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Hot-injection synthesis and characterization of quaternary Cu2ZnSnSe4 nanocrystals
Materials Letters 64 (2010) 1424–1426
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 e v i e r. c o m / l o c a t e / m a t l e t
Hot-injection synthesis and characterization of quaternary Cu2ZnSnSe4 nanocrystals Hao Wei a,⁎, Wei Guo b, Yijing Sun b, Zhi Yang a, Yafei Zhang a,⁎ a National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication Technology of Ministry of Education, Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China b School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 8 January 2010 Accepted 16 March 2010 Available online 19 March 2010 Keywords: Cu2ZnSnSe4 Hot-injection Solar energy materials Nanomaterials Semiconductors
a b s t r a c t Cu2ZnSnSe4 (CZTSe) is one of promising materials in the use of absorber layers of solar cells. It contains earth-abundant elements of zinc and tin, a near-optimal direct band gap of ∼ 1.5 eV, as well as a large absorption coefficient ∼ 104 cm-1. The CZTSe nanocrystals in oleylamine (OLA) was successfully prepared via hot-injection method. The characterization of its structure, composition, morphology and absorption spectra were done using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV–vis absorption spectra. The results revealed that the monodispersed nanocrystals were single phase polycrystalline within the range of 15–20 nm. Optical measurements showed a direct band gap of 1.52 eV, which was optimal for low cost solar cells. The capping property of OLA was also demonstrated by examining Fourier Transform Infrared Spectroscopy (FTIR) feature peaks of CZTSe and OLA, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cu2-II–IV–VI4 quaternary semiconductors such as Cu2ZnSnSe4 (CZTSe), Cu2ZnSnS4 (CZTS), and Cu2ZnGeSe4 have been extensively investigated as effective light-absorbing materials in solar cells recently. As one of this system, CZTSe proves itself to be one of the most interesting materials for application in solar cell owing to its optical band gap of 1.4–1.56 eV and large absorption coefficient over 104 cm− 1 [1,2]. In contrast to well known solar cell materials such as CdTe and Cu2InGaSe4 (CIGS) which employs expensive indium (In) and gallium (Ga), constituent elements Zn and Sn in CZTSe are relatively abundant and cheap [3,4]. There are a few reports on preparation of CZTSe absorber material by sputtering and thermal evaporation techniques using costly, high temperature and low-throughput fabricating techniques [1–8]. The highest conversion efficiency of 3.2% was yielded in CZTSe-based solar cells by magnetron sputtering method [3]. There is a great demand for the development of low-temperature deposition techniques of thin film solar cells. A new preparation method for CZTSe solar cells without high temperature and pressure is to chemically synthesize CZTSe nanocrystals with controlled stoichiometry and disperse them in an “ink”. This solution-based approach will alleviate the need for high-temperature annealing in selenium atmosphere which may cause selenium loss. In particular, photovoltaic devices based on
⁎ Corresponding authors. Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel./fax: + 86 21 3420 5665. E-mail addresses: [email protected] (H. Wei), [email protected] (Y. Zhang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.03.034
nanocrystals ink have recently been reported, including CuInS2, CuInSe2, CIGS, and CZTS [9–16]. But to the best of our knowledge, synthesis of CZTSe nanocrystals has not been reported previously. Herein, we report the synthesis of CZTSe nanocrystals in oleylamine (OLA) using hotinjection method. Besides, its structure, composition, morphology and absorption spectra were also characterized extensively. 2. Experimental 2.1. Synthesis of CZTSe nanocrystals All of the chemical reagents and solvents were used without further purification. 6 mmol selenium (Se, 99.98%) and 15 mL OLA (N70%) were added to a 100 mL two-neck flask followed with vacuum pumping and N2 bubbling. The mixture was kept at 150 °C for 1 h; 3 mmol copper acetate dehydrate (Cu(OAc)2 , analytic-grade), 1.5 mmol zinc chloride dehydrate (ZnCl2, analytic-grade), 1.5 mmol tin (II) chloride dihydrate (SnCl2·2H2O, analytic-grade) and 15 mL OLA were mixed in another 100 mL two-neck flask under N2 atmosphere. When the mixture was heated to 100 °C, it was rapidly injected into selenium solution. Temperature was raised to 240 °C and kept for 2 h under vigorous stirring. After the mixture was cooled to room temperature, 20 mL ethanol was added to the flask, treated by ultrasonic dispersion for 30 min, and then centrifugated at 8000 rpm for 10 min. The supernatant solution was discarded and the precipitate was redispersed in 15 mL tetrachloroethylene (TCE). After centrifugated at 7000 rpm for 5 min, the supernatant was mixed with 0.6 mL OLA and 15 mL ethanol, then centrifugated at 8000 rpm for 10 min. The precipitate was redispersed in 15 mL ethanol and
H. Wei et al. / Materials Letters 64 (2010) 1424–1426
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centrifugated again to yield a high-purity CZTSe nanocrystals. The CZTSe nanocrystals could be dispersed in non-polar organic solvents, including chloroform, toluene, and TCE.
