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Synthesis of nanostructured sol–gel ITO films at different temperatures and study of their absorption and photoluminescence properties
Optical Materials 31 (2008) 429–433
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis of nanostructured sol–gel ITO films at different temperatures and study of their absorption and photoluminescence properties Susmita Kundu 1, Prasanta K. Biswas * Sol–Gel Division, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Kolkata 700032, West Bengal, India
a r t i c l e
i n f o
Article history: Received 29 August 2007 Received in revised form 23 May 2008 Accepted 3 June 2008 Available online 23 July 2008 PACS: 42.70.Qs 81.40.tv 73.61.Le 73.63.Bd
a b s t r a c t Nanostructured indium tin oxide (ITO) films were deposited on silica glass by sol–gel dipping method from salt derived PVA based aqueous precursor. The films were cured at 250 °C, 350 °C, 450 °C, 600 °C, 700 °C and 900 °C and characterized by XRD, SEM, AFM techniques to observe heating effect on nanostructured feature. Nanocluster sizes were determined by TEM study. Different crystal phases of ITO were existed in the temperature range 250–900 °C. Quantum confinement behavior of the nanoclusters was observed for their size being near Bohr radius. Absorption, band gap and photoluminescence behavior of the nanstructured ITO films supported excitonic transitions due to the formation of electron hole pair generated by interaction of electromagnetic radiation. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Indium tin oxide Sol–gel Nanoclusters Absorption Photoluminescence
1. Introduction Indium tin oxide (ITO) is an important material as it is one of the most widely used transparent conducting oxides (TCO) required as a component in photovoltaic cells, sensor modules, transparent electrodes in plasma display panels, etc. [1–4]. Particularly its application in organic photovoltaics has given much attention to the material scientists [5–9]. However, efforts are also going on for the development of cost effective materials such as Al– ZnO [10], Sb–SnO2 [11] having similar properties. We have also started sol–gel synthesis of the above type cost effective materials. But in the present context, we like to focus the basic study of nanostructured ITO prepared under different conditions as extensive work has already been done on ITO for various applications as stated above. If ITO be synthesized in nanostructured forms having cluster size close to the Bohr radius [12] then it may be used as a promising candidate for nonlinear optical (NLO) applications due to its quantum confinement effect. Recently several attempts have
been made to synthesize nano ITO by different methods [4,13–15]. In our previous paper [16] we have reported the synthesis of nanostructured ITO film on silica glass at a particular temperature using sol–gel procedure. We have also studied the photoluminescence property of the above-synthesized ITO with variation of excitation wavelength to know the quantum confinement effect in it. The nanostructured feature of any system depends on its cluster size and shape, which again may be controlled by variation of processing temperature [15]. Hence it is important to understand the characteristics of nanostructured materials of different size and shapes if it can be developed from the same material. We have observed variation of nanostructural feature in ITO if the processing temperature be varied and this type of study on sol–gel ITO system was not done earlier. In this paper we are reporting the synthesis and characterization of sol–gel dip coated nanostructured ITO films prepared at different temperatures. The characterizations were done by examining X-ray diffraction pattern, microstructure, absorption, band gap and photoluminescence properties. 2. Experimental
* Corresponding author. Tel.: +91 33 2483 8082x3303; fax: +91 33 2473 0957. E-mail addresses: [email protected] (S. Kundu), [email protected] (P.K. Biswas). 1 Presently in Central Mechanical Engineering Research Institute, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India. 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.06.005
