J. Phys. Chem. C 2009, 113, 15037–15042
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Phase-Selective Synthesis of CuInS2 Nanocrystals Sudip K. Batabyal,† Lu Tian,† N. Venkatram,‡ Wei Ji,*,‡ and Jagadese J. Vittal*,† Department of Chemistry, National UniVersity of Singapore, Singapore 117543, and Department of Physics, National UniVersity of Singapore, Singapore 117542 ReceiVed: June 4, 2009; ReVised Manuscript ReceiVed: July 8, 2009
Single-source precursor, [(Ph3P)CuIn(SC{O}Ph)4] (1), and dual-source precursors, [Cu(SC{O}Ph)] (2) and [In(bipy)(SC{O}Ph)3] (3), have been used to obtain the monodispersed wurtzite (hexagonal) and zincblende (also called sphalerite, cubic) phases of copper indium sulfide nanocrystals (CIS NCs). The NCs have been characterized by X-ray powder diffraction patterns, transmission electron microscopy, selected area electron diffraction, and energy-dispersive X-ray analysis. It is shown that the relative ratios of surfactants have influence on the formation of the wurtzite or zincblende phase of CIS. Moreover, the reaction temperature plays a role in stabilizing the high-temperature metastable zincblende cubic phase at room temperature. In the presence of trioctylphosphine oxide (TOPO) and dodecanethiol (DT), 1 generates the wurtzite phase of CIS when the reaction temperature is below 275 °C, but above this temperature the obtained product belongs to zincblende (cubic). The morphology of the CIS also changes from nanoplates to nanoparticles when it undergoes phase transformation. In the wurtzite phase, monodispersed nanoplates are formed at 175 °C and nanorods (NRs) produced at 250 °C along with the plates. Wurtzite and zincblende CIS nanocrystals exhibit intense emission in the ultraviolet region and weak emission in the visible region. The nonlinear optical (NLO) properties of the CIS NCs have also been characterized with femtosecond laser pulses at a wavelength of 780 nm. Introduction Chalcopyrite-based solar cells have attracted immense interest in photovoltaic research as these materials have promising properties and provide maximum efficiency in solar energy conversion. The solar cell consisting of CuInS2 (CIS) thin film shows 12.5% efficiency.1 The chalcopyrite solar cell based on Cu-In-Ga Se exhibits the maximum performance with a efficiency of 18.8%.2 In general I-III-VI2 ternary semiconductor compounds are an important class of materials with wide applications in solar cells,3,4 light emitting devices,5,6 pigments, and phosphors.7 In particular, CIS, a semiconductor with a direct band gap of 1.5 eV, is a promising solar cell material.8 Because of its potential applications, there have been strong research interests in the fabrication of micro- and nanostructures of CIS. Qian’s group reported the preparation of CIS nanocrystals (NCs) via solvo/hydrothermal routes from dual multiple sources.9-12 Castro et al. synthesized ultrafine CIS NCs by thermal decomposition of the single-source precursor, [(PPh3)2CuIn(SEt)4].13,14 The micro-/submicrometer scale novel morphologies, such as flower-vase-like nanostructures,15 microspheres composed of nanoplates or nanoparticles (NPs),16,17 and nanowires18 were also achieved. Monodispersed CIS NCs have been reported only in the past two years.19-22 But controlled synthesis of nanomaterials with new crystal structures attracted attention due to the development of structure-dependent properties.23-26 Recently Pan et al. reported CIS NCs in two new phases, namely compositionally disordered sphalerite or zincblende (cubic) and wurtzite (hexagonal) for the first time.20 Since then fewer attempts have been made to stabilize this metastable state at room temperature. Qi et al. prepared the * To whom correspondence should be addressed. J.J.V.: E-mail
[email protected]; fax +65-6779-1691. W.J.: e-mail
[email protected]; fax +65-6777-6126. † Department of Chemistry, National University of Singapore. ‡ Department of Physics, National University of Singapore.