2.2. Structural and optical characterization of CZTSe nanocrystals The crystal structure of CZTSe nanocrystals was characterized by powder X-ray diffraction (XRD) using Cu Kα radiation, λ = 1.54 Å. An Oxford INCA energy-dispersive X-ray spectroscopy (EDS) detector was used to analyse element composition and proportion. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were taken using a JEM 2100 microscope at 200 kV accelerating voltage. The function of OLA as a surfactant in CZTSe nanocrystal fabrication was revealed by Fourier Transform Infrared Spectroscopy (FTIR) observation. UV–vis absorption spectra were carried out to evaluate the optical properties of CZTSe nanocrystals by using a Lambda 20 UV–vis spectrometer.
3. Results and discussion CZTSe nanocrystals were synthesized by hot-injection method. The synthesis process is as follows: 2CuðOAcÞ2 + ZnCl2 + SnCl2 ⋅ 2H2 O + OLA 2404Se → Cu2 ZnSnSe4 nanocrystals + side products: The injection of metal precursors to selenium solution will lead to rapid nucleation and growth of CZTSe nanocrystals, while one-pot method will lead to poor monodispersion property. Fig. 1 shows the XRD pattern of the as-synthesized nanocrystals which are composed of tetragonal CZTSe nanocrystals (JCPDS 52-0868). We can identify three major peaks attribute to (112), (204), and (312). The crystal domain size estimated from full width at half maximum (FWHM) of the (112) peak by the Scherrer equation is 16.7 nm. The average composition of the nanocrystals determined by EDS analysis is Cu2Zn0.84Sn1.24Se4.08, considering the error of the EDS detector (approximately ±2 at.%). The slightly Zn poor and Sn rich composition deviated from stoichiometric CZTSe may be due to different reactivities of metal precursors. Upon calculation, the interplanar spacing of CZTSe (112) plane is 0.327 nm, larger than ideal distance of CZTS (112) plane (0.3126 nm, JCPDS 52-0868). This shift is due to the crystal volume expansion with the replacement of S by Se. Fig. 2 shows the TEM and SAED images of as-synthesized CZTSe nanocrystals dispersed in chloroform. The average size of CZTSe nanocrystals is 17±2 nm, which corresponds well with the particle size calculated from XRD. The morphology of CZTSe nanocrystals is a mixture of triangular, hexagonal, and plate like nanocrystals. SAED reveals its polycrystalline nature indicated by the presence of diffraction spots of (112), (204), and (312) planes. OLA plays a key role in fabrication of CZTSe nanocrystals. Its function as a surfactant is demonstrated by FTIR spectra of both as-synthesized CZTSe nanocrystals and pure OLA. The presence of OLA group on CZTSe nanocrystals is indicated by NH2 bending modes at 909, 964 and 993 cm− 1, C–H bending mode at 1468 cm− 1, NH2 scissor mode at 1568 cm− 1 and C–H stretching mode at 2865, 2921 and 2955 cm− 1. It suggests that oleylamine ligands remain intact, capping CZTSe nanocrystals. The feature peaks of as-synthesized CZTSe nanocrystals coincide with OLA in Fig. 3, indicating the existence of OLA molecules as a surfactant in CZTSe nanocrystals. Removal of these OLA molecules is necessary because it will impede electricity conductivity in solar cell devices. Fig. 4 shows UV–vis absorption spectrum of absorption coefficient (α) versus wavelength (λ). The optical band gap is determined by extrapolating straight line of the plot (αhν)2 versus hν as shown in the inset of Fig. 4. The observed band gap of 1.52 eV corresponds well with that reported in the literature of 1.4–1.56 eV [1–7]. This observation also eliminates existence of secondary phase of ZnSe and Cu2SnSe3, whose band gaps are 2.82 and 0.84 eV, respectively [17].