The aqueous precursor solution of indium tin oxide was prepared maintaining the ratio of In:Sn as 90:10 using the same procedure as described in our earlier report [16] where hydrated
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S. Kundu, P.K. Biswas / Optical Materials 31 (2008) 429–433
indium nitrate was synthesized from indium ingots (SRL, India, purity 99.9%) by reaction with concentrated nitric acid (E. Merck India Ltd.) and the recrystallized SnCl4, 5H2O (Loba Chemie, India, purity, 98%) was the starting material for dopant. Concentration of the sol/solution was kept at 6.0 wt% equivalent In2O3. PVA (mol. wt. approx. 22,000, BDH, UK,) was used as an organic binder. Thin films of 500–2000 ±5 Å thickness (measured by Gartner autogain L116B ellipsometer) were developed on Heraus make (Germany) Suprasil grade pure silica glass by the dipping technique (Chemat 200, USA). The nanostructured feature of ITO was controlled by changing initially the curing temperature from 250 °C to 450 °C and then annealing at 600 °C, 700 °C and 900 °C with a soaking of 30 min in each case. The morphology, particle size distribution and surface topography were studied by SEM (Leo 400c), contact mode AFM (Nanoscope IV) and HRTEM (Jeol 2110) experiments, respectively. Carbon coated Cu-grid was used for HRTEM image. Crystalline phases were identified by XRD and ED patterns using X-ray diffractometer (Philips PW-1730 (Ni-filtered Cu Ka radiation)) and transmission electron microscopy, respectively. Absorption and fluorescence spectra were recorded by using Shimadzu UV–VIS-NIR (model 3101PC) spectrophotometer (resolution, 0.10 nm; photometric accuracy, 0.004 (1A)) and
Perkin Elmer fluorimeter (LS55), respectively. The wavelength accuracy of LS55 is ±1.0 nm and its sensitivity is signal to noise, 500:1 r.m.s. using the Raman band of water with excitation at 350 nm; excitation and emission band pass is 10 nm. The source for LS55 is Xenon discharge lamp, equivalent to 20 kW for 8 ls duration. Pulse width at half height is <10 ls. Spectra were recorded using computer selectable cut off (high-pass) filters at 290 and 430 nm for excitations at 278 and 423 nm, respectively. Excitation width and power density was 10 nm and 775 V, respectively, in each case. 3. Results and discussion Scanning electron microscopy (SEM) images of the nanostructured ITO films developed by heating the dip coated samples at different temperatures reveals different features (Fig. 1). The film cured at 250 °C shows two types of cluster size; on increasing the temperature to 350 °C, cubic ITO of single mode size distribution of relatively low dimension was obtained [16]. Generally particle size increases with temperature but in this case the observed anomalous behavior may be ascribed to initial formation of shell like microsphere of PVA for its surfactant behavior [17]. The hydro-
Fig. 1. SEM images of nanostructured sol–gel ITO films cured at (1) 250 °C, (2) 350 °C, (3) 450 °C, (4) 600 °C, (5) 700 °C and (6) 900 °C.
S. Kundu, P.K. Biswas / Optical Materials 31 (2008) 429–433
lysed Sn-doped indium was possibly condensed with the hydroxyl group of PVA and existing in the core of the shell which was distinctly observed in the AFM image (not shown here). During curing to 250 °C the surface layer of PVA was possibly breaking partially generating clusters of two size distributions as it decomposes near this temperature. On increasing the temperature to 350 °C, the PVA is fully decomposed and homogeneous nanoclusters produced. In case of 450 °C, grains of relatively large dimensions along with the small one were found to be present in an amorphous medium as observed in SEM image. On the other hand, 600 °C sample depicted the growth of small grains in the film which transformed to dense homogeneous nanoclusters of rhombohedral phase at
Fig. 2. AFM images of nanostructured sol–gel ITO films cured at (a) 450 °C and (b) 600 °C.
b
<222>(c)
Intensity (a.u.)
<400>(c)
<440>(c)
(6) (5)
(6)
Intensity (a.u.)
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-200
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-300
30
40
50
2 theta in degree
60
30
32
34
2 theta in degree
Fig. 3. X-ray diffraction (XRD) pattern of nanostructured sol–gel ITO films cured at different temperatures in the 2h range 28–65° (a); expanded XRD in the 2h range 29–35° (b).
