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Structural and Spectroscopic Characterizations of ZnO Quantum Dots Annealed at Different Temperatures
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ScienceDirect J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
Structural and Spectroscopic Characterizations of ZnO Quantum Dots Annealed at Different Temperatures Geeta Rani*, P.D. Sahare Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India [Manuscript received October 28, 2012, in revised form February 4, 2013, Available online 30 August 2013]
Here, we report the synthesis and characterizations of solegel derived zinc oxide (ZnO) quantum dots (QDs) using zinc acetate dihydrate (Zn(CH3COO)2$2H2O) and lithium hydroxide monohydrate (LiOH$H2O) as raw material. The as-prepared ZnO QDs was annealed at different temperature (400, 700, and 900 C) and the structural, optical properties were investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), UVeVis and photoluminescence (PL) spectroscopy. The powder XRD patterns of the obtained samples showed the formation of single-phase wurtzite structure and the morphological changes have been observed with increasing annealing temperature. In the absorption spectra, the optical band gap of nanocrystalline ZnO QDs decreased from 3.18 to 3.11 eV and the particle size increased with increasing temperature. In the PL spectra, a broad green emission peak related to defect levels in the visible range of the spectra have been recorded. KEY WORDS: ZnO quantum dots; Photoluminescence; UVeVis spectroscopy
1. Introduction Structural and optical properties of semiconductor nanocrystals are important from both a fundamental science and a proposed photonic application point of view. Zinc oxide (ZnO) is the richest family of nanostructures including nanorods, nanopencils, nanotubes and so on. It is an important IIeVI semiconductor with a direct wide band gap (3.37 eV) at room temperature and a large exciton binding energy (60 MeV). ZnO is currently attracting worldwide intense interests because of its importance in fundamental studies and its numerous applications especially as optoelectronic materials[1]. Recent researches show that ZnO nanomaterials can be used as promising piezoelectric nanogenerators, which triggers a wide range of subsequent research in searching for new synthetic methods to synthesize ZnO nanomaterials[2]. Among various ZnO nanostructures, ZnO nanoparticles are the most frequently studied[3] because of their interest in fundamental study and also their applied aspects such as in solar energy conversion, photo catalysis, light-emitting materials[4], transparent UV protection films and chemical sensors[5]. Hitherto, searching new methodology to synthesize Corresponding author. Tel.: þ91 9560752969; E-mail address: [email protected] (G. Rani). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.08.015 *
uniform ZnO nanoparticles is still of great importance for both fundamental study and application of ZnO nanomaterials. Various methods have been developed for the synthesis of ZnO nanoparticles with uniform morphology and size[6,7]. Among these synthetic routes, solution chemical methods provide a promising way for low cost and large-scale production, which do not need expensive raw materials and complicated equipments[8,9]. However, most solution methods to use Zn salts as the starting material and the reactions are generally carried out in water or alcohol by the hydrolysis of Zn salts in the presence of a base[10,11]. It has been demonstrated that the anion of the zinc salts and the cation of the base have great effect on the formation of the ZnO nanocrystals[12,13]. Moreover, most methods exploit surface modification or protecting reagents to prevent from agglomeration and to control the morphology development of ZnO nanocrystals. There are still few reports about synthesizing ZnO nanoparticles at mild condition that do not need any protecting reagents and complicated process control. ZnO QDs are of great interest because the three-dimensional confinement of carrier and phonon leads to not only continuous tuning of the optoelectronic properties but also improvement in device performance. The special application advantages of ZnO QDs and nanoparticles in the area of bioscience also improve more attention due to its “survival lifetime” of a few hours, the slow solubility and high compatibility in biofluid and necessaries of Zn for human beings everyday[14]. Therefore, a number of reports of fabrication, structural, and optical characterization of ZnO QDs have been done in recent years[15e21].
