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Synthesis, structural and photoluminescence properties of SiOx nanospheres prepared by evaporation of silicon monoxide
Materials Letters 65 (2011) 3202–3204
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, structural and photoluminescence properties of SiOx nanospheres prepared by evaporation of silicon monoxide Sanjay K. Srivastava ⁎, Vikas Sareen, Mukul Sharma, Vandana, P.K. Singh National Physical Laboratory, Council of Scientific & Industrial Research, Dr. K.S. Krishnan Road, New Delhi-110012, India
a r t i c l e
i n f o
Article history: Received 11 March 2011 Accepted 3 July 2011 Available online 8 July 2011 Keywords: Amorphous materials Nanoparticles Microstructure Luminescence Electron microscopy
a b s t r a c t We report synthesis of amorphous silicon oxide nano-spheres in bulk quantity by using a simple non-catalytic process based on thermal evaporation of silicon monoxide. Structural characteristics of the nano-spheres were investigated using high resolution scanning and transmission electron microscopes equipped with energy dispersive X-ray spectroscopy (EDAX). Selected area electron diffraction and EDAX analyses revealed that nano-spheres were amorphous and comprised of silicon oxide only with Si and O in an atomic ratio of ~ 1:2. Photoluminescence measurements showed that the SiOx nano-spheres had strong emission band in the blue region. Possible growth mechanism based on vapor-solid model has been discussed. The SiO vapors generated at high temperature react with O2 to form SiOx vapors which subsequently condense on the substrates (placed in the appropriate temperature zone) to form SiOx nano-spheres. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There has been increasing interest in the synthesis of nanoscale lowdimensional materials such as zero-dimensional (0-D) (nanospheres, nanoparticles) and one-dimensional (1-D) (nanowires, nanotubes) nanostructures owing to their important applications in optical, catalytic and sensing devices [1]. Variety of 0-D oxide nanostructures of materials such as, ZnO [2], TiO2 [3], TeO2 [4], etc. have been synthesized by different techniques, for example, by hydrothermal and solvothermal methods, sol–gel method, vapor transport or vapor-solid method, solid state pyrolytic reaction, etc. [2–6]. Among these, silicon oxide (SiOx) nano-particles have attracted great interest owing to their novel catalytic properties [7]. The SiOx nano-particles are used to make electronic substrates, humidity sensors, electrical and thermal insulators, etc. and in each of these applications their roles are different. In addition, The SiOx nanostructures show intensive blue light emission, which may be a candidate material for high-resolution optical heads of scanning near-field optical microscopes, nano interconnection integrated optical devices, low-dimensional wave-guides, etc. [8–10]. There are several methods to synthesize SiOx nano-spheres primarily based on chemical routes [11,12]. However, these methods are complicated, time consuming and difficult to scale up. SiOx nano-spheres have also been fabricated by thermal evaporation of Si/SiO2 mixture [13] but as a byproduct of silicon nanowires. Therefore, synthesis of SiOx nanospheres by using a simple and inexpensive process up-gradable to bulk production is of great interest. Further, not much attention has been
⁎ Corresponding author. Tel.: + 91 11 45608617; fax: + 91 11 45609310. E-mail address: [email protected] (S.K. Srivastava). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.006
given to photoluminescence (PL) properties of the SiOx nano-spheres, though PL of high purity silica glasses with lifetime between 0.1 and 10 ms (depending on the origin of the luminescence bands) has been reported by Nishikawa et al. [14]. In this letter, we report synthesis of pure SiOx nano-spheres using a non-catalytic approach based on thermal evaporation of silicon monoxide (SiO). Key features of this approach are: (i) it is a single step process, (ii) does not require expensive vacuum equipment, and (iii) free of metallic impurities neither in pure nor in oxide form. Here, SiO vapors are transported from hot zone (~1300 °C) to downstream lower temperature zone where they are allowed to condense on the substrates. Morphology, structural and PL properties of the as-deposited material has been investigated and growth mechanism of SiOx nanospheres is discussed.