Fig. 1. (a) Powder XRD patterns and (b) elemental composition measured by EDS of CZTSe nanocrystals.
4. Conclusions In this study, we have reported for the first time that monodisperse CZTSe nanocrystals have been synthesized in solution through hotinjection method. XRD, EDS, TEM, and SAED reveal their structure, composition and morphology. UV–vis data indicates CZTSe nanocrystals
Fig. 2. (a) TEM images of CZTSe nanocrystals dispersed in chloroform with (b) HRTEM image and (c) SAED pattern.
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have an optical band gap of 1.52 eV, which proves itself to be a promising candidate in future fabrication of cost-effective, high efficiency solar cells. It has also been demonstrated that CZTSe nanocrystals are passivated and protected by surfactant OLA. Acknowledgements This work is supported by the NSFC (Grant no. 50730008 and 50902092), Shanghai Science and Technology Grant No: 09JC1407400 and Shanghai-Applied Materials Collaborative Research Program No: 09520714400. References
Fig. 3. FTIR spectra of (a) pure OLA and (b) as-synthesized CZTSe nanocrystals.
Fig. 4. UV–vis absorption spectrum of CZTSe nanocrystals. The inset image shows an obtained band gap of 1.52 eV.
[1] Babu GS, Kumar YBK, Bhaskar PU, Raja VS. Semicond Sci Tech 2008;23:85023. [2] Wibowo RA, Lee ES, Munir B, Kim KH. Phys Status Solidi A Appl Mater 2007;204: 3373. [3] Zoppi G, Forbes I, Miles RW, Dale PJ, Scragg JJ, Peter LM. Prog Photovoltaics: Res Appl 2009;17:315. [4] Babu GS, Kumar YBK, Bhaskar PU, Raja VS. J Phys D Appl Phys 2008;41:205305. [5] Hahn H, Schulze H. Die Naturwissenschaften 1965;52:426. [6] Matsushita H, Maeda T, Katsui A, Takizava T. J Cryst Growth 2000;208:416. [7] Ito K, Nakazawa T. Jpn J Appl Phys, Part 1 1988;27:2094. [8] Long F, Wang WM, Tao HC, Jia TK, Li XM, Zou ZG, Fu ZY. Mater Lett 2010;64:195. [9] Guo Q, Hillhouse HW, Agrawal R. J Am Chem Soc 2009;131:11672. [10] Dutta PD, Sharma GA. Mater Lett 2006;60:2395. [11] Panthani MG, Akhavan V, Goodfellow B, Schmidtke JP, Dunn L, Dodabalapur A, Barbara PF, Korgel BA. J Am Chem Soc 2008;49:16770. [12] Tang J, Hinds S, Kelley SO, Sargent EH. Chem Mater 2008;20:6906. [13] Riha SC, Parkinson BA, Prieto AL. J Am Chem Soc 2009;131:12054. [14] Guo Q, Kim SJ, Kar M, Shafarman WN, Birkmire RW, Stach EA, Agrawal R, Hillhouse HW. Nano Lett 2008;8:2982. [15] Katagiri H, Jimbo K, Yamada S, Kamimura T, Maw WS, Fukano T, Ito T, Motohiro T. Appl Phys Express 2008;1:0412011. [16] Steinhagen C, Panthani MG, Akhavan V, Goodfellow B, Koo B, Korgel BA. J Am Chem Soc 2009;131:12554. [17] Marcano G, Rincón C, De Chalbaud LM, Bracho DB, Sánchez PG. J Appl Phys 2001;90:1847.
