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Dependence of Microstructure and Luminescence on Shell Layers in Colloidal CdSe/CdS Core/Shell Nanocrystals Jiaming Zhang,‡ Xuke Zhang,‡ and J. Y. Zhang*,† College of Animal Husbandry and Veterinary, Henan Agricultural UniVersity, Zhengzhou 450002, China, and Veterinary Medicine Engineering & Technology Research Center of Henan ProVince, Luoyang 471000, China, College of Electronic Science and Engineering, Southeast UniVersity, Nanjing 210096, China ReceiVed: December 20, 2009
Colloidal CdSe nanocrystals (NCs) were successively overcoated with CdS shells, and the X-ray diffraction and Raman measurements of the yielded CdSe/CdS core/shell NCs revealed that stress in the CdSe core was dramatically increased with the growth of the CdS shell but was released when the shell became thicker. The CdSe lattice contraction, photoluminescence (PL) quantum yield, and PL average lifetime exhibit similar dependences on the shell thickness. The PL lifetime undulation with temperature is indicative of the spatial distribution of trap states. 1. Introduction In recent years, colloidal semiconductor nanocrystals (NCs) as biological probes have been attracting considerable interest because they have the potential to complement the wellestablished in vitro/vivo markers (e.g., organic dyes and fluorescent proteins) for applications needing better photostability over long time scales.1-3 Surface-localized states, which are due to the presence of surface atoms with reduced coordination number, occur on them. Their optical properties are significantly affected by these inhomogeneous surface traps because of their high surface-to-volume ratio.4-6 Steady-state and time-resolved photoluminescent (PL) spectra of colloidal NCs in multifarious situations have been examined in detail, but comparatively little is understood about the carrier dynamics involved with the surface traps.7,8 Several kinetic schemes, in which three9 or more states10,11 are involved, have been proposed to elucidate the interplay among intrinsic excitons and surface states. Recently, Chergui et al.7 explained the multiexponential PL decay kinetics of CdSe NCs by using stochastic groundstate dipole moments which are dominantly contributed from the surface effect. Scholes et al.8 developed a scheme, based on Marcus electron transfer theory, to understand how excitons in NCs interact with their surroundings. To improve the luminescent efficiency and colloidal stability of these nanostructured particles, surface modification of colloidal NCs becomes necessary.1-3 The growth of a shell of a higher band gap inorganic material on a core of another lower band gap material to form a heterostructure has been one of the successful routes in the surface modification.12,13 CdSe NCs are attractive for use in emissive applications due to their sizedependent PL tunable across the visible spectrum, and core/ shell CdSe/CdS,13 CdSe/ZnS,12 and CdSe/Zn0.5Cd0.5S/ZnS14 NCs have been synthesized. The shell-growth can eliminate nonradiative defects on the core’s surface, so the PL quantum yield (QY) is greatly enhanced, but the PL QY is not increased monotonically with increasing the shell thickness.14 Because of the lattice mismatch between the core and the shell, the interface * To whom correspondence should be addressed. ‡ Henan Agricultural University and Veterinary Medicine Engineering & Technology Research Center of Henan Province. † Southeast University.
