Deposition of Indium-Tin-Oxide (ITO) Nanoparticles

Proceedings of the 4th International Conference on Nanostructures (ICNS4) 12-14 March, 2012, Kish Island, I.R. Iran

Deposition of Indium-Tin-Oxide (ITO) Nanoparticles using a Sol-Electrophoretic Technique N. Yasrebia, B. Bagheria, P. Yazdanfara, B. Rashidianab* a b

Department of Electrical and Electronics Engineering, Sharif University of Technology, Tehran, Postal code, Iran Institute for Nanoscience and Technology, Centre of Excellence for Nanostructures, PO Box 11155-9363, Tehran, Iran * [email protected]

Abstract: A method is presented to simultaneously synthesize and deposit transparent conductive oxides, specifically indium tin oxide (ITO), on a metallic substrate. The method is based on electrophoretic deposition. Effects of presented two stage filtering method, together with the effect of filter type on average size of the deposited particle, are discussed. It is also shown that omission of filters can be considered a method to deposit relatively dense thin films containing larger nanoparticles on the desired surface, and incorporation of filter types which preserve their shape and position relative to the substrate, can be used to get nanostructures with desired shapes, such as pyramids, nanorods, etc.. Films containing these forms can have many applications in nanoelectronics and field emission devices.

Keywords: ITO Nanoparticles; Transparent electrodes; Electrophoretic Deposition (EPD); Sol-Gel method; Optical Devices.

Introduction As a transparent, conductive material, Indium-Tin-Oxide (ITO) thin film have been used as antistatic material for cathode ray tubes (CRT) displays [1] in the last decade. Recently, applications like field emission devices [2], organic solar cells [3], and light emitting diodes [3] have also been found. Sputter deposition followed by annealing was the main method for deposition of these transparent conductive oxides, which had the limitation to deposit on somehow flat substrates, and materials which could withstand high temperatures. Production of ITO nanoparticles, which show the same properties as ITO thin films, has become the centre of attention to overcome these difficulties, and has also given rise to newer applications like ITO nanoparticles as channel material in transparent transistors [4], ink material for Ink-Jet printing [5], and near-infrared reflective film[6]. Different materials and methods like spin coating [4, 6], hydrolysis of metal carboxylates [3], etc. has been previously used to synthesis of ITO nanoparticles. Here, we introduce the application of sol-electrophoretic deposition [7] using a citric acid sol to formation of ITO nanoparticles. This method has been previously used by Limmer, et. al., in order to synthesize ITO nanorods [8]. Modifications to the method have been made in order to get nanoparticles instead of nanorod. We present the exact effect of filters on particle size and distribution, when deposited on a metallic substrate, and discuss the phenomenon which initiates nanoparticle formation.

Experimental The chemicals used in sol preparation were: Indium Chloride (Aldrich co.), absolute ethanol, citric acid

monohydrate, ethylene glycol (Merck), and deionized water (prepared in the lab). Sol is prepared by first mixing a 3:2 mass ratio of ethanol and ethylene glycol, then heating the mixture to 40oC. After that, citric acid is added to the solution in a manner that the final mass ratio of total In+Sn to the citric acid would be 1:2. The solution is then stirred using an automated system, and meanwhile SnCl2 is added. Finally, InCl3 is added and the resulting mixture is stirred for 45 minutes, before deionized water is introduced. An additional 45 minutes of stirring finalizes the process. The solution is then cooled at room temperature, and filtered with a standard Whatmann paper filter. Volume of the prepared sol is 40cc with a molarity of approximately 0.12. The electrochemical cell is formed by pouring the mixture into a beaker and inserting cathode and anode electrodes inside the solution. Cathode electrodes are selected to be aluminium sheets. A polytetrafluoroethylene (PTFE) holder was designed and fabricated, and a 1 inch diameter cellulose filter (pore size of 150-200nm, Whatmann co.) was mounted over the aluminium cathode, sealing the cathode from the solution (excepting at the filter’s surface) using vacuum o-rings. Scanning Electron Microscope (SEM) picture of filter’s pore shapes are shown in Fig. 1. Plastic screws were used to fix the holder on Al sheet. Anode electrode was a steel rod. Prior to experiment, Al sheets, cathode holder, and steel rod, were cleaned using a standard RCA cleaning procedure [9] to prevent contamination. Anode to cathode distance inside the cell is 3cm. In order to start the deposition process, a constant voltage of 5V is applied to the electrodes for 1 hour. Cell current is measured to be approximately 10mA. The resulting sample is then dried at 110oC for 3 hours. Finally, the cellulose filter is

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removed by putting the sample in a furnace at 700oC, where the filter is burned without any ashes.

randomly. The filter has prohibited passing large particles (>200nm). So that these particles are most common in the solution (see Fig. 2), the resulting film contains a smaller number of ITO nanoparticles compared to the previous case where no filter was present. Moreover, sizes of the particles deposited when using filter, are smaller than the particles deposited without cellulose filter, and are distributed more sparsely than the particles in previous case. Average size of particles in Fig. 3 is measured to be between 70-200 nm (average radius between 35-100nm), which is 3-9 times smaller than Fig. 2. This shows successful synthesis of the desired nanoparticles.

