Fluorescence, thermoluminescence, and photostimulated luminescence of nanoparticles

Chapter 5 FLUORESCENCE, THERMOLUMINESCENCE, AND PHOTOSTIMULATED LUMINESCENCE OF NANOPARTICLES Wei Chen Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, People's Republic of China

Contents 1. 2.

3. 4.

5.

6.

7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement of Split Levels of Quantum Confinement . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Optical Absorption Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Photoluminescence Excitation Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Excitation Energy Dependence of Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Absorption and Luminescence of Surface States in Nanoparticles . . . . . . . . . . . . . . . . . . . . . 4.1. General Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Excitonic and Trapped Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Absorption of Surface States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Size Dependence of Trapped Luminescence from Surface States . . . . . . . . . . . . . . . . . . Thermoluminescence of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Thermoluminescence of CdS Clusters in Zeolite-Y . . . . . . . .................. 5.3. Thermoluminescence of ZnS Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. A Schematic Luminescence Model of Surface States . . . . . . . . . . . . . . . . . . . . . . . . Photostimulated Luminescence of Ag and AgI Clusters in Zeolite-Y . . . . . . . . . . . . . . . . . . . 6.1. Photostimulated Luminescence of Silver Clusters in Zeolite-Y . . . . . . . . . . . . . . . . . . . 6.2. Photostimulated Luminescence of AgI Clusters in Zeolite-Y . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

325 328 328 329 339 346 346 348 364 366 371 371 371 376 378 379 381 384 390 390 390

1. I N T R O D U C T I O N O n e o f t h e m o s t i n t e r e s t i n g s u b j e c t s in m a t e r i a l s s c i e n c e is a q u a n t u m c o n f i n e m e n t e f f e c t in l o w - d i m e n s i o n a l s y s t e m s as q u a n t u m w e l l s , q u a n t u m w i r e s , a n d q u a n t u m dots. T h e int e r e s t o f this s u b j e c t r e l i e s o n t w o a s p e c t s , o n e is t h e d e s i r e to u n d e r s t a n d t h e t r a n s i t i o n f r o m m o l e c u l a r to b u l k e l e c t r o n i c p r o p e r t i e s , t h e o t h e r is t h e p r o s p e c t o f p r a c t i c a l a p p l i c a t i o n o f t h e s e m a t e r i a l s to o p t o e l e c t r o n i c d e v i c e s [ 1 ], p h o t o c a t a l y s t s [2, 3], a n d c h e m i c a l

Handbook of NanostructuredMaterials and Nanotechnology, edited by H.S. Nalwa Volume4: OpticalProperties Copyright 9 2000 by Academic Press All rights of reproduction in any form reserved.

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sensors [4]. The most striking property of low-structured semiconductors is the massive change in optical properties as a function of the size. This is the quantum confinement that is observed as a blue shift in absorption spectra with a decrease of particle size [5]. As the size is reduced to approach the exciton Bohr radius, there is a drastic change in the electronic structure and physical properties, such as a shift to higher energy, the development of discrete features in the spectra, and concentration of the oscillator strength into just a few transitions. The electronic states in the limiting three-dimensional confinement lead to molecular orbitals (strong confinement). The electronic states of a quantum dot are better described with a linear combination of atomic orbitals than bulk B loch functions in momentum space [6]. However, it is still unknown how the bulk electronic properties develop with the size of the nanoparticles. Thus it is interesting to investigate three-dimensional evolution of molecular to bulk properties in a quantum box. Theoretical models based on the effective-mass approximation have established two limiting regimes [7]: the weak and the strong confinement regimes. The first one occurs when the particle radius is larger than the exciton radius. In these regime, the exciton translation motion is confined. The second limiting regime occurs when the particle radius is smaller than the exciton radius. In these regimes, the individual motions of electrons and holes are independently quantized. In both regimes the main experimental effect of the confinement is the blue shift of the absorption edge, which is roughly proportional to the inverse of the square of the particle radius and the appearance of a structured absorption spectrum due to the presence of discrete energy levels. However, some electronic properties are expected to be modified only in the strong confinement regime, for example, the enhancement of electron-hole interaction. This is due to the increase of spatial overlap of electron and hole functions with decreasing size. As a consequence, the splitting between the radiative and nonradiative exciton state is enhanced largely in the strong confinement regime. The quantum confinement not only causes the increase of the energy gap (blue shift of the absorption edge) and the splitting of the electronic states, but also changes the densities of states and the exciton oscillator strength [6]. It was revealed that many of the differences between the electronic behaviors of the bulk and of the quantum-confined low-dimensional semiconductors are due to their difference in densities of states [8-11]. Figure 1 [ 11 ] shows the variation of density of states with dimensionality. Passing from three dimensions to two dimensions the density N ( E ) of states changes from a continuous dependence N ( E ) ~ E 1/2 to a steplike dependence. Thus the optical absorption features are different for the bulk and for the quantum well structure. The optical absorption edge for a quantum well is now at a higher photon energy than for the bulk semiconductor and,

2d ul

Energy Fig. 1. Variationof density of states of electrons withincrease of the quantization dimensionin quantum structures. (Source: Reprinted with permissionfrom [11]. 9 1996AmericanChemicalSociety.)

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PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

above the absorption edge, the spectrum is stepped rather than smooth, the steps corresponding to allowed transitions between valence-band states and conduction-band states, while, at each step, sharp peaks appear corresponding to confined electron-hole (exciton) pairs states. In the case of zero-dimensional systems (quantum boxes, dots, nanocrystallites, clusters, nanoparticles and colloids, etc.), the density of states is illustrated as a delta function. Optically, large absorption coefficients are observed, which is illustrated in Section 2.1. The low-dimensional structure has proven to be very promising for application to semiconductor lasers, which is mainly due to the quantum confinement of the carriers and the variation of the density of states with dimensionality [12]. The density of states has a more peaked structure with the decrease of the dimensionality. This leads to a change in the gain profile, a reduction of threshold current density and a reduction of the temperature dependence of the threshold current. Furthermore, improvements of the dynamic properties are also expected. Owing to the steplike density of states, high gain with lower spontaneous emission rate has been realized in a GaAs/A1GaAs graded index-separate confinement heterostructure single quantum well (GRIN-SCH SQW) laser [13]. The quantum structured laser is a promising light source for various applications and many new optical devices based on low-dimensional structures have been proposed and demonstrated. These include optical modulators [ 14, 15], optical bistable devices [ 16], tunable p-i-n QW photodetectors [ 17, 18], size effect modulation light sources [ 19], Q-switching laser light source [ 12], modulation-doped detectors [20, 21 ], and single-electron devices [22-26] (single-electron transistors [23], single-electron storage [24], single-electron computing [25], etc.). The preceding text demonstrates that low-dimensional structured materials are interesting, both for basic research and for practical applications. Low-dimensional semiconductor structures are usually fabricated by highly sophisticated growth techniques like molecular beam epitaxy (MBE) and metallorganic chemical vapor deposition (MOCVD). Those methods may provide low-dimensional structures of high quality. However, some difficulties or problems existed [ 10]. On the other hand, quantum dots can be grown in a relatively easy way, that is the chemical methods, including the colloidal method, sol-gel, Langmuir-Blodgett (LB) thin films, self-assembly, embedding in polymers, encapsulation in zeolites or in glasses, and so forth. Many terms have been used to describe these ultrasmall particles, such as quantum dots, Q-particles, clusters, nanoparticles, nanocrystals, and others. Usually, the zero-dimensional structures prepared in physical methods like MBE and MOCVD are called quantum dots by physicists, while the small particles formed in chemical methods are called nanoparticles, nanoclusters, Q-particles, or nanocrystallites by chemists. We think that the quantum dot is the same in physics as the nanoparticle (nanocrystal, nanocluster, etc.), by definition a system, where the motion of the carriers is confined in all three dimensions. Here we adapt the term nanoparticles to loosely describe semiconductors in the size regime from a few to hundreds of angstroms, which possess hybrid molecular and bulk properties and truly represent a new class of materials. Obviously, the study of these systems is a new interdisciplinary branch of the materials science developing on a border between chemistry and physics. For example, the linear combination of atomic orbitals-molecular orbitals (LCAOMO) approach provides a natural framework to understand the evolution of nanoparticles from molecule to bulk and the size dependence of the lowest excited-state energy (energy gap), while the enhancement in the excited-state oscillator strength can be best appreciated with the exciton concept in semiconductor physics [6]. There are many publications on nanoparticles, mostly on their preparation and optical characterization. Fluorescence of nanoparticles is the subject that is studied most widely but still has some problems that are not clear even at present. Here we review the experimental results and we discuss the fluorescence mechanism. In our group, we have studied the absorption and photoluminescence excitation spectra of nanoparticles systematically [43, 51, 96]. The absorption spectra of the surface states in ZnS nanoparticles were first observed by us [29] and the energy transfer from the exciton

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to the surface states was discussed. We also observed the thermoluminescence (TL) and the photostimulated luminescence (PSL) of nanoparticles [27-31 ]. The thermoluminescence is related to the surface states and the PSL is caused by the carrier migration or energy transfer from the matrix to the particles or between the two domains of the clusters. The appearance of PSL indicates that nanoparticles may find application as a medium for erasable optical storage. This is a new direction of nanoparticle application which is introduced here.

2. M E A S U R E M E N T OF SPLIT L E V E L S OF QUANTUM C O N F I N E M E N T

2.1. Optical Absorption Spectra Optical spectroscopy has played a key role in the study of nanoparticles. The quantum confinement is clearly evidenced by the blue shift of the absorption edge with decreasing size. There will be a series of discrete features appearing near the onset of absorption in the spectra. As the size is becoming smaller, these transitions occur at higher energies and are more widely spaced, as predicted by standard models of quantum confinement. These features are shown clearly in the absorption spectra of CdSe nanoparticles with very narrow size distribution [32] (Fig. 2). The quantum-size effects not only include the blue shift of the exciton energy but they also cover the increase of the exciton oscillator strength and the increase of the binding energy. The exciton absorption in bulk materials is not able to be observed at room temperature, but becomes visible in nanoparticles. This is attributed to the increase of the exciton binding energies and of the oscillator strength. The size dependence of the exciton oscillator strength is one of the most interesting subjects. In a bulk semiconductor, the electron and the hole are bounded by the screened Coulomb interaction with a binding energy of a few to tens of millielectron volts. This exciton is easily ionized at thermal energies, which accounts for the absence of a strong exciton absorption band in a bulk semiconductor at room temperature. By confining the electron and the hole in a nanoparticle, the binding energy and the oscillator strength can increase due to the enhanced spatial overlap between the electron and the hole wavefunction and the coherent motion of the exciton. This confinement effect is responsible for the appearance of the exciton absorption in nanoparticles at room temperature which is illustrated as follows [6].

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Fig. 2. Low-temperature(10 K) linear-absorption spectra of CdSe nanocrystals.Mean particle diameters are shown. These spectra have been scaled for clarity. (Source: Reprinted with permission from [32]. 9 1994 American Physical Society.)

328

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

The exciton oscillator strength is given by [33],

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(1)

where E is the transition energy, M is the transition dipole moment and is concerned with the probability of finding an electron and hole on the same site (the overlap). The oscillator strength per unit volume f l y (v being the cluster volume) determines the magnitude of the absorption coefficient [6]. In the strong confinement (R < a s ) regime, there is an increased overlap of the electron and the hole wavefunctions, and increases with decreasing cluster volume. As a result, f is now only weakly dependent on cluster size [34]. However, the oscillator strength f l y per unit volume now increases with decreasing cluster size and scales roughly as (aB/R) 3 [35, 36]. The exciton absorption band should therefore become stronger as R decreases, and so the excitonic-type features in the absorption spectra become visible even at room temperature [6]. Thus the absorption features of semiconductor nanoparticles are totally different from that of the bulk.

2.2. Photoluminescence Excitation Spectra Optical absorption spectroscopy is of course an effective method to study the quantum-size effects, particularly the enlargement of the energy gap and the enhancement of the exciton energy. However, some difficulties exist in the study of quantum-size effects by optical absorption spectroscopy. Puzzling is the fact that many of the semiconductor nanoparticles synthesized so far show no exciton absorption bands at all, in spite of the positive identification of their existence by X-ray. One of the original and basic experimental questions about quantum dots--how their electronic spectra evolve with size in the strong confinement regimemremains largely unanswered. Early work on this question was constrained by difficulties in preparing high-quality, monodisperse samples. Inhomogeneities such as distribution in size and shape cancel the higher transitions. This size fluctuation gives rise to an inhomogeneous broadening of absorption and luminescence bands that mask information about a single size. Such difficulties have been settled down. Extremely monodisperse (<5% rms) colloidal CdSe quantum dots have been obtained successfully [37] and some of the higher states were able to be resolved in the optical absorption spectra. Now, the problem is that, even though the quality of the samples approaches the highest, inhomogeneities remain that broaden absorption and conceal transitions. It was reported [38] that, in the samples with apparently very narrow size distribution, the absorption spectra show a broad rising absorption to the blue and that the discrete features in the spectra are riding on this background. Broad transitions and unresolved steps are more typically observed for semiconductor nanoparticles. Size inhomogeneity is conveniently invoked as the explanation of the inhomogeneous broadening. To further study the exciton features and to resolve the discrete states by quantum confinement, more effective methods are required. Transient differential absorption (TDA) [39], photoluminescence excitation (PLE) [7, 38, 40-44], hole-borning [45], and fluorescence line-narrowing spectroscopy [46] are the techniques which can reveal the fine structures of the absorption features efficiently. For example, TDA is an effective way to reveal the size-dependent absorption of semiconductor nanoparticles in the strong confinement regime, because it effectively increases the resolution of the spectrum by optically selecting and bleaching a subset of the quantum dots. However, in TDA the competition between bleach features and induced absorption complicates the analysis [44]. PLE may avoid such situations, therefore it has become a standard technique to obtain quantum dot absorption information which is illustrated as follows. In prior studies of luminescence in II-VI nanoparticles, it has been assumed that the excitation profile exactly parallels the absorption spectra. This assumption is of some practical importance because it implies that the most effective excitation of nanoparticles occurs with high photon energy above the absorption edge [38]. However, in both nanoparticles

329

CHEN

and quantum wells, it is often observed that luminescence efficiency drops off at high energy excitation (see Fig. 22 [96]), presumably because of the presence of nonradiative pathways [96, 103]. Thus it is of considerable interest to study the excitation spectra of semiconductor nanoparticles more fully and to compare the excitation spectrum with the absorption spectrum. PLE spectrum is obtained by monitoring the emission in a very narrow spectral region while scanning the excitation energy. The PLE spectra of semiconductor nanoparticles have been reported by several investigators [7, 38, 40-44]. It was reported by Wang and Herron [66] that the exciton transition of CdS clusters in zeolites is more pronounced in the excitation spectrum than in the absorption spectrum. Systematic studies of PLE of semiconductor nanoparticles have been carried out by Hoheisel et al. [38], Oliveria et al. [40], Rodrigues et al. [41], Norris et al. [42], Norris and Bawendi [44], and Chen et al. [43]. Here we introduce the main results of these reports. The PLE spectra of CdSe nanoparticles were first reported by Norris and Bawendi [44]. The PLE along with absorption and luminescence spectra for ~28 A radius CdSe dots are shown in Figure 3. As seen, transitions that overlap in direct absorption are resolved by PLE. The result shows clearly, that by monitoring a narrow spectral band of the full luminescence while scanning the excitation energy, PLE reveals absorption features with inhomogeneous broadening greatly reduced. Due to this increase in resolution, PLE may be used to study the evolution of quantum dot absorption features as a function of dot radius. Hoheisel et al. [38] have explored the PLE to study the electronic states of CdSe nanoparticles ranging in size from 9 to 26 ,~ radius at 77 K. It was shown [38] that in addition to the discrete manifold of quantum-confined electronic excitations, there is a threshold for continuum absorption. Excitation into this continuum, the luminescence efficiency is reduced substantially. An excitation profile or spectrum of emission is obtained

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330

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

Absorbance

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by collecting a fixed window of the emission spectrum and scanning the excitation energy. The CdSe nanoparticles show two emission bands at 2.175 and 1.65 eV, which are assigned to the band-edge and deep trap emission, respectively, [38]. The excitation spectra for the band-edge emission (2.175 eV) and for the deep trap emission (1.65 eV) are shown and compared with the optical absorption spectra in Figure 4. As each particle size emits at a characteristic energy luminescence light, a very small fraction of the size distribution is selected in this kind of measurement [38]. This leads to a narrowed, almost homogeneous spectrum. This is the advantage of PLE over reflectance absorption (ABS) for characterization of nanoparticles. The advantage of PLE over ABS is due to the difference between these two spectroscopy techniques. In PLE measurement, by monitoring a narrow spectral band of the full luminescence, while scanning the excitation energy, the inhomogeneous broadening by size distribution may be reduced greatly. Thus, the absorption features of a "single particle" may be reflected in PLE and the transitions that overlapped in direct absorption spectrum are resolved in PLE. PLE has become a standard technique to obtain quantum dot absorption information [38, 40, 41, 45]. The previous text illustrates the difference between PLE and ABS and the advantage of PLE in the study of nanoparticles. In the following we will demonstrate in more detail the difference between PLE and ABS. The fact that the discrete states could not be detected in the absorption spectra is probably due to the inhomogenous broadening. The absorption intensity is determined primarily by the numbers of occupied states in the ground level and the numbers of unoccupied states in the excited level and on the transition probability. The absorption constant is given by [47], Tt,e 2 K -- ~ fij N(E) nm

(2)

where j~j is the oscillator strength governing the transition probability. N ( E ) is the density of states function. The size distribution of the clusters may cause the N ( E ) variate in a certain region which may cause the overlap of the transitions to discrete states and may result in the broadening of the absorption band. By the principle of balance, the rate of photo-excitation in any elementary frequency interval ( d r ) must be exactly equal to the rate of generation of photons (in corresponding energy interval) by electron-hole recombination. The photo-excitation rate is given by [47], R-

f

C'q(v) K dv

331

(3)

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where q(v) is the photon density of radiation, C I is the velocity of radiation within the material, and K is the absorption constant. As q(v) increases very rapidly with increasing wavelength, the significant part of the integral in Eq. (3) is concentrated around the absorption edge where the absorption index is fairly low and the dispersion is small [47]. This is quite well in agreement with our observations [43]. Comparing Eqs. (2) and (3), one may see that even though the absorption transition between two states occurs, the transition may not be detected in the PLE if q (v) is zero. The transition between two states may be detected in absorption spectra as long as the transition is not forbidden, no matter whether the transition is radiative or not. However in PLE only the radiative transition can be measured. This is the difference between ABS and PLE. The inhomogeneity of nanoparticles may cause the absorption and emission to broaden. The overlap of transitions for particles of different sizes makes it impossible to differentiate the discrete states of a given size of clusters. However in PLE measurement, the wavelength of emitted light may be monitored within a very narrow region corresponding to nanoparticles of a certain size interval, so that the inhomogenous broadening can be avoided. Thus, in PLE discrete states of clusters may be detected and the transition bands are sharpened. This is the advantage of PLE over ABS for the characterization of nanoparticles [43, 45]. We have mentioned that, most II-VI nanoparticles show two emission bands, which are assigned to the band-edge and deep-trap emissions, respectively. The PLE for the bandedge emission is different from that for the deep-trap emission. The excitation spectra of CdSe nanoparticles for emissions at 2.175 eV (band edge) and at 1.65 eV (deep trap) are shown and compared with the absorption spectrum in Figure 4 [38]. It is noted that the excitation spectrum for the deep-trap emission at 1.65 eV more closely resembles the inhomogeneous absorption spectrum. Because, unlike the shallow traps responsible for the band-edge emission, the energy levels of the deep traps are widely dispersed within one particle size. Consequently, the entire size distribution contributes to the excitation spectrum. This is why the PLE for the deep-trap emission is closer to the inhomogeneous absorption spectrum. It can be seen from Figure 4 that the quantum yield from the first two states is the same for both the excitation spectra of the band-edge and of the deep-trap emissions, and that the amount of luminescence depends only on the relative strength of the absorption. The relative yield drops by about 50% for the third and even more for higher states [38]. This drop in quantum yield is remarkable as this means that excitation into the higher states does not result in relaxation to the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap with the same efficiency as excitation into the lower energy part of the spectrum. This has the important consequence that high quantum yields can only be obtained when excited within the lowest two states and not as is usually done by excitation in the far blue or UV for the bulk materials [38]. It is shown in Figure 4 that the excitation spectra of CdSe nanoparticles are nearly identical to the absorption spectrum near the band edge, indicating that both the first and second excited states have equal quantum yields [38]. Near 0.3 eV above the first transition, the excitation spectrum sharply deviates from the absorption spectrum, indicating that fluorescence from the higher states is not as efficient as emission near the band edge. This demonstrates that the states near the band edge are different from those further in the blue. This may be confirmed by other experiments, for example, Stark effect studies of the absorption spectrum in the presence of an external electronic field indicated that the higher excited states were much less sensitive to the electronic field than to those near the absorption edge [38]. It was shown by Oliveira et al. [40] that photoluminescence excitation (PLE) spectroscopy can be used as a probe of the quantum dot size distribution of nanoparticles. Figure 5 [40] shows the photoluminescence and absorption spectra of CdTe nanoparticles in glasses. Note that the photoluminescence spectrum shows just a shoulder at the absorption band edge, indicating that the main peak is from recombination of carriers in the surface

332

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

1.4

1.6

1.8

2.0

2.2

Energy (eV) (b)

IX.tettio,

Fig. 5. (a) Photoluminescence (PL) and absorption (ABS) spectra of CdTe nanocrystals in glasses (radius 5.0 nm). The PL spectrum was measured at 2 K with excitation at 514 nm. The ABS spectrum was measured at room temperature. (b) Schematic showing the first confined (thick lines) and trap energy levels (thin lines) for quantum dots of two different sizes. (Source: Reprinted with permission from [40]. 9 1995 American Institute of Physics.)

i,o&.

