiments. This becomes even more necessary if the intensity variations with the photon energy are found to be consistent with distinctly different descriptions of the internal structure of the NC system within the experimental error, therefore, lacking a uniqueness of the solution. This is illustrated schematically in Figure 6, where we show the simulated spectra of S 2p and Se 3p from a hypothetical sulfide/selenide NC with a 2.2 nm radius selenide core and a 0.8 nm thick sulfide shell. It turns out that essentially the same spectral intensity ratio of S 2p and Se 3p will be observed for a wide range of other NC systems, including homogeneous alloy, graded alloys, and core/shell structures of various thicknesses, as illustrated in Figure 6. For example, the spectrum recorded at one photon energy, hν1, is also compatible with (A) homogeneous alloy of anion composition (Se0.17S0.83) for any 1229
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
properties.19 Specifically, it has been pointed out19 that such a graded structure avoids any interface defect formation that tends to be present in systems with sharp interfaces due to the lattice mismatch between the core and the shell material; such interface defect states, if present; provide fast (less than nanoseconds) nonradiative decay pathways, thereby significantly reducing the PL quantum efficiency. Additionally, it has been shown that the graded alloy formation at the interface leads to an interesting differential collapse of the electron and the hole wave functions toward the center of the NC compared to the core/shell structure. This helps in further improving the PL efficiency by reducing the overlap of the valence band maximum (or the HOMO) and the conduction band minimum (or the LUMO) with surface states. Interestingly, it was found that the collapse of the HOMO wave function is more pronounced compared to that of the LUMO wave function in such graded alloy systems, whereas these HOMO and LUMO wave functions are similarly extended in the case of a core/shell system. As a consequence, the PL lifetime of the graded system is expected to be larger due to the reduced wave function overlap between the HOMO and the LUMO. This conclusion was in fact supported by lifetime measured in corresponding systems, establishing a radiative lifetime in the graded alloy system about twice that of the core/shell system. In recent times, there have been more reports of investigating the internal structure of complex NC systems using variable photon energies. One such study investigated an InP/ZnS NC system, synthesized in two different ways, namely a single step reaction with all reactants simultaneously being present and a two-step process where InP was first synthesized, followed by a second step in synthesis to grow a shell of ZnS on top. Indium core level spectra were analyzed in terms of component spectra arising from In in different chemical environments, following closely the analysis of S spectra in refs 39 and 40. This investigation41e points to the formation of a homogeneous alloy in the single step synthesis and a graded alloy structure in the case of the two step synthesis. The graded alloy formation in the two step synthesis is akin to the graded alloy formation in the two-step synthesis of the QDQW structure of ZnS/CdSe/ ZnS20b at the inner ZnS/CdSe interface (see Figure 7). We note here that all two-step syntheses do not necessarily lead to such graded alloy formation; for example, the outer interface between CdSe and ZnS in the QDQW structure is found to be sharp with no evidence of any significant intermixing. Additionally when we grow a CdS layer on top of a CdSe core19 in a two-step synthesis, a sharp interface is formed. Similarly, a single step synthesis does not necessarily ensure a homogeneous alloy formation; for example, CdSeS19 NC has been shown to form an inhomogeneous, graded alloy structure, unlike in the case of the InP/ZnS NC system, where a homogeneous alloy formation is reported in ref 41e. While these observations can be rationalized in terms of the reactivities of different species present in the reaction mixture and solubility products, such counterintuitive examples help us to appreciate the diversity encountered in the internal structure of nanoparticles under different synthetic conditions and the unique efficacy of XPS in unraveling such internal structures. Such detailed and quantitative understanding of the internal structure, in turn, helps us to understand the origin of spectacular electronic and optical properties of these NC systems. We have already discussed the remarkable example of the CdSeS system with its very high PL quantum efficiency.19 There are other examples too where XPS has helped one to