strain accumulates dramatically during the shell-growth, and eventually can be released through the formation of dislocations,15 indicating that the spatial distribution of defects in core/ shell NCs can be controlled by the shell-growth. Therefore, the series of PL spectra from NCs with successive shell-growths can reveal the nature of traps in NCs. In this work, zinc-blende CdSe NCs were successively overcoated with CdS monolayers, and the microstructure and the temperature-dependent timeresolved PL spectra of the obtained CdSe/CdS core/shell NCs were measured. The experimental results indicate clearly a strong link between the PL decay dynamics and the stress in the CdSe core. The PL average lifetime undulates with temperature, and the interplay between intrinsic exciton and trap state is discussed. Aside from fundamental scientific interest, a better understanding of the shell effect on NC’s optical properties is important for optimizing nanocrystal synthesis, and can also lead to improvements in nanocrystal performance in applications. 2. Experimental Section Chemicals. Cadmium oxide (99.99%), selenium (99.5%, 100 mesh), sulfur (99.98%, powder), trioctylphosphine oxide (TOPO, 90%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), octadecylamine (ODA, 97%), tributylphosphine (TBP, 97%), and Stearic acid (99%) were purchased from Alfa Aecscar, and all the chemicals were used directly without any further purification. Synthesis of CdSe Core NCs. The CdSe cores were synthesized via a well-established organometallic route.14,16 A mixture of 2 mmol ofCdO, 8 mmol of stearic acid, and 10 g of ODE was heated to 200 °C to make a homogeneous solution and cooled to room temperature. A 15 g sample of ODA and 5 g of TOPO were added and the solution was heated to 280 °C under argon atmosphere, then a mixture of 10 mmol of Se, 2.36 g of TBP, and 6.85 g of ODE was injected quickly. The solution was maintained at 250 °C during the CdSe growth. The size of the CdSe NCs was controlled by the growth time, and two CdSe samples, whose average diameters were 3.5 and 4.4 nm, were obtained. These CdSe NCs were purified and dissolved in hexane for the following shell growth.
10.1021/jp9120194 2010 American Chemical Society Published on Web 02/15/2010
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Figure 1. TEM and HRTEM images of the CdSe cores (a) and the CdSe/CdS core/shell NCs with CdS thickness of 4 MLs (b).
Synthesis of CdSe/CdS Core/Shell NCs. The CdS shell growth was done by using a successive-ion-layer adsorption and reaction method,14,16 which was based on alternating injections of the Cd- and S-precursors into the solution containing the CdSe-core NCs. The cadmium precursor solution was prepared by dissolving CdO (0.8 mmol) in OA (6.4 mmol) and ODE (18.0 mL) at 250 °C. The sulfur precursor solution was prepared by dissolving sulfur (0.4 mmol) in ODE (10 mL) at 200 °C. The two precursor solutions were freshly prepared under Aratmosphere. The Cd-precursor solution was kept at about 60 °C, while the sulfur injection solution was cooled to room temperature. The concentration of CdSe-core NCs was estimated on the basis of the UV-vis spectra. The CdSe cores (1 × 10-4 mmol of particles) in hexane were mixed with 5 g of ODE and 1.5 g of ODA. After removal of the hexane under vacuum at 60 °C, the mixture was heated to 240 °C under Ar-flow where the shell growth was performed. The core/shell CdSe/CdS NCs were formed by alternating addition of Cd-precursors and S-precursors, respectively. A period of 10 min between each addition was chosen since the UV-vis and PL-spectra showed no further changes after this time period. The amount of the precursors was calculated from the respective volumes of concentric spherical shells with a thickness of one hypothetical monolayer, and the average thickness of one CdS monolayer was taken as 0.35 nm. Core/shell NCs with different shell thickness were synthesized by using CdSe cores with the same size. All these NCs were purified for the following characterization. Microstructural and Optical Characterization. A JEOL 4000EX transmission electron microscope (TEM) was used to analyze the size distribution, and structure of the NCs, and the size distribution was obtained by analyzing more than 200 nanocrystals in each sample. X-ray diffraction (XRD) spectra were obtained with a Rigaku D/max 2500 VL/PC X-ray diffractometer. Raman spectra were recorded with the 514.5 nm line of an Ar+ laser as the excitation source in a backscattering configuration at room temperature. Absorption spectra were measured with an UV-3600 spectrophotometer, and PL spectra and time-resolved PL spectra were measured with a F900 fluorescence spectrophotometer. The time-resolved PL spectra was obtained by using a time-correlated single photon counting (TCSPC) technique, and the emission was monitored at the PL peak wavelength for each sample. 3. Results and Discussion Figure 1 shows typical TEM images of these NCs. These obtained NCs are close to spherical with good monodisperisity, regardless of “bare” cores or core/shell NCs. According to the TEM measurement, the average diameter (the size distribution) is 3.5 (9%) for the CdSe core, and 4.2 (10%), 4.9 (10%), 5.5 (11%), and 6.1 (11%) for the four core/shell NCs, so the CdS
Figure 2. The absorption (solid line) and PL spectra (dotted line) of CdSe cores and CdSe/CdS core/shell NCs. The inset shows the PL quantum yield (QY) versus the number of CdS monolayer(s).
