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Photoluminescence of oxidized porous silicon treated by sodium borohydride aqueous solution
Materials Letters 75 (2012) 115–117
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoluminescence of oxidized porous silicon treated by sodium borohydride aqueous solution Yan Mo a, Anshu Ren a, Jianfei Mao a, Zaide Zhou a, Hongyan Yuan b, Juan Du a, Dan Xiao a, b,⁎ a b
College of Chemistry, Sichuan University, 610064, Chengdu, China College of Chemical Engineering, Sichuan University, 610065, Chengdu, China
a r t i c l e
i n f o
Article history: Received 18 November 2011 Accepted 29 January 2012 Available online 3 February 2012 Keywords: Luminescence Porous materials Oxidized porous silicon
a b s t r a c t Alkaline solutions are usually used to remove porous layer from porous silicon. However, novel star-like photoluminescent oxidized porous silicon (PS) particles have been successfully fabricated in sodium borohydride (NaBH4) solution in this work. It is interesting that porous layer has been oxidized in NaBH4 alkaline solution, rather than being removed. Silicon oxide is formed due to the constant reactions of silicon nanocrystals and NaOH. Then these oxides are adjusted to star-like morphology by the pressure of H2. Both NaOH and H2 are generated from the hydrolysis of NaBH4. Oxidized PS shows bright orange photoluminescence (PL) at 550 nm. This emission can be simply tuned into red or blue by illumination of high pressure mercury lamp or high temperature treatment. The PL of NaBH4-treated PS is originated from quantum confinement and surface state of silicon nanocrystallites. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the light emission of porous silicon (PS) has been reported by Canham in 1990 [1], the material has been extensively investigated due to its potential applications in fully integrated optoelectronics, silicon microelectronics and biomaterials. Many researches investigated post-treatment techniques of electro-chemical etched PS, both PL stability and intensity were improved [2,3]. Bard and other researchers have found that surface modification was a key factor to improve PL emission efficiencies [4]. Oxidation is one of the most studied approaches. The oxidation reaction provides some advantages in PL stability and yield [5,6], but many of the oxidation processes need ion beam, high-temperature treatment, complex processing, and toxic organic reagents which are costly and harmful to the environment. In this work, we developed a convenient approach to obtain luminescent oxidized PS simply by NaBH4 aqueous solution treatment. Alkaline solutions are usually used to remove porous layer from PS then porosity can be calculated by gravimetric measurements [7]. However, such alkaline solutions are beneficial to the formation of silicon oxide instead of removing porous layer in this experiment.
anode and Pt as the counter electrode in a Teflon cell. PS films were made with a current density of 35 mA/cm2 for 20 min in HF: C2H5OH of 1:1 volume ratio. Freshly prepared PS wafers were rinsed with anhydrous ethanol and deionized water several times and then placed respectively in NaBH4, NaOH solutions and deionized water for 24 h. At last all samples were rinsed by deionized water and dried in a stream of nitrogen. One of the NaBH4-treated samples was calcined at 550 °C for 30 min and another one was illuminated by a 120 W high pressure mercury lamp with a distance of 20 cm for 24 h.
2. Expeimental PS was prepared by anodic etching of phosphorus doped (100) silicon wafers (2–4 Ω cm, 500 μm thick). Silicon wafer was used as the
⁎ Corresponding author at: 29 Wangjiang Road, Sichuan University, Chengdu 610064, China. Tel.: + 86 28 85415029; fax: +86 28 85416029. E-mail address: [email protected] (D. Xiao). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.130
Fig. 1. PL spectra of PS wafers placed in NaBH4, NaOH and deionized water.