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G. Rani and P.D. Sahare: J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
In the present work, nearly uniform spherical ZnO QDs with no agglomeration were synthesized. The growth process and morphology evolution of the ZnO QDs were investigated. The results indicated that the morphology evolution depends on the annealing temperature. Some spectroscopic studies were also discussed. 2. Experimental 2.1. Sample preparation Zinc acetate dihydrate (99.2%) [Zn(CH3COO)2$2H2O], lithium hydroxide monohydrate (LiOH$H2O), ethanol were procured from CDH and Merck, India, and were used to synthesize ZnO QDs. All the chemicals were of AR grade and were directly used without special treatment. In a typical preparation method, 1 mol/l Zn(CH3COO)2$2H2O was prepared in 50 ml of ethanol using vigorous stirring at 50 C for 90 min, followed by cooling at room temperature. LiOH (0.59 g) was dissolved in 50 ml of ethanol and kept in ultrasonic bath then cooled at room temperature. This solution was slowly added to the Zn2þ solution with vigorous stirring at room temperature. The precipitation of ZnO QDs from the transparent sol was carried out by addition of n-heptane followed by vigorous stirring. The ratio of volume of n-heptane to sol was 3:2. After centrifugation, the precipitate was collected and re-dispersed into ethanol. The removal of excess acetate and lithium ions from the dispersed ZnO was accomplished by repetitive washing of ZnO with ethanol and n-heptane. The obtained precipitate was dried at room temperature resulting in white powders of ZnO QDs. The obtained powder was annealed in air at different temperatures (400, 700, and 900 C) for 3 h.
Fig. 1 XRD patterns of the ZnO QDs of as-prepared sample (a) and samples annealed at: (b) 400 C, (c) 700 C and (d) 900 C.
XRD, all the diffraction peaks can be readily indexed to a wurtzite structure of ZnO (space group P63mc), which is well matched with the standard data (JCPDS, CARD No. 80-0075, a ¼ b ¼ 0.3253 nm, c ¼ 0.5209 nm). The results indicate that the products consisted of pure phases after annealing and the diffraction peaks related to the any impurities were not observed in the XRD pattern confirming the high purity of the synthesized product. It could be seen that the diffraction peaks shown in Fig. 1(d) were more intensive and narrower, implying a good crystalline nature and larger particle size of the sample annealed at 900 C. The average crystallite size was estimated using FWHM (full width at half maximum) value from the line broadening of the XRD peaks using Scherrer’s formula[22]:
2.2. Sample characterizations The structural and optical properties of the obtained powder samples were characterized by different techniques. The X-ray diffraction (XRD) was done on the machine D8-Advance model of Bruker Inc. using monochromatic CuKa line (l ¼ 0.154056 nm) and was compared with the available PCPDF data files to confirm the formation of different phases in the material. The high resolution transmission electron microscopy (HRTEM) images were taken to study the morphology (shape and size) of the nanocrystalline material. The photographs of the images were taken on a transmission electron microscope (FEI TEM model TECNAI G2 T30, U-TWIN) at 300 kV. The TEM samples were prepared by putting a drop of the solution of suspended nanoparticles in anhydrous ethanol on a carbon coated copper grid (200 meshes). The UVevisible spectra of the material were taken in wavelength range from 250 to 600 nm using Evolution UVe Vis. spectrophotometer, Thermo Scientific Inc. to estimate the band gaps of the material. Photoluminescence (PL) spectra were carried out on a fluorescence spectrophotometer (Fluorolog Horiba JOBIN YVON) using Xe lamp with excitation wavelength 350 nm to study the different kinds of defects in the material. 3. Result and Discussion 3.1. Structural and morphological investigations Fig. 1 represents the typical XRD pattern of the samples annealed at different temperatures. Within the detection limits of
D ¼
0:9l bcos qB
(1)
where D is the average crystallite size, l is the incident wavelength, qB is the Bragg angle and b is the diffracted FWHM (radian). The average particle sizes of the ZnO QDs calculated from the most intense peak are approximately 3e 5 nm, 50 nm, 170 nm, 250 nm for the as-prepared and samples annealed at 400, 700 and 900 C, respectively as expected from the sharpened and enhanced diffraction peaks. The TEM images and SAED (selected-area electron diffraction) pattern of the as-prepared and samples annealed at different temperature are shown in Fig. 2 revealing the particles growth with increasing temperature. The SAED patterns manifested that crystallinity of ZnO nanoparticles increased and some bright and dark rings with diffraction spots also appeared in the diffraction patterns at higher temperatures. The increase in crystallinity of the nanoparticles might be due to the long range ordering of the unit cells with increasing annealing temperature. The ZnO QDs are found to be spherical in shape and almost mono-dispersed in size (w5 nm) and the shape and size of nanoparticles changed with increasing temperature. However, with the increase of the annealing temperature from 400 to 900 C, the shape of the nanoparticles distorted and randomly shaped nanoparticles were obtained. Apart from the change in shape of the nanoparticles, the size of nanoparticles increased (also changed). The average crystallite size from these micrographs has been calculated by taking the average of highest, smallest and medium size particles of ZnO. The particle sizes were found to be w3, 45, 155, and
G. Rani and P.D. Sahare: J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
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Fig. 2 TEM images with the SAED patterns of ZnO QDs of as-prepared (a) and annealed samples at: (b) 400 C, (c) 700 C, and (d) 900 C.