A schematic representation of this procedure is shown in Figure 6, where the use of two photon energies and the TEM derived size is shown to eliminate any simple graded alloy structure from possible solutions of the internal structure of the given NC. This methodology was followed in refs 41a−d. However, analysis in some of these studies41a−c also fixed the preformed core sizes from TEM studies, performed prior to putting a shell on the core; in general, this is not a reliable method, in view of the possibility of a partial dissolution of the core or the formation of an alloy during the subsequent steps in the synthesis. At this point, we note that a quantitative description of a complex NC fixes the overall composition. Thus, a better route to validating or constraining the XPS analysis will be to require the overall composition determined from the XPS analysis to be consistent with the actual composition of the NC that can be easily fixed by independent experiments. This methodology is illustrated in the lowest entry in Figure 6, which shows that the use of spectra recorded at two photon energies in conjunction with the overall size of the NC from TEM and the S/Se composition ratio from ICP/AES experiments determine the internal structure of the given NC uniquely, picking out the correct description from many possibilities. This route was indeed used in ref 20b to determine precisely the internal structure of a targeted core/shell/shell ZnS/CdSe/ZnS quantum dot/quantum well structure, and it was shown that the core region is a graded alloy of ZnS and CdSe rather than being made of pure ZnS as targeted. This study20b clearly showed that the reaction to put a shell of CdSe on the preformed ZnS core invariably causes diffusion of Cd and Se ions toward the center of the core, leading to the formation of the graded alloy structure, whereas the synthesis procedure to place a shell of ZnS on top of the outer CdSe in the final step leads to a sharp CdSe/ZnS interface, as shown schematically in Figure 7. In this study, it was further shown that the quantum
Figure 7. (a) Schematic model of the heterostructured nominal ZnS/ CdSe/ZnS NC, which has three main layers: a graded alloy core till radius r1 followed by a shell of CdSe till r2 and finally a ZnS layer till r3. A sharp interface exists between CdSe and the outer ZnS layer. (b) Schematic representation of the variation of the chemical composition for the final model derived from XPS analysis along with other indirect experimental supports. The orange line shows the variation of Se concentration along the radius of the NC, while the thin blue line represents the sulfide concentration. (Adapted with permission from ref 20b. Copyright 2009 American Chemical Society.)
mechanically calculated band gap of the proposed internal structure is in very good agreement with the experimentally observed band gap using UV−visible absorption spectra. Similar analyses for highly luminescent CdSeS NCs have established that these systems are also characterized by a graded alloy interface, instead of a sharp interface, between CdSe core and CdS shell with very important consequences on its optical 1230
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
structure information from strongly correlated bulk systems,43 or in estimating the thickness of a surface oxide layer grown in the process of oxidation of a metal.44 In summary, we have presented here the powerful technique of photoelectron spectroscopy in elucidating the internal structure of complex NC systems in great detail and with accuracy. This is indeed an apparently improbable route to determine structure and size with a spectroscopic technique instead of using a microscopic one. In effect, the acute sensitivity of the mean escape depth of the electrons ejected by the photoelectric process on the photon energy used allows one to effectively slice and probe the NC in planes perpendicular to the direction of the electron detection (see Figure 2b). This slicing and the sensitivity of the photoelectron spectra to composition as well as different environments of a given element provide the microscopic information, replacing the lateral resolution of a conventional microscope with a vertical or depth resolution achievable with XPS. We have discussed how such spectroscopic investigations provide understanding of physical and chemical properties, particularly of PL quantum efficiency, via a description of the internal and the surface structure of the NCs. We have also discussed the limitations of this approach and how these can be, at least partially, circumvented.
understand variations in the PL efficiencies via quantitative analysis of the composition of NCs. For example, ref 41c and 41f established the defect structure and its composition near the surface of CdTe and PbSe NCs, respectively, showing that these results are able to explain observed significant variations in PL quantum efficiencies within a related series of samples quite successfully.
