CHARACTERIZATION AND PROPERTIES OF TITANIUM-VANADIUM OXIDE THIN FILMS PREPARED BY ArF LASER ABLATION

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1 CHARACTERIZATION AND PROPERTIES OF TITANIUM-VANADIUM OXIDE THIN FILMS PREPARED BY ArF LASER ABLATION Radek FAJGAR a, Jaroslav KUPČÍK a, Jan ŠUBRT b and Filip NOVOTNÝ c a Institute of Chemical Process Fundamentals, ASCR, Prague 6, Czech Republic b Institute of Inorganic Chemistry, ASCR, Husinec-Řež, Czech Republic c Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Prague 1, Czech Republic Abstract Thin films of vanadium-titanium complex oxides were prepared by ArF excimer laser ablation of TiO 2 and VO 2 targets. The light grey films with thickness of 155 nm were grown on glass and Au substrates and characterized by microscopy, spectroscopy and diffraction techniques. The as-prepared films were nanocrystalline and heating at 450 o C in vacuum led to formation of mixture of crystalline V 0.95 Ti 0.05 O and TiO nanoparticles. The film shows reversible thermochromic behaviour between 3.0 and 1.4 micrometers. The films, annealed in air oxidize and the colour of the film changes to light yellow. Electrical and optical properties of the films are demonstrated. Keywords: vanadium-titanium oxide, thermochromic effect 1. INTRODUCTION Thermochromic effect is a temperature dependent structural transformation of various transition metal compounds (usually oxides or sulfides). The transformation is accompanied with a remarkable change of their optical and electronic properties. Most widely studied material is vanadium dioxide, which changes at transition temperature from monoclinic, semiconducting and IR transparent form to tetragonal, metallic and IR reflecting one. The transition temperature 68 o C is too high for practical application as smart windows. The transition temperature can be decreased by doping with tungsten [1,2] or titanium oxides [3-5] or with magnesium [3]. Vanadium has various valence states resulting in many oxide forms. A lot of them were widely studied for their optical, electrical, electrochemical, thermochromic or thermal switching properties. In this work, vanadium titanium oxide films were deposited by excimer laser ablation of VO 2 /TiO 2 targets in a vacuum. The deposited film was characterized and its thermochromic and electrical properties were studied. 2. EXPERIMENTAL

2 Thin layers of titanium-vanadium oxides were prepared by ArF laser ablation of VO 2 and TiO 2 targets (Dia 13 x 2 mm), mounted on a rotating holder. The experiments were conducted in a 1l Pyrex reactor equipped with a quartz entrance window, a rotating target holder and a substrate holder. The ArF laser (Semento, Estonia) was operated with a repetition frequency 10 Hz and an energy (55 mj/pulse, 193 nm). The laser beam was focused by a quartz window (focus length 17 cm) on the surface of the targets. Glass and gold were used as substrates for deposition of thin films. For resistance measurements a gold electrode array on the 2 x 2 cm glass substrate was used. The electrode spacing was typically 0.2 mm. The ablation was conducted under vacuum (10-3 Pa, Pfeiffer HiCube turbostation). The deposited films were analyzed and characterized by FTIR (a Nicolet Impact 400 spectrometer), Raman (a Nicolet Almega XR spectrometer, excitation wavelength 473 nm) and UV-VIS (Shimadzu UV 1601) spectroscopies. Scanning electron microscopy and EDX elemental analysis were performed on XL30 CP instrument (Philips) using accelerating voltage in the range from 5 to 25 kv. The chemical composition was taken from several µm 2. Transmission electron microscopy and selected area electron diffraction was measured using a Philips EM 201 microscope. ProcessDiffraction V B software was used to solve the diffraction patterns [6]. Atomic force microscopy (XE-100 Park Systems) was used to measure a deposit thickness and surface roughness. The thermochromic properties were studied using 30 o specular reflection equipment (Nicolet Impact 400). Thin films were studied as prepared and annealed in vacuum and in air. Thermal treatment was performed in a Tube furnace (Thermolyne). A quartz tube with deposit was evacuated by a turbostation (Pfeiffer Vacuum TCP 380) and a temperature ramp was set at 10 o C/min. The samples were maintained at the desired temperature (450 o C) for 90 min and then they were cooled to the room temperature. 3. RESULTS AND DISCUSSION The thin films were prepared using ArF laser pulses, highly focused on a target surface. The ablation was performed for 12 min and both VO 2 and TiO 2 targets were ablated alternately by rotation of target holder. The VO 2 target was ablated for 18 seconds and TiO 2 one for 12 seconds which means that film was formed by 24 layers of vanadium and titanium oxides. The chemical composition of the films was studied by the EDX technique. The bulk composition shows vanadium/titanium ratio 0.59/0.41 for the asprepared sample. This result is in a good agreement with irradiation time of the targets, which means that ablation of the both targets has a similar efficiency. Heating to 450 o C at pressure 10-3 Pa has no influence on a sample stoichiometry. The reactor was evacuated to 10-3 Pa before the ablation and in the

