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A Theoretical Study on the Electronic Structure of ZnSe/ZnS and ZnS/ZnSe Core/Shell Nanoparticles Biplab Goswami, Sougata Pal, and Pranab Sarkar* Department of Chemistry, VisVa-Bharati UniVersity, Santiniketan, 731235, India ReceiVed: February 29, 2008; ReVised Manuscript ReceiVed: May 26, 2008

By making use of a density-functional tight-binding (DFTB) method we studied the electronic and optical properties of ZnSe/ZnS and ZnS/ZnSe core/shell nanoparticles. Our emphasis will be on the atomic charge distribution, electronic energy levels, band gap, excitation spectra, and their variation with the thickness of the shell for both ZnSe/ZnS and ZnS/ZnSe core/shell systems. We have made a qualitative comparison of our theoretical results with those of experimental observation on these core/shell systems. Finally, we studied the energetics of diffusion of one Se(S) and S(Se) atoms between the core and the shell in these nanostructures to understand the stabilities of these systems against chemical degradation. I. Introduction Colloidal semiconductor nanocrystals, or quantum dots (QDs), that exhibit quantum confinement effects have drawn significant attention for last couple of decades since they will provide key building blocks for nanodevices.1–5 The size-dependent properties of semiconductor QDs afforded by quantum confinement effects offer numerous novel applications of these materials in light-emitting devices,6–9 lasers,10–12 biological fluorescence marking,13–18 and solar cells.19–21 Extensive experimental and theoretical investigations have been carried out on sizedependent properties of binary II-VI semiconductor nanocrystals. To increase the stability of colloidal nanocrystals against chemical degradation and to improve the luminescence efficiency from band edge states, approaches have been developed to epitaxially grow a higher band gap semiconductor material around the QDs, resulting in so-called core/shell systems. Core/ shell semiconductor QDs exhibit novel properties and have become the subject of recent study from both fundamental and practical points of view. Particles passivated with inorganic shell structures are more robust than organically passivated QDs and have greater tolerance to processing conditions necessary for incorporation into solid state structures. In addition, the shell type and shell thickness of such composite core/shell nanostructures provide further control for tailoring the optical and electronic properties. Some examples of core/shell nanostructures reported include CdSe/ZnS, ZnS/CdS, CdS/HgS, CdS/PbS, CdSe/CdS, CdSe/ZnSe, etc.22–41 and the inverse structures with accounts of improved luminescence quantum yields, decreased fluorescence lifetimes, and benefits related to the tailoring of the relative band gap positions between the two materials. When the narrower band gap QDs are overcoated with a wider band gap semiconductor material, both the electron and the hole are mostly confined in the core region, resulting in localization of charge carriers along with the passivation of surface radiative recombination sites. Wide band gap II-VI semiconductor nanocrystals, such as ZnSe, are particularly appealing due to their ability of emitting from the blue to the UV spectral range. The ZnSe QDs and ZnSe-based nanostructures such as ZnSe/ ZnS core/shell semiconductor QDs have been the subject of intense investigation in recent times. There are a number of * Corresponding author E-mail: [email protected].