275 nm corresponding to as-prepared and samples annealed at 400, 700 and 900 C, respectively. The average size of the nanoparticles measured from the TEM analysis was corroborated with the XRD analysis. 3.2. Optical studies Fig. 3 shows the UVevisible absorption spectra of ZnO QDs annealed at different temperature. It can be seen that the absorbance of the samples increases with increasing annealing temperature. As the transparency of the samples have a direct consequence with the bands gap i.e. the band gap of the sample increases with increasing transparency or decreasing absorbance. The band gap of the sample also depends on the particles size
and decreases with increasing particle size. Thus, the increase in absorbance and particle size infer the decrease of the band gap of the samples with increasing temperature. Furthermore, a red-shift was also observed in the characteristic absorption peak of ZnO QDs, with increasing temperature, and was attributed to the decrease of the band gap of the samples. For the allowed direct transition, the variation of a with photon energy (hn) obeys Tauc’s plot[23] ðahnÞ2 ¼ A hn Eg
(2)
where A is a constant, Eg is band gap, h is Plank constant and a is the absorption coefficient. Fig. 4 illustrates the (ahn)2 vs hn plot used for the evaluation of the band gap of ZnO QDs.
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G. Rani and P.D. Sahare: J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
Fig. 3 Optical absorption spectra of ZnO QDs annealed samples at: (a) 400 C, (b) 700 C, (c) 900 C.
Fig. 5 PL emission spectra of the ZnO QDs annealed samples at: (a) 400 C, (b) 700 C, (c) 900 C. The samples were measured at room temperature with excitation wavelength of 350 nm.
Inset of Fig. 4 revealed that the band gap of the obtained samples decreased from 3.18 to 3.11 eV with increasing temperature. The qualitative informations about the band gap evaluated from Fig. 3 are well corroborated with the quantitative results of Fig. 4. The PL properties of the ZnO QDs were investigated by a excitation of 350 nm and a well-known broad green emission band in the range 400e650 nm was observed as shown in Fig. 5, which is considered to be related to the single ionized oxygen vacancy due to the recombination of a photon-generated hole with the single ionized charged state of the defect in ZnO QDs[24,25] and it was reported that the oxygen vacancies responsible for the green emission are mainly located at the surface[26e29]. It is also found from the spectra that the peak at 528 nm in the sample annealed at 400 C changes its position and a red-shift of about 4 nm was observed with increasing temperature, probably related to the decrease of the band gap. The shift in PL peak due to size is quite common in the case of semiconductor QDs studies[30e32] because the samples grew in size due to annealing in an oxygen atmosphere and this could be assigned to too many interdependent factors, such as electrone phonon coupling, lattice distortion, localization of charge carriers due to interface effects and point defects. The increase in the peak intensity indicates that surface to volume ratio decreases; therefore surface defects decreases with increasing annealing
temperature and resulting in the enhancement of the ZnO QDs crystallinity. As a result, the PL investigation shows the influence of annealing temperature on the PL properties of the ZnO QDs and therefore its potential application in UV emission devices.
Fig. 4 Plot of (ahn)2 vs hn of ZnO QDs annealed samples at: (a) 400 C, (b) 700 C, (c) 900 C. Inset shows the variation of band gap with temperature.