course of irradiation a pressure increase to 10-2 Pa was observed.

The films prepared are light grey and possess extremely good adhesion to the glass substrate.

$The deposit shows very poor degree of crystallization, as demonstrated by broad diffusion rings in the electron diffraction pattern (Fig. 1a, inset). Annealing at 450 o C for 1.$

3 course of irradiation a pressure increase to 10-2 Pa was observed. It means that the highly energetic laser beam evaporates the targets, decomposes oxides and the film is enriched in heavier components [7] (vanadium and titanium), resulting in formation of suboxides. The films prepared are light grey and possess extremely good adhesion to the glass substrate. Annealing at 450 o C in vacuum has no influence on colour and adhesion, whereas air annealing at changes the colour to light yellow as a consequence of the oxidation. The as-prepared deposit is very compact and transmission electron microscopy shows that it forms a flat plates (Fig. 1a). The deposit shows very poor degree of crystallization, as demonstrated by broad diffusion rings in the electron diffraction pattern (Fig. 1a, inset). Annealing at 450 o C for 1.5 hrs in the vacuum leads to the pronounced crystallization. The nanocrystalline film (Fig. 1b) shows the discrete rings in the SAED pattern (Fig. 1b, inset). a b c Fig. 1. TEM images of the (a) as-prepared sample, (b) the sample annealed at 450 o C in vacuum; insets are corresponding electron diffraction patterns and (c) ED spectrum of the annealed sample (red line) with cubic V 0.95 Ti 0.05 O The software used to solve the SAED data produces a XRD-like distribution from the measured SAED ring patterns. Comparison of the electron diffraction results with a PDF 2 data library revealed presence of V 0.95 Ti 0.05 O nanocrystallites together with TiO. Both metals have a lower valence state in the oxides prepared by ArF laser ablation. This result was expected due to observed pressure increase, caused by releasing of gaseous oxygen. Both vanadium-titanium oxide (PDF ) and titanium monoxide are cubic (PDF , hongquiite). Weak reflections should be ascribed to the titanium dioxide probably as a consequence of a surface oxidation. The AFM analysis was performed on the as-prepared sample deposited on the glass substrate. A small part of the sample was removed by a cutting edge. The thickness of the sample was found to be

4 155 nm (Fig. 2a), which means that growth rate of the deposit is about 13 nm per minute. The surface is very smooth with mean roughness less than 15 nm, as demonstrated by the three dimensional AFM image (Fig. 2b). a b Fig. 2. AFM (a) profile and (b) topography of the as-prepared deposit on the glass substrate. Fig. 3. Raman spectra of the films Fig. 4. Transmission spectra of the as-prepared film in the NIR region The Raman spectrum of the deposit (Fig. 3b) revealed bands 995, 700, 413, 290 and 145 cm -1. Such bands are typical for vanadium oxide material and represent a different stretching and bending vibrations of the V-O-V framework [8]. The VO 2 and TiO 2 targets alone were used for ablation and vanadium oxide and titanium oxide films were prepared separately. The Raman spectra of the resulted films are depicted in the Fig. 3c (vanadium oxide film) and Fig. 3d (titanium oxide film). Comparison of the spectra shows, that only vanadium oxide bands contribute to the V/Ti/O spectrum. The vacuum annealed film shows a set of different bands between cm -1, that is attributable to V 0.95 Ti 0.05 O (Fig. 3a).

