Aare Floren

Photo-activated oxygen sensitivity of graphene at room temperature

Photo-induced changes in the electrical conductivity and the sensitivity to oxygen gas of graphene sheets grown by chemical vapor deposition and transferred onto Al2O3 and SiO2 thin film substrates were studied at ambient conditions. The pristine graphene sensors were initially completely insensitive to oxygen gas at room temperature but showed significant (up to 100%) response when illuminated with weak ultraviolet (300 nm or 365 nm) light. Oxygen response was governed by Langmuir law and its activation was insensitive to humidity. The mechanism of sensitization is analyzed together with other photo-induced effects—negative persistent photo-conduction and photo-induced hysteresis of field effect transistor characteristics. While the reduction of conductivity in air is persistent effect, the oxygen sensitization and enlargement of hysteresis take place only under the direct influence of light. It is concluded that the charge traps with differently adsorbed oxygen and water are involved in these phenomena.

Stability of luminescence decay parameters in oxygen sensitive polymer films doped with Pd-porphyrins

An essential factor for the creation of oxygen sensors based on luminescence quenching is the long-term stability of the parameters determining the luminescence decay. In the present work, the effects of material aging and photobleaching on decay parameters are studied for Pd-tetraphenylporphyrin and Pd-pentafluoro-tetraphenylporphyrin molecules embedded into different polymer films. In polymethylmethacrylate host the decay was well described by stretched exponent function. During annealing of PMMA films at different temperatures between 20 and 70 °C during 9 months a logarithmic decrease of the material sensitivity occurred due to the physical aging of the polymer. The photostability studies of PMMA films showed an opposite effect: the photobleaching was accompanied by a decrease in natural decay time, whereas the oxygen sensitivity remained practically constant. Luminescence quenching in polystyrene and polycarbonate films had different character as compared to PMMA, and was interpreted with a two-site model, combined with a nonlinear gas transport model. In these polymer hosts the oxygen sensitivity increased during the first stage of aging, which was ascribed to the changes in the Langmuir component of gas transport, i.e., to the evaluation of microvoids in these glassy polymers.

Preparation and Characterization of Transparent Electrodes Based on CNT-s Doped Metal Oxides

In the present work, it is shown that carbon nanotube-doped transition metal oxides are potential candidates for use as ceramic transparent electrode materials. Used carbon nanotubes (CNT-s) are synthesized by using chemical vapor deposition (CVD) method. Electrodes in shape of fibers are obtained via inexpensive and low temperature sol-gel method. Due to extraordinary electrical and optical properties of CNT-s and good chemical and physical stability of metal oxide ceramics, resulting composites could be an interesting subject for industry.

Modelling of gas sensitivity for p-type semiconducting thin films

A model is presented for describing the conductivity dependencies of p-type thin film sensors from oxygen and CO pressures. The model basis on the kinetic equations, composed for the processes of dissociative ionization of O2 and oxidation of CO on the surface, and taking into the account the charge balance between the adsorbed species and the hole accumulation layer. The model results are in agreement with the experimental data obtained on thin film sensors with two different p-type materials: Co3O4 and CoWO4. Both the experimental and model data differ essentially from the data obtained previously for n-type materials like SnO2. In particular, it was found that the sensitivity to CO increases significantly with decreasing O2 content in ambient environment. It is predicted that a similar effect (i.e. an increased sensitivity) can be obtained by engineering the intrinsic surface bending of the valence band.

Electrical and Gas Sensing Properties of Cr2O3-TiO2 Thin Films made by Pulsed Laser Deposition

Pulsed laser deposition was applied to chromia-titania solid solutions (CTO) with the aim to study electrical and gas sensing properties on (ultra)thin films of this material. The films with different compositions (5% to 33.3% of Ti from total cationic amount) were characterized by XRD, XRR, AFM, and EPMA. The electrical conductivity of the films was studied in clean air and in the presence of reducing gases (H2, CO). The gas sensitivity was found to depend strongly on growth and annealing conditions but only slightly from thickness (10-70 nm) or composition of the films prepared under similar conditions.