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700 °C. On further heating the sample at 900 °C, cluster size increased again due to high temperatures that are not distinct in the SEM image. The anomalous feature at 450 °C film was possibly for the initiation of change in crystal phase from cubic to rhombohedral. The AFM images (Fig. 2) of 450 °C and 600 °C samples also show the similar features i.e. at 450 °C the film contains a melting phase whereas at 600 °C, small crystallites of different shapes started to generate from the melting region. The X-ray diffraction patterns of the films (Fig. 3a and b) are also in agreement with the SEM and AFM features. The 250 °C films have the amorphous feature due to formation of PVA shell like microsphere containing very small sizes of the nanoclusters. The films heated at higher temperatures shows bixbyite cubic (c) structure of indium oxide [18] with oxygen deficient (V0) cubic In2xSn2O7x entity [19]. At 450 °C, although SEM features does not reveal any grains but XRD gives a cubic feature probably due to the presence of some crystals which still remain in the cubic forms. Rhombohedral (r) [20] phase started to grow from 600 °C, and at 700 °C sample the r phase becomes dominant but further increasing the annealing temperature to 900 °C, the cluster size increased and the rhombohedral phase transformed to cubic phase. The high resolution TEM (Fig. 4a and b) of 350 °C sample having average 4.7 nm cluster size [16] exhibited the separation of 222 planes of cubic phase in the fringe pattern [19] of the crystal domain. The ED patterns also show the bixbyite cubic structure of ITO. On increasing the curing temperature to 700 °C the transformation of cubic phase to rhombic phase is also evident from the high resolution TEM of 700 °C cured sample. It depicts almost uniform shape having hexagonal nanocluster size of around 12 nm. Fringe pattern at the crystal domain exhibited the separation of 104 planes of rhombohedral [20] phase. The TEM image of 600 °C cured sample shows a mixed phase of cubic and rhombohedral crystal symmetry having cluster size around 8 nm (not shown here). It is evident that the nanocluster size of all the ITO cured at different temperatures as obtained from the TEM images are quite close to exciton Bohr radius of In2O3 [16,21]. As the size of nanocluster of a semiconductor approaches to Bohr radius, a blue shift in energy occurs due to the quantum confinement phenomenon resulting in hydrogen like exciton formation [10,22]. In our previous report we have already observed the quantum confinement effect of nanoclusters examining their size by TEM and studying their absorption, band gap and photoluminescence properties. Generally in any nanoclusters the increase in particle size due to increase in processing temperature would lead to lowering of exciton transition energy. We have taken the absorption spectra (Fig. 5a) of all the ITO films cured at different
Fig. 4. HRTEM images of nanostructured sol–gel ITO films with lattice fringes and ED patterns, cubic (a) 350 °C sample, rhombohedral (b) 700 °C sample.
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1.0
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d (O. D) / dλ (a.u)
a
400 450 500 550 600
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Fig. 5. (a) Absorption spectra, (b) second derivative (d(OD)2/dk2) of the absorption spectra of nanostructured sol–gel ITO films cured at different temperatures.
ahv / ðhv EÞ1=2
ð1Þ
Due to formation of the nanoclusters blue shift of the bulk band gap of ITO (3.65 eV) [23] was observed. The band gap at 4.0 eV is for cubic nanoclusters and corresponds to the excitonic transition appearing at 308 nm for all the samples. In addition, presence of another band gap around 3.5 eV is equivalent to the bulk band gap of ITO as mentioned above. This suggests the existence of both free carrier and quantum confinement phenomena in nanoclustered system. This observation implies the effectiveness of using the relation (Eq. (1)) even for nanoclusters. Besides this, in our previous work [16] the nanocluster size calculated using the band gaps obtained from the above relation was quite close to the average particle size of ITO determined by the TEM experiment. We obtained another band gap value at 4.5 eV possibly for the existence of rhombohedral phases in 600 °C and 700 °C cured samples. We have recorded fluorescence spectra of the samples. When we excited the samples (Fig. 6) at 278 nm, the emission peaks at 423, 484 and 530 nm with a hump at 439 nm were observed for bound excitons (i.e. excitons [24] trapped at the defect centers of ITO [25,26]). The peak at 400 nm in each case is possibly for the emissions of free excitons [16,24] i.e. they are not trapped in defect
1000 253 Excitonic Transition
800
PLIntensity (a.u)
423 (2)
393
440
PL Intensity
temperatures from 250 °C to 900 °C but we did not observe any distinct feature of red or blue shift of absorption peak due to the excitonic transition. We found a broad peak at around 300 nm in all the samples. To identify the peak positions we have taken the second derivatives (Fig. 5b) of the absorption spectra and found that in every samples there is a peak at 307.5 nm except for 700 °C sample, for which the peak is red shifted to 316 nm. For 600 °C sample there is another peak at 281 nm which becomes more prominent with little shift to 286 nm for 700 °C. This peak was not observed for other samples. It can be inferred from the previous work [16] that the peak at 308 nm is for cubic nanoclusters. The derivatives of the absorption spectra of 350 °C and 450 °C cured samples exhibit that 308 nm peak splits into 302 and 312 nm and this predicts different size distribution of the samples. As phase transformation started at 450 °C and the rhomobohedral phase dominated at 600 °C and 700 °C samples, the peak at 280 nm for 600 °C shifted to 286 nm for 700 °C heated sample. This may be assigned for the formation of rhombohedral phase of nanoclusters. For this particular sample, the red shifted value at 316 nm may be due to the increase in size of some of the cubic clusters partially which is expected to exist even at 700 °C cured sample. We evaluated effective band gap of the nanoclusters cured at different temperatures from the plot, (ahm)2 versus hm (Eq. (1)). It is the direct band gap of typical semiconductors, where a is the absorption co-efficient in the edge region, hm is the photon energy and E is the band gap.