experimental studies reporting the synthesis and properties of this core/shell structure.42–48 Thus Nikesh et al.42 reported the synthesis of ZnSe QDs and the ZnSe/ZnS core/shell system and have shown that this core/shell system yields a remarkable enhancement of photoluminescence (PL) quantum efficiency without affecting the spectral distribution. Lomascolo et al.43 investigated the ZnSe/ZnS system by means of time-resolved PL spectroscopy and have shown that these nanocrystals are stable against photo-oxidation and have PL quantum efficiencies around 15% even months after synthesis. Kim et al.44 studied the strain effects on sizes and emission energies of ZnSe/ZnS QDs, and they observed a consistent blue shift of the groundstate emission with increasing thickness of the ZnS shell. Very recently, Ali et al.45 reported the synthesis of both ZnSe/ZnS and reverse-type ZnS/ZnSe core/shell nanostructures. Their study revealed that when ZnSe nanocrystals were passivated with higher band gap ZnS, then the integrated PL intensity increases 2.6 fold to that of the ZnSe core. The reverse-type core/shell system ZnS/ZnSe exhibited a significant red shift in its absorption spectra. Although there are a number of experimental studies on ZnSe/ZnS and ZnS/ZnSe core/shell nanostructures now available in the literature, theoretical studies addressing the electronic structure of these particular systems and core/shell systems in general are scarce.49–51 However, theoretical investigations of the problem of interest are of crucial importance in optimizing the nanostructured devices, guiding relevant experiments, providing rational explanations of experimental observation, and sometimes offering new results to be experimentally verified. In what follows, we propose here to study the electronic structure of ZnSe/ZnS and ZnS/ZnSe semiconductor nanostructures using a density-functional tightbinding (DFTB) method.52,53 Although nanocrystallites have not yet completed their evolution into bulk solids, structural studies indicate that they retain the bulk crystal structure and lattice parameter.54 Therefore, we have considered the spherical parts of both zinc-blende and wurtzite crystal structure with one semiconductor compound outside the other one and, subsequently, let the structures relax to their total energy minimum. We will present results for both zinc-blende and wurtzite derived ZnSe/ZnS and ZnS/ZnSe core/shell systems. In particular, our emphasis will be on the distribution of charge, density of states, band gap, and excitation spectra of the core/shell system. We

10.1021/jp801781s CCC: $40.75  2008 American Chemical Society Published on Web 07/11/2008

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will discuss the variation of these properties with the size (thickness) of the shell. Finally, we will discuss the energetics of simultaneous diffusion of one S(Se) atom from core to shell and one Se(S) atom from shell to core in ZnS/ZnSe system and vice versa for the ZnSe/ZnS system. The article is organized as follows. In Section II we briefly discuss the density-functional method as used in the present study. We devote Section III to present and discuss the results of our calculation on various core/shell systems. Section IV contains a brief summary of our findings. II. Theoretical Method In this work we have employed the parametrized DFTB method of Porezag et al.,52,53 which has been described in detail elsewhere, and, therefore, here shall be only briefly outlined. The approximate DFTB method is based on the densityfunctional theory of Hohenberg and Kohn in the formulation of Kohn and Sham.55,56 In this method, the single-particle wave functions Ψi(r) of the Kohn-Sham equations are expanded in a set of atomic-like basis functions (φm), with m being a compound index that describes the atom at which the function is centered, the angular dependence of the function, as well as its radial dependence. These functions are obtained from selfconsistent density-functional calculations on the isolated atoms employing a large set of Slater-type basis functions. The effective Kohn-Sham potential Veff(r b) is approximated as a simple superposition of the potentials of the neutral atoms Veff (r b) ) ∑j V0j (|r b- b Rj|). Furthermore, we make use of a tightbinding approximation, so that 〈φm|V0j |φn〉 is non-vanishing only when φm and/or φn is centered at b R j. Then the binding energy is approximated as the differences between the single-particle energies of the occupied orbitals of the compound and those of isolated atoms augmented with shortrange pair potentials, we obtain eq 1, occ

EB ≈

∑ εi - ∑ ∑ εjm + 21 ∑ Ujj (|Rbj - bRj |) ′

i

m

j

′

(1)

j*j′

where, εi is the energy of the ith orbital for the system of interest, and εjm is the energy of the jth orbital for the isolated mth atom. bj - b Finally, Ujj′(|R Rj′|) is a pair potential and is determined from exact density-functional calculations on diatomics, that is, in our study on the ZnS/ZnSe systems, Zn2, S2, and Se2 molecules. In our calculations only the 3d and 4s electrons of Zn, 3s and 3p electrons of S, and 4s and 4p electrons of Se are explicitly included whereas the other electrons are treated within a frozencore approximation. Because the computational method is parametrized, we therefore test its transferability to larger systems by first performing calculations on infinite, periodic, crystalline structures of ZnS57 and ZnSe.58,59 This led to optimized lattice constant 5.46 (5.61) Å for the zinc-blende ZnS (ZnSe) crystal and lattice constants 3.84 (4.02) Å and 6.27 (6.57) Å for wurtzite ZnS (ZnSe) crystal. The corresponding experimental values for the zinc-blende and wurtzite structures are 5.41 (5.67), 3.811 (3.98), and 6.134 (6.53) Å respectively. Thus, the structural parameters of the infinite crystalline material are well reproduced. In addition, we have reproduced the band structure of bulk ZnS and ZnSe crystals. Furthermore, we used an extension performing within time-dependent densityfunctional response theory (TD-DFRT) for the calculation of the excitation spectra. The calculations performed by this program package are referred to as TD-DFRT-TB.60 III. Results and Discussions A. ZnSe/ZnS core/shell system. Let us first consider the ZnSe/ZnS core/shell system where a narrower band gap ZnSe