4. Conclusion The QDs of wurtzite ZnO of about 3e5 nm in diameter were synthesized by a solegel process, which showed the quantum size effect as a blue shift of the band gap energy. The properties of the ZnO nanoparticles including photo catalysis, gas-sensing, and PL are the subject of future investigations. In our preliminary study, the aggregates of ZnO quantum dots showed a high photo catalytic activity, promising for a wide range of photochemical applications. It is expected that this synthesis method may have unique morphologies. Acknowledgements The authors would like to thank University Science Instrumentation Centre (USIC), University of Delhi for providing instrumentation facility and UGC for financial assistance. REFERENCES [1] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. 43 (2004) 4774e4777. [2] Z.L. Wang, J. Song, Science 312 (2006) 242e246. [3] R. Turgeman, S. Tirosh, A. Gedanken, Chem. Eur. J. 10 (2004) 1845e1850. [4] G.R. Gattorno, P.S. Jacinto, L.R. Va’zquez, J. Phys. Chem. B 107 (2003) 12597e12604. [5] E. Meulenkamp, J. Phys. Chem. B 102 (1998) 5566e5572. [6] L. Spanhel, M.A. Anderson, J. Am. Chem. Soc. 113 (1991) 2826e 2833. [7] E.M. Wong, J.E. Bonevich, P.C. Searson, J. Phys. Chem. B 102 (1998) 7770e7775. [8] Q. Tang, W. Zhou, J. Shen, W. Zhang, L. Kong, Y. Qian, Chem. Commun. (2004) 712e713. [9] C. Wang, E. Shen, E. Wang, L. Gao, Z. Kang, C. Tian, Mater. Lett. 59 (2005) 2867e2871. [10] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott, J. Am. Chem. Soc. 124 (2002) 12954e12955. [11] C. Wang, E. Wang, E. Shen, L. Gao, Mater. Res. Bull. 41 (2006) 2298e2302. [12] H. Du, F. Yuan, S. Huang, J. Li, Y. Zhu, Chem. Lett. 33 (2004) 770e771.
G. Rani and P.D. Sahare: J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
[13] E. Hosono, S. Fujihara, T. Kimura, Electrochem. Solid State Lett. 7 (2004) C49eC51. [14] J. Zhou, N.S. Xu, Z.L. Wang, Adv. Mater. 18 (2006) 2432e 2435. [15] B.L. Zhu, C.S. Xie, D.W. Zeng, W.L. Song, A.H. Wang, Mater. Chem. Phys. 89 (2005) 148e153. [16] S.W. Kim, T. Kotani, M. Ueda, S. Fujita, S. Fujita, Appl. Phys. Lett. 83 (2003) 3593e3595. [17] S.W. Kim, M. Ueda, M. Funato, S. Fujita, J. Appl. Phys. 97 (2005) 104316e104324. [18] K.K. Kim, N. Koguchi, Y.W. Ok, T.Y. Seong, S.J. Park, Appl. Phys. Lett. 84 (2004) 3810e3812. [19] J.K. Lee, C.R. Tewell, R.K. Schulze, M. Nastasi, D.W. Hamby, D. A. Lucca, H.S. Jung, K.S. Hong, Appl. Phys. Lett. 86 (2005) 183111. [20] E.W. Seeling, B. Tang, A. Yamilov, H. Cao, R.P.H. Chang, Mater. Chem. Phys. 80 (2003) 257e263. [21] L. Madler, W.J. Stark, S.E. Pratsinis, J. Appl. Phys. 92 (2002) 6537e6540.
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[22] B.D. Cullity, Element of X-ray Diffraction, second ed., AddisonWesley, New York, 1956, p. 99. [23] J. Tauc, A. Menthe, J. Non-Cryst. Solids 56 (1972) 8e10. [24] A. Van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 90 (2000) 123e128. [25] Z. Li, Y. Xiong, Y. Xie, J. Inorg. Chem. 42 (2003) 8105e8109. [26] D. Li, Y.H. Leung, A.B. Djurisic, Z.T. Liu, M.H. Xei, S.L. Shi, S.J. Xu, W.K. Chan, Appl. Phys. Lett. 85 (2004) 1601e1603. [27] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983e7990. [28] M. Liu, A.H. Kitai, P. Mascher, J. Lumin. 54 (1992) 35e42. [29] X.L. Wu, G.G. Siu, C.L. Fu, H.C. Ong, Appl. Phys. Lett. 78 (2001) 2285e2287. [30] A.I. Ekimov, I.A. Kudryavtsev, M.G. Ivanov, A.L. Efros, J. Lumin. 46 (1990) 83e95. [31] U. Woggon, in: R. Helbig (Ed.), Advances in Solid State Physics, vol. 35, Vieweb, Braunschweig, 1996, p. 145. [32] U. Woggon, M. Saleh, A. Uhring, M. Portune, C. Klingshrin, J. Cryst. Growth 138 (1994) 988e992.