600
(3)
PLE Spectra 269 278 286 292 298 308 250
275
300
Wavelength (nm)
485 400
(5)
530
(4)
200
(6) 0 400
450
500
Wavelength (nm) Fig. 6. Photoluminescence (PL) spectra of nanostructured sol–gel ITO films cured at different temperatures excited at kex, 278 nm (inset shows the photoluminescence excitonic (PLE) spectrum when the emission be fixed at kem, 423 nm).
centers. If the excitons be bound with the phonons then a number of inflexions may appear in the broad emission band due to the exciton–phonon interaction [27]. This is prominent for the samples cured at relatively high temperatures, from onwards 450 °C as evident from the appearance of a number of inflexions at the broad 400 nm band (Fig. 6). The excitation wavelength was obtained [16] from the photoluminescence excitation (PLE) spectrum shown at the inset of Fig. 6. The appearance of number of PLE bands at higher energy levels was possibly for exciton–phonon interaction. Usually this interaction may be inferred from the PLE and PL spectra unlike the absorption spectra. The structural difference due to the change in curing temperature does not have any influence on the emission peaks of the bound excitons. From the intensity ratio of the free exciton peak at 400 nm and the bound excitonic peak at 423 nm in each case, it can be seen that the intensity of the free excitons is relatively high for the samples where the rhomobohedral phases dominates. Rhomobohedral phase is the distorted cubic structure and the defect sites in the rhombohedral system may not be strong enough to trap the free excitons. 4. Conclusion Nanostructured feature of ITO as evident from SEM, TEM and AFM studies was changed if the curing temperature be changed. On increasing temperature to 700 °C, the bixbyite cubic ITO
S. Kundu, P.K. Biswas / Optical Materials 31 (2008) 429–433
transformed to rhombohedral which again transformed to cubic form at 900 °C. All the samples revealed the generation of excitonic transitions by excitation with electromagnetic radiation. Absorption, band gap and PL properties supported the quantum confinement behavior. Exciton–phonon interaction was predominated in the samples heated above 350 °C as revealed by PL study. Acknowledgement Authors are thankful to Dr. H.S. Maiti, Director, CGCRI, Kolkata for his constant encouragement to carry out this work under CTSM Programme [# CMM 0022 (1)]. References [1] [2] [3] [4] [5] [6] [7]
A. Antony, M. Nisha, R. Manoj, M.K. Jayaraj, Appl. Surf. Sci. 225 (2004) 294. K. Utsumi, O. Matsunaga, T. Takahata, Thin Solid Films 334 (1998) 30. N.G. Patel, P.D. Patel, V.S. Vaishnav, Sensors Actuators B 96 (2003) 180. S.J. Limmer, K. Takahashi, G. Cao, Proc. SPIE 5224 (2003) 25. H. Spanggaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 83 (2004) 125. E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954. H. Hoppe, N.S. Sariciftci, J. Mater. Res. 19 (2004) 1924.