Figure 1. Radial distribution of Mulliken gross populations for the valence electrons of Zn, Se, and S atoms for zinc-blende core/shell systems of different sizes: (a)(ZnSe)37(ZnS)21, (b)(ZnSe)37(ZnS)31, (c)(ZnSe)37(ZnS)46, (d)(ZnSe)37(ZnS)58, and (e) (ZnSe)37(ZnS)67. The horizontal dashed lines mark the values for the neutral atoms, that is, 12 for Zn and 6 for Se and S. Triangles, crosses, and squares mark the populations of Zn, Se, and S atoms, respectively.

QD is coated with a higher band gap ZnS shell. To describe several properties of the systems we first define the center of the core/shell system through eq 2, n

∑

1 b b R0 ) R n j)1 j

(2)

where the summation goes over all the atoms of the cluster. Subsequently, the radial distance for the jth atom is defined by eq 3.

bj - b rj ) |R R0|,

j ) 1, 2,....n

(3)

In Figure 1 we have shown the atomic Mulliken gross population for the individual atoms as a function of their radial distance of eq 3 for some representative zinc-blende ZnSe/ZnS core/shell systems. The behavior of wurtzite core/shell systems are similar. Because only the valence electrons are included, for neutral atoms, these numbers would be 12 for Zn and 6 for Se and S.

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Figure 2. Density of states (DOSs) for some representative zinc-blende (left column) and wurtzite-derived (right column) core/shell system of different sizes: (a, d) (ZnSe)37(ZnS)21, (b, e) (ZnSe)37(ZnS)31, (c, f) (ZnSe)37(ZnS)58. The thick lines represent the DOSs of the (ZnSe)37 system. The vertical dashed line marks the Fermi energy.

From the figure it is readily seen that there is small electron transfer from Zn to Se and S in the inner part of the clusters, whereas there is relatively large electron transfer in the outer part of the system. The width of the region where relatively large charge transfer takes place depends on the thickness of the shell. In general, there is larger electron transfer from Zn to S than from Zn to Se in agreement with their electrongetivities difference. The other feature from the figure is that the outermost atoms in the core/shell systems are also anion similar to that of pure ZnS or ZnSe clusters.57–59 A close inspection of the Mulliken populations of zinc-blende and wurtzite (not shown here) ZnSe/ZnS core/shell systems reveals that the extent of charge transfer is relatively more in wurtzite core/shell systems than the zinc-blende core/shell structure. So the crystal structure plays an important role in the charge transfer for the core/shell system. The detail analysis of the charge transfer of core and shell atoms reveal that there is essentially net charge transfer from the core to the shell region. The extent of net charge transfer from the core to the shell region decreases with increasing shell size. Ali et al.45 in their experimental study on the same system also observed a similar behavior. Figure 2 shows the density of states (DOSs) for three different sizes of both zinc-blende (left panel) and wurtzite (right panel) core/shell nanoparticles, and the contribution of the ZnSe core is also shown (thick lines) in the same figure. The DOS (thin line) of (ZnSe)37(ZnS)21 is shown on the top panel. The middle panel refers to (ZnSe)37(ZnS)31 and the bottom panel to (ZnSe)37(ZnS)58 core/shell nanoparticles. The graphs with a thick line represents the DOSs of pure (ZnSe)37 clusters. The figure reveals that the conduction band edge of core/shell clusters are dominated by a ZnSe core and a little contribution from the shell. In contrast, the valence band edge states are dominated equally by both the ZnSe core and the ZnS shell. The contribution of the ZnS shell in the conduction band edge states are evident from the fact that the width of the conduction band edge grows in size as thickness of the shell increases. For wurtzite core/shell clusters, the presence of a shell causes some states to appear in the band gap region, thereby decreasing the