shell thickness is approximately 1, 2, 3, and 4 ML(s), respectively. Hereafter, these CdSe/CdS core/shell NCs are labeled as CS1, CS2, CS3, and CS4, respectively. Obviously, the size distribution is not broadened dramatically even though the particles are coated with 4 MLs of shell material. The particles in the HRTEM images (the insets in Figure 1) show wellresolved lattice fringes. The blurry appearance at the rim of this particle might be due to the molecular ligands.14 The lattice spacing is measured as ∼3.5 Å, which is approximately consistent with the lattice spacing between {111} planes for bulk zinc-blende CdSe. A well-defined interface between CdSe core and ZnS shell was not observed in any of the core/shell samples. The lattice fringes persist throughout the entire nanocrystal, an indication of epitaxial growth.17 Figure 2 shows the absorption and PL spectra, and the inset shows the PL quantum yield (QY) of these NCs. Due to partial leakage of exciton into the shell, the growth of the CdS shell results in a considerable shift of the emission peak from ∼573 nm (bare-core NCs) to ∼598 nm (core/shell NCs, 4 ML), and a corresponding shift of the first exciton absorption peak from ∼566 nm to ∼588 nm. The PL QY is dramatically enhanced as the shell is grown toward 3 ML CdS, but it is slightly decreased with further growth. Therefore, the sample with the 3-ML CdS shell is the “brightest”. Figure 3 shows the XRD patterns for the CdSe core and the core/shell CdSe/CdS NCs. The CdSe cores are of zinc-blende (ZB) phase, and the XRD pattern of the core/shell NCs is roughly the same as that of bare CdSe cores, but the diffraction angles are gradually shifted to larger 2θ values with the shell growth toward 3 ML CdS, and then shifted back with further shell growth. The gradual shift of XRD peaks rules out the possibility of phase separation or separated nucleation of CdS NCs.17 With the CdS shell growth on the CdSe core, these peak widths are not narrowed according to the Scherrer’s formula. There have been several explanations for the gradual shift of NC’s XRD peaks. Dabbousi et al.17 suggested that such X-ray patterns of the composite particles might be fit by adding the weighted contributions to the scattering from both the core and the shell, so the XRD diffraction angles would shift between those inherent to CdSe and CdS ZB phases. Lim et al.18 suggested that the gradual shift of XRD peaks might result from the presence of the lattice contraction in the CdSe core. Murray et al.19 proposed that the prolate shape could induce some shifts in XRD peaks of CdSe NCs. Since TEM images of our NCs indicate that these NCs have a spherical shape, we believe that the shifts measured in our samples are not due to a prolate shape, but from the lattice contraction. The Raman spectra, as shown
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Figure 5. The time-resolved PL spectra of the CdSe cores (open circle) and the core/shell NCs with 3 CdS MLs (triangle). The inset shows the PL average lifetime (solid square) versus the shell monolayer(s), and the CdSe lattice contraction (open square), calculated by the XRD spectrum, is shown also for a comparison. Figure 3. The XRD patterns for the CdSe cores and the CdSe/CdS core/shell NCs. Vertical lines indicate zinc-blende CdSe (solid line) and CdS (dotted line) bulk reflections.
Figure 4. The Raman spectra for the CdSe cores and the CdSe/CdS core/shell NCs.
