Solid State Phenomena Vols. 99-100

Abstract: Two types of silver-doped glass were used for direct laser recording of 2D and 3D photonic crystals. The first contained a diffusion layer (20 microns thick) with embedded silver nanoclusters of 20-nm average radius. 2D and 3D photonic crystals of submicron lattice parameters were fabricated by nanosecond pulsed laser irradiation (l, = 355 nm) using four or five coherent intersecting beams. Under irradiation the clusters absorbing light energy are heated to high temperatures and become mobile due to the formation of liquid shells around them. Adjacent clusters move towards each other and towards the irradiated surface under local temperature gradients, form agglomerates and merge in periodically located "spots" of high light intensity in the interference field. The second type of glass, photosensitive to UV irradiation, contained in the bulk Ag+ and Ce3+ ions. Under UV irradiation excited electrons passed from Ce3+ to Ag+. The Ag atoms became neutral and under subsequent heat treatment of the glass at elevated temperatures have a tendency to form nanoclusters, thus “developing” the UV recorded patterns. Using nanosecond pulsed irradiation of 308 nm we have recorded 3D photonic crystals in the bulk of such glass.

Abstract: Templating methods provide an alternative approach for fabricating macroporous solids with three-dimensionally ordered arrays of pores with diameters from tens to hundreds of nanometers. The main goal of this work is the fabrication of thermoelectric-opal composite and inverse opals made of thermoelectric material in order to obtain enhanced performance by reducing the thermal conduction. Chemical bath deposition (CBD) has been employed and thermodynamic modelling was used to obtain optimum conditions for the desired phase of particle formation within the channels of the opal-like structures. The wetting behaviours of opals were considered in the choiceof the appropriate solvents. When loading is finished, the precursor is decomposed and reduced when appropriate. The opal template is then removed by pyrolysis to obtain the macroporous structures. The structural properties of materials produced were investigated.

Abstract: We investigated the lasing spectra of multiple scattering active media based on solid polymer solutions of rhodamin 6G with embedded scattering silica particles of mean diameter 500nm. Lasing in these samples appears without any cavity and arises even in rather small active volume 0.05*0.5*0.5mm3. In a thick sample positive feedback for lasing is provided mainly by multiple scattering of light