2. Experiments Growth was carried out in a conventional horizontal alumina tube furnace (ID: 58 mm, hot zone 200 mm with a total tube length: 600 mm). SiO powder (purity ~99.9%, Pure Tech. Inc. New York, USA) was kept in an alumina boat at the center of the tube. Cleaned silicon substrates (size: 2 × 2 cm2) were placed downstream (in the direction of carrier gas flow) in the tube on an alumina sample holder. The source SiO and the substrates temperatures were measured to be ~1300 °C and ~1100–1150 °C respectively. The alumina tube was purged with Ar (99.95% purity with traces of oxygen and moisture), which is also used as the carrier gas, for three hours prior to heating up to 1300 °C under a constant flow rate of 50 l per hour. Growth was carried out for 1 h. Then the tube was cooled down to room temperature under flowing Ar. Thick
S.K. Srivastava et al. / Materials Letters 65 (2011) 3202–3204
wool-like white material was deposited on silicon substrates, substrates holder as well as on the tube inner wall. Morphology of the as-deposited films was examined by scanning electron microscope (SEM; model LEO 440 VP). Structural and compositional analysis of the material was carried out by using high-resolution transmission electron microscope (HR-TEM; model FEI, Technai G20-stwin, 200 kV) equipped with energy dispersive X-ray spectroscopy (EDAX; model EDAX Company, USA). PL measurements of the sample were carried out at room temperature using double mono-chromator based spectrometer (model Perkin Elmer LS55) with xenon flash lamp as excitation source. 3. Results and discussion 3.1. Micro-structural analysis Fig. 1 shows a representative SEM image of wool-like white film deposited on the silicon substrates. It consists of a high density of spherical nano-particles. Similar nanostructures were also found in the material deposited over alumina sample holder as well as in the powder collected from the alumina tube. No other structures were observed in the temperature region 1100–1150 °C demonstrating that purely spherical structure and not a mixture of nanowires, nanoparticles, chains etc. can be grown with this process. Fig. 2(a) shows representative TEM micrograph of the nanospheres. Magnified view of the nano-spheres is shown in the inset of Fig. 2(a). Most of the nano-spheres have diameter in the range of 40–300 nm with center of the distribution at ~ 60 nm. Some large spheres (diameter: 500–850 nm) were also found in the deposits. A representative magnified view of large spheres is shown in Fig. 2(b). These micrographs clearly show that the microstructures have perfect spherical shape with remarkably clean and smooth surface. 3.2. Structural and compositional analysis Highly diffusive ring pattern in selected area electron diffraction (SAED) pattern (shown in the inset of Fig. 2(c)) reveals that the nano-spheres are of a purely amorphous nature. The HR-TEM study further confirmed that no crystalline structure exists in the nanospheres. No lattice fringes could be resolved by the HR-TEM across the diameter of the nanospheres (Fig. 2(c)). Furthermore, no Si–SiO2 core–shell structure was observed. This reveals that the nano-spheres have uniform amorphous structure across the diameter. The EDAX spectrum shown in Fig. 2(d) for a single nano-sphere shows that they are composed of mainly two elements Si and O. Strong C and Cu
3203
signals are attributed to the carbon coated Cu micro-grid. The quantitative analysis shows that the atomic ratio of Si:O is about 1:1.93. Based on above observations it can be concluded that the deposited material comprised of pure and amorphous SiOx nanospheres. The exact mechanism of SiOx nano-spheres formation by vapor transport of SiO is not fully understood. However, our results of nano-sphere formation could be explained on vapor-solid growth based mechanism proposed in the following paragraph. The SiO vapors, generated at ~1300 °C, are transported towards the substrates by the carrier gas and during traversal they get converted into SiOx molecules directly by reacting with O2 traces available in Ar. This is because no special arrangement was used to remove moisture or residual O2 from the carrier gas or the process tube (either by creating vacuum, using hydrogen gas or putting O2 traps). However, care was taken to avoid large O2 concentration by purging the process tube with Ar prior to heating. Therefore, following reaction could take place: SiOðgÞ þ O2 ðgÞ→SiOx ðsÞ The SiOx molecules so formed condense on the substrates in the temperature zone (~1150 °C) to form SiOx nano-clusters which then act as nucleation centers. The SiOx nano-clusters have high surface energy at such a high temperature and subsequently would aggregate to induce SiOx nano-spheres to minimize their systemic energy [15] since for a given volume system energy of a sphere is the minimum.