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Figure 3. HOMO-LUMO gap as a function of number of ZnS pair (size of the shell) for (a) zinc-blende and (b) wurtzite-derived ZnSe/ ZnS core/shell nanostructures. [fixed core is (ZnSe)37].

band gap compared to pure clusters. Another feature that is very specific for wurtzite core/shell clusters is that the conduction band edge states are dominated by a ZnS shell whereas the valence band edge states are dominated by both the ZnSe core and the ZnS shell. We have shown in Figure 3 the variation of band gap of both zinc-blende and wurtzite ZnSe/ZnS core/shell nanostructures as a function of the shell size. The band gap value of pure unpassivated zinc-blende Zn37Se37 nanocrystal is 1.723 eV.58 The figure clearly indicates that for zinc-blende core/shell systems the presence of ZnS shell increases the band gap only to some extent and also that the band gap increases with increasing shell size and decreases after passing through a maximum. Because the band gap values are only slightly affected by the ZnS shell, one would expect only a small blue shift in the absorption spectra. This prediction of our theoretical calculation on ZnSe/ZnS core/shell nanostructures is in good agreement with recent experimental observation of Ali et al.45 and Hwang et al.48 These authors have only observed a very small shift in the UV-vis absorption spectra of the zinc-blende ZnSe/ZnS core/shell system compared to ZnSe nanocrystals. The increasing trend in band gap of the ZnSe/ZnS core/shell system with increasing shell size is also agreement with the experimental result of Kim et al.44 on ZnSe/ZnS QDs. The actual magnitudes of band gap values are, of course, different because they have studied ZnSe/ZnS QDs of relatively large size. For the wurtzite ZnSe/ZnS core/shell system, we see there is an overall increasing trend (with oscillation) in the band gap values with the size of the shell. However the band gap values are smaller than that of pure wurtzite Zn37Se37 nanocrystals (3.615 eV).58 The smaller values of band gap is evident from the DOSs figure where it is clearly seen that the presence of a shell causes some states to appear in the band gap region. So our theoretical results suggests that the absorption spectra of wurtzite ZnSe/ ZnS core/shell systems will exhibit a red shift compared to pure ZnSe QDs. Our theoretical result is in good agreement with

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Figure 4. TD-DFRT-TB spectra of zinc-blende (left column) and wurtzite-derived (right column) ZnSe/ZnS core/shell systems of different sizes: (a, d) (ZnSe)37, (b, e)(ZnSe)37(ZnS)21, (c,f) (ZnSe)37(ZnS)31. (All y-axes have the same scale, and all curves are broadened with Gaussian 0.27 eV).

the experimental observation of Chen et al.47 where the PL spectra of the wurtzite ZnSe/ZnS core/shell system clearly showed a red shift compared to ZnSe QDs. This red shift may occur because the conduction band offset between ZnSe and ZnS is smaller than the confinement energy of the electron, and as the shell grows, the electronic wave function extends to a larger box, and its confinement energy is lowered. This red shift may also be due to the dominance of nonradiative recombination mechanisms, more likely involving surface states. However, the band gap of this wurtzite core/shell system increases with increasing shell size, so the absorption spectra of the wurtzite core/shell system with a thicker shell would show a blue shift compared to a core/shell system with a thinner shell. Electronic excitation as computed with TD-DFRT-TB60 for six different clusters is shown in Figure 4. The panels a-c of the figure correspond to zinc-blende Zn37Se37, (ZnSe)37(ZnS)21, and (ZnSe)37(ZnS)31), respectively, whereas panels d-f correspond to wurtzite Zn37Se37, (ZnSe)37(ZnS)21, and (ZnSe)37(ZnS)31 core/shell systems, respectively. The interesting feature of the figure is that the lowest excitation energies of the zincblende ZnSe/ZnS core/shell system (left panel) remains almost constant with the increasing shell size. The intensity of this transition as evident from the figure is less intense. Several strong response peaks, that is, high intensity peaks, are in the range of shorter wavelengths, corresponding to the higher excited states. The near invariance of lowest excitation energy can be explained if we look back to the DOSs (Figure 2a-b) of these systems. The DOSs show that the band alignment of core/shell systems are similar as that of a ZnSe core. The ZnS shell only influences the valence band; hence, the growing of a ZnS shell cannot affect the lowest excitation energies of the core/shell nanocrystals. So, from the DOSs it is clear that the electron transition orbitals (i.e., HOMO and LUMO) of these zinc-blende core/shell nanocrystals, especially the part of the ZnSe core, are similar, showing that the contributions of the ZnS shell in these orbitals are quite small. However, the lowest excitation energy of wurtzite ZnSe/ZnS core/shell systems (right panel) showed a clear red shift compared to a pure wurtzite