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[8] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [9] S. Gunes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324. [10] H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makinen, Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420–421 (2002) 539. [11] L.K. Dua, A. De, S. Chakraborty, P.K. Biswas, Mater. Charact. 59 (2008) 578. [12] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [13] Z. Guisheng, X. Huarui, L. Chuntu, J. Inorg. Mater. 20 (2005) 479. [14] B. Balamurugana, F.E. Kruis, Appl. Phys. Lett. 86 (2005) 083102. [15] S. Li, X. Qiao, J. Chen, H. Wang, F. Jia, X. Qiu, J. Cryst. Growth 289 (2006) 151. [16] S. Kundu, P.K. Biswas, Chem. Phys. Lett. 414 (2005) 107. [17] B. Yu, C. Zhu, F. Gan, Opt. Mater. 7 (1997) 15. [18] S.S. Kim, S.Y. Choi, C.G. Park, H.W. Jin, Thin Solid Films 347 (1999) 155. [19] A. Soloveva, V. Zhdanov, Inorg. Mater. 21 (1985) 828. [20] B.C. Kim, Sung-Min Kim, Joon-HyangLee, Jeong-Joo Kim, J. Am. Ceram. Soc. 85 (2002) 2083. [21] X.S. Peng, G.W. Meng, J. Zhang, X.F. Wang, Y.W. Wang, C.Z. Wang, L.D. Zhang, J. Mater. Chem. 12 (2002) 1602. [22] S. Jana, P.K. Biswas, Mater. Lett. 32 (1997) 263. [23] G.D. Gilliland, Mater. Sci. Eng. R 18 (1997) 99. [24] A.E. Hichou, A. Kachouane, J.L. Bubendorff, M. Addou, J. Ebothe, M. Troyon, A. Bougrine, Thin Solid Films 458 (2004) 263. [25] H.Y. Peng, C.W. Lee, H.O. Everitt, D.S. Lee, A.J. Stecki, J.M. Zavada, Appl. Phys. Lett. 86 (2005) 051110-1. [26] K. Nanda, S.N. Sahu, Adv. Mater. 13 (2001) 280. [27] S.F. Ren, Z.Q. Gu, D. Lu, Solid State Commun. 113 (2000) 273.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Synthesis of nanostructured sol–gel ITO films at different temperatures and study of their absorption and photoluminescence properties Susmita Kundu 1, Prasanta K. Biswas * Sol–Gel Division, Central Glass and Ceramic Research Institute, 196 Raja S.C. Mullick Road, Kolkata 700032, West Bengal, India
a r t i c l e
i n f o
Article history: Received 29 August 2007 Received in revised form 23 May 2008 Accepted 3 June 2008 Available online 23 July 2008 PACS: 42.70.Qs 81.40.tv 73.61.Le 73.63.Bd
a b s t r a c t Nanostructured indium tin oxide (ITO) films were deposited on silica glass by sol–gel dipping method from salt derived PVA based aqueous precursor. The films were cured at 250 °C, 350 °C, 450 °C, 600 °C, 700 °C and 900 °C and characterized by XRD, SEM, AFM techniques to observe heating effect on nanostructured feature. Nanocluster sizes were determined by TEM study. Different crystal phases of ITO were existed in the temperature range 250–900 °C. Quantum confinement behavior of the nanoclusters was observed for their size being near Bohr radius. Absorption, band gap and photoluminescence behavior of the nanstructured ITO films supported excitonic transitions due to the formation of electron hole pair generated by interaction of electromagnetic radiation. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Indium tin oxide Sol–gel Nanoclusters Absorption Photoluminescence
1. Introduction Indium tin oxide (ITO) is an important material as it is one of the most widely used transparent conducting oxides (TCO) required as a component in photovoltaic cells, sensor modules, transparent electrodes in plasma display panels, etc. [1–4]. Particularly its application in organic photovoltaics has given much attention to the material scientists [5–9]. However, efforts are also going on for the development of cost effective materials such as Al– ZnO [10], Sb–SnO2 [11] having similar properties. We have also started sol–gel synthesis of the above type cost effective materials. But in the present context, we like to focus the basic study of nanostructured ITO prepared under different conditions as extensive work has already been done on ITO for various applications as stated above. If ITO be synthesized in nanostructured forms having cluster size close to the Bohr radius [12] then it may be used as a promising candidate for nonlinear optical (NLO) applications due to its quantum confinement effect. Recently several attempts have
been made to synthesize nano ITO by different methods [4,13–15]. In our previous paper [16] we have reported the synthesis of nanostructured ITO film on silica glass at a particular temperature using sol–gel procedure. We have also studied the photoluminescence property of the above-synthesized ITO with variation of excitation wavelength to know the quantum confinement effect in it. The nanostructured feature of any system depends on its cluster size and shape, which again may be controlled by variation of processing temperature [15]. Hence it is important to understand the characteristics of nanostructured materials of different size and shapes if it can be developed from the same material. We have observed variation of nanostructural feature in ITO if the processing temperature be varied and this type of study on sol–gel ITO system was not done earlier. In this paper we are reporting the synthesis and characterization of sol–gel dip coated nanostructured ITO films prepared at different temperatures. The characterizations were done by examining X-ray diffraction pattern, microstructure, absorption, band gap and photoluminescence properties. 2. Experimental
