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J. Phys. Chem. B 2000, 104, 6514-6528
FEATURE ARTICLE Structure and Photophysics of Semiconductor Nanocrystals Alexander Eychmu1ller Institut fu¨ r Physikalische Chemie, UniVersita¨ t Hamburg, Bundesstr. 45, 20146 Hamburg, Germany ReceiVed: December 10, 1999; In Final Form: April 6, 2000
Nanoparticle research is still a rapidly growing field of science. Semiconductor nanocrystals of a large variety of compositions have been prepared in exceedingly good quality. State-of-the-art preparations and standard characterization methods are discussed. The build-up of more complex nanoheterostructures is reported, and their structural and electronic properties are outlined. An overview is given on recent achievements in the photophysical investigations of single nanoparticles and the application of EXAFS spectroscopy to the study of structural properties of semiconductor nanoparticles.
1. Introduction “Physics and Chemistry on the Nanometer Scale” could be the title of an attractive textbook. Interested readers should, however, realize that the work would be incomplete or even become obsolete when being published. The reason for this is that the investigation of the physical and chemical properties of nanoscaled materials is one of the fields of research in natural sciences with the most dynamic progress. One can, of course, claim that “nano” is “in”, but it is much more than a vogue word. The immense preparatory and technical progress made during the past years arouses the interest of industrial companies. This sector is being investigated by solid-state physicists, inorganic chemists, physical chemists, colloid chemists, material scientists, and recently, even biological scientists, medical scientists, and engineers. Depending on his or her scientific background, each scientist has his or her own special view of nanometer-sized structures. Physicists notice that decreasing the size of the object investigated leads to the loss of typical solid state properties, e.g., the band gap of a semiconductor. Chemists may start from the atomic orbital and preferably see an increasing complexity of the electronic structure, which finally leads to orbitals of very big molecules. Both ways of looking at nanometer-sized structures are, of course, correct; the phenomenon in question has been described with the term “size quantization effect”. Presumably, the simplest picture familiar to nearly everyone is based on the quantum-mechanical “particle-in-the-box” problem, in which the smaller the box becomes, the higher the lowest energy eigenvalue becomes. This explains the fact observed that the energy of the first electronic transition of a semiconductor material such as cadmium sulfide shifts from a solid-state value of 2.5 eV to a value of 4.5 eV for molecule-like aggregates. Furthermore, phase transition pressures, melting points, optical, optoelectronic, catalytic, magnetic, and electric properties of nanomaterials differ from those of the solid and from those of the molecular species of which they consist. Therefore, one could speak of a new state of matter. The activities in this field of research may be seen in the
considerably increased number of publications and in special conferences being held more and more frequently. In contrast to structures in which the nanometer scale is reached in only one direction in space (quantum wells), or in two (quantum wires), no common name has been found, until now, for those that show such reduced dimensions in all three directions in space. These structures, which are the object of this paper, have been named quantum dot, quantum sphere, quantum crystallite, nanocrystal, microcrystal, colloidal particle, nanoparticle, (nano-)cluster, Q-particle, and “artificial atom”. The terms are widely used synonymously, sometimes with misleading results. The methods of producing nanostructures can be divided into roughly “physical” and “chemical” ones. The “physical” methods use increasingly sophisticated molecular beam techniques and lithographic methods, thus achieving very defined structures that are especially suitable for high precision electric and magnetic measurements (see, for example, refs 1 and 2). The following deals merely with “chemically” produced samples. These fall into at least two categories. Starting from the commercial production of colored glass filters, the specific preparation of nanocrystals, especially of I-VII and II-VI semiconductors, is achieved by diffusion-controlled growth in silicate and borosilicate glass matrixes. The early papers came from Al. L. Efros and A. L. Efros3, as well as from Ekimov and Onushchenko,4 who