Pavol Durina

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.

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.

Structure of Hydrogen Gas Sensing TiO2 Thin Films Prepared by Sol-Gel Method and their Comparison with Magnetron Sputtered Films

TiO2 thin films with a thickness of about 150 nm were deposited by spin coating method on sapphire substrate from a sol-gel system. The hydrogen sensing properties of TiO2 films annealed at various temperatures were studied and correlated with their structure, optical and electrical properties. The annealing temperatures in the range of 600 800 °C lead to anatase films with a roughness in the range of 0.6 0.9 nm. Their sensitivity towards hydrogen is low. The thin films annealed at temperatures in the range 900 1000 °C consist of rutile phase and their roughness increased to 11.7 13.5 nm. They showed good hydrogen sensitivity with optimal operating temperature 200 250 °C. The structure and sensing properties of the prepared films are compared with those synthesized with magnetron sputtering. The maximum of sensitivity was measured on the thin films with diameter of the grains about 100 nm in both cases, i.e. on thin films prepared by sol-gel method as well as on thin films prepared by magnetron sputtering. The maximum sensitivity correlates with the diameter of the grains and dont depend on the allotropy of the titanium dioxide anatase or rutile.

Metal oxide gas sensors on the nanoscale

Low cost, low power and highly sensitive gas sensors operating at room temperature are very important devices for controlled hydrogen gas production and storage. One of the disadvantages of chemosensors is their high operating temperature (usually 200 – 400 °C), which excludes such type of sensors from usage in explosive environment. In this report, a new concept of gas chemosensors operating at room temperature based on TiO2 thin films is discussed. Integration of such sensor is fully compatible with sub-100 nm semiconductor technology and could be transferred directly from labor to commercial sphere.

Properties of Metal Oxide Gas Sensors with Electrodes Placed below the Sensing Layer

In this work, we investigate the influence of position of electrodes on the sensitivity of hydrogen gas sensors based on TiO2 thin films. We have prepared two types of sensors with platinum comb-like electrodes deposited on top and under the TiO2 layer. Response of these sensors to hydrogen gas in the concentration range of 0 10 000 ppm at temperature of 350 °C has been studied. The sensors with electrodes placed under the TiO2 layer showed two orders of magnitude lower sensitivity for 10 000 ppm compared to sensors with electrodes on top of the layer, but it was considerably increased when thickness of the TiO2 layer was lowered. This gives a possibility to improve the sensitivity of gas sensors in which the electrodes must be placed below the sensing layer for their protection from harsh environment.

Patterning of nanometer structures by using direct-write e-beam lithography for the sensor development

In this work, the optimalisation of e-beam parameters and the writing strategy have been performed. Various positive and negative e-beam resists have been evaluated for high resolution e-beam lithography and pattern transfer. Both, lift-off method and ion beam etching have been investigated for the pattern transfer into thin Pt and MoC layer on saphire substrate.

Simple patterning method of sub-micro- and nanometer structures for gas sensor

In this study, a simple patterning method of submicrometer structures is proposed for gas sensor development. Comb-like electrodes patterned in thin Pt layer were proposed to measure gas sensor electrical conductivity. Negative resist SU-8 was used as a masking layer for ion etching of electrodes in 35 nm Pt layer on sapphire substrate. This method was applied for the patterning of the comb-like structures with submicrometre dimensions for gas sensor conductivity measurements.

Structure and Epitaxial Behavior of Rutile TiO2 Thin Films Prepared by DC Magnetron Sputtering and Ex-Situ Annealing

Titanium dioxide gas sensors are typically employing metastable anatase nanocrystalline phase. Operation at high temperature can thus negatively affect their long term stability. Employment of rutile phase with strong texture and larger grain size may ensure better reliability and longer lifetime. Therefore in this work we study the possibility to utilize stable rutile phase thin films prepared at relatively low temperature on c-cut sapphire substrates. Technological conditions have been chosen in order to obtain highly oriented titanium dioxide rutile thin films using reactive DC magnetron sputtering on unheated substrates. Subsequent ex-situ annealing in temperature range from 500°C to 800°C leads to increase of crystallite size and improvement of in-plane preferential orientation. Surface topography has been characterized by atomic force microscopy. Structure, texture and the strain evolution has been investigated using x-ray diffraction measurements. All investigated thin films showed epitaxial relationship with respect to the substrate: rutile-TiO2(100)[00 || Al2O3(0001)[10. Sensitivity of such rutile films to hydrogen has been measured and compared with our previous results on anatase thin films.

P2.0.5 Studies of Hydrogen Gas Sensing Properties of Anatase TiO2 Thin Films Prepared by Magnetron Sputtering

Titanium dioxide thin films were deposited on sapphire substrates by reactive dc magnetron sputtering method. The prepared thin films were then annealed in air at various temperatures (600 – 800 °C) to achieve crystalline thin films having anatase phase. The comb-like Pt electrodes, with a distance of 10 μm, were prepared on the top of the films to measure the electrical and gas sensing properties. The films prepared in this work showed high electrical response for various concentrations of H2/air, ranging from 20 to 10 000 ppm. The response was tested in the working/operating temperature range 150 – 350 °C and it was observed that these thin films are the most sensitive at temperatures below 200°C.