Figure 5. Radial distribution of Mulliken gross populations for the valence electrons of Zn, S, and Se atoms for zinc-blende core/shell systems of different sizes: (a) (ZnS)37(ZnSe)21, (b) (ZnS)37(ZnSe)31, (c) (ZnS)37(ZnSe)46, (d) (ZnS)37(ZnSe)58, and (e) (ZnS)37(ZnSe)67. The horizontal dashed lines mark the values for the neutral atoms, that is, 12 for Zn and 6 for Se and S. Triangles, squares, and crosses mark the populations of Zn, S, and Se atoms, respectively.

cluster. From the DOSs (Figure 2d-e) it is seen that the presence of a ZnS shell results in some states with energies around the Fermi energy, and these states cause lowering in the lowest excitation energies. So, the lowest excitation energies are very much dependent on the crystal structure of the core/ shell system, and our results are in qualitative agreement with experimental results44,45,47,48 of this particular core/shell system. B. ZnS/ZnSe Core/Shell System. Let us consider the reverse type ZnS/ZnSe core/shell system where a wider band gap material ZnS is coated with lower band gap ZnSe shell. In Figure 5 we have shown the Mulliken gross population of individual atoms as a function of their radial distance for a few ZnS/ZnSe core/shell systems of different sizes. The general features are more-or-less the same as that of the ZnSe/ZnS core/ shell system, but the extent of charge transfer from Zn to the anions are greater for zinc-blende structures compared to wurtzite (not shown here) structures and is exactly reverse what we observed for the ZnSe/ZnS system. Another important finding of our study is that the extent of net charge transfer from Zn to the anions for the ZnS/ZnSe core/shell system is smaller than that of the ZnSe/ZnS core/shell system. In the

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Figure 6. Density of states for some representative zinc-blende (left column) and wurtzite-derived (right column) core/shell system of different sizes: (a, d) (ZnS)37(ZnSe)21, (b, e) (ZnS)37(ZnSe)31, (c, f) (ZnS)37(ZnSe)58. The thick lines represent the DOSs of the (ZnS)37 system. The vertical dashed line marks the Fermi energy.

detailed analysis of the charge transfer of core and shell atoms here, we also found a net charge transfer from core to shell region, and the magnitude is less than that of the ZnSe/ZnS system. In Figure 6, the DOSs of six representative ZnS/ZnSe core/ shell systems, similar to that of Figure 2, are shown. From the figure it is evident that, similar to ZnSe/ZnS core/shell nanocrystals, the valence bands of ZnS/ZnSe nanocrystals are also dominated by both core and shell atoms, whereas conduction band are mainly from core atoms, although the contribution of atoms from shell to the conduction bands slowly increases with the increase in size of the shell. The band alignment of the zincblende ZnS/ZnSe core/shell system is very much similar to that of the ZnS core, whereas for wurtzite nanoparticles some states appear in the band gap region. In Figure 7 we have plotted the band gap values of both (a) zinc-blende and (b) wurtzite ZnS/ZnSe core/shell systems as a function of the shell size. The values of the band gap of the zinc-blende ZnS/ZnSe core/shell systems are smaller than that of an unpassivated Zn37S37 cluster. Therefore, our theoretical results suggest that the ZnS/ZnSe core/shell nanostructures exhibit a red shift in its absorption spectra compared to pure ZnS QDs. Ali et al.45 in their experimental study with a ZnS/ ZnSe core/shell system observed the red shift in the absorption spectra. This red shift in the band gap values of the core/shell nanoparticles in relation to the parent material are attributed to relaxation of quantum confinement resulting from the growth of the shell. Our study also suggests that the band gap values of ZnS/ZnSe core/shell system increases with the increase in shell size. For wurtzite core/shell systems there is an overall increasing trend in the band gap values with increasing shell size. The low value for the largest system (with thicker shell) we studied may be because of the appearance of surface states from ZnSe shell. The electronic excitation spectra, as calculated by TD-DFRT, of a few representative ZnS/ZnSe core/shell systems are shown in Figure 8. The general features are more-or-less the same as that of ZnSe/ZnS core/shell systems. From the figure it is seen

