Shell Nanocrystals via

Dec 21, 2010 - (a) Absorption and PL spectra; (b) relative kinetics of the CdS NCs .... Municipal Science and Technology Commission (0952 nm06200) and...
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Size Controlled Synthesis of Blue Emitting Core/Shell Nanocrystals via Microreaction Zhen Wan, Weiling Luan,* and Shan-tung Tu Key Laboratory of Pressure Systems and Safety (MOE), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, People's Republic of China ABSTRACT: Quantum dots emitted from 450 to 480 nm are considered as promising materials for the fabrication of blue and white light-emitting diodes, where core/shell structured CdS/ ZnS nanocrystals (NCs) are among the ideal candidates. In this work, highly luminescent blue emission CdS/ZnS NCs were first synthesized in an accelerated fashion via capillary microreactor. The addition of oleylamine in the source solution was demonstrated as an efficient way to realize the size controlled synthesis of bare CdS NCs, while the application of zinc diethyldithiocarbamate as a single molecular precursor of shell material displayed a facile method to overcoat the bare CdS NCs and enhance the quantum efficiency. With the control of grain size and the optimization of capping reaction parameters, CdS/ZnS NCs emitted from 424 to 483 nm with a quantum yield as high as 80% were obtained, while maintaining narrow full width at half-maximum of photoluminescence from 21 to 24 nm. Furthermore, the continuous ultraviolet irradiation observation proved the excellent photochemical stability of CdS/ZnS NCs.

’ INTRODUCTION Due to unique size dependent optical properties, colloidal semiconductor nanocrystals (NCs), commonly known as quantum dots (QDs), are of great interest for both fundamental research1-3 and technical applications. Their excellent photoluminescence (PL) and electroluminescence (EL) properties make QDs an attractive alternative to organic molecules in applications such as lightemitting diodes (LEDs)4-8 solar cells,9-13 biological fluorescence labels,14-16 lasers,17 and so on. Due to the improvements of synthetic processes18,19 and surface modifications20,21 of NCs with organic or inorganic overlayers, QD-based LEDs, lasers, and photovoltaic devices are becoming possible for commercialization. To date, highly efficient green, red, and near-infrared emitting QDs have been facilely realized with traditional CdSe-based core/shell NCs22-24 or environmental-benign InP/ZnS25,26 and InAs/ZnSe27 core/shell NCs. However, pure blue luminescent NCs (PL from 450 to 480 nm) were rarely reported, which restricted the fabrication of NC-based blue LEDs and white light generation. Recently, several methods for the synthesis of blue emitting NCs were demonstrated with small-sized CdSe28 and CdZnS29 NCs, but still showed poor size distribution and quantum efficiency. Theoretically, blue luminescence with narrow emission bandwidths can be obtained from large-sized CdS30-34 or ZnTe NCs. However, it is difficult to prepare large-size (>4.5 nm diameter) ZnTe NCs with narrow size distribution. CdS is considered as an ideal candidate to achieve blue emission. With a band gap of 2.42 eV, varying the size of CdS NCs could result in PL covering the most UV to blue spectral region from 360 to 470 nm, covering the pure blue region. Moreover, the high defect concentrations on the surface of large CdS NCs results in a low r 2010 American Chemical Society

quantum yield (QY). In order to resolve this issue, the capping of CdS NCs with ZnS (Type I) will confine the electron and hole in the core region, which allows the significant improvement of emission efficiency. The common synthetic method for CdS/ZnS NCs relies on the injection process in a batch reactor under inert atmosphere. Until now, the CdS/ZnS NCs with optimal quality were synthesized by Peng's group.34 A “thermal-cycling coupled single precursor” method was developed, while high efficiency with PL QY up to 50%, good color purity with full width at half-maximum (fwhm) as narrow as 18 nm, near mondisperse, and stable CdS/ZnS NCs with their emission peak tunable between 375 to 475 nm were synthesized. However, the limited heat and mass transfer as well as discrete operations intrinsic in batch reactions usually lead to long reaction time and poor reaction yield. In a typical batch reaction, about 100 mg of purified NCs could be obtained in one run, which restricted the large-scale production of the NCs. Furthermore, tedious purifications for CdS NCs are required to eliminate the regrowth of CdS QDs during the overcoating of ZnS. Thus, synthetic procedures of multiple steps would cause the waste of raw materials and poor reproducibility. Microreaction provided a facile way to synthesize core/shell structure NCs in a continuous steady fashion under open air.35,36 The enhanced heat and mass transfer existed in microchannel and shortened the nucleation and growth time of NCs to several seconds, while the precise control of grain growth resulted in narrow size distribution and high stability of the as-prepared NCs. Received: September 17, 2010 Revised: December 5, 2010 Published: December 21, 2010 1569

