Chapter 3

Zinc Selenide Quantum Dots

3.1 Introduction:

Nanoscale materials have been extensively studied in recent years to study the evolution of the electronic structure from molecules to bulk. The properties or the electronic structure of nanoparticles is not a simple extrapolation of its corresponding molecular or bulk state. Quantum dots show unique features arising from the variation in crystal structure parameters, discretization of electron energy levels, concentration of oscillator strength for particular transitions, high polarizability of electron energy levels and increased surface to volume ratio [1-4]. Quantum confinement in semiconductors leads to discrete transitions that are blue shifted in energy from the bulk. Inhomogeneous broadening of the optical spectra due to size distribution and shape variations of the nanoparticles, conceal the fine structure in the energy states of quantum dots. In order to study the evolution of the electronic, optical and structural properties of material with size it is essential to synthesize nanoparticles of different sizes (with narrow size distribution) and controlled surface properties. II-VI semiconductors are relatively easy to synthesize, yielding free standing colloidal quantum dots by wet chemical routes. The evolving electronic structure as a function of size can be determined by non-contact optical probes in case of direct band gap II-VI semiconductors. The quantum size effects in CdSe nanoparticles with wurtzite structure are thoroughly investigated [5], while relatively less attention is being paid to other II-VI nanocrystals such as zinc selenide or zinc oxide. ZnSe is a II-VI direct band gap, zinc blende semiconductor with a bulk band gap of 2.7 eV at room temperature. ZnSe has a bulk Bohr exciton radius of 4.5 nm and bulk

49 exciton binding energy of 18 meV [6]. ZnSe is an attractive material to fabricate optoelectronic devices such as light emitting diodes or laser diodes in the blue to UV spectral range. Few reports demonstrate the possibility of fabricating such devices [7, 8]. The synthesis and characterization of ZnSe nanoparticles [9-23] have been reported. The earliest report on ZnSe nanoparticles by Chestnoy et al. [9] describes the synthesis of the nanocrystallites in alcoholic as well as aqueous media. Along with a strong transition for the lowest excited state, a weaker transition at higher energy was observed which was attributed to spin orbit splitting in the valence band. These nanoparticles were stable only at low temperatures. Li and Nogami [10] prepared ZnSe nanoparticles by dip-coating method for sol-gel derived complex solution. A strong photoluminescence peak observed at around 600 nm is attributed to Se vacancies. ZnSe nanoparticle formation embedded in glass prepared from supersaturated glass solution by annealing and rapid quenching are reported [12, 17]. The quantum confined band edge emission and the size dependence of two excited state energies, the spin orbit interaction, and the red shifted emission were observed for the first time in the ZnSe quantum dots in a glass matrix. Naturally formed self organized quantum dot arrays (QDAs) of ZnSe were grown by Stranski-Krastanov mode by Zhang et al. [13]. The confinement of carriers results from the difference in the band gaps of the strained ZnSe layer and the strain-relaxed ZnSe QDA. Liao et al. [14] have fabricated ZnSe quantum dots in a Volmer-Weber mode (an island growth) employing metalorganic chemical vapor deposition. Band gap emission at about 3.1 eV has been observed at 10 K for smaller crystallites. Kumbhojkar et al. [15] prepared free standing ZnSe quantum dots in an aqueous media capped by sodium hexameta phosphate showing luminescence at about 440 nm. Self activated luminescence due to Zn vacancies was seen in aged and uncapped ZnSe quantum dots. Highly luminescent in UV-blue region, and relatively monodisperse, colloidal ZnSe nanocrystals having HOMO-LUMO gap between 2.8 eV and 3.4 eV were synthesized from organometallic precursors in a hot alkylamine

