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Synthesis and characterization of phase-purity Cu9BiS6 nanoplates
Materials Letters 64 (2010) 1091–1094
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and characterization of phase-purity Cu9BiS6 nanoplates Yaping Zeng a,b, Hongxing Li a,b, Boyuan Xiang a,b, Haiqing Ma a,b, Baihua Qu a,b, Mingxia Xia a,b, Yicheng Wang a,b, Qinglin Zhang a,b, Yanguo Wang a,b,c,⁎ a b c
Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 24 December 2009 Accepted 8 February 2010 Available online 12 February 2010 Keywords: Copper bismuth sulfide Nanomaterials Hydrothermal Semiconductors
a b s t r a c t Single crystalline Cu9BiS6 nanoplates have been synthesized via a hydrothermal route. The X-ray diffraction (XRD) showed these resultant nanoplates having cubic symmetry with the cell constant of 0.556 nm. The morphology of these nanoplates identified by scanning electron microscopy (SEM) was the hexagonal and octagonal configurations respectively. Microstructure characterized by transmission electron microscopy (TEM) revealed six side surfaces of the hexagonal nanoplate coincident with the {110} lattice planes and the octagonal nanoplate enclosed by the four {100} and four {110} lattice planes. Optical diffuse reflectance measurement demonstrated that these Cu9BiS6 nanoplates have a band gap of 1.25 eV. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Ternary chalcogenides of I–III–VI2, such as CuInS2 and CuInSe2 with the reported band gap ranging from 0.9 eV to 1.9 eV, have attracted considerable attention in recent years due to commercial thin film photovoltaic devices based on Cu(In,Ga)(Se2,S2) in recent years [1,2]. But the element Indium in natural reserves is relatively rare [3], which limits these copper indium sulfides or selenides for application in photovoltaic device. Therefore, development of new semiconductors instead of copper indium sulfides is imminently required. Bi is more abundant than In in global reserves. Copper bismuth sulfides with the similar optical and electrical properties as copper indium sulfides are the promising substitutions [4]. Up to now, the Cu3BiS3 [4,5], Cu4Bi4S9 [6], and CuBiS2 [7] nanostructures including nanowires, nanorods and thin films have been synthesized by various methods. However, few reports are about the synthesis of Cu9BiS6 nanostructures [8]. Therefore, further studies of Copper bismuth sulfides are still highly desirable, especially for investigation of mass production of copper bismuth sulfides. In this article the experimental fabrication of Cu9BiS6 nanoplates via hydrothermal route is reported. We reveal that mass production of the Cu9BiS6 can be obtained and the resultant Cu9BiS6 nanoplates are highly dominated by the hexagonal and octagonal disks. The band gap of these Cu9BiS6 nanoplates was evaluated to be 1.25 eV, which made the Cu9BiS6 to be a suitable semiconductor for application in solar absorber layer.
All reagents used in this synthesis including CuCl, BiCl3, sulfur powder, and polyethylene glycol 1000 (PEG-1000) were commercial products. The analytical grade ethanol was used as receiver. CuCl (0.446 g), BiCl3 (0.158 g) (according to the stoichiometric ratio of 9:1) and PEG-1000 (1.0 g) were dissolved into 60 ml deionized water under vigorous stirring for 30 min and PEG-1000 was used as the ligand [9]. Sulfur powder (0.096 g) was added into the solution directly. The whole mixture solution was then sealed into a Teflon-lined stainless-steel autoclave (100 ml capacity). The autoclave was heated and maintained at 180 °C for 18 h and then cooled to room temperature naturally. The as-obtained product was collected by centrifugation, and washed with deionized water and ethanol for three times, respectively. The yield was then dried under vacuum at 60 °C for 8 h. X-ray diffraction instrument (XRD, Siemens D-5000, and Cu Ka, λ = 1.5405 Å) was used to examine the crystal structure of the assynthesized product. The morphologies and microstructures of the final product were characterized by field emission scanning electron microscope (FE-SEM, model S-4800) operated at 5 kV and transmission electron microscopy (TEM, model JEOL-2010) operated at 200 kV. Optical diffuse reflectance measurement was performed in the 200– 1600 nm region at room temperature using PE Lambda 750 UV Spectrophotometer equipped with an integrating sphere attachment and BaSO4 as reference.