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 e v i e r. c o m / l o c a t e / m a t l e t
Hot-injection synthesis and characterization of quaternary Cu2ZnSnSe4 nanocrystals Hao Wei a,⁎, Wei Guo b, Yijing Sun b, Zhi Yang a, Yafei Zhang a,⁎ a National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication Technology of Ministry of Education, Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China b School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 8 January 2010 Accepted 16 March 2010 Available online 19 March 2010 Keywords: Cu2ZnSnSe4 Hot-injection Solar energy materials Nanomaterials Semiconductors
a b s t r a c t Cu2ZnSnSe4 (CZTSe) is one of promising materials in the use of absorber layers of solar cells. It contains earth-abundant elements of zinc and tin, a near-optimal direct band gap of ∼ 1.5 eV, as well as a large absorption coefficient ∼ 104 cm-1. The CZTSe nanocrystals in oleylamine (OLA) was successfully prepared via hot-injection method. The characterization of its structure, composition, morphology and absorption spectra were done using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV–vis absorption spectra. The results revealed that the monodispersed nanocrystals were single phase polycrystalline within the range of 15–20 nm. Optical measurements showed a direct band gap of 1.52 eV, which was optimal for low cost solar cells. The capping property of OLA was also demonstrated by examining Fourier Transform Infrared Spectroscopy (FTIR) feature peaks of CZTSe and OLA, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Cu2-II–IV–VI4 quaternary semiconductors such as Cu2ZnSnSe4 (CZTSe), Cu2ZnSnS4 (CZTS), and Cu2ZnGeSe4 have been extensively investigated as effective light-absorbing materials in solar cells recently. As one of this system, CZTSe proves itself to be one of the most interesting materials for application in solar cell owing to its optical band gap of 1.4–1.56 eV and large absorption coefficient over 104 cm− 1 [1,2]. In contrast to well known solar cell materials such as CdTe and Cu2InGaSe4 (CIGS) which employs expensive indium (In) and gallium (Ga), constituent elements Zn and Sn in CZTSe are relatively abundant and cheap [3,4]. There are a few reports on preparation of CZTSe absorber material by sputtering and thermal evaporation techniques using costly, high temperature and low-throughput fabricating techniques [1–8]. The highest conversion efficiency of 3.2% was yielded in CZTSe-based solar cells by magnetron sputtering method [3]. There is a great demand for the development of low-temperature deposition techniques of thin film solar cells. A new preparation method for CZTSe solar cells without high temperature and pressure is to chemically synthesize CZTSe nanocrystals with controlled stoichiometry and disperse them in an “ink”. This solution-based approach will alleviate the need for high-temperature annealing in selenium atmosphere which may cause selenium loss. In particular, photovoltaic devices based on
⁎ Corresponding authors. Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel./fax: + 86 21 3420 5665. E-mail addresses: [email protected] (H. Wei), [email protected] (Y. Zhang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.03.034
nanocrystals ink have recently been reported, including CuInS2, CuInSe2, CIGS, and CZTS [9–16]. But to the best of our knowledge, synthesis of CZTSe nanocrystals has not been reported previously. Herein, we report the synthesis of CZTSe nanocrystals in oleylamine (OLA) using hotinjection method. Besides, its structure, composition, morphology and absorption spectra were also characterized extensively. 2. Experimental 2.1. Synthesis of CZTSe nanocrystals All of the chemical reagents and solvents were used without further purification. 6 mmol selenium (Se, 99.98%) and 15 mL OLA (N70%) were added to a 100 mL two-neck flask followed with vacuum pumping and N2 bubbling. The mixture was kept at 150 °C for 1 h; 3 mmol copper acetate dehydrate (Cu(OAc)2 , analytic-grade), 1.5 mmol zinc chloride dehydrate (ZnCl2, analytic-grade), 1.5 mmol tin (II) chloride dihydrate (SnCl2·2H2O, analytic-grade) and 15 mL OLA were mixed in another 100 mL two-neck flask under N2 atmosphere. When the mixture was heated to 100 °C, it was rapidly injected into selenium solution. Temperature was raised to 240 °C and kept for 2 h under vigorous stirring. After the mixture was cooled to room temperature, 20 mL ethanol was added to the flask, treated by ultrasonic dispersion for 30 min, and then centrifugated at 8000 rpm for 10 min. The supernatant solution was discarded and the precipitate was redispersed in 15 mL tetrachloroethylene (TCE). After centrifugated at 7000 rpm for 5 min, the supernatant was mixed with 0.6 mL OLA and 15 mL ethanol, then centrifugated at 8000 rpm for 10 min. The precipitate was redispersed in 15 mL ethanol and
H. Wei et al. / Materials Letters 64 (2010) 1424–1426
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centrifugated again to yield a high-purity CZTSe nanocrystals. The CZTSe nanocrystals could be dispersed in non-polar organic solvents, including chloroform, toluene, and TCE.
