J. Phys. Chem. C 2008, 112, 8229–8233
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Optically Bifunctional Heterostructured Nanocrystals Angshuman Nag,† Akshay Kumar,†,⊥ P. Prem Kiran,‡ S. Chakraborty,§ G. Ravindra Kumar,| and D. D. Sarma*,†,§ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India, AdVanced Centre of Research in High Energy Materials, UniVersity of Hyderabad, Hyderabad 500 046, India, Centre for AdVanced Materials, Indian Association for the CultiVation of Science, Kolkata 700032, India, and Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India ReceiVed: January 4, 2008; ReVised Manuscript ReceiVed: March 4, 2008
We show that an attempt to synthesize nanocrystal alloys of Cd1-xZnxSySe1-y in a single-step reaction leads naturally to a heterostructured nanocrystal system with CdSe-rich core and CdS-ZnS hybrid shell because of differences in chemical reactivities of the different constituents. These nanocrystals exhibit very high (∼65%) photoluminescence (PL) efficiency. The band-edge excitonic energy and the corresponding band-edge emission can be tuned over a range of the visible spectrum by changing the composition. These samples show very strong two-photon absorption cross section (σ2 ) 1.923 × 10-47 cm4 · s/photon or 1923 GM), establishing a very rare example of an optically bifunctional material with tunability and a very high quantum efficiency for linear optical properties, namely, PL, and strong nonlinear properties, namely, two-photon absorption. Introduction Semiconducting nanocrystals (NCs), also known as quantum dots, are considered as artificial atoms, large enough to hold crystallinity.1 Owing to unique size-dependent properties due to the quantum confinement of excitons in all three dimensions,2–6 such NCs not only allow the study of evolution of bulk properties from the molecular limit but also have a great technological impact,7–9 triggering a huge interest10–16 in synthesizing good quality NCs. Specifically, NCs have been shown to have very high quantum efficiency (QE) for linear optical properties, such as photoluminescence (PL), which has already found important technological applications. Quantum confinement of charge carriers in NCs may also remarkably enhance optical nonlinearity compared to their bulk counterparts, generating considerable interest in the study of nonlinear optical properties of such NCs, particularly for their potential applications in nonlinear photonic devices.17–21 Interestingly enough, a material with both a high PL efficiency and a strong twophoton absorption (TPA) is very rare. Recently, it has been shown that a wide variety of functionalities can be added to these NCs by making heterostructured NCs instead of homogeneous NCs. One of the first investigated and most popular in this category is core-shell NCs, where a higher band-gap inorganic shell is prepared on top of the core material, passivating dangling bonds on the core surface and hence enhancing the luminescence efficiency drastically.22 Subsequently, a variety of heterostructured NCs have been introduced, establishing spherical quantum well structure,23 typeII core-shell structure,24 inverted core-shell structure,25 and core-barrier-shell struture.26 These heterostructured NCs give * Corresponding author. E-mail:
[email protected]. Also at Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560054, India. † Indian Institute of Science. ‡ University of Hyderabad. § Indian Association for the Cultivation of Science. | Tata Institute of Fundamental Research. ⊥ Present address: Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089.