3.3. Photoluminescence Fig. 3 shows the room temperature PL spectrum of SiOx nanospheres recorded with a 241 nm (~5.1 eV) excitation. A strong blue emission band is observed with peak centered at ~390 nm (~3.15 eV). In the past, PL in SiOx nanostructures, mostly in nanowires, have been reported. The occurrence of a single or two PL bands have been correlated with material structural properties [8,9]. For example, Yu et al. [8] observed two broad PL peaks at ~ 2.65 eV and ~ 3.0 eV, whereas Wang et al. [9] reported a single broad PL peak at ~ 2.78 eV from the SiOx nanowires. These emissions have been attributed to the structural defects related to oxygen deficiency in SiOx nanowires which act as radiative recombination centers. Nishikawa et al. [14] have observed several luminescence bands in the range of 1.9–4.3 eV in different high purity silica glasses with PL lifetime between 0.1 and 10 ms (depending on the origin of luminescence bands). For example, lifetime of PL band at ~ 3.1 eV has been observed to be ~ 0.1 ms and attributed to intrinsic diamagnetic defects centers, such as two-fold co-ordinated silicon lone pair centers (O\Si\O) caused by high oxygen deficiency in the samples [14]. Therefore, the observed blue emission from SiOx nano-spheres at ~ 3.15 eV could also have its origin to the structural defects such as oxygen deficiency, possibility of which is high in the present experiment as we have tried to exploit the limited presence of O2/moisture in the carrier gas during the growth. It is also supported by 1:1.93 atomic ratio of Si:O as determined by EDAX analysis. However, more precise and controlled experiments are required to establish this fact and present results may be a guide for further investigations.
4. Conclusions
Fig. 1. Representative SEM micrograph of spongy-white film deposited on silicon substrates showing spherical particles.
Amorphous SiOx nano-spheres of an average diameter ~60 nm were synthesized by using thermal evaporation of SiO. The nanospheres were free from metal contaminations and showed blue photoluminescence. In-situ SiOx vapors formation via reaction of SiO vapors with O2 resulted into the formation of SiOx nano-clusters which subsequently formed nanospheres on the substrates placed in the appropriate temperature zone. The present simple and low cost
3204
S.K. Srivastava et al. / Materials Letters 65 (2011) 3202–3204
(a)
(b) 130 nm
830 nm 50 nm
250 nm 500 nm
100 nm
(c)
800 700 600
Counts (a.u.)
400
(d)
Si Si
C
500 O
300 200 Cu
100 Cu
Cu
0
5 nm
0
2
4
6
8
10
Energy (KeV) Fig. 2. (a) TEM micrograph showing uniformly dispersed nano-spheres, magnified view of which is shown in the inset, (b) TEM micrograph of large nano-spheres, (c) HR-TEM micrograph of nano-spheres showing amorphous nature (inset shows the corresponding SAED pattern), and (d) EDAX spectrum of a nano-sphere (quantitative data given in the inset).
process for the synthesis of pure SiOx nano-spheres may lead to their potential applications in catalysis, optical and sensing devices.
Acknowledgments Authors are thankful to Dr. V.N. Singh for HRTEM and Dr. D. Haranath for PL measurements of the samples. The work was partially supported by the CSIR project (SIP-17).
350
PL Intensity (a.u.)
300 250 200 150 100 50 0 300
400
500
600
Wavelength (nm) Fig. 3. Room temperature PL spectrum of nano-spheres sample.
700
References [1] Rao CNR, Govindaraj A, Vivekchand SRC. Annu Rep Prog Chem Sect A 2006;102: 20–45. [2] Meulenkamp EA. J Phys Chem B 1998;102:5566–72. [3] Yang HG, Zeng HC. J Phys Chem B 2004;108:3492–5. [4] Qin B, Bai Y, Zhou Y, Liu J, Xie X, Zheng W. Mater Lett 2009;63:1949–51. [5] Chen X, Mao SS. Chem Rev 2007;107:2891–959. [6] Wang ZL. J Phys Condens Matter 2004;16:R829–58. [7] Gole JL, White MG. J Catal 2001;204:249–52. [8] Yu DP, Hang L, Ding Y, Zhang HZ, Bai ZG, Wang JJ, et al. Appl Phys Lett 1998;73: 3076–8. [9] Wang YW, Liang CH, Meng GW, Peng XS, Zhang LD. J Mater Chem 2002;12:651–3. [10] Pang CL, Cui H, Wang CX. Cryst Eng Comm 2011;13:4082–5. [11] Pei LZ. Mater Charact 2008;59:656–9. [12] Rao KS, El-Hami K, Kodaki T, Matsushige K, Makino K. J Colloid Interface Sci 2005;289:125–31. [13] Gole JL, Stout JD, Rauch WL, Wang ZL. Appl Phys Lett 2000;76:2346–8. [14] Nishikawa H, Shiroyama T, Nakamura R, Ohki Y. Phys Rev B 1992;45:586–91. [15] Zhang Y, Wang N, He R, Liu J, Zhang X, Zhu J. J Cryst Growth 2001;233:803–8.