⁎ Corresponding author. Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China. Tel./fax: + 86 010 82648009. E-mail address: [email protected] (Y. Wang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.022
3. Results and discussion Morphology of the as-product has been examined by FE-SEM (Fig. 1a) and shows a general view of nanostructures with the plate-
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Y. Zeng et al. / Materials Letters 64 (2010) 1091–1094
Fig. 1. (a) SEM image shows a general view of the product. (b) EDS spectrum of the product depicts the presence of Cu, Bi and S elements. (c) XRD pattern of the product and (d) the atomic arrangement in unit cell of the Cu9BiS6 crystal.
like morphology randomly dispersed on the surface of substrate. The SEM-equipped energy-dispersive X-ray spectroscopy (EDX) analysis (Fig. 1b) demonstrates that all the as-synthesized samples are composed of elements Cu, Bi, and S. The atomic ratio of Cu:Bi:S is about 9:1:6, which is consistent with the stoichiometry of Cu9BiS6. Fig. 1c shows a typical XRD of the product. All the diffraction peaks can be matched well with the face-centered cubic Cu9BiS6 structure with lattice constant of 5.563 Å and space group Fm3m [8]. No extra diffraction peaks corresponding to other binary and/or ternary sulfides such as Cu2S and CuBiS2 are observed, indicating that the product obtained in this study is of high phase-purity. The atomic arrangement of the Cu9BiS6 crystal is shown in Fig. 1d, where the sulfur atoms occupy the nodes of a face-centered cubic lattice, bismuth and copper atoms occupy the centers of the sulfur octahedra and tetrahedra respectively [8]. Careful inspection on individual Cu9BiS6 nanoplates depicts that their morphology can be divided into two categories as shown by highly magnified SEM images in Fig. 2a and b respectively, hexagonal and octagonal configurations. Edge length of these nanoplates is ranging from 100 nm to 300 nm. The thickness of these Cu9BiS6 nanoplates depicted in Fig. 2c is about 40 nm. The angles between the neighboring edges of the hexagonal and octagonal disks in Fig. 2a and b are measured to be about 120° and 135° respectively. The hexagonal and octagonal disks are all single crystals which are identified by the corresponding electron diffraction (ED) patterns. The ED pattern taken from the hexagonal disk displayed in Fig. 3a is inserted in Fig. 3b, and confirms that the hexagonal disk is perpendicular to the [111] direction. The 0.32 nm lattice fringes in the high resolution TEM (HRTEM) image shown in Fig. 3b coincide with the (110) lattice plane of Cu9BiS6 crystal, which verifies that the side surface of this disk is parallel to the (110) lattice plane. Therefore, the hexagonal disk is enclosed by the six {110} side surfaces and the {111} top and bottom surfaces. The pattern inserted in Fig. 3d is the homologous ED pattern of the octagonal disk in Fig. 3c, proving it perpendicular to the [001] direction. The interplanar spacing of 0.56 nm shown in HRTEM image (Fig. 3d) corresponds to the (100) plane of the face-centered cubic crystal of Cu9BiS6. The octagonal disk appears to be truncated from regular rectangular plate. The neighboring edge of the (100) edge coincides to the {220} planes. Consequently, eight side surfaces of the octagonal disk correspond to the
Fig. 2. SEM images identify the morphological characteristics of the Cu9BiS6 nanoplates, (a) hexagonal configuration, (b) octagonal shape and (c) the thickness of a typical nanoplate which is about 40 nm.