rise to several additional or improved properties, such as higher brightness, localization of charge carriers, spatial separation of charge carriers, and light harvesting, compared to their binary homogeneous analogues.22–26 Here, we report a single-step synthesis of heterostructured NCs with CdSe-rich core and CdS-ZnS hybrid shell, making use of different reactivities of the constituents. Furthermore, these samples are shown to have a very high PL efficiency (∼65%) as well as a strong TPA cross section (1923 GM). Experimental Section A solution with various compositions of Se and S (1.9 mmol) in trioctylphosphine (1.0 mL) was injected into a mixture of CdO (0.386 mmol), ZnO (0.609 mmol), oleic acid (7.340 mmol), and technology-grade 1-octadecene (10 mL) and maintained at 300 °C with vigorous stirring, and the reaction was stopped after 1 min. Samples with 0.23, 0.08, and 0.02 mmol of Se precursors are referred to as sample I, II, and III, respectively, throughout this report. Elemental composition for Zn, Cd, and Se was obtained by inductively coupled plasma atomic emission spectroscopy (ICPAES) by using a Perkin-Elmer Optima 2100 DV spectrometer. A JEM 2010 HRTEM, JEOL microscope at an accelerating voltage of 200 kV was used for transmission electron microscopy (TEM). UV-visible absorption and PL experiments were performed on a Perkin-Elmer Lambda 35 UV-visible spectrometer and Perkin-Elmer LS 55 Luminescent spectrometer, respectively. Photoelectron spectroscopy (PES) was performed by using synchrotron radiation from the vacuum-ultraviolet beamline at Elettra, Trieste, Italy, as well as with a Al KR laboratory photon source. Experimental spectrum at a given photon energy was decomposed to S 2p and Se 3p contributions by using a least-squared-error fitting procedure.27–29 Fundamental (1064 nm) and second-harmonic (532 nm) radiations from a hybrid mode-locked Nd:YAG laser, giving 35 ps pulses with a 10 Hz repetition rate, were used as the excitation sources for open aperture z-scan studies to investigate the nonlinear absorption (NLA) properties. Synthesis and theoretical fitting
10.1021/jp800063f CCC: $40.75 2008 American Chemical Society Published on Web 05/10/2008
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Nag et al.
Figure 1. (a) TEM micrograph for sample I with corresponding SAED pattern and HRTEM image in the insets; (b) size distribution histogram.
procedures have been described in detail in the Supporting Information (SI). Results and Discussion Figure 1a shows a TEM image for sample I along with the corresponding selected area electron diffraction (SAED) pattern and a high-resolution TEM (HRTEM) image. TEM clearly establishes an abundance of spherical, highly crystalline particles with a narrow size distribution. The corresponding histogram for the size distribution, shown in Figure 1b, shows that these nanocrystals have an average size of 6.7 nm with a size distribution of 9%. Similar results were also obtained for the other two samples. We find the average size of all three samples to be 6.5 ( 0.3 nm. Given a size distribution of about 8-9% for each of the three samples, we conclude that all three samples have essentially the same size. The SAED pattern, shown in the inset of Figure 1a, exhibits a three ring characteristic of a zinc-blende (ZB) crystal structure.30 Interplanar distances obtained from SAED patterns for sample I are 3.44, 2.10, and 1.80 Å; HRTEM in the inset of Figure 1a shows an interplanar separation of 3.45 Å, which is in good agreement. These three interplanar distances can be assigned to (111), (220), and (311) diffractions of the ZB phase. Interestingly, these numbers are in between the standard (ICSD) interplanar distances for the ZB phase of CdSe (3.50, 2.14, and 1.83 Å) and CdS (3.36, 2.06, and 1.76 Å); however, the measured interplanar distances for sample I are much larger than those of ZnSe (3.26, 2.00, and 1.70 Å) or ZnS (3.13, 1.92, and 1.64 Å), suggesting a primary composition close to CdSe/S. The interplanar distances exhibit a systematic dependence on the Se-precursor amount in the synthesis of the sample. For example, sample III, with the smallest Se/S ratio in the precursor, shows corresponding interplanar distances of 3.34, 2.03, and 1.73 Å. This systematic decrease in interplanar distances with decreasing Se is easy to understand in view of the larger size of Se2- compared to that of S2- ions. Powder X-ray diffraction (XRD) patterns, shown in Figure S1 of the SI, agree with the conclusions made from SAED patterns, with the diffraction peaks shifting systematically to higher angles with decreasing Se content. The diffraction patterns also confirm the formation of the cubic ZB structure in each case with lattice parameters between those of CdS and CdSe. Figure 2 shows the absorption and PL spectra of samples I, II, and III dispersed in heptane. Band-edge excitonic energies were determined from the point of inflection of each of the absorption spectra as 2.02 eV (614 nm), 2.07 eV (598 nm),
Figure 2. UV-visible absorption and PL spectra with λex ) 2.75 eV for samples I, II, and III.