50 coordinating solvent by Hines et al.[\6]. Lomascolo et al. [19] followed the same method to synthesize ZnSe/ZnS core shell nanoparticles. Quinlan et al. [20] have used inverse micelles technique using ion exchange reaction to produce monodisperse, cubic ZnSe quantum dots. Zhan et al. [21] have carried out a reaction in which zinc powder reacts with equivalent amount of elemental selenium to produce a complex which can be converted into wurtzite ZnSe nanoparticles. Zhu et al. [22] synthesized ZnSe nanoparticles of about 3 nm diameter by sonochemical irradiation of an aqueous solution of selenourea and zinc acetate under argon. Kim et al. [23] have grown self- assembled ZnSe quantum dots on a ZnS matrix in the Stranski-Krastanov mode using metalorganic chemical vapor deposition technique. Quantum confined carriers in the ZnSe quantum dots showed narrowing of the photoluminescence line width and a large red shift of the emission peak energy with increasing temperature. In the present study, ZnSe nanoparticles were synthesized by multiple injections of Zn and Se precursors in a hot coordinating solvent hexadecyl amine (HDA). Different sizes of the nanoparticles were obtained by injecting the precursors at different temperatures. Cubic zinc blende crystallites with sizes ranging from 2.5 nm to 4.5 nm with well passivated surfaces and hence showing only band gap luminescence were obtained. Low temperature photoluminescence excitation (PLE) measurements are carried out to determine the energy of the higher excited states in ZnSe nanoparticles. PLE studies at 10 K reveal up to three different excited states showing size dependent variation. Tentatively, these transitions were attributed using effective mass approximation.

3.2 Experiment:

Study of quantum size effects requires synthesis of highly monodisperse nanocrystallites with good crystallinity, and electronic as well as chemical passivation of its surface. In the present study, ZnSe nanoparticles were synthesized according to the method described by Hines et al. [16] and modified by the method of Peng et al.

51 [24] to achieve different sizes. In this procedure a high boiling point solvent, hexadecylamine (HDA) was dried and degassed under vacuum at 125°C for few hours. This solution was purged with nitrogen gas during this process. HDA was heated to 310°C under nitrogen atmosphere. Appropriate amounts of diethyl zinc and tri octyl phosphine selenide were diluted with 2 ml of tri octyl phosphine (TOP). The Zn and Se precursors in TOP solution was divided into two parts of equal amount and injected into the hot HDA at different temperatures depending on the desired size. The smallest size was obtained by injecting at a temperature of 260°C while the largest size was obtained by injecting at 300°C. In the case of the smallest sized samples heating was stopped within few seconds of the injection whereas for higher sizes heating was continued for few minutes after which the solution was allowed to cool slowly. At 80°C butanol was added to the solution to prevent HDA from solidifying at room temperature. Proper amount of methanol was added to the cooled solution for precipitation of the nanocrystals. The ZnSe nanocrystals were isolated and washed with butanol. The isolated nanocrystals were dried in vacuum and dispersed in hexane for characterization. The solvents butanol, methanol and hexane were used after distillation by standard procedures. Elemental analysis of ZnSe quantum dots was carried out using energy dispersive x-ray analysis. Studies indicate presence of Zn and Se in the samples. X ray diffraction (XRD) measurements were obtained using Philips PW 1840 powder x ray diffractometer, using Cu K

52 Computer Xe Lamp

Excitation Monochromator

4 Sample chamber and Cryostat f Emission He Lines Data PMT Monochromator Acquisition System Compressor

Figure 3.1: Schematic diagram of the photoluminescence setup.

The focal length of both the monochromators was 0.19 m while their resolution was 0.1 nm. Light from the Xenon arc lamp is incident on the entrance slit of excitation monochromator. Monochromatic light from the excitation monochromator is focused on the sample kept in the cryostat using quartz lenses. Light emitted from the sample is collected by quartz lenses and focused onto the entrance slit of emission monochromator. The PMT placed at the exit slit of the emission monochromator detects the light and the signal from the PMT is input to the data acquisition system. In all the experiments the overall resolution of the photoluminescence setup was kept at -0.5 nm. The sample was placed between two quartz plates mounted on a closed cycle helium

53 cryostat (Janis CCS-150). The photoluminescence (PL) and photoluminescence excitation (PLE) measurements were carried out at 10 K.

