I.G. Dimitrov

Al doped ZnO thin films for gas sensor application

Highly textured pure and Al doped ZnO thin films have been produced by pulsed laser deposition for optical gas sensor application. The influence of the processing parameters such as substrate temperature and oxygen pressure applied during depositions, and dopant concentration on the structural, morphological, and optical properties of the films were investigated. All deposited films are textured along the (002) direction. The substrate temperature and the oxygen pressure have stronger influence on the film crystallinity compared to the presence of the dopants. The grain size of the films prepared from 2 wt% Al2O3 doped ZnO target is approximately the same as the one produced from pure ZnO tagret. The increase in the dopant concentration into the ZnO target (to 5 wt% Al2O3) leads to an increase of the in-plane grain size of the as-deposited films which is well known to increase the gas sensitivity. At the same time, the use of doped targets increase the droplets on the film surface which deteriorates the optical detection of the gas sensing effect. The increase of the dopant concentration reduces the film transmission in the visible range and the transmission cut-off edge is shifted to the shorter wavelengths. The films deposited from 2 wt% Al2O3 doped ZnO target at oxygen pressure of 0.05 mbar and 300°C substrate temperature have good mode properties which makes them good candidates for optical sensors.

Silver nanoparticles produced by PLD in vacuum

In this work, we report the fabrication of silver (Ag) nanoparticles on silica substrate in vacuum by PLD using a third harmonic of Nd:YAG laser (λ = 355 nm). The existence of Ag nanoparticles was evidenced by transmission electron microscopy (TEM) and an analysis of the size of the nanoparticles as a function of the number of laser pulses was performed. The Ag nanoparticles produced had mean diameters in the range from 1 nm to 15 nm, with the size distribution centered near 5 nm. The presence of the Ag nanoparticles was also evidenced from the appearance of a strong optical absorption band into the measured UV-VIS spectra, associated with surface plasmon resonance. Broadening of the absorption peak and its shifting to the longer wavelengths was observed as the number of laser pulses increased. A theoretical study of the optical properties of Ag nanoparticles predicts the presence of a thin oxide shell covering the nanoparticles.

Growth of anatase TiO2 thin films by laser ablation

Thin TiO2 films were grown on (001) SiO2 substrates using excimer KrF laser ablation of ceramic targets. The influence of deposition temperatures at fixed oxygen pressure of 10 Pa on the crystal and optical properties of the films was investigated. Structural characterisation by X-ray diffraction and Raman spectroscopy shows preferential crystallization of the anatase TiO2 films were obtained at high temperatures of 500 and 600 °C, respectively. Optical transmission as high as 92% in the visible spectral region was measured for films grown at temperatures higher than 400 °C. The refractive index and the thickness of the films measured by m-line spectroscopy shows the highest values of 2/41 and 250 nm, respectively, at deposition temperature of 600 °C.

Ultraviolet annealing of thin Y2O3 films grown by pulsed laser deposition

We report on the results of ultraviolet (UV)-assisted annealing of thin Y2O3 films produced by pulsed laser deposition. An excimer XeCl laser was used for ablation of Y2O3 ceramic target. The films were grown on (001) SiO2 substrates at 500 °C and oxygen pressure of 0.05 mbar. The effect of UV-assisted laser annealing on the structure, morphology, and optical properties was investigated. The UV-assisted annealing was performed by the same laser. The beam was directed parallel or toward the surface of the as-deposited films. The influence of the ambient gas (O2 or N2O) is explored. The ambient atmosphere has an influence on the preferential (cubic or monoclinic) phase of gowth while it has no significant effect on the surface morphology. The absorption coefficient in the VIS range has a lower value for the films annealed with laser directed parallel to the surface independently on the gas environment. Annealing of the films with laser beam directed at the film surface slightly increases the refractive index, independently of the gas ambient.