and 2.18 eV (570 nm), for sample I, II, and III, respectively, thereby exhibiting a distinct and systematic blue-shift in the band-edge excitonic energy with a decrease of the Se concentration. Because TEM results do not show any significant change in the nanocrystal size with composition, the observed change in the excitonic energies is related to the change in the composition. This point is discussed further later in the text. All three samples, I, II, and III, exhibit band-edge PL emission (Figure 2) with peaks at 2.01, 2.07, and 2.17 eV, respectively, with narrow (∼0.1 eV) full width at half-maximum, indicating well passivated nanocrystal samples with a small spread in either size or composition within a given sample. QE of these NCs dispersed in heptane was measured to be ∼65% by using rhodamine 6G as a reference dye. The observed high QE is comparable to the best reported12,22 QEs exhibited by any nanocrystalline system. We have also collected PL spectra with a wide wavelength scan, 350-800 nm, after excitation at 330 nm (not shown here); in all such cases, only the band-edge emission is observed, confirming the formation of only one kind of emitting species rather than a mixture, suggesting a common composition for all the NCs in a given sample. Band gaps31 of bulk ZnS, ZnSe, CdS, and CdSe are 3.60, 2.67, 2.42, and 1.74 eV, respectively. Clearly, the band gaps of bulk ZnS, ZnSe, and CdS are larger than those observed for any of the three nanocrystalline samples. Because quantum confinement in a nanocrystal increases the band gap as well as the excitonic binding energy compared to the bulk value, it is clear that the band gap, and consequently the composition, of these nanocrystals must be related primarily to CdSe, the only compound among the possible four, that
Optically Bifunctional Heterostructured Nanocrystals has a bulk band gap smaller than those found for the nanocrystals. However, by using established2 size correlation, the observed band-edge excitonic energy of 2.02, 2.07 and 2.18 eV for sample-I, II, and III can be attributed to CdSe NC with a size of 5.5, 5.2, and 4.5 nm, respectively, these size estimates being significantly smaller than the average diameter (6.5 ( 0.3 nm) of any of these samples. We also used a different32 size-correlation curve, which gives the largest possible size of CdSe NCs as 4.8 nm corresponding to the smallest band-edge excitonic energy of 2.02 eV among those of the three samples, thus reconfirming that the observed band-edge excitonic energy cannot arise from ∼6.5 nm CdSe NCs. Therefore, the observed excitonic energies of these samples are surely not due to the absorption of any possible binary nanocrystals, but the excitons must be from CdSe or CdSeS blocks in a heterostructured NC. Considering the effect of solubility product (KSP),33 nucleation of CdSe core will be much faster compared to any other possible composition, because CdSe has ∼10-6 times smaller KSP value compared to that of CdS, which in turn, is nearly one and three orders of magnitude smaller than those of ZnSe and ZnS, respectively. Thus, it is reasonable to expect a heterostructured NC with CdSe core and CdS-ZnS hybrid shell, summing up to a total size of 6.7 nm for sample I, explaining the observed band-edge excitonic energy. However, a huge decrease in Se/S precursor ratio can add some amount of S in the CdSe core, thus giving a systematic blueshift in the band gap for samples II and III. ICP-AES data show that Zn:Cd:Se ratios in the final product NCs are 0.5:3.4:1.0, 1.5:8.4:1.0, and 3.3:15.4:1.0 for samples I, II, and III, compared to the Zn:Cd:S:Se precursor ratios of 2.7:1.7:8.3:1.0, 10.2:6.4:31.7:1.0, and 30.5:19.3:95.0:1.0, respectively. This shows that precursors for Cd and Se are more reactive compared to those of Zn and S, respectively, under the reaction conditions. These findings also indicate heterostructured NCs as suggested by the consideration of their band-edge excitonic energies. To establish this point on a firm footing, we have recorded photoemission core-level spectra of S 2p and Se 3p levels with different photon energies in order to delineate the relative S/Se composition at various depths from the sample surface.27,29,34 Typical spectra for sample I are shown in Figure 3. Experimental spectra, represented by filled circles, exhibit a strong overlap of S 2p and Se 3p core-level spectral features arising from similar binding energies of these two levels. Relative contributions of S 2p and Se 3p components to the overall spectrum at each photon energy were obtained by performing a least-squared error analysis, similar to the ones reported earlier.27–29 We found that the spectra in Figure 3 can be consistently described in terms of single components of S and Se, shown by broken blue and red lines, respectively. The resulting best fit to each experimental spectrum is represented by a black solid line overlapping the experimental data points, showing a good agreement. The relative contributions from S and Se signals at each photon energy is determined by obtaining the ratio of the integrated areas under the blue (S contribution) and red (Se contribution) broken lines in Figure 3a-d and by normalizing the ratio with the corresponding photoionization cross sections of S 2p and Se 3p for a given photon energy; this allows us to compare the S/Se contributions obtained at different photon energies independently of the photoionization cross-section variation. The resulting relative Se/S contribution is shown as a function of photon energy in the inset to Figure 3a. Clearly,
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Figure 3. Photoemission spectra of S 2p and Se 3p core levels (filled circle) falling in the same binding energy region for sample I. The blue and red dashed lines are the simulated spectra of S 2p and Se 3p, respectively, and the black solid line is the overall fit to the experimental spectrum for four different incident photon energies: (a) 365.8, (b) 601.1, (c) 731.2, and (d) 1486.6 eV. The inset to panel a shows the variation of Se/S ratio with photon energies, probed by PES.