3.3 Results and Discussion:

The crystalline phase and average size of the nanoparticles were determined by X-ray diffraction (XRD). A comparison with standard JCPD data indicates cubic zinc blende phase of ZnSe nanocrystals. 1 .

V(a) V Ulw? . *1(b) w !%(c) 220 31

_l> 1 iI . Ii 1I • •I 20 30 40 50 60 29

Figure 3.2: X-ray diffraction patterns of ZnSe nanoparticles showing cubic phase. Average diameters determined using Scherrer formula for (a) ZnSe-I, (b) ZnSe-II and (c) ZnSe-III nanoparticles were 2.5 ± 0.4 nra, 3.5 ± 0.4 run and 4.5 ± 0.6 nrrfrespectively

54 The average size D of the nanoparticles is estimated using Scherrer formula considering the line broadening of the XRD patterns (D = 0.9A//3cos6), where X is the wavelength of the incident x-rays, /? is the full width at half maxima of the diffraction peaks, and 6 is the angle of diffraction. Figure 3.2 shows the XRD patterns of the ZnSe nanoparticles with sizes as 2.5 ± 0.4 nm, 3.5 ± 0.4 nm and 4.5 ± 0.6 nm for ZnSe-I, ZnSe-II and ZnSe-III respectively. The diffraction lines of cubic zinc blende phase for bulk ZnSe is also shown for comparison.

The size, shape, and crystal structure of the nanoparticles can be precisely determined by transmission electron microscopy (TEM). Figure 3.3(a) shows the TEM image of ZnSe-I nanoparticles. Highly crystalline spherical shaped nanoparticles are seen from the figure. The histogram based on the TEM image of ZnSe-I sample is shown in figure 3.3(b). The average size of the nanoparticles is found to be 2.8 nm with a standard deviation of 1.03 nm. The selected area diffraction pattern shows diffraction rings corresponding to (111), (200), (220), (311), and (400) lattice planes of the zinc blende crystal structure of ZnSe. Figure 3.4(a) shows the TEM picture of ZnSe-II nanoparticles demonstrating highly crystalline spherically shaped nanoparticles. The size distribution of ZnSe-II nanoparticles is shown in the histogram in figure 3.4(b). The average size of the nanoparticles is determined as 3.9 nm with a standard deviation of 1.04 nm. The electron diffraction pattern shows zinc blende structure for ZnSe-II nanoparticles (figure 3.4(c)) as the diffraction rings match with the (111), (200), and (311) planes. Figure 3.5(a) shows the TEM image of ZnSe-III nanoparticles. Highly crystalline spherical shaped nanoparticles are seen from the figure. The histogram giving the average size and the size distribution of ZnSe-III nanoparticles is shown in figure 3.5(b). The average size of the nanoparticles is found to be 4.6 nm with a standard deviation of 1.19 nm. The selected area diffraction patterns ZnSe-III nanoparticles show the diffraction rings corresponding to the (111), (200), (220), and (311) lattice planes of the zinc blende structure of ZnSe.

55 • ' (a) 1 *•' .

• • • .. /- :_' ' ' 1 • u "^

V'

• > i ... > -

• * - * /V»'*i H"- •

— '•'

• -f;Hl ._ ' . »* V •' ' • - < - • . ... • .- : -. '%• ' - ••:•.'..'*• > *• B --.•• "••*«."•-"• * * ' "V * - - "» 2 • ' ~ *rf 'J~., ,•*'•

* ** «- IS*3*-.. . " ' fry • - • T *

• (b)

-

0123456789 10 Particle size (nm) Figure 3.3: (a) Transmission Electron Microscopic images ofZnSe-I nanoparticles, (b) Histogram based on the TEM image, (c) Electron diffraction pattern of ZnSe-I.

56 1 ; • ;—^ T - •. ,'. £fk (a> k' •.:%• .

