Raman spectra of different thickness MoO 3 films 

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... trioxide (MoO 3 ) is metal oxide that has excellent photochromic and electrochromic properties. As some other transition metal oxides, MoO 3 shows good adsorption of ammonia, carbon oxide, nitric oxides and hydrogen. It is also sensitive to many organic compounds such as hydrocarbonic and aromatic gases, ethanol, gasoline, trimethylamine and many other. However, MoO 3 has a low melting point of 795oC. There has been limited research on MoO 3 for gas sensing applications. The sensing behavior of sputtered MoO 3 thin films was first studied by Mutschall and coworkers [1]. Thin films of MoO 3 were prepared by RF sputtering. These films were found to be highly sensitive to ammonia in the temperature range of 400-450°C. The possibility of nanostructured MoO 3 films to build advanced chemical sensors, is very perspective. MoO 3 films of various thickness (300 nm – 1,5 μ m) were deposited by RF reactive sputtering on quartz resonators with silver (Ag) and gold (Au) electrodes. The aim is to use the quartz crystal microbalance (QCM) method for studying gas sensing properties of MoO 3 thin films. QCM is an extremely sensitive mass sensing method, capable of measuring mass changes in the nanogram range. This means that QCM sensors are capable of measuring mass changes as small as a fraction of a monolayer or a single layer of atoms. The high sensitivity and the real-time monitoring of mass changes on the sensor crystal make QCM a very attractive technique for gas sensors. Some difficulties in reactive sputtering deposition of MoO 3 on Ag, were established and the technology of deposition on resonators was optimized. Test sensor devices were built. They are based on quartz resonators with Au electrodes and resonant frequency of about 14 MHz. These MoO 3 thin film gas sensors are being tested to find the feasibility of using this sensor type for on-line monitoring of the concentration of ammonia, carbon oxide, nitric oxides and other gases. In the present research, in order to deposit MoO 3 thin films reactive sputtering of molybdenum target is used in the presence of oxygen as reactive gas. Technological parameters were optimized to obtain films with good quality on different substrates. The influence of technological conditions during deposition, such as the oxygen partial pressure and deposition time, on the molybdenum oxide structure and properties, have been studied. The optical properties were also examined by visual and infra-red spectroscopy and elipsometric analysis. The consequent thermal treatment in a different medium has also been studied. Deposition of the films was carried out using vacuum installation A-400VL. The main parameters of the RF reactive sputtering was precisely tuned to get films with optimum properties. The thickness was controlled by the RF power (cathode voltage) and the deposition time. The oxide structure was controlled by the oxygen partial pressure. To obtain stoichiometric MoO 3 films were used values of the oxygen partial pressure more than 1.10 -3 Pa. This means that the MoO 3 films are formed in excess of oxygen in the chamber. The films were deposited on unheated substrates. The structure was modified by consequent thermal treatment at temperatures 250-300oC. The films’ structure is identified by Resonant Raman scattering analysis. Raman spectra showed that MoO 3 film structure strongly depends on the thickness. Raman spectra of MoO 3 films with three different values of thickness (0,9 μ m – 1,4 μ m) are showed on figure 1. The Raman spectrum of as-deposited films of MoO 3 with thickness of 0.9 μm (N1) had very well expressed Raman peaks at positions, characteristic of the monoclinic modification. Some peaks of orthorhombic structure were also shown in thinner films spectrum. These films with orthorhombic structure that presupposes good sensitivity to many gases have been studied. Thicker films (N2, N3) are amorphous-like with large, low-intensity Raman peaks. Various sub-stoichiometric oxide species are also observed in different films. Processing and microstructure play a key role in determining the gas sensing behavior of the MoO 3 thin films. In order to be more sensitive the films should be porous and crystal, preferably with orthorhombic modification. As-deposited MoO 3 films with thickness more than 1,2 μm are almost amorphous. After heat treatment at temperatures higher than 250oC MoO 3 crystallizes. However, processing the quartz resonators thermally causes problems. That is why, thinner films (0.5 – 1 μm) should be deposited on the resonators. Another reason for the films to be thinner is the necessity for less weight in order to obtain better selectivity of the QCM sensor. These thinner films, however, need special deposition technology to obtain good thickness uniformity. Some difficulties in RF sputtering deposition through aperture were established and the technology of deposition on resonators was optimized by using special masks. QCM crystals are used as sensors to determine mass changes as a result of frequency changes. Through deposition on the QCM crystal of gas sensing film, such as MoO 3 , can be build high sensitive gas sensor. QCM gas sensor construction is showed on figure 2. The heart of the QCM is the piezoelectric AT-cut quartz crystal sandwiched between a pair of electrodes. When the electrodes are connected to an oscillator and an AC voltage is applied over the electrodes the quartz crystal starts to oscillate at its resonance frequency due to the piezoelectric effect. This oscillation is generally very stable due to the high quality of the oscillation (high Q factor). If a rigid layer is evenly deposited on one or both of the electrodes the resonant frequency will decrease proportionally to the mass of the adsorbed layer according to the Sauerbrey ...