2.2. Structural and optical characterization of CZTSe nanocrystals The crystal structure of CZTSe nanocrystals was characterized by powder X-ray diffraction (XRD) using Cu Kα radiation, λ = 1.54 Å. An Oxford INCA energy-dispersive X-ray spectroscopy (EDS) detector was used to analyse element composition and proportion. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were taken using a JEM 2100 microscope at 200 kV accelerating voltage. The function of OLA as a surfactant in CZTSe nanocrystal fabrication was revealed by Fourier Transform Infrared Spectroscopy (FTIR) observation. UV–vis absorption spectra were carried out to evaluate the optical properties of CZTSe nanocrystals by using a Lambda 20 UV–vis spectrometer.
3. Results and discussion CZTSe nanocrystals were synthesized by hot-injection method. The synthesis process is as follows: 2CuðOAcÞ2 + ZnCl2 + SnCl2 ⋅ 2H2 O + OLA 2404Se → Cu2 ZnSnSe4 nanocrystals + side products: The injection of metal precursors to selenium solution will lead to rapid nucleation and growth of CZTSe nanocrystals, while one-pot method will lead to poor monodispersion property. Fig. 1 shows the XRD pattern of the as-synthesized nanocrystals which are composed of tetragonal CZTSe nanocrystals (JCPDS 52-0868). We can identify three major peaks attribute to (112), (204), and (312). The crystal domain size estimated from full width at half maximum (FWHM) of the (112) peak by the Scherrer equation is 16.7 nm. The average composition of the nanocrystals determined by EDS analysis is Cu2Zn0.84Sn1.24Se4.08, considering the error of the EDS detector (approximately ±2 at.%). The slightly Zn poor and Sn rich composition deviated from stoichiometric CZTSe may be due to different reactivities of metal precursors. Upon calculation, the interplanar spacing of CZTSe (112) plane is 0.327 nm, larger than ideal distance of CZTS (112) plane (0.3126 nm, JCPDS 52-0868). This shift is due to the crystal volume expansion with the replacement of S by Se. Fig. 2 shows the TEM and SAED images of as-synthesized CZTSe nanocrystals dispersed in chloroform. The average size of CZTSe nanocrystals is 17±2 nm, which corresponds well with the particle size calculated from XRD. The morphology of CZTSe nanocrystals is a mixture of triangular, hexagonal, and plate like nanocrystals. SAED reveals its polycrystalline nature indicated by the presence of diffraction spots of (112), (204), and (312) planes. OLA plays a key role in fabrication of CZTSe nanocrystals. Its function as a surfactant is demonstrated by FTIR spectra of both as-synthesized CZTSe nanocrystals and pure OLA. The presence of OLA group on CZTSe nanocrystals is indicated by NH2 bending modes at 909, 964 and 993 cm− 1, C–H bending mode at 1468 cm− 1, NH2 scissor mode at 1568 cm− 1 and C–H stretching mode at 2865, 2921 and 2955 cm− 1. It suggests that oleylamine ligands remain intact, capping CZTSe nanocrystals. The feature peaks of as-synthesized CZTSe nanocrystals coincide with OLA in Fig. 3, indicating the existence of OLA molecules as a surfactant in CZTSe nanocrystals. Removal of these OLA molecules is necessary because it will impede electricity conductivity in solar cell devices. Fig. 4 shows UV–vis absorption spectrum of absorption coefficient (α) versus wavelength (λ). The optical band gap is determined by extrapolating straight line of the plot (αhν)2 versus hν as shown in the inset of Fig. 4. The observed band gap of 1.52 eV corresponds well with that reported in the literature of 1.4–1.56 eV [1–7]. This observation also eliminates existence of secondary phase of ZnSe and Cu2SnSe3, whose band gaps are 2.82 and 0.84 eV, respectively [17].