Y. Zeng et al. / Materials Letters 64 (2010) 1091–1094
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Fig. 3. (a) TEM image showing a typical hexagonal nanodisk and its six side surfaces are illustrated by an inset on left top corner. (b) HRTEM image and corresponding electron diffraction pattern is inserted at the top right corner of the hexagonal nanodisk. (c) TEM image of the octagonal nanodisk and the eight side surfaces illustrated in the inset on the top left corner. (d) HRTEM image and the inset is the electron diffraction pattern of the octagonal nanodisk.
four {100} and four {110} lattice planes respectively. The octagonal and hexagonal nanodisks with the large top/bottom surfaces are favorable for the improvement of the performance in display and photovoltaic devices [10]. Formation of the Cu9BiS6 nanodisks has close relation to the PEG, because organic surfactants can act as structure-directing agents or “soft templates” and have been widely used to prepare nanostructured materials with peculiar morphologies [9,11]. Without addition of PEG the prepared Cu9BiS6 nanocrystals are mainly dominated by the microspheres gathered by irregular sheets (to be reported elsewhere). Because the formation of faceted nanocrystals is related to selective adsorption of ions and small nanoparticles on the crystal faces during the growth [12], a possible explanation of the Cu9BiS6 nanoplates is that the PEG long chain molecules have effect on fixing the primary Cu9BiS6 nuclei seeds and initializing the selective coalescence of the adjacent nuclei seeds, leading to the crystal growth with the preferred facets. The band gap of the Cu9BiS6 nanoplates has been estimated from the generated reflectance-versus-wavelength data via converting reflectance to absorption data according to the Kubelka–Munk equation as follows, α/S = (1 − R)2 / (2R), where R is the reflectance and α and S are the absorption and scattering coefficients, respectively. By extrapolating the linear region of a plot (ahv)2 against photon energy, optical diffuse reflectance measurement reveals the presence of optical band gap at 1.25 eV as shown in Fig. 4. Semiconductors with a band gap ranging from 0.9 eV to1.9 eV are suitable for use as the solar absorber layer in thin film photovoltaic devices [4]. Therefore, the as-synthesized Cu9BiS6 nanoplates may
have potential application in the photoelectric converter. The photovoltaic property of the Cu9BiS6 nanoplates is under study. 4. Conclusion In summary, we have presented the synthesis of the single crystal Cu9BiS6 nanoplates via a simple hydrothermal route. The as-prepared Cu9BiS6 nanoplates are high phase-purity in crystal structure and the octagonal and hexagonal nanodisks in morphology. The prepared Cu9BiS6 nanoplates with an optical band gap 1.25 eV may be applied as the solar absorber layer in thin film photovoltaic devices. The
Fig. 4. The optical band gap of Cu9BiS6 nanoplates.
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hydrothermal route employed in this study could be used for synthesis of other low dimensional ternary chalcogenides with distinct morphology. Acknowledgements The authors are grateful to the NSF of China (Grant Nos. 60796078, 10774174, and 20903038), “973” National Key Basic Research program of China (Grant Nos. 2007CB310500 and 2007CB936301) and the Opening Project of Key Laboratory of Low Dimensional Quantum Structures and Quantum Control (Hunan Normal University), Ministry of Education (No. QSQC0912) for the financial support.
References [1] Connor ST, Hsu CM, Weil B, Aloni DS, Cui Y. J Am Chem Soc 2009;131(13). [2] Ramakrishna Reddy KT, Reddy PJ. Mater Lett 1990;10(6):275–9. [3] George MW. Mineral Commodity Summaries, U.S. Geological Survey Report. Washington, DC: U.S. Government Printing Office; 2005. [4] Gerein NJ, Haber JA. Chem Mater 2006;18:6297–302. [5] Estrella VO, Nair MTS, Nair PK. Semicond Sci Technol 2003;18:190–4. [6] Kryukova G, Heuer M, Doering Th, Bente K. J Crystal Growth 2007;306:212–6. [7] Razmara MF, Henderson CMB, Pattrick RAD. J Mater Res 1997;61:79–88. [8] Torneoka K. Am Mineral 1982;67:360–72. [9] Huynh WU, Dittmer JJ, Alivisatos AP. Science 2002;295:2425–7. [10] Gou XL, Cheng FY, Shi YH, Zhang L, Peng SJ, Chen J, et al. J Am Chem Soc 2006;128: 7222–9. [11] Wang WZ, Poudel B, Yang J, Wang DZ, Ren ZF. J Am Chem Soc 2005;127:13792–3. [12] Pileni MP. Nature Mater 2003;2:145–50.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis and characterization of phase-purity Cu9BiS6 nanoplates Yaping Zeng a,b, Hongxing Li a,b, Boyuan Xiang a,b, Haiqing Ma a,b, Baihua Qu a,b, Mingxia Xia a,b, Yicheng Wang a,b, Qinglin Zhang a,b, Yanguo Wang a,b,c,⁎ a b c
Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history: Received 24 December 2009 Accepted 8 February 2010 Available online 12 February 2010 Keywords: Copper bismuth sulfide Nanomaterials Hydrothermal Semiconductors
a b s t r a c t Single crystalline Cu9BiS6 nanoplates have been synthesized via a hydrothermal route. The X-ray diffraction (XRD) showed these resultant nanoplates having cubic symmetry with the cell constant of 0.556 nm. The morphology of these nanoplates identified by scanning electron microscopy (SEM) was the hexagonal and octagonal configurations respectively. Microstructure characterized by transmission electron microscopy (TEM) revealed six side surfaces of the hexagonal nanoplate coincident with the {110} lattice planes and the octagonal nanoplate enclosed by the four {100} and four {110} lattice planes. Optical diffuse reflectance measurement demonstrated that these Cu9BiS6 nanoplates have a band gap of 1.25 eV. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Ternary chalcogenides of I–III–VI2, such as CuInS2 and CuInSe2 with the reported band gap ranging from 0.9 eV to 1.9 eV, have attracted considerable attention in recent years due to commercial thin film photovoltaic devices based on Cu(In,Ga)(Se2,S2) in recent years [1,2]. But the element Indium in natural reserves is relatively rare [3], which limits these copper indium sulfides or selenides for application in photovoltaic device. Therefore, development of new semiconductors instead of copper indium sulfides is imminently required. Bi is more abundant than In in global reserves. Copper bismuth sulfides with the similar optical and electrical properties as copper indium sulfides are the promising substitutions [4]. Up to now, the Cu3BiS3 [4,5], Cu4Bi4S9 [6], and CuBiS2 [7] nanostructures including nanowires, nanorods and thin films have been synthesized by various methods. However, few reports are about the synthesis of Cu9BiS6 nanostructures [8]. Therefore, further studies of Copper bismuth sulfides are still highly desirable, especially for investigation of mass production of copper bismuth sulfides. In this article the experimental fabrication of Cu9BiS6 nanoplates via hydrothermal route is reported. We reveal that mass production of the Cu9BiS6 can be obtained and the resultant Cu9BiS6 nanoplates are highly dominated by the hexagonal and octagonal disks. The band gap of these Cu9BiS6 nanoplates was evaluated to be 1.25 eV, which made the Cu9BiS6 to be a suitable semiconductor for application in solar absorber layer.
All reagents used in this synthesis including CuCl, BiCl3, sulfur powder, and polyethylene glycol 1000 (PEG-1000) were commercial products. The analytical grade ethanol was used as receiver. CuCl (0.446 g), BiCl3 (0.158 g) (according to the stoichiometric ratio of 9:1) and PEG-1000 (1.0 g) were dissolved into 60 ml deionized water under vigorous stirring for 30 min and PEG-1000 was used as the ligand [9]. Sulfur powder (0.096 g) was added into the solution directly. The whole mixture solution was then sealed into a Teflon-lined stainless-steel autoclave (100 ml capacity). The autoclave was heated and maintained at 180 °C for 18 h and then cooled to room temperature naturally. The as-obtained product was collected by centrifugation, and washed with deionized water and ethanol for three times, respectively. The yield was then dried under vacuum at 60 °C for 8 h. X-ray diffraction instrument (XRD, Siemens D-5000, and Cu Ka, λ = 1.5405 Å) was used to examine the crystal structure of the assynthesized product. The morphologies and microstructures of the final product were characterized by field emission scanning electron microscope (FE-SEM, model S-4800) operated at 5 kV and transmission electron microscopy (TEM, model JEOL-2010) operated at 200 kV. Optical diffuse reflectance measurement was performed in the 200– 1600 nm region at room temperature using PE Lambda 750 UV Spectrophotometer equipped with an integrating sphere attachment and BaSO4 as reference.