wurtzite CIS by solvothermal method with a mixture of morphologies from nanoplates to microspheres.27 Connor et al. reported the wurtzite CIS nanorods formation from Cu2S nanoplates.28 It has been observed that the wurtzite phase has been formed in the presence of dodecane thiol (DT) whereas the zincblende phase resulted in the presence of oleylamine (OL).20 Recently Xie et al. synthesized high-quality CuInS2 nanocrystals in the size range between about 2 and 20 nm by adjusting the relative reactivity of Cu precursor with indium precursor in a noncoordinating solvent.29 Nose et al. observed that the phase of the CIS product is determined by the complex ligand spacies.30 Here we report the influence of temperature on the phase formation, and describe the observation that the zincblende phase can also stabilized in the presence of DT at high temperature. Single-source and dual-source precursor methods are two common synthetic strategies for ternary chalcogenides. Choi et al. synthesized acorn-, bottle-, and larva-shaped Cu-In sulfide heterostructure nanocrystals from the thermal decomposition of a mixture of Cu-oleate and In-oleate complexes in dodecanethiol.31 However, there are many examples available for multiple reactants giving phase-pure CIS and CuInxGa(1-x)S2 (CIGS) compounds. For example, very recently Koo et al. have reported the synthesis of highly monodispersed wurtzite CIS nanodisks by heating metal chlorides and thiourea in OL.32 Herein single-molecular precursor [(Ph3P)CuIn(SC{O}Ph)4] (1) as well as dual sources [Cu(SC{O}Ph)] (2) and [In(bipy)(SC{O}Ph)3] (3) have been employed to make the wurtzite and zincblende phases of CIS. Nonlinear optical (NLO) materials have potential applications in multiphoton imaging, optical limiting, optical switching, etc. Semiconductor NCs have attracted much attention in the field of nonlinear optics due to their unique quantum confinement effects. The optical nonlinearities of these NCs and their potential applications are being extensively investigated.33-35 One of the current challenges is
10.1021/jp905234y CCC: $40.75 2009 American Chemical Society Published on Web 07/23/2009
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TABLE 1: Description of the Reaction Conditions, Phases, and Morphology of the Products ratio of sample code precursors:TOPO:DDT reaction temp, °C CIS1 CIS2 CIS3
1:50:50 1:50:50 1:50:50
150 175 200
CIS4
1:50:50
250
CIS5
1:50:50
275
CIS6
1:50:50
300
CIS7
1:50:50
350
CIS8 CIS9 CIS10 CIS11
1:75:25 1:75:25 1:75:25 1:75:25
150 200 300 350
crystallinephase
comments on size and shape
wurtzite wurtzite wurtzite
plates (diameter, 30-60 nm) plates (diameter, 6-20 nm) mixture of plates (major product) (30-60 nm) and nanorods (length and width ∼50 nm and ∼10 nm, respectively wurtzite mixture of nanorods (major product) (length and diameter, ∼50 nm and ∼10 nm) and plates (diameter 30-60 nm) zincblende (major) + mixture of very small spherical particles (5-10 nm) wurtzite and plates (30-60 nm) zincblende (major) + mixture of very small spherical particles (5-10 nm) wurtzite and plates (30-60 nm) zincblende very small spherical particles (5-10 nm) wurtzite plates (diameter 20-50 nm) wurtzite plates (diameter 20-60 nm) wurtzite plates (diameter 10-30 nm) wurtzite plates (diameter 10-30 nm)
