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Preparation of CuI particles and their applications in carbon nanotube-Si heterojunction solar cells
Materials Letters 79 (2012) 106–108
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of CuI particles and their applications in carbon nanotube-Si heterojunction solar cells Hongguang Wang, Xi Bai, Jinquan Wei ⁎, Peixu Li, Yi Jia, Hongwei Zhu, Kunlin Wang, Dehai Wu Key Lab for Advanced Materials Processing Technology of Education Ministry, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 4 February 2012 Accepted 31 March 2012 Available online 9 April 2012 Keywords: Carbon nanotubes Particles Solar energy materials
a b s t r a c t Copper iodide (CuI) has good electrical conductivity and is usually used as hole-injector, collector and transporter in solar cells. CuI particles are prepared on silicon wafer by metal-assisted etching method in Cu(NO3)2/HF solution and then iodizing in I2/ethanol solution. The size and density of the CuI particles are well controlled by tuning the etching time in Cu(NO3)2/HF solution. It forms a CNT–CuI–Si solar cell when a carbon nanotube (CNT) film covers on the Si substrate modified by CuI particles. The efficiency of the CNT–Si heterojunction cell modified by CuI particles reaches 6% in the initial test. The quantum efficiency of the CNT–Si cell with CuI modification is higher than that without CuI modification, which shows that CuI particles can enhance hole-collecting and transporting ability in the CNT-Si heterojunction solar cell. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Copper (I) iodide (CuI), a p-type semiconductor with good electrical conductivity, is likely to show promising applications in light emitting diodes and solar cells [1,2]. Crystalline CuI has a large band gap of 3.1 eV, which was usually used as buffer layer and holeinjection layer in solar cells [2,3]. Recently, CuI was used to construct fully solid-state dye sensitized solar cell [3,4]. It was reported that ptype CuI can form high photovoltage heterojunction solar cell with ntype Si, where CuI layer were used as hole-injectors, collectors and transporters [5]. We also substituted carbon nanotube (CNT) film for ITO (indium tin oxide) as transparent electrodes in the CuI–Si solar cell [6]. The efficiency of the CuI–Si solar cell reaches to 6%– 10% with CNTs as transparent electrodes and acid doping. The condensed CuI layer was deposited onto Si wafer by vacuum evaporating the high purity CuI powder, which is an expensive and energy intensive process. On the other hand, CNTs have great potential applications in solar cells because of its flexible structure and unique properties, such as ultra-high light absorption ability and high electrical conductivity [7,8]. In fact, CNTs have been introduced into many kinds of solar cells, where CNT acts as hole-collector and electrode [9,10]. Recently, we developed a novel heterojunction solar cell between the flexible CNT film and n-type Si, which shows a moderate efficiency of 5%– 7% [11]. The efficiency of the CNT–Si solar cell depends on the quality of heterojunction [12].
Fig. 1a showed a schematic diagram of CuI modification on Si wafer and fabrication of CNT–CuI–Si solar cells. Typically, a n-type Si wafer (4 in., 2–4 Ω·cm) with a thermal oxide layer (300 nm in thickness) was patterned by photolithography and wet-etching, using a mask containing square-shaped windows of 3 mm×3 mm. Au/Ti layer (thickness of 100 nm) was deposited on the back side of the Si wafer. Copper particles were doped on the Si wafer by wet etching in 0.1 M Cu(NO3)2/0.2 M HF solution. The Si wafer was then immersed in 1 mM I2/ethanol solution for more than 10 min to completely convert the Cu into CuI particles. The Si surface was slightly oxidized after iodizing, and then washed in diluted HF before assembling the solar cells. Macroscopic CNT films used in the experiments were prepared by floating chemical vapor deposition method and then treated in H2O2 and HCl solution to remove the catalyst particles [13]. The fabrication of CNT–CuI–Si solar cells was similar to our recent report [11]. The morphology and structure of CuI particles were characterized and evaluated by scanning electron microscopy (SEM) (LEO-1530) and X-ray diffraction (XRD) (D/max 2500). The current versus voltage (I–V) characteristics of the solar cells were measured with a Keithley 2601 sourcemeter. A solar simulator (K Newport 91195 class A, AM 1.5 G, 100 mW cm− 2) was used as the light source in I–V test. The quantum efficiency (QE) of the solar cells was tested by using a Qtest Station 2000. 3. Results and discussion
⁎ Corresponding author. Fax: + 86 10 62770190. E-mail address: [email protected] (J. Wei). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.114
The deposition of Cu particles to the Si wafer was actually a metalassisted etching process, where Si was etched into Si 4+ and Cu 2+ was reduced into Cu particles (see Eq. (1)). The metal-assisted etching
H. Wang et al. / Materials Letters 79 (2012) 106–108
107
Fig. 1. (a) The schematic diagram of fabricating CNT–CuI–Si solar cell. (b) SEM image of Cu particles, (c) SEM image of CuI particles, (d) SEM image of the CNT-CuI-Si solar cell.