in Figure 4, indicate the presence of compressive strain in the CdSe cores also. The coherency strain, which enables the shell material at the interface to adapt the lattice parameters of the core, can induce the gradual shift of the XRD peaks, and the distribution of bond lengths resulting from the strain is responsible for maintaining the XRD peak width constant though there is an increase in the NC’s diameter due to the shell growth.20 In addition, there is little lattice contraction in the CdSe core, which is induced from surface tension due to surface reconstruction. Herron et al.21 reported that free-standing CdS NCs show a bond contraction of ∼0.5% compared to the bulk. The CdSe lattice contraction, which is calculated from the position of the (220) peak, is 0.31%, 0.53%, 1.17%, 2.2%, and 1.55% for the core, CS1, CS2, CS3, and CS4, respectively. The contraction is increased with the increase of the shell thickness, but the compression strain becomes relaxed when the shell thickness is increased toward 4 ML CdS. In addition, Core/ shell CdSe/CdS NCs with the larger core size (∼4.5 nm) were synthesized also. Lattice contraction occurs during the coherency growth of the CdS shell also, but its value is smaller than that in the case with the small cores. The Raman peaks corresponding the CdSe longitudinal optical (LO) and 2LO phonons appear clearly in the bare CdSe cores, as shown in Figure 4. Compared to the symmetric LO band of the bulk CdSe, that of the CdSe cores shows low-frequency
asymmetric broadening and a downward shift (∆ω) of ∼3.5 cm-1. The shift expected by phonon localization model [∆ωL(r)], which is based on the relaxation of the q ) 0 selection rule in semiconductor NCs, is estimated to be ∼6 cm-1 for the CdSe core.22 The experimental results definitely demonstrate the presence of an additional effect competing with the phonon localization in determining the Raman peak positions of the CdSe core, and the additional effect is proposed to be the lattice contraction,23 which has been demonstrated by the XRD measurements in Figure 3. With the CdS shell growth on the CdSe core, the line of the CdSe LO phonon does not become narrower, but it shows a gradual upward shift, indicating the increase of such lattice contraction. In addition, a new scattering band, corresponding to CdS LO phonons, appears around 290 cm-1 for the core/shell NCs, and it becomes stronger and narrower with the shell growth. According to Scamarcio et al.,22 the phonon frequency shift due to the lattice contraction (∆ωS) is given by
[(
∆ωS ) ωLO 1 + 3
∆a a
-γ
)
]
-1
(1)
where γ is the Gruneisen parameter, whose value is about 1.1 for CdSe, ωL is the LO phonon frequency of the bulk, and ∆a/a is the lattice contraction. The lattice contraction, which is calculated according to eq 1, is 0.39%, 0.56%, 1.03%, 1.74%, and 1.35% for the core, CS1, CS2, CS3, and CS4, respectively. The evolution of the lattice contraction with the shell growth is consistent with the results from the XRD measurements. Figure 5 shows the typical time-resolved PL spectra of these NCs in hexane solution at room temperature, which are not single exponential but multiexponential. It has been suggested that the single-exponential dynamics for a single NC will be changed to be the multiexponential decay in the NC’s ensemble due to fluctuation in the fluorescence decay24 or the stochastic nature of the ground-state dipole moment.7 Another possibility is that the multiexponential dynamics means more than one radiative recombination channel of excitons.25-27 These spectra can be biexponentially fitted with the reduced χ2 e 1.2. Although the fit with a triexponential decay is much better, a biexponential fit is chosen in this work since the numerical fit with χ2 e 1.2 is proposed to be acceptable in the TCSPC case.28 The fast decay (τ1) is of several nanoseconds and the slow one (τ2) is of several
Colloidal CdSe/CdS Core/Shell Nanocrystals
Figure 6. The τ1 value (square), the τ2 value (triangle), and the PL average lifetime (open circle) versus temperature of these NCs.