5 The optical measurements were carried out using the FTIR and UV spectrophotometers in the UV- VIS and IR regions. Optical transmission in the UV-VIS region shows no change with increasing temperature, while in the NIR region the deposit exhibits a strong dependence on the temperature in the range from 22 to 88 o C. Fig. 4 shows the optical changes induced by increasing of the substrate temperature. The maximum change of the transmission is located at 1.47 µm. The changes observed are reversible, but much slower than that reported in the bulk VO 2 films. Electrical resistivity of the as-prepared sample was measured from 22 to 88 o C. The film was deposited on the gold electrode array situated on the insulating glass substrate. The measurements were performed in an apparatus under dry nitrogen in a dark and a temperature ramp was set to 0.5 o C/min. The resistivity of the as-prepared sample was 190 MΩ and heating up to 88 o C led to a remarkable deceasing of this value (Fig. 5, blue markers). A subsequent cooling process has followed a different curve (Fig. 5, red markers), and the initial resistivity has not been reached within a two hours cooling cycle. Further two hours at room temperature were needed to achieve the initial resistivity. The same hysteresis effect was observed in the NIR transmittance of the film and it is a sign of the phase change proceeding between room and increased temperatures or the strain appearing in the multilayer structure of the film [3]. Resistivity, MOhm Heating Cooling Temperature, o C Fig. 5. Dependence of the electrical resistance on the temperature 4. CONCLUSION V/Ti/O based multilayer thermochromic films were prepared by the excimer laser ablation onto glass, gold and electrode substrates. The oxide films were characterized by electron diffraction and Raman spectroscopy. NIR spectrum of the as-prepared film shows the decrease of the transmission in the temperature region from 22 to 88 o C. Electrical resistance drops with increased temperature and the hysteresis behaviour was observed. The temperature related changes both optical and electrical are

6 found to be reversible. Annealing at 450 o C leads to structural changes resulting in formation of V 0.95 Ti 0.05 O and TiO nanocrystallites. ACKNOWLEDGEMENT The support of the Grant agency of the Czech Republic No. 203/09/1117 is gratefully acknowledged. LITERATURE [1] JUNWEI, Y., LI, Z., FENGJUAN, L. et al., Prepararation, Characterization and Properties of Thermochromic Tungsten-doped Vanadium Dioxide by Thermal Reduction and Annealing. Journal of Alloys and Compounds 504 (2010) [2] JIN, P., YOSHIMURA, Y., IGARASHI, K. Et al., Epitaxial growth of W-doped VO 2 /V 2 O 3 multilayer on α-al 2 O 3 (110) by reactive magnetron sputtering. Thin Solid Films 375 (2000) [3] GRANQVIST, C.G., GREEN, S., NIKLASSON et al.,.advances in Chromogenic Materials and Devices. Thin Solid Films 518 (2010) [4] KAKIUCHIDA, H., JIN, P., TAZAWA, M., Optical Characterization of Vanadium-titanium Oxide Films. Thin Solid Films 516 (2008) [5] TSUYUMOTO, I., NAWA, K., Thermochromism of Titanium-vanadium Oxide Films Prepared From Peroxotitanate and Peroxovanadate Solutions. Solid State Ionics 179 (2008) [6] LÁBÁR, J.L., Consistent indexing of a (set of) SAED pattern(s) with the ProcessDiffraction program. Ultramicroscopy, 103 (2005) [7] ASHFOLD, M. N. R., CLAYSSENS, F., Fuge G. M., et al., Pulsed Laser Ablation and Deposition of Thin Films. Chem. Soc. Rev. 33 (2004), [8] RAMA, N, RAMACHANDRA RAO, M. S., Synthesis and Study of Electrical and Magnetic Properties of Vanadium Oxide Micro and Nanosized Rods Using Pulsed Laser Deposition Technique. Solid State Commun. 150 (2010)

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