was reported to prepare Si nanowire array in AgNO3/HF solution [14]. Fig. 1b showed a SEM image of the Si wafer after immersing in Cu(NO3)2/HF solution for 30 s. The size of Cu particle varied from 50 nm to 300 nm. Eq. (2) depicted the conversion of Cu to CuI in iodine/ethanol solution. Fig. 1c showed a SEM image of Si wafer with CuI particles. The size of CuI particle was ~ 500 nm. The shape of CuI was different from that of Cu particles. It needed to point out that Cu and CuI particles were only observed on the Si surface, not on the SiO2 layer. Fig. 1d showed a SEM image of the CuI particles on Si covered by CNT film. The CNT bundles contacted not only to the Si wafer, but also to the CuI particles. After etching in the Cu(NO3)2/HF solution, there were lots of etching dents on the Si surface. Si þ 4HF þ 2Cu
2þ
2Cu þ I2 →2CuI
þ
→SiF4 þ 4H þ 2Cu
ð1Þ ð2Þ
In the experiments, the size and coverage rate of the CuI particles depended mainly on the etching conditions of Si wafer in the Cu(NO3)2/HF solution. Fig. 2a and 2b showed two SEM images of the CuI particles obtained by immersing Si wafer in Cu(NO3)2/HF solution for 20 s and 50 s, respectively. Large CuI clusters, composing several grains, were observed in the samples with etching time of 50 s
(Fig. 2b). Fig. 2c depicted the dependence of mean size and coverage rate of CuI particles on the etching time in Cu(NO3)2/HF solution. It is clearly shown that both of the size and coverage rate CuI particles on the Si wafer increase almost linearly with the etching time. When the etching time extends from 30 s to 50 s, the mean size of CuI particles increase from 250 nm to 400 nm, and the coverage rate increases from 40% to 70%, respectively. Fig. 2d showed the XRD patterns of the Si substrate with and without CuI modification. The strong peak centered at 2θ = 69.1° derived from (100) crystal plane of Si substrate. The structure of CuI was identified on the basis of the three distinguishable diffraction peaks at 2θ = 25.5°, 42.3° and 49.9°, corresponding to the {111}, {220} and {311} crystal plane of the cubic CuI (JCPDS card 06-0246), respectively. No signal of Cu was detected in the XRD pattern, indicating that the copper nanoparticles had converted to CuI completely after reaction in iodine/ethanol solution. It formed a CNT–CuI–Si solar cell when the CNT film covered on n-Si substrate modified by CuI. For comparison, we also fabricated CNT–Si solar cells in the same batch on Si substrate without CuI modification. Fig. 3a showed the light I–V curves of a CNT–CuI–Si solar cell as well as a CNT–Si solar cell under one solar equivalent (100 mW/cm2) illustration. It showed a short-circuit current density (Jsc) of 20.6 mA/cm2, an open-circuit voltage (Voc) of 0.5 V, a fill factor (FF) of 58.4% and a power conversion efficiency (η) of 6.0% for the CNT–CuI–Si solar cell.
Fig. 2. SEM images of the CuI particle obtained by etching in Cu(NO3)2/HF solution for (a) 20 s and (b) 50 s. (c) Dependence of coverage rate and particle size of CuI particles on the etching time in Cu(NO3)2/HF solution. (d) XRD patterns of Si and CuI–Si.
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H. Wang et al. / Materials Letters 79 (2012) 106–108
Fig. 3. Photovoltaic properties of the CNT–Si and CNT–CuI–Si solar cells. (a) Light I–V curves, (b) quantum efficiency spectra, (c) differential coefficient spectra. (d) Light I–V curves of the CNT–CuI–Si solar cells fabricated by varying the etching time.
