J. Phys. Chem. C 2009, 113, 3939–3944
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Synthesis and Characterization of Nanostructured Wurtzite CuInS2: A New Cation Disordered Polymorph of CuInS2 Yunxia Qi,†,‡,# Qiangchun Liu,†,‡,§,# Kaibin Tang,*,†,‡ Zhenghua Liang,†,‡ Zhibiao Ren,‡ and Xianming Liu† Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China, Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, P. R. China, Department of Physics, Huaibei Coal Industry Teachers college, Huaibei, Anhui 235000, P. R. China ReceiVed: September 9, 2008; ReVised Manuscript ReceiVed: October 27, 2008
A new wurtzite (WZ) structure CuInS2, space group P63mc, a ) 3.90652(13) Å, c ) 6.42896(23) Å, has been synthesized by a one-step solvothermal method. Analogous with the disordered zinc-blende structure, wurtzite structure is metastable at room temperature and considered as a disordered polymorph of chalcopyrite (CH) structure, where Cu and In atoms randomly occupy the cation sublattice positions. It is believed that the solvent of ethanolamine plays an important role in the synthesis of WZ-CuInS2. The coordination between Cu2+ and -NH2 of ethanolamine molecules favors the nucleation and growth of WZ-CuInS2. Differential scanning calorimeter, together with X-ray diffraction analysis, revealed a phase transition from WZ-CuInS2 to CH-CuInS2 when WZ-CuInS2 was heated to certain temperature. The visible and near-infrared absorption spectra show that the as-prepared nanostructured WZ-CuInS2 has distinct optical properties compared with conventional CH-CuInS2. 1. Introduction Among the various categories of materials, AIBIIICVI ternary semiconductors have received increased interest because of their potential applications in light-emitting diodes, photovoltaic cells, and nonlinear optical devices.1 Normally, AIBIIICVI compounds crystallize in the chalcopyrite structure, which may be regarded as a superstructure of the zinc-blende type. Because A and B atoms ordered occupy the cation sublattice positions, this leads to doubling of the zinc-blende subcell in the chalcopyrite structure. When the temperature is higher than a critical value Tc, the A and B atoms disordering in cation sublattice, the chalcopyrite structure reverts to the zinc-blende structure.2 In addition, another order phase of AIBIIICVI compounds, the orthorhombic phase, has been observed in AgInS2 and AgInSe2 compounds.3 Analogous with chalcopyrite structure, orthorhombic phase AIBIIICVI compounds are regarded as the superstructure of the wurtzite type. Similarly, when the A and B atoms are disordered, orthorhombic phase could revert to wurtzite structure. Hence, a new disorder polymorph, wurtzite structure, could be rationally predicted. To illuminate the difference related to the behavior of cation occupation about the two pairs of order-disorder structures, the A and B atoms arrangements on the cation sublattices are schematically shown in Figure 1. Particularly, in the disordered structure, the occupation probability of A or B atoms on every cation positions is 50% (Figure 1b). In the case of chalcopyrite structure, the polymorphic order-disorder transition has been studied extensively by both * To whom correspondence should be addressed. E-mail: kbtang@ ustc.edu.cn. Phone: 86-551-3601791. Fax: 86-551-3601791. † Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China. ‡ Department of Chemistry, University of Science and Technology of China. § Department of Physics, Huaibei Coal Industry Teachers college. # These authors contributed equally to this work.
Figure 1. Schematic diagrams of the A and B atoms arrangement on the (a) (112) plan of chalcopyrite structure or (001) plan of orthorhombic phase. (b) (111) plan of zinc-blende structure or (001) plan of wurtzite structure. A atoms are indicated by (O) and B atoms are denoted by (b).