⁎ Corresponding author. Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, Hunan University, Changsha 410082, China. Tel./fax: + 86 010 82648009. E-mail address: [email protected] (Y. Wang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.02.022
3. Results and discussion Morphology of the as-product has been examined by FE-SEM (Fig. 1a) and shows a general view of nanostructures with the plate-
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Fig. 1. (a) SEM image shows a general view of the product. (b) EDS spectrum of the product depicts the presence of Cu, Bi and S elements. (c) XRD pattern of the product and (d) the atomic arrangement in unit cell of the Cu9BiS6 crystal.
like morphology randomly dispersed on the surface of substrate. The SEM-equipped energy-dispersive X-ray spectroscopy (EDX) analysis (Fig. 1b) demonstrates that all the as-synthesized samples are composed of elements Cu, Bi, and S. The atomic ratio of Cu:Bi:S is about 9:1:6, which is consistent with the stoichiometry of Cu9BiS6. Fig. 1c shows a typical XRD of the product. All the diffraction peaks can be matched well with the face-centered cubic Cu9BiS6 structure with lattice constant of 5.563 Å and space group Fm3m [8]. No extra diffraction peaks corresponding to other binary and/or ternary sulfides such as Cu2S and CuBiS2 are observed, indicating that the product obtained in this study is of high phase-purity. The atomic arrangement of the Cu9BiS6 crystal is shown in Fig. 1d, where the sulfur atoms occupy the nodes of a face-centered cubic lattice, bismuth and copper atoms occupy the centers of the sulfur octahedra and tetrahedra respectively [8]. Careful inspection on individual Cu9BiS6 nanoplates depicts that their morphology can be divided into two categories as shown by highly magnified SEM images in Fig. 2a and b respectively, hexagonal and octagonal configurations. Edge length of these nanoplates is ranging from 100 nm to 300 nm. The thickness of these Cu9BiS6 nanoplates depicted in Fig. 2c is about 40 nm. The angles between the neighboring edges of the hexagonal and octagonal disks in Fig. 2a and b are measured to be about 120° and 135° respectively. The hexagonal and octagonal disks are all single crystals which are identified by the corresponding electron diffraction (ED) patterns. The ED pattern taken from the hexagonal disk displayed in Fig. 3a is inserted in Fig. 3b, and confirms that the hexagonal disk is perpendicular to the [111] direction. The 0.32 nm lattice fringes in the high resolution TEM (HRTEM) image shown in Fig. 3b coincide with the (110) lattice plane of Cu9BiS6 crystal, which verifies that the side surface of this disk is parallel to the (110) lattice plane. Therefore, the hexagonal disk is enclosed by the six {110} side surfaces and the {111} top and bottom surfaces. The pattern inserted in Fig. 3d is the homologous ED pattern of the octagonal disk in Fig. 3c, proving it perpendicular to the [001] direction. The interplanar spacing of 0.56 nm shown in HRTEM image (Fig. 3d) corresponds to the (100) plane of the face-centered cubic crystal of Cu9BiS6. The octagonal disk appears to be truncated from regular rectangular plate. The neighboring edge of the (100) edge coincides to the {220} planes. Consequently, eight side surfaces of the octagonal disk correspond to the
Fig. 2. SEM images identify the morphological characteristics of the Cu9BiS6 nanoplates, (a) hexagonal configuration, (b) octagonal shape and (c) the thickness of a typical nanoplate which is about 40 nm.