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Y. Mo et al. / Materials Letters 75 (2012) 115–117
NaBH4 solution is a kind of alkaline liquor since it can hydrolyze to produce NaOH as follows:
Table 1 The change of pH values with time in various solutions. Time
t=0 t=1 h t=2 h t=3 h t=4 h t = 17 h t = 23 h t = 24 h
pH value 1.0 M NaBH4 solution
1.0 M NaBH4 + PS wafer
0.1 M NaOH solution
0.1 M NaOH + PS wafer
8.75 10.70 10.83 10.96 11.01 11.15 11.22 11.25
8.78 10.56 10.67 16.76 10.86 10.97 11.00 11.02
11.78 11.76 11.77 11.78 11.79 11.71 11.64 11.67
11.77 11.57 11.45 11.36 11.32 11.25 11.10 10.93
3. Results and discussion Visual observation shows that oxidized PS films placed in NaBH4 solution appear more stronger PL emission under UV lamp, while oxidized PS films formed in deionized water show only faint PL. Similar results were obtained by fluorescence spectra (Fig. 1). NaBH4treated sample shows an intense emission peak at 550 nm. Its PL intensity is enhanced to 22 times compares to that of PS wafer placed in deionized water.
NaBH4 þ 2H2 O→NaBO2 þ 4H2 ↑
ð1Þ
NaBO2 þ H2 O→NaOH þ HBO2
ð2Þ
It is well known that porous layer on PS wafer can be removed by alkaline solution [7]. But porous layer has not been peeled off by alkaline solution-treatment in this experiment. On the contrary, a layer of photoluminescent oxidized PS is formed. This is very different from other studies. We suppose that the surface of freshly-prepared PS is highly reactive and particularly sensitive to alkaline solutions. Silicon nanocrystallites will react with NaOH to form Na2SiO3. A possible mechanism is proposed: Si þ 2NaOH þ H2 O→Na2 SiO3 þ 2H2 ↑
ð3Þ
Silicon oxide is coated on the surface of PS via hydrolysis of Na2SiO3 in the same alkaline solution [8,9]. In order to investigate the role of alkaline solution in the formation of silicon oxide particles, a control experiment has been done. Freshly prepared PS wafers were respectively placed in NaOH solutions with different concentrations for 24 h. PL
Fig. 2. SEM and TEM images of freshly-prepared and 1.0 M NaBH4-treated PS. (a–b) SEM images of NaBH4-treated sample: at low (a) and high magnification (b). (c) Freshly-prepared PS. (d) 0.1 M NaOH-treated PS wafer. (e) TEM image and selected area electron diffraction patterns of silicon nanocrystallites along the [110] zone axis shown in the insets.
Y. Mo et al. / Materials Letters 75 (2012) 115–117
117
Fig. 3. (a) PL spectra. (b–c) SEM images of NaBH4-treated sample and the one illuminated by high pressure mercury lamp.
results are also shown in Fig. 1. As can be seen, PL intensity increases with NaOH concentration in the beginning. However, when the concentration of NaOH is increased to 1.0 M, the PL intensity is completely quenched. Porous layer is peeled off from PS wafer. From Fig. 1 we can see that oxidized PS is indeed formed in alkaline solution with low concentrations, but PL intensity of NaOH-treated PS wafer is much weaker than that of NaBH4-treated sample. We assumed that relative slow and constant formation speed of NaOH may be a crucial factor in the formation of intense photoluminescent silicon oxide. In NaBH4 solution, NaOH is provided by the hydrolysis of NaBH4 slowly, thus silicon nanoparticles can react with NaOH to form silicon oxide constantly. For the purpose of confirming such surmise, we monitored pH values of NaBH4 and NaOH solutions in real time. Results are shown in Table 1. Formation of NaOH leads to pH value increases with time in NaBH4 solution. But the increasing speed of pH value is much slower for the NaBH4 solution containing PS wafer, indicating that the reaction of Eq. (3) consumes NaOH and gives rise to lower pH value. As a comparison, pH values for NaOH solution decrease with time. The much faster decreasing speed of the NaOH solution containing PS wafer also confirms the reaction between silicon nanocrystallites and NaOH. Above results shows that pH value is increased with time for NaBH4 solution, while pH value for NaOH solution is decreased, confirming that slow and stable formation speed of NaOH is very important in the formation of silicon oxide in this experiment. According to Fritsch and his coworkers' report [10], porous or rough oxidized silicon will show photoluminescence. From SEM images we can see that the surface of freshly-prepared PS wafer is porous while oxidized PS placed in NaBH4 solution is covered by star-like particles. As shown in Fig. 2a, a regular oxide array is