Fig. 1. SEM picture of Cellulose filter used for synthesis of nanoparticles. Pore size varies between 100nm-1μm.

Results and Discussion Above procedure were repeated with and without cellulose filter, and the effect of the filter on deposited nanostructures was studied using SEM. Fig. 2 shows SEM picture of nanoparticles deposited on the Al sheet without using the filter on a 5μm scales. As can be seen, the deposited ITO particles have an average diameter of approximately 660nm. The particles are packed together and have formed a relatively dense film on the Al surface.

Fig. 3. SEM pictures of ITO nanoparticles deposited through cellulose filter, (a) 5μm scale, (b) 2μm scale. Note that maximum particle size is approx. 220 nm, and minimum size is below 70nm in diameter.

Fig. 2. SEM picture of the deposited ITO nanoparticles on Al sheet without cellulose filter on a 5μm Scale. Note the film density, and average particle size. On the other hand, Fig. 3 shows SEM picture of nanoparticles deposited on the Al sheet on 5μm (Fig. 3a), and 2μm (Fig. 3b) scales. As was expected from the shape of the filter (Fig. 1) particles are deposited

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Another point that is worth noting is that cellulose filter’s shape deforms when left in a solution containing water for a long period of time. As a result, there will be a small gap between Al sheet and the filter around the centre of the filter. This is exactly the opposite of what happens when membrane filters are used (as in [8]), where the filter preserves its shape in vicinity of water based solutions. Fig. 4 shows SEM picture of the Al sheet on places where the filter stock to the Al substrate, and was forced to preserve its original shape because of the pressure from the o-rings (used to force the deposition to


happen through filter holes). As can be seen, pyramids are formed showing the tendency towards formation of ITO nanorods rather than ITO nanoparticles, in the places where the filter is located exactly on top of the substrate (no gap).

[5] S. Hong, Y. Kim, J. Han, “Development of Ultrafine Indium Tin Oxide (ITO) Nanoparticle for Ink-Jet Printing by Low Temperature Synthetic Method”, IEEE Transactions on Nanotechnology, Vol.7, No. 2 (2008) 172. [6] J. Song, Y.H. Kim, Y.S. Kang, “Preparation of indium tin oxide nanoparticles and their application to near IRreflective films”, Current Applied Physics, 6 (2006) 791795. [7] G. Cao, “Growth of Oxide Nanorod Arrays through Sol Electrophoretic Deposition”, J. Phys. Chem. B, 108 (2004) 19921-19931. [8] S.J. Limmer, S.Y. Cruz, G.Z. Cao, “Films and nanorods of transparent conducting oxide ITO by a citric acid sol route”, Appl. Phys. A, 79 (2004) 412-424. [9] K. Reinhardt, W. Kern, “Silicon Wafer Cleaning Technology”, William Andrew Inc., 2008, pp. 24-27

Fig. 4. Pyramids formation on the places where there was no gap between substrate and the filter.

Conclusions ITO nanostructures were synthesized and deposited using Sol-EPD Technique. It was shown that incorporation of a cellulose filter with nanometer pore size results in nanometer sized particles, compared to 600-700nm sized particles formed in the opposite case. It is also shown that by forcing the cellulose filter to stick to the substrate (Al sheet) other shapes of nanostructures like pyramids can be synthesized. In addition, possibility to formation of a dense particle layer on metallic substrates was verified, and the required method was presented.

References [1] Y.S. Cho, G. Yi, J.J. Hong, S.H. Jang, S.M. Yang, “Colloidal indium tin oxide nanoparticles for transparent conductive films”, Thin Solid Films, 515 (2006) 18641871. [2] H.J. Jeong, H.K. Choi, G.Y. Kim, Y.I. Song, Y. Tong, S.C. Lim, Y.H. Lee, “Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method”, Carbon, 44 (2006) 2689-2693. [3] R. Gilstrap Jr., C. Capozzi, C. Carson, R. Gerhardt, C. Summers, “Synthesis of a nanoagglomerated Indium Tin Oxide Nanoparticle Dispersion”, Advanced Material, 20 (2008) 4163-4166. [4] S. Dasgupta, S. Gottschalk, R. Kruk, H. Hahn, “A nanoparticulate indium tin oxide field-effect transistor with solid electrolyte gating”, Nanotechnology, 19 (2008) 455203.

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