ScienceDirect J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
Structural and Spectroscopic Characterizations of ZnO Quantum Dots Annealed at Different Temperatures Geeta Rani*, P.D. Sahare Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India [Manuscript received October 28, 2012, in revised form February 4, 2013, Available online 30 August 2013]
Here, we report the synthesis and characterizations of solegel derived zinc oxide (ZnO) quantum dots (QDs) using zinc acetate dihydrate (Zn(CH3COO)2$2H2O) and lithium hydroxide monohydrate (LiOH$H2O) as raw material. The as-prepared ZnO QDs was annealed at different temperature (400, 700, and 900 C) and the structural, optical properties were investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), UVeVis and photoluminescence (PL) spectroscopy. The powder XRD patterns of the obtained samples showed the formation of single-phase wurtzite structure and the morphological changes have been observed with increasing annealing temperature. In the absorption spectra, the optical band gap of nanocrystalline ZnO QDs decreased from 3.18 to 3.11 eV and the particle size increased with increasing temperature. In the PL spectra, a broad green emission peak related to defect levels in the visible range of the spectra have been recorded. KEY WORDS: ZnO quantum dots; Photoluminescence; UVeVis spectroscopy
1. Introduction Structural and optical properties of semiconductor nanocrystals are important from both a fundamental science and a proposed photonic application point of view. Zinc oxide (ZnO) is the richest family of nanostructures including nanorods, nanopencils, nanotubes and so on. It is an important IIeVI semiconductor with a direct wide band gap (3.37 eV) at room temperature and a large exciton binding energy (60 MeV). ZnO is currently attracting worldwide intense interests because of its importance in fundamental studies and its numerous applications especially as optoelectronic materials[1]. Recent researches show that ZnO nanomaterials can be used as promising piezoelectric nanogenerators, which triggers a wide range of subsequent research in searching for new synthetic methods to synthesize ZnO nanomaterials[2]. Among various ZnO nanostructures, ZnO nanoparticles are the most frequently studied[3] because of their interest in fundamental study and also their applied aspects such as in solar energy conversion, photo catalysis, light-emitting materials[4], transparent UV protection films and chemical sensors[5]. Hitherto, searching new methodology to synthesize Corresponding author. Tel.: þ91 9560752969; E-mail address: [email protected] (G. Rani). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.08.015 *
uniform ZnO nanoparticles is still of great importance for both fundamental study and application of ZnO nanomaterials. Various methods have been developed for the synthesis of ZnO nanoparticles with uniform morphology and size[6,7]. Among these synthetic routes, solution chemical methods provide a promising way for low cost and large-scale production, which do not need expensive raw materials and complicated equipments[8,9]. However, most solution methods to use Zn salts as the starting material and the reactions are generally carried out in water or alcohol by the hydrolysis of Zn salts in the presence of a base[10,11]. It has been demonstrated that the anion of the zinc salts and the cation of the base have great effect on the formation of the ZnO nanocrystals[12,13]. Moreover, most methods exploit surface modification or protecting reagents to prevent from agglomeration and to control the morphology development of ZnO nanocrystals. There are still few reports about synthesizing ZnO nanoparticles at mild condition that do not need any protecting reagents and complicated process control. ZnO QDs are of great interest because the three-dimensional confinement of carrier and phonon leads to not only continuous tuning of the optoelectronic properties but also improvement in device performance. The special application advantages of ZnO QDs and nanoparticles in the area of bioscience also improve more attention due to its “survival lifetime” of a few hours, the slow solubility and high compatibility in biofluid and necessaries of Zn for human beings everyday[14]. Therefore, a number of reports of fabrication, structural, and optical characterization of ZnO QDs have been done in recent years[15e21].