formed. The pressure which is supplied by the evolution of H2 in Eqs. (1) and (3) may be responsible for the distinctive appearances. The surface morphologies depend on the deviation degree of the formation conditions from the thermodynamic equilibrium or the driving force for crystallization [11]. The high reactive silicon oxide particles may be forced to rearrange under the strong pressure generated by H2. The free oxide particles may stick to any parts of the formed clusters, but they are more likely to encounter the tips than to penetrate deep into clusters' inner regions, which leading to outward growth from the initial location of seed particles [12]. Therefore distinctive morphology is formed successfully with time evolution. In comparison with NaOH solution, less H2 bubbles were observed, resulting to less silicon oxide and weaker PL intensity. SEM image of 0.1 M NaOH-treated PS sample only shows a thin oxide layer surrounding the pores of PS (Fig. 2d). Silicon oxide particles (~50 nm) appear on the surface. It is concluded that NaBH4 plays the role of foaming agent and provides NaOH at a slow and constant speed, thus silicon oxide with star-like morphology and intense PL intensity can be obtained. It is suggested that PL is a result of quantum confinement within the nanometer size silicon branches of the PS [13]. The requirement of silicon nanocrystallites for PL from oxidized PS has since been demonstrated by TEM results measurement. From the selected area electron
diffraction patterns (Fig. 2e, lower left) along the [110] zone axis, the 111 and 220 reflections are observed. High-resolution TEM analysis shows the presence of nanocrystalline silicon with diameter smaller than 5 nm (Fig. 2e). Quantum confinement induced PL is evidenced by the observation of blue-shift of PL wavelength by reducing the dimensions of silicon nanocrystallites through calcining NaBH4-treated sample at 550 °C for 30 min (Fig. 3a). It is interesting that PL can also be tuned into red when the NaBH4-treated sample was illuminated by high pressure mercury lamp (Fig. 3a). This red-shift was attributed to the trapping of an electron or exciton by Si_O bonds on the surface [14]. SEM images show that the sizes of these star-like oxides become larger (Figs. 3b–c), which is proposed to be associated with the formation of more oxides (see Supplementary data). It illustrated that PL of NaBH4-treated PS may originated from both quantum confinement and surface states. 4. Conclusion In summary, a simple, inexpensive, solution-phase method was used to fabricate oxidized PS layer at room-temperature. NaBH4 solution works as a foaming agent and provides a large amount of H2 which acts an important role in the formation of star-like rough morphology. Both quantum confinement and surface states may be responsible to the PL origination. Since emission wavelength is tunable, silicon device based on triple PL wavelength can be fabricated easily. Acknowledgments Funding for this work was provided by the National Science Foundation of China (Nos. 20803050 and 20927007). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2012.01.130. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Canham LT. Appl Phys Lett 1990;57:1046–8. Nakamura T, Ogawa T, Hosoya N, Adachi S. J Lumin 2010;130:682–7. Gelloz B, Koshida N. Appl Phys Lett 2009;94:201903. Ding ZF, Quinn BM, Haram SK, Pell LE, Korgel BA, Bard AJ. Science 2002;296:1293–7. Gelloz B, Koyama H, Koshida N. Thin Solid Films 2008;517:376–9. Kayahan E. J Lumin 2010;130:1295–9. Ruminski AM, Barillaro G, Chaffin C, Sailor MJ. Adv Funct Mater 2011;21:1511–25. Duan JH, Feng YQ, Yang G, Xu WL, Li XG, Liu Y, et al. Ind Eng Chem Res 2009;48: 1468–75. Wang ZJ, Wu LN, Qi YL, Jiang ZH. Appl Surf Sci 2010;256:3443–7. Fritsch E, Mihut L, Baibarac M, Baltog I, Ostrooumov M, Lefrant S, et al. J Appl Phys 2001;90:4777–82. Oaki Y, Imai H. Cryst Growth Des 2003;3:711–6. Ng CHB, Tan H, Fan WY. Langmuir 2006;22:9712–7. Lee SG, Koh Y, Jang S, Kim J, Woo H-G, Kim S, et al. J Nanosci Nanotechnol 2010;10: 3266–70. Ray M, Bandyopadhyay NR, Ghanta U, Klie RF, Pramanick AK, Das S, et al. J Appl Phys 2011;110:094309.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Photoluminescence of oxidized porous silicon treated by sodium borohydride aqueous solution Yan Mo a, Anshu Ren a, Jianfei Mao a, Zaide Zhou a, Hongyan Yuan b, Juan Du a, Dan Xiao a, b,⁎ a b