experimental and theoretical approaches. Many chalcopyrite compoundshavebeenfoundundergoingareversibleorder-disorder transition to the zinc-blende structure as the temperature is above Tc,2,4 whereas wurtzite structure of AIBIIICVI compounds has been rarely discussed.5 Controllable synthesis of materials with new crystal structures, especially metastable structures, is of great significance in the exploration of novel structure-determined properties and in the enrichment of crystallography.6 Metastable materials are often considered as counterparts that break away from its natural preferencesthe thermodynamically stable state. During the past decades, in virtue of synthesis nanocrystals, a few metastable semiconductors such as rock salt GaN7a and CdS,7b wurtzite ZnS,7c,d γ-Ga2O3 quantum dots,7e and monoclinic AgInSe23d and AgInS2,7f-i have been obtained under milder conditions, eg., solvothermal method, pyrolysis of organometallic precursors, and so on. Recently, CuInS2 has been the subject of intense interest due to its unique properties, including high absorption coefficient of 1 × 105 cm-1 at 500 nm,8 a direct band gap of 1.5 eV, which matches well with the solar spectrum,9 easy conversion of n/p carrier type,10 and low toxicity. Consequently, diverse methods
10.1021/jp807987t CCC: $40.75 2009 American Chemical Society Published on Web 02/12/2009
3940 J. Phys. Chem. C, Vol. 113, No. 10, 2009 have been developed to synthesis CuInS2.11 On the other hand, experiment and theory observations demonstrated that above Tc, a polymorphic order-disorder transition occurred in the CuInS2 crystals. Binsma discovered that CuInS2 exists in three polymorphic modifications, that is: (1) chalcopyrite structure, from ambient temperature up to 1253 K, (2) zinc-blende structure, between 1253 and 1318 K, (3) an unknown phase, assumed wurtzite structure, from 1318 K to the congruent melting temperature (1363 K).2a Also, experiments revealed that both the zinc-blende structure and unknown phase transform into chalcopyrite phase as the temperature recurred to the room temperature.2a This indicates that they are metastable, highenergetic structures relative to chalcopyrite structure. Very recently, Pan et al. first reported the synthesis of zinc-blende and wurtzite structure CuInS2 nanocrystals by a hot-injection method.5 However, it is still remains a challenge for the synthesis of metastable CuInS2 exiting stably at room temperature. Here, we report a simple one-step preparation for a new phase of nanostructured CuInS2, which was indexed as wurtzite structure, a metastable, cation disordered polymorph of CuInS2, by Rietveld refinement technique. On the discussing of synthesis mechanism, it is found that the coordination between Cu2+ and a ligand, such as ethanolamine, is especially beneficial for the nucleation and growth of WZ-CuInS2. In addition, DSC analysis was used to record the temperature induced phase change from WZ-CuInS2 to CH-CuInS2. The band gaps of the as-prepared WZ-CuInS2 samples, which come from different copper sources, were also investigated by fitting the optical absorption results. Importantly, this study has uncovered and studied a novel disordered polymorph of AIBIIICVI compoundsswurtzite structure. Consequently, a series of wurtzite structure AIBIIICVI compounds can be rationally predicted. 2. Experimental Section Preparation. Nanostructured WZ-CuInS2 samples were prepared by a simple solvothermal route. All the reactants were of analytical grade and used without further purification. Stoichiometric amounts of InCl3 · 4H2O, CuCl2 · 2H2O, 20% molar excess of thiourea, and 40 mL of ethanolamine were added to a 100 mL breaker, which was then put into an ordinary ultrasonic cleaner using 56 kHz operating frequency (J&L Co., China, JL-60DT) for a few minutes to form a deep-blue solution. Then the solution was transferred into a 50 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 180 °C for 24 h and then cooled down to room temperature naturally. The precipitate was separated by centrifugation, wished with distilled water and absolute ethanol several times, and dried at 60 °C for 4 h. To investigate the influence of copper source on the morphology, CuSO4 · 5H2O and Cu(NO3)2 · 5H2O were employed to replace CuCl2 · 2H2O with other synthetic conditions unchanged. Characterization. The as-prepared products was characterized by powder X-ray diffraction (XRD) on a Philips X’pert X-ray diffractometer equipped with Cu KR radiation (λ ) 1.5418 Å). The XRD data for indexing and cell-parameter calculation was recorded in step-scanning mode in the angular range 25 < 2θ < 120° with 0.02° step width and 5 s count time. The XRD pattern was analyzed by the Rietveld method using the FULLPROF program. The detail of profile refinement and the results are presented in the Supporting Information (Figure S1 and Table S1). Transmission electron microscope (TEM) images were taken with a Hitachi H-800 transmission electron microscope at an acceleration voltage of 200 kV. The copper grid covered with a holey carbon for TEM observation was gilded, and the
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Figure 2. XRD pattern of nanostructured WZ-CuInS2 synthesized when CuCl2 · 2H2O is used as a copper source.