1

,

1.6

2.0

2.4

2.6

2.11

~ergy (eV) Fig. 6. Photoluminescenceexcitation spectra of CdTe nanocrystals in glasses (radius 5.0 nm). Detection energies range from below to the high-energy tail of the photoluminescence spectrum (see Fig. 5). (Source: Reprinted with permission from [40]. 9 1995 American Institute of Physics.)

traps [45, 48, 49]. Figure 5b illustrates these transitions. Due to size distribution, both the luminescence and the absorption bands are quite broad. While the PLE of the same sample they obtained shows several well-resolved lines (Fig. 6 [40]), which were only suggested by the usual absorption spectra [50]. PLE is also able to be used to study the confinement as a function of quantum dot size with only one sample, just by changing the energy used to detect the photoluminescence. It was reported [40] that the PLE spectra of different samples detected at the same energy are similar, whereas spectra detected at different energies

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1.97 eV

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2.0

2.2

2.4

2.6

2.$

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E n e r ~ (eV)

Fig. 7. Photoluminescenceexcitation spectra of CdTe nanocrystals in glasses formed by annealing at 580 ~ for 25, 50, 95, and 170 min and the average sizes of the four samples are 3.6, 4.2, 4.6, and 5.0 nm, respectively. (Source: Reprinted with permission from [40]. (g) 1995 American Institute of Physics.)

for the same sample are blue shifted as detection energy increases. These characteristics will be described as follows. Four samples of CdTe nanoparticles in glasses were formed by annealing at 580 ~ for 25, 50, 95 and 170 min, respectively, [40]. The sizes of the four samples are 3.6, 4.2, 4.6, and 5.0 nm, respectively. A series of PLE spectra were recorded for each sample only by tuning the detection energy over the spectral range of the photoluminescence (PL) spectrum [40]. Figure 6 [40] shows the PLE spectra of the sample with a radius of 5.0 nm. It was found that, for detection energy below the main PL peak, the PLE spectrum has a broad feature, whereas for detection energy at the high tail of the PL spectrum, the PLE spectrum has well-resolved peaks. In CdTe-glasses, the PL peak is mainly due to the recombination of carriers in surface states [40], therefore the PLE detects photoluminescence coming from all quantum dots with the first band-to-band transition energy higher, but not lower, than the detection energy. As a result the PLE spectra obtained at the high-energy side of the luminescence peak probe a narrower size distribution with better resolved lines than the low-energy one (see Fig. 6). Different PLE spectra for the same sample may be obtained just by changing the PLE detection energy, that is, the quantum dot size distribution may be probed within the sample. Figure 7 [40] shows the PLE spectra of the four samples. The PLE detection is the same at 1.97 eV. It is seen clearly that the PLE spectra at the same detection are similar for all the samples. Although the samples present different nanocrystallite size distribution, the probing size in each sample is the same provided that the detection energy is the same. Furthermore, we found that the blue shift of the nanoparticles upon decreasing size may be reflected in the PLE spectra if the detection energy is selected to the maximum of the luminescence peak. The PLE spectra of three samples of CdS nanoparticles with radius of 18.5, 22.0, and 23.5 * are displayed in Figure 8, respectively. The nanoparticles were encapsulated in a mesoporous zeolite [51]. It is clearly seen that, as a whole, the PLE spectrum shifts to the blue as the average size is decreased, and the separation between the excitation peaks is wider for smaller particles. The detection energy at the PL peak maximum is to probe the average size of the particles that are most concentrated. Thus the discrete states can be resolved and the blue shift can be observed. A similar result was also reported by Norris and Bawendi [44] on CdSe nanoparticles. Figure 9 displays the PLE spectra of seven samples, ranging from "--15 to *--43/~ in mean

334

P H O T O L U M I N E S C E N C E A N D STIMULATED L U M I N E S C E N C E OF NANOPARTICLES

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400

450

Wavelength(nm) Fig. 8. Photoluminescence excitation spectra of CdS clusters in a mesoporous zeolite. The sizes (from top to bottom) are 1.85, 2.20, and 2.35 nm, respectively. (Source: Reprinted from [51], with permission from Elsevier Science.)

6OO i-

5OO -

1

Wavelength (nm) 4OO

"--

""

1-

3OO

9 . . . . .

I--

_

.

-

-

1

_

I:

.&, r

C:

2.0

2.4

2.8

3,2

3.6

4.0

4,4

Energy (eV) Fig. 9. Normalized PLE scans for seven different size CdSe quantum dot samples. Size increases from top to bottom and ranges from ---1.5 to ~4.3 nm in radius. (Source: Reprinted with permission from [44]. 9 1996 American Physical Society.)

335

CHEN

Table I. Experimentaland Theoretical Results of the Interband Transitions of CdS Nanoparticles in Mesoporous Zeolite [51] Size (~) 18.5

22.0

23.5

PLE (eV)

Theoretical(eV) Assignment

3.20

3.19

1S-IS

3.66

3.82

1S-1P

4.70

4.63

1S-1D

2.93

3.01

IS-IS

3.41

3.46

1S-1P

4.49

4.03

1S-1D

2.90

2.95

IS-IS

3.32

3.35

1s-1P

4.26

3.85

1S-1D

radius (from top to bottom) [44]. It is seen clearly that the PLE as a whole shifts to the blue and the separation between the excitation peaks is increased with decreasing size. This is in agreement with the estimation by quantum confinement. As many as eight absorption features may be resolved in a single spectrum, this is due to the high quality of their samples. Interesting is that, band-edge exciton fine structure is also able to be observed in the PLE spectrum (see the additional structure on the first PLE peak of Fig. 9 [44]). This indicates that PLE is really a very good way to reveal the intrinsic absorption features of nanoparticles. The discrete peaks may be caused by the splitted states by quantum confinement and may be explained well based on the effective-mass approximation. The lowest three discrete states are IS, 1P, and 1D, who's energy may be estimated with the following formula [52, 53], h2 Enl - Eg + 2meR 2

(1921

(4)

where Eg is the bulk bandgap (Eg -- 2.58 eV for CdS), me is the effective mass of the electrons (me "-0.18 m for CdS), ~Onl are the roots of the Bessel function (qglS = 3.14, q91p = 4.49, and q91D = 5.76) [ 19, 20]. h is the Planck constant, R is the average size of the clusters. The theoretical and experimental results of CdS nanoparticles are shown in Table I [51 ]. The theoretical estimation is in agreement with the observation approximately. Thus the three absorption bands in the PLE spectrum are assigned to the IS-IS (exciton), 1S-1P, and 1S-1D transitions, respectively. The theoretical estimation based on the effective-mass approximation may explain the size dependence of the excited states well. Similar results were observed for CdS nanoparticles prepared in colloids [54] (Fig. 10 and Table II) and ZnS clusters in zeolite-Y [43]. (Fig. 11 and Table III.) What we must point out is that the calculation [Eq. (4)] is only an estimation. Because the effective-mass approximation oversimplifies the description of the crystal potential as a spherical well of infinite depth [52, 53]. However we do not have spherical clusters or nanoparticles. A rigorous treatment should consider the valence-band degeneracy and should include a better description of the valence band. The mixing between the bulk valence bands should be taken into account. As the transition strengths in quantum dots are determined by the overlap between electron and hole envelope functions [55]. Due to the valence-band mixing, the simple selection rules, that is, An = 0 and AL = 0, are

336

P H O T O L U M I N E S C E N C E A N D STIMULATED L U M I N E S C E N C E OF NANOPARTICLES

PLE ,~ PL

ABS

:5

tO 0 |

v

.__>, t'0 ..=,., r-

,l III I I

.> n,'

$

I |

~_,,...J

_0

i

i

I

i

i

250

300

350

400

450

.....

i

500

Wavelength(nm) Fig. 10. Reflectance absorption (ABS), photoluminescence excitation (PLE), and emission (PL) spectra of 2.38 nm radius CdS nanoparticles. (Source: Reprinted with permission from [54].)

Table II. Experimental and Theoretical Results of the Interband Transitions of CdS Nanoparticles with a Radius of 23.8 ,~ [54] PLE (eV)

120 100

t

Calculated (eV)

Assignment

3.35

3.22

1S-IS

4.06

3.90

1S-1P

4.84

4.75

1S-1D

ZnS/Y

ABS -"t 9

,"7,.

PLE

80 60

PL

40

i

\ 20 0

I

I

I

400

500

600

I

200

300

Wavelength(rim) Fig. 11. Reflectance absorption (ABS), photoluminescence excitation (PLE, ~,em -- 375.5 nm), and emission (PL, ~,ex = 319 nm) spectra of ZnS clusters in zeolite-Y. (Source: Reprinted with permission from [43].)

no longer valid [56]. This is why we can see the 1S-1P transitions that are forbidden in spherical approximation. It was reported by Norris and Bawendi [44] that the absorption features are most efficiently resolved in PLE when the emission is monitored on the blue edge of the lumines-

337

CHEN

Table III. Experimental and Calculated Excitation Peaks of ZnS Clusters in Zeolite-Y Experimental (eV)

Calculated (eV)

Assignment

5.35

1S-1D

4.66

1S-1P

4.12

1S-1S

--

Surface states

5.42* 5.45 t 4.50* 4.50 t 3.94* 3.75 t 3.15t * ~.em = 357.5 nm. t ~.em = 535 nm [43].

Wavelength(nm) 550 500 450 400 350 a)

PLE - - - " FIT

- - -

.,..

"1

i

xl

i

i

i

o

I

. . .

I ~ L . .

!

..

9 I

~.

- l -

- ~ 1

9 -

9 I

b) ~a)

~,i (b)

(a)

.................

=-0

2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 Energy(eV) Fig. 12. (a) Demonstration of the fitting procedure used to extract PLE peak positions. The PLE scan (solid line) is compared to the fit (dashed line) for a "~1.8 nm radius CdSe nanoparticle sample. (b) The individual Gauss,an components (solid line) and the cubic background peak is decomposed into two narrow features lightly to the red of a broader absorption peak. (Source: Reprinted with permission from [44]. 9 1996 American Physical Society.)

cence. Thus in their P L E m e a s u r e m e n t , the detection energy was selected to the e m i s s i o n e n e r g y w h e r e the blue e d g e intensity is ~ 1 / 3 of the p e a k height. As m a n y as eight features were o b s e r v e d in their P L E spectrum of C d S e nanoparticles (Fig. 3). T h r o u g h comparison of the spectra with theoretical predictions (Figs. 12 and 13), the P L E lines were assigned as follows: (a) 1S3/21Se, (b) 2S3/21Se, (c) 1S1/21Se, (d) 1P3/21Pe, (e) 2S1/21Se,

338

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

Decreasing Radius 1.2 -

a~

,.."' .,~

,.,

>

(h)

(i)

j.-

1.0

10

(O)

..

..:::.

.-

.~

*"~

/~'

o,

'~

J'"

0.6

0

~0.4

C I

~ w

.... ......

0.2

,

i 0.0 . . . . .i . 9

.,~..+.**.~..~,~-.~

'

(el .

' .

.

!

I L

9

9

I

.

,,

.

I

9

,,,.

9

I

I

2.0 2.2 2.4 2.6 2.8 Energy of 1st ExcitedState (eV)

Fig. 13. Transitionenergies (relative to the first excited states) versus the energy of the first excited state. Strong (weak) transitions are denoted by circles (crosses). The solid (dashed) lines are visual guides for the strong (weak) transitions to clarify their size evolution. (Source: Reprinted with permission from [44]. 9 1996 American Physical Society.) (f) 1Ps/21Pe (1P]/21Pe), (g) 3SI/21Se, (i)4S3/22Se (1S1/22Se, 1P~21Pe). However the assignments of lines h and j are not certain, the possible assignments are: (h) (1S1/21De, 2S3/22Se, 1S3/22Se, 2S3/21De, IDs/21De, and 4P3/21Pe), and (j) (2P~/22P e, 3S3/23Se, 2P3/22Pe, 4P3/22Pe, 2Ps/22Pe, 4Ps/22Pe, and 3S3/22De). It can be seen from Figure 13 that as the radius is decreased, the energy of the excited states is increased and the separation between the excited states is wider. Five PLE lines in Figures 6 and 7 were reported in CdTe nanocrystals doped in glasses by Oliveira et al. [40]. According to the calculation based on a modified multiband envelope function model [57, 58], the five PLE lines are assigned to h 1- --+ e +, h 2- --+ e +, h 1+ --+ e - , h 2+ ~ e - , s o - --+ e - , respectively, [40]. The calculated results also show that the excited-state energy is increased as the radius is decreased and the separation between the excited states becomes wider except for that between the two highest excited states [40]. The previous results show clearly, that the splitted absorption features by quantum-size confinement may be revealed by PLE spectroscopy due to the effective reduction of the inhomogeneous broadening. PLE has become a standard technique to obtain quantum dot absorption information.

3. EXCITATION E N E R G Y D E P E N D E N C E OF F L U O R E S C E N C E We can see in the foregoing text that in the same sample the PLE spectrum is dependent on the monitored emission wavelength. Similarly, the photoluminescence spectrum is dependent on the excitation energy or wavelength. In the following, we will introduce the dependence of the fluorescence on the excitation energy. The dependence of nanocrystal luminescence on the excitation energy has been pointed out by several researchers [7, 38, 41, 54]. However, the detailed results were reported by Chamarro et al. [7], Hoheisel et al. [38], and Rodrigues et al. [41]. It was pointed out that [38], while size is an important factor in determining the energy and relative type

339

CHEN

of emission from the nanocrystals, an equally important issue is the excitation energy or wavelength. By tuning the excitation source to the far red edge of the cluster absorption the extent of inhomogeneous versus homogeneous broadening can be ascertained because only the largest nanocrystals in the sample are excited, and line-narrowing effects are observed [59]. An opposite effect, "line-broadening," can also be expected when the excitation source is tuned far blue of the absorption edge where the homogeneous and inhomogeneous linewidths are comparable. In this limit all the nanocrystals are excited simultaneously, and the complete inhomogeneous profile of the luminescence is observed. Between the far red and deep blue excitation it is possible to generate emission starting with different initially prepared excited states leading to complex but predicated changes in the emission spectrum. This is predicated on the assumption that each nanocrystal luminescence at an energy that is determined only by its size, regardless of what photon energy it is excited with. The features of the nanocrystal fluorescence on the excitation energy may be reflected in Figure 14 and the variation follows these expected patterns [38]: line narrowing in the red, followed by complex behavior when the excitation occurs on top of overlapping states, culminating in an emission spectrum independent of the excitation wavelength. In Figure 14, as the photon energy is increased above the absorption edge of 2.1 eV one peak is observed whose emission energy shifts smoothly with excitation energy. Such a result is expected for an inhomogeneous, single state system (2.175-2.225 eV). However, because there are multiple states present, there is a photon energy (2.29 eV) where the largest nanocrystals can be excited into their second excited state, while a smaller size is excited into their first excited state. Because each nanocrystal emits at one photon energy, regardless of excitation energy, at this point the emission spectrum shows two peaks in it, one from the smaller set of clusters absorbing into their first state and the second from the larger clusters excited into their second state. As the photon energy is tuned further to the blue, this pattern is repeated between states 2 and 3. Unlike the first double peak structure, this spectrum has only a disappearing shoulder on the blue edge due to the limited range of the size distribution (at 2.34 eV). The pattern that emerges from this data is that whenever excitation

E x c i t a t i o n [eV]: 2.175

2.175

2.214

2.214

2.255 2.296

^

2.318

2.

2.340

I1

2.385 2.431 2.455

~

5

2.480

J c

2.531 2.583 2.695 , 2.818 2.952

r-1

~ 71- ' | ~ i

9

.i

9

9" 9

9 1

s" ~

i

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 $ Photon E n e ~ (eV)

fll !

1 II Wlil !

W

I

m

9

1.8 2 2.2 2.4 2.6 Photon Energy (,V)

Fig. 14. (a) Experimental fluorescence spectra for CdSe-nanocrystals (R = 16 A) for various excitation energies at 77 K. The spectrum at the bottomrepresents the absorption spectrum. (b) Simulatedfluorescence spectra to modelthe evolutionof the multiplepeak structure of the particle ensemble in the previous text. All considered excitation energies correspondto those of the experimental spectra. (Source: Reprinted with permission from [38]. 9 1994American Institute of Physics.)