':'\--- •: ''''•;':• ' ,.;-\ 3

* . A"" *T «.^ •

•':• . .^^••-.^M? 1 ' ^ ' :v:;'<^'''": ^i^^ : WiptJil

0 12 3 4 5 6 Size (nm)

Figure 3.4: (a) TEM images of ZnSe-II nanoparticles, (b) Histogram showing the size distribution ZnSe-II nanoparticles. (c) Electron diffraction pattern of ZnSe- II

57 '• > . • • » ' • "iKfm »~~~

70 — (b) 60 1 50 .1 40

JO III

20 III.

10 Jill

0 ™™™™'™> 012345678lllll 9 10 Size (nm)

Figure 3.5: (a) Transmission Electron Microscopic images of ZnSe-III nanoparticles. (b) Histogram derived from the TEM image, (c) Electron diffraction pattern of ZnSe-III

58 Figure 3.6 shows a comparison of size calculated using tight binding formalism [25] with present work on ZnSe quantum dots. The values agree within an experimental uncertainty. The experimental values are taken from the work reported on ZnSe quantum dots from the references 16, 19, 17, 10, 15, 18 along with the present work.

3.0 ___ Tight Binding Calculations • Hines I • 2.5 Lomascolo 1 T Smith I * Nogami I X Kumbhojkar 2.0 I O Nikeshi + > Nikesh2 (Present Work) £ 1.5 x: w Q. CO O) "g 1-0 CO o \ CO X \ *

0.5 o x \ *

• \^^s • * • ^•s^ •

^""-"'---^^Y 0.0 •

,1,1,1. 1 . 1 . 1 . 1 . 1 . 1 . 1 0 10 20 30 40 50 60 70 80 90 100 Size in Diameter (A)

Figure 3.6: A comparison of experimentally estimated size of ZnSe nanoparticles with calculated values by tight binding formalism [25]. Reported experimental work is also indicated.

59 The sizes of the ZnSe samples determined by x-ray diffraction, TEM and as estimated from the optical absorption data [using tight binding calculations [25]] are given in the table 3.1. The sizes match within the experimental error. TEM indicates slightly higher sizes compared to XRD. This is a consequence of the fact that XRD measures the size of the crystalline core while TEM measures the size of the core along with the organic capping agent.

Table 3.1: Sizes of ZnSe samples determined by XRD, TEM, and Tight binding (TB) calculations [25].

Estimated sizes of ZnSe nanoparticles (nm) Optical

Sample TB absorption XRD TEM calculations (eV)

ZnSe-I 2.5 2.8 2.37 3.50

ZnSe-II 3.5 3.9 3.25 3.22

ZnSe-III 4.5 4.6 4.25 3.02

Figure 3.7 illustrates the absorption and the photoluminescence spectra of ZnSe- I nanoparticles. Quantum confinement effect is clearly seen in these spectra. The absorption feature showing the lowest excited state is seen at 355 nm (3.5 eV) blue shifted from the band gap energy of bulk ZnSe (2.7 eV). The PL emission maximum is observed at 394 nm (3.15 eV).

60 Energy (eV) 6 5.5 5 4.5 4 3.5 3 2.5 I ' I I I ' i—i—i—' 1 1 r

ZnSe-l

200 250 300 350 400 450 500 550 Wavelength (nm)

Figure 3.7: The absorption (solid line) and the photoluminescence (dashed line) spectra for 2.5 nm ZnSe-I nanoparticles. For PL spectra the sample was excited at 300 nm. Absorption was carried out at room temperature. PL measurements were carried out at 10 K.

The absorption and photoluminescence spectra of ZnSe-II nanoparticles are shown in Figure 3.8. The absorption feature is observed at 385 nm (3.22 eV) while the PL emission peak is red shifted with respect to the absorption feature and is located at 414 nm (2.99 eV). The absorption feature of ZnSe-III nanoparticles is observed at 410 nm (3.02 eV) and the PL emission peak is located at 418 nm (2.96 eV) (Figure 3.9).