■
CONCLUSION AND FUTURE OUTLOOK Before closing, it is worthwhile to recall limitations and possible pitfalls of this otherwise powerful technique to determine the internal structure of a variety of complex NC systems. We have already touched upon the uncertainties that arise from various assumptions and approximations involved in modeling such NCs for the purpose of quantitative analysis. We have also pointed out that the use of different core levels of the same element, the use of core levels of different elements in the system, as well as the use of several photon energies may help in reducing chances of a wrong conclusion concerning the internal structure (see Figure 6). Most of the studies reported so far involve soft X-rays, with the photon energy being restricted below 1.5 keV; the main reason for this has been the absence of high photon energy photoemission beam lines at synchrotron centers globally. However, in recent times, there has been an increased availability of very high energy photoemission beam lines, where photon energies even above 8 keV can be employed to carry out XPS. Since this makes available the study of a large number of core levels, inaccessible to lower photon energies, it is expected that there will be increasing investigation of internal structures of NCs using these high photon energy techniques. There are some intrinsic advantages of using such high photon energies. At such energies, the calculated photoionization cross sections and mean free paths of electrons are more reliable, as approximations involved in estimating these become more valid at higher electron energies. However, the use of such multitude of spectral information also invariably involves use of an increasing number of free parameters such as the value of the mean free path and the photoionization cross section. Thus, it always remains highly desirable to validate the quantitative model description arrived at from the spectral analysis with the use of stable and independent experimental inputs, such as the shape and the overall size of NCs from TEM and the average composition of the system determined from physical or chemical methods. Even after such caution being exercised, it should be kept in mind that the entire discussion has been on the basis of monodispersed NC systems; therefore, the application of these techniques will be rendered ineffective if there is a wide variation in the size, shape, composition, or internal structure of different NCs within a given ensemble of NCs. Additionally, it is also important to keep in mind that the analysis does not take into account shadowing of one NC by another and, therefore, photoelectron experiments should be carried out in the dilute limit to minimize such shadowing. This is effectively achieved by dispersing a small quantity of NCs in graphite powder,19,20b,39,40,41f as this also helps in reducing the charging problem often encountered while recording photoelectron spectra from large-band-gap, highly insulating systems. It is also instructive to see the quantitative analysis of photoelectron spectra of NCs within a larger perspective. While we have attempted to cover most such studies on NCs in this review, similar analysis has been performed in very different contexts, such as to extract the surface and bulk electronic
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses †
Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India. ‡ Currently at Department of Chemistry, Indian Institute of Science Education & Research (IISER), Pune-411008, Maharashtra, India. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) (a) Efros, A. L. E., A. L. Sov. Phys. Semicond. 1982, 16, 772. (b) Brus, L. E. J. Chem. Phys. 1983, 79 (11), 5566−5571. (c) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95 (2), 525−532. (2) Klimov, V. I., Semiconductor and Metal Nanocrystals, Synthesis and Electronic and Optical Properties; Marcel Dekker, Inc.: New York, 2004. (3) (a) Sapra, S.; Sarma, D. D. Phys. Rev. B 2004, 69 (12), 125304. (b) Viswanatha, R.; Sapra, S.; Saha-Dasgupta, T.; Sarma, D. D. Phys. Rev. B 2005, 72 (4), 045333. (c) Sapra, S.; Shanthi, N.; Sarma, D. D. Phys. Rev. B 2002, 66 (20), 205202. (4) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15 (14), 2854−2860. (5) (a) Coe, S.; Woo, W.-K.; Bawendi, M.; Bulovic, V. Nature 2002, 420 (6917), 800−803. (b) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Nature 2006, 442 (7099), 180−183. (c) Nag, A.; Kumar, A.; Kiran, P. P.; Chakraborty, S.; Kumar, G. R.; Sarma, D. D. J. Phys. Chem. C 2008, 112 (22), 8229−8233. (d) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2009, 110 (1), 389−458. (6) (a) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100 (2), 468−471. (b) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119 (30), 7019−7029. (c) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101 (46), 9463−9475. (7) Lee, J.-S.; Shevchenko, E. V.; Talapin, D. V. J. Am. Chem. Soc. 2008, 130 (30), 9673−9675. 1231