340

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

2.8

~" 2.6 c9 I,U r

~

2.4 2.2

2

2

2.2

2.4

2.6

2.8

3

3.2

Excitation Energy (eV)

Fig. 15. Positionof the emission peak versus excitation energy. This plot is composed from five different samples with various particle radii (9, 13, 16, 21, 26/~). There are three different line groups indicating the emission peak due to excitation of the first (a), second (b), third and higher [all (c)] excited states. Within each line group the peak positions measured of a single sample are connected by thin lines. Note that the emission energies of all samples upon excitation of the first two excited states line up very well. They spread out for higher energy (smaller particles) indicating the effect of quantum confinement. The excitation of the third state merges into the continuum excitation indicated by the almost horizontal line at higher excitation energy. (Source: Reprinted with permission from [38]. 9 1994 American Institute of Physics.)

occurs at an energy where the sample is absorbing into two different, yet overlapping electronic states, a dual peak emission spectrum is observed. At still higher excitation energies a different behavior is observed. The new peak apparently from state 3 moves due to its inhomogeneous width to 2.175 eV, but rather than being followed by a new state from 4 at higher excitation, the emission wavelength becomes independent of excitation even when the sample is illuminated with more than 2.6 eV [38]. By plotting the peak of the emission spectrum versus its corresponding excitation source (Fig. 15) [38], the shift of the emission spectra versus excitation wavelength can be seen more graphically. A sample with no Stokes shift in this graph gives a line of slope 1 (solid line), while a size-independent Stokes shift also has a slope of 1, but with a different intercept [38]. If the emission is independent of excitation, as it is when the sample is excited in the blue of its absorption, then a horizontal line is observed. Figure 15 contains the data from each state in the nanocrystal spectrum. Even though this data was compiled from many different sizes of clusters, a consistent pattern emerges. The first group of lines (labeled as a in Fig. 15) shows [38] the position of the luminescence peak due to excitation near the red edge for all particle sizes. Like the second peak (labeled b) the behavior is nearly linear. At higher energies, corresponding to excitation of smaller particles, all the line groups fan out. The third emission peak (c) stops shifting when the excitation energy in increased further, giving roughly horizontal lines (dashed) [38]. A similar result was reported in CdSe nanocrystals embedded in a silicate glass matrix [41]. The absorption spectrum of the CdSe doped glass sample is shown in Figure 16 [41 ], it has a broad peak centered at ~ 2 . 2 4 eV and a broad shoulder at ,~2.65 eV. The blue shift of the absorption edge relative to the band gap of bulk CdSe indicates the strong confinement of the carriers. From the shift of the absorption edge, the average particle radius is estimated to be ~ 2 . 7 nm. The PL spectrum of the sample consists of a very broad band centered at ~ 1.7 eV and additional structures in the photon energy range of 2.0 ,-~ 2.3 eV. The former band is usually attributed to deep traps [60, 61 ], it is not considered further here. The sharper structure between 2.0 ,-~ 2.3 eV close to the low-energy absorption peak is referred to as band-edge emission. These structures are found to be dependent on the excitation energy (hvL) as seen in Figure 16. The behavior of the PL spectra in CdSe doped glass may be divided into three regions according to the excitation energy (hvL) [41].

341

CHEN

CdSe T-80K

~W 0.0 C

"e 0.0 v

9~raw

m C

o

t

0.0

._=

j 2.o

2.3

~ 0.0 O.,t- :1.1111eV

0.0

Owt,,~W

| I 0.0 t O.O

2.0

2.2

2.4

2.6

2.8

3.0

PhOtOn energy (eV)

Fig. 16. Photoluminescence(PL) spectra of the CdSe doped glass sample at 80 K (curves a-f). For each spectrum the excitation energy (hvL) is markedby an arrow. The identifiable peaks have been labeled AI, A-D. Curve g is the linear absorption spectrum of the same sample. The inset shows the fitting of the PL spectrumwith hvL = 2.41 eV: open circles are the experimental data, dotted lines are the individual Gaussian to which each peak was fitted, and the full line in the sum of the Gaussians. (Source: Reprinted from [41], with permission from Elsevier Science.)

When h vL < 2.2 eV (region I) the PL spectra contain two peaks whose positions vary with h vL in almost the same way [41 ]. In this region the lower energy peak (A') is weaker than the higher energy peak (A) but its intensity increases faster with hvL. Also, the widths of both peaks increase with h vL. In this region the excitation energies lie within the tails of the first absorption peak and therefore only the largest nanocrystals are excited. For hvL in this region, the allowed electronic transition in the nanocrystals must involve the lowest energy confined electrons and holes. Hence, the smaller is h vL the larger is the radius of the resonantly excited nanocrystals. Therefore, the energies of peaks A and A" shift with h VL due to change in the size of the nanocrystals that are resonantly excited. In region II (2.4 eV < hvL < 2.6 eV) [41 ] the behavior of the PL spectra are more complex. When hvL = 2.41 eV the PL has two intense peaks at -~2.09 eV (B) and ,~2.19 eV (C) and a much weaker one at ~2.31 eV (D). For higher hvL peak D disappears while peak C shifts to higher energies while its intensity decreases relatively to that of peak B. Peak B often overlaps with peak A ~. The two appear to be partly resolved for h VL = 2.497 eV. The earlier complex behavior of the PL spectra may be explained in terms of the selectively excited PL. The energy levels in CdSe nanocrystals were calculated theoretically using an envelope function approximation [55, 56]. Combining the theoretical eigenenergy values and a bulk CdSe bandgap (1.84 eV at 80 K), the size of nanocrystals which are excited by incident photos with h vL in region II can be estimated. For example, while a nanocrystal with a radius of 2.3 nm would be excited via the 1S3/2 ~ 1Se transition by a 2.41 eV photon another one with a radius of 4.0 nm would be excited by the same photon via the 1S3/2 --+ 1De transition. The excited electron-hole pairs in both nanocrystals would relax and would recombine radiatively through the 1Se ~ 1S3/2 transition. The resultant PL spectrum will consist of two peaks at ,~2.41 eV and ,-~2.03 eV due to the 2.3 and the 4.0 nm

342

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

nanocrystals, respectively. As h VL is varied, some crystals with different radii are brought into resonance while others go out of resonance. As a result a complex and rich variety of dependence of the PL spectra on h vL is observed. In region III where hvL > 2.55 eV there are so many transitions allowed for nanocrystals with different radii that the PL spectra contain signals from almost the entire distribution of nanocrystals. Therefore only a very small dependence of the PL spectra on hvL is observed [41]. Photoluminescence of CdSe nanocrystals in a glass matrix was also investigated by Chamarro et al. [7] at low temperature with size-selective excitation. Figure 17 [7] shows low-temperature absorption spectra of CdSe nanoparticles in oxide glasses at 5 K. The radii of the four samples are 15, 17, 18, and 27 A, respectively. Absorption bands from confined states at higher energy are clearly resolved, indicating a narrow size distribution. The size selectively excited PL spectra of sample 3 are shown in Figure 18 [7]. It is seen that luminescence mainly originates from band-edge state transitions. Curve a in Figure 18

1.2 /

.

'

" ...... "

'-

"

4'

/

1.0 0.8

3

'

~

0.6

2

"

~

0.4

~.8

-,

2.O

2.2

2.4

2.6

2.8

3.O

3.2

ENERGY (eV} Fig. 17. Absorptionspectra of CdSe nanocrystals in oxide glasses at 5 K. The mean radii are 1.5, 1.7, 1.8, and 2.7 nm for samples 1, 2, 3, and 4, respectively. (Source: Reprinted with permission from [7]. 9 1996 American Physical Society.)

II

),

c

W U)

=L

o

,a t,, t v

=0 "a

-I

m it 17 I= we. Im

1.9

2

2.1 2 . 2 2 . 3 ENERGY (eV)

2.4

2.5

Fig. 18. Photoluminescence spectra of CdSe nanocrystals in oxide glasses (sample 3 of [7]) obtained at 5 K with excitation at (curve a) 3.645 eV, (curve b) 2.39 eV, (curve c) 2.344 eV, (curve e) 2.26 eV, (curve f) 2.216 eV, (curve g) 2.17 eV, and (curve h) 2.142 eV. The black dots indicate the position of the F-line. The dashed curve is the absorption spectrum. (Source: Reprinted with permission from [7]. 9 1996 American Physical Society.)

343

CHEN

is obtained with an excitation energy well above the band edge, thus the emission is contributed from the entire size distribution. Curves b-h in Figure 18 are the PL spectra measured by tuning the excitation energy through the first absorption peak. For low-energy excitation, the luminescence spectrum, close to the laser position, is dominated by three lines: a line (denoted as an F-line) with an energy shift of a few meV from the laser and two lines about 26 and 52 meV from the F-line energy position. These two lines are assigned to the 1LO and 2LO phonon replica of the F-line [7]. The three lines are hardly distinguishable and the spectrum consists of two broad bands (curves b and c). For an excitation energy higher than 2.31 eV, the F-line is not observed anymore. This is similar to the results observed by Rodrigues et al. [41 ]. The size-selective technique makes it possible to study the size dependence of the PL spectra [7]. The energy difference A E between the laser energy and the F-line is plotted as a function of the particle radius. It shows clearly that AE is increased with decreasing size. From the analysis of the time decay and the steady-state and time-dependent degree of linear polarization, the F-line is attributed to the recombination of the optically forbidden A exciton [7]. The size dependence of the F-line shift (AE) was considered to be originated mainly from the size dependence of the electron-hole exchange energy which varies as 1/R 3 for small nanocrystals owing to the increasing overlap of the electron and hole wavefunctions. The small deviation of the F-line shift from the 1/R 3 dependence [7] was considered to be caused by the size dependence of the acoustical-phonon energy. Because the radiation recombination was considered to be possible through a phonon-assisted virtual transition to the confined B-exciton state [7]. This work tells us that size selectively excited PL may provide much information to the intrinsic properties of single size nanocrystals. Similar results were observed by us [51, 54] in CdS nanoparticles deposited from chemical colloids or embedded in a mesoporous zeolite. The reflectance absorption (ABS), photoluminescence (PL), and excitation (PLE) spectra of the CdS nanoparticles deposited from chemical colloids are shown in Figure 10. The average size of the particles estimated from an X-ray diffraction pattern is around 23.8 ,~ [54]. Two emission bands at 385 and 465 nm were observed and were attributed to the excitonic and trapped luminescence, respectively, [54]. The excitonic emission is much stronger than the trapped emission, demonstrating a good surface passivation of the nanoparticles. Actually, the excitonic emission band consists of two peaks and the relative intensity of the two peaks is dependent on the photoexcitation energy (Fig. 19) [54]. Excitation at 370 nm, the high-energy peak is stronger than the low-energy peak, but the low-energy peak is stronger when the excitation is at 260 or 310 nm [54]. Figure 20 shows the reflectance absorption (ABS), photoluminescence excitation (PLE), and emission (PL) spectra of CdS clusters in a mesoporous zeolite [51 ]. The average size of the clusters is around 18.5/~. The strong emission around 550 nm is assigned to the trapped luminescence arising from surface states. A small shoulder around 420 nm is probably the excitonic fluorescence. The trapped luminescence is much stronger than the excitonic fluorescence, indicating poor passivation of the surface states. Figure 21 [51] shows the emission spectra excited at 288, 368, and 434 nm, respectively. These excitation energies are corresponding to the three excitation bands in Figure 20, respectively. At high-energy excitation, both the excitonic and the trapped emission bands were observed. While at lowenergy excitation, only the trapped emission band was observed and the band was sharpened largely as the excitation energy was lower. Our observations demonstrate that not only the excitonic, but also the trapped fluorescence are dependent on the excitation energy [51 ]. It was pointed out [41] that the behavior of these selectively excited PL spectra is dependent on the nanocrystal size distribution. If the distribution is very broad, a large number of particles of different sizes are always excited. Hence a broad PL spectrum with no distinct features will be observed independent of excitation energy (hvL). On the other hand if the distribution is extremely narrow, the emission peak always occurs at the same energy determined by the unique crystal size. Only in the intermediate case of a distribution can

344

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

v>,, .i t,t) ri

r-

J

.i

EX: 310nm

i

n,"

EX: 260nm

360

I

I

I

1

I

I

380

400

420

440

460

480

500

Wavelength(nm) Fig. 19. Photoluminescence spectra of 2.38 nm-CdS nanoparticles excited at 260 nm (bottom), 310 nm (middle), and 370 nm (top), respectively. (Source: Reprinted with permission from [54].) Size:l.85nm PL

"C3 (D ,IN

ABS

PLE

600 Z

4ow eIu

-= 2 0 ,I It l

/%~"~ex?tons

'k

lg

0

I

I

I

I

300

400

500

600

700

Wavelength(nm)

Fig. 20. Reflectance absorption (ABS), photoluminescence excitation (PLE, ~emission --553 nm), and emission (PL, ~.excitation= 312 nm) spectra of CdS clusters in a mesoporous zeolite. (Source: Reprinted from [51], with permission from Elsevier Science.)

appropriate excitation energy excited several nanocrystals simultaneously producing a PL spectra which contains more than one peak and whose relative intensities and peak positions vary with h vL. We [96] have studied the excitation-energy dependence of the photoluminescence of CdS clusters in zeolite-Y in which the size distribution is very narrow. We found that the emission position is not varied with excitation energy but the emission intensity is related to the excitation energy. The luminescence intensity excited at 250 nm is lower than that excited at 310 nm (Fig. 22 [96]). This indicates that the luminescence efficiency is lower at higher energy excitation, which is contrary to that in the bulk from which the luminescence efficiency is higher at higher excitation energy. Because more nonradiative channels occurred at upper levels of the quantum box [ 104], the radiative recombination rate is lower by higher excitation into higher levels at higher excitation energy and thus the luminescence is weaker. Our observation supports the intrinsic mechanism proposed by Benisty et al. [104] for the poor luminescence properties of quantum-box systems.

345

CHEN

80 Size:1.85nm 34nm

.~ 60 :5 .,~ e-

40-

r

._> n,'

20Ex:288nm

\

I

I

I

400

500

600

700

Wavelength(nm)

Fig. 21. Photoluminescence spectra of CdS clusters in a mesoporous zeolite excited at 288, 368, and 434 nm, respectively. (Source: Reprinted from [51], with permission from Elsevier Science.) 120 ABS

,-:,. l O 0 -

._z- 8 O -

PLE

PL

~'~/

Ex: 310nm

e-

--" c

60-

ID g9

t~

40

-

20-

0

200

I

I

I

300

400

500

600

Wavelength(nm)

Fig. 22. Absorption (ABS), photoluminescence excitation (PLE), and emission (PL) spectra of CdS clusters in zeolite-Y (5 wt%). (Source: Reprinted from [96], with permission from Elsevier Science.)

From the selectively excited PL results the size distribution and the energies of excitedstate transitions can be determined by comparing with the theoretical calculation [41]. These observations tell us that selectively excited PL is a good method to study the sizedependent optical properties of nanoparticles and from which much useful information may be obtained.

4. A B S O R P T I O N A N D L U M I N E S C E N C E O F S U R F A C E STATES IN N A N O P A R T I C L E S 4.1. G e n e r a l I n t r o d u c t i o n

Optical excitation of semiconductor nanoparticles leads to band-edge and deep trap luminescence. The size dependence of the exciting or band-edge fluorescence has been studied extensively and is now well understood [10]. It is noted that the fluorescence process in

346

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

semiconductor nanoparticles is very complex and that most nanoparticles exhibit broad and Stokes-shifted luminescence arising from the deep traps of surface states [59, 62-66]. Only clusters with good surface passivation may show high band-edge emission [37]. The absence of band-edge emission has been previously attributed to a large nonradiative decay rate of the free electrons to the deep-trapped states [67]. As the particles become smaller, the surface-volume ratio and the surface states increase rapidly, thus reducing the excitonic emission via nonradiative surface recombination [68]. The earlier text indicates that the surface states are very important to the physical properties, especially the optical properties, of the nanoparticles. However, little is known about the physical properties of the surface states. Some reports [69, 70] said that the trapped fluorescence of the surface states does not vary as much upon decreasing size, while others [28, 29, 38, 61, 71 ] showed that the surface luminescence shifts to the blue as the size is decreased. These results indicate that the surface states of semiconductor nanoparticles should be investigated further. Furthermore, no absorption properties of the surface states have been reported, even if the fluorescence of surface states has been widely reported. It has been pointed out [61 ] that the surface states are not able to be detected in the optical absorption spectra. The absorption features of the surface states in ZnS nanoparticles were reported by us [29]. Here we will summarize the absorption and luminescence of the surface states in semiconductor nanoparticles. First, we introduce some basic concepts of the surface states and then we show how the surface states influence the intrinsic properties of the nanoparticles. For particles in such a small size regime, a large percentage of the atoms is on or near the surface, for example, 99% of the atoms are on the surface for a 1 nm sized particle (Table IV). The existence of this vast interface between the nanoparticles and the surrounding medium can have a profound effect on the particle properties. The imperfect surface of the nanoparticles may act as electron and/or hole traps upon optical excitation. Thus the presence of these trapped electrons and holes can in turn modify the optical properties of the particles. They can also lead to further photochemical reactions which are of considerable interest in the field of photocatalysts [72]. For example, the presence of surface-trapped electron-hole pairs can reduce the exciton oscillator strength [73], thus may modify the absorption and luminescence of excitons. The effect of a surface-trapped electron-hole pair on the optical absorption spectrum of CdS nanoparticles has been studied with timeresolved laser spectroscopic techniques [73]. Experimentally [6], it was found that the exciton absorption is bleached during the presence of the trapped electron-hole pair and recovers as the trapped electron-hole pair decays away. It was pointed out [6] that one trapped electron-hole pair can bleach the exciton absorption for the whole cluster. There must exist, therefore, a strong interaction between the trapped electron-hole pair and the exciton to cause the loss of the exciton oscillator strength [6]. The effective mass models consider crystallite internal molecular orbitals that evolve into the continuous valence and conduction bands in the bulk crystal. It may illustrate the

Table IV. Relationbetween Size and Surface Atoms

Size (nm)

Atoms

Percentage of Atoms at Surface (%)

10

3* 10 4

20

4

4* 103

40

2

2.5* 102

80

1

30

99

347

CHEN

Fig. 23. Sketchshowinginfluence of surface states on the spatial overlap of the electron and the hole wavefunction. (Source: Reprintedwith permissionfrom [6]. 9 1991AmericanChemical Society.)

blue shift of the absorption edge upon size qualitatively but it cannot explain many of the other physical properties, such as the fluorescence efficiency, because it ignores possible surface states and surface construction. Each 32 ~ diameter CdSe QC has an internal F = 3/2 highest occupied molecular orbital (HOMO). This state lies about 0.1 eV below the top of the bulk valence band [59]. Theory predicts a surface HOMO band of narrow width, composed of lone pairs on Se atoms that have three covalent bonds into the bulk lattice [74]. This surface band actually lies within the bandgap if the surface geometry is held unchanged from bulk tetrahedral angles and bond lengths. That is, an internal HOMO hole could localize spontaneously on the surface. Because there is a large increase in effective mass from a delocalized hole to one localized in the surface band, the kinetic energy of the hole is not increased substantially upon localization, as would be expected from bulk arguments alone [59]. The behavior of the surface states within the energy bandgap just like the impurity levels within the bandgap of bulk materials, will influence the physical properties largely [61]. It was predicted by theoretical calculation that in the presence of a surface trapped electron-hole pair, the exciton energy shifts to the red by only "~50 meV but its oscillator strength is reduced by 90% for a 25 A-radius CdS particle [6]. The basic physics can be easily understood by considering the overlap of the electron and hole wavefunction [6]. Without the trapped electron-hole pair (Fig. 23a [6]), the hole wavefunction (smaller, dark circle) is located at the center of the cluster and has good overlap with the electron wavefunction (shade circle). By introduction of a trapped electron and a hole to the cluster surface, the hole wavefunction is now localized in the presence of the trapped electron because of the behavior is still delocalized (Fig. 23b [6]). This may reduce the spatial overlap of the electron and the hole wavefunction and thus of the oscillator strength of the exciton. It was also expected [6] that the trapped electron is more efficient than the trapped hole in bleaching the exciton absorption because the trapped hole is not capable of localizing the electron (with small effect mass) and therefore is inefficient in reducing the electron-hole overlap. This prediction does quite well with the pulse radiolysis experiments performed by Henglein [75] and Henglein et al. [76]. It clearly seems that the surface effects of nanoparticles play a key role in their properties, from structural transformation to light emission and to solubility [ 11 ]. Furthermore, it was predicted [46, 59] that surface states near the gap can mix with interior levels to a substantial degree, and these effects may also influence the spacing of the energy levels. It was established that in many cases it is the surface of the particles rather than the particle size which determines their properties. It is, therefore, a major goal to characterize the surface states and to control them chemically [6, 11, 71].

4.2. Excitonic and Trapped Fluorescence In the following we will introduce the fluorescence of semiconductor nanoparticles with emphasis in the fluorescence of surface states and the influence of the surface states in the

348

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

1

\

J!

iI

,,..a

I!