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, structural and photoluminescence properties of SiOx nanospheres prepared by evaporation of silicon monoxide Sanjay K. Srivastava ⁎, Vikas Sareen, Mukul Sharma, Vandana, P.K. Singh National Physical Laboratory, Council of Scientific & Industrial Research, Dr. K.S. Krishnan Road, New Delhi-110012, India
a r t i c l e
i n f o
Article history: Received 11 March 2011 Accepted 3 July 2011 Available online 8 July 2011 Keywords: Amorphous materials Nanoparticles Microstructure Luminescence Electron microscopy
a b s t r a c t We report synthesis of amorphous silicon oxide nano-spheres in bulk quantity by using a simple non-catalytic process based on thermal evaporation of silicon monoxide. Structural characteristics of the nano-spheres were investigated using high resolution scanning and transmission electron microscopes equipped with energy dispersive X-ray spectroscopy (EDAX). Selected area electron diffraction and EDAX analyses revealed that nano-spheres were amorphous and comprised of silicon oxide only with Si and O in an atomic ratio of ~ 1:2. Photoluminescence measurements showed that the SiOx nano-spheres had strong emission band in the blue region. Possible growth mechanism based on vapor-solid model has been discussed. The SiO vapors generated at high temperature react with O2 to form SiOx vapors which subsequently condense on the substrates (placed in the appropriate temperature zone) to form SiOx nano-spheres. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There has been increasing interest in the synthesis of nanoscale lowdimensional materials such as zero-dimensional (0-D) (nanospheres, nanoparticles) and one-dimensional (1-D) (nanowires, nanotubes) nanostructures owing to their important applications in optical, catalytic and sensing devices [1]. Variety of 0-D oxide nanostructures of materials such as, ZnO [2], TiO2 [3], TeO2 [4], etc. have been synthesized by different techniques, for example, by hydrothermal and solvothermal methods, sol–gel method, vapor transport or vapor-solid method, solid state pyrolytic reaction, etc. [2–6]. Among these, silicon oxide (SiOx) nano-particles have attracted great interest owing to their novel catalytic properties [7]. The SiOx nano-particles are used to make electronic substrates, humidity sensors, electrical and thermal insulators, etc. and in each of these applications their roles are different. In addition, The SiOx nanostructures show intensive blue light emission, which may be a candidate material for high-resolution optical heads of scanning near-field optical microscopes, nano interconnection integrated optical devices, low-dimensional wave-guides, etc. [8–10]. There are several methods to synthesize SiOx nano-spheres primarily based on chemical routes [11,12]. However, these methods are complicated, time consuming and difficult to scale up. SiOx nano-spheres have also been fabricated by thermal evaporation of Si/SiO2 mixture [13] but as a byproduct of silicon nanowires. Therefore, synthesis of SiOx nanospheres by using a simple and inexpensive process up-gradable to bulk production is of great interest. Further, not much attention has been
⁎ Corresponding author. Tel.: + 91 11 45608617; fax: + 91 11 45609310. E-mail address: [email protected] (S.K. Srivastava). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.006
given to photoluminescence (PL) properties of the SiOx nano-spheres, though PL of high purity silica glasses with lifetime between 0.1 and 10 ms (depending on the origin of the luminescence bands) has been reported by Nishikawa et al. [14]. In this letter, we report synthesis of pure SiOx nano-spheres using a non-catalytic approach based on thermal evaporation of silicon monoxide (SiO). Key features of this approach are: (i) it is a single step process, (ii) does not require expensive vacuum equipment, and (iii) free of metallic impurities neither in pure nor in oxide form. Here, SiO vapors are transported from hot zone (~1300 °C) to downstream lower temperature zone where they are allowed to condense on the substrates. Morphology, structural and PL properties of the as-deposited material has been investigated and growth mechanism of SiOx nanospheres is discussed.