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
Chemistry of Materials
Review
(8) Lee, J.-S.; Bodnarchuk, M. I.; Shevchenko, E. V.; Talapin, D. V. J. Am. Chem. Soc. 2010, 132 (18), 6382−6391. (9) (a) Karakoti, A. S.; Sanghavi, S.; Nachimuthu, P.; Yang, P.; Thevuthasan, S. J. Phys. Chem. Lett. 2011, 2 (22), 2925−2929. (b) Jasieniak, J.; Mulvaney, P. J. Am. Chem. Soc. 2007, 129 (10), 2841− 2848. (10) Nag, A.; Sapra, S.; Nagamani, C.; Sharma, A.; Pradhan, N.; Bhat, S. V.; Sarma, D. D. Chem. Mater. 2007, 19 (13), 3252−3259. (11) (a) Sengupta, S.; Ganguli, N.; Dasgupta, I.; Sarma, D. D.; Acharya, S. Adv. Mater. 2011, 23 (17), 1998−2003. (b) Acharya, S.; Sarma, D. D.; Golan, Y.; Sengupta, S.; Ariga, K. J. Am. Chem. Soc. 2009, 131 (32), 11282−11283. (12) Buck, M. R.; Bondi, J. F.; Schaak, R. E. Nature Chem. 2012, 4 (1), 37−44. (13) Pradhan, N.; Sarma, D. D. J. Phys. Chem. Lett. 2011, 2 (21), 2818−2826. (14) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Nano Lett. 2001, 1 (4), 207−211. (15) Ivanov, S. A.; Nanda, J.; Piryatinski, A.; Achermann, M.; Balet, L. P.; Bezel, I. V.; Anikeeva, P. O.; Tretiak, S.; Klimov, V. I. J. Phys. Chem. B 2004, 108 (30), 10625−10630. (16) (a) Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.; Bussian, D. A.; Klimov, V. I.; Hollingsworth, J. A. J. Am. Chem. Soc. 2008, 130 (15), 5026−5027. (b) Mahler, B.; Spinicelli, P.; Buil, S.; Quelin, X.; Hermier, J.-P.; Dubertret, B. Nat. Mater. 2008, 7 (8), 659− 664. (17) Ivanov, S. A.; Piryatinski, A.; Nanda, J.; Tretiak, S.; Zavadil, K. R.; Wallace, W. O.; Werder, D.; Klimov, V. I. J. Am. Chem. Soc. 2007, 129 (38), 11708−11719. (18) Reiss, P.; Protière, M.; Li, L. Small 2009, 5 (2), 154−168. (19) Sarma, D. D.; Nag, A.; Santra, P. K.; Kumar, A.; Sapra, S.; Mahadevan, P. J. Phys. Chem. Lett. 2010, 1 (14), 2149−2153. (20) (a) Eychmüller, A.; Mews, A.; Weller, H. Chem. Phys. Lett. 1993, 208 (1−2), 59−62. (b) Santra, P. K.; Viswanatha, R.; Daniels, S. M.; Pickett, N. L.; Smith, J. M.; O’Brien, P.; Sarma, D. D. J. Am. Chem. Soc. 2009, 131 (2), 470−477. (21) Battaglia, D.; Blackman, B.; Peng, X. J. Am. Chem. Soc. 2005, 127 (31), 10889−10897. (22) Liz-Marzán, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12 (18), 4329−4335. (23) Yu, Z.; Guo, L.; Du, H.; Krauss, T.; Silcox, J. Nano Lett. 2005, 5 (4), 565−570. (24) (a) Baranov, A. V.; Rakovich, Y. P.; Donegan, J. F.; Perova, T. S.; Moore, R. A.; Talapin, D. V.; Rogach, A. L.; Masumoto, Y.; Nabiev, I. Phys. Rev. B 2003, 68 (16), 165306. (b) Tschirner, N.; Lange, H.; Schliwa, A.; Biermann, A.; Thomsen, C.; Lambert, K.; Gomes, R.; Hens, Z. Chem. Mater. 2011, 24 (2), 311−318. (25) Zheng, W.; Wang, Z.; van Tol, J.; Dalal, N. S.; Strouse, G. F. Nano Lett. 2012, 12 (6), 3132−3137. (26) (a) Norris, D. J.; Efros, A. L.; Erwin, S. C. Science 2008, 319 (5871), 1776−1779. (b) Nag, A.; Chakraborty, S.; Sarma, D. D. J. Am. Chem. Soc. 2008, 130 (32), 10605−10611. (27) Karan, N. S.; Sarkar, S.; Sarma, D. D.; Kundu, P.; Ravishankar, N.; Pradhan, N. J. Am. Chem. Soc. 2011, 133 (6), 1666−1669. (28) Balet, L. P.; Ivanov, S. A.; Piryatinski, A.; Achermann, M.; Klimov, V. I. Nano Lett. 2004, 4 (8), 1485−1488. (29) Jang, E.; Jun, S.; Pu, L. Chem. Commun. 2003, 24, 2964−2965. (30) Cho, J.; Jung, Y. K.; Lee, J.-K. J. Mater. Chem. 2012, 22 (21), 10827−10833. (31) (a) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1988, 11 (11), 577−589. (b) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1991, 17 (13), 927−939. (32) Vijayakrishnan, V.; Chainani, A.; Sarma, D. D.; Rao, C. N. R. J. Phys. Chem. 1992, 96 (22), 8679−8682. (33) Roy, A.; Chainani, A.; Sarma, D. D.; Sood, A. K. Appl. Phys. Lett. 1992, 61 (14), 1655−1657. (34) Roy, A.; Sarma, D. D.; Sood, A. K. Mol. Spectrosc. 1992, 48 (11− 12), 1779−1787.