TABLE 1: Crystal Data and Rietveld Refinement for WZ-CuInS2a atom
site
x
y
z
g
biso (A2)
Cu In S
2b 2b 2b
1/3 1/3 1/3
2/3 2/3 2/3
0 0 0.375
0.5 0.5 1.0
1.205(37) 1.205(37) 0.744(62)
a P63mc, a ) 3.90652(13) Å, c ) 6.42896(23) Å, V ) 84.967(5) Å3. Z ) 1, Rp ) 11.6%, Rwp ) 10.6%.
obtained Au diffraction ring was used as inner standard for indexing. High-resolution transmission electron microscope (HRTEM) images were performed on a JEOL-2010 transmission electron microscope. The scanning electron microscopy (SEM) images were taken using a Sirion 200 field emission scanning electron microscope (FESEM, 20 kV). The visible and nearinfrared spectra were recorded on a JGNA Specord 200 PC UV-visible spectrophotometer. Differential scanning calorimetry (DSC) analysis was carried out on a differential scanning calorimeter model 60 (Shimadzu) with a heat rate of 10 °C min-1 under a flow of nitrogen gas. 3. Results and Discussion 3.1. Structure Characterization. Nanostructured WZCuInS2 synthesized by using CuCl2 · 2H2O as a copper source was studied by XRD, and the structure was refined by the Rietveld technique. The XRD pattern (Figure 2) shows the characteristic of hexagonal cell with space group of P63mc. The calculated cell parameters and corresponding structural parameters are shown in Table 1. XRF and XPS analyses (Supporting Information Figures S2, S3) indicate that the chemical composition of the product is composed of Cu, In, and S with mole ratio of 1.06:1:2.24, which is close to the stoichiometric compositions of CuInS2. The morphology and crystal structure of the sample were examined by TEM, HRTEM, and selected area electron diffraction (SAED), respectively. Figure 3a shows that the synthesized WZ-CuInS2 is aggregated with irregular morphologies. In addition, a few of approximate hexagonal nanostructure (Figure 3c) with edge lengths of 100-200 nm are observed in the obtained product. Figure 3b shows the HRTEM image and the corresponding SAED pattern obtained from a quasiflaky nanostructure marked with an arrowhead in Figure 3a. The HRTEM image exhibits clear lattice fringes with spacing about 6.43 Å, which is in good agreement with the interplanar spacing of (001) plane of the WZ-CuInS2. The corresponding SAED pattern shows well-crystallized single crystal nature of the flaky
Nanostructured Wurtzite CuInS2
Figure 3. (a) TEM image of the as-prepared WZ-CuInS2 synthesized when CuCl2 · 2H2O is used as a copper source. (b) HRTEM image of a quasiflaky nanostructure marked with an arrow in (a) and the corresponding SAED (inserted in (b). (c) TEM image of a hexagonal nanostructure in the sample. (d) SAED pattern along the [0001] zone axis. (e) SAED pattern along the [11j00] zone axis. The weak bright rings marked by arrowheads in (d) and (e) are assigned to Au diffraction ring.