may claim the fact that they were the first to correctly interpret the “size quantization” as a quantummechanical phenomenon. Descriptions that summarize the properties of nanoparticles synthesized in glasses are given in the survey of Henneberger and Puls5 as well as in the monographs of Woggon6 and Gaponenko.7 The second access to “chemical” quantum dots is a colloidochemical one, which is mostly achieved by carrying out a precipitation reaction in homogeneous solution in the presence of so-called stabilizers. The “stabilizers” prevent the colloids from agglomerating or from further growing, thus making sure that the particles remain in solution. The advantage of these kind of preparations is their versatility. By choosing primary materials and stabilizers, one can produce nearly any semiconductor material as nanometer-
10.1021/jp9943676 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/21/2000
Feature Article sized particles, some of them even in considerable quantities. A. Henglein and L. Brus can certainly be considered as pioneers in this sector. With their papers, published in 1982 and 1983,8-10 they supplied the fundamental principles concerning synthesis, as well as the understanding of electronic structure.11-14 On the basis of the early papers, very successful preparations have been developed in their laboratories and in the laboratories of their former collaborators. A method often cited and frequently employed for the production of CdS particles in aqueous solution uses polyphosphate as stabilizer.15 M. Bawendi reported on the trioctyl phosphine/trioctyl phosphine oxide synthesis (TOP/ TOPO) of CdE (E ) S, Se, Te),16 and the group of H. Weller succeeded in synthesizing a series of different II-VI semiconductor particles by using various thiols as stabilizers.17 This article will repeatedly deal with these three methods of synthesis. Interestingly, colloidochemical preparations and advanced inorganic syntheses converge (cf. esp. chapter 3 in ref 18). The latter produce larger and larger aggregates, such as [Cd17S4(SPh)28],2-19 [Cd17Se4(SePh)24(PPh3)4]2+, and [Cd32Se14(SePh)36(PPh3)4].20 The crystallization of the following compounds was achieved with colloidochemical methods: [Cd32S14(SPh)36]*4DMF,21 [Cd17S4(SCH2CH2OH)26],22 and [Cd32S14(SCH2CH(OH)CH3)36]*4H2O23 (Ph ) C6H5, DMF ) dimethyl formamide). The structural characteristic that these compounds have in common is that they are tetrahedral sections cut out of the cubic zinc blende phase. In addition, there are analogous compounds, such as [Cd16(SePh)32(PPh3)2],24 that “lack” a central atom or that may be structurally formed in a manner similar to that of Koch pyramids.25 Examples of this kind may justify the fact that nanocrystals are described by the expression “artificial atoms”. The monodisperse particles become useful as constituents of big supramolecular units.26-28 Reports on the, in this sense, “homoatomic” self-organization of larger semiconductors29-31 and metal particles already exist.18,32-34 “Heteroatomic”, possibly even semiconductor-metal superstructures are expected soon. The following four chapters take into account the character and historic development of the topics with which they deal. The following chapter “Synthesis and Characterization” is limited, apart from some fundamental aspects, to the recent works of our group. In “Nanoheterostructures”, it is especially the quesiton of core-shell-type semiconductor particles prepared in different laboratories, and the chapters “single-particle fluorescence” and “EXAFS spectroscopy” are almost review articles by themselves. Both our own contributions and those of others are given in the text for better understanding the electronic structure of the particles. More detailed information on the theoretical aspects may be taken from the monographs of Banyai and Koch,35 Woggon,6 Gaponenko,7 and the survey of Yoffe.36 2. Synthesis and Characterization The breath-taking development of “nanochemistry” is reflected in an immense number of publications on the synthesis of semiconductor nanoparticles. Nearly all of the II-VI semiconductors have been prepared in colloidal form, some of them in a variety of different approaches. A great number of these developments have been summarized in surveys, in special issues of journals and in books.12,13,27,34,37-52 The achievement of desired particle sizes over the largest possible range, narrow size distributions, good crystallinity, desired surface properties and, should the occasion arise, high luminescence quantum yields, as well as adjustable electronic properties, are all results that are considered to be characteristics of a “good preparation”.