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Figure 1. Schematic graph for the setup of capillary microreactor.

In this work, a capillary microreactor was developed to realize the preparation of highly luminescent blue emitting CdS/ZnS core/shell structure NCs in open air. The emission wavelength, corresponding to the size of CdS NCs, can be facilely tuned by the addition of oleylamine (OLA), and qualitative investigation of the influence of OLA on the PL property as well as size distribution was conducted to optimize the synthesis. Furthermore, CdS NCs were directly used without any purification steps in the overcoating process, while zinc diethyldithiocarbamate (ZDC) was utilized as an eviromentally-benign ZnS source. Finally, the PL stability of the as-prepared CdS/ZnS NCs was also carried out through continuous irradiation under 365 nm UV light. The formation of the core/shell structure and high quality of CdS/ ZnS NCs has been confirmed by PL spectroscopy, powder X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX), and transmission electron microscopy (TEM).

’ EXPERIMENTAL SECTION Chemicals. Cadmium oxide (CdO, 99.9%) and sulfur powder (S, 99.5%) were purchased from Shanghai Chemical Reagent (SCR). Oleic acid (OA, 90%, SCR), OLA (70%, Fluka), trioctylphosphine (TOP, 90%, Fluka), 1-octadecene (ODE, 90%, Fisher), ZDC (99%, Shanghai Dunhuang Chemical Plant), chloroform (analytical reagent, SCR), methanol (analytical reagent) and acetone (analytical, SCR) were used directly without any further purification and the whole operations were carried out in the open-air. Set-Up of the Capillary Microreactor. The microreactor consists of a precursor delivery system and a convective micromixer as well as a 70 cm length of polytetrafluoroethylene (PTFE) capillary (462 μm I.D.), as shown in Figure 1. The convective mixer comprising a magnetic bar and a miniature PTFE chamber (36 μL) was utilized as the efficient mixing for the anion and cation precursors under low flow rate. The nucleation and growth of NCs were formed in the PTFE capillary channel, while the stable heating source was provided by a thermal-stable oil bath. Synthesis of CdS Core QDs. A suspension of 1.2 mmol CdO, 2.4 mL OLA, 1.92 mL OA, and 3.68 mL ODE were heated together at 150 °C for 1 h with vigorous stirring to prepare a clear yellow cadmium precursor solution. Meanwhile, an S stock solution was prepared by dissolving 0.4 mmol S powder in 2.4 mL OLA, and then diluted with 5.6 mL ODE at 150 °C under magnetic stirring for 1 h. Equal-volume solutions of Cd and S precursors were delivered by a syringe pump (Harvard 22, U.S.A.) under the same flow rate, then combined and mixed in a convective micromixer. After that, the mixed flow entered into a certain length of heated capillary for the nucleation and growth of NCs. The whole process was performed in an open air. Growth of ZnS Shell on CdS Core. The as-formed CdS QDs was utilized without any further treatment. Single-molecular

Figure 2. (a) Absorption and PL spectra; (b) relative kinetics of the CdS NCs prepared under various residence time (T = 280 °C).