I' II

(D r

9 ca~ 9

i

4O0

~ I ~ ~f~,i Ct 9

_~mple II "~'~ample I%. i 600

,

~--

800 wavelength [nm]

,..d

-i'~-

1000

Fig. 24. Absorptionand fluorescence spectraof CdS nanoparticles (samplesI and II). (Source: Reprinted from [71], with permission from Elsevier Science.) fluorescence of the particles. As we know, the spectral position, the ratio of excitonic to trapped fluorescence, as well as the absolute intensity are influenced not only by the particle size but also by the surface modification. Usually the band-edge emission due to the exciton recombination is very weak, and the red-shifted emission from the deep-trapped states is strong. As pointed out that, in many cases, it is the surface of the particles rather than the particle size which determines their properties. In semiconductor particles an electron-hole pair is generated by absorption of light. After being trapped on the surface, the charge carriers may recombine either under the emission of light or nonradiatively. Fluorescence spectroscopy is, therefore, an easy and sensitive method of studying the surface states of nanoparticles [71]. It is also a convenient method with which to obtain information on the energetics and dynamics of photogenerated charge carriers in small particles. The spectral distribution may give information on the occurrences, population, and depths of the surface traps. The relaxation kinetics of the charge carriers can be obtained from timeresolved measurements. It was concluded from the temperature-dependent fluorescence of quantum-sized CdS and CdSe particles that one of the charge carriers is trapped in shallow traps and the other one in deep traps [71 ]. However, which one is trapped in the shallow or in the deep traps should be studied further. The fluorescence and absorption spectra of two samples of CdS nanoparticles are shown in Figure 24 [71 ]. The sizes of the two samples estimated from the absorption edge are 34 and 43/k, respectively. Both of the two samples exhibit a sharp excitonic fluorescence band (at 435 nm for sample 1 and 480 nm for sample 2), as well as a broad fluorescence band at longer wavelengths arising from the recombination of trapped charge carriers at the surface states. The fluorescence quantum yield of the samples is in the order of 0.1 [71 ]. Cooling the sample to 4 K increases the quantum yield to 1 without significantly changing the spectral shape. Thus, it was concluded that the fluorescence spectra at room temperature may reflect the energetic distribution of the entirety of the photoexcited charge carriers [71 ]. As seen from Figure 25 [71] the addition of nitromethane (or methylviologen can quench the fluorescence of CdS nanoparticles effectively and may shift the emission to the red. In order to explain the fluorescence quenching experiments, it was assumed that the electron traps are fixed relative to the conduction band of differently sized particles. Therefore, energetically, the main difference between the two samples is the bandgap energy. Considering these assumptions and the positive reduction potentials, the fluorescence quenching behavior of CdS nanoparticles by nitromethane or by methylviologen may be explained reasonably [71 ]. It may be seen later that this assumption is consistent reasonably with the thermoluminescence results of semiconductor nanoparticles [27-29]. Although there are many publications on the fluorescence of nanoparticles [38, 59, 63, 64, 77-81], it is not clear now about the nature of the emitting states and the lumines-

349

CHEN

[

I

AI

i

sample I o~,,~ r~

~D

,l,,,a

o~,,~

10 -3 M C H 3 N O 2

r r r~

o

sample II All

,-d ~D

9

9

400

S

I

10-3 M CH3NO2

600 800 wavelength [nm]

1000

Fig. 25. Fluorescencespectraof CdS nanoparticlesbefore (solidline) and after (dottedline) the addition of 10-3 M nitromethane. (Source: Reprinted from [71], with permissionfrom Elsevier Science.)

cence mechanism. Rosseti et al. [5] and Wang and Herron [66, 82] have published data on low-temperature fluorescence measurements on colloidal CdS and CdSe particles in solution and in zeolite frameworks. The fluorescence quantum yields of their samples were extremely low and even at liquid helium temperature they were far from unity. Therefore, fluorescence was only a side reaction when compared to the main process of radiationless recombination. Due to the broad size distribution, the samples showed only a broad fluorescence band from the recombination of trapped charge carriers. Detailed conclusions on the nature of the emitting states and on the trap depths for electrons and holes could hardly be drawn. O'Neil et al. [77] have published results on both static and time-resolved fluorescence experiments performed with CdS particles which exhibited higher fluorescence quantum yield but rather broad size distribution. However, the clear differentiation between excitonic and trapped fluorescence could be made. In the meantime some highly mondisperse CdS particles have been prepared and showed high fluorescence quantum yields at room temperature [63, 83]. Eychmuller et al. [64] had reported the temperature-dependent static and the time-resolved fluorescence of CdS nanoparticles and they showed how surface chemistry influence the trap depths and how the population of traps with electrons and holes changes as a function of temperature. Their work is representative of one type of semiconductor nanoparticle fluorescence which is introduced as follows. Two samples of CdS nanoparticles were studied by Eychmuller et al. [64], sample 1 shows only the excitonic fluorescence (Fig. 26, upper), while sample 2 shows both the excitonic and the trapped fluorescence (Fig. 26, lower). The temperature dependence of the fluorescence of the two samples is shown in Figures 27 and 28, respectively. The most obvious features are a general increase in fluorescence intensity and a blue shift of the fluorescence maximum with decreasing temperature. Both the excitonic and the trapped fluorescence increase with decreasing temperature. Figure 29 [64] shows the spectral position of the maxima of the two fluorescence bands as a function of temperature. It is seen that the spectral shift of the trapped emission is much larger than that of the excitonic fluorescence. It is noted that the energetic difference between the excitonic and trapped

350

P H O T O L U M I N E S C E N C E A N D S T I M U L A T E D L U M I N E S C E N C E OF N A N O P A R T I C L E S

Fig. 26. Absorption and fluorescence spectra of CdS nanoparticles at room temperature. (~,ex -- 360 nm, upper: sample I, lower: sample II). (Source: Reprinted with permission from [64].)

Fig. 27. Fluorescence spectra of CdS nanoparticles (sample I) at different temperatures. ()~ex -- 360 nm). (Source: Reprinted with permission from [64].)

Fig. 28. Fluorescence spectra of CdS nanoparticles (sample II) at different temperatures. (~,ex -- 360 nm). (Source: Reprinted with permission from [64].)

351

CHEN

~" 22.0 ~

........

....

17.4 .

0

excitonic E 21.5-

-16.9

o

-

.

N ~

trapped

o .

"16.4

21.0

O ~

,

0

0

"

= 20.5 -, , , , , ~ " , , , , ~ . ; , , ~ - , ' . . . . . , ,,15.9 ~, 0 50 100 150 200 250 ...7 Temperature (K) Fig. 29. Spectralposition of the trapped (circles) and excitonic (dots) fluorescence maxima as a function of temperature (sample II). (Source: Reprinted with permission from [64].)

r~

700 600 500

"~_~=~=~400300

~ , 8 ~

= 200 O

100 IJ

9

0

"

"

~ ."."~ '.:,, . . . . . ~..~......... . 50 100 150 200 250 Temperature (K) "

i

i

i

"

"

9

300

Fig. 30. Excitonic(dots) and trapped (circles) fluorescence intensities as a function of temperature (sample II). (Source: Reprinted with permission from [64].)

fluorescence amounts to 0.6 "~ 0.7 eV which is considered mainly due to the energy loss of hole trapping and it was concluded that the electron traps are much more shallow than the hole traps [64]. Figure 30 [64] shows the intensity of the excitonic fluorescence (dots) and the trapped fluorescence (circles) of sample 2 as a function of temperature. It is seen that the temperature dependence of the fluorescence is very complex and that the dependence of the trapped fluorescence is different from that of the excitonic fluorescence, the latter is more complex. The decay curves of the excitonic and trapped fluorescence at different temperatures are shown in Figure 31 a and b, respectively, [64]. The decay curves of both the excitonic and the trapped fluorescence are multiexponential, but, there are some differences in the decay behavior of the two types of fluorescence [64]. It was noted [64] that a simple explanation, that is, the photogenerated electron-hole pair (exciton) gives rise to spontaneous fluorescence, cannot explain the complex behavior of the temperature dependence of the fluorescence and decay rate. It was also found that in CdS and ZnS nanoparticles the trapping of electrons is an extremely fast process occurring in the 10-13-10 -14 s time range [78, 84]. Therefore, spontaneous fluorescence has no chance to compete with this process. Thus, excitonic fluorescence was considered to be arisen via detrapping of the trapped electrons. In this sense, the traps work as a reservoir for electrons and they lead to a delayed fluorescence [71, 64]. Considering the preceding

352

PHOTOLUMINESCENCE AND STIMULATEDLUMINESCENCE OF NANOPARTICLES

4K

t/)

.,::; eo

.,.,.,

i/} 0,1 .4. r.....,.

0

100

200

300

T~me (ns)

~4s

_3o~~

b

er

C

,I

0

100 2oo The Ins)

300

Fig. 31. Decaycurvesfor the excitonic fluorescence (a) and for the trapped fluorescence (b) at different temperatures (sampleII). (Source: Reprinted with permissionfrom [64].)

measurement, a model was proposed by Eychmuller et al. [64] to explain the excitonic and the trapped fluorescence of CdS nanoparticles. They [64] proposed that the trap population depends on the temperature as schematically shown in Figure 32 for two temperatures. They distinguished between shallow and deeper traps and they considered that nonradiative processes (Knr) mainly take place out of deeper traps. The fluorescence decay rate is determined by detrapping (K d) and Knr. With decreasing temperature two effects influence the fluorescence decay in opposite ways: detrapping out of a given trap becomes slower as it is a thermally activated process, on the other hand the trap population shifts closer to the conduction band, that is, most of the electrons are then trapped in very shallow traps. It seems that the temperature dependence of the fluorescence may be explained reasonably based on the previous assumptions. Some interesting results on the luminescence of CdSe nanocrystallites have been observed by Bawendi et al. [59] who used time-, wavelength-, temperature-, polarization-

353

CHEN

m u

shallow traps .-!.-'~I

270 K

4K

Fig. 32. Modelfor the excitonic fluorescence in a CdS quantum dot with a temperature-dependent trap population. (Source: Reprinted with permission from [64].)

LumlMm:uwe

,A~orpt.~n

[,,llllrtll'14~l~t

~tlcm

Fig. 33. (Top) Absorption and luminescence spectra of 3.2 nm CdSe nanocrystals at 15 K ( ~ . e x " 440 nm); (Bottom) The same absorption spectrum but the luminescence excitation wavelength is on the red edge of the absorption band (~.ex= 550 nm) for size selection. (Source: Reprinted with permission from [59]. 9 1992 American Institute of Physics.)

resolved luminescence to elucidate the nature of the absorption and "band-edge" luminescing states in 3 2 / ~ wurtzite CdSe particles. Figure 33 shows the absorption and excitation dependent luminescence spectra of the CdSe particles [59]. The results shows that excitation in the blue gives a symmetric band-edge emission with high quantum yield (Fig. 33, upper). The entire size distribution contributes to this emission. In contrast, only the "largest" crystallites are excited selectively on their zero phonon lines by spectrally narrow excitation on the red edge of the absorption spectrum. Thus the same sample shows the cw structured emission by 550 nm excitation (Fig. 33, lower). This emission approaches

354

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

that of a perceptively monodisperse sample. There is a Frank-Condon LO phonon emission progression with bulk spacing of 200 cm -1, with a shift of ~75 cm -1 between the excitation energy and the peak of the LO zero phonon line in emission. The shape of the structured emission spectrum is nearly insensitive to excitation wavelengths on the red side of absorption, but a blur light is used, the apparent ratio of the intensities of the zero to first LO phonon decreases until the first LO phonon appears more intense than the zero LO phonon. This is a direct result of the size distribution_bluer wavelength exciting a broader distribution by exciting larger crystallites on the first LO phonon and smaller ones on their zero phonon line [59]. Such phenomena have been discussed in Section 3. Figure 34 [59] shows the time-resolved emission spectra constructed from decay curves at different emission wavelengths. There is a clear red shift of the structured spectrum during the first nanosecond. In Figure 34, the highest intensity at each time is scaled to the same absolute value; there is no resolved rise time in emission at any wavelength in this region. Figures 35 and 36 show the emission decay at the peak of the zero LO phonon lines

Time Resolved Luminescence ,,1.

9

50-250 ps

u

~50-650 !

~

550

-

ps

!

5

ns

5;5

560

565

Wavelength (nm) Fig. 34. Time-resolvedluminescence spectra showing a red slide (75 cm-1) of the spectrum as a function of time during the first nanosecond. The intensities are all scaled to the same value for comparison of line shapes. (~.ex = 549 nm). (Source: Reprinted with permission from [59]. 9 1992 American Institute of Physics.)

I.uminwoenc~m

~uto

Intenmltios) ~OK 20K 101<

..o c

0

1

2

5

4

5

6

7

Time (nsec)

Fig. 35. Luminescencedecays of the LO zero phonon line during the first nanosecond (ns) as a function of temperature. (Source: Reprinted with permission from [59]. 9 1992 American Institute of Physics.)

355

CHEN

,,, ,,

,

_i'

,,

i

ml

i

-,-w $:::t

2 e:xo 9

0

500

1000 1500 Time (nsec)

2000

Fig. 36. Luminescencedecays of the LO zero phonon line during the first 2/xs as a function of temperature. (Source: Reprinted with permission from [59]. 9 1992 American Institute of Physics.) l

w

w

CW LUMINESCENCE =10K 130 K

i

440

520

L

i

600 680 760 WAVELENGTH (nm)

840

Fig. 37. Steady-stateluminescence of CdSe nanoparticles as a function of temperature. The band-edge emission decreases but the deep-trap emission increases with temperature. (Source: Reprinted with permission from [59]. 9 1992 American Institute of Physics.)

as a function of temperature [59]. There are two distinct componentsma short, temperature insensitive component on the 100 ps scale and a long multiexponential microsecond component whose average lifetime decreases by a factor of about ~ 10 from 10 to 50 K. The temperature dependence of the continuous wave (cw) emission is shown in Figure 37 [59]. There are two luminescence bandsmthe band-edge emission and the deep-trap emission in the near IR. The relative quantum yield of the band-edge emission decreases, while the deep-trap emission increases with decreasing temperature. Figure 38 [59] shows the temperature dependence of the LO phonon structured edge emission. It can be seen that as temperature increases, the spectrum shifts to higher energy, and emission occurs to the blue of the exciting wavelength, consisting with populating states within KT (K-Boltzmann constant, T-temperature) of each other. The LO phonon linewidths also increase with temperature. The foregoing experimental results provide much information about the luminescence states and the process of CdSe nanoparticles and they indicate that the dynamic process is very complex. It was pointed out [59] that the fast component is "resonance emission," while the slow component is red shifted by about 75 cm -1 at 10 K. It is also shown that the long component is not directly excited by the laser, but is produced by radiationless transition from some other initially excited state. It was also suggested [59] that a fast ther-

356

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

~k

Gated Luminescence

~ r

t 1

15K'

Pump

545 Wavelength (nm) Fig. 38. Temperaturedependence of the luminescence spectrum. The shift between absorption and the zero phonon peak in luminescence decreases with temperature. The pump laser was almost all gained out except for a small amount which was kept in for calibration. (Source: Reprinted with permission from [59]. 9 1992 American Institute of Physics.) lb>

?oc lc>

Yea

la> Fig. 39. The three-state model. ~tbais the radiative decay from b to a, ~c is the radiationless decay from b to c, and Ycb represents the rate of repopulation of b fixed by microscopic reversibility. There is an energy offset E between b and c. A nonradiative pathway Ynr has been put to account for the decrease in quantum yield with temperature and b and c were assumed to have equal degeneracies. (Source: Reprinted with permissionfrom [59]. 9 1992 American Institute of Physics.)

mal equilibrium is excited between a weakly emitting, long-lived lower state, and a strong emitting, upper state. These results can be modeled phenomenologically by assuming that both fast and slow components occur in one nanoparticle and do not represent emission from a separate group of nanoparticles. The model is shown in Figure 39 [59], "(a)" is the ground state, "(b)" represents initially populated states which carry most of the oscillator strength in absorption, while "(c)" represents "darker" states populated principally by radiationless transitions from (b). State c has an extremely long lifetime implying that at least one carrier is localized or "trapped," producing a very poor overlap with the other carriers. It was proposed that [59] the hole is surface localized, essentially at the same energy as the internal 1Sh MO, as suggested as the surface band calculation. The experimental results can be understood if there is strong resonant mixing among the zero-order A and B internal MOs and the zero-order surface Se lone lair states. Some of the surface Se atoms will couple strongly and some will couple weakly to the interior A and B MOs. The model based on the resonance between interior and surface localized states is able

357

CHEN

i

ii

ii

LUMINESCENCE

,

i

i

it

_

S'PEC TR A

/

-- 22~,~

DIAMETER

)..

I.-tn z w tz:

J

, ,

_

I

_

I

d

360

J

440 EMISSION

"r

v

t

L

I

~

~

s2o

. e ~

/

"

6oo

WAVELENGTH

DIAMETER

seo

~so

(nm}

Fig. 40. Time-resolvedemission spectra of CdS nanoparticles at 10 K. The dashed lines refer to integrated emissionin the 0 ~ 1.5/zs time period after excitation. The solid lines refer to the 16 --~32/zs time period. (Source: Reprinted with permission from [61]. 9 1986 American Chemical Society.)

to illustrate the influence of the surface states on the intrinsic properties of the particles reasonably. Similar fluorescence behavior in CdS clusters was investigated by Chestnoy et al. [61 ]. They also measured the time, wavelength, temperature, and physical size dependence of the CdS cluster luminescence, focusing on the excited relaxation and luminescence process following optical excitation. Excitation of the colloidal clusters produces a broad luminescence band in the visible region of the spectrum as shown in Figure 40 [61]. Through analysis, they attributed the luminescence to a photogenerated, trapped electron tunneling to a preexisting, trapped hole and they pointed out that the range of tunneling distance is almost independent of cluster size. The optical line shape and the temperature dependence of the lifetime indicate that the carriers are very strongly coupled to lattice phonons. All these phenomena indicate that the luminescence processes in these particles are quite complex and await to be studied further. A growing part of the present activities on semiconductor nanoparticles is motivated by the search for mechanism of energy relaxation, because the study of the energy or carrier relaxation by time-resolved PL is helpful for understanding the luminescence process that is still a controversial subject. For example, Bawendi et al. [59] have argued that the bandedge emission comes from the recombination of a shallow surface trapped hole with a delocalized electron, whereas Eychmuller et al. [64] considered that in CdS nanoparticles it is the electron that becomes surface trapped. These need to be revealed further. In bulk semiconductors, the energy relaxation of nonequilibrium electrons and holes is mainly mediated through interaction with an LO phonon or, in highly excited materials, proceeds by carrier-cartier scattering. In systems with a discrete energy level structure, the relaxation process is modified when the level spacing grows with increasing confinement. It has been suggested that the cartier relaxation is suppressed unless the level separation equals the LO-phonon energy. This slowing down of the relaxation rate of nonequilibrium charge carriers is referred to as the phonon bottleneck [79]. The survival of a hot carriers

358

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

9

II 9 ' . r " 1

.

I

. . . .