Y. Zeng et al. / Materials Letters 64 (2010) 1091–1094
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Fig. 3. (a) TEM image showing a typical hexagonal nanodisk and its six side surfaces are illustrated by an inset on left top corner. (b) HRTEM image and corresponding electron diffraction pattern is inserted at the top right corner of the hexagonal nanodisk. (c) TEM image of the octagonal nanodisk and the eight side surfaces illustrated in the inset on the top left corner. (d) HRTEM image and the inset is the electron diffraction pattern of the octagonal nanodisk.
four {100} and four {110} lattice planes respectively. The octagonal and hexagonal nanodisks with the large top/bottom surfaces are favorable for the improvement of the performance in display and photovoltaic devices [10]. Formation of the Cu9BiS6 nanodisks has close relation to the PEG, because organic surfactants can act as structure-directing agents or “soft templates” and have been widely used to prepare nanostructured materials with peculiar morphologies [9,11]. Without addition of PEG the prepared Cu9BiS6 nanocrystals are mainly dominated by the microspheres gathered by irregular sheets (to be reported elsewhere). Because the formation of faceted nanocrystals is related to selective adsorption of ions and small nanoparticles on the crystal faces during the growth [12], a possible explanation of the Cu9BiS6 nanoplates is that the PEG long chain molecules have effect on fixing the primary Cu9BiS6 nuclei seeds and initializing the selective coalescence of the adjacent nuclei seeds, leading to the crystal growth with the preferred facets. The band gap of the Cu9BiS6 nanoplates has been estimated from the generated reflectance-versus-wavelength data via converting reflectance to absorption data according to the Kubelka–Munk equation as follows, α/S = (1 − R)2 / (2R), where R is the reflectance and α and S are the absorption and scattering coefficients, respectively. By extrapolating the linear region of a plot (ahv)2 against photon energy, optical diffuse reflectance measurement reveals the presence of optical band gap at 1.25 eV as shown in Fig. 4. Semiconductors with a band gap ranging from 0.9 eV to1.9 eV are suitable for use as the solar absorber layer in thin film photovoltaic devices [4]. Therefore, the as-synthesized Cu9BiS6 nanoplates may
have potential application in the photoelectric converter. The photovoltaic property of the Cu9BiS6 nanoplates is under study. 4. Conclusion In summary, we have presented the synthesis of the single crystal Cu9BiS6 nanoplates via a simple hydrothermal route. The as-prepared Cu9BiS6 nanoplates are high phase-purity in crystal structure and the octagonal and hexagonal nanodisks in morphology. The prepared Cu9BiS6 nanoplates with an optical band gap 1.25 eV may be applied as the solar absorber layer in thin film photovoltaic devices. The
Fig. 4. The optical band gap of Cu9BiS6 nanoplates.
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hydrothermal route employed in this study could be used for synthesis of other low dimensional ternary chalcogenides with distinct morphology. Acknowledgements The authors are grateful to the NSF of China (Grant Nos. 60796078, 10774174, and 20903038), “973” National Key Basic Research program of China (Grant Nos. 2007CB310500 and 2007CB936301) and the Opening Project of Key Laboratory of Low Dimensional Quantum Structures and Quantum Control (Hunan Normal University), Ministry of Education (No. QSQC0912) for the financial support.
References [1] Connor ST, Hsu CM, Weil B, Aloni DS, Cui Y. J Am Chem Soc 2009;131(13). [2] Ramakrishna Reddy KT, Reddy PJ. Mater Lett 1990;10(6):275–9. [3] George MW. Mineral Commodity Summaries, U.S. Geological Survey Report. Washington, DC: U.S. Government Printing Office; 2005. [4] Gerein NJ, Haber JA. Chem Mater 2006;18:6297–302. [5] Estrella VO, Nair MTS, Nair PK. Semicond Sci Technol 2003;18:190–4. [6] Kryukova G, Heuer M, Doering Th, Bente K. J Crystal Growth 2007;306:212–6. [7] Razmara MF, Henderson CMB, Pattrick RAD. J Mater Res 1997;61:79–88. [8] Torneoka K. Am Mineral 1982;67:360–72. [9] Huynh WU, Dittmer JJ, Alivisatos AP. Science 2002;295:2425–7. [10] Gou XL, Cheng FY, Shi YH, Zhang L, Peng SJ, Chen J, et al. J Am Chem Soc 2006;128: 7222–9. [11] Wang WZ, Poudel B, Yang J, Wang DZ, Ren ZF. J Am Chem Soc 2005;127:13792–3. [12] Pileni MP. Nature Mater 2003;2:145–50.