J. Phys. Chem. B, Vol. 104, No. 28, 2000 6515 With the increasing interest of industrial companies in these materials, criteria such as price, quantity, production speed, or something similar will become more significant. The nature and concentration of the initial materials; the stabilizer, which keeps the particles in solution; the solvent; the pH-value; the reaction time; the temperature; and the atmosphere are some of the parameters that influence a colloidochemical synthesis of nanocrystals. Certain methods, such as exclusion chromatography, gel electrophoresis, or size-selective precipitation, are used for postpreparatory size fractionation. The most successful preparations, both in the above-mentioned sense and in terms of reproducibility, are based on the reliable separation of nucleation and growth with following size-selective precipitation. Presently, the characterization of nanoparticles is achieved by standard procedures, such as absorption and emission spectroscopy, powder X-ray diffractometry (P-XRD), and transmission electron microscopy (TEM). To settle special questions, nearly all methods imaginable have been referred to: time-resolved luminescence, fluorescence line narrowing (FLN), hole burning (HB), flash photolysis and pulse radiolysis, nonlinear-optical procedures, nuclear magnetic resonance, electron paramagnetic resonance, ODMR (optically detected magnetic resonance), photo electron spectroscopy, X-ray small angle scattering, X-ray absorption and emission (including EXAFS (extended X-ray absorption fine structure spectroscopy)), Auger electron spectroscopy, mass spectroscopy, and various chromatographic methods. The above-mentioned literature survey deals with most of these methodical advances.12,13,27,34,37-52 Table 1 shows some nanocrystalline semiconductors with a range of their characteristics prepared in our group. Reference 17 is a paper that deals with the synthesis, the characterization and various photophysical properties of both thiol- and polyphospate-stabilized CdS particles. The prediction according to which the oscillator strength of the first electronic transition (or “1s-1s-transition”) should be independent of the particle size53 could be experimentally confirmed because of the quality of the nanocrystals, especially relating to their narrow size distributions. Furthermore, the temperature-dependent shift of the 1s-1s transition could be measured. It is observed that the absolute value of the so-called temperature coefficient dEg/dT (Eg ) band gap energy in the solid, 1s-1s transition energy in nanocrystals) increases with decreasing particle size. The discussion in ref 17 gives some hints on possible causes of this phenomenon. It has, however, not yet been understood. Much the same applies to the shift of the first absorption transition to lower energies, which is observed when cluster films are made from the solutions of very small CdS particles. It seems a likely supposition that this is a consequence of a particle-particle interaction, and the first results of investigations that are being carried out seem to confirm this.54 The study of luminescence properties of polyphosphatestabilized CdS nanoparticles (in ref 17, samples g and h) led to some publications55-57 in the beginning of the 1990s. Their essential results are reported in the summary, ref 47. The fluorescence decay is clearly prolonged compared with that of the solid material. The observation that this (multiexponential) decay becomes faster with decreasing temperature, although the quantum yield increases, required the development of an unusual fluorescence mechanism. A distribution of electron traps (presumably at the particle surface) is assumed. Energetically, these traps are located relatively near the conduction band of the particles. The population of these states is temperaturedependent (the lower the temperature, the smaller the energy
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Eychmu¨ller
TABLE 1: Semiconductor Nanocrystals Prepared in the Hamburg group (“QY”: Luminescence Quantum Yield, “?”: Crystal Structure not yet Determined, ML: Monolayer, “without”: no Additional Stabilizers) crystal structure
details
CdS
compound thiols
stabilizer
mean diameter (nm) 1.3-4.0
cubic (zinc blende)
CdS
polyphosphate
4.0-17.0
cubic
CdS
sulfonic acids
4.0
?
CdSe CdTe HgTe
thiols thiols thiols
2.0-4.0 2.0-4.0 5.0
cubic (zinc blende) cubic (zinc blende) cubic (Coloradoite)
very narrow fractions, crystals from Cd17S4(SR)26 and Cd32S14(SR)36 narrow fractions, QY 10% stabilizer exchange, nonpolar solvents very narrow fractions very narrow fractions IR - fluorescence, up to QY 50%
CdS on HgTe Cd1-xHgxTe
thiols thiols
7.0 4.0
cubic ?