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G. Rani and P.D. Sahare: J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
In the present work, nearly uniform spherical ZnO QDs with no agglomeration were synthesized. The growth process and morphology evolution of the ZnO QDs were investigated. The results indicated that the morphology evolution depends on the annealing temperature. Some spectroscopic studies were also discussed. 2. Experimental 2.1. Sample preparation Zinc acetate dihydrate (99.2%) [Zn(CH3COO)2$2H2O], lithium hydroxide monohydrate (LiOH$H2O), ethanol were procured from CDH and Merck, India, and were used to synthesize ZnO QDs. All the chemicals were of AR grade and were directly used without special treatment. In a typical preparation method, 1 mol/l Zn(CH3COO)2$2H2O was prepared in 50 ml of ethanol using vigorous stirring at 50 C for 90 min, followed by cooling at room temperature. LiOH (0.59 g) was dissolved in 50 ml of ethanol and kept in ultrasonic bath then cooled at room temperature. This solution was slowly added to the Zn2þ solution with vigorous stirring at room temperature. The precipitation of ZnO QDs from the transparent sol was carried out by addition of n-heptane followed by vigorous stirring. The ratio of volume of n-heptane to sol was 3:2. After centrifugation, the precipitate was collected and re-dispersed into ethanol. The removal of excess acetate and lithium ions from the dispersed ZnO was accomplished by repetitive washing of ZnO with ethanol and n-heptane. The obtained precipitate was dried at room temperature resulting in white powders of ZnO QDs. The obtained powder was annealed in air at different temperatures (400, 700, and 900 C) for 3 h.
Fig. 1 XRD patterns of the ZnO QDs of as-prepared sample (a) and samples annealed at: (b) 400 C, (c) 700 C and (d) 900 C.
XRD, all the diffraction peaks can be readily indexed to a wurtzite structure of ZnO (space group P63mc), which is well matched with the standard data (JCPDS, CARD No. 80-0075, a ¼ b ¼ 0.3253 nm, c ¼ 0.5209 nm). The results indicate that the products consisted of pure phases after annealing and the diffraction peaks related to the any impurities were not observed in the XRD pattern confirming the high purity of the synthesized product. It could be seen that the diffraction peaks shown in Fig. 1(d) were more intensive and narrower, implying a good crystalline nature and larger particle size of the sample annealed at 900 C. The average crystallite size was estimated using FWHM (full width at half maximum) value from the line broadening of the XRD peaks using Scherrer’s formula[22]:
2.2. Sample characterizations The structural and optical properties of the obtained powder samples were characterized by different techniques. The X-ray diffraction (XRD) was done on the machine D8-Advance model of Bruker Inc. using monochromatic CuKa line (l ¼ 0.154056 nm) and was compared with the available PCPDF data files to confirm the formation of different phases in the material. The high resolution transmission electron microscopy (HRTEM) images were taken to study the morphology (shape and size) of the nanocrystalline material. The photographs of the images were taken on a transmission electron microscope (FEI TEM model TECNAI G2 T30, U-TWIN) at 300 kV. The TEM samples were prepared by putting a drop of the solution of suspended nanoparticles in anhydrous ethanol on a carbon coated copper grid (200 meshes). The UVevisible spectra of the material were taken in wavelength range from 250 to 600 nm using Evolution UVe Vis. spectrophotometer, Thermo Scientific Inc. to estimate the band gaps of the material. Photoluminescence (PL) spectra were carried out on a fluorescence spectrophotometer (Fluorolog Horiba JOBIN YVON) using Xe lamp with excitation wavelength 350 nm to study the different kinds of defects in the material. 3. Result and Discussion 3.1. Structural and morphological investigations Fig. 1 represents the typical XRD pattern of the samples annealed at different temperatures. Within the detection limits of
D ¼
0:9l bcos qB
(1)
where D is the average crystallite size, l is the incident wavelength, qB is the Bragg angle and b is the diffracted FWHM (radian). The average particle sizes of the ZnO QDs calculated from the most intense peak are approximately 3e 5 nm, 50 nm, 170 nm, 250 nm for the as-prepared and samples annealed at 400, 700 and 900 C, respectively as expected from the sharpened and enhanced diffraction peaks. The TEM images and SAED (selected-area electron diffraction) pattern of the as-prepared and samples annealed at different temperature are shown in Fig. 2 revealing the particles growth with increasing temperature. The SAED patterns manifested that crystallinity of ZnO nanoparticles increased and some bright and dark rings with diffraction spots also appeared in the diffraction patterns at higher temperatures. The increase in crystallinity of the nanoparticles might be due to the long range ordering of the unit cells with increasing annealing temperature. The ZnO QDs are found to be spherical in shape and almost mono-dispersed in size (w5 nm) and the shape and size of nanoparticles changed with increasing temperature. However, with the increase of the annealing temperature from 400 to 900 C, the shape of the nanoparticles distorted and randomly shaped nanoparticles were obtained. Apart from the change in shape of the nanoparticles, the size of nanoparticles increased (also changed). The average crystallite size from these micrographs has been calculated by taking the average of highest, smallest and medium size particles of ZnO. The particle sizes were found to be w3, 45, 155, and
G. Rani and P.D. Sahare: J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
1037
Fig. 2 TEM images with the SAED patterns of ZnO QDs of as-prepared (a) and annealed samples at: (b) 400 C, (c) 700 C, and (d) 900 C.