College of Chemistry, Sichuan University, 610064, Chengdu, China College of Chemical Engineering, Sichuan University, 610065, Chengdu, China
a r t i c l e
i n f o
Article history: Received 18 November 2011 Accepted 29 January 2012 Available online 3 February 2012 Keywords: Luminescence Porous materials Oxidized porous silicon
a b s t r a c t Alkaline solutions are usually used to remove porous layer from porous silicon. However, novel star-like photoluminescent oxidized porous silicon (PS) particles have been successfully fabricated in sodium borohydride (NaBH4) solution in this work. It is interesting that porous layer has been oxidized in NaBH4 alkaline solution, rather than being removed. Silicon oxide is formed due to the constant reactions of silicon nanocrystals and NaOH. Then these oxides are adjusted to star-like morphology by the pressure of H2. Both NaOH and H2 are generated from the hydrolysis of NaBH4. Oxidized PS shows bright orange photoluminescence (PL) at 550 nm. This emission can be simply tuned into red or blue by illumination of high pressure mercury lamp or high temperature treatment. The PL of NaBH4-treated PS is originated from quantum confinement and surface state of silicon nanocrystallites. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the light emission of porous silicon (PS) has been reported by Canham in 1990 [1], the material has been extensively investigated due to its potential applications in fully integrated optoelectronics, silicon microelectronics and biomaterials. Many researches investigated post-treatment techniques of electro-chemical etched PS, both PL stability and intensity were improved [2,3]. Bard and other researchers have found that surface modification was a key factor to improve PL emission efficiencies [4]. Oxidation is one of the most studied approaches. The oxidation reaction provides some advantages in PL stability and yield [5,6], but many of the oxidation processes need ion beam, high-temperature treatment, complex processing, and toxic organic reagents which are costly and harmful to the environment. In this work, we developed a convenient approach to obtain luminescent oxidized PS simply by NaBH4 aqueous solution treatment. Alkaline solutions are usually used to remove porous layer from PS then porosity can be calculated by gravimetric measurements [7]. However, such alkaline solutions are beneficial to the formation of silicon oxide instead of removing porous layer in this experiment.
anode and Pt as the counter electrode in a Teflon cell. PS films were made with a current density of 35 mA/cm2 for 20 min in HF: C2H5OH of 1:1 volume ratio. Freshly prepared PS wafers were rinsed with anhydrous ethanol and deionized water several times and then placed respectively in NaBH4, NaOH solutions and deionized water for 24 h. At last all samples were rinsed by deionized water and dried in a stream of nitrogen. One of the NaBH4-treated samples was calcined at 550 °C for 30 min and another one was illuminated by a 120 W high pressure mercury lamp with a distance of 20 cm for 24 h.
2. Expeimental PS was prepared by anodic etching of phosphorus doped (100) silicon wafers (2–4 Ω cm, 500 μm thick). Silicon wafer was used as the
⁎ Corresponding author at: 29 Wangjiang Road, Sichuan University, Chengdu 610064, China. Tel.: + 86 28 85415029; fax: +86 28 85416029. E-mail address: [email protected] (D. Xiao). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.01.130
Fig. 1. PL spectra of PS wafers placed in NaBH4, NaOH and deionized water.
116
Y. Mo et al. / Materials Letters 75 (2012) 115–117
NaBH4 solution is a kind of alkaline liquor since it can hydrolyze to produce NaOH as follows:
Table 1 The change of pH values with time in various solutions. Time
t=0 t=1 h t=2 h t=3 h t=4 h t = 17 h t = 23 h t = 24 h
pH value 1.0 M NaBH4 solution
1.0 M NaBH4 + PS wafer
0.1 M NaOH solution
0.1 M NaOH + PS wafer
8.75 10.70 10.83 10.96 11.01 11.15 11.22 11.25
8.78 10.56 10.67 16.76 10.86 10.97 11.00 11.02
11.78 11.76 11.77 11.78 11.79 11.71 11.64 11.67
11.77 11.57 11.45 11.36 11.32 11.25 11.10 10.93
3. Results and discussion Visual observation shows that oxidized PS films placed in NaBH4 solution appear more stronger PL emission under UV lamp, while oxidized PS films formed in deionized water show only faint PL. Similar results were obtained by fluorescence spectra (Fig. 1). NaBH4treated sample shows an intense emission peak at 550 nm. Its PL intensity is enhanced to 22 times compares to that of PS wafer placed in deionized water.