Figure 7. HOMO-LUMO gap as a function of number of ZnSe pairs (size of the shell) for (a) zinc-blende and (b) wurtzite-derived ZnS/ ZnSe core/shell nanostructures [fixed core is (ZnS)37].

Figure 8. TD-DFRT-TB spectra of zinc-blende (left column) and wurtzite-derived (right column) ZnS/ZnSe core/shell systems of different sizes: (a, d) (ZnS)37, (b, e) (ZnS)37(ZnSe)21, (c, f) (ZnS)37(ZnSe)31. (All y-axes have the same scale, and all curves are broadened with Gaussian 0.27 eV).

that for both zinc-blende and wurtzite ZnS/ZnSe core/shell nanostructures the lowest excitation energies showed a clear red shift compared to pure Zn37S37 crystals and is in agreement with the recent experimental observation of Ali et al.45 The red shift in lowest excitation energies can be well explained from the band alignment of DOSs of these systems (Figure 6). From the DOSs of both zinc-blende (Figure 6a-b) and wurtzite nanocrystals (Figure 6d-e), we have seen that the presence of

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Figure 9. The variation of relative total energy accompanying an interchange of a close pair of Se(S) and S(Se) atoms for some selected core/shell systems; for Zinc-blende (a) (ZnSe)37(ZnS)58 and (b) (ZnS)37(ZnSe)58 and for Wurtzite (c) (ZnSe)37(ZnS)58 and (d) (ZnS)37(ZnSe)58. The reaction coordinate is defined as being 0 before and 1 after interchange.

a ZnSe shell results in some orbitals in the band gap region, and these orbitals essentially cause the reduction of the lowest excitation energies. The DOSs (Figure 6f) of wurtzite core/shell nanocrystals with a relatively thicker ZnSe shell reveals that there is no orbitals in the band gap region and correspondingly would have higher band gap or lowest excitation energies. Finally, to understand the stability of these core/shell nanostructures against degradation, we studied the energetics of diffusion of one Se(S) atom from the core to the shell and, simultaneously, one S(Se) atom from shell to the core. First, we simply interchanged two close Se atom S atoms in various core/shell systems and subsequently allowed the structure to relax to the closest total energy minimum. We have calculated the minimum energy path (MEP) between the structures before and after diffusion by using the nudged elastic band (NEB) method.61–63 The NEB method provides an efficient recipe for finding the MEP between a given initial and final state of transition. The MEP is found by constructing a set of images of the system between the initial and final states. A spring interaction between adjacent images is added to ensure continuity of the path, thus making an elastic band. An optimization of the band involving the minimization of the force acting on the images brings the band to the MEP. The resulting total energy curves for four representative core/shell systems(ZnSe/ ZnS and ZnS/ZnSe core/shell systems of both zinc-blende and wurtzite structures) are shown in Figure 9. It is seen that the diffusion is connected with a fairly large energy barrier for each system we studied here, making the diffusion highly improbable. The activation barrier for wurtzite core/shell nanostructures are a little higher than that of the corresponding zinc-blende structures. Finally, the large energy barrier for the transition in both ZnSe/ZnS and ZnS/ZnSe core/shell systems suggest that these QDs are, in principle, stable against degradation. IV. Conclusions The growth of a shell of a second material on a core of another material to form a core/shell system has been a successful route in the surface modification of nanostructured materials. The study of these core/shell nanostructures are particularly interesting and significant because the choice of shell material and control of the thickness of the shell allows one to control the function of the complex nanocrystals and to tune it to the desired behavior. In this paper we have presented results