The efficiency of the CNT–Si solar cell was 4.8%, with Jsc = 18.5 mA/cm2, Voc = 0.48 V and FF= 53.9%, which was in the same level of our recent report [11]. It showed that the efficiency of CNT–Si cell enhanced about 20% after modifying by CuI particles at the Si surface, which resulted mainly from the high ability of collecting and transporting holes of CuI particles [5]. Quantum efficiency (QE) curves of the CNT–Si and CNT–CuI–Si cells were shown in Fig. 3b. The QE of the CNT–Si with CuI modification (CNT–CuI–Si) was higher than that without CuI modification in the whole spectra, indicating the higher ability of conversion of incident photon to charge of the CNT–CuI–Si cell than that of the CNT–Si cell. The differential quantum efficiency curves of the CNT–Si cells with and without CuI modification were shown in Fig. 3c. It shows that the two differential quantum efficiency curves had the same peak centered at 1.2 eV. The work function of the CNTs was about 4.8 eV [15], which equaled to the Fermi level of the cubic CuI (Eg =3.1 eV and Ev =5.2 eV, EF =4.8 eV). The band gap of the CNT–Si cell coincidentally equaled to that of the CNT–CuI–Si. Thus the two differential quantum efficiency curves had the same peak position. Fig. 3d showed the light I–V curves of the CNT–CuI–Si solar cells fabricated by varying the etching time. It was clearly shown that the performance of the CNT–CuI–Si solar cells depended on the CuI particle size and coverage rate significantly. It showed that the Jsc of the solar cells was sensitive to the etching time, but the Voc was almost maintained at ~ 0.5 V. At high coverage rate of 70% (etching time 50 s), the Jsc of solar cell was 4 mA/cm 2, which was only 1/5 to that of the solar cells with coverage rate lower than 40% (etching time b30 s). At high coverage rate, some of the CuI particles do not contact Si surface, thus the hole collecting capacity of CuI layer declines significantly. 4. Conclusions CuI particles are prepared on the Si wafer by etching in Cu(NO3)2/HF and reacting in I2/ethanol solution. The size and density of CuI particles
are well controlled by tuning the etching time. The efficiency of CNT–Si solar cells is improved by suitable CuI modification. The efficiency of the CNT–Si solar cells modified with CuI particles reaches to 6.0%.
Acknowledgments This work is supported by National Natural Science Foundation of China (51172122), Foundation for the Author of National Excellent Doctoral Dissertation (2007B37), and Program for New Century Excellent Talents in University. The author also acknowledged Dr. Liu Jiang in Tsinghua for helpful discussion.
References [1] Liu ZW, Qayyum MF, Wu C, Whited MT, Djurovich PI, Hodgson KO, et al. J Am Chem Soc 2011;133:3700–3. [2] Stakhira P, Cherpak V, Volynyuk D, Ivastchyshyn F, Hotra Z, Tataryn V, et al. Thin Solid Films 2010;518:7016–8. [3] Kumara GRA, Konno A, Shiratsuchi K, Tsukahara J, Tennakone K. Chem Mater 2002;14:954–5. [4] Meng QB, Takahashi K, Zhang XT, Sutanto I, Rao TN, Sato O, et al. Langmuir 2003;19:3572–4. [5] Iimori H, Yamane S, Kitamura T, Murakoshi K, Imanishi A, Nakato Y. J Phys Chem C 2008;112:11586–90. [6] Li PX, Wang SS, Jia Y, Li Z, Ji CY, Zhang LH, et al. Nano Res 2011;4:979–86. [7] Yang ZP, Ci LJ, Bur JA, Lin SY, Ajayan PM. Nano Lett 2008;8:446–51. [8] Dürkop T, Getty SA, Cobas E, Fuhrer MS. Nano Lett 2004;4:35–9. [9] Kymakis E, Amaratunga GAJ. Appl Phys Lett 2002;80:112–4. [10] Pasquier AD, Unalan HE, Kanwal A, Miller S, Chhowalla M. Appl Phys Lett 2005;87: 203511. [11] Jia Y, Wei JQ, Wang KL, Cao AY, Shu QK, Gui XC, et al. Adv Mater 2008;20:4594–8. [12] Jia Y, Cao AY, Bai X, Li Z, Zhang LH, Guo N, et al. Nano Lett 2011;11:1901–5. [13] Wei JQ, Ci LJ, Jiang B, Li YH, Zhang XF, Zhu HW, et al. J Mater Chem 2003;13: 1340–4. [14] Peng KQ, Lu AJ, Zhang RQ, Lee ST. Adv Funct Mater 2008;18:3026–35. [15] Liu P, Sun Q, Zhu F, Liu K, Jiang KL, Liu L, et al. Nano Lett 2008;8:647–51.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation of CuI particles and their applications in carbon nanotube-Si heterojunction solar cells Hongguang Wang, Xi Bai, Jinquan Wei ⁎, Peixu Li, Yi Jia, Hongwei Zhu, Kunlin Wang, Dehai Wu Key Lab for Advanced Materials Processing Technology of Education Ministry, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 4 February 2012 Accepted 31 March 2012 Available online 9 April 2012 Keywords: Carbon nanotubes Particles Solar energy materials
