Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Indium Tin Oxide (ITO) Transparent Conductive Thin Films Elaborated by Sol-Gel Routes M. Tümerkan Kesim, Hakan Yavaş and Caner Durucan* Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06531, Turkey Abstract—Commercial grade ITO thin films are widely produced by sputtering. On the other hand, aqueous based coating techniques such as sol-gel, which is simple and easily applicable in the economic sense, has the potential for developing ITO thin films. In this study, sols prepared from indium and tin salts are employed to form ITO thin films on soda lime silica float glass. The effects of processing parameters such as film deposition parameters, the number of coating layers and heat treatment on the morphological, electrical, and optical properties of ITO coatings are reported. Indium tin oxide (ITO) thin films have been used in many optoelectronic applications such as display panels, solar cells, touch screens and electrochromic devices. Among different processing methods, sol-gel approaches offer several advantages for preparation of ITO films. These include feasibility in coating large area substrates, low equipment cost and ease in controlling chemical homogeneity and tin-dopant incorporation [1-3]. Sol-gel derived ITO films can be prepared by using organic metal alkoxide precursors or inorganic metal salts. Organic based routes offer higher chemical homogeneity and are also advantageous in achieving desired film thickness (in the order of 200-250 nm) by single coating operation. But, the need for highly expensive starting chemicals and poor storage stabilities of organic sols limit industrial scale production. In this study, sol-gel processing routes for developing ITO thin films utilizing inorganic precursors has been established. The sols were prepared from InCl 3 .4H 2 O and SnCl 4 . The solvents were ethanol (EtOH) and acetylacetone (AcAc) which may also serve as chelating agent [4]. ITO sol preparation was achieved by dissolving 8.00 g of InCl 3 4H 2 O in 45 mL AcAc at 25 C followed by refluxing at 60 C for 3 h. Meanwhile, 0.33 mL SnCl 4 were mixed with 4.50 mL ethanol at 25 C. After complete homogenization of In-solution, two solutions were mixed under stirring. Final coating sols were aged for at least 2 days prior to coating and then were deposited on precleaned glass substrates by spinning. In case of multiple coating operations 10 min 150 C air drying was performed in between the subsequent operations. Finished samples were heat treated in air for 1h at 600 C. The characterization of the films was performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) examinations. The performance assessments were achieved by UV-Vis spectroscopy for examining the optical properties and by four-probe conductivity measurements. XRD analyses revealed that tin-incorporated indium oxide films were obtained after heat treatment at 600 C. The SEM micrographs in Figure 1 exhibit the effect of post coating heat treatments on film formation behavior. The films exposed to open air cooling were severely cracked as shown in Figure 1a. However, a firm film build up can be achieved for coatings formed by furnace cooling. The micrograph in Figure 1b shows the representative detailed microstructure of the sol-gel derived ITO films imaged from pristine region of a 4-layered film heat treated at 600 C followed by furnace cooling. The ITO crystals vary in size (10-40 nm) with an average size of around 20±5 nm. (a) 5 m 250 nm Figure 1. (a) SEM micrographs of microcracked ITO coating (4- layered) heat treated at 600 C for 1h followed by air cooling, and (b) the details of the microstructure of furnace cooled coating with similar processing history. Figure 2a shows that as the film thickness increases from a single layer to 10 layers, the resistance decreases 2 orders of magnitude, reaching to value of several thousands ohms/sq. This value is comparable with the resistance reported for commercial quality sputtered ITO films. However, the film thickness also greatly changes the transmittance. As shown in Figure 2b, the transmittance of the films produced by 1-, 4- and 7-step coating operations are comparable and are in the range of 90±5%, and there is only slight reduction in visible light transmittance with increasing film thickness. However, the transmittance remarkably decreases for the film produced by 10 coating operation, reaching to a value of 70%. Figure 2. (a) Variation in sheet resistance and (b) the visible spectral transmittance of ITO coatings as a function of number of coating operations. In summary, ITO films were prepared by sol-gel using lowcost non-alkoxide precursors. A comparison of resistivity and transparency has been performed for the films obtained by multiple spin coating operations. The properties of the films are promising and potentially applicable for coating of large area substrates. This work is supported by METU-BAP-03-08-2010-04. *corresponding author: cdurucan@metu.edu.tr [1] R. B. Hadj Tahar et al., J. Appl. Phys. 82, 15 (1997). [2] Y. Djaoued et al., Thin Solid Films 293, 108 (1997). [3] S. R. Ramanan et al., Thin Solid Films 389, 207 (2001). [4] H. Uchihashi et al., J. Ceram. Soc. Jpn. 97, 396 (1989). (b) 6th Nanoscience and Nanotechnology Conference, zmir, 2010 409
Poster Session, Tuesday, June 15 Theme A1 - B702 Graphene and Single Walled Carbon Nanotubes As High Performance Thin Film Semiconductors Coskun Kocabas 1 * 1 Department of Physics, Bilkent University, Ankara 06800, Turkey Abstract-Recent advances of the growth and assembly of low dimensional carbon provides promises for thin film electronic applications. Chemical vapor deposition of single walled carbon nanotube (SWNT) arrays on quartz substrates provides a scalable approach for carbon nanotube based electronics. Similar growth process of graphene on metallic substrates could also provide possibilities to fabricate large area thin film transistors. This presentation will provide a brief summary of the chemical vapor deposition of SWNT arrays and graphene on single crystal quartz and metallic substrates. Furthermore, we will provide some of the recent results on the experimental and theoretical study of high frequency performance of these devices. This material is based upon work supported by the TUBITAK under the Grant No. 109T259 and FP7 under the Grant No. 0T256458 (GrapheneRF) . 6th Nanoscience and Nanotechnology Conference, zmir, 2010 410
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