the preparation of the NCs having high multiphoton absorption cross section. Since wurtzite and zincblende phases of CIS NCs have been obtained, the NLO properties have also been investigated. Experimental Procedures The precursors, 1-3, were synthesized according to known literature methods.36-38 The precursor [(Ph3P)CuIn(SC{O}Ph)4] (1, 50 mg, 0.05 mmol) was added to dodecanethiol (DT, C12H25SH, Aldrich, 0.60 mL, 2.5 mmol) and trioctylphosphine oxide (TOPO, C24H51PO, Aldrich, 0.97 g, 2.5 mmol) at room temperature (the molar ratio of precursor:DT:TOPO ) 1:50: 50) and the contents were heated at 175 °C for 15 h with gentle stirring under nitrogen atmosphere. The solution was cooled to ca. 70 °C and then an excess of ethanol was added to obtain a flocculent precipitate. The solid was separated by centrifugation, washed with ethanol, and dried. The product can be easily redispersed in toluene and hexane. In the dual-source method the precursors [Cu(SC{O}Ph)] (2, 14.7 mg, 0.073 mmol) and [In(bipy)(SC{O}Ph)3] (3, 50 mg, 0.073 mmol) were added to DT (0.88 mL, 3.65 mmol) and TOPO (1.41 g, 3.65 mmol) at room temperature (the molar ratio of precursor:DT:TOPO ) 1:50:50) and the contents was heated at 175 °C for 16 h with gentle stirring under nitrogen atmosphere. The workup procedure was similar to that described for the single-source precursor method. X-ray powder diffraction (XRPD) patterns were obtained by using a D5005 Bruker X-ray diffractometer equipped with Cu KR radiation. The accelerating voltage and current were 40 kV and 40 mA, respectively. Samples were prepared on glass slides. Concentrated toluene solutions were slowly evaporated at room temperature on a glass slide to obtain samples for analysis. Photophysical measurements were made by dispersing the CIS NPs in toluene. The UV-vis absorption spectra were obtained from Shimadzu UV-2401PC. Transmission Electron Microscopic (TEM) images were acquired with use of a JEOL JSM2010 microscope operating at 200 KV. High-resolution transmission electron microscopy (HRTEM) images and electron diffraction patterns were obtained from a JEOL JSM-3010 instrument. The samples for TEM were prepared by placing a drop of dilute solution of the sample in toluene onto a Cu grid (300 mesh) and then it was completely dried under vacuum. The NLO properties of the CIS NCs dispersed in toluene were investigated by femtosecond (fs) Z-scan at a wavelength of 780 nm, 300 fs laser pulses at 1 kHz repetition rate. The laser pulses
were generated by a mode-locked Ti:sapphire laser (Quantronix, IMRA), which seeded a Ti:sapphire regenerative amplifier (Quantronix, Titan). The laser pulses were focused onto a 1-mmthick quartz cuvette that contained the CIS solution with a minimum beam waist of 30 µm. The linear transmittance of all the solution was adjusted to 80% at 780 nm. The incident and transmitted laser powers were monitored as the cuvette was moved (or Z-scanned) along the propagation direction of the laser pulses. Results and Discussion The molecular precursor [(Ph3P)CuIn(SC{O}Ph)4] has been successfully employed to synthesize both wurtzite and zincblende phases of CIS, using DT and TOPO in the ratio of 50:50 (v/v). The zincblende phase was stabilized by a mixture of oleic acid and oleylamine (OL) while DT can stabilize the wurtzite phase.20 While this paper was being reviewed, Koo et al. also showed that oleylamine can stabilize wurtzite CIS nanocrystals.32 Here it is found that the phase formation of CIS in mixed surfactant can be controlled by the reaction temperature. The details of the CIS NCs synthesized at different temperatures with different surfactant ratios are given in Table 1. It is observed that the wurtzite phase has been stabilized below 250 °C while the zincblende phase started forming above 250 °C. XRPD patterns of as-synthesized CIS (with precursor:
Figure 1. XRPD patterns of the synthesized CIS at different temperatures for TOPO:DT ) 50:50 (The red and green lines at the bottom refer to the peak positions of wurtzite and zincblende phases, respectively, from ref 20).
Phase Selective Synthesis of Copper Indium Sulfide
Figure 2. XRPD patterns of synthesized CIS at different temperatures for TOPO to DT ratio 75:25 (w/w). The red and green lines at the bottom refer the peak positions of wurtzite and zincblende phases, respectively, from ref 20.