2. Experiments Growth was carried out in a conventional horizontal alumina tube furnace (ID: 58 mm, hot zone 200 mm with a total tube length: 600 mm). SiO powder (purity ~99.9%, Pure Tech. Inc. New York, USA) was kept in an alumina boat at the center of the tube. Cleaned silicon substrates (size: 2 × 2 cm2) were placed downstream (in the direction of carrier gas flow) in the tube on an alumina sample holder. The source SiO and the substrates temperatures were measured to be ~1300 °C and ~1100–1150 °C respectively. The alumina tube was purged with Ar (99.95% purity with traces of oxygen and moisture), which is also used as the carrier gas, for three hours prior to heating up to 1300 °C under a constant flow rate of 50 l per hour. Growth was carried out for 1 h. Then the tube was cooled down to room temperature under flowing Ar. Thick
S.K. Srivastava et al. / Materials Letters 65 (2011) 3202–3204
wool-like white material was deposited on silicon substrates, substrates holder as well as on the tube inner wall. Morphology of the as-deposited films was examined by scanning electron microscope (SEM; model LEO 440 VP). Structural and compositional analysis of the material was carried out by using high-resolution transmission electron microscope (HR-TEM; model FEI, Technai G20-stwin, 200 kV) equipped with energy dispersive X-ray spectroscopy (EDAX; model EDAX Company, USA). PL measurements of the sample were carried out at room temperature using double mono-chromator based spectrometer (model Perkin Elmer LS55) with xenon flash lamp as excitation source. 3. Results and discussion 3.1. Micro-structural analysis Fig. 1 shows a representative SEM image of wool-like white film deposited on the silicon substrates. It consists of a high density of spherical nano-particles. Similar nanostructures were also found in the material deposited over alumina sample holder as well as in the powder collected from the alumina tube. No other structures were observed in the temperature region 1100–1150 °C demonstrating that purely spherical structure and not a mixture of nanowires, nanoparticles, chains etc. can be grown with this process. Fig. 2(a) shows representative TEM micrograph of the nanospheres. Magnified view of the nano-spheres is shown in the inset of Fig. 2(a). Most of the nano-spheres have diameter in the range of 40–300 nm with center of the distribution at ~ 60 nm. Some large spheres (diameter: 500–850 nm) were also found in the deposits. A representative magnified view of large spheres is shown in Fig. 2(b). These micrographs clearly show that the microstructures have perfect spherical shape with remarkably clean and smooth surface. 3.2. Structural and compositional analysis Highly diffusive ring pattern in selected area electron diffraction (SAED) pattern (shown in the inset of Fig. 2(c)) reveals that the nano-spheres are of a purely amorphous nature. The HR-TEM study further confirmed that no crystalline structure exists in the nanospheres. No lattice fringes could be resolved by the HR-TEM across the diameter of the nanospheres (Fig. 2(c)). Furthermore, no Si–SiO2 core–shell structure was observed. This reveals that the nano-spheres have uniform amorphous structure across the diameter. The EDAX spectrum shown in Fig. 2(d) for a single nano-sphere shows that they are composed of mainly two elements Si and O. Strong C and Cu
3203
signals are attributed to the carbon coated Cu micro-grid. The quantitative analysis shows that the atomic ratio of Si:O is about 1:1.93. Based on above observations it can be concluded that the deposited material comprised of pure and amorphous SiOx nanospheres. The exact mechanism of SiOx nano-spheres formation by vapor transport of SiO is not fully understood. However, our results of nano-sphere formation could be explained on vapor-solid growth based mechanism proposed in the following paragraph. The SiO vapors, generated at ~1300 °C, are transported towards the substrates by the carrier gas and during traversal they get converted into SiOx molecules directly by reacting with O2 traces available in Ar. This is because no special arrangement was used to remove moisture or residual O2 from the carrier gas or the process tube (either by creating vacuum, using hydrogen gas or putting O2 traps). However, care was taken to avoid large O2 concentration by purging the process tube with Ar prior to heating. Therefore, following reaction could take place: SiOðgÞ þ O2 ðgÞ→SiOx ðsÞ The SiOx molecules so formed condense on the substrates in the temperature zone (~1150 °C) to form SiOx nano-clusters which then act as nucleation centers. The SiOx nano-clusters have high surface energy at such a high temperature and subsequently would aggregate to induce SiOx nano-spheres to minimize their systemic energy [15] since for a given volume system energy of a sphere is the minimum.