ZDC (2 mmol) dissolved in TOP (2 mL) and OLA (2 mL) was chosen as Zn and S source to form ZnS shell. The setup exhibited in Figure 1 was applied in this process with a variation of reaction temperature and residence time. The obtained CdS/ ZnS core/shell QDs from the outlet was purified by centrifugation and decantation with acetone. The precipitation was then dissolved in chloroform and extracted with an equal volume of methanol. Finally, the precipitation was redispersed in chloroform for the further characterization. Characterization. UV-vis and PL spectra were recorded on a Cary 100 UV-vis spectrometer (Varian) and Cary Eclipse fluorescence spectrophotometer (Varian), respectively. QY of PL was determined by comparing the integrated emission of the QDs samples in chloroform with that of a fluorescent dye Rhodamine 6G with identical optical density in the excitation wavelength. High resolution transmission electron microscope (HRTEM) images and EDX were acquired using a JEOL JEM2100F operated at an acceleration voltage of 200 kV, and the sample was prepared by dipping an amorphous carbon-copper grid in a dilute solution of QDs dispersed in chloroform. XRD pattern was taken by a D/max2550 V diffractometer (Rigaku) operating with Cu KR radiation (λ = 0.154056 nm), with samples deposited as a thin layer on an Si (100) wafer.

’ RESULTS AND DISCUSSIONS In batch reactions, the long response time for temperature stabilization as well as the vibration in reaction conditions makes it difficult to achieve NCs with precise size control. However, microreaction provides a reproducible tool to synthesize NCs in an improved controllable manner. Furthermore, the increased reactivity of the precursors resulting from the enhanced heat and mass transfer in a microreactor would greatly improve the reaction 1570

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Figure 3. Temporal evolution of the absorption spectrum of CdS NCs growth at various OLA content: (a) 0.0 vol. %; (b) 15.0 vol. %; (c) 22.5 vol. % ; (d) 30.0 vol. %; (e) 37.5 vol. %; and (f) 45.0 vol. % (OLA in both side, T = 280 °C).

efficiency. Here, a capillary microreactor was applied for the continuous synthesis of CdS NCs. High reaction temperature is needed in the preparation of CdS NCs based on an organometallic approach to fulfill the requirement of dynamic bonding (“on” and “off” the surface periodically) on the surface ligands of NCs. Furthermore, the annealing of NCs at a high temperature is necessary for the crystallization of nanoparticles with desired surface structures. However, gas will be generated from the solvent when the reaction temperature is over its boiling point, hence lowered the reproducibility. In this work, a reaction temperature of 280 °C was adopted to achieve the balance between the process stability and the reactivity of precursors. Figure 2 compared the absorption and PL spectra as well as the relative kinetics data of the samples prepared under various residence times from 10 to 90 s. Therefore, the evaluation of residence time was achieved by changing the flow rate from 21.12 to 2.35 mL h-1. The sharp band-edge absorbance and several higher transitions were clearly observed from the absorption spectra. The increased residence time from 10 to 90 s induced a 30 nm red-shift of absorption peak from 411 to 441 nm as shown in Figure 2b. In the initial 30 s, the high concentration of precursors caused the fast growth rate inducing the rapid red-shift from 411 to 439 nm. Under the prolonged residence time from 30 to 90 s, the depletion of precursors resulted in the decreased growth rate, leading to a slight red-shift. As the half width at half-maximum (hwhm) of the first absorption peak serves as an index of size

distribution of the as-formed NCs, a residence time of 50 s was observed as the optimal parameter. Furthermore, during the first 50 s, the hwhm of the absorption peak shifted from 26.2 to 11.4 nm, and then broadened to 13.4 nm with the prolonged residence time. The “focusing of size distribution” model raised by Peng et al.37 well explained these phenomena. For the diffusion-controlled growth of a spherical NC, a definition point of critical size (CS) was formed. NCs that were smaller than the CS show negative growth rates, while larger ones grow at rates depending strongly on their particle sizes. Furthermore, the CS is not a constant value which is dependent on the monomer concentration. In the early reaction stage from 10 to 50 s, the high monomer concentration led to a small CS, which is smaller than the normal size of the NCs presented in the solution. In this period, small NCs exhibited a rapid growth rate than larger ones, thus the size distribution of the NCs gradually focused. The prolonged growth time to 90 s resulted in low monomer concentration, which led to a large CS. As a result, large CdS kept growing based on the sacrifice of dissolved small NCs (also called Ostwald ripening), caused boarder size distributions. The active surface ligand was found to play a critical role for determining the size, size distribution, growth rate, and even Ostwald ripening in the synthesis system of CdS NCs. In order to realize the preparation of CdS NCs emitted from purple to blue luminescence, the size controlled synthesis of monodisperse CdS is a prerequisite. On the basis of the conventional batch reactions, 1571

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Figure 4. (a) Mean diameter and (b) particle concentration of the CdS samples prepared during the evolution of residence time and volume percentage of OLA (OLA in both side, T = 280 °C).