HgS CdS on HgS CdS/HgS/CdS
polyphosphate polyphosphate polyphosphate
2.0 3.0 5.0 core 1-3 ML HgS
cubic (β-HgS) cubic cubic
ZnO TiO2 SnO2(:Sb) YVO4:Eu or Sm or Dy
“without” “without” “without” “without”
1-3 ML CdS 4.0-8.0 4.0-5.0 5.0 20.0
hexagonal Anatas Rutil tetragonal
LaPO4:Eu or Tb or Ce LaPO4:Eu or Tb or Ce CePO4:Tb CePO4:Tb Cd3P2 InP InAs
“without” tris-ethylhexyl phosphate “without” tris-ethylhexyl phosphate “without” TOP/TOPO TOP/TOPO
20.0 5.0 20.0 5.0 2.5 3.0-6.0 3.0-6.0
monoclinic monoclinic monoclinic monoclinic tetragonal cubic cubic
difference between conduction band and populated traps). The repopulation of the conduction band from these traps represents the rate-determining step in the photocycle. The observations are satisfactorily explained by the reciprocity of the temperaturedependence of these countercurrent processes; at higher temperatures, relatively deep traps are populated with relatively small repopulation rates according to the energy difference, whereas at lower temperatures, only shallow traps are populated, which are slowly depopulated according to the low temperature. Thus, it is a question of the competition between trapping and detrapping whether the overall fluorescence decay becomes slower or faster with decreasing temperature. A summary of the discussion of this model and relative alternatives can be found in ref 47. More recent papers on this subject are discussed in the two following chapters. The synthesis presented by Kornowski et al.58 provides a sample of Cd3P2 nanocrystals that is ideally suitable for photophysical investigations. The sample exhibits a narrow particle-size distribution, absorption, and emission in the visible spectral range, near band-edge fluorescence, with 25% quantum yield at room temperature (still the highest quantum yield of uncoated particles with emission in the visible spectral range), photostability, and transparency of the sample over the temperature range of 4 to 295 K. The particles have diameters of 2.7 ( 0.4 nm, exhibit an extremely high extinction coefficient in the range of the first absorption transition (about 106 lmol-1cm-1 at 600 nm), and a band gap of about 2 eV, which is considerably enlarged in comparison with the solid (Eg(Cd3P2) ) 0.5 eV). These observations are in accordance with the size quantization effect. Measurements of the absorption, the static and the time-resolved fluorescence as a function of temperature, and the low-temperature fluorescence excitation spectra have been carried out. The absorption and the near band-edge
Cd:Hg ) 1:0 to 1:1.2, QY 44% “epitaxial” “epitaxial” quantum wells in quantum dots
n-conducting line fluorescence of bulk material different morphologies very narrow fractions very narrow fraction QY 25% very narrow fractions very narrow fractions
emission again show a temperature coefficient that is a bit larger than that of the solid material. Below about 200 K, a further luminescence band is observed that is supposed to originate from traps. As can be seen from the fluorescence excitation spectra, this band comes mainly from the smaller particles of the size distribution. The fluorescence decay, which is multiexponential at room temperature, clearly decomposes at lower temperatures into a nanosecond component and a micro- to millisecond component. Although the temperature, excitation, and observation wavelength were varied, it was not possible to unambiguously assign these obviously different luminescence channels to peculiarities of the sample that had been spectroscopically observed. Rogach et al.59 describe the synthesis and characterization of some thiol-stabilized CdTe nanocrystals. The reaction of sodium hydrogentelluride with cadmium perchlorate, in the presence of 1-thioglycerol or 2-mercaptoethanol as stabilizers, leads to the desired precipitation reaction. The medium size of the particles is influenced by the ratio of concentrations of the reactants, the following thermal treatment, and the stabilizer that is chosen. After the initial preparation, still narrower fractions are achieved by size-selective precipitation. Sample “b” (in ref 59) is of such quality that we could propose a tetrahedral structure with the empirical formula [Cd54Te32(SCH2CH2OH)52]8by means of EXAFS spectroscopy60,61 (see below). The particles are extremely small (1.3 to 2.4 nm) and crystallize in the cubic zinc blende phase of CdTe. As pointed out in ref 61, the “size” determination of such nanoparticles is delicate; the methods in principle suitable like TEM and P-XRD in the small- and wideangle range (if necessary with Rietveld analysis) produce varying results. Because of the tiny contrast of the smallest particles, TEM emphasizes rather larger nanocrystals of a distribution, small-angle XRD measures the periodicity inside
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J. Phys. Chem. B, Vol. 104, No. 28, 2000 6517
Figure 1. Absorption spectra of very small Cd-chalcogenide nanoparticles, from left to right, two samples of CdS, two samples of CdSe, and two samples of CdTe.