Fig. 1. (a) Powder XRD patterns and (b) elemental composition measured by EDS of CZTSe nanocrystals.
4. Conclusions In this study, we have reported for the first time that monodisperse CZTSe nanocrystals have been synthesized in solution through hotinjection method. XRD, EDS, TEM, and SAED reveal their structure, composition and morphology. UV–vis data indicates CZTSe nanocrystals
Fig. 2. (a) TEM images of CZTSe nanocrystals dispersed in chloroform with (b) HRTEM image and (c) SAED pattern.
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have an optical band gap of 1.52 eV, which proves itself to be a promising candidate in future fabrication of cost-effective, high efficiency solar cells. It has also been demonstrated that CZTSe nanocrystals are passivated and protected by surfactant OLA. Acknowledgements This work is supported by the NSFC (Grant no. 50730008 and 50902092), Shanghai Science and Technology Grant No: 09JC1407400 and Shanghai-Applied Materials Collaborative Research Program No: 09520714400. References
Fig. 3. FTIR spectra of (a) pure OLA and (b) as-synthesized CZTSe nanocrystals.
Fig. 4. UV–vis absorption spectrum of CZTSe nanocrystals. The inset image shows an obtained band gap of 1.52 eV.
[1] Babu GS, Kumar YBK, Bhaskar PU, Raja VS. Semicond Sci Tech 2008;23:85023. [2] Wibowo RA, Lee ES, Munir B, Kim KH. Phys Status Solidi A Appl Mater 2007;204: 3373. [3] Zoppi G, Forbes I, Miles RW, Dale PJ, Scragg JJ, Peter LM. Prog Photovoltaics: Res Appl 2009;17:315. [4] Babu GS, Kumar YBK, Bhaskar PU, Raja VS. J Phys D Appl Phys 2008;41:205305. [5] Hahn H, Schulze H. Die Naturwissenschaften 1965;52:426. [6] Matsushita H, Maeda T, Katsui A, Takizava T. J Cryst Growth 2000;208:416. [7] Ito K, Nakazawa T. Jpn J Appl Phys, Part 1 1988;27:2094. [8] Long F, Wang WM, Tao HC, Jia TK, Li XM, Zou ZG, Fu ZY. Mater Lett 2010;64:195. [9] Guo Q, Hillhouse HW, Agrawal R. J Am Chem Soc 2009;131:11672. [10] Dutta PD, Sharma GA. Mater Lett 2006;60:2395. [11] Panthani MG, Akhavan V, Goodfellow B, Schmidtke JP, Dunn L, Dodabalapur A, Barbara PF, Korgel BA. J Am Chem Soc 2008;49:16770. [12] Tang J, Hinds S, Kelley SO, Sargent EH. Chem Mater 2008;20:6906. [13] Riha SC, Parkinson BA, Prieto AL. J Am Chem Soc 2009;131:12054. [14] Guo Q, Kim SJ, Kar M, Shafarman WN, Birkmire RW, Stach EA, Agrawal R, Hillhouse HW. Nano Lett 2008;8:2982. [15] Katagiri H, Jimbo K, Yamada S, Kamimura T, Maw WS, Fukano T, Ito T, Motohiro T. Appl Phys Express 2008;1:0412011. [16] Steinhagen C, Panthani MG, Akhavan V, Goodfellow B, Koo B, Korgel BA. J Am Chem Soc 2009;131:12554. [17] Marcano G, Rincón C, De Chalbaud LM, Bracho DB, Sánchez PG. J Appl Phys 2001;90:1847.