nanostructure. Interestingly, the forbidden diffraction spot (001) appeared in this diffraction pattern, which was not observed in all the XRD patterns. It is usually attributed to the double diffraction of the incident electron whose scattering is much greater than that assumed in the kinematic theory.12 Figure 3d is the SAED pattern taken along the [0001] zone axis. The diffraction spots show the expected 6-fold symmetry, and the most inner set of diffraction spots can be indexed to the {101j0} reflection of WZ-CuInS2. Figure 3e is the SAED pattern along the [11j00] zone axis. The (0002) is consistent with the observed c parameter from the XRD pattern. Finally, the refinement results of the XRD pattern together with the SAED patterns confirm that the as-obtained products crystallize in the hexagonal wurtzite structure. Structural models of WZ-CuInS2 are shown in Figure 4. Cu and In atoms randomly occupy one set of hexagonal positions and S atoms reside the other. Thus, the structure of WZ-CuInS2 can be described as the hexagonal close packing of anions (S) layers and cations (Cu, In) layers, stacking in an Aa Bb Aa Bb sequence. In addition, a very weak peak of impurity (marked with star in Supporting Information Figure S1) was detected by powder X-ray diffraction in the step-scanning mode with 0.02° step width and 5 s count time. This peak was indexed on chalcopyrite phase with a ) 5.52199(210) Å, c ) 11.11258(821) Å, and the detailed calculation results shown that about 7.45% chalcopyrite phase coexists with wurtzite phase (Supporting Information Table S2). Further research will be performed on the well control the purity of wurtzite phase. 3.2. Morphology Modulation. Further experiments indicate that copper sources strongly influence the morphology of WZCuInS2. Using other copper sources such as CuSO4 · 5H2O and Cu(NO3)2 · 5H2O yielded WZ-CuInS2 nanoparticles with quite different morphologies. Figure 5 exhibits the morphologies of the products obtained from when CuSO4 · 5H2O was used. A panoramic FESEM image in Figure 5a reveals that the obtained sample comprises several types of structures. Figure 5b presents the most typical structure of nanoplates observed in the sample.
J. Phys. Chem. C, Vol. 113, No. 10, 2009 3941 Further observation reveals that many of these nanoplates are hexagons or approximate hexagons. In addition, two other types of shapes have also been identified. As shown in Figure 5c, many uniform hollow microspheres with diameter of about 1 µm are found in the product. An apparent cavity of a broken sphere can be observed in the inset of Figure 5c. Some flowerlike microspheres with diameter of 2-3 µm have also been detected (Figure 5d). These flower-like structures are comprised of nanoplates, which have smooth surfaces. The detailed structures were further examined by TEM, HRTEM, and SAED. Figure 6a presents a single CuInS2 nanoplate with curving edge. SAED pattern (Figure 6b) was obtained by focusing the electron beam perpendicular to the flat surface of the plate. The obtained hexagonal pattern is attributed to [0001] zone axis diffraction of a single crystal WZ-CuInS2. HRTEM image (Figure 6c) also shows the clearly hexagonal lattice planes with lattice spacing of 3.4 Å, which corresponds to the (101j0) plane of the WZCuInS2. Moreover, detailed investigation showed that the nanoplate has superlattice structures with a periodicity of 6.8 Å, which is double period of (101j0) planes. Surprisingly, the superlattice exhibits densely distributed stacking faults/twin planes. Such phenomenon is also confirmed by the corresponding SAED pattern. As shown in Figure 6b, at half the distance between the bright spots, there are many weaker extra spots, which almost linked with each other to form many parallel bright lines owing to the consecutive reflection splitting induced by the occurrence of large amount of superlattice stacking faults/ twin planes. Shown in Figure 7a is a TEM image recorded from CuInS2 hollow microspheres (Figure 5c), confirming these spheres of the hollow nature. The SAED pattern taken from the edge of the microspheres demonstrates the polycrystalline characteristic of these microspheres (Figure 7a, inset). HRTEM image shows that the lattice spacings are determined to be 3.2 and 3.0 Å, corresponding to the (0002) and (101j1) planes, respectively (Figure 7b). The XRD pattern of the sample synthesized when CuSO4 · 5H2O used as copper source is shown in Supporting Information Figure S4. All the strong peaks are well indexed to the WZ-CuInS2. In addition, when Cu(NO3)2 · 5H2O was used as a copper source, WZ-CuInS2 with nanoplates and microspheres were obtained by similar solvothermal method (Supporting Information Figure S5). 3.3. Discussions of Phase Formation Mechanism. As we know, many reports have demonstrated that some surfactants were utilized to adjust the chemical environment so that the metastable, high-energy structures, relative to their thermodynamics stable phase, can be trapped.7d,13 In the present study, results indicated that ethanolamine was pivotal for the formation of nanostructured WZ-CuInS2. In detail, it plays three important roles: (1) solvent, (2) reducing agent, Cu2+ was reduced to Cu+, and (3) ligand. Dissolving all the reactants in ethanolamine to form a blue homogeneous solution before heating is crucial for the successful synthesis of WZ-CuInS2, in which the blue solution indicates the formation of coordination between Cu2+ and -NH2 of ethanolamine molecules. In contrast, if the coordination has not fully formed before heating, the as-obtained sample was a mixture of WZ-CuInS2 and CH-CuInS2 (Supporting Information Figure S6a). This result is also consistent with the previous reports of a similar solvothermal reaction, in which the starting materials were elements and the reactants did not form a homogeneous solution, so the obtained products were CH-CuInS2.14 Moreover, other solvents with similar coordination properties also proved to be suitable for the formation of WZ-CuInS2 such as ethylenediamine and isopropanolamine (Supporting Information
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Figure 4. Structural model of WZ-CuInS2 (a) crystal structural of WZ-CuInS2 and (b) stacking of tetrahedrons.