'I

;" " ' ; '

"

~

""

';

"

I

"

"

"

"

I

-

"

to I

/

/

~

-50 fs

40

'~4 t,r

~30 :::I 0

t~

~ 20 r

l.==l

lps

,-.l I0

cw' t O

ItJtl,.l

2.3

i = . . l l l . l l *

2.4

2.5

2.6

~_

I I L J l l

2.7

l,J*

2.8

!.9

Energy (eV) Fig. 41. Time-resolved(solid lines) and cw (dashed line) photoluminescence spectra of CdS nanoparticles measured at pump flounce COp= 5 #J cm-1 . (Source: Reprinted with permission from [80]. 9 1996American Physical Society.)

system over a long time increases the probability of carriers escaping from the quantum dot into the matrix and therefore would cause nonradiative recombination. The hindered relaxation could provide an intrinsic mechanism for low luminescence efficiency [79]. Femtosecond measurement is important in the study of such processes as well as the carrier dynamics in semiconductor nanoparticles. Such measurements have been carried out on CdS [80], CdSe [7, 79, 81, 85-87] and lnAs [88] quantum dots. Here we introduce some of these results. The time-resolved spectra of the band-edge emission of CdS nanocrystals recorded at different delay times are shown in Figure 41 (solid lines) along with a cw (time-integrated) spectrum (dashed line) recorded at the same pump intensity [80]. The up-converted PL spectra exhibit a number of discrete features which are not resolved in the cw spectrum. The striking feature of the recorded spectra is the extremely fast buildup dynamics in the whole spectral range. The initial relaxation time of spectrally integrated PL derived from the spectra in Figure 41 is ~ 1 ps. The analysis of the relaxation dynamics performed shows that transitions ~r2 and or4 (as denoted in Fig. 41) have relatively slow and nearly the same relaxation dynamics with time constant 20-23 ps. Whereas the transitions Orl and nr3 are characterized by much shorter relaxation times of about 1 and 3 ps, respectively. As seen from Figure 41 as well as reported in the literature, there is a red shift from the lowest absorption peak to the band-edge emission. The shift amounts to several tens of meV and has been explained either by strong electron-phonon interaction [89] or by the presence of localized states (surface and/or defects) involved in the band-edge emission [90]. Two maxima were observed in the femtosecond spectra of CdS particles (Fig. 41 [80]): one (short-lived) at the transition between the lowest extended states (or1), and the other (long-lived) at the position of the cw-emission maximum (~2). This observation excludes the explanation of the PL red shift by strong coupling of extended states to lattice vibrations and it supports the presence of the localized states in the energy bandgap which are involved in the band-edge emission. A model (Fig. 42 [80]) was proposed to illustrate the femotsecond PL discrete features in CdS nanocrystals. The trapping of carriers

359

CHEN

e

Is e

~2 = 20 - 30 ps Band-Edge Emission

El' t o a

% [c~

!%

o e e

'Deep-Center, ' Emission t

t I n

A~ _

Ao

i

:,

A i

~n= lps h

I sh

Fig. 42. The tentative scheme of energy levels in CdS nanocrystals which accounts for the PL spectra: lse and lsh are the lowest electron and hole extended states, A0 and A- are the levels of the neutral and the

charged acceptors, respectively. ET is a deep electron trap. Thin solid and dashed arrows show the transitions leading to the band-edge and the deep-center emission, respectively.Thick solid arrows show the carrier trapping. (Source: Reprinted with permission from [80]. 9 1996 American Physical Society.)

to deep centers or surface states is an ultrafast process and the carrier trapping is directly related to the relaxation time. The involvement of the surface states in the edge or excitonic emission may explain the time-resolved PL of the nanocrystals reasonably. The dynamics of photoluminescence in CdSe quantum crystallites was studied by Lefebvre et al. [81 ] via time-resolved photoluminescence measurement. The medium-size CdSe nanocrystallites with diameters larger than the bulk exciton Bohr diameter were chosen for the studies because it was considered that size distribution is a severe obstacle for absorption and luminescence properties of small particles and that in the case of small crystal radii, even small size fluctuation causes important variations of transition energies [81]. It appeared that in such small crystallites, extrinsic states localized at the interface between the semiconductor and the matrix play a major role in recombination dynamics and it has been shown that near band-edge emission in small CdSe or CdS crystallites results from quantum mechanical resonance between exceptions and "extrinsic" surface states. In the medium-size particles, both the size fluctuation and the surface effects may be reduced efficiently, thus the medium-size particles should be good candidates for detailed studies of the dynamics of recombination processes [81 ]. The absorption and PL spectra of four samples of CdSe nanoparticles are shown in Figure 43 [81]. The absorption blue shift, particle sizes, and decay time constants of corresponding PL peak are given in Table V. The radii were deduced and the free exciton radiative lifetime was estimated by using the model of Kayanuma [35]. Sample 4 corresponds to a stronger confinement regime, while samples 1-3 belong to a moderate quantum confinement. In samples 1-3, the excitonic lines (A-C) of hexagonal CdSe were observed in the absorption spectra and sharp excitonic emission lines were measured. While in sample 4 excitonic lines are no longer resolved. This indicates that in moderate-size nanocrystallites, the inhomogeneous broadening due to size distribution is quite small. In samples 1-3, the sharp lines above 1.85 eV are mainly due to recombination of "free" excitons, while in sample 4 (also sample 3), the broad low-energy bands are assigned to the interface or surface states [81 ].

360

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

9

I

~

|

,.

9

i

"

1

9

s

"

I

"

!

,~_~

,-'* .... %,.

.~

.'.;2:::"'"" ....

-

r,13

<

Z

9 B J :. ~

,

t T=2K

..... : .... ~ .... .._-",

. .... ~

.

,

. !

1.6

1.7

9

t

9

1.8 1.9 2.0 2.1 2.2 PHOTON ENERGY (eV)

I

2.3

.

2.4

Fig. 43. Static low-temperature (T = 2 K) absorption (solid curves) and PL (dotted curves) spectra of four glass samples containing CdSe nanocrystals. The energy of the A line in the bulk CdSe is 1.826 eV. (Source: Reprinted with permission from [81].)

Table V. MeasuredBlue Shift of the A Excitonic Absorption Lines and Decay Time Constants of the Corresponding PL Peak for the Four Samples of CdSe Nanocrystallites Absorption Blue Shift (meV)

Crystallite Radius(aB)

1

16

2.33

32

79

2

44

1.89

31

117

3

64

1.76

54

135

4

220

1.55

170

165

Sample

Measured Calculated rl (ps) rlr (ps)

(Source: Reprinted with permission from [81].)

Two PL spectra of sample 2, corresponding to different time-scale integration are shown in Figure 44 [81 ]. It shows clearly that the low-energy emission band (below ,-~ 1.84 eV) exhibits a radiative decay much slower than the excitonic lines. It was observed that a strong enhancement of the intensity ratio between the high-energy and low-energy contributions, when increasing the excitation power density. This proves that the low-energy band can be attributed to a relatively small number of interface states [81 ]. The decay curves of the three spectral bands as indicated in Figure 44 are shown in Figure 45 [81]. The decay time constants of the three bands are given in Table V. It was demonstrated [81] that the decay behaviors are dependent on the particle size and size

361

CHEN

Fig. 44. Emission spectra of CdSe nanocrystals in glasses (sample 2 of Fig. 43) under pulsed laser excitation at Jk = 400 nm. Solid circles represent the PL signal integrated during 7 ns after the beginning of the emission. Open circles show the part of the previous signal which is collected after the first nanosecond of emission. The B 1 and B2 bands have almost disappeared after delay, while the B3 band is much weaker, but with a much slower decay. (Source: Reprinted with permission from [81].)

Fig. 45. Normalized intensities of B1, B2, and B3 emission bands versus time. Lines show biexponential decays for B 1 and B2 with the same time constants of 31 and ~300 ps, with different relative starting amplitudes. The much slower decay of the B3 band can be fitted by using a time constant of ~14 ns. (Source: Reprinted with permission from [81].)

distribution. T h e m e a s u r e d d e c a y s b e c o m e m o r e and m o r e m u l t i e x p o n e n t i a l u p o n d e c r e a s ing the a v e r a g e crystal radius. This is o b v i o u s l y d u e to the s u p e r i m p o s i t i o n o f v a r i o u s contributions o f d i f f e r e n t p h y s i c a l origins and f r o m different crystallites. This effect is v e r y

362

PHOTOLUMINESCENCE AND STIMULATEDLUMINESCENCE OF NANOPARTICLES

evident on the time-resolved PL study of sample 4 [81]: the observed decay can be fitted by using a slow exponential component with a time constant increasing progressively from ~ 2 ns at higher energies (up to 2.0 eV) to ~15 ns around 1.55 eV. In sample 4, the fast contribution of intrinsic states has almost disappeared. From the earlier results, three different mechanisms with differentiable time regimes, were identified by the authors [81 ]. The first one corresponds to near-band-edge emission with very fast decay rate (rl "~ 30 ps) and probably involves "volume" states. The second is the slow regime at lower energies that most probably involves radiative and nonradiative transitions between pairs of trapped electrons and holes at surface sites. The long exponential decay suggests both carriers localized on surface states with small overlap between the electron and hole wavefunctions (donor-acceptorlike). This is supported by the observation that the lower the energy (the deeper the traps) the slower the recombination rate, which is typical of donor-acceptor recombination. The third, the intermediate regime, with decay times of "~ 150-300 ps, is observed within a range of a few tens of meV "on top of" and below the principal peak. This component is believed to involve the radiative decay of trapped excitons at surface sites. It was also observed that [81], as temperature increases, the r3 time constant, corresponding to the slow-rate emission (B3), decreases quickly and becomes of the order of 400-500 ps, for T > 40 K. This behavior may result from thermal detrapping of electrons from the shallow surface states. Considering the previous results, a luminescence model is proposed by Lefebvre et al. [81] and is shown in Figure 46, (0) being the ground state. The luminescence processes may be illustrated well according to this model. The incoming photons result in the formation of electron-hole pairs which thermalize into bulk excitons and surface states. The free excitons partly decay radiatively with a radiative lifetime fir and partly relax toward surface-trapped excitons with time constant rl. Once the excitons are surface trapped they recombine radiatively with a time constant r2. Lastly, the surface-trapped electron-hole pairs recombine either radiatively or nonradiatively with a global decay time r3. There is much work dedicated to the luminescence of nanoparticles that is not mentioned here and there are many models that were proposed to explain the luminescence mechanism in nanoparticles [93-95]. However, there is no common agreement on the luminescence process. Usually, the PL spectra of CdS, CdSe, and CdSxSel_x nanoparticles consist of two broad bands: one (high energy) at the band-edge spectral energies and the other (low energy) in the near-IR-red spectral range [59, 62-66]. The low-energy band has been suggested to originate from donor-acceptor recombination involving deep defect states associated with sulfur vacancies or surface states [61, 66]. The high-energy band (band-edge emission) has been explained by a different recombination mechanism, such

Electron-hole pairs Free ex,citons ~t~ - ~.zk-E I t

Ir

i excltons Trapped 1:2

I

I Trapped electrons 9& holes ,1;3nr

B1

1

IB3

(o) Fig. 46. A sketch of a four-level modeling of recombination dynamics in the present medium-size nanocrystals. The recombination of trapped excitons does not correspond simply to the B2 line of Figure 43. (Source: Reprinted with permissionfrom [81].)

363

CHEN

as the recombination of the delocalized electron-hole (e-h) pairs strongly coupled to lattice vibration [89] or recombination through localized states, possibly, of surface origin [44, 59, 90, 91 ]. In a word, the luminescence process is very complicated and is not very clear even at present, partly the fluorescence behavior is closely related to the conditions and methods of the sample preparation. However, it is agreed that surface states are involved and they play a key role in the luminescence of nanoparticles. 4.3. Absorption of Surface States [29] Mostly, the involvement and the influence of surface states in the excitonic or band-edge fluorescence of nanoparticles are deduced from the time-resolved measurement. Direct observation on the interaction between the excitons and the surface states (trapped carriers) may be obtained from their absorption spectra. The absorption feature of surface states of nanoparticles is probably important to understand their luminescence process. However no report was dedicated to their absorption features and it was pointed out [61] that the surface states of small particles are not able to be measured in the optical absorption spectra. However, no one knows why the surface states are not able to be detected in the absorption measurement. Probably the contents of the surface states are "too low" that they are not able to be measured in the optical absorption spectra, because most reported nanoparticles are prepared and are stabilized by capping or by passivation with organic or oxide materials. The surface states may be increased largely in the small particles without capping or passivation and may be measured in the absorption spectra. In this way, we have measured the absorption spectra of ZnS nanoparticles successfully [29]. The uncapped ZnS nanoparticles were prepared with the mixing of Zn(NO3)2 with Na2S in ionized water [29]. Figure 47 shows the X-ray diffraction (XRD) patterns of the ZnS nanoparticles. It was revealed that the particles exhibit a zinc-blend crystal structure. The three diffraction peaks are corresponding to (111), (220), and (311) planes of the cubic crystalline ZnS, respectively. Due to size effect, the XRD peaks are broadened and their widths become larger as the particles become smaller. The average sizes of the particles prepared at room temperature (RT), 50 and 80 ~ estimating from the Debey-Scherrer formula, are 1.24, 1.65, and 2.28 nm, respectively. Two broad absorption bands appear in the reflectance absorption spectra of the ZnS nanoparticles (Fig. 48). Obviously, the absorption band around 300 nm is the interband

(111)

ffl e,e-

n,, 2.28nm

20

I

I

I

1

30

40

50

60

70

2theta(deg.)

Fig. 47. X-raydiffraction patterns of the uncappedZnS nanoparticles. (Source: Reprintedwith permission from [29]. 9 1997AmericanInstitute of Physics.)

364

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

100

80Z~ v ti} tO E t~ d~ i,... 0 U} ..O

60-

40-

<

surface states

20ZnS/Y

0 200

I

I

I

I

300

400

500

600

700

Wavelength(nm)

Fig. 48. Reflectanceabsorption spectra of the uncapped ZnS nanoparticles and of the ZnS clusters in zeolite-Y (ZnS/Y). (Source: Reprinted with permission from [29]. 9 1997 American Institute of Physics.)

transition or exciton absorption of the particles, which has been studied extensively [ 10, 53]. Our interest is in the second absorption band in the long wavelength region which has not been reported previously. We think they are caused by the surface states of the ZnS nanoparticles. Because the absorption is lying below the absorption edge of the particles, that is, the absorption energy is lower than the bandgap of the particles. As the ZnS nanoparticles were prepared in the same way mentioned in this chapter, but were encapsulated into the cages of zeolites (zeolite-Y), the absorption in the long wavelength region disappears totally (Fig. 48), since the surface of the particles may be passivated well within the cages of the zeolite and the contents of the surface states may be reduced largely [ 1, 91 ]. What we should point out is that, we say the contents of the surface states in a capped or passivated nanoparticle are too low. Here the too low has only a relative meaning. Because strong trapped fluorescence arising from surface states may be observed from the capped or passivated nanoparticles [63-66, 77, 92]. The surface states in these small capped particles are still more abundant as compared with that in bulk materials. We think that the fluorescence efficiency of the surface states is higher than that of the band-edge or excitonic emission. This is why the trapped fluorescence of the capped particles is so strong, even if the contents of the surface states in the particles are relatively low. At least two factors may make the surface luminescence efficiency higher than that of the band-edge or excitonic emission. One is that, in small particles, the trapping of electrons to the deep traps (surface states) is an extremely fast process occurring in 10 -13 to 10 -14 s time range [78, 84], spontaneous fluorescence has no chance to compete with it, band-edge or excitonic fluorescence can only arise via detrapping of the trapped electrons. Another is that, as the absorption of the surface states is extended to the absorption edge of the interband transition or to the exciton absorption and, due to the strong electron-phonon interaction [38, 88], the band-edge or excitonic emission is shifted to the low-energy side of their corresponding absorption edge, thus the band-edge or excitonic emission is effectively overlapped with the absorption of the surface states. The energy transfer from the interband states or excitons to the surface states may occur easily via reabsorption. This may explain why most nanoparticles exhibit the fluorescence of surface states excited by the band-toband absorption or by the exciton absorption (see Fig. 49 and Refs. [61-65, 98]). We think that the energy transfer from the interband states or excitons to the surface states is more significant in determining the luminescence efficiency of the surface states.

365

CHEN

12 1.24nm "7", ::=

r

9nm

r162 r r

6l

ID

n

~

~

2.28nm

._>

re

3-

300

,/ /..,.: I

I

I

I

I

I

350

400

450

500

550

600

650

Wavelength(nm)

Fig. 49. Photoluminescencespectra of the uncapped ZnS nanoparticles. (~excitation Reprinted with permissionfrom [29]. 9 1997 AmericanInstitute of Physics.)

=

280 nm). (Source:

It can be seen from Figure 48 that as the size decreases, the absorption of the surface states becomes more intensive and the absorption peak shifts to the blue. This indicates that the contents of the surface states increase as the size of the particles decreases. Because the surface-volume ratio increases as the size decreases, ions at the surface increase rapidly, surface states (dangling bonds, defect sites, or traps) increase rapidly via surface reconstruction. Figure 48 also tells us that the energy of the surface states is correlated to the particle size, which is in agreement with the luminescence observations which is illustrated as follows. 4.4. Size Dependence of Trapped Luminescence from Surface States Although there are a lot of publications on the luminescence of nanoparticles, only a few were dedicated to the size dependence of the trapped fluorescence from the surface states. Some reports [69, 70] mentioned that the trapped fluorescence is not dependent on the particle size, while others [28, 29, 38, 61, 71 ] demonstrated it is. It is important to reveal whether the trapped fluorescence is dependent on size or not. Because by examining the size dependence of the two main emission features, that is, the excitonic and the trapped emissions, it is possible to determine to what extent the carriers are confined in the emitting state. If the fluorescence and the absorption of the surface states is dependent on size, it indicates not only the excitons but also the trapped carders at the surface states are confined by the quantum-size effect. If the fluorescence of the surface states is not dependent on size, reflecting that the trapped carriers are not confined by quantum-size effect. This is also important, it may tell us whether we can adjust the energy of surface states via quantum-size effect or not. To reveal this, we studied systematically the size dependence of the surface fluorescence. Our results support that the size dependence of the nanoparticle fluorescence. We prepared ZnS and CdS nanoparticles in two ways, encapsulation in zeolites and encapsulation by organic compounds. The size of particles in zeolites is controlled by loading and the size of the organic capped particles is adjusted by the reaction temperature. The PL emission spectra of ZnS clusters in zeolite-Y are shown in Figure 50 [43]. Two emissions around 535 and 355 nm are observed from ZnS clusters in zeolite-Y (Fig. 50). The emission at 535 nm is strong and broad, while that at 355 nm is weak and sharp and near the onset of the absorption edge (Fig. 11). The 355 nm emission was assigned to the excitonic and the 535 nm emission was assigned to the trapped luminescence. Both the excitonic and the trapped luminescence bands are size dependent, shifting to the blue as

366

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

100 80r

60-

oo37181

r--

.-, _=

40

20-

0

I

I

I

I

I

I

I

350

400

450

500

550

600

Wavelength(nm) Fig. 50. Emissionspectra of ZnS clusters in zeolite-Y. ( ~ , e x (Source: Reprinted with permission from [43].)

319 nm and 1-1, 2-3, 3-5, and 4-10 wt%).

--

(111)

A

(220)

(311)

Q_ r

r(D r

._> (D

r~

20

t

I

I

30

40

50

30

2Theta(deg.) Fig. 51. X-raydiffraction patterns of CdS clusters in a mesoporouszeolite. (Source: Reprinted from [51], with permission from Elsevier Science.) the particle size is decreased. The intensities of both the two emissions first increase then decrease upon decreasing size. This indicates that both the excitons and the trapped carriers at surface states are confined by the quantum-size effect. Zeolite-Y belongs to the microporous zeolites in which the size of the cages is smaller than 1.5 nm. In the mesoporous zeolites the cages are bigger. Thus it is easy to adjust the cluster size and to form clusters with different sizes in the mesoporous zeolites. Figure 51 [51 ] shows the X-ray diffraction (XRD) patterns of the CdS clusters in a mesoporous zeolite. The XRD lines of the zeolite host have been removed artificially from Figure 51. It is revealed that the clusters exhibit a zinc-blend crystal structure. The three diffraction peaks are corresponding to (111), (220), and (311) planes of the cubic crystalline CdS, respectively. Due to size effect, the XRD peaks are broadened and their widths become larger as the clusters become smaller. The sizes of the clusters calculated from the Debey-

367

CHEN

60

50-

/ ~

\

/2"20"r~ .35nm

40-

ee-

--

A

0.86nm I1~ 0.92nm | 1.36nm | 1.72nm / 1.85nm

30-

IID

/

20

10

0 350

i

i

i

I

i

i

400

450

500

550

600

650

Wavelength(nm)

Fig. 52. Photoluminescencespectra of CdS clusters in a mesoporous zeolite. (~excitation (Source: Reprintedfrom [51], with permissionfromElsevier Science.)