* Corresponding author. Tel.: +91 33 2483 8082x3303; fax: +91 33 2473 0957. E-mail addresses: [email protected] (S. Kundu), [email protected] (P.K. Biswas). 1 Presently in Central Mechanical Engineering Research Institute, Mahatma Gandhi Avenue, Durgapur 713209, West Bengal, India. 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.06.005
The aqueous precursor solution of indium tin oxide was prepared maintaining the ratio of In:Sn as 90:10 using the same procedure as described in our earlier report [16] where hydrated
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S. Kundu, P.K. Biswas / Optical Materials 31 (2008) 429–433
indium nitrate was synthesized from indium ingots (SRL, India, purity 99.9%) by reaction with concentrated nitric acid (E. Merck India Ltd.) and the recrystallized SnCl4, 5H2O (Loba Chemie, India, purity, 98%) was the starting material for dopant. Concentration of the sol/solution was kept at 6.0 wt% equivalent In2O3. PVA (mol. wt. approx. 22,000, BDH, UK,) was used as an organic binder. Thin films of 500–2000 ±5 Å thickness (measured by Gartner autogain L116B ellipsometer) were developed on Heraus make (Germany) Suprasil grade pure silica glass by the dipping technique (Chemat 200, USA). The nanostructured feature of ITO was controlled by changing initially the curing temperature from 250 °C to 450 °C and then annealing at 600 °C, 700 °C and 900 °C with a soaking of 30 min in each case. The morphology, particle size distribution and surface topography were studied by SEM (Leo 400c), contact mode AFM (Nanoscope IV) and HRTEM (Jeol 2110) experiments, respectively. Carbon coated Cu-grid was used for HRTEM image. Crystalline phases were identified by XRD and ED patterns using X-ray diffractometer (Philips PW-1730 (Ni-filtered Cu Ka radiation)) and transmission electron microscopy, respectively. Absorption and fluorescence spectra were recorded by using Shimadzu UV–VIS-NIR (model 3101PC) spectrophotometer (resolution, 0.10 nm; photometric accuracy, 0.004 (1A)) and
Perkin Elmer fluorimeter (LS55), respectively. The wavelength accuracy of LS55 is ±1.0 nm and its sensitivity is signal to noise, 500:1 r.m.s. using the Raman band of water with excitation at 350 nm; excitation and emission band pass is 10 nm. The source for LS55 is Xenon discharge lamp, equivalent to 20 kW for 8 ls duration. Pulse width at half height is <10 ls. Spectra were recorded using computer selectable cut off (high-pass) filters at 290 and 430 nm for excitations at 278 and 423 nm, respectively. Excitation width and power density was 10 nm and 775 V, respectively, in each case. 3. Results and discussion Scanning electron microscopy (SEM) images of the nanostructured ITO films developed by heating the dip coated samples at different temperatures reveals different features (Fig. 1). The film cured at 250 °C shows two types of cluster size; on increasing the temperature to 350 °C, cubic ITO of single mode size distribution of relatively low dimension was obtained [16]. Generally particle size increases with temperature but in this case the observed anomalous behavior may be ascribed to initial formation of shell like microsphere of PVA for its surfactant behavior [17]. The hydro-
Fig. 1. SEM images of nanostructured sol–gel ITO films cured at (1) 250 °C, (2) 350 °C, (3) 450 °C, (4) 600 °C, (5) 700 °C and (6) 900 °C.
S. Kundu, P.K. Biswas / Optical Materials 31 (2008) 429–433
lysed Sn-doped indium was possibly condensed with the hydroxyl group of PVA and existing in the core of the shell which was distinctly observed in the AFM image (not shown here). During curing to 250 °C the surface layer of PVA was possibly breaking partially generating clusters of two size distributions as it decomposes near this temperature. On increasing the temperature to 350 °C, the PVA is fully decomposed and homogeneous nanoclusters produced. In case of 450 °C, grains of relatively large dimensions along with the small one were found to be present in an amorphous medium as observed in SEM image. On the other hand, 600 °C sample depicted the growth of small grains in the film which transformed to dense homogeneous nanoclusters of rhombohedral phase at
Fig. 2. AFM images of nanostructured sol–gel ITO films cured at (a) 450 °C and (b) 600 °C.
b
<222>(c)
Intensity (a.u.)
<400>(c)
<440>(c)
(6) (5)
(6)
Intensity (a.u.)
a
(r) (5)
0
0
(4)
(vo) (4)
-100
(3)
-200
(2)
-200
(3) (c) (2)
-300
30
40
50
2 theta in degree
60
30
32
34
2 theta in degree
Fig. 3. X-ray diffraction (XRD) pattern of nanostructured sol–gel ITO films cured at different temperatures in the 2h range 28–65° (a); expanded XRD in the 2h range 29–35° (b).