a b s t r a c t Copper iodide (CuI) has good electrical conductivity and is usually used as hole-injector, collector and transporter in solar cells. CuI particles are prepared on silicon wafer by metal-assisted etching method in Cu(NO3)2/HF solution and then iodizing in I2/ethanol solution. The size and density of the CuI particles are well controlled by tuning the etching time in Cu(NO3)2/HF solution. It forms a CNT–CuI–Si solar cell when a carbon nanotube (CNT) film covers on the Si substrate modified by CuI particles. The efficiency of the CNT–Si heterojunction cell modified by CuI particles reaches 6% in the initial test. The quantum efficiency of the CNT–Si cell with CuI modification is higher than that without CuI modification, which shows that CuI particles can enhance hole-collecting and transporting ability in the CNT-Si heterojunction solar cell. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Copper (I) iodide (CuI), a p-type semiconductor with good electrical conductivity, is likely to show promising applications in light emitting diodes and solar cells [1,2]. Crystalline CuI has a large band gap of 3.1 eV, which was usually used as buffer layer and holeinjection layer in solar cells [2,3]. Recently, CuI was used to construct fully solid-state dye sensitized solar cell [3,4]. It was reported that ptype CuI can form high photovoltage heterojunction solar cell with ntype Si, where CuI layer were used as hole-injectors, collectors and transporters [5]. We also substituted carbon nanotube (CNT) film for ITO (indium tin oxide) as transparent electrodes in the CuI–Si solar cell [6]. The efficiency of the CuI–Si solar cell reaches to 6%– 10% with CNTs as transparent electrodes and acid doping. The condensed CuI layer was deposited onto Si wafer by vacuum evaporating the high purity CuI powder, which is an expensive and energy intensive process. On the other hand, CNTs have great potential applications in solar cells because of its flexible structure and unique properties, such as ultra-high light absorption ability and high electrical conductivity [7,8]. In fact, CNTs have been introduced into many kinds of solar cells, where CNT acts as hole-collector and electrode [9,10]. Recently, we developed a novel heterojunction solar cell between the flexible CNT film and n-type Si, which shows a moderate efficiency of 5%– 7% [11]. The efficiency of the CNT–Si solar cell depends on the quality of heterojunction [12].
Fig. 1a showed a schematic diagram of CuI modification on Si wafer and fabrication of CNT–CuI–Si solar cells. Typically, a n-type Si wafer (4 in., 2–4 Ω·cm) with a thermal oxide layer (300 nm in thickness) was patterned by photolithography and wet-etching, using a mask containing square-shaped windows of 3 mm×3 mm. Au/Ti layer (thickness of 100 nm) was deposited on the back side of the Si wafer. Copper particles were doped on the Si wafer by wet etching in 0.1 M Cu(NO3)2/0.2 M HF solution. The Si wafer was then immersed in 1 mM I2/ethanol solution for more than 10 min to completely convert the Cu into CuI particles. The Si surface was slightly oxidized after iodizing, and then washed in diluted HF before assembling the solar cells. Macroscopic CNT films used in the experiments were prepared by floating chemical vapor deposition method and then treated in H2O2 and HCl solution to remove the catalyst particles [13]. The fabrication of CNT–CuI–Si solar cells was similar to our recent report [11]. The morphology and structure of CuI particles were characterized and evaluated by scanning electron microscopy (SEM) (LEO-1530) and X-ray diffraction (XRD) (D/max 2500). The current versus voltage (I–V) characteristics of the solar cells were measured with a Keithley 2601 sourcemeter. A solar simulator (K Newport 91195 class A, AM 1.5 G, 100 mW cm− 2) was used as the light source in I–V test. The quantum efficiency (QE) of the solar cells was tested by using a Qtest Station 2000. 3. Results and discussion
⁎ Corresponding author. Fax: + 86 10 62770190. E-mail address: [email protected] (J. Wei). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.114
The deposition of Cu particles to the Si wafer was actually a metalassisted etching process, where Si was etched into Si 4+ and Cu 2+ was reduced into Cu particles (see Eq. (1)). The metal-assisted etching
H. Wang et al. / Materials Letters 79 (2012) 106–108
107
Fig. 1. (a) The schematic diagram of fabricating CNT–CuI–Si solar cell. (b) SEM image of Cu particles, (c) SEM image of CuI particles, (d) SEM image of the CNT-CuI-Si solar cell.