NaBH4 þ 2H2 O→NaBO2 þ 4H2 ↑
ð1Þ
NaBO2 þ H2 O→NaOH þ HBO2
ð2Þ
It is well known that porous layer on PS wafer can be removed by alkaline solution [7]. But porous layer has not been peeled off by alkaline solution-treatment in this experiment. On the contrary, a layer of photoluminescent oxidized PS is formed. This is very different from other studies. We suppose that the surface of freshly-prepared PS is highly reactive and particularly sensitive to alkaline solutions. Silicon nanocrystallites will react with NaOH to form Na2SiO3. A possible mechanism is proposed: Si þ 2NaOH þ H2 O→Na2 SiO3 þ 2H2 ↑
ð3Þ
Silicon oxide is coated on the surface of PS via hydrolysis of Na2SiO3 in the same alkaline solution [8,9]. In order to investigate the role of alkaline solution in the formation of silicon oxide particles, a control experiment has been done. Freshly prepared PS wafers were respectively placed in NaOH solutions with different concentrations for 24 h. PL
Fig. 2. SEM and TEM images of freshly-prepared and 1.0 M NaBH4-treated PS. (a–b) SEM images of NaBH4-treated sample: at low (a) and high magnification (b). (c) Freshly-prepared PS. (d) 0.1 M NaOH-treated PS wafer. (e) TEM image and selected area electron diffraction patterns of silicon nanocrystallites along the [110] zone axis shown in the insets.
Y. Mo et al. / Materials Letters 75 (2012) 115–117
117
Fig. 3. (a) PL spectra. (b–c) SEM images of NaBH4-treated sample and the one illuminated by high pressure mercury lamp.
results are also shown in Fig. 1. As can be seen, PL intensity increases with NaOH concentration in the beginning. However, when the concentration of NaOH is increased to 1.0 M, the PL intensity is completely quenched. Porous layer is peeled off from PS wafer. From Fig. 1 we can see that oxidized PS is indeed formed in alkaline solution with low concentrations, but PL intensity of NaOH-treated PS wafer is much weaker than that of NaBH4-treated sample. We assumed that relative slow and constant formation speed of NaOH may be a crucial factor in the formation of intense photoluminescent silicon oxide. In NaBH4 solution, NaOH is provided by the hydrolysis of NaBH4 slowly, thus silicon nanoparticles can react with NaOH to form silicon oxide constantly. For the purpose of confirming such surmise, we monitored pH values of NaBH4 and NaOH solutions in real time. Results are shown in Table 1. Formation of NaOH leads to pH value increases with time in NaBH4 solution. But the increasing speed of pH value is much slower for the NaBH4 solution containing PS wafer, indicating that the reaction of Eq. (3) consumes NaOH and gives rise to lower pH value. As a comparison, pH values for NaOH solution decrease with time. The much faster decreasing speed of the NaOH solution containing PS wafer also confirms the reaction between silicon nanocrystallites and NaOH. Above results shows that pH value is increased with time for NaBH4 solution, while pH value for NaOH solution is decreased, confirming that slow and stable formation speed of NaOH is very important in the formation of silicon oxide in this experiment. According to Fritsch and his coworkers' report [10], porous or rough oxidized silicon will show photoluminescence. From SEM images we can see that the surface of freshly-prepared PS wafer is porous while oxidized PS placed in NaBH4 solution is covered by star-like particles. As shown in Fig. 2a, a regular oxide array is formed. The pressure which is supplied by the evolution of H2 in Eqs. (1) and (3) may be responsible for the distinctive appearances. The surface morphologies depend on the deviation degree of the formation conditions from the thermodynamic equilibrium or the driving force for crystallization [11]. The high reactive silicon oxide particles may be forced to rearrange