=

312 nm).

Scherrer formula are 0.86, 0.92, 1.36, 1.72, 1.85, 2.20, and 2.35 nm, respectively. As expected, the size of the clusters increases as the CdS loading increases. It is also found that as the size becomes bigger, the XRD peaks shift to higher in 20 (for (111) peak, from 26.3 ~ for 0.86 nm to 27.2 ~ for 2.35 nm), indicating a contraction in lattice constants upon increasing size. This contraction is probably caused by the static pressure of the zeolite framework, because as the clusters grow bigger, the static pressure from the zeolite framework is higher. The emission spectra of the CdS clusters in the mesoporous zeolite are shown in Figure 52 [51 ]. The emission consists of two bands, a strong one around 550 nm and a small shoulder around 420 nm. The band around 550 nm is much stronger, broader, and Stokesshifted. Obviously it is the so-called trapped luminescence arising from the surface states. The shoulder around 420 nm is at the absorption edge (Fig. 20) [51 ] and is attributed to the band-edge or excitonic emission. Both the two emission bands become more intensive and shift to the blue as the size of the clusters is decreased. This demonstrates further that both the exciton and the trapped carders at surface states are confined by quantum-size effect. The size of organic capped clusters may be controlled by the reaction temperature. Figure 53 [28] shows the X-ray diffraction (XRD) patterns of the ZnS nanoparticles prepared at different temperatures. The ZnS particles were formed by the reaction of Zn(NO3)2 with Na2S in a stabilizer agent. It is revealed that the particles exhibit a zinc-blend crystal structure. The three diffraction peaks are corresponding to (111), (220), and (311) planes of the cubic crystalline ZnS, respectively. The average sizes of the particles prepared at room temperature (RT), 50, 100, and 200 ~ estimating from the Debey-Scherrer formula, are 18.1, 25.0, 27.4, and 30.1 A, respectively. The reflectance absorption and the emission spectra of the ZnS nanoparticles prepared at RT are shown in Figure 54. Obviously, the emission in Figure 54 belongs to the trapped luminescence arising from surface states. No excitonic emission is observed, reflecting that the surface passivation of the particles is not good. The luminescence intensity increases as the size is decreased (Fig. 55 [28]), indicating the increase of surface states upon decreasing size. The trapped emission shifts to the blue as the size decreases, demonstrating that the emission energy of the surface states is also correlated to the quantum-size effects.

368

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

6OOO

(tU) SO00

(zzo) (}it)

J 2oo0 111(10

20.00

30.00

~.00

~0.00

(0.00

~l~eta Id~.l Fig. 53. X-ray diffraction patterns of ZnS nanoparticles prepared at room temperature (RT), 50, 100, and 200~ respectively. (Source: Reprinted with permission from [28]. 9 1997 American Institute of Physics.) 35 ""-.

28-

/)

ABS

PL

"

:3

tO C

21-

e-

o c

/

,.Q L

o w

14-

,.Q

'

/

",

C tO

E

:3 --I

_

o

0 200

I

I

I

I

I

I

250

300

350

400

450

500

550

Wavelength(nm)

Fig. 54. The reflectance absorption (ABS) and the luminescence (PL) spectra of ZnS nanoparticles prepared at room temperature. (~.excitation-- 340 nm).

In Section 4.3 it was demonstrated that uncapped ZnS clusters may be deposited from colloids and from which the absorption of surface states was observed. The photoluminescence spectra of the uncapped ZnS nanoparticles are shown in Figure 49 [29]. Obviously, the broad and Stokes-shifted emission band is the so-called trapped luminescence arising from the surface states [64]. It can be seen that the surface emission becomes more intensive and shifts to the blue as the size of the particles is decreased. The size dependence of the surface emission is in agreement with that of the long-wavelength absorption band in Figure 48 [29]. Both the absorption and the emission results show that the energy of the surface states is size dependent. The results reported by Hoheisel et al. [38] show that both the excitonic and the trapped emission peaks shift in energy as a function of size (Fig. 56 [38]). This indicates that one or the other or both of the two carriers must be in a confined interstate of the nanoparticles

369

CHEN

35

1-RT I 2-50~ I 3-100~ I 4-200 ~ I

1 28-

=.

(g

/

21-

M C

9C

14-

/

,s

3 "',,

~

4

_

I

I

I

I

I

35O

400

450

500

550

Wavelength(nm)

Fig. 55. Luminescence spectra of ZnS nanoparticles prepared at room temperature (RT), 50, 100, and 200~ respectively. (~.excitation"- 340 nm). (Source: Reprinted with permission from [28]. 9 1997 American Institute of Physics.)

Radius: 21 A ~

~

16A

C

0A 1.5

2

2.5

3

Photon Energy (eV) Fig. 56. Absorption (thick lines) and fluorescence (thin lines) spectra for CdSe nanocrystals at T = 77 K. The excitation energy is indicated by the high-energy starting point of the fluorescence spectrum. (Source: Reprinted with permission from [38]. 9 1994 American Institute of Physics.)

when recombination occurs [38]. That is the trapped carders in the surface states are also confined by the quantum-size effect. This is consistent with our observations and supports that the surface fluorescence is size dependent. The shift of the surface emission due to size variation has been reported and discussed by Chestnoy et al. [61]. The correlation between R (the distance between two trapping sites) and emission wavelength of the defects in bulk II-VI semiconductor crystals was borrowed to explain the size dependence of the surface emission. The Coulomb potential e 2/e R was considered to be an important term determining the emission energy of the surface states and it was pointed out that close

370

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

pairs with small R emit at higher energy than distant pairs, for fixed diameter D [61 ]. This model is not able to illustrate the size dependence of the surface emission reasonably, because the distance between two trapped carriers is not related to the size of the particles but to the contents of the trapped carriers. It was also pointed out in Ref. [61] that the short-range well depths of the charged carriers (D+ and D_) and the lowest delocalized state (Ex) of the crystallite are dependent on the diameter D, however no definite relations between these parameters and D were pointed out. We are confused with the two emission bands with different wavelengths from the surface states of a given size particle [61 ] and how such an estimation is related to the size dependence of the surface emission. Here we propose a new model to explain the size dependence of the surface states. Our model is based on the observation of the thermoluminescence (TL) of semiconductor nanoparticles [27-29].

5. T H E R M O L U M I N E S C E N C E OF NANOPARTICLES [27-29]

5.1. Introduction First, we illustrate simply what is thermoluminescence and the difference between thermoluminescence and photoluminescence. Whenever a semiconductor is irradiated, electrons and holes are created. If electron-hole pairs recombine immediately and emit a photon that is known as fluorescence. If the electrons and holes created do not recombine rapidly, but are trapped in some metastable states separately, they need energy to be released from the traps and recombine to give luminescence. If the detrapping process is caused by heating or thermostimulation, the luminescence is called thermoluminescence (TL). Thermoluminescence is a good way to detect the recombination emission caused by detrapping of carriers thermally. The energy corresponding to the glow peak is equal to the trap depth. What we must point out is that traps and carriers (electrons and holes) may be produced by irradiation, they are also able to be created during sample processing. Because as the particles become smaller, ions at the surface increase rapidly, and most ions at the surface are not saturated in coordination, electrons or holes may be excited easily and may escape from ions, then are trapped at surface states located in the forbidden gap [53, 61 ]. Carriers trapped at the surface states or defect sites may be released by heating to recombine to give the so-called thermoluminescence (TL). Obviously the TL process is different from the PL process, as the energy of thermostimulation is not sufficiently high to excite the electrons from their ground states to their excited states. Only the carriers ionized from the surface states or defect sites are involved in the TL process, that is, the thermoluminescence is arisen from surface states. Surface states are abundant in nanoparticles and it is the surface states that cause the fluorescence of nanoparticles quite weak, because carriers may be trapped in the surface states where nonradiation recombination occurs, and the trapping is a very fast process [78, 84], spontaneous fluorescence has no chance to compete with it. Luminescence might be improved if the trapped carriers at the surface states are effectively released from their traps by stimulation with energy equal to the trap depth. Therefore, TL is a good way to study the trapped carriers and the surface states. The TL behavior was first investigated in my laboratory. Here we introduce our works on ZnS nanoparticles and CdS clusters in zeolite-Y.

5.2. Thermoluminescence of CdS Clusters in Zeolite-Y [27] CdS clusters in zeolite-Y were formed by ion exchange followed by sulfurization with Na2S. The glow curves were recorded on a Harshaw 2000A thermoluminescence dosimeter with a heating rate of 12 K/s. An equal amount (15 mg) of sample was taken for each measurement. No irradiation was carried out before the measurements.

371

CHEN

,~. i00 ~.~ ....... "'""",. =

80

o

40

<

20

4

0

200

I

t

I

300

400

500

600

Wavelength(nm)

Fig. 57. Reflectanceabsorptionspectra of CdS clusters in zeolite-Y (CdS/Y).The CdS loadings from 1 to 4 are 1, 3, 5, and 20 wt%, respectively. (Source: Reprinted from [27], with permissionfrom Elsevier Science.)

As for comparison, the glow curves of the following samples were also measured [27]: 1. bulk CdS, deposited from Cd(NO3)2 and Na2S solution. 2. mechanical mixture of equal CdS and zeolite-Y powders. 3. CdZ+-exchanged zeolite-Y (Cd2+/Y), prepared by exposing of zeolite-Y powders to Cd(NO3)2 solution followed by drying in a vacuum at 200 ~ for 5 h. 4. zeolite-Y powders exposed to NazS solution for 5 h (NazS/Y). The absorption edges of CdS clusters in zeolite-Y are clearly blue shifted as compared with that of the bulk and shift to the red as CdS loading increases (Fig. 57). This is caused by the so-called quantum-size effect, because as the loading increases, more CdS would enter into the pores of zeolite, more and bigger clusters would be formed [96, 97]. Curve-4 in Figure 57 is similar to the absorption spectrum of the bulk CdS, reflecting that at high loading some bulk CdS may be formed outside the pores of the zeolite. It was suggested in a previous work on CdS in zeolite-Y [98] that as CdS loading increases, the cluster size remains constant, but cluster density goes up. The increase in cluster density (and the existence of clusters in near neighbor pores), not the change of cluster size, accounts for the shift in absorption spectra. The shift of the absorption spectra is hard to explain by suggesting that only the cluster density increases as loading increases. The increase of the cluster density may increase the absorption intensity, but only the change of the cluster size may cause the shift of the absorption edges. It is also difficult to imagine that the cluster size remains constant when discrete CdS clusters interconnect to form superclusters. Furthermore, it is suggested in Ref. [98] that CdS clusters are located within the sodalite cages (6.6 A). However, it is revealed late that CdS clusters are formed in the supercages (13 ,~), because the entry aperture of the sodalite cages is too small (2.1 ,~) for S 2- anions ('--3.6 ~) to penetrate to form sulfides [99]. It is easy to understand that the cluster size increases as the loading increases provided the clusters are formed in the supercages. The fluorescence of CdS clusters in zeolite-Y with low loading is extremely weak. Samples with high CdS loading exhibit obvious luminescence and the luminescence intensity first increases then decreases as loading increases (Fig. 58 [27]). The emission around 400 nm that appeared in our samples is attributed to the recombination of bounded excitons [96]. The glow curves of CdS clusters in zeolite-Y are shown in Figure 59. Every sample exhibits an obvious glow peak around 375 K. No glow peak is detected in this temperature region from the pure zeolite, while a glow peak around the same temperature is measured

372

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

CdS/Y

40=.

30Thermoluminescence

t,~

t(D

20-

C =..,.

10Photoluminescence

r 0

0

I

I

I

5

10

15

20

CdS Ioading(wt%)

Fig. 58. Intensityvariation of thermoluminescence and photoluminescence upon CdS loading in CdS/Y. (Source: Reprinted from [27], with permission from Elsevier Science.) 20

1

15-

2

,,-.>.

3

d 9-~ 10r

4 5

e,(D

_= 5

0 250

I

I

I

300

350

400

450

Temperature(K)

Fig. 59. Glowcurves of CdS clusters in zeolite-Y with CdS loading of 1, 3, 5, and 20 wt% (from 1 to 4), respectively. Curves 5 and 6 represent the bulk CdS and the mechanical mixture of CdS with zeolite-Y powders, respectively. (Source: Reprinted from [27], with permission from Elsevier Science.)

from pure CdS powders, but much weaker in intensity. The TL intensity of CdS in zeolite mechanically mixed is even lower than that of the pure CdS. In order to reveal the cause of the thermoluminescence, the glow curves of Cd2+-exchanged zeolite-Y (Cd2+/Y) and zeolite-Y exposed to Na2S solution (Na2S/Y) are also measured (Fig. 60 [27]). The glow peak of Cd2+/Y is higher than that of CdS/Y, while no obvious signal is detected from zeolite-Y exposed to Na2S solution. The foregoing results indicate that the TL of CdS/Y is caused by CdS clusters. As no irradiation was made before the measurement of the samples, the TL must be caused by the intrinsic defects or surface states of the clusters. It can be seen from Figures 58 and 59 that as the CdS loading in zeolite-Y decreases, the TL intensity increases. One may think that the glow peak of CdS/Y is caused by the bulk CdS, because the glow peak of CdS is located around the same temperature region. In fact, it is not true, although some CdS may be formed outside the zeolite pores, but it only occurs at sufficient high loading. At low loading, all CdS would be confined in the pores of zeolite as clusters. In our experience, more CdS are formed outside the pores as the loading increases. Suppose the glow curve is caused by the bulk CdS, it becomes stronger as the CdS

373

CHEN

=. 6 o~

CdS/Y(2Owt%)

Cd2+/Y

~ A

rr ~D r 4,,.=,

(D

._> "~ n,' 2 -

0

I

I

I'

J

i

'

250 300 350 400 450 500 550 Temperature(K)

Fig. 60. Glowcurves of CdS clusters in zeolite-Y (20 wt%), Cd2+-exchangedzeolite-Y (Cd2+/Y) and zeolite-Y powders exposed to Na2S solution (Na2S/Y). (Source: Reprinted from [27], with permission from Elsevier Science.) loading increases, just like the situation of curves 5 and 6 (Fig. 59), which represent the bulk CdS and the mechanical mixture of equal CdS with zeolite-Y powders, respectively. The inverse dependence of the TL intensity on the CdS loading indicates that the TL efficiency is dependent on the cluster properties. As we point out, as the loading increases, the cluster density goes up and the sizes of clusters are bigger [96, 97]. This means that the TL intensity is not dependent on the cluster density but is dependent on the cluster sizes. The luminescence intensity increases as the cluster size decreases. We have mentioned in the Introduction that the trapping of electrons in semiconductor clusters is a very fast process, spontaneous fluorescence has no chance to compete with it [78, 84]. Thus the quantum yield of fluorescence of clusters is very low [63]. However, traps and trapped carders in clusters are abundant, luminescence might be improved if the trapped carders are effectively released from the traps by stimulation with energy equal to the trap depth. Thermoluminescence is a good method to detect the recombination emission caused by the detrapping of carriers thermally. The energy corresponding to the glow peak is equal to the trap depth. The glow peaks in Figure 59 are caused by trapped carders produced during sample processing. Because in nanoparticles or clusters, most ions at the surface are not saturated in coordination, electrons or holes may be excited easily and may escape from the ions, then are trapped at surface states located in the forbidden gap [53]. In the thermoluminescence process, electrons or holes may be ionized from the surface states and may recombine to emit luminescence. According to the theories of thermoluminescence [ 101,102], the TL intensity of clusters may be given by dm I -- - ~ dt

= mnA

(5)

where m and n represent the density of holes and electrons for recombination, respectively. A is the carder recombination probability. Obviously, m and n are proportional to the surface states. As we have pointed out, as the size of clusters decreases, surface ions and states increase rapidly, thus enhancing TL efficiency. Furthermore, in clusters the wavefunctions of the electron and the hole are overlapped effectively, which may result in the increase of their recombination probability (A). Theses two effects may make the TL of clusters much stronger than that of the bulk and may increase as cluster size decreases. Besides the two effects we discussed previously there may be some other possibilities that may cause the TL of CdS/Y to decrease as the loading increases. One is the reabsorption of luminescence by the bulk CdS, because as the loading increases, some bulk CdS may be formed outside the zeolite pores. However, this effect is significant only at

374

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

high loading. According to our works [96, 97], at low loading, no or very little bulk CdS are formed. Below 10 wt%, the photoluminescence (PL) increases as loading increases, indicating reabsorption is not important. Above 10 wt%, the PL decreases (Fig. 58), reflecting strong reabsorption, thus the increase of TL as loading increases is not so obvious. Another possibility is the carrier transport between neighboring clusters, because it was reported [98] that as the loading increases, the clusters in the cages of the zeolite may be interconnected and carriers may transport or may transfer between the neighboring cages. The carrier transport reduces the recombination rate of carriers and thus reduces the TL efficiency and causes the TL decrease as the loading or connectivity increases. However, it may be deduced from the change of the PL upon the CdS loading that this effect is minor. Suppose that the carrier transport reduces the carrier recombination, it also affects the PL of the clusters and causes the PL decrease as the loading or connectivity of the clusters increases. However, we observed that the PL first increases then decreases as the loading increases. It was also reported in Ref. [98] that at low CdS concentration, no emission can be seen even down to 4 K, but obvious emission exhibits from the so-called "superclusters" at high loading. It seems that the increase of the "connectivity" of the clusters does not decrease the rate of the carrier recombination. We think that the CdS clusters formed in the supercages of zeolite-Y cannot be connected. Because the supercages are isolated or separated, each supercage is surrounded by several sodalite cages. It is hard for them to be interconnected. It was found that the dependence of PL on CdS loading or on cluster size is different from that of TL. Because these two processes are different, in the PL process, the increase of surface states enhances nonradiation and thus decreases PL efficiency [96]. While in the TL process, the increase of surface states provides more accessible carriers for recombination and therefore enhances TL efficiency. It can be seen from Figure 59 that the clusters of different sizes have almost the same glow peak positions and shapes, reflecting that the properties of traps or surface states are not sensitive to the cluster size. In summary, the thermoluminescence of CdS clusters encapsulated in zeolite-Y is observed successfully and is revealed to be caused by the CdS clusters, not by other defects in the zeolite. The TL intensity increases as CdS loading decreases. The thermoluminescence is considered to be related to the surface states of the clusters. Carriers trapped at the surface states may be ionized by heating and may recombine to give luminescence. The increase of the TL intensity upon CdS loading is mainly caused by two effects. The clusters formed are smaller at lower loading, having higher surface to volume ratio and containing more accessible trapped carriers for TL. Meanwhile in smaller clusters, the recombination probability increases due to stronger quantum-confinement effect. Thus it is not surprising that the TL increases as the cluster size increases. However, in this kind of material, we cannot rule out the quenching of the luminescence by reabsorption of bulk CdS, although this effect is not so important as the size effect and only occurs at sufficient high loading. The position and shape of the glow curves do not change very much upon the cluster loading or size, indicating that traps or surface states are not sensitive to the cluster sizes. However the glow peak shape of the clusters is a little different from that of the bulk, revealing that the probabilities and dynamics of carriers recombination in CdS clusters and in the bulk are different. All these await to be accomplished. In fact, much work awaits to be done, such as the precise correlation between the TL efficiency and the cluster sizes, emission or energy spectra of thermostimulation, the assignment of the glow peaks, the estimation of physical parameters of the traps, the change of TL upon preparation conditions, etc. We also found that the TL of semiconductor clusters is very sensitive to light irradiation with different energy. The dependence of TL upon irradiation might be an interesting subject, it may provide an opportunity for searching new and more sensitive materials for optical storage and dosimeter.