3.3. Photoluminescence Fig. 3 shows the room temperature PL spectrum of SiOx nanospheres recorded with a 241 nm (~5.1 eV) excitation. A strong blue emission band is observed with peak centered at ~390 nm (~3.15 eV). In the past, PL in SiOx nanostructures, mostly in nanowires, have been reported. The occurrence of a single or two PL bands have been correlated with material structural properties [8,9]. For example, Yu et al. [8] observed two broad PL peaks at ~ 2.65 eV and ~ 3.0 eV, whereas Wang et al. [9] reported a single broad PL peak at ~ 2.78 eV from the SiOx nanowires. These emissions have been attributed to the structural defects related to oxygen deficiency in SiOx nanowires which act as radiative recombination centers. Nishikawa et al. [14] have observed several luminescence bands in the range of 1.9–4.3 eV in different high purity silica glasses with PL lifetime between 0.1 and 10 ms (depending on the origin of luminescence bands). For example, lifetime of PL band at ~ 3.1 eV has been observed to be ~ 0.1 ms and attributed to intrinsic diamagnetic defects centers, such as two-fold co-ordinated silicon lone pair centers (O\Si\O) caused by high oxygen deficiency in the samples [14]. Therefore, the observed blue emission from SiOx nano-spheres at ~ 3.15 eV could also have its origin to the structural defects such as oxygen deficiency, possibility of which is high in the present experiment as we have tried to exploit the limited presence of O2/moisture in the carrier gas during the growth. It is also supported by 1:1.93 atomic ratio of Si:O as determined by EDAX analysis. However, more precise and controlled experiments are required to establish this fact and present results may be a guide for further investigations.
4. Conclusions
Fig. 1. Representative SEM micrograph of spongy-white film deposited on silicon substrates showing spherical particles.
Amorphous SiOx nano-spheres of an average diameter ~60 nm were synthesized by using thermal evaporation of SiO. The nanospheres were free from metal contaminations and showed blue photoluminescence. In-situ SiOx vapors formation via reaction of SiO vapors with O2 resulted into the formation of SiOx nano-clusters which subsequently formed nanospheres on the substrates placed in the appropriate temperature zone. The present simple and low cost
3204
S.K. Srivastava et al. / Materials Letters 65 (2011) 3202–3204
(a)
(b) 130 nm
830 nm 50 nm
250 nm 500 nm
100 nm
(c)
800 700 600
Counts (a.u.)
400
(d)
Si Si
C
500 O
300 200 Cu
100 Cu
Cu
0
5 nm
0
2
4
6
8
10
Energy (KeV) Fig. 2. (a) TEM micrograph showing uniformly dispersed nano-spheres, magnified view of which is shown in the inset, (b) TEM micrograph of large nano-spheres, (c) HR-TEM micrograph of nano-spheres showing amorphous nature (inset shows the corresponding SAED pattern), and (d) EDAX spectrum of a nano-sphere (quantitative data given in the inset).
process for the synthesis of pure SiOx nano-spheres may lead to their potential applications in catalysis, optical and sensing devices.
Acknowledgments Authors are thankful to Dr. V.N. Singh for HRTEM and Dr. D. Haranath for PL measurements of the samples. The work was partially supported by the CSIR project (SIP-17).
350
PL Intensity (a.u.)
300 250 200 150 100 50 0 300
400
500
600
Wavelength (nm) Fig. 3. Room temperature PL spectrum of nano-spheres sample.
700
References [1] Rao CNR, Govindaraj A, Vivekchand SRC. Annu Rep Prog Chem Sect A 2006;102: 20–45. [2] Meulenkamp EA. J Phys Chem B 1998;102:5566–72. [3] Yang HG, Zeng HC. J Phys Chem B 2004;108:3492–5. [4] Qin B, Bai Y, Zhou Y, Liu J, Xie X, Zheng W. Mater Lett 2009;63:1949–51. [5] Chen X, Mao SS. Chem Rev 2007;107:2891–959. [6] Wang ZL. J Phys Condens Matter 2004;16:R829–58. [7] Gole JL, White MG. J Catal 2001;204:249–52. [8] Yu DP, Hang L, Ding Y, Zhang HZ, Bai ZG, Wang JJ, et al. Appl Phys Lett 1998;73: 3076–8. [9] Wang YW, Liang CH, Meng GW, Peng XS, Zhang LD. J Mater Chem 2002;12:651–3. [10] Pang CL, Cui H, Wang CX. Cryst Eng Comm 2011;13:4082–5. [11] Pei LZ. Mater Charact 2008;59:656–9. [12] Rao KS, El-Hami K, Kodaki T, Matsushige K, Makino K. J Colloid Interface Sci 2005;289:125–31. [13] Gole JL, Stout JD, Rauch WL, Wang ZL. Appl Phys Lett 2000;76:2346–8. [14] Nishikawa H, Shiroyama T, Nakamura R, Ohki Y. Phys Rev B 1992;45:586–91. [15] Zhang Y, Wang N, He R, Liu J, Zhang X, Zhu J. J Cryst Growth 2001;233:803–8.