J. Phys. Chem. C, Vol. 112, No. 31, 2008 11635 of our theoretical calculation on structural and electronic properties of ZnSe/ZnS and ZnS/ZnSe core/shell systems of both zinc-blende and wurtzite modifications. The study of Mulliken population analysis suggest that there is charge transfer from core to the shell for both ZnSe/ZnS and ZnS/ZnSe core/shell systems but that the magnitude is higher for the former. The values of the band gap for zinc-blende ZnSe/ZnS core/shell systems are a little larger than the corresponding ZnSe cluster, suggesting that these nanostructures will exhibit a small blue shift in their absorption spectra. However, the band gap values of wurtzite ZnSe/ZnS core/shell systems are smaller than that of wurtzite ZnSe clusters. So our results suggest that these core/ shell systems will exhibit red shift in their absorption spectra. The band gap values of both zinc-blende and wurtzite ZnS/ ZnSe core/shell nanostructures are smaller than that of pure ZnS nanocrystal; thus, the absorption spectra of these core/shell systems will exhibit red shift. Our theoretical results are in good agreement with the recent experimental observations. The high diffusion barrier as revealed from the study of the energetics of simultaneous diffusion of Se(S) and S(Se) from core to the shell and vice-versa confirms the stability of these core/shell systems against degradation. To the best of our knowledge, this is the first systematic theoretical study addressing the structural, electronic, and optical properties of both ZnSe/ZnS and ZnS/ ZnSe core/shell nanostructures, which we believe will be helpful in understanding the properties of these nanocrystals and also for further exploration of these materials. Acknowledgment. The financial supports from CSIR, Government of India [01(2148)/07/EMR-II] and UGC(SAP), New Delhi, through research grants are gratefully acknowledged. One of the authors (S.P.) is grateful to CSIR, New Delhi for the award of a Senior Research Fellowship (SRF). References and Notes (1) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Eur. J. 2002, 8, 28. (2) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (5) Banin, U.; Cao, Y. W.; Katz, D.; Millo, O. Nature 1999, 400, 542. (6) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (7) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506. (8) Chen, H. S.; Wang, S. J. J.; Lo, C. J.; Chi, J. Y. Appl. Phys. Lett. 2005, 86, 131905. (9) Muller, A. H.; Petruska, M. A.; Achermann, M.; Werder, D. J.; Akhadov, E. A.; Koleske, D. D.; Hoffbauer, A. A.; Klinov, V. I. Nano Lett. 2005, 5, 1039. (10) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314. (11) Artemyev, M. V.; Woggon, U.; Wannemacher, R.; Jaschinski, H.; Langbein, W. Nano Lett. 2001, 1, 309. (12) Chan, Y.; Steckel, J. S.; Snee, P. T.; Caruge, J.-M.; Hodgkiss, J. M.; Nocera, D. G. Appl. Phys. Lett. 2005, 86, 073102. (13) Bruchez, M.; Morronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2046. (14) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (15) Mitchel, G. P.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1999, 121, 8122. (16) Han, M. Y.; Gao, X. H.; Su, J. Z.; Nie, S. M. Nat. Biotechnol. 2001, 19, 631. (17) Chan, W. C. W.; Maxwell, D. J.; Gao, X. H.; Balley, R. E.; Han, M. Y.; Nie, S. M. Curr. Opin. Biotech 2002, 13, 40. (18) Alivisatos, A. P. Nat. Biotechnol. 2004, 22, 47. (19) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462. (20) Landia, B. J.; Castrob, S. L.; Rufa, H. J.; Evansa, C. M.; Baileyc, S. G.; Raffacllea, R. P. Sol. Energy Mater. Sol. Cells 2005, 87, 733. (21) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425.

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