Figure 5. FESEM images of nanostructured WZ-CuInS2 synthesized when CuSO4 · 5H2O is used as a copper source: (a) a panoramic FESEM image, (b) nanoplates, (c) hollow spheres. Inset: image of the cavity of a single microsphere. (d) Flower-like spheres.
Figure 7. WZ-CuInS2 hollow spheres synthesized when CuSO4 · 5H2O is used as a copper source: (a) TEM image, inset is SAED pattern; (b) HRTEM image. Figure 6. WZ-CuInS2 nanoplates synthesized when CuSO4 · 5H2O is used as a copper source: (a) TEM image, (b) HRTEM image, (c) SAED pattern.
Table S3). Contrary to solvents having weaker coordination properties, such as N,N-dimethylformamide (DMF), ethylene glycol proved to be in favor of CH-CuInS2 (Supporting Information Table S3). The result suggests that different coordination properties of solvent effect the phase selection. To better understand the formation mechanism of the WZCuInS2, samples collected at different reaction times were characterized by XRD. A fast kinetics-controlled nucleation and growth process can be clearly observed. As shown in Supporting
Information Figure S6b, at the early stage (0.5 h), the sample comprises well crystallized WZ-CuInS2. When the reaction time was prolonged to 1 h, Supporting Information Figure S6c shows that the sample was pure WZ-CuInS2. Such a process is quite different from the previous report on the preparing of CHCuInS2 hollow spheres in a similar solvothermal method in the solvent of DMF, which involves a formation of amorphous primary particles followed by an Ostwald ripening process.11j Moreover, if the amorphous primary particles were separated and crystallized in ethanolamine, besides CH-CuInS2, WZCuInS2 appeared as shown in Supporting Information Figure S6d, which confirms that the ligand effect of ethanolamine affects the nucleation and growth. In addition, it is found that
Nanostructured Wurtzite CuInS2
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Figure 8. Nanostructured WZ-CuInS2 synthesized when CuSO4 · 5H2O is used as a copper source, (A) DSC curve and (B) XRD patterns: (a) DSC-340 °C, (b) DSC-500 °C, (c) 550 °C, 4 h, (d) 600 °C, 4 h. (XRD patterns c and d were recorded on the samples heated in a sealed vacuum quartz capillary at 550 °C for 4 h, and 600 °C for 4 h, respectively. A little of In2O3 marked by arrowheads was detected. It is attributed to the oxidation of WZ-CuInS2 during the DSC heat treatments.)
Figure 9. Optical absorption spectra and band gap of the as-obtained CuInS2 samples. (a-b) Sample I, CuCl2 · 2H2O as copper source. (c) Sample II, CuSO4 · 5H2O as copper source (black lines); sample III Cu(NO3)2 · 5H2O as copper source (red lines). The absorption peaks appeared at 800 nm is assigned to the apparatus.
most of WZ-CuInS2 remained even if WZ-CuInS2 further grew in DMF (Supporting Information Figure S6e), suggesting that the crystal growth stage has very small influence on phase formation. On the basis of the experimental results, we explain the WZCuInS2 formation process as follows: (1) A kinetics-controlled process leads to the nucleation of metastable phase on the ligand effect of solvent. (2) The phase selection of WZ-CuInS2 or CHCuInS2 is greatly determined by the nucleation stage, and the following crystal growth stage has very small influence on it. Further studies are still demanded to understand the effect of ligands on the nucleation and growth of WZ-CuInS2 to achieve a more detailed formation mechanism.