TOPO:DT ) 1:50:50) at different temperatures are shown in Figure 1. Patterns a to g in Figure 1 represent the XRPD patterns of the samples CIS1 to CIS7 referred to in Table 1. The diffraction patterns of samples CIS1, CIS2, CIS3, and CIS4 (Figure 1a-d) are identical with the wurtzite phase of the CIS.20 However, the XRPD patterns of sample CIS5 (Figure 1e) reveals the presence of both wurtzite and zincblende phases. Even at 300 °C sample CIS6 exhibits small diffraction peaks due to the wurtzite phase along with the predominant zincblende phase (Figure 1f). But the sample obtained at 350 °C CIS7 has a pure zincblende phase.20 Hence it may be concluded that the wurtzite phase is formed up to 250 °C and the pure zincblende phase is produced only at 350 °C. In other words, the formation of these two phases can be controlled by changing the temperature of the reaction while keeping the other experimental conditions the same. The ligand-dependent phase stabilization of nanocrystals is now well-known for a few binary semiconductors. Zincblende or wurtzite phase formation for ZnSe, ZnS, and CdTe nanocrystals has been observed due to the influence of surfactants as ligands.39 Pan et al. reported that pure DT stabilized the wurtzite phase;20 however, addition of TOPO to DT above 250 °C stabilized the zincblende phase. The bond strength between the metallic monomers and the surfactant ligand molecules, as well as the steric size of the ligand molecules influenced the growth rate. The kinetically stable wurtzite phase of CIS appearing from the OL or 1-hexadecylamine complex of the metallic monomer has been attributed to faster growth rate.30 In addition to the temperature, the surfactant also has a crucial role to play on the phase selectivity of CIS. There is no formation of pure wurtzite phase above 250 °C when the capping agents TOPO and DT were used in the ratio of 50:50 (w/w). However, if the amount of TOPO is increased the wurtzite phase is observed predominantly even at 350 °C. Figure 2 shows the
Figure 3. TEM images of the samples CIS2 (a and b) and CIS3 (c and d).
J. Phys. Chem. C, Vol. 113, No. 33, 2009 15039 XRPD patterns of product CIS obtained at different temperatures for the precursor:TOPO:DT ratio of 1:75:25 for 15 h reaction time. Figure 2a-d for samples CIS8 to CIS11 clearly shows that the crystalline phase of CIS obtained in the temperature range 150-350 °C is the wurtzite phase. Morphological Investigation. From the morphological studies it is observed that the optimum temperature for monodispersed CIS NPs formation is 175 °C. Figure 3 shows the TEM micrographs of the sample CIS2 referred to in Table 1. The diameters of these plate-like particles are in the range 8-20 nm with an average diameter of 10 ( 2.9 nm. The HRTEM analysis shows the growth of the plates is along the (100) planes (Figure S1, Supporting Information). The interplanar spacing of such nanoplates is 3.39 Å, which corresponds to the (100) plane of the wurtzite phase. The EDAX analysis provides the relative ratios of Cu:In:S to be 1:0.9:1.8 (Figure S2, Supporting Information). At relatively low temperature the reaction yielded wider size dispersity with the increase of average diameter. Further, at 125 °C only partial decomposition of the precursor occurs as confirmed by XRPD patterns, but the diameter of the CIS is in the range of 40-80 nm. The 30-60 nm range of particle size (Figure S3, Supporting Information) resulted at 150 °C. If the reaction temperature is increased further the NPs are transformed to nanorods. At 200 °C the product is a mixture of nanorods and nanoplates with fewer of the former (Figure S4, Supporting Information). At 250 °C the percentage of CIS nanorods increases as shown in Figure 3c. The lengths and widths of these nanorods are ∼50 nm and ∼10 nm, respectively. The HRTEM analysis of these rods shows that the lattice fringes along the length of the rods which correspond to the (101) plane of the wurtzite phase (Figure S5, Supporting Information). The temperature is a key factor to induce the morphology change of the nanomaterials.40,41 The elevated reaction temperature provides more energy for the preferential growth of specific crystal planes of CIS NPs.42 When the reaction temperature is increased to 250 °C, CIS nanorods instead of NPs are formed. The phase transformation is also accompanied by a change in morphology. At high temperatures (300 and 350 °C) very small particles of zincblende CIS were obtained. The diameter of these NPs is in the range of 4-10 nm with an average diameter of 5.1 ( 1.1 nm. Figure 4 shows the TEM images of zincblende CIS NPs. The selected area electron diffraction patterns (SAED) of the CIS2 and CIS7 are totally different (Figure S6, Supporting Information), which supports the different phase formation of the product. The spots could be indexed according to the reported interplanar distances (calculated from the XRPD patterns) before.20 As seen above from XRPD in Figure 2, the wurtzite crystal phase has been stabilized even at 350 °C by changing the TOPO: DT ratio from 50:50 to 75:25 (samples CIS8-CIS11 in Table
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Figure 4. TEM images of the CIS obtained at 350 °C in different magnifications.