the Se/S ratio increases rapidly with an increase in photon energy from 365.8 to 601.1 eV; further increase in photon energy still continues to increase the ratio but with a relatively slower rate. The implication of this interesting behavior can be qualitatively understood by considering the fact that the PES signal becomes exponentially bulk-sensitive with an increasing photon energy.27,29 Therefore, the observed increase in the Se/S ratio with an increase in the photon energy clearly shows that the core of the NC is richer with Se, but the number density of S increases as we move from the core to the surface region of the NC. These results establish the hetero structure of the NCs and rules out the possibility of a homogeneous alloy; in the latter case, Se/S ratio would have been independent of the photon energy. Open aperture z-scan data to probe NLA properties are shown for sample I in Figure 4a at different input intensities. The excitation energy, 2.33 eV (532 nm), for the main frames of Figure 4a, being greater than the band-edge excitonic energy, 2.02 eV, defines a resonant condition and generating an electron-hole pair. These excited electron and hole then decay nonradiatively to the bottom of the conduction band or the lowest unoccupied molecular orbital (LUMO) and to the top of the valence band or the highest occupied molecular orbital (HOMO), respectively, prior to recombining radiatively with a characteristic decay time. But a sufficiently high input intensity can lead to nonlinear optical properties, such as excited-state absorption (ESA), where an excited carrier reabsorbs one more photon before decaying to lower states or TPA, where carriers absorb two photons simultaneously instead of one. At a lower input irradiance, 5.3 GW cm-2, Figure 4a shows an increase in the normalized transmission as we approach z ) 0 because of the process known as saturable absorption. Here, the carriers absorb a photon and get trapped in the excited state, and with an increase in input intensity as we move toward z ) 0, the ground state gets depleted, resulting in fewer carriers available for the absorption of light, leading to an increase in the normalized transmittance. However, a sharp dip in the normalized transmittance, representing an increased absorption by the system, near z ) 0 is observed with further increase in input irradiance (12.1 GW cm-2); the magnitude of this dip increases with a further increase in the excitation intensity to 15.3 GW
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Nag et al. cross section increases with a decrease in the NC size, whereas in others,37 it decreases with a decrease in particle size. For both samples I and II, the contribution from ESA is observed to be negligible (10-23 cm2); this may be due to a considerable extent of faster depopulation of the excited states because of the picosecond-scale nonradiative phenomena arising from both the surface defects and the auger recombination. Also, the observed effective two-photon coefficient is an order of magnitude higher compared to that of CdS NCs of comparable sizes38,39 observed with picosecond laser pulses. Moreover, the onset of TPA appears at a lower input irradiance in the present case. There are a few reports for higher σ2, obtained from binary ZnS,17 CdTe,37 and CdS40 NCs, but these works do not report any significant PL efficiency. However, there is only one report by Larson et al.41 exhibiting both high TPA and high PL efficiency from a typical core-shell NC prepared according to a multistep reaction, in contrast to our single-step reaction scheme, preparing a new material with a compositional variation across the radius of the NC controlled by tuning the reactivity of different precursors. Further, experimental conditions to perform TPA in our samples are completely different from those in ref 41. Additionally, input intensity-dependent observation of peak and dip in normalized transmittance at z ) 0 is obtained because of the intensity-dependent TPA channel present in these samples, allowing one to choose the required input intensity depending on the application. Conclusions
Figure 4. Open aperture z-scan curves measured for (a) sample I and (b) sample II with different excitation intensities under resonant condition by using a 532 nm light source. Symbols of different colors are experimental data at different excitation intensities, as mentioned in the plots, and the solid lines of the corresponding color, overlapped with the symbols, are simulated data. The written input intensities are measured at z ) 0. For clear presentation, the red and black curves in both panels (a and b) are shifted vertically up by 0.06 and 0.09, respectively. Inset to panel a shows z-scan data for sample I in nonresonant excitation condition, with a 1064 nm light source.