3.4. Thermal Stability. The DSC curve of WZ-CuInS2 nanostructures is shown in Figure 8A, which indicates two endothermic peaks and an exothermic peak. The first endothermic peak below 100 °C can be attributed to the dehydration of adsorbed water. The second endothermic peak around 274 °C can be attributed to the desorption of chemically adsorbed ethanolamine molecules. This is clearly evidenced by Supporting Information Figure S7, which shows that the as-prepared sample has adsorbed ethanolamine. It is noticeable that the exothermic peak with maximal at 406 °C may be attributed to phase transition from WZ-CuInS2 to CH-CuInS2. To confirm this speculation, the samples before and after heating at 406 °C were collected and investigated by X-ray diffraction. Figure 8a reveals
3944 J. Phys. Chem. C, Vol. 113, No. 10, 2009 that the sample was nearly pure WZ-CuInS2 before the heating, while Figure 8b reveals that heating to 500 °C leads to the formation of CH-CuInS2 as the major crystalline phase. Accordingly, the exothermic peak at 406 °C resulted from phase transition from WZ-CuInS2 to CH-CuInS2. In addition, Figure 8b also reveals that the phase transition from WZ-CuInS2 to CH-CuInS2 was not completely, and partial WZ-CuInS2 still remained. Further studies show that the wurtzite phase completely disappeared as the temperature reached 600 °C (Figure 8d). The result confirms that WZ-CuInS2 is a metastable, highenergetic structure relative to CH-CuInS2 at room temperature. 3.5. Optical Characterization. Figure 9 shows the visible and near-infrared absorption of the WZ-CuInS2 nanostructures with different shapes and crystal structures. Sample I exhibits a strong absorption edge around 1000 nm (Figure 9a). The optical properties of the sample II and sample III were notably different, showing broad absorption peaks at 1075 and 1261 nm, respectively (Figure 9c), indicating broad size distributions, which mostly arise from the shape diversity of the two samples. In addition, we consider that the superlattice structures of the two samples also affect their absorption behaviors. The calculated band gaps for sample I-III are 1.23, 1.18, and 1.10 eV, respectively, which are smaller than that of CH-CuInS2 (Eg ) 1.5 eV). Furthermore, the band gap of WZ-CuInS2 is slightly higher than sphalerite phase (1.07 eV) formed at high pressure,15 which is consistent with conventional difference between wurtzite and sphalerite structure. It is well-known that the band structure of a semiconductor is strongly dependent on the manner of orbital interactions, which is affected by the symmetry of crystal structure. Hence, the observed optical behavior not only reflects influences of the crystal structures on the optical properties but also suggests the novel properties and expanded applications resulted from this new phase structure. On the other hand, the results are also consistent with the theoretical calculation that disordering of cations in chalcopyrite structure induces band gap decrease.4a-d 4. Conclusions In conclusion, a simple one-step synthesis method is described to fabricate metastable WZ-CuInS2 nanostructures. It is suggested that coordination between Cu2+ and -NH2 reduced the energy barrier, allowing the formation of a metastable phase. In addition, by adjusting copper sources, WZ-CuInS2 with different morphologies such as nanoplates, hollow spheres, and solid microspheres have been obtained. The metastability of WZ-CuInS2 was confirmed by DSC analysis, which showed that WZ-CuInS2 transformed into CH-CuInS2 when it was heated to a certain temperature. We have also uncovered remarkably different optical properties in visible and near-infrared regions of the as-obtained WZ-CuInS2 compared with those of CH-CuInS2. Importantly, the present study offers an effective strategy to expand the range of available solid-state materials because it not only exhibits a new phase of CuInS2 but also opens up a new disordered polymorph of AIBIIICVI compounds. On the basis of the current synthesis method and further understanding of formation mechanism, we expect to lead to the strategy expanding some other AIBIIICVI compounds, e.g., CuGaS2, CuInSe2, etc., to capture their corresponding WZ structures. Moreover, it is considered that orthorhombic CuInS2 can be obtained if Cu and In atoms of WZ-CuInS2 are ordered at a given growth condition. Acknowledgment. Financial support by the National Natural Science Foundation of China (no. 20671086, 20621061), the Program for New Century Excellent Talents in University (NCET), and the 973 Projects of China are gratefully acknowledged.
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