the particle powder and, hence, the size of the particles including the ligand shell. The determination of the particle size by means of wide-angle measurements, via the width of the reflexes, becomes vague when the reflexes become too wide and unstructured. Our group tries to approach this problem by carrying out computer simulations of TEM pictures62 as well as by an increasing extent of Rietveld analyses of powder X-ray diffractograms.63 In a further paper,64 it could be shown that with continuing heat supply, the absorption onset and the emission band of the CdTe particle solutions can be steadily shifted into the red spectral range. As a consequence of these properties, the particles were incorporated into more complex structures, such as LEDs (light emitting devices)65 and photonic crystals.66 The inner structure and composition of the particles used in this way are being investigated very carefully in our group. There are hints indicating that the thiols, which are initially covalently bound to the particle surface, are decomposed by prolonged heat supply and possibly become a sulfide source. So one cannot rule out the formation of mixed crystals such as Cd(Te,S). Preparatory and electron microscopic investigations support this supposition.67 By applying thioglycolic acid instead of 1-thioglycerol or 2-mercaptoethanol, M. Gao was able to increase the fluorescence quantum yield of CdTe particles to about 18%.68 Interestingly enough, the quantum yield depends on the pH value of the solution and reaches the mentioned 18% at pH ) 4.5. The formation of a layer of cadmium thiol complexes surrounding the particles is concluded from systematic absorption and emission studies. The preparatory approach described above, which results exclusively in samples of very small particles with narrow size distribution, was recently extended to the synthesis of CdSe particles.69 Figure 1 shows six absorption spectra of thiolstabilized cadmium chalcogenide particles, to be precise, from the left to the right two fractions of CdS followed by two fractions of CdSe and finally by two fractions of CdTe. All of the samples are in the range of strong size quantization and show, with the exception of the one absorbing at the longest wavelengths, moleculelike absorption spectra. Finally, some results of an analogous HgTe preparation shall be presented.70 The special interest in this compound, which as a solid material exhibits a negative band gap and can therefore be considered as a semimetal, is due to the fact that by size quantization, it should be possible to reach the spectral range between 1.3 and 1.5 µm, which is important for the telecommunication industry. This interest is reflected in a cooperation between our group and the British Telecom Laboratories in
Figure 2. P-XRD of small HgTe particles. Line spectrum gives the bulk HgTe coloradoite reflections.
Figure 3. TEM image of small HgTe particles. Insets show a single particle in high resolution together with the corresponding Fourier transform.
Ipswich. Figure 2 shows the P-XRD of the thiolglycerolstabilized HgTe particles. As can be seen from the width of the reflexes, very small particles were again prepared that undoubtedly belong to the cubic coloradoite phase. Transmission electron microscopy (Figure 3) again illustrates the crystallinity of the sample in the survey. In the inset of Figure 3, a highresolution image of one particle together with its Fourier transform is shown. These studies confirm the assignment to the cubic phase and enable us to ascertain the particle-size distribution. It covers the range of 3 to 6 nm, and so, it is rather wide. These findings are also reflected in the unstructured absorption spectrum beginning at 1400 nm and in the width of the observed fluorescence band. The latter has a maximum at about 1100 nm and is consequently not far from the desired spectral range. The enormous luminescence quantum yield of 48% at room temperature and the fact that up to now the preparation is not yet optimized, offer the possibility of coming to applications relevant to industry. Very recently, a further step
6518 J. Phys. Chem. B, Vol. 104, No. 28, 2000
Figure 4. Typology of semiconductor nanoheterostructures, VB: valence band, CB: conduction band.