275 nm corresponding to as-prepared and samples annealed at 400, 700 and 900 C, respectively. The average size of the nanoparticles measured from the TEM analysis was corroborated with the XRD analysis. 3.2. Optical studies Fig. 3 shows the UVevisible absorption spectra of ZnO QDs annealed at different temperature. It can be seen that the absorbance of the samples increases with increasing annealing temperature. As the transparency of the samples have a direct consequence with the bands gap i.e. the band gap of the sample increases with increasing transparency or decreasing absorbance. The band gap of the sample also depends on the particles size
and decreases with increasing particle size. Thus, the increase in absorbance and particle size infer the decrease of the band gap of the samples with increasing temperature. Furthermore, a red-shift was also observed in the characteristic absorption peak of ZnO QDs, with increasing temperature, and was attributed to the decrease of the band gap of the samples. For the allowed direct transition, the variation of a with photon energy (hn) obeys Tauc’s plot[23] ðahnÞ2 ¼ A hn Eg
(2)
where A is a constant, Eg is band gap, h is Plank constant and a is the absorption coefficient. Fig. 4 illustrates the (ahn)2 vs hn plot used for the evaluation of the band gap of ZnO QDs.
1038
G. Rani and P.D. Sahare: J. Mater. Sci. Technol., 2013, 29(11), 1035e1039
Fig. 3 Optical absorption spectra of ZnO QDs annealed samples at: (a) 400 C, (b) 700 C, (c) 900 C.
Fig. 5 PL emission spectra of the ZnO QDs annealed samples at: (a) 400 C, (b) 700 C, (c) 900 C. The samples were measured at room temperature with excitation wavelength of 350 nm.
Inset of Fig. 4 revealed that the band gap of the obtained samples decreased from 3.18 to 3.11 eV with increasing temperature. The qualitative informations about the band gap evaluated from Fig. 3 are well corroborated with the quantitative results of Fig. 4. The PL properties of the ZnO QDs were investigated by a excitation of 350 nm and a well-known broad green emission band in the range 400e650 nm was observed as shown in Fig. 5, which is considered to be related to the single ionized oxygen vacancy due to the recombination of a photon-generated hole with the single ionized charged state of the defect in ZnO QDs[24,25] and it was reported that the oxygen vacancies responsible for the green emission are mainly located at the surface[26e29]. It is also found from the spectra that the peak at 528 nm in the sample annealed at 400 C changes its position and a red-shift of about 4 nm was observed with increasing temperature, probably related to the decrease of the band gap. The shift in PL peak due to size is quite common in the case of semiconductor QDs studies[30e32] because the samples grew in size due to annealing in an oxygen atmosphere and this could be assigned to too many interdependent factors, such as electrone phonon coupling, lattice distortion, localization of charge carriers due to interface effects and point defects. The increase in the peak intensity indicates that surface to volume ratio decreases; therefore surface defects decreases with increasing annealing
temperature and resulting in the enhancement of the ZnO QDs crystallinity. As a result, the PL investigation shows the influence of annealing temperature on the PL properties of the ZnO QDs and therefore its potential application in UV emission devices.
Fig. 4 Plot of (ahn)2 vs hn of ZnO QDs annealed samples at: (a) 400 C, (b) 700 C, (c) 900 C. Inset shows the variation of band gap with temperature.
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