Figure 5. (a) Absorption spectra; (b) PL spectra; and (c) PL peak location and fwhm of PL for the CdS/ZnS NCs obtained under various temperatures with the same residence time as 170 s.

the size of CdS NCs is usually achieved via the variation of reaction time and concentration of OA. However, the promoted Ostwald ripening under a high OA concentration and long reaction time led to large CdS NCs with a broad size distribution. Here we found that the utilization of OLA as a coligand could also lead to monodisperse CdS NCs. More importantly, the variation of OLA exhibited as a facile method to tune the size of NCs, without the sacrifice of size distribution. The UV-vis spectra at various residence times were recorded for CdS NCs synthesized under different volume percentages of OLA (the whole OLA volume percentage in both Cd and S side), as shown in Figure 3. With the increase of residence time from 10 to 90 s and the variation of OLA volume concentrations from 0 to 45 vol. %, a red shift of UV wavelength from 371 to 449 nm was observed, coupling with sharp absorption peak and several excitionic transition features. The kinetic data with regard to CdS molar concentration and mean particle diameter were obtained by the methods reported by Yu.38 The data shown in Figure 4 supplies the further evidence for OLA-induced kinetic control. The increased volume percentage of OLA in the source solution resulted in large particle size and low particle concentration. Take the residence time of 50 s for an example, the addition of 45 vol. % OLA in the source solution resulted in a 1.4 nm increase of mean diameter compared with the samples prepared without OLA (from 3.7 to 5.1 nm), coupling with the dramatic decrease of particle concentration

from 8.1  1019/L to 0.49  1019/L. The addition of OLA will induce two competitive mechanisms, decelerating the growth by providing increased ligand coverage for the surface of NCs and accelerating the growth by activating the precursor. According to the report on CdSe NCs synthesis, the addition of OLA enhanced the activity of Cd precursor, leading to the increased particle concentration and reduced size of CdSe NCs.39 Here, an opposite trend was observed in our experiment. Under the reaction temperature of 280 °C, the ligands apart from the surface of NCs were dominant in the quasi-gas state. Thus, a high reaction temperature would promote the enhancement of the activation capability of the OLA toward the reactivity of the precursors. The resulting fast growth rate led to small particle concentration and large CdS NCs.40 On the basis of the above-mentioned recipe, the organic ligands were not sufficient enough to passivate the surface defects in CdS NCs. As a result, bare CdS cores tend to emit deep-trap white luminescence that overwhelm the whole blue spectrum. Here type I core-shell structured NCs were applied for the effective passivation of carriers in the cores, which would result in enhanced probability of the PL efficiency and photochemical stability. In addition, compared with the bare NCs, the capping with shell also reduced the cytotoxicity of NCs. ZnS was chosen as the shell material because of its large band gap, and potential for forming type I configurations, which would confine the exciton on the CdS cores. Furthermore, the relatively 1572

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Figure 6. (a) Absorption spectra; (b) PL spectra; (c) PL peak location and fwhm of PL for the CdS/ZnS NCs synthesized under various residence times at the same temperature as 140 °C.

Figure 7. (a) PL spectra; (b) PL peak location and fwhm of PL; and (c) QY for the CdS/ZnS NCs prepared under various OLA content (T = 140 °C, t = 170 s).

Figure 9. TEM image of (a) CdS cores with a size of 4.3 ( 0.2 nm and (b) CdS/ZnS core-shell NCs with a size of 5.2 ( 0.3 nm.