into this direction has been accomplished by using deuterated water (D2O) as a solvent. Strong luminescence has been observed over the full strategic 1.3 and 1.5 µm telecoms window.71 3. Nanoheterostructures Since the mid-1980s, there have been reports on experiments carried out in order to cover nanocrystalline semiconductor particles of a given composition by another material, mostly of larger band gap (often described as type I-nanoheterostructures, cf. Figure 4). Another approach is to bring particles of different materials into such close contact (“sandwich colloids” or type II-nanoheterostructures) that charge carrier transport between the particles becomes possible (cf. Figure 4). When compared with the uncoated material, the type I-particles often exhibit increased luminescence quantum yields, which is mostly explained by the passivation of the surface of the initial colloid. This passivation consists of the saturation of free valences at the particle surface. As a result, the radiationless recombination occurring here is suppressed. Obviously, it is more effective to coat the particles with (epitaxially growing) inorganic material than with ionically or van der Waals-bound organic substances. Type II-particles are of special interest in connection with questions on photocatalysis, as efficient charge separation can be achieved by suitably choosing the original materials to be brought into contact. By this, the durability of the charge carriers is increased to a desirable extent. Structures of both kinds have repeatedly been dealt with in summaries (among others, see refs 13, 38, 41, 42, 44, and 72-75, as well as in ref 47, pp 178188, in which our own endeavors relating to the system HgS on CdS and CdS on HgS have been put together). The following discussion shall therefore deal only with some recent developments, starting with a short summary of one of our papers, in which we present a simple and successful theoretical model that describes spherical core-shell-, that is type I-, particles.76 Originally developed for quantum dot quantum wells (QDQWs) that are built up like onions and consisting of three domains, the model can, of course, easily be applied to more simple structures. On the basis of the papers of Kortan et al.77 and Haus et al.,78 the charge carriers in the nanoheterostructures are treated as “particle-in-the-box”. Energy eigenvalues and functions are gained in the framework of the “effective mass approximation”. Coulombic interactions between the electron and the hole, as well as finite potentials at the particle boundaries, have been included in the model. The physical constants effective mass of electron and hole, high-frequency dielectric constant, band gap of the core and shell materials, and band gap of the matrix surrounding the particles are taken from tables. The relative positions of the conduction and valence bands important for the understanding of the electronic properties of the nanocomposites can, so far as they have been measured, be incorporated directly or can be taken from eletronegativity data. With the
Eychmu¨ller aid of this model, it is possible to predict whether a combination of materials, core dimensions, and thicknesses of the layers leads to the result that both charge carriers are localized inside the nanocomposites or that one of them is localized, whereas the other keeps free mobility over the total structure. In the mid-1990s, the group of M. Bawendi hit upon the idea of incorporating CdSe particles by means of an electrospray procedure (electrospray organometallic chemical vapor deposition, ES-OMCVD) into a thin layer of zinc selenide to produce a nanocomposite for optoelectronic applications. It turned out, however, that in this procedure, the luminescence properties of the initial colloid are to a great extent lost. Danek et al.79 developed a way out of this dilemma, by first coating the preformed CdSe particles in solution with some monolayers of ZnSe before incorporating them into the ZnSe matrix. On the basis of the previously mentioned, very successful TOP/TOPO synthesis of CdSe nanocrystals,16 the desired coating of the initial colloid is achieved by means of diethyl zinc and trioctyl phosphine selenide. Their characterization is made by absorption, emission, Augerelectron, and X-ray fluorescence spectroscopy as well as by P-XRD and high-resolution transmission electron microscopy (HRTEM). After coating, a small red shift can be observed in the absorption and emission spectra that is explained by the relaxation of the charge carriers in the sizequantized semiconductor particles. The small fluorescence quantum yield (
