Anna. Og. Dikovska

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.

Pulsed laser deposited ZnO film on side-polished fiber as a gas sensing element

A simple sensor element consisting of a side-polished single-mode fiber and a planar metal oxide waveguide is described. The thin ZnO planar waveguide was produced on the polished fiber surface by pulsed laser deposition at optimized processing parameters. A measurement scheme for in situ control of the film thickness during the deposition process was developed and used. X-ray diffraction measurements and scanning electron microscopy were used to characterize the structure and the surface morphology of the planar waveguide, respectively. The numerical evaluation of the sensor sensitivity predicts the possibility to detect refractive index changes of less than 10(-4). Furthermore, preliminary gas sensor tests were performed by using a mixture of 1.5% butane diluted in N(2) and pure butane. A shift of the spectral position of the resonance points was observed from 3 to 5 s after gas exposure, which corresponds to refractive index changes of 3 x 10(-5) and 1.2 x 10(-3) for 1.5% butane and for pure butane, respectively.

Properties of plasmonic arrays produced by pulsed-laser nanostructuring of thin Au films

Summary A brief description of research advances in the area of short-pulse-laser nanostructuring of thin Au films is followed by examples of experimental data and a discussion of our results on the characterization of structural and optical properties of gold nanostructures. These consist of partially spherical or spheroidal nanoparticles (NPs) which have a size distribution (80 ± 42 nm) and self-organization characterized by a short-distance order (length scale ≈140 nm). For the NP shapes produced, an observably broader tuning range (of about 150 nm) of the surface plasmon resonance (SPR) band is obtained by renewal thin film deposition and laser annealing of the NP array. Despite the broadened SPR bands, which indicate damping confirmed by short dephasing times not exceeding 4 fs, the self-organized Au NP structures reveal quite a strong enhancement of the optical signal. This was consistent with the near-field modeling and micro-Raman measurements as well as a test of the electrochemical sensing capability.

Structural and photoluminescent properties of Ag/ZnO nanocomposite heterostructures

We describe the preparation of Ag nanoparticles covered by ZnO and investigation of the Ag/ZnO nanocomposite heterostructures. The silver nanoparticles were produced by pulsed laser deposition (PLD) on quartz substrates in a vacuum chamber and were post-deposition annealed by a Nd:YAG laser for surface nanostructuring. The morphology and optical properties of Ag nanoparticles were studied and the mean particles diameter and size distribution were estimated. Laser ablation was used for the subsequent deposition of ZnO. In order to study the Ag nanoparticles influence on ZnO, X-ray diffraction (XRD) and photoluminescent (PL) spectra were obtained and analyzed. It was found that the band-gap emission of ZnO grown on Ag nanoparticles can be enhanced by about a factor of five compared to the emission of ZnO film without Ag undercoating. The silver nanoparticles are also responsible for the UV shift in the PL emission.

Au nanostructure fabrication by pulsed laser deposition in open air: Influence of the deposition geometry

We present a fast and flexible method for the fabrication of Au nanocolumns. Au nanostructures were produced by pulsed laser deposition in air at atmospheric pressure. No impurities or Au compounds were detected in the resulting samples. The nanoparticles and nanoaggregates produced in the ablated plasma at atmospheric pressure led to the formation of chain-like nanostructures on the substrate. The dependence of the surface morphology of the samples on the deposition geometry used in the experimental set up was studied. Nanocolumns of different size and density were produced by varying the angle between the plasma plume and the substrate. The electrical, optical, and hydrophobic properties of the samples were studied and discussed in relation to their morphology. All of the nanostructures were conductive, with conductivity increasing with the accumulation of ablated material on the substrate. The modification of the electrical properties of the nanostructures was demonstrated by irradiation by infrared light. The Au nanostructures fabricated by the proposed technology are difficult to prepare by other methods, which makes the simple implementation and realization in ambient conditions presented in this work more ideal for industrial applications.

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.

Magnetic-particles-composed wire structures produced by pulsed laser deposition in a magnetic field

We demonstrate the possibility to fabricate wire structures composed by arranged magnetic particles using pulsed laser deposition (PLD) in the presence of a magnetic field. Ablation of Ni and Co targets was performed in air by nanosecond laser pulses delivered by a Nd:YAG laser system oscillating at 355 nm. Due to the high density of the ambient, particles and clusters were formed by condensation in the plasma plume close to the target. The strong deceleration of the ablated material under these conditions further benefited the efficiency of applying a magnetic field to the plume. We also studied the effect of the target-to-substrate distance and the ambient pressure on the morphology of the deposited structures.

Fabrication of Au nanostructures by pulsed laser deposition in air

Results on fabrication of Au nanostructures by laser ablation in open air are presented. The ablation of the Au target is performed in air environment by nanosecond laser pulses delivered by Nd:YAG laser system operated at λ = 355 nm. Due to the high density of the ambient atmosphere, the intensive collisions of the plume spices result in formation of nanoparticles and aggregates by condensation close to the target. The produced nanoagregates are deposited on a quartz substrate where grow in a specific nanostructure. Diagnostics of the laser-generated plasma for the laser fluences used in this study is performed. Study based on change of ambient conditions shows that the increase of the air pressure from 10 Torr to atmospheric one leads to transition from thin film to porous structures. It is found that the surface morphology of the structures produced by pulsed laser deposition (PLD) in open air strongly depends on the substrate-target distance. The electrical properties of the obtained structures are studied by measurement of their electrical resistance. It is found that the conductivity of the structures strongly depends on their morphology. The fabricated structures have potential for application in the field of electronics and sensors.

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.

Structural and optical properties of thin indium oxide films produced by pulsed laser deposition

Thin indium oxide (In2O3) films were grown on (001) SiO2 by pulsed laser deposition techniqe at oxygen pressure from 1 to 20 Pa and substrate temperature between 100 and 500 °C from ceramic targets. The structural and optical properties of the films were investigated as a function of the growth conditions: oxygen pressure and substrate temperature. Polycrystalline In2)O3 films with prefer3ential (111) orientation were produced at oxygen pressure higher than 5 Pa and temperature higher than 100 °C. The Raman spectra confirm the cubic structure of the films. The films have transparency between 85% and 92% in the 400-2400 nm spectral range. The lowest optical waveguide loss measured is 9 dB/cm for the film grown at the optimum conditions: P(O2) = 10 Pa and Ts = 300 °C.