375

CHEN

5.3. Thermoluminescence of ZnS Nanoparticles [28, 29] Two methods were used to prepare the ZnS nanoparticles. One [28] is capped with organic stabilizer agents, the other [29] is deposited from the colloids directly without capping. In both cases, the sizes of the clusters were adjusted by the reaction temperature. There are some differences in the physical properties of the "two" ZnS nanoparticles. Their structure, optical absorption, and photoluminescence have been introduced in Sections 2.4.1 and 2.4.2, respectively. Here we illustrate their thermoluminescence.

5.3.1. Thermoluminescence of Capped ZnS Nanoparticles [28] Figure 61 shows the glow curves of the capped ZnS nanoparticles which were recorded without any irradiation. An equal amount (15 mg) of sample was taken for each measurement. The sizes of the particles are 18.1, 25.0, 27.4, and 30.1 ,~, respectively, [28]. An obvious glow peak around 380 K is exhibited from the nanoparticles. All four samples show the glow peaks at almost the same position and the TL intensity increases as the particle size decreases. The change of the TL is consistent with that of the surface fluorescence. It is reasonable to consider that the TL of the nanoparticles is correlated to the surface states. The glow signal is caused by the recombination of trapped carriers released by heating. The glow peaks in Figure 61 are caused by the trapped carriers which are produced during the sample processing, because no irradiation was carried out before the measurement. Carriers trapped at the surface states or defect sites may be released by heating to recombine to give out the so-called thermoluminescence (TL). According to Eq. (5), the increase of TL upon decreasing size may be explained reasonably. Because the decrease of the particle size may enhance the surface ions and states rapidly and, as the content of the surface states increase, the particles may provide more accessible carriers (holes and electrons) for the TL recombination, that is, the m and n are proportional to the surface states. Furthermore, in nanoparticles the wavefunctions of electron and hole are overlapped effectively, which may result in the increase of their recombination probability or rate (A). These two effects may make the TL of small particles much stronger than that of the bulk and may increase as the size is decreased. We plot the TL intensity versus the particle radius (Fig. 62), hopefully to figure out which of the two effects is more important. Because surface effects should vary as surfacevolume (i.e., 1/ R), while quantum confinement should vary as 1/vol (i.e., 1/ R 3). However,

10-

8 -

::= ~C

6-

_=

esSS"~

//

W 4-

,/s

99

,"

X

,

3

,, ',, \

2

_

300

r

1

I

320

340

360

T 380

400

Temperature(K) Fig. 61. Glow curves of capped ZnS nanoparticles. (Source: Reprinted with permission from [28]. 9 1997 American Institute of Physics.)

376

PHOTOLUMINESCENCE AND STIMULATEDLUMINESCENCE OF NANOPARTICLES

70 IV I / R 3

56. . . . . .

:5

"7".

.'~.~,.. .

(11

m-

=~ 4 2 -

tll v

r

._Z"

ffl e--

C

|C

-=

28-

..A IX.

,,.I

I--

14-

0

0

I

I

I

I

I

1

2

3

4

5

1/Rn 10~*1,(n=1,3)

Fig. 62. Curvesof TL and PL intensities versusparticle size (R) in capped ZnS nanoparticles. (Source: Reprinted with permissionfrom [28]. 9 1997AmericanInstitute of Physics.)

the results in Figure 62 cannot tell us which of the two effects is more important. Probably, the surface and the quantum confinement effects do the same contribution to the increase of the TL upon decreasing size. We have mentioned that the measurement of TL is desiring to get some useful information about the surface states of the particles. Here we attempt to figure out what we can learn about the surface states from the TL investigations. The appearance of TL prior to radiation indicates that some trapped carriers have been pre-existed. The pre-existing carriers or occupation of the localized states have been proposed by Chestnoy et al. [61] to explain the very weak dependence of the fluorescence decay lifetime upon CdS cluster size and have been explained by Klimov et al. [80] to explain the extremely fast buildup dynamics of the low-energy emission bands of CdS nanocrystals. TL from semiconductor nanoparticles without any radiation has been observed in CdS clusters in zeolite-Y [27], CdS [103], and ZnS [28, 29] nanoparticles. Our observations provide direct evidence for the pre-existing trapped carriers and support the suggestions of Chestnoy et al. [61 ] and of Klimov et al. [80].

5.3.2. Thermoluminescence of Uncapped ZnS Nanoparticles [29] Figure 63 shows the glow curves of the uncapped ZnS nanoparticles [29]. An obvious glow peak around 396 K is observed. The three samples show the glow peaks at almost the same position and the TL intensity increases as the particle size decreases. The change of the glow curves of the uncapped ZnS particles upon size is similar to that of the capped ZnS particles. However, the glow peak maximum is different, indicating that the trap depth related to the glow peak is correlated to the condition of sample preparation. Furthermore, the TL intensity-size curves of the uncapped particles are different from that of the capped particles, indicating a different TL mechanism occurring in the two particles. As pointed out earlier, there are two effects, that is, surface effect and quantum confinement, that may make the TL of small particles increase as the size decreases. In the capped ZnS particles, the surface and the quantum confinement effects do the same contribution to the increase the TL upon decreasing size. To figure out which of the two effects is more significant in the uncapped ZnS particles, we plot the TL intensity versus the particle radius (Fig. 64). As pointed out, the surface effect should vary as surface-volume (i.e., 1/ R), while quantum confinement should vary as 1/vol (i.e., 1/R3). The results in Figure 64 show

377

CHEN

10 1.24nm _

,,.->

~,,

_

.65nm

ell)

_=

'~t ,~2.;28nm

_

/'

.--I

_

#.#

j

~,

i

I

I

I

I

I

I

340

360

380

400

420

440

Temperature(K)

Fig. 63. Glowcurves of uncapped ZnS nanoparticles. (Source: Reprinted with permission from [29]. 9 1997 American Institute of Physics.) 12

~

::i

.._~.

9-

.._~.. ,,-:,. :3 v

e-

~

t~ r tD r

6

e,_,1

_.1

g-

_

0

0

IX.

g-

I

I

I

1

2

4

6

8

10

1/R".10"*~,(n=1,3)

Fig. 64. Curvesof TL and PL intensities versus particle size (R) in uncapped ZnS nanoparticles.

that the increase of TL and of PL upon 1/R is faster than that upon 1/R 3, indicating that the surface effect does more contribution to the luminescence of the particles. It is also indicated that the surface effect is more significant in the uncapped particles than in the capped particles. This is reasonable, because in the uncapped particles the content of surface states is higher than in the capped particles, therefore the surface effect is more important.

5.4. A S c h e m a t i c L u m i n e s c e n c e M o d e l of Surface States

Surface states or defect sites in nanoparticles are so abundant that the trapping of carders is a very fast process [78, 84]. Fluorescence has been considered to occur via detrapping of carriers from the surface states or traps. The detrapping of carriers is an important process to determine the fluorescence efficiency and to reveal the luminescence mechanism. The detrapping rate is correlated to the trap depth which is measured with respect to the bottom of the conduction band for electron traps and to the top of the valence band for hole traps

378

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

Dp

,,

S

"

,,,,m

~trap depth~~._ I surface fluorescence I

o,,

P ft.- small size

big size

Fig. 65. A schematicfor the sizedependenceof the trapped fluorescencefrom surface states in semiconductor nanoparticles. (Source: Reprinted with permissionfrom [28, 29]. 9 1997American Institute of Physics.)

in bulk materials. In nanoparticles (clusters, nanocrystals, and quantum dots), we define the trap depth to the lowest excited state or exciton state for electron traps. The trap depth determining the activation energy of detrapping may be estimated from the glow curves. The glow peak maximum does not change upon size, reflecting that the trap depth does not vary as much upon decreasing size. As in the conventional TL theories [101, 102], only the electron traps were involved. Also, in our work, we only consider the electron traps. The physical properties of these traps may be reflected from the temperature, shape or symmetry of the glow peak. It can be seen that the glow peak temperature and shape (or symmetry) of all the samples are similar, indicating that the physical properties of the traps (i.e., surface states and/or defect sites) are not sensitive to the particle size. The trap depths of the capped and of the uncapped ZnS nanoparticles, estimating from the glow peaks according to the methods in Ref. [101], are around 0.75 [28] and 0.96 eV [29], respectively. Considering these results, the size dependence of the surface fluorescence may be explained reasonably. The schematic is shown in Figure 65 [28, 29]. As the trap depth does not change as much upon the size, while the bandgap increases as the size decreases, the separation between the electron-hole states (similar to the donor-acceptor pairs [61 ]) increases upon decreasing size. Thus the luminescence of surface states shifts to the blue as the size is decreased. This is the case for most nanoparticles reported. In summary, the thermoluminescence of ZnS nanoparticles is studied. Both the TL and the surface fluorescence increase as the particle size decreases. The consistence of the TL with the surface fluorescence indicates that the TL is related to the surface states. TL may occur via detrapping of carriers by heating. As the size becomes smaller, the surface to volume ratio increases, the particles contain more accessible trapped carriers for TL. This is one factor to make the TL increase upon decreasing size. Another factor is the increase of the carrier recombination rate upon decreasing size due to the increase of the overlap between the electron and the hole wavefunctions. In the capped ZnS particles, the two effects are equally important, while in the uncapped particles the surface effect is more significant than the quantum confinement, because in the uncapped particles, the content of surface states is higher. The appearance of TL prior to any radiation reveals that trapped carriers have existed previously. The glow peak of the particles is not sensitive to the size, indicating the trap depth does not change as much upon decreasing size. The size dependence of the surface fluorescence may be explained well with the model based on the TL results.

6. PHOTOSTIMULATED LUMINESCENCE OF Ag AND CLUSTERS IN ZEOLITE-Y

AgI

A strong luminescence should be observed in quantum structured materials, because the carrier recombination rate should be increased due to the increase of the overlap of the

379

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Energy c~176

- -

nonr~iative channels

---I c -e-

il adisttve ;hannel$ llmul

iil

111111111111

61,m,.=O

_ -

I,m,n

"..':'..'..::~ Fig. 66. (a) Energyrelaxationin a continuum. Radiative recombinationis possible even though K = Kt is needed because both band edges are populated. (b) Energyrelaxationthrough a fully quantized box level can be very slow. (l, m, n) = (lt, mt, n~) then is needed to decay radiatively, but this rarely occurs because nonradiative channels are efficient on electrons storedmorethan nanoseconds. (Source:Reprinted with permission from [104]. 9 1991 American Physical Society.)

electron and the hole wavefunctions. However, the fluorescence efficiency of most quantum crystals is very low. Three models have been proposed to explain the low luminescence efficiency of nanoparticles. One is attributed to the fast trapping of carriers to the surface states where nonradiation recombination occurs [59, 61, 71, 78, 80, 81, 84]. However, it was considered by Benisty et al. [ 104] that the poor radiative efficiency in quantum boxes in photoluminescence and laser action is due to the combination of inefficient energy relaxation and orthogonality of carrier quantum states, rather than to a major increase in extrinsic defect density. It was proposed that electrons captured from barriers in the upper levels of quantum boxes are retained in their cascade to the fundamental states for more than nanoseconds. Due to the mutual orthogonality of quantum states in a box, no luminescence, or much less than bulk, can be obtained from these stored electrons with reasonable assumptions for the hole population. The model in Figure 66 [104] shows clearly why the radiative rate is low due to the efficient nonradiative channels occurring in the upper levels of the quantum box. The third mechanism [79] for low luminescence efficiency in nanoparticles is the trapping of carriers escaping from the quantum dot into the matrix. This therefore, would cause nonradiative recombination processes. If the weak luminescence is caused by the intrinsic effect proposed by Benisty et al. [ 104], it is probably hard to enhance the luminescence efficiency. If the luminescence is quenched by surface states or defects, it may be improved via surface passivation. As the fluorescence of most nanoparticles may be enhanced by surface passivation, thus we believe that surface states play a key role in determining the fluorescence efficiency. To avoid the obstacle of the surface states, Bhargava et al. [68] and Bhargava [105] have worked out a new way to enhance the fluorescence efficiency of nanoparticles by incorporating an impurity into the quantum-confined structure, because the dominant recombination route can be transferred from the surface states to the impurity states by doping. These doped nanocrystals may make an impact on the next generation bright, high resolution, and high contrast emissive displays [105]. Because most clusters are stabilized in a matrix like polymers [ 106], zeolites [ 1], and glasses [79, 52], and others, charge or carrier transfer from quantum dots and their surround matrix have been noted [30, 31, 79]. Electrons or holes trapped at the defect sites of the matrix are metastable, they may be ionized and they may return to the quantum dot states [30, 31]. This indicates that charge transfer between quantum dots and the matrix

380

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

is reversible. This reversible process may find applications for optical storage. The photostimulated luminescence (PSL) of Ag [30] and AgI [31 ] clusters in zeolite-Y were observed supporting the preceding views.

6.1, Photostimulated Luminescence of Silver Clusters in Zeolite-Y [30] Silver clusters have been studied extensively [3, 4, 52, 107, 108], because they are very sensitive to light stimulation and they may find applications as photocatalysts for solar energy conversion [3] and as optical materials for information or image storage [4]. It was reported [108] that silver microclusters on silver halide grains may work as latent images and reduction sensitization centers which play a dominant role in the photographic process. Silver clusters may be formed simply by encapsulation in the cages of zeolites. The structure and chemistry of silver clusters in zeolites have been reviewed by Sun and Serf [107]. The spectroscopy and photoprocesses of silver clusters in zeolites have been discussed by Ozin et al. [109]. Ozin et al. [4] also have a patent entitled "photosensitive, radiation sensitive, thermally sensitive, and pressure sensitive silver sodalite materials." A Japanese patent (SHO-61-61894, Yokono et al.) describes an optical recording medium consisting of a silver halide and a large pore zeolite compound. The material darkens when exposed to light and may fade back to its original color on heating. These indicate that silver clusters are novel materials with potential applications in optical and chemical sensors. For the first time, we observed the photostimulated luminescence (PSL) of silver clusters encapsulated in zeolite-Y [30]. PSL is different from PL (photoluminescence), in PL the excitation energy is higher than the emission energy, while in PSL the stimulation (excitation) energy is lower than the emission energy. The PSL is caused by the recombination of luminescence centers with electrons released from their traps by photostimulation [ 1 !0]. The photostimulatable materials may find applications as image plates for X-ray computerized radiography [ 111 ] and as a medium for erasable optical memory [112]. Our observations here indicate that silver clusters encapsulated in zeolite-Y (Ag-zeolite-Y) exhibit PSL and may be used as erasable optical storage materials. In the preparation of silver clusters inside the zeolite, the zeolite powder was slurried in deionized water with pH adjusted to 6 with nitric acid. The silver nitrate was added and the mixture was stirred at room temperature for 30 h. The Ag+-ion-exchanged zeolite was collected by filtration, washed with deionized water until no Ag + was detected in the filtrate by C1- solution, then suction dried to a damp powder, and finally dried and calcined at 250 ~ in dark and in vacuum for 10 h. The PL and PSL spectra were recorded with a Hitachi M-850 fluorescence spectrophotometer. The light source for stimulation or excitation was a 150 watt Xe lamp, and the emitted light was detected with a R3788 photomultiplier tube with resolution of 0.15 nm. In the PL measurement, the excitation light is chosen at 305 nm, which is shorter in wavelength than the emitted light (505 nm). In the PSL measurement, the stimulation (or excitation) light is chosen at 840 nm, which is longer in wavelength than the emitted light. The reported spectra were corrected automatically for the photomutiplier response. All measurements were carried out at room temperature. The silver clusters in the zeolite show an excitation maximum at 305 nm and an emission peak at 505 nm (Fig. 67) [30], respectively. The fluorescence of sliver clusters in zeolite-Y has been studied extensively [109]. The appearance of an absorption band at 306 nm, a fluorescence emission at 490 nm, as well as an excitation spectrum with maximum at 306 nm are considered to be strong evidence for the existence of zeolite entrapped silver atoms from comparison with the corresponding data for gaseous and rare gas matrix isolated Ag o atoms [ 113]. The excitation and emission spectra of our sample are consistent with the observation of Kellerman and Texter [113]. Thus the fluorescence is tentatively

381

CHEN

6 a

5 4

~

2 1 0

l

i

i

i

i

J

240

320

400

480

560

640

Wavelength(nm)

Fig. 67. Photoluminescenceexcitation (a, ~em = 505 nm) and emission (b, ~.ex = 305 nm) spectra of silver clusters in zeolite-Y. (Source: Reprinted from [30], with permission from Elsevier Science.) 6 1

5

2 3

4

, ~

5

ttt

2

4

0

350

I

I

400

450

I

500

550

600

650

Wavelength(nm)

Fig. 68. Photoluminescenceof silver clusters in zeolite-Y. 1. sample without UV-irradiation; 2. sample after UV-irradiation at 254 nm for 1 min; 3. sample after UV-irradiation at 254 nm for 10 min; 4. sample after UV-irradiation at 254 nm for 30 rain; 5. sample after UV-irradiation at 254 nm for 30 min and then optically bleached at 840 nm for 10 min. (Source: Reprinted from [30], with permission from Elsevier Science.)

assigned to the transition between the ground state (2S1/2) and the excited state (2p1/2) of Ag atoms [ 109]. It was found that the emission from Ag o decreased under the UV-irradiation at 254 n m (Fig. 68) [30]. Figure 69 shows the photostimulation spectra of Ag-zeolite-Y after UVirradiation for 10 min and for 30 min, respectively. An absorption appears at around 840 n m in the stimulation spectrum under UV-irradiation. A similar absorption band around 850 n m was reported in the optical stimulation spectra of natural silicates (feldspar [ 114]) after y-irradiation. However, no assignment was made to the stimulation spectra of the natural minerals [ 114]. We think the absorption is related to the electron centers or F-centers correlated to the oxygen vacancies in the zeolite framework. An obvious fluorescence is detected by stimulation with the light corresponding to the absorption band in the stimulation spectrum (Fig. 70). This is the so-called photostimulated luminescence (PSL). The

382

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

2.0 A

1.5

ai

1.o

m t-.

_= 0.5

0.0 750

780

840

810

870

900

Wavelength(nm)

Fig. 69. Photostimulatedspectra of silver clusters in zeolite-Y after UV-irradiation at 254 nm for 10 min (dotted) and for 30 min (solid), respectively. (Source: Reprinted from [30], with permission from Elsevier Science.) 2.0

1.5

-

::3

d -~ 1 . 0 9 te....,

0.5

--

t

0.0

1

1

1

I

I

350 400 450 500 550 600 650 Wavelength(nm)

Fig. 70. Photostimulated luminescence spectra of silver clusters in zeolite-Y after UV-irradiation at 254 nm for 10 min (dotted) and for 30 min (dash), respectively. (Source: Reprinted from [30], with permission from Elsevier Science.)