Figure 5. TEM images of the CIS obtained at 175 °C in three different magnifications (a-c) and at 250 °C in two different magnifications (d, e).
of the zincblende phase as the major product. However, at 350 °C the sample CIS16 did not show a recognizable XRPD pattern due to poor crystallinity. The results for dual-source experiments are tabulated in Table S8 in the Supporting Information. The morphology of the obtained product is optimum at 175 °C with identical dimensionality of the obtained product for a singlesource experiment.
Figure 6. UV-vis absorption spectra of wurtzite (CIS2) and zincblende (CIS7) CIS nanostructures.
1). Accordingly, there is no drastic morphological change observed in this temperature range. The plate-like shape still persists for these NCs (Figure S7, Supporting Information), which is consistent with the XRPD results. In addition to the single-source precursor, the dual sources, namely 2 and 3, were also employed to synthesize CIS NCs. The crystal phase obtained below 250 °C for samples CIS13 and CIS14 is predominantly wurtzite. At 300 °C the NCs become less crystalline but XRPD patterns indicate the presence
The morphological changes accompanied by the change of reaction were also observed when dual sources were used. For the reaction temperature at 250 °C the dual sources also yielded nanorods mixed with some dispersed spherical particles (Figure 5). The aspect ratios of these rods are comparatively more than that of the rods obtained from the single-source experiment. The lengths are in the range 50-70 nm and the diameters are in the range 5-10 nm. Under the experimental conditions used the synthesis did not yield chalcopyrite (tetragonal) CIS and it appears that these single molecular precursors are suitable for making wurtzite and sphalerite phases of CIS. Although there are reports attempting to rationalize the stabilization of the high-temperature phase at room temperature43,44 due to its complexity we are unable to account for the stability of the wurtzite (hexagonal) phase at room temperature.
Phase Selective Synthesis of Copper Indium Sulfide
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Figure 7. (a) Open- and (b) closed-aperture Z-scans of CIS NCs (sample CIS7) performed at a wavelength of 780 nm. The solid lines are the best fits with the Z-scan theory.
Optical Properties. Optical properties of CIS NCs are of great importance as they provide information for the applications of these materials in optoelectronic devices and solar cells. Though there are few reports on the optical properties of CIS NCs, most of the studies have been carried out on the chalcopyrite phase.22,45-47 The optical properties of ternary nanocrystals depend on the size and the shape of the nanocrystals, surface states, and composition.22,45-47 The UV-vis absorption spectra of the as-prepared CIS nanostructures have been measured at room temperature (∼25 °C). Figure 6 displays typical spectra for samples CIS2 and CIS7. No sharp absorption peak was observed for both samples but the CIS NCs show a broad absorption band with an absorption tail up to ∼800 nm, signifying that they are suitable for solar cell applications. No pronounced excitonic peaks are observed in the wavelength range from 600 to 800 nm, though there is weak shoulder located at ∼760 nm. Due to the quantum confinement, the band gap energy of CIS nanostructures is expected to be blue-shifted, compared to the band gap energy of bulk CuInS2.48 Since the average diameter of CIS7 is 10 nm, a blue shift can be estimated by using an finite-depth-well effective mass approximation calculation.48 The calculated band gap for the 10 nm sized NCs is 1.67 eV (details of the calculations are given in S11 of the Supporting Information) by taking the band gap value for matrix TOPO as 5 eV.49 Nonlinear Optical Properties. The NLO properties of these nanostructures show interesting behaviors, depending on both size and