(35) Hoener, C. F.; Allan, K. A.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Phys. Chem. 1992, 96 (9), 3812−3817. (36) Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994, 98 (15), 4109−4117. (37) Kim, T.; Kim, S. W.; Kang, M.; Kim, S.-W. J. Phys. Chem. Lett. 2011, 3 (2), 214−218. (38) (a) Winkler, U.; Eich, D.; Chen, Z. H.; Fink, R.; Kulkarni, S. K.; Umbach, E. Chem. Phys. Lett. 1999, 306 (1−2), 95−102. (b) Kulkarni, S. K.; Winkler, U.; Deshmukh, N.; Borse, P. H.; Fink, R.; Umbach, E. Appl. Surf. Sci. 2001, 169−170 (0), 438−446. (39) Nanda, J.; Kuruvilla, B. A.; Sarma, D. D. Phys. Rev. B 1999, 59 (11), 7473−7479. (40) Nanda, J.; Sarma, D. D. J. Appl. Phys. 2001, 90 (5), 2504−2510. (41) (a) Borchert, H.; Haubold, S.; Haase, M.; Weller, H.; McGinley, C.; Riedler, M.; Möller, T. Nano Lett. 2001, 2 (2), 151−154. (b) Borchert, H.; Dorfs, D.; McGinley, C.; Adam, S.; Möller, T.; Weller, H.; Eychmüller, A. J. Phys. Chem. B 2003, 107 (30), 7486− 7491. (c) Borchert, H.; Talapin, D. V.; Gaponik, N.; McGinley, C.; Adam, S.; Lobo, A.; Möller, T.; Weller, H. J. Phys. Chem. B 2003, 107 (36), 9662−9668. (d) Bernardi, F.; Fecher, G. H.; Alves, M. C. M.; Morais, J. J. Phys. Chem. Lett. 2010, 1 (6), 912−917. (e) Huang, K.; Demadrille, R.; Silly, M. G.; Sirotti, F.; Reiss, P.; Renault, O. ACS Nano 2010, 4 (8), 4799−4805. (f) Sapra, S.; Nanda, J.; Pietryga, J. M.; Hollingsworth, J. A.; Sarma, D. D. J. Phys. Chem. B 2006, 110 (31), 15244−15250. (42) (a) Scofield, J. H. Theoretical Photoionization Cross Sections from 1 to 1500 keV; Lawrence Livermore National Laboratory Report; 1973. (b) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32 (1), 1− 155. (43) (a) Maiti, K.; Mahadevan, P.; Sarma, D. D. Phys. Rev. Lett. 1998, 80 (13), 2885−2888. (b) Maiti, K.; Sarma, D. D. Phys. Rev. B 2000, 61 (4), 2525−2534. (c) Maiti, K.; Manju, U.; Ray, S.; Mahadevan, P.; Inoue, I. H.; Carbone, C.; Sarma, D. D. Phys. Rev. B 2006, 73 (5), 052508. (d) Maiti, K.; Sarma, D. D.; Rozenburg, M. J.; Inoue, I. H.; Makino, H.; Goto, O.; Pedio, M.; Cimino, R. Europhys. Lett. 2001, 55 (2), 246−252. (44) Sen, P.; Sarma, D. D.; Budham, R. C.; Chopra, K. L.; Rao, C. N. R. J. Phys. F: Met. Phys. 1984, 14 (2), 565−577.
1232
dx.doi.org/10.1021/cm303567d | Chem. Mater. 2013, 25, 1222−1232