61 Energy (eV) 6 5.5 5 4.5 4 3.5 3 2.5 1 ' 1 ' 1 ' l\ ' 1 i ' 1 • 1

ZnSe-ll

• 415

385 I u \

\ \ /

• S-"

I.I. ..L i 1 . 1 i 200 250 300 350 400 450 500 550 Wavelength (nm)

Figure 3.8: The absorption (solid line) and the photoluminescence (dashed line) spectra for 3.5 nm ZnSe-II nanoparticles. The sample was excited at 300 nm. Absorption was carried out at room temperature. PL measurements were done at 10K.

ZnSe nanoparticles showed only band edge luminescence and no defect luminescence was observed in the samples. The luminescence of ZnSe nanoparticles at 10 K showed a blue shift in the emission compared to the room temperature emission peaks. ZnSe-I exhibits a blue shift of 0.09 eV, ZnSe-II and ZnSe-III showed a blue shift of 0.03 eV and 0.04 eV respectively. A similar blue shift has been observed in the low temperature luminescence of CdSe nanoparticles [26]. A size dependent Stokes shift is observed between the absorption and the emission peak. This Stokes shift is large for the smallest ZnSe nanoparticles and decreases as the size increases. This has also been

62 Energy (eV) 6 5.5 5 4.5 4 3.5 3 2.5 I' I ' I • I •—I—'—I—r

I . 1 1 1 . 1 . L__, I . I , I 200 250 300 350 400 450 500 550 Wavelength (nm)

Figure 3.9: The absorption (solid line) and the photoluminescence (dashed line) spectra, excited at 350 nm for 4.5 nm ZnSe-III nanoparticles. Absorption was carried out at room temperature. PL was done at 10K.

observed in other II-VI nanocrystal systems. Various models have been proposed to explain the Stokes shift with respect to the optical absorption [18, 27-29]. The red shift in emission with respect to the optical absorption in nanoparticles is often attributed to luminescence decay from surface localized trap states. In zinc blende type crystals like ZnSe, the top of the valence band is six fold degenerate due to the p-type character of the atomic orbitals [3, 9]. The schematic showing the band structure of zinc blende type and wurtzite type semiconductors is given in Appendix-II. Spin orbit coupling reduces the valence band degeneracy. In quantum confined structures, the exchange interaction between the electron and hole is significant as the overlap of the carriers is increased which also affect the degeneracies. In the case of quantum dots the energy levels are

63 highly degenerate and each excited state is split into a multiplet of levels. The optical transitions that follow the electric dipole approximation (Al = ±1, 0) are allowed. In CdSe nanoparticles the presence of optically forbidden state or the dark state has been proposed to be responsible for the Stokes shift [27]. The band edge emitting state is the optically forbidden level or the dark excitonic state in quantum dots. With decreasing size, the energy difference between the first optically allowed state and the optically forbidden dark state increases. A size dependent exchange interaction has been reported to be responsible for the Stokes shift in InP quantum dots [30]. Smith et al. [17] suggest that size dependent phonon progression may be partly responsible for the PL emission Stokes shift. In photoluminescence excitation (PLE), a spectrally narrow portion of the emission is monitored while the excitation energy is scanned. The spectrum of the emission amplitude as a function of the excitation energy provides an absorption spectrum for the subset of the particles which emit at the monitored wavelength. Thus PLE can be used as a size selective optical technique. Figure 3.10 shows the PLE spectra of ZnSe samples when the emission is monitored at different wavelengths on the blue side of the full luminescence spectra [27]. When the emission is monitored at a particular wavelength a subset of the particles which emit at that wavelength and hence having a certain size are probed [5]. In figure 3.10, the emission wavelength is varied from 375 nm to 430 nm. The PLE spectra for different emission wavelength correspond to different sizes ranging from 21 A to 36 A. A clear blue shift of the first excited state is observed as the particle size decreases. Here, we estimate the size of these nanoparticles from the tight binding calculations carried out by Sapra et al. [25] for ZnSe nanoparticles. Size estimated by XRD as well as transmission electron microscopy (TEM) gives the average size of the particles. TEM can measure the size of the particles accurately. The samples that are studied have finite size distribution. Whereas the features observed in the PLE spectra give a more precise value of the transition energy of the first excited state from which the size of the ZnSe nanoparticles

64 can be estimated using the tight binding calculations. The sizes studied here are in the strong confinement regime.