was reported to prepare Si nanowire array in AgNO3/HF solution [14]. Fig. 1b showed a SEM image of the Si wafer after immersing in Cu(NO3)2/HF solution for 30 s. The size of Cu particle varied from 50 nm to 300 nm. Eq. (2) depicted the conversion of Cu to CuI in iodine/ethanol solution. Fig. 1c showed a SEM image of Si wafer with CuI particles. The size of CuI particle was ~ 500 nm. The shape of CuI was different from that of Cu particles. It needed to point out that Cu and CuI particles were only observed on the Si surface, not on the SiO2 layer. Fig. 1d showed a SEM image of the CuI particles on Si covered by CNT film. The CNT bundles contacted not only to the Si wafer, but also to the CuI particles. After etching in the Cu(NO3)2/HF solution, there were lots of etching dents on the Si surface. Si þ 4HF þ 2Cu
2þ
2Cu þ I2 →2CuI
þ
→SiF4 þ 4H þ 2Cu
ð1Þ ð2Þ
In the experiments, the size and coverage rate of the CuI particles depended mainly on the etching conditions of Si wafer in the Cu(NO3)2/HF solution. Fig. 2a and 2b showed two SEM images of the CuI particles obtained by immersing Si wafer in Cu(NO3)2/HF solution for 20 s and 50 s, respectively. Large CuI clusters, composing several grains, were observed in the samples with etching time of 50 s
(Fig. 2b). Fig. 2c depicted the dependence of mean size and coverage rate of CuI particles on the etching time in Cu(NO3)2/HF solution. It is clearly shown that both of the size and coverage rate CuI particles on the Si wafer increase almost linearly with the etching time. When the etching time extends from 30 s to 50 s, the mean size of CuI particles increase from 250 nm to 400 nm, and the coverage rate increases from 40% to 70%, respectively. Fig. 2d showed the XRD patterns of the Si substrate with and without CuI modification. The strong peak centered at 2θ = 69.1° derived from (100) crystal plane of Si substrate. The structure of CuI was identified on the basis of the three distinguishable diffraction peaks at 2θ = 25.5°, 42.3° and 49.9°, corresponding to the {111}, {220} and {311} crystal plane of the cubic CuI (JCPDS card 06-0246), respectively. No signal of Cu was detected in the XRD pattern, indicating that the copper nanoparticles had converted to CuI completely after reaction in iodine/ethanol solution. It formed a CNT–CuI–Si solar cell when the CNT film covered on n-Si substrate modified by CuI. For comparison, we also fabricated CNT–Si solar cells in the same batch on Si substrate without CuI modification. Fig. 3a showed the light I–V curves of a CNT–CuI–Si solar cell as well as a CNT–Si solar cell under one solar equivalent (100 mW/cm2) illustration. It showed a short-circuit current density (Jsc) of 20.6 mA/cm2, an open-circuit voltage (Voc) of 0.5 V, a fill factor (FF) of 58.4% and a power conversion efficiency (η) of 6.0% for the CNT–CuI–Si solar cell.
Fig. 2. SEM images of the CuI particle obtained by etching in Cu(NO3)2/HF solution for (a) 20 s and (b) 50 s. (c) Dependence of coverage rate and particle size of CuI particles on the etching time in Cu(NO3)2/HF solution. (d) XRD patterns of Si and CuI–Si.
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H. Wang et al. / Materials Letters 79 (2012) 106–108
Fig. 3. Photovoltaic properties of the CNT–Si and CNT–CuI–Si solar cells. (a) Light I–V curves, (b) quantum efficiency spectra, (c) differential coefficient spectra. (d) Light I–V curves of the CNT–CuI–Si solar cells fabricated by varying the etching time.