PSL is the same in wavelength as the PL (photoluminescence), but much weaker in intensity. The PSL is also assigned to the fluorescence of Ag atoms. It was found that the PSL intensity of Ag-zeolite-Y increased with increasing irradiation time. The PSL phenomenon in BaFBr:Eu 2+ phosphors has been studied extensively [ 100, 110, 111, 115]. In BaFBr:Eu 2+, the PSL occurs via the recombination of [Eu 2+ + h] [ 115] or Eu 3+ [110] with the electrons released from the F-centers by photostimualtion. In Agzeolite-Y, it was observed that the Ag o emission decreased under UV-irradiation, indicating the decrease of Ag ~ Under photostimulation at 840 nm, the emission of Ag o increases slightly (Fig. 68). These observations tell us that under irradiation, some Ag o atoms converted to Ag +, while the electrons were captured by the oxygen vacancies in the zeolite framework to form electron centers. Under photostimulation, the electrons are released from the centers and recombine with Ag + to give the fluorescence of Ag ~ In fact, the charge transfer from the zeolite framework to the entrapped silver atoms or clusters has been discussed previously [116]. It was reported that dehydrated Ag +exchanged zeolite-A (Ag-A) is very sensitive to even small amounts of moisture [ 117]. Upon absorption of water, the brick red of dehydrated Ag-A changed to orange, then to yellow, and finally to white. The color changes were attributed to a charge transfer from the framework oxygen to silver cations by Kim and Serf [ 116]. A similar phenomenon was described in a Japanese patent (SHO-61-61894), that is, the materials composed by silver

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CHEN

halides and zeolite compound darkens when exposed to light and fades back to its original color by heating. These results support that charge can transfer from the zeolite framework to the entrapped Ag + by thermo- or photostimulation. In the following, we attempt to illustrate why Ag atoms can be formed within the zeolite and how can the charge transfer between the framework and the encapsulated silver cations. The molecular formula of zeolite-Y is Na64[A164Si1280384].256H20, some silicon ions (Si 4+) in the tetrahedra are substituted by aluminum ions (A13+). Because of the valence difference between aluminum and silicon, the zeolite lattice possesses a negative charge equal to the number of aluminum atoms. Thus the zeolite crystallites have large electrostatic potentials that may absorb some cations in their cavities. Ion exchange is a popular way to encapsulate cations into the cavities of zeolites. In the solution exchange, it should be Ag + cations rather than Ag o atoms that enter into the zeolite cages. However, as indicated by the fluorescence, it is Ag o atoms rather than Ag + cations that appear in the zeolite. We think that the "autoreduction" mechanism proposed in the Ag +exchanged-zeolite-A [ 118] may be borrowed to explain the appearance of Ag o atoms in Ag+-exchanged-zeolite-Y. According to the temperature-programmed desorption experiment on hydrated Ag+-exchanged-zeolite-A, oxygen gas was evolved by heat treatment at a high temperature above 400 K and the Ag + may be "autoreduced" to Ag ~ The overall stoichiometry of this reaction can be represented symbolically as [118], 2Ag + +

ZO 2- ~

1 0 2 --[--2Ag ~ + Z

where ZO 2- represents a zeolite framework and one of its oxygens, and Z represents a zeolite framework with a missing oxygen link, that is, with a Lewis acid site (oxygen vacancy). The autoreduction may occur in Ag+-exchanged zeolite-Y by heat treatment in dark and in vacuum. Thus it was Ag o atoms rather than Ag + cations that appeared in the zeolite. Ag o may be ionized to Ag + by UV-irradiation (Ag o ~ Ag+). The ionized electron may be captured in the Lewis acid sites (oxygen vacancies) that are the acceptors of electrons. The trapped electrons in the Lewis sites are metastable and may be released by light stimulation. The photoreleased electron may recombine with Ag + to give the emission of Ag ~ These processes are revisable, reflecting that Ag-zeolite-Y has potential application as a medium for erasable optical memory. Here we propose that the production of UV-irradiation is Ag + rather than [Ag + + h] complex, because the irradiation energy (4.96 eV) is much lower than the energy gap of the zeolite (10.5 eV as estimated from the empirical method proposed by Maj [ 119]). No electron-hole pairs could be created under UV-irradiation. The irradiation can only ionize the electrons from Ag o atoms. In summary, the photostimulated luminescence of silver-exchanged zeolite-Y is reported. Under UV-irradiation, the PL intensity of silver atoms decreased and an absorption band showed up around 840 nm. Under photostimulation at 840 nm, the fluorescence of silver atoms was observed and the PL intensity of silver atoms increased slightly. These phenomena were considered to be caused by the charge transfer from the zeolite framework to the entrapped silver atoms. These reversible processes make the material have potential applications in erasable optical memory.

6.2. Photostimulated Luminescence of AgI Clusters in Zeolite-Y [31] Silver halide clusters have been studied extensively [3, 107, 120-125], because they are very sensitive to light stimulation and they may find applications as photocatalysts for solar energy conversion [3] and as a medium for optical information or image storage [4]. Silver halide clusters may be formed simply by encapsulation in the cages of zeolites. The synthesis of silver halide clusters in zeolites has been reported by several investigators [ 121-124]. An optical recording medium consisting of a silver halide and a large pore

384

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

zeolite compound has been described in a Japanese patent (SHO-61-61894, Yokono et al.). The synthesis, structure, and optical properties of sliver halide clusters in halosodalites have been studied by Stein et al. [121-123]. Calculations showed that electronic transitions took place between the clusters and the sodalite framework [ 122]. The photosensitive properties of AgI inclusion in modenite have been reported by Hirono et al. [ 124] and were considered to be caused by a photoinduced intermediate state. The roles of silver and silver iodide nanoclusters in silver halide photographic emulsion grains have been discussed widely [108, 125]. It is agreed that silver and silver iodide microclusters play a very important role in the photographic process in mixed-halide emulsions and have a great influence on their spectral sensitization, light absorption, chemical sensitization, pressure sensitivity, and developability [ 125]. All these indicate that silver iodide clusters are novel optical materials with potential applications. For the first time, we observed the photostimulated luminescence (PSL) of silver iodide clusters in zeolite-Y. Our observations demonstrate that silver iodide clusters encapsulated in zeolites may work as a medium for erasable optical memory. In the preparation of AgI clusters in zeolite-Y (henceforth Agl/Y), Ag + ions were first exchanged into the cages of the zeolite. The zeolite powders were slurred in deionized water with pH adjusted to 6 with nitric acid. The silver nitrate was added and the mixture was stirred at room temperature for 30 h. The Ag+-ion-exchanged zeolite was collected by filtration, washed with deionized water until no Ag + was detected in the filtrate by C1- solution. Then the Ag+-ion-exchanged zeolite powder was slurried in a sodium iodide solution by stirring at 100 ~ for 20 h. The color of the materials changed from gray to yellow as the interaction was going on. Then the materials were collected by filtration and washed extensively with deionized water and finally dried and calcined at 250 ~ in dark and in vacuum for 20 h. The transmission electron microscopy (TEM) observations were made on the ultrathin films which were prepared as follows: the powder samples were mixed with a diluted organic binder which was made from nitrocellulose, ethyl acetate, and n-butanol. Then the films were formed simply by spin coating or formed on a water surface by dropping. The films were moved to a copper grid and were thinned by low energy ion-beam bombardment. The Auger electron spectra were measured on a PHI 4~550-AES spectrophotometer. The PL and PSL spectra were recorded with a Hitachi M-850 fluorescence spectrophotometer. In the PL measurement, the excitation light was chosen at 274 or 303 nm, which is shorter in wavelength than the emitted light. In the PSL measurement, the stimulation (or excitation) light was chosen at 625,675, or 840 nm, which is longer in wavelength than the emitted light. In the measurement of the PL excitation spectrum, the scan was taken from 200 to 400 nm by monitoring the emitted light at 474 or 510 nm. In the measurement of the photostimulation spectrum, the scan was taken from 550 to 900 nm by monitoring the emitted light at 474 or 510 nm. The reported spectra were corrected automatically for the photomutiplier response. All measurements were carried out at room temperature. The TEM observations show that the clusters are quite uniform and even in size. The mean size of the clusters is 1.0-2.0 nm which is in agreement with the space in the zeolite supercages (1.3 nm in diameter), indicating that the clusters are formed inside the cavities of the zeolite. It is worth noting that the actual size of a cluster or a quantum dot is less than that measured by TEM because the distributions measured by TEM are affected by strain, which tends to overestimate the size. In order to determine the formation of AgI clusters, the energy spectrum corresponding to the TEM picture was recorded (Fig. 71 [31 ]). Iodine was detected in the reaction products of Ag+-exchanged zeolite-Y with NaI solution, but no iodine was measured in the zeolite exposed to NaI solution, because the I - ions can be reacted with Ag + to form the AgI deposit. Otherwise, the iodine could not be deposited in the zeolite. It can be seen from the energy spectrum that the content of iodide is lower than that of silver. This means that silver clusters may also be formed along with the formation of AgI

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CHEN

Fig. 71. Energyspectrum of AgI clusters in zeolite-Y. (Source: Reprinted with permission from [31]. 9 1998 AmericanInstitute of Physics.)

0

:3 v

200

I

I

I

I

300

400

500

600

700

Electron Energy (eV)

Fig. 72. Augerelectron spectra of zeolite-Y loaded with AgI and of zeolite-Y exposed to NaI solution. (Source: Reprinted with permission from [31]. 9 1998 AmericanInstitute of Physics.)

clusters. Because in preparation, the samples were annealed at 250 ~ in vacuum, this may cause some loss of iodine and may convert part of the AgI into Ag. It was found that the color of the sample changed during the TEM observation, and finally some clusters or dots "disappeared" or became colorless, indicating that the clusters are not stable under electron beam irradiation. The Auger electron spectra (AES) of the zeolite loaded with AgI (AgI/Y) and of the zeolite exposed to NaI solution were measured (Fig. 72). After sputtering for 60 s, both Ag and I were detected in Agl/Y samples, but no I signal was found in the latter. These results indicate further that AgI clusters are really formed in the zeolite. The PL excitation and emission spectra of Agl/Y are displayed in Figures 73 and 74, respectively. In order to reveal the origin of the fluorescence, the PL excitation and emission spectra of silver clusters in zeolite (Ag/Y) prepared in the same way are displayed along with that of Agl/Y, respectively. We also made the measurement on zeolite-Y powders exposed to NaI solution and zeolite-Y loaded with 12 by vapor inclusion, but no fluorescence was detected from these samples. These observations show clearly that the emission at 474 nm and the excitation at 274 nm are attributable to the AgI clusters, while the emission at 510 nm and the excitation at 303 nm are assigned to silver clusters. The fluorescence results indicate that both Ag and AgI clusters were formed inside the zeolite. This is in agreement with the TEM observations.

386

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

10 Emission

Wavelength S~

II

8

474nm--~/~ "3".

,'/

6-

t'r-

~

/'

_ _

0 200

I

I

250

300

350

Wavelength(nm)

Fig. 73. Photoluminescenceexcitation spectra of Agl/Y (dash and dotted lines) and Ag~ (solid line). (Source: Reprinted with permission from [31]. 9 1998 American Institute of Physics.) 12

citationWavelength

10--:5

8-

9=~

6-

303nm /

.~274nm

,

t

4

2

I

I

I

I

I

400

450

500

550

600

650

Wavelength(nm)

Fig. 74. Photoluminescenceemission spectra of AgI/Y (dash and dotted lines) and Ag/Y (solid line). (Source: Reprinted with permission from [31]. (~) 1998 AmericanInstitute of Physics.)

It is reported [ 108, 125] that silver microclusters usually appear on silver halide emulsion grains and may work as a latent image and reduction sensitization centers which play a dominant role in the photographic process. Here we find that some interactions exist between the AgI and Ag clusters in zeolite-Y. Because when silver clusters are excited, not only the emission of silver clusters, but also the emission of AgI clusters are observed. Similarly, when AgI clusters are excited, both the emission of Ag and AgI clusters are detected. This demonstrates that energy or charge carriers can transfer from Ag clusters to AgI clusters and vice versa. It is interesting to find that the emission of AgI clusters is stronger than that of Ag clusters when excitation is at the absorption band of Ag clusters. The emission of Ag clusters is stronger by indirect excitation into the excited states of AgI clusters than that by direct excitation into the excited states of Ag clusters. Similarly, the emission of AgI clusters is stronger by indirect excitation into the excited states of Ag clusters than that by direct excitation into the excited states of AgI clusters. These phenomena indicate that,

387

CHEN

1.0 0 . 8

--

~i 0.6

0.4-

~

3

,

,

0.2 1 ..

o.o|

,

; T , -

550 600 650 700 750 800 850 900 Wavelength(nm)

Fig. 75. Photostimulationspectra of AgI/Y. (1) ~.em = 474 nm, UV-irradiation for 5 min; (2) Xem - 510 nm, UV-irradiation for 5 min; (3) ~.em = 5 1 0 n m , UV-irradiationfor 5 min. (Source: Reprinted with permission from [31]. 9 1998AmericanInstitute of Physics.)

in photoexcitation, energy transfer or carrier migration between Ag and AgI clusters is a dominant process. Details about the interaction and energy transfer between the two clusters are not very clear now, but they are probably related to the structure of the two clusters. We have demonstrated the presence of both Ag and AgI clusters in the zeolite and the conversion of part of AgI into Ag by annealing at 250 ~ in vacuum. Thus a type of composite clusters, that is, the clusters each containing some Ag as well as AgI, may be formed. Each of these composite clusters may have two domains with a sharp boundary separating the Ag from AgI. The structure of the composite AgI-Ag clusters is probably similar to that of mixed CdS-CdSe nanoparticles [126]. As Ag and AgI clusters have different energy gaps, there must be a potential difference between the Ag and the AgI clusters, and therefore the dominant mobile carrier is the electron in one and the hole in the other. Thus, under light stimulation, charge carrier migration or energy transfer between the two clusters can be expected. This is similar to the result observed in the coupled composite CdS-CdSe and in the core-shell type mixed (CdS) CdSe and (CdSe) CdS nanoparticles [126]. Three absorption bands at 625,675, and 840 nm were measured in the photostimulation spectra of Agl/Y (Fig. 75) and the absorption increased with increasing UV-irradiation time. Two absorption bands around 630 and 850 nm were reported in the optical stimulation spectra of natural silicate (feldspar [ 114]) after v-irradiation. However, no assignment was made to the optical stimulation spectra of the natural mineral. We think these absorption bands are related to the electron centers or F-centers correlated to the oxygen vacancies in the framework of the silicate. Thus the two absorption bands at 625 and 840 nm of Agl/Y are tentatively assigned to the electron centers in the zeolite framework. The absorption at 675 nm was not observed in the optical stimulation spectra of pure zeolite-Y or Ag/Y [15]. This indicates that this absorption is not related to the zeolite, but it is probably caused by the AgI clusters. However, at present, we do not know clearly what causes the 675 nm absorption. One possibility is the interstitial silver ions produced by UV-irradiation, the other is F-centers, because an absorption at 672 nm in KI crystal [127] was assigned to F-centers. We think the interstitial silver ions are the most likely, because they are easily produced by irradiation. The irradiation energy at 254 nm is probably not high enough to create F-centers in silver halides. Luminescence was detected by photostimulation at 675 or at 840 nm, which is called photostimulated luminescence (PSL). The PSL intensity increased with the UV-irradiation time (Fig. 76). The PSL spectrum is different from the emission spectrum of Agl/Y, but it is consistent with the emission spectrum of Ag/Y. The PSL spectrum by stimulation at 625 nm cannot be measured, probably because the absorption band at 625 nm is partly overlapped with the emission band and thus the emission signal is masked by the stimula-

388

PHOTOLUMINESCENCE AND STIMULATED LUMINESCENCE OF NANOPARTICLES

0.8 3

,\ 0.6

-

l

'

,,->.

=.

: 2_

\

i _">, \

0.4-

II

0.2

0.0

I

I

I

I

450

500

550

600

Wavelength(nm) Fig. 76. Photostimulated luminescence spectra of AgI/Y. (1))~em = 474 nm, UV-irradiation for 5 min; (2))~em -- 510 nm, UV-irradiation for 5 min; (3) ~.em = 510 nm, UV-irradiation for 5 min. (Source: Reprinted with permission from [31]. 9 1998 American Institute of Physics.)

tion light. The appearance of PSL from AgI/Y indicates that these materials may find an application as a medium for optical memory. In AgI/Y, the PSL may be caused by the recombination of luminescence centers with electrons released by optical stimulation. However, in AgI/Y the PSL process is probably more complex, because there are two luminescence centers in AgI/Y, that is, AgI and Ag clusters, and energy or charge carriers may transfer or may migrate between the two kinds of clusters. Besides, electrons may be stored in the zeolite framework, and they can also be trapped in the AgI clusters. As the emission from Ag clusters is dominant in the PSL, we mainly consider the PSL process involving Ag clusters. It is known from comparison with the fluorescence of silver clusters in zeolites [109] that the emission of Ag cluster in AgI/Y is mainly from Ag o atoms, because the absorption band at 306 nm, a fluorescence emission at 490 nm, as well as an excitation spectrum with maximum at 306 nm are considered to be strong evidence for the existence of zeolite entrapped silver atoms [113]. The clusters were prepared by ion exchange. In the solution exchange, it should be Ag + cations rather than Ag o atoms that enter into the zeolite cages. However, as indicated by the fluorescence, it is Ag o atoms rather than Ag + cations that appear in the zeolite. The autoreduction mechanism proposed in the Ag+-exchanged zeolite-A [118] may be borrowed to explain the appearance of Ag o atoms in the zeolite as we mentioned in the previous section. We found that the emission from silver clusters was decreased by UV-irradiation, but it was recovered by optical stimulation at 840 or at 675 nm. This indicates that part of the Ag o atoms might be ionized to Ag + cations by UV-irradiation. The ionized electrons may be captured in the zeolite Lewis acid sites (oxygen vacancies) that are the electron acceptors, or trapped at the surface states or defects in AgI clusters. The trapped electrons may be released by optical stimulation and the photoreleased electrons may recombine with Ag + to give the emission of Ag o clusters. In summary, AgI clusters were formed in zeolite-Y. The fluorescence of AgI/Y consists of the emission of both AgI and Ag clusters. Reversible energy transfer or charge cartier migration may occur between the two types of clusters. Photostimulated luminescence (PSL) from AgI/Y was observed by photostimulation at 675 or at 840 nm. The PSL spectrum of AgI/Y is consistent with the emission of Ag clusters. It was considered to be caused by the charge transfer from the zeolite framework or from the AgI clusters to Ag clusters. These observations indicate that clusters of silver halides or silver clusters encapsulated in zeolites may be used for optical storage.

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CHEN

7. SUMMARY Here the fluorescence, thermoluminescence, and photostimulated luminescence of nanoparticles have been reviewed. The quantum-size and surface effects and their influence on the exciton oscillator strength, absorption, and fluorescence were also discussed. The photoluminescence excitation spectra and the excitation energy dependence of fluorescence of nanoparticles were illustrated. Photoluminescence excitation spectrum is better than absorption spectrum in the detection of splitted levels by quantum-size effect and has become a standard technique to obtain quantum dot absorption features, while the size selectively excited PL technique provides a good method to study the size dependence of the photoluminescence of nanoparticles in only one sample. Thus the photoluminescence excitation spectrum and the size selectively excited PL technique may provide much information of the intrinsic properties of nanoparticles. The fluorescence process in nanoparticles is very complex and is not clear even at present, but it is agreed that surface states are involved and play an important role in the luminescence of nanoparticles. Our results demonstrate further that not only the exciton, but also the trapped carders at the surface states are confined by quantum-size effect. Both the exciton and the surface luminescence may be adjusted by quantum-size confinement. Thermoluminescence from semiconductor nanoparticles was observed and was discussed. The appearance of thermoluminescence of nanoparticles before any irradiation demonstrates the pre-existed trapped carders and the little change of the glow peak upon size indicates that the trap depth of surface states is not sensitive to particle size. Based on the thermoluminescence, a model proposed may explain the size dependence of the trapped luminescence from surface states of nanoparticles reasonably. The observations of photostimulated luminescence from Ag and AgI clusters in zeolites indicate that these materials may find an application as a medium for erasable optical memory. This represents a new direction for the practical applications of nanoparticles. As these phenomena have just been observed, much work is to be accomplished, both for basic research and for practical applications.

Acknowledgments I thank my students, Zhaojun Lin and Yan Xu, and my co-workers, for their help in experiments. I thank Jianhui Zhang for his help in the reprinting of the literature figures. I am grateful to Professors Lanying Lin, Zhanguo Wang at my Institute and Professor Mianzeng Su at Peking University, for their continued encouragement and support. I also thank my wife, Chunying Liang and my lovely daughter, Dandan Chen, for their support of my research works. This subject is supported by the President's Foundation of the Chinese Academy of Sciences and the National Natural Science Foundation of China.

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