tens of nanoseconds, corresponding to the recombination of intrinsic excitons and the interplay between excitons and surface traps, respectively.8,25-28 As shown in Figure 5, compared with the CdSe core, the core/shell NC has a slightly larger value of τ1, but its τ2 component has a lower relative fluorescence intensity,28 resulting in a smaller average lifetime (τav). The τav value, as shown in the inset of Figure 5, is decreased as the shell is grown toward 3 ML CdS and it is slightly increased with further growth. The inset of Figure 5 shows the CdSe lattice contraction versus the CdS shell thickness also. Both the lattice contraction and the τav value reach their extrema when the shell is three CdS monolayers (i.e., the “brightest” sample), and as shown in the inset of Figure 2, the PL QY is the same way. In the case with a larger core size (∼4.5 nm), experimental results are similar. The shell-growth, which happens in an epitaxial manner, eliminates nonradiative traps at the core/shell interface, and the intrinsic excitons can be confined well within NCs by a thick shell, so there is a lower possibility that excitons are trapped at surface states around NCs’ surroundings. On the other hand, the interface strain accumulates dramatically during the shellgrowth, as manifested by the presence of the CdSe lattice contraction, and its release will lead to nonradiative defects near the core in the case of a thicker shell. Therefore, Figures 2-5 indicate clearly a strong link between the fluorescence properties and surface (interface) traps in NCs. To explore the PL decay dynamics, temperature-dependent time-resolved PL spectra were measured also. For the temperature-varied PL measurements, a small number of NCs were adsorbed to a quartz substrate by spin coating a dilute NC solution, and the sample was placed in an Oxford cryostat in a vacuum. All the temperature-dependent time-resolved PL spectra are also biexponential decays for the CdSe cores and the core/shell NCs. Figure 6 shows the τ1 value, the τ2 value, and the τav value as the function of temperature, and there are three notable features: (1) The τ1 value is slightly deceased with the increase of temperature for all five samples, indicating that there are some nonradiative recombination processes of intrinsic excitons that are thermally activated,26 which is evidenced by the weakening of PL intensity with temperature. (2) The τav value of the CdSe cores is increased with the increase of temperature, and then is gradually decreased. The initial rise of τav is not caused by the splitting of dark and bright exciton fine structure states because the temperature range (>77 K) is above the splitting energy
J. Phys. Chem. C, Vol. 114, No. 9, 2010 3907 (several meV for the NCs used in this work).29 Such undulation in τav appears in the core/shell samples also, but the temperature (Tmax) corresponding to the maximum of τav is varied with the shell thickness. The Tmax value becomes higher with the shell growth toward 3 MLs (i.e., the “brightest” sample), and then is slightly lower with a thicker shell. (3) The temperature dependence of the τ2 value is also of undulation, similar to the τav case, though its Tmax is different from that of τav, indicating that the undulation in the PL lifetime results from the radiative recombination related to surface traps. The numerical simulation, based on Marcus electron transfer theory, reveals that a complex interplay between an activated trapping/detrapping process and the density of accessible trap states can induce a PL lifetime undulation with temperature in an ensemble of NCs.8 The PL lifetime rise shown in Figure 6 is indicative of a growing trap state population, which is determined by the free-energy difference (∆G) between exciton and trap, the reorganization energy (λ), and the electronic coupling integral (V) between the exciton and the trap state. Larger λ and/or ∆G values imply a higher temperature to activate thermally a surface state to trap/ detrap an exciton. With the shell growth, the distance between the surface trap and the exciton’s center is increased, so the reorganization energy, which enables the environment to accommodate the change due to carrier trapping, will be increased, and the Tmax value will become larger. However, in the case of a thicker shell (i.e., 4 MLs CdS shell), the release of stress yields some traps near the CdSe core, so the λ value for carrier trapping at these traps will be lower, and the Tmax value will become smaller. 