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700 °C. On further heating the sample at 900 °C, cluster size increased again due to high temperatures that are not distinct in the SEM image. The anomalous feature at 450 °C film was possibly for the initiation of change in crystal phase from cubic to rhombohedral. The AFM images (Fig. 2) of 450 °C and 600 °C samples also show the similar features i.e. at 450 °C the film contains a melting phase whereas at 600 °C, small crystallites of different shapes started to generate from the melting region. The X-ray diffraction patterns of the films (Fig. 3a and b) are also in agreement with the SEM and AFM features. The 250 °C films have the amorphous feature due to formation of PVA shell like microsphere containing very small sizes of the nanoclusters. The films heated at higher temperatures shows bixbyite cubic (c) structure of indium oxide [18] with oxygen deficient (V0) cubic In2xSn2O7x entity [19]. At 450 °C, although SEM features does not reveal any grains but XRD gives a cubic feature probably due to the presence of some crystals which still remain in the cubic forms. Rhombohedral (r) [20] phase started to grow from 600 °C, and at 700 °C sample the r phase becomes dominant but further increasing the annealing temperature to 900 °C, the cluster size increased and the rhombohedral phase transformed to cubic phase. The high resolution TEM (Fig. 4a and b) of 350 °C sample having average 4.7 nm cluster size [16] exhibited the separation of 222 planes of cubic phase in the fringe pattern [19] of the crystal domain. The ED patterns also show the bixbyite cubic structure of ITO. On increasing the curing temperature to 700 °C the transformation of cubic phase to rhombic phase is also evident from the high resolution TEM of 700 °C cured sample. It depicts almost uniform shape having hexagonal nanocluster size of around 12 nm. Fringe pattern at the crystal domain exhibited the separation of 104 planes of rhombohedral [20] phase. The TEM image of 600 °C cured sample shows a mixed phase of cubic and rhombohedral crystal symmetry having cluster size around 8 nm (not shown here). It is evident that the nanocluster size of all the ITO cured at different temperatures as obtained from the TEM images are quite close to exciton Bohr radius of In2O3 [16,21]. As the size of nanocluster of a semiconductor approaches to Bohr radius, a blue shift in energy occurs due to the quantum confinement phenomenon resulting in hydrogen like exciton formation [10,22]. In our previous report we have already observed the quantum confinement effect of nanoclusters examining their size by TEM and studying their absorption, band gap and photoluminescence properties. Generally in any nanoclusters the increase in particle size due to increase in processing temperature would lead to lowering of exciton transition energy. We have taken the absorption spectra (Fig. 5a) of all the ITO films cured at different
Fig. 4. HRTEM images of nanostructured sol–gel ITO films with lattice fringes and ED patterns, cubic (a) 350 °C sample, rhombohedral (b) 700 °C sample.
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1.0
(4)
0.4
(6)
0.2
(3) (4) (5)
281
280
0.0 400
500
600
286 300
250
Wavelength (nm)
(6)
316
(5)
300
(1) (2)
(2)
(3)
308
2
0.6
(1)
b
(4)=600˚C (5)=700˚C (6)=900˚C
2
Absorbance
0.8
(1)=250˚C (2)=350˚C (3)=450˚C
d (O. D) / dλ (a.u)
a
400 450 500 550 600
Wavelength (nm)
Fig. 5. (a) Absorption spectra, (b) second derivative (d(OD)2/dk2) of the absorption spectra of nanostructured sol–gel ITO films cured at different temperatures.