Figure 8. (a) UV-vis absorption and PL spectra of the as-prepared CdS/ZnS NCs with various mean sizes (b) Time evolution of the normalized PL intensity of CdS/ZnS core-shell samples (PL peak location: 451 nm).

small lattice mismatch between CdS and ZnS, which is about 8%, facilitates epitaxial growth of ZnS on CdS NCs. Here, ZDC was utilized as the zinc sulfide single precursor in the preparation of

CdS/ZnS core/shell structure NCs, while the as-formed CdS NCs were directly applied as core materials without purification. After the capping process, the deep-trap white luminescence changes to a bright blue emission, as shown in Figures 5 and 6. In this case, the temperature and residence time shows significant influence on the capping process, especially in the control of shell thickness. A high temperature will induce the formation of a CdZnS alloy complete with a separate nucleation of ZnS, which results in poor size distribution and low luminescence efficiency. Otherwise, a low temperature cannot overcome the energy 1573

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Figure 10. (a)XRD patterns and (b) EDX spectra of CdS (bottom) and CdS/ZnS NCs (top).

barrier to form a reactive monomer for the growth of the shell. Furthermore, the thickness of the ZnS shell was found to largely depend on the residence time. A thick shell formed under a long reaction time would result in the loss of carriers to the shell, leading to the reduced QY of core/shell NCs. Hence, the capping temperature and residence time were systematically investigated. Figure 5 shows the absorption and PL spectra as well as relative kinetics data of CdS/ZnS NCs synthesized under various temperatures at the same residence time as 170 s. With the increase of capping temperature from 100 to 140 °C, the PL peak slightly red-shifted from 434 to 439 nm, indicating the improved growth rate under high temperature. The formation of a complete ZnS shell led to the successful confinement of the exciton in the CdS cores, as indicated by the improved PL intensity. However, the evolution of gas which was formed by the rapid decomposition of ZDC under further increased reaction temperature to 180 °C led to the shortened residence time. Furthermore, a high reaction temperature induced the alloying of CdS with Zn. All these justified the tiny blue-shift of PL peak (from 439 to 437 nm). Furthermore, when the thickness of ZnS shell exceeds a threshold, the mismatch of the lattice parameters between CdS and ZnS can induce strain at the interface between the core and shell, and the resulting defects in the ZnS shell throws a negative effect on the PL efficiency. As the fwhm of PL under various temperatures was basically the same, the PL intensity was taken as a parameter for the optimization of process. The “crossover” temperature for PL intensity was observed as 140 °C, which indicated the optimal thickness under this temperature. Under the optimal temperature as 140 °C, the residence time was investigated by varying the flow rate of precursors as shown in Figure 6. The increased residence time has a weak influence on the PL peak location and fwhm. However, the PL intensity was obviously enhanced, indicating the successful formation of ZnS shell. The residence time below 170 s would result in thin shell that was not enough to passivate large CdS cores, while the slowly increased rate of PL intensity existed under the residence time over 170 s. In consideration of the reaction yield and PL efficiency, 170 s was applied for the continuous synthesis of CdS/ZnS NCs. As a result, with the optimal temperature as 140 °C and residence time as 170 s, efficient blue CdS/ZnS NCs were obtained using CdS NCs prepared under various OLA content, as shown in Figure 7. The highest QY of PL as 76.1% was observed using CdS NCs prepared with the OLA percentage as 22.5 vol. %. In order to prepare pure blue luminescent NCs with an emission from 450 to 480 nm, large-size CdS/ZnS NCs were under investigation. CdS NCs with large-size can be obtained by increasing reaction temperature, or improving the concentration of precursors.