240 260 280 300 320 340 360 380 400 420 Wavelength (nm)

Figure 3.10: PLE spectra of ZnSe nanoparticles monitored at different emission wavelengths (a) 375 nm (b) 384 nm (c) 395 nm (d) 407 nm (e) 414 nm (f) 430 nm.

In the PLE spectra, transitions from higher excited states were also observed. The first excited state transition is the strongest compared to the higher excited state transitions. In some of the spectra the second, third and fourth excited states are also seen. The PLE spectra were fitted with Gaussians to determine the exact energies of higher excited states. A typical Gaussian fit is shown in Figure 3.11. The energy of the excited state transitions obtained by Gaussian deconvolution of the PLE spectra along

65 240 260 280 300 320 340 360 380 400 420 Wavelength (nm)

Figure 3.11: Gaussian fit of PLE spectra of 33 A ZnSe nanoparticles. (Km = 407 nm).

with the corresponding sizes of the ZnSe nanoparticles is given in Table 3.2. Figure 3.12 shows the transitions from the first, second, third and fourth excited states for different sizes of ZnSe nanoparticles. The figure clearly shows the increase in blue shift in the first excited state and the energy separation between the states with decrease in nanocrystal size. ZnSe has a zinc blende structure and a spin-orbit splitting of 0.43 eV in the valence band [9, 31]. The excited state transitions were deduced for ZnSe nanoparticles from the reported EMA calculations [31]. The outline of the variation observable in the optical speetra of quantum dots is given in Chapter 1. The excited e e e states transitions are assigned as lS -153*/2, 1S'-2S*2, \P -l/»*2 and \S - \S so

[5, 19, 31] which are the first, second, third and the fourth excited state transitions

e respectively. Xia et al. [31] have calculated the energy values for \S -lS^/2 and

\Se ~\Sso transitions for ZnSe spheres of diameter ~35 A and -50 A using effective

66 mass approximation for infinite potential spherical quantum well and compared it with experimentally obtained values of Chestnoy et al. [9]. The calculated energies were found to match well with the experimental transition energies. However, in the present

e h e S0 case for the 34 A ZnSe nanoparticles the \S -\S v2 and \S -\S transitions do not match with the values calculated by Xia which are higher compared to that obtained by PLE. Smith et al. [17] also conclude that the quantum confinement effects in ZnSe cannot be explained on the basis of reported EMA calculations. Valence band mixing may be much more complicated than that can be handled by EMA formulations.

Table 3.2: Table giving the energy of the excited states of ZnSe quantum dots with different sizes.

1st excited state 2nd excited state 3rd excited state 4th excited state Size (A) (eV) (eV) (eV) (eV)

36 3.15 3.28 3.45 3.58

34 3.17 3.40 3.74 4.06

32 3.20 3.49 3.90 4.59

26 3.44 3.75 -- ~

21 3.61 ~ ~ --

In summary, stable ZhSe quantum dots with size as small as 2.9 nm were prepared. More importantly, the quantum dots are of high quality and show only strong band edge luminescence. No defect state luminescence is seen. Low temperature, high resolution, photoluminescence excitation spectroscopy has been employed to experimentally determine the higher excited states in ZnSe nanoparticles.

67 20 22 24 26 28 30 32 34 36 38 Dot Diameters (A)

Figure 3.12: PLE energy peak positions for different transition states versus size of the nanoparticles.

Using PLE, effectively five samples having diameters ranging between 2.1 nm to 3.6 nm were investigated. The transition energies of the electronic \Se and hole IS/,, and split off hole \SS0 states obtained by effective mass approximation using infinite potential spherical quantum well overestimates the excited state transition energies obtained by PLE. In short, valence band wrapping is much more complicated in case of q-size ZnSe. Further work on estimating the electron-hole energy levels in ZnSe quantum dots is underway.

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70