T. Plecenik

Hydrogen gas sensors based on nanocrystalline TiO2 thin films

Titanium dioxide thin films are extensively studied for applications in solid state gas sensor devices. Their gas sensing properties are strongly dependent on deposition technique, annealing temperature, film thickness and consequent properties like crystalline structure, grain size or amount of defects and impurities. In this work we report the gas sensing properties of TiO2 thin films prepared by reactive magnetron sputtering technique and subsequently annealed at temperatures 600°C and 900°C. The films were exposed to different concentrations of H2 gas up to 10 000 ppm. Their sensitivity to gas at various operating temperatures, ranging from 250°C to 450°C, was obtained by measuring their resistance.

Studies of resistance switching effects in metal/YBa2Cu3O7−x interface junctions

Current-voltage characteristics of planar junctions formed by an epitaxial c-axis oriented YBa2Cu3O7-x thin film micro-bridge and Ag counter-electrode were measured in the temperature range from 4.2 K to 300 K. A hysteretic behavior related to switching of the junction resistance from a high-resistive to a low-resistive state and vice-versa was observed and analyzed in terms of the maximal current bias and temperature dependence. The same effects were observed on a sub-micrometer scale YBa2Cu3O7-x thin film - PtIr point contact junctions using Scanning Tunneling Microscope. These phenomena are discussed within a diffusion model, describing an oxygen vacancy drift in YBa2Cu3O7-x films in the nano-scale vicinity of the junction interface under applied electrical fields.

Effect of crystallographic anisotropy on the resistance switching phenomenon in perovskites

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented YBa2Cu3O6+c (YBCO) thin films with different crystallographic orientation have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.

Charge specific protein placement at submicrometer and nanometer scale by direct modification of surface potential by electron beam.

The understanding and the precise control of protein adsorption is extremely important for the development and optimization of biomaterials. The challenge resides in controlling the different surface properties, such as surface chemistry, roughness, wettability, or surface charge, independently, as modification of one property generally affects the other. We demonstrate the creation of electrically modified patterns on hydroxyapatite by using scanning electron beam to tailor the spatial regulation of protein adsorption via electrostatic interactions without affecting other surface properties of the material. We show that domains, presenting modulated surface potential, can be created to precisely promote or reduce protein adsorption.

Effect of Post-Deposition Annealing Treatment on the Structural, Optical and Gas Sensing Properties of TiO2 Thin Films

One of the potential applications of TiO2 is its use in gas sensor technology. The aim of this work was to study the gas sensing properties of TiO2 thin films in combination with the effect of post-deposition annealing treatment. Titanium dioxide thin films with thickness 100 nm were prepared by the reactive dc magnetron sputtering. The thin films were deposited on sapphire substrate from a titanium target in an oxygen atmosphere. The samples were then post-annealed in air in the temperature range 600 °C 1000 °C. Crystal structure, surface topography and absorption edge of the thin films have been studied by X-ray Diffraction technique, Atomic Force Microscopy and UV-VIS Spectroscopy. It was found that the phase gradually changed from anatase to rutile, the grain size and roughness tended to increase with increasing post-annealing temperature. The effect of these factors on gas sensing properties was discussed. For electrical measurements comb-like Pt electrodes were prepared by standard photolithography and the films were exposed to different concentrations of H2 gas up to 10000 ppm in synthetic air at various operating temperatures from 200 °C to 350 °C.

Direct creation of microdomains with positive and negative surface potential on hydroxyapatite coatings

A method for the direct patterning of electrostatic potential at the surface of hydroxyapatite is presented here. Microdomains of surface potential have been created on hydroxyapatite coatings by a 20 keV focused electron beam with minimal alterations of surface chemistry. The success of such approach has been confirmed by Kelvin probe force microscopy measurements, which show that this method is capable of creating micron sized positive and negative local electrostatic potential. The shape and potential difference of these domains were found to depend on the dose of total injected charge from the electron beam as well as the speed with which such charge is injected.

Inverse polarity of the resistive switching effect and strong inhomogeneity in nanoscale YBCO-metal contacts

We have studied a bipolar resistive switching phenomenon in c-axis oriented normal-state YBa2Cu3O7-c (YBCO) thin films at room temperature by scanning spreading resistance microscopy (SSRM) and scanning tunneling microscopy (STM) techniques. The most striking experimental finding has been the opposite (in contrast to the previous room and low-temperature data for planar metal counter-electrode-YBCO bilayers) voltage-bias polarity of the switching effect in all SSRM and a number of STM measurements. We have assumed that the hysteretic phenomena in current-voltage characteristics of YBCO-based contacts can be explained by migration of oxygen-vacancy defects and, as a result, by the formation or dissolution of more or less conductive regions near the metal–YBCO interface. To support our interpretation of the macroscopic resistive switching phenomenon, a minimalist model that describes radical modifications of the oxygen-vacancy effective charge in terms of a charge-wind effect was proposed. It was shown theo...

Directly created electrostatic micro-domains on hydroxyapatite: probing with a Kelvin Force probe and a protein

Micro-domains of modified surface potential (SP) were created on hydroxyapatite films by direct patterning by mid-energy focused electron beam, typically available as a microprobe of Scanning Electron Microscopes. The SP distribution of these patterns has been studied on sub-micrometer scale by the Kelvin Probe Force Microscopy method as well as lysozyme adsorption. Since the lysozyme is positively charged at physiological pH, it allows us to track positively and negatively charged areas of the SP patterns. Distribution of the adsorbed proteins over the domains was in good agreement with the observed SP patterns.

Surface transport properties of Fe-based superconductors: The influence of degradation and inhomogeneity

Surface properties of Co-doped BaFe2As2 epitaxial superconducting thin films were inspected by X-ray photoelectron spectroscopy, scanning spreading resistance microscopy (SSRM), and point contact spectroscopy (PCS). It has been shown that surface of Fe-based superconductors degrades rapidly if being exposed to air, what results in suppression of gap-like structure on PCS spectra. Moreover, SSRM measurements revealed inhomogeneous surface conductivity, what is consistent with strong dependence of PCS spectra on contact position. Presented results suggest that fresh surface and small probing area should be assured for surface sensitive measurements like PCS to obtain intrinsic properties of Fe-based superconductors.

Surface Charge and Carbon Contamination on an Electron-Beam-Irradiated Hydroxyapatite Thin Film Investigated by Photoluminescence and Phase Imaging in Atomic Force Microscopy

The surface properties of hydroxyapatite, including electric charge, can influence the biological response, tissue compatibility, and adhesion of biological cells and biomolecules. Results reported here help in understanding this influence by creating charged domains on hydroxyapatite thin films deposited on silicon using electron beam irradiation and investigating their shape, properties, and carbon contamination for different doses of incident injected charge by two methods. Photoluminescence laser scanning microscopy was used to image electrostatic charge trapped at pre-existing and irradiation-induced defects within these domains, while phase imaging in atomic force microscopy was used to image the carbon contamination. Scanning Auger electron spectroscopy and Kelvin probe force microscopy were used as a reference for the atomic force microscopy phase contrast and photoluminescence laser scanning microscopy measurements. Our experiment shows that by combining the two imaging techniques the effects of trapped charge and carbon contamination can be separated. Such separation yields new possibilities for advancing the current understanding of how surface charge influences mediation of cellular and protein interactions in biomaterials.