4. Conclusion In this work, ZB CdSe cores were overcoated with 1, 2, 3, and 4 monolayer(s) of CdS shell, respectively. The ZB structure has a more isotropic distribution of facets than the wurtzite structure, which can incorporate shell materials more isotropically and lead to a more uniform shell distribution. The XRD and Raman measurements were done to study the evolution of the lattice contraction with the shell thickness, and the temperature-dependent time-resolved PL spectra were taken out to explore the carrier recombination process. The experimental results indicate clearly a strong link between the fluorescence properties and surface (interface) traps in the NCs, and the interplay between intrinsic excitons and trap states is discussed. Acknowledgment. The NSF of China, 10401024 and 10774023, provided financial support. References and Notes (1) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2016. (2) Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A. M.; Gambhir, S. S.; Weiss, S. Science 2005, 307, 538–544. (3) Larson, D. R.; Zipfel, W. R.; Williams, R. M.; Clark, S. W.; Bruchez, M. P.; Wise, F. W.; Webb, W. W. Science 2003, 300, 1434– 1436. (4) Bryant, G. W.; Jaskolski, W. J. Phys. Chem. B 2005, 109, 19650– 19656. (5) Dayal, S.; Burda, C. J. Am. Chem. Soc. 2007, 129, 7977–7981. (6) de Mello Donega, C.; Hickey, S. G.; Wuister, S. F.; Vanmaekelbergh, D.; Meijerink, A. J. Phys. Chem. B 2003, 107, 489–496. (7) Van Mourik, F.; Giraud, G.; Tonti, D.; Chergui, M.; Van der Zwan, G. Phys. ReV. B 2008, 77, 165303. Al Salman, A.; Tortschanoff, A.; Van Der Zwan, G.; Van Mourik, F.; Chergui, M. Chem. Phys. 2009, 357, 96– 101. (8) Jones, M.; Lo, S. S.; Scholes, G. D. Proc. Natl. Acad. Sci. U.S.A. 2009, 106 (9), 3011–3016. Jones, M.; Lo, S. S.; Scholes, G. D. J. Phys. Chem. C 2009, 113, 18632–18642.
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(9) Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L. E. J. Chem. Phys. 1992, 96, 946–954. (10) Jones, M.; Nedeljkovic, J.; Ellingson, R. J.; Nozik, A. J.; Rumbles, G. J. Phys. Chem. B 2003, 107, 11346–11352. (11) Morello, G.; Anni, M.; Cozzoli, P. D.; Manna, L.; Cingolani, R.; De Giorgi, M. J. Phys. Chem. C 2007, 111, 10541–10545. (12) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468– 471. (13) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019–7029. (14) Xie, R.; Kolb, U.; Li, J.; Basch, T.; Mews, A. J. Am. Chem. Soc. 2005, 127 (20), 7480–7488. (15) Liang, D.; Shen, L.; Wang, Z.; Cui, Y.; Zhang, J.; Ye, Y. Chin. Phys. Lett. 2008, 25, 4431–4434. (16) Battaglia, D.; Li, J. J.; Wang, Y. J.; Peng, X. G. Angew. Chem., Int. Ed. 2003, 42, 5035–5039. (17) Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. J. Phys. Chem. B 1997, 101, 9463–9475. (18) Lim, S.; Chon, B.; Joo, T.; Shin, S. J. Phys. Chem. C 2008, 112, 1744–1747. (19) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715.
Zhang et al. (20) Chen, X.; Lou, Y.; Samia, A. C.; Burda, C. Nano Lett. 2003, 3, 799–803. (21) Herron, N.; Calabrese, J.; Farneth, W.; Wang, Y. Science 1993, 259, 1426. (22) Scamarcio, G.; Lugara, M.; Manno, D. Phys. ReV. B 1992, 45, 13792. (23) Zhang, J.; Wang, X.; Xiao, M.; Qu, L.; Peng, X. Appl. Phys. Lett. 2002, 81, 2076. (24) Fisher, B. R.; Eisler, H. J.; Stott, N. E.; Bawendi, M. G. J. Phys. Chem. B 2004, 108, 143–148. (25) Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L. E. J. Chem. Phys. 1992, 96, 946–954. (26) Valerini, D.; Cretı´, A.; Lomascolo, M.; Manna, L.; Cingolani, R.; Anni, M. Phys. ReV. B 2005, 71, 235409. (27) Lee, W. Z.; Shu, G. W.; Wang, J. S.; Shen, J. L.; Lin, C. A.; Chang, W. H.; Ruaan, R. C.; Chou, W. C.; Lu, C. H.; Lee, Y. C. Nanotechnology 2005, 16, 1517–1521. (28) Zhang, J. M.; Zhang, X. K.; Zhang, J. Y. J. Phys. Chem. C 2009, 113, 9512–9515, where the relative fluorescence intensity is defined. (29) Califano, M.; Franceschetti, A.; Zunger, A. Nano Lett. 2005, 5, 2360–2364.
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