cm-2. This indicates the presence of an intensity-dependent absorption channel in addition to the linear absorption. In the resonant condition, both ESA and TPA can take place from different allowed transitions of the NC system (Figure S2 in the SI). Measurements in the nonresonant condition with an excitation energy (1.65 eV, 1064 nm) much smaller than the HOMO-LUMO gap (2.02 eV) also exhibit a strong dip near z ) 0 as shown in the inset to Figure 4a. This observation clearly suggests TPA as the underlying mechanism contributing to the observed NLA in nonresonant condition, because ESA cannot take place with a photon energy lower than the HOMO-LUMO gap. Furthermore, experimental spectra both in the resonant and nonresonant conditions were simulated by using a spectroscopic model as described in the SI by carrying out a least-squared error analysis, taking into account contributions from both ESA and TPA. These theoretical fits show that NLA is due to TPA with a cross section35 (σ2) of 1.923 × 10-47 cm4 · s/photon (or 1923 GM). Sample II also shows a similar NLA behavior, as shown in Figure 4b, but with a slightly smaller TPA cross section, 1.39 × 10-47 cm4 · s/photon, compared to that of sample I, although the difference between TPA cross sections for the two samples is not very significant. Because TPA varies with the size, composition, and extent of surface passivation, it is difficult to ascribe any specific origin to or to determine the exact nature of such a variation at this stage. In fact, there are contradicting trends in the literature; in some cases,17,36 the TPA
By utilizing vastly different reactivities between different ions, heterostructured NCs with CdSe-rich core and CdS-ZnS hybrid shell have been synthesized by using a simple single-step reaction. It is shown that the band-edge excitonic energy can be varied controllably by changing the composition without changing the size of the synthesized NCs; this allows us to tune the emitted light over a range of the visible spectrum via sharp, band-edge emissions, with a very high (∼65%) QE. Thus, these systems can be used for light-emitting devices and biological labeling. Independently, these samples also exhibit remarkable nonlinear optical properties with very high cross sections of TPA and can be used for optical switches and limiters in the picosecond regime. Acknowledgment. Authors acknowledge the Department of Science and Technology, Government of India, for funding the project. D.D.S. acknowledges the National J. C. Bose Fellowship. A.N. acknowledges CSIR, Government of India, for a fellowship. Supporting Information Available: Synthesis, fitting of photoelectron spectra, XRD patterns, and fitting of NLA data. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Sapra, S.; Sarma, D. D. Phys. ReV. B 2004, 69, 125304. (3) Viswanatha, R.; Sapra, S.; Saha-Dasgupta, T.; Sarma, D. D. Phys. ReV. B 2005, 72, 045333. (4) Viswanatha, R.; Amenitsch, H.; Sarma, D. D. J. Am. Chem. Soc. 2007, 129, 4470. (5) Robel, I.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2007, 129, 4136. (6) Viswanatha, R.; Santra, P. K.; Dasgupta, C.; Sarma, D. D. Phys. ReV. Lett. 2007, 98, 255501. (7) Yang, H.; Holloway, P. H. J. Phys. Chem. B. 2003, 107, 9705. (8) Ali, M.; Chattopadhyay, S.; Nag, A.; Kumar, A.; Sapra, S.; Chakraborty, S.; Sarma, D. D. Nanotechnology 2007, 18, 075401.
Optically Bifunctional Heterostructured Nanocrystals (9) Nag, A.; Sarma, D. D. J. Phys. Chem. C 2007, 111, 13641. (10) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (11) Nanda, J.; Sapra, S.; Sarma, D. D.; Chandrasekharan, N.; Hodes, G. Chem. Mater. 2000, 12, 1018. (12) Jang, E.; Jun, S.; Pu, L. Chem. Commun. 2003, 2964. (13) Jun, S.; Jang, E. Chem. Commun. 2005, 4616. (14) Wang, Y. S.; Thomas, P. J.; O’Brien, P. J. Phys. Chem. B 2006, 110, 21412. (15) Viswanatha, R.; Chakraborty, S.; Basu, S.; Sarma, D. D. J. Phys. Chem. B 2006, 110, 22310. (16) Nag, A.; Sapra, S.; Nagamani, C.; Sharma, A.; Pradhan, N.; Bhat, S. V.; Sarma, D. D. Chem. Mater. 2007, 19, 3252. (17) Nikesh, V. V.; Dharmadhikari, A.; Ono, H.; Nozaki, S.; Kumar, G. R.; Mahamuni, S. Appl. Phys. Lett. 2004, 84, 4602. (18) Jia, W.; Douglas, F. G.; Sun, W. Appl. Phys. Lett. 2004, 85, 6326. (19) Vasa, P.; Ayyub, P.; Singh, B. P. Appl. Phys. Lett. 2005, 87, 063104. (20) Savitski, V. G.; Malyarevich, A. M.; Demchuk, M. I.; Yumashev, K. V.; Raaben, H.; Zhilin, A. A. J. Opt. Soc. Am. B 2005, 22, 1660. (21) Wang, X.; Du, Y.; Ding, S.; Wang, Q.; Xiang, G.; Xie, M.; Chen, X.; Peng, D. J. Phys. Chem. B 2006, 110, 1566. (22) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (23) Dorfs, D.; Henschel, H.; Kolny, J.; Eychmueller, J. Phys. Chem. B 2004, 108, 1578. (24) Kim, S.; Fisher, B.; Eisler, H.-J.; Bawendi, M. J. Am. Chem. Soc. 2003, 125, 11466. (25) Balet, L. P.; Ivanov, S. A.; Piryatinski, A.; Achermann, M.; Klimov, V. I. Nano Lett. 2004, 4, 1485.
J. Phys. Chem. C, Vol. 112, No. 22, 2008 8233 (26) Dias, E. A.; Sewall, S. L.; Kambhampati, P. J. Phys. Chem. C 2007, 111, 708. (27) Nanda, J.; Kuruvilla, B. A.; Sarma, D. D. Phys. ReV. B 1999, 59, 7473. (28) Krishnakumar, S. R.; Sarma, D. D. Phys. ReV. B 2003, 68, 155110. (29) Sapra, S.; Nanda, J.; Pietryga, J. M.; Hollingsworth, J. A.; Sarma, D. D. J. Phys. Chem. B 2006, 110, 15244. (30) Deng, Z.; Cao, L.; Tang, F.; Zou, B. J. Phys. Chem. B 2005, 109, 16671. (31) Kittel, C. Introduction to Solid State Physics, 7th ed.; John Wiley & Sons: New York, 1995; Chapter 8. (32) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854. (33) Joshi, R. K. Appl. Phys. Lett. 2006, 84, 4602. (34) Borchert, H.; Talapin, D. V.; McGinley, C.; Adam, S.; Lobo, A.; de Castro, A. R. B.; Moller, T.; Weller, H. J. Chem. Phys. 2003, 119, 1800. (35) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Prasad, P. N. Appl. Phys. Lett. 1995, 67, 2433. (36) Lad, A. D.; Kiran, P. P.; More, D.; Kumar, G. R.; Mahamuni, S. Appl. Phys. Lett. 2008, 92, 43126. (37) Pan, L.; Tamaia, N.; Kamada, K.; Deki, S. Appl. Phys. Lett. 2007, 91, 051902. (38) Han, M. Y.; Huang, W.; Chew, C. H.; Gan, L. M.; Zhang, X. J.; Ji, W. J. Phys. Chem. B. 1998, 102, 1884. (39) Gong, H-M.; Wang, X-H.; Du, Y-M.; Wang, Q. Q. J. Chem. Phys. 2006, 125, 024707. (40) He, J.; Mi, J.; Li, H.; Ji, W. J. Phys. Chem. B 2005, 109, 19184. (41) 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.
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