structure. Here, we present the discussion on the NLO properties of sample CIS7 prepared at a temperature of 350 °C with 50 TOPO:50 DT. Figure 7 displays typical open- and closed-aperture Z-scans for the sample CIS7. Both absorptive and refractive nonlinearities show positive signs. We attribute the positive sign of the nonlinear absorption to two-photon absorption. The band gap energy for bulk CuInS2 is known to be 1.53 eV. However, the band gap energy for the CIS7 NCs is blue-shifted to 1.67 eV, as discussed above. This band gap energy is between the singlephoton energy (1.58 eV or 780 nm) and two-photon energy (3.16 eV). Therefore, the two-photon absorption should manifest itself in the observed nonlinear absorption. Associated with the twophoton absorption, there is an optical Kerr nonlinearity (n2) due to the bound electronic effect. Our Z-scans confirm that the measured nonlinear absorption and refraction should be independent of the light intensity, consistent with the characteristics of two-photon absorption and Kerr nonlinearity which are third-order nonlinear processes. Thus, the nonlinear absorption and refraction can be described by ∆R ) βI, and ∆n ) n2I, respectively, where β and n2 are the nonlinear absorption coefficient and nonlinear refractive
index, and I is the light intensity. Both β and n2 values can be extracted from the best fit between the Z-scan theory and the data.50 The βNC and n2NC values are calculated from the following: βNC ) β/(|f |4Vf) and n2NC ) [n2 - (1 - Vf)n2 sol]/ (|f|4Vf), where βNC is the nonlinear absorption coefficient of NCs, n2NC is the nonlinear refractive index of NCs, n2 sol is the nonlinear refractive index of toluene, f is the local field factor given by 3εsol/(εNC + 2εsol), where εsol is the dielectric function of toluene and εNC is the dielectric function of CuInS2, and Vf is the volume fraction of NCs relative to toluene. In our case, the calculated value for f is 0.585, where εsol ) 2.2351 and εNC ) 7,52 and Vf ) 0.001. With these values, we obtain βNC ) 279.2 cm GW-1 or two-photon absorption cross section σNC ) 7.11 × 10-44 cm4 s photon-1 particle-1. Similarly, the n2NC value is calculated to be 1.3 × 10-2 cm2 GW-1, with using the n2 value of toluene (n2 sol ) 1.1 × 10-6 cm2 GW-1). The NLO properties of semiconducting NPs have been studied extensively. The reported value35,53,54 for the two-photon absorption cross section of the most recent studies is ∼10-46 cm4 s photon-1 particle-1. Higher two-photon absorption cross sections of ∼10-44 cm4 s photon-1 particle-1of CIS NCs show the importance of these kinds of materials in NLO device applications. Higher cross sections should enhance the performance of optical limiting and multiphoton excited fluorescence, for example. Conclusions Wurtzite and zincblende phases of copper indium sulfide, which were reported recently, exhibiting interesting optical properties, have been synthesized. Surfactants and synthesis temperature have a strong influence on phase selectivity of the CIS NCs. With precursor:TOPO:DT ratio of 1:50:50, the wurtzite phase has been stabilized below 250 °C, the zincblende phase started forming above 250 °C, and the pure phase of zincblende can be obtained at 300 °C. However, we observed that the wurtzite phase has been stabilized even at 300 °C, if the amount of TOPO is increased. The morphology of the assynthesized NCs depends upon the phase and synthesis temperature. On changing the phase by varying the temperature, the morphology of the produced NCs has also changed from plate-like particles (wurtzite) to tiny circular nanodisks (zincblende). This appears to be the first report to demonstrate that the reaction temperature changes the phase of CIS nanocrystals from wurtzite to sphalerite in a systematic way. The high two-photon absorption cross section of these materials demands their application in ultrafast photoswitching, optical limiting, and other optical devices.
