Arūnas Šetkus

Properties of CuxS thin film based structures: influence on the sensitivity to ammonia at room temperatures

Abstract Surface and electrical properties of thin CuxS films grown on different substrates (polyethylene and micro-porous-Si layer) sensitive to ammonia at room temperature are investigated. The resistance response to ammonia is measured in the CuxS in air with relative humidity from 15 to 100% at temperatures from 290 to 350 K. The surface chemical composition and the morphology are analysed. A honeycombed construction of the CuxS films deposited on a micro-porous Si-layer and proportions between components in the chemical composition are found to correlate with the higher sensitivity to ammonia in air at room temperature.

Copper-doping level effect on sensitivity and selectivity of tin oxide thin-film gas sensor

Abstract The influence of doping level of basic material on electrical resistance controlled by gas chemisorption is investigated experimentally for chemically deposited tin oxide thin films with different amounts of Cu and Sb additives. Significant modification of dependences of resistance versus temperature in oxygen-rich atmosphere and anomalous change of selectivity of resistance response to CO and H 2 gas are obtained by different doping levels of the films. The main features of the experimental results are explained by the variation of the position of the Fermi level with the concentration of bulk impurities.

Peculiarities of surface doping with Cu in SnO2 thin film gas sensors

It is demonstrated that a small amount of Cu impurity (less than 1 at.%) significantly modifies the properties of tin oxide thin films used for the gas sensors. Different amount of Cu (between 0.5 and 7 at.%) was sputtered on the top of the films. The structure, surface chemical composition, optical and electrical properties are studied for these films. It is shown that the surface doping with the sputtered Cu leads to nearly constant doping level in all the volume of the polycrystalline tin oxide thin film. It is shown experimentally that the selectivity of the resistance response to CO, H2 and Cl2 gases is improved for the tin oxide sensors by the small amount of the sputtered Cu. A correlation is revealed between the Cu effect and the electronic surface states that were detected by the XPS and the optical analysis.

Influence of Cu overlayer on the properties of SnO2-based gas sensors

Abstract The series of gas sensors based on SnO 2 thin films and doped with different amount of Cu was prepared by using dc magnetron sputtering. The surface properties and chemical composition of the sensors covered with Cu and Pt overlayers were studied by XPS, SIMS and optical UV–VIS spectroscopy. The electrical conductivity controlled by surface interaction with gases (CO, H 2 , Cl 2 ) was also investigated experimentally. Obtained depth profiles demonstrate that the surface doping with Cu results in nearly uniform Cu distribution through the whole thickness of the SnO 2 film, while the Pt-doping modifies only the surface of the sample. The origin of different Cu and Pt distributions in depth and their influence to the surface chemical reactions during gas treatments are discussed. The chemical state of Cu-dopant and its effect to the valence band (XPS) and optical (UV–VIS) spectra are analysed.

Copper on-top-sputtering induced modification of tin dioxide thin film gas sensors

Abstract Thin film gas sensors based on pure tin dioxide were modified by sputtering of copper on the surface of the films. A gradual increase of an amount of the on-top-Cu produced increasing doping with uniform distribution of copper throughout entire tin oxide film. An increase of an amount of Cu caused continuous variation of the clean air resistance and the resistance response to CO and H 2 gases at various temperatures (290–550 K). For certain Cu amounts, samples show a large sensitivity towards one gas component while the cross-sensitivity remains small. The Cu-effect is discussed considering the variation of the Fermi level induced by doping and the clusters of copper on the surfaces of the grains.

Gas sensitivity studies by optical spectroscopy below the absorption edge in tin oxide thin film sensors

Optical transmission spectra are investigated experimentally below the fundamental absorption edge in tin oxide thin film-based gas sensors. The spectra are studied under different conditions for gas chemisorption when the resistance response to gas is modified by heating of the sensors in various atmospheres or adding of impurity metals into tin oxide. Optical bandgap, refractive coefficient and optical absorption index are deduced from the experimental spectra at the conditions tested. The changes of the optical parameters are related with the electronic properties of the films that depend on surface defect states created by chemisorbed oxygen species. The resistance response to the gaseous surrounding is discussed on the basis of the analysis of the optical parameters. Some aspects of the mechanism of gas sensitivity are commented for tin oxide gas sensors.

Thickness effect of constituent layers on gas sensitivity in SnO2/[metal]/metal multi-layers

Abstract The multi-layer sensors based on the SnO2/[metal]/metal thin layers are produced and investigated at temperatures from 20°C to 300°C in clean and contaminated air (CO, H2, NOx). Various metals Pt, Au, Mo and Ni were used for the layer at the bottom of the sensor. Thickness of the constituent layers is varied and the electrical resistance and the X-ray photoelectron spectroscopy (XPS) depth profiles are measured. It is determined that the resistance response to gases is dependent on the thickness of each of the constituent layers. The response signal was set to maximum by an optimal thickness of the top- and bottom-layers. The selectivity to gas was changed by the metallic inter-layer grown in the interface between the top- and bottom-layers up to appropriate thickness. The metallic character of the conductance in the base film is associated with a unique response to gas in the sensors tested.

CO-gas-induced resistance switching in SnO2/ultrathin Pt sandwich structure

Abstract A novel sandwich structure based on an insulating metal oxide top layer and an ultrathin metal bottom layer is proposed. The experiments are carried out on a model structure consisting of thin films of Pt and SnO2. A resistance switching effect induced by CO gas is obtained in these sandwich structures. The effect is proved to originate from the properties of the ultrathin Pt film. The large amplitude of the effect is related to the tin oxide top layer. Some features of the switching effect are discussed on the basis of the proposed model.

Stability and oxidation of the sandwich type gas sensors based on thin metal films

Abstract The stability of the properties is investigated in the new family of the solid-state gas sensors based on various thin metal films (Pt, Au, Ni, Mo) with the modifying top layer of tin oxide. The resistance response to exposure to gas is demonstrated being stable within certain interval of temperatures. The limits of the interval depend on the metal selected for the basic layer. The variation of the chemical composition is also studied in the sensors after overheating in the clean or contaminated air. The drift of the electric resistance and the mechanism of gas sensitivity are discussed on the basis of the oxidation of the metal film.

Expression and purification of active recombinant equine lysozyme in Escherichia coli

Equine lysozyme (EL) is a calcium (Ca)-binding lysozyme and is an intermediary link between non-Ca-binding C-type lysozyme and alpha-lactalbumin. The feature of lysozymes to assemble into the fibrils has recently gained considerable attention for the investigation of the functional properties of these proteins. To study the structural and functional properties of EL, a synthetic gene was cloned and EL was overexpressed in Escherichia coli as a fused protein. The His-tagged recombinant EL was accumulated as inclusion bodies. Up to 50 mg/l of the recombinant EL could be achieved after purification by Ni(2+) affinity chromatography, refolding in the presence of arginine, CM-Sepharose column purification following TEV protease cleavage. The purified protein was functionally active, as determined by the lysozyme activity, proving the proper folding of protein. The purified lysozyme was used for the oligomerisation studies. The protein formed amyloid fibrils during incubation in acidic pH and elevated temperature. The recombinant EL forms two types of fibrils: ring shaped and linear, similar to the native EL.