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Tailoring crystalline structure in N-doped TiO2 thin films: application to photo-catalytic and biological reactions. Rod Boswell, Christian Sarra-Bournet and Christine Charles Space Plasma Power and Propulsion Group PRL/PSPE, Australian National University Canberra, ACT, Australia e-mail: [email protected] Abstract: Photocatalytic reactions at the surface of titanium dioxide (TiO2) have been attracting much attention in view of their practical use in environmental, energy and biomedical applications [1]. However, the bandgap of TiO2 corresponds to maximal wavelength absorptions in the UV. Thus, the development of photocatalysts exhibiting high absorption under visible light should allow a more efficient use of the solar spectrum. The objective of this study was to obtain TiO2 doped with nitrogen (N-doped) by plasma sputtering in a helicon reactor. Changing the impinging ion energy resulted in changes in the crystalline nanocrystals from anatase to rutile. Moreover, introduction of nitrogen also resulted in changes in the crystalline structure. Results will be presented on the photon absorbance and interaction with blood simulants. Keywords: bio-active thin films, photo catalysis, plasma deposition 1. Introduction Titanium dioxide (TiO2) is one of the most studied coating materials due to its many outstanding properties. This material is hard with a high thermal and chemical stability. Moreover, it presents a wide electron energy band gap, transparency throughout the visible spectrum and a high refractive index from UV to far infrared. These properties led to the use of TiO2 for a wide range of applications such as in electronic devices, optical and hard coatings, photocatalysts, and biomedical applications. Titanium dioxide is found naturally in three crystalline phases: anatase, rutile and brookite, as well as in an amorphous phase. The amorphous phase is sought after for good quality optical films while anatase phase is known to have a higher photocatalytic activity than the other crystal types of TiO2, due to its larger bandgap energy (3.2 eV vs. 3.0 eV) and rutile is characterized by higher refractive index and hardness. Plasma sputtering deposition techniques allow the substrate to be kept at low temperature (< 200°C) during the process, which allows low melting point materials to be used as substrates, such as plastics. It also allows control and modification of the structure and properties of the TiO2 films via the process parameters like power, pressure, gas mixtures, partial pressures, bias, temperature, etc. The present paper studies the potential of using lowfield helicon plasma for a sputtering process of titanium dioxide. The influence of the ion densities on the characteristics of the TiO2 coatings, including the density, refractive index, crystallinity, crystallite size and surface elemental composition have been investigated using a number of analytical techniques. The overall objective of this study was to develop an efficient one-step plasma deposition process to obtain crystalline TiO2. 2 Experiment The Helicon-Assisted Reactive Sputtering (HARES) experimental setup is shown in Figure 1. The diffusion chamber (180 mm High, 340 mm i.d. aluminium) is equipped with vacuum gauges, optical ports and holders for target and substrate (10 cm diameter). The bottom chamber (350 mm High, 400 mm i.d. stainless steel) houses the pumping bench. The two chambers are connected by a glass tube (150 mm long and 150 mm diameter). A doublesaddle field type helicon RF antenna is located around the tube, which is supplied with RF power between 50-1000W at 13.56 MHz coupled to the plasma via a RF matchbox. High-purity oxygen (99.9 %) and argon (99.997 %) was injected in the diffusion chamber by two 100-sccm mass flow controller. The base pressure in the diffusion chamber before deposition was 1x10-3 Pa. Figure 1.Schematic of the HARES setup. The substrate and target holders are situated 5 cm above the exit of the source tube (z = 5 cm) and can be easily moved in the radial direction. For the experiment presented here, the target was set in place directly in the centre of the diffusion chamber (z = 5 cm, r = 0 cm) while the substrate holder was moved to different target-to-substrate distance (TSD) (z = 5cm, r = 3-11 cm). This configuration allows a constant ion density on the target, thus a constant sputtering rate, while changing the ion density impinging on the substrate during the sputter deposition process. A DC bias voltage was applied to the target while the substrate was placed on an electrical-floated substrate holder with no cooling. 3. Surface characterization techniques As-deposited samples were analyzed while some samples were also annealed after the plasma deposition process in an oven furnace in an air atmosphere at 300 °C for three hours. This temperature was chosen as it is low enough as to favour crystallisation of the metastable anatase phase and not the rutile phase. The film refractive index and thickness was measured by a spectroscopic reflectometer using a Cauchy model. Validation of the thickness measurements with the ellipsometer was performed with a mechanical profiler. Density of the deposited coatings was calculated by measuring the mass difference of the sample before and after the plasma deposition and coating volume was integrated from a thickness mapping using the spectroscopic reflectometer. The crystalline structure of the TiOx samples was probed by x-ray diffraction (XRD) using a conventional x-ray diffractometer in a 2θ scan. Measurements were obtained at a 0.5degree angle, 40kV, 30mA with a Co Anode (Kα = 0.17889 nm) with detections scans between 20-100 degrees. To determine the stochiometry of the TiOx films, Rutherford Backscattering Spectroscopy (RBS) measurements were performed. For the TiOx samples, A 2-MeV He ion beam was produced by the accelerator and directed onto the coating. The back-scattered beam was collected and analyzed. 3. Plasma Characterisation Using an electron temperature of 4 eV, typical for this type of discharge, a threefold increase in plasma density can be found over a narrow range of magnetic field values (0.8 mT < B0 < 5 mT) (Figure 2). This peak in density is thought to be due to a helicon wave being launched in the system. In argon, using the helicon dispersion relation for a magnetic field of 2 mT and a plasma density of 1.5 x 1017 m-3, the calculated wavelength of the helicon wave is ∼ 27 cm, twice the antenna length (13 cm), which is a well known matching condition for efficient helicon wave excitations. This suggests a direct transition from capacitive (E) to helicon wave (W) mode. The transition back to E mode at higher fields is not so well understood, but appears to be related to a decrease in radiation resistance from the antenna, and thus a lower power transfer efficiency. It can also be observed that the plasma density obtained in the 10% O2 / 90% Ar discharge are relatively similar to the pure argon case while a pure oxygen discharge sees a modest peak and a decrease in density for higher magnetic field. Figure 2. Langmuir probe ion densities measurements at the exit of the source (z = 0, r = 0) as a function of the magnetic field in the source for various gases ( 100% Ar, 10% O2, 90% Ar, 100% O2) To operate in the low magnetic field helicon mode it was elected to work at a magnetic field in the source of 3.2mT. The coil in the diffusion chamber was also used to confine the plasma and enhances the uniformity of the process. Figure 3. Ion density (m-3) mapping in the diffusion chamber for the optimum sputtering conditions (10% O2, 90% Ar, 3.2 mT in the source) A plasma density mapping of the diffusion chamber is drawn in Figure 5. As it can be observed, the plasma density is roughly uniform in the axial direction with a decreasing profile in the radial direction. 4. Thin film properties Thin films of TiO2 were sputtered in various configurations of plasma density and the results of RBS and XPS will be presented in the talk. When the positive ion bombardment on the substrate is high, TiO2 rutile crystalline films can be obtained directly from a one-step low temperature plasma sputtering process, thus no post high-temperature treatment step is required. Since those films are already highly crystallized, they are thermally stable and conversion into anatase phase is highly unfavourable. TiO2 amorphous coatings can also be obtained by lowering the ion density over the substrate. Flowing nitrogen through the system lead to changes in the band gap and improvements in the photo-catalytic properties of the films. The results demonstrate that helicon-assisted reactive sputtering using a low-field helicon mode is a convenient technique for the deposition of TiO2 films which enables tailoring of the crystalline structure by controlling the deposition parameters in a wide range to application demands.