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Acknowledgment. The Ministry of Education, Singapore is gratefully acknowledged for financial support through NUS (Grant Nos. R-143-000-283-112 and R-143-000-371-112). Supporting Information Available: TEM, HRTEM pictures, SAED, and EDAX data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Klaer, J.; Bruns, J.; Henninger, R.; Top¨per, K.; Klenk, R.; Ellmer, K.; Bra¨ unig, D. A tolerant two step process for efficient CuInS2 solar cells. In Proceedings of the Second World conference on PhotoVoltaic Solar Energy ConVersion; Schmid, J., Ossenbrink, H. A., Helm, P., Ehmann, H., Dunlop, E. D., Eds.; EC Joint Research Center: Luxembourg, July 1998; pp 537-540. (2) Contreras, M. A.; Egaas, B.; Ramanathan, K.; Hiltner, J.; Swartzlander, A.; Hasoon, F.; Noufi, R. Prog. PhotoVolt. Res. Appl. 1999, 7, 311. (3) AbuShama, J.; Johnston, S.; Moriarty, T.; Teeter, G.; Ramanathan, K.; Noufi, R. Prog. PhotoVoltaics: Res. Appl. 2004, 12, 39. (4) Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Schmidtke, J. P.; Dunn, L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. J. Am. Chem. Soc. 2008, 130, 16770. (5) Aksenov, I.; Sato, K. Jpn. J. Appl. Phys. 1992, 31, 2352. (6) Shay, J. L.; Tell, B.; Kasper, H. M.; Schiavone, L. M. Phys. ReV. B 1972, 5, 5003. (7) Onose, C.; Onose, C. S.; Mihaly, M.; Badea, N.; Niciu, H.; Niciu, D.; Elisa, M. J. Optoelectron. AdV. Mater. 2007, 9, 3206. (8) Guille´na, C.; Herreroa, J.; Gutie´rreza, M. T.; Briones, F. Thin Solid Films 2005, 19, 480. (9) Lu, Q.; Hu, J.; Tang, K.; Qian, Y.; Zhou, G.; Liu, X. Inorg. Chem. 2000, 39, 1606. (10) Xiao, J.; Xie, Y.; Tang, R.; Qian, Y. J. Solid State Chem. 2001, 161, 179. (11) Jiang, Y.; Wu, Y.; Mo, X.; Yu, W.; Xie, Y.; Qian, Y. Inorg. Chem. 2000, 39, 2964. (12) Cui, Y.; Ren, J.; Chen, G.; Qian, Y. T.; Xie, Y. Chem. Lett. 2001, 236. (13) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Chem. Mater. 2003, 15, 3142. (14) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. J. Phys. Chem. B 2004, 108, 12429. (15) Das, K.; Datta, A.; Chaudhuri, S. Cryst. Growth Des. 2007, 7, 1547. (16) Peng, S.; Liang, J.; Zhang, L.; Shi, Y.; Chen, J. J. Cryst. Growth 2007, 305, 99. (17) Gou, X.; Cheng, F.; Shi, Y.; Zhang, L.; Peng, S.; Chen, J.; Shen, P. J. Am. Chem. Soc. 2006, 128, 7222. (18) Wakita, K.; Iwai, M.; Miyoshi, Y.; Fujibuchi, H.; Ashida, A. Compos. Sci. Technol. 2005, 65, 765. (19) Wang, D.; Zheng, W.; Hao, C.; Peng, Q.; Li, Y. Chem. Commun. 2008, 2556. (20) Pan, D.; An, L.; Sun, Z.; Hou, W.; Yang, Y.; Yang, Z.; Lu, Y. J. Am. Chem. Soc. 2008, 130, 5620. (21) Du, W. M.; Qian, X. F.; Yin, J.; Gong, Q. Chem.sEur. J. 2007, 13, 8840. (22) Nairn, J. J.; Shapiro, P. J.; Twamley, B.; Pounds, T.; Wandruszka, R. V.; Fletcher, T. R.; Williams, M.; Wang, C.; Norton, M. G. Nano Lett. 2006, 6, 1218.
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