ahv / ðhv EÞ1=2
ð1Þ
Due to formation of the nanoclusters blue shift of the bulk band gap of ITO (3.65 eV) [23] was observed. The band gap at 4.0 eV is for cubic nanoclusters and corresponds to the excitonic transition appearing at 308 nm for all the samples. In addition, presence of another band gap around 3.5 eV is equivalent to the bulk band gap of ITO as mentioned above. This suggests the existence of both free carrier and quantum confinement phenomena in nanoclustered system. This observation implies the effectiveness of using the relation (Eq. (1)) even for nanoclusters. Besides this, in our previous work [16] the nanocluster size calculated using the band gaps obtained from the above relation was quite close to the average particle size of ITO determined by the TEM experiment. We obtained another band gap value at 4.5 eV possibly for the existence of rhombohedral phases in 600 °C and 700 °C cured samples. We have recorded fluorescence spectra of the samples. When we excited the samples (Fig. 6) at 278 nm, the emission peaks at 423, 484 and 530 nm with a hump at 439 nm were observed for bound excitons (i.e. excitons [24] trapped at the defect centers of ITO [25,26]). The peak at 400 nm in each case is possibly for the emissions of free excitons [16,24] i.e. they are not trapped in defect
1000 253 Excitonic Transition
800
PLIntensity (a.u)
423 (2)
393
440
PL Intensity
temperatures from 250 °C to 900 °C but we did not observe any distinct feature of red or blue shift of absorption peak due to the excitonic transition. We found a broad peak at around 300 nm in all the samples. To identify the peak positions we have taken the second derivatives (Fig. 5b) of the absorption spectra and found that in every samples there is a peak at 307.5 nm except for 700 °C sample, for which the peak is red shifted to 316 nm. For 600 °C sample there is another peak at 281 nm which becomes more prominent with little shift to 286 nm for 700 °C. This peak was not observed for other samples. It can be inferred from the previous work [16] that the peak at 308 nm is for cubic nanoclusters. The derivatives of the absorption spectra of 350 °C and 450 °C cured samples exhibit that 308 nm peak splits into 302 and 312 nm and this predicts different size distribution of the samples. As phase transformation started at 450 °C and the rhomobohedral phase dominated at 600 °C and 700 °C samples, the peak at 280 nm for 600 °C shifted to 286 nm for 700 °C heated sample. This may be assigned for the formation of rhombohedral phase of nanoclusters. For this particular sample, the red shifted value at 316 nm may be due to the increase in size of some of the cubic clusters partially which is expected to exist even at 700 °C cured sample. We evaluated effective band gap of the nanoclusters cured at different temperatures from the plot, (ahm)2 versus hm (Eq. (1)). It is the direct band gap of typical semiconductors, where a is the absorption co-efficient in the edge region, hm is the photon energy and E is the band gap.
600
(3)
PLE Spectra 269 278 286 292 298 308 250
275
300
Wavelength (nm)
485 400
(5)
530
(4)
200
(6) 0 400
450
500
Wavelength (nm) Fig. 6. Photoluminescence (PL) spectra of nanostructured sol–gel ITO films cured at different temperatures excited at kex, 278 nm (inset shows the photoluminescence excitonic (PLE) spectrum when the emission be fixed at kem, 423 nm).
centers. If the excitons be bound with the phonons then a number of inflexions may appear in the broad emission band due to the exciton–phonon interaction [27]. This is prominent for the samples cured at relatively high temperatures, from onwards 450 °C as evident from the appearance of a number of inflexions at the broad 400 nm band (Fig. 6). The excitation wavelength was obtained [16] from the photoluminescence excitation (PLE) spectrum shown at the inset of Fig. 6. The appearance of number of PLE bands at higher energy levels was possibly for exciton–phonon interaction. Usually this interaction may be inferred from the PLE and PL spectra unlike the absorption spectra. The structural difference due to the change in curing temperature does not have any influence on the emission peaks of the bound excitons. From the intensity ratio of the free exciton peak at 400 nm and the bound excitonic peak at 423 nm in each case, it can be seen that the intensity of the free excitons is relatively high for the samples where the rhomobohedral phases dominates. Rhomobohedral phase is the distorted cubic structure and the defect sites in the rhombohedral system may not be strong enough to trap the free excitons. 4. Conclusion Nanostructured feature of ITO as evident from SEM, TEM and AFM studies was changed if the curing temperature be changed. On increasing temperature to 700 °C, the bixbyite cubic ITO
S. Kundu, P.K. Biswas / Optical Materials 31 (2008) 429–433
transformed to rhombohedral which again transformed to cubic form at 900 °C. All the samples revealed the generation of excitonic transitions by excitation with electromagnetic radiation. Absorption, band gap and PL properties supported the quantum confinement behavior. Exciton–phonon interaction was predominated in the samples heated above 350 °C as revealed by PL study. Acknowledgement Authors are thankful to Dr. H.S. Maiti, Director, CGCRI, Kolkata for his constant encouragement to carry out this work under CTSM Programme [# CMM 0022 (1)]. References [1] [2] [3] [4] [5] [6] [7]
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