The efficiency of the CNT–Si solar cell was 4.8%, with Jsc = 18.5 mA/cm2, Voc = 0.48 V and FF= 53.9%, which was in the same level of our recent report [11]. It showed that the efficiency of CNT–Si cell enhanced about 20% after modifying by CuI particles at the Si surface, which resulted mainly from the high ability of collecting and transporting holes of CuI particles [5]. Quantum efficiency (QE) curves of the CNT–Si and CNT–CuI–Si cells were shown in Fig. 3b. The QE of the CNT–Si with CuI modification (CNT–CuI–Si) was higher than that without CuI modification in the whole spectra, indicating the higher ability of conversion of incident photon to charge of the CNT–CuI–Si cell than that of the CNT–Si cell. The differential quantum efficiency curves of the CNT–Si cells with and without CuI modification were shown in Fig. 3c. It shows that the two differential quantum efficiency curves had the same peak centered at 1.2 eV. The work function of the CNTs was about 4.8 eV [15], which equaled to the Fermi level of the cubic CuI (Eg =3.1 eV and Ev =5.2 eV, EF =4.8 eV). The band gap of the CNT–Si cell coincidentally equaled to that of the CNT–CuI–Si. Thus the two differential quantum efficiency curves had the same peak position. Fig. 3d showed the light I–V curves of the CNT–CuI–Si solar cells fabricated by varying the etching time. It was clearly shown that the performance of the CNT–CuI–Si solar cells depended on the CuI particle size and coverage rate significantly. It showed that the Jsc of the solar cells was sensitive to the etching time, but the Voc was almost maintained at ~ 0.5 V. At high coverage rate of 70% (etching time 50 s), the Jsc of solar cell was 4 mA/cm 2, which was only 1/5 to that of the solar cells with coverage rate lower than 40% (etching time b30 s). At high coverage rate, some of the CuI particles do not contact Si surface, thus the hole collecting capacity of CuI layer declines significantly. 4. Conclusions CuI particles are prepared on the Si wafer by etching in Cu(NO3)2/HF and reacting in I2/ethanol solution. The size and density of CuI particles
are well controlled by tuning the etching time. The efficiency of CNT–Si solar cells is improved by suitable CuI modification. The efficiency of the CNT–Si solar cells modified with CuI particles reaches to 6.0%.
Acknowledgments This work is supported by National Natural Science Foundation of China (51172122), Foundation for the Author of National Excellent Doctoral Dissertation (2007B37), and Program for New Century Excellent Talents in University. The author also acknowledged Dr. Liu Jiang in Tsinghua for helpful discussion.
References [1] Liu ZW, Qayyum MF, Wu C, Whited MT, Djurovich PI, Hodgson KO, et al. J Am Chem Soc 2011;133:3700–3. [2] Stakhira P, Cherpak V, Volynyuk D, Ivastchyshyn F, Hotra Z, Tataryn V, et al. Thin Solid Films 2010;518:7016–8. [3] Kumara GRA, Konno A, Shiratsuchi K, Tsukahara J, Tennakone K. Chem Mater 2002;14:954–5. [4] Meng QB, Takahashi K, Zhang XT, Sutanto I, Rao TN, Sato O, et al. Langmuir 2003;19:3572–4. [5] Iimori H, Yamane S, Kitamura T, Murakoshi K, Imanishi A, Nakato Y. J Phys Chem C 2008;112:11586–90. [6] Li PX, Wang SS, Jia Y, Li Z, Ji CY, Zhang LH, et al. Nano Res 2011;4:979–86. [7] Yang ZP, Ci LJ, Bur JA, Lin SY, Ajayan PM. Nano Lett 2008;8:446–51. [8] Dürkop T, Getty SA, Cobas E, Fuhrer MS. Nano Lett 2004;4:35–9. [9] Kymakis E, Amaratunga GAJ. Appl Phys Lett 2002;80:112–4. [10] Pasquier AD, Unalan HE, Kanwal A, Miller S, Chhowalla M. Appl Phys Lett 2005;87: 203511. [11] Jia Y, Wei JQ, Wang KL, Cao AY, Shu QK, Gui XC, et al. Adv Mater 2008;20:4594–8. [12] Jia Y, Cao AY, Bai X, Li Z, Zhang LH, Guo N, et al. Nano Lett 2011;11:1901–5. [13] Wei JQ, Ci LJ, Jiang B, Li YH, Zhang XF, Zhu HW, et al. J Mater Chem 2003;13: 1340–4. [14] Peng KQ, Lu AJ, Zhang RQ, Lee ST. Adv Funct Mater 2008;18:3026–35. [15] Liu P, Sun Q, Zhu F, Liu K, Jiang KL, Liu L, et al. Nano Lett 2008;8:647–51.