under the strong pressure generated by H2. The free oxide particles may stick to any parts of the formed clusters, but they are more likely to encounter the tips than to penetrate deep into clusters' inner regions, which leading to outward growth from the initial location of seed particles [12]. Therefore distinctive morphology is formed successfully with time evolution. In comparison with NaOH solution, less H2 bubbles were observed, resulting to less silicon oxide and weaker PL intensity. SEM image of 0.1 M NaOH-treated PS sample only shows a thin oxide layer surrounding the pores of PS (Fig. 2d). Silicon oxide particles (~50 nm) appear on the surface. It is concluded that NaBH4 plays the role of foaming agent and provides NaOH at a slow and constant speed, thus silicon oxide with star-like morphology and intense PL intensity can be obtained. It is suggested that PL is a result of quantum confinement within the nanometer size silicon branches of the PS [13]. The requirement of silicon nanocrystallites for PL from oxidized PS has since been demonstrated by TEM results measurement. From the selected area electron
diffraction patterns (Fig. 2e, lower left) along the [110] zone axis, the 111 and 220 reflections are observed. High-resolution TEM analysis shows the presence of nanocrystalline silicon with diameter smaller than 5 nm (Fig. 2e). Quantum confinement induced PL is evidenced by the observation of blue-shift of PL wavelength by reducing the dimensions of silicon nanocrystallites through calcining NaBH4-treated sample at 550 °C for 30 min (Fig. 3a). It is interesting that PL can also be tuned into red when the NaBH4-treated sample was illuminated by high pressure mercury lamp (Fig. 3a). This red-shift was attributed to the trapping of an electron or exciton by Si_O bonds on the surface [14]. SEM images show that the sizes of these star-like oxides become larger (Figs. 3b–c), which is proposed to be associated with the formation of more oxides (see Supplementary data). It illustrated that PL of NaBH4-treated PS may originated from both quantum confinement and surface states. 4. Conclusion In summary, a simple, inexpensive, solution-phase method was used to fabricate oxidized PS layer at room-temperature. NaBH4 solution works as a foaming agent and provides a large amount of H2 which acts an important role in the formation of star-like rough morphology. Both quantum confinement and surface states may be responsible to the PL origination. Since emission wavelength is tunable, silicon device based on triple PL wavelength can be fabricated easily. Acknowledgments Funding for this work was provided by the National Science Foundation of China (Nos. 20803050 and 20927007). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2012.01.130. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Canham LT. Appl Phys Lett 1990;57:1046–8. Nakamura T, Ogawa T, Hosoya N, Adachi S. J Lumin 2010;130:682–7. Gelloz B, Koshida N. Appl Phys Lett 2009;94:201903. Ding ZF, Quinn BM, Haram SK, Pell LE, Korgel BA, Bard AJ. Science 2002;296:1293–7. Gelloz B, Koyama H, Koshida N. Thin Solid Films 2008;517:376–9. Kayahan E. J Lumin 2010;130:1295–9. Ruminski AM, Barillaro G, Chaffin C, Sailor MJ. Adv Funct Mater 2011;21:1511–25. Duan JH, Feng YQ, Yang G, Xu WL, Li XG, Liu Y, et al. Ind Eng Chem Res 2009;48: 1468–75. Wang ZJ, Wu LN, Qi YL, Jiang ZH. Appl Surf Sci 2010;256:3443–7. Fritsch E, Mihut L, Baibarac M, Baltog I, Ostrooumov M, Lefrant S, et al. J Appl Phys 2001;90:4777–82. Oaki Y, Imai H. Cryst Growth Des 2003;3:711–6. Ng CHB, Tan H, Fan WY. Langmuir 2006;22:9712–7. Lee SG, Koh Y, Jang S, Kim J, Woo H-G, Kim S, et al. J Nanosci Nanotechnol 2010;10: 3266–70. Ray M, Bandyopadhyay NR, Ghanta U, Klie RF, Pramanick AK, Das S, et al. J Appl Phys 2011;110:094309.