However, the Ostwald ripening could be promoted under a high temperature and a long residence time, led to the defocusing of size distribution. Thus, increasing the concentration of precursors under the optimal OLA volume percentage as 22.5 vol. % is effective. Finally, as shown in Figure 8a, just by varying of residence time and OLA content as well as the concentration of precursors, a size series CdS/ZnS NCs with good color tenability in the optical window from 424 to 483 nm were obtained. As-prepared samples demonstrated high color purity (fwhm is 21-24 nm) and high PL QY up to 80%. Furthermore, a UV irradiation experiment was carried to investigate the photochemical stability of the as-formed CdS/ ZnS NCs. The CdS/ZnS sample with identical optical density (∼0.1) was loaded in a quartz cuvette and was irradiated by a commercial UV lamp (6 W, 365 nm) under atmospheric conditions at room temperature for a certain period of time. The PL spectra were recorded after different intervals of irradiation time. The time evolved PL intensities of the CdS/ZnS NCs was shown in Figure 8b. The sample shows well photochemical stability. After the consecutive illumination for 10 h, the PL intensity of the CdS/ZnS NCs kept a value of 30%. With the prolonged illumination period, the fluorescent quenching rate became slowly. After the illumination for 15 h, the PL intensity can still remain as 25%. Furthermore, the original CdS/ZnS NCs without any dilution stored in atmosphere can keep the performance for several months. On the basis of the capillary microreactor and reaction recipe used here, the production yield could reach about 200 mg/h. For large-scale production, the enhancement of the production yield could be facilely realized via the integration of multi reaction channel or the increment of the reaction channel length. Figure 9 shows HRTEM images of the as-prepared CdS and CdS/ZnS NCs under the above-mentioned experimental conditions. Near monodisperse particles were obtained. Therefore, the average diameter of core and core-shell NCs were 4.3 and 5.2 nm, respectively, and the distinguishable lattice plane reveals the high crystallinity of the dots. The monolayer of ZnS shell is illustrated as 0.3 nm.22 Therefore, in this research, the optimal QY of PL was achieved for shell thickness as 1.5 monolayers. Furthermore, the successful growth of ZnS shell around the CdS cores was demonstrated by the XRD and EDX measurement, as shown in Figure 10. Figure 10a shows the corresponding XRD spectra of the CdS NCs and CdS/ZnS NCs. Compared with the bare CdS NCs, the diffraction peaks of the overcoated sample shifted to high angle, which is close to the pattern of Zinc Blend phase. This phenomenon confirms the successful formation of ZnS shell around the surface of CdS NCs. Moreover, the growth of ZnS shell around the CdS cores was clearly demonstrated by the EDX 1574

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’ CONCLUSIONS In summary, a capillary microreactor was developed to synthesize highly luminescent blue emitting CdS/ZnS core/shell structure NCs in a controllable manner under atmospheric conditions. Asprepared NCs exhibit strong PL QY (up to 80%) with narrow spectral bandwidth (fwhm < 24 nm) at room temperature. The OLA showed great influence on the as-formed CdS NCs; the size of bare CdS NCs can be precisely tuned from 3.7 to 5.1 nm by varying the OLA concentration under the reaction temperature as 280 °C. Furthermore, a single molecular ZDC precursor was utilized as the shell material to improve PL intensity and reduce the cytotoxicity, while bare CdS NCs was directly applied for the overcoating process without any purification process, which offered the feasibility to synthesize CdS/ZnS NCs in a large-scale. Finally, under an overcoating temperature like 140 °C, excellent optical performance of CdS/ZnS NCs with high photochemical stability over the range from purple to pure blue (424 nm-483 nm) was obtained.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ86 21 6425 3513; E-mail: [email protected].

’ ACKNOWLEDGMENT The authors appreciated the financial support of the National Nature Science Foundation of China (No. 50772036), the Focus of Scientific and Technological Research Projects (109063), the Nano-Project of Shanghai Municipal Science and Technology Commission (0952 nm06200) and the Fundamental Research Funds for the Central Universities (WJ0913001). ’ REFERENCES (1) Brus, L. J. Phys. Chem. 1986, 90, 2555–2560. (2) Alivisatos, A. P. Science 1996, 271, 933–937. (3) Peng, X. G. Nano Res. 2009, 2, 425–447. (4) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354–357. (5) Schlamp, M. C.; Peng, X. G.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837–5842. (6) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506–1508. (7) Coe, S.; Woo, W.-K.; Bawendi, M. G.; Bulovic, V. Nature 2002, 420, 800–803. (8) Anikeeva, P. O.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V. Nano Lett. 2009, 9, 2532–2536. (9) Plass, R.; Pelet, S.; Krueger, J.; Gr€atzel, M. J. Phys. Chem. B 2002, 106, 7578–7580. (10) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385–2393. (11) Lee, H. J.; Yum, J.-H.; Leventis, H. C.; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. I.; Gr€atzel, M.; Nazeeruddin, M. K. J. Phys. Chem. C 2008, 112, 11600–11608. (12) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737–18753. (13) Bang, J. H.; Kamat, P. V. ACS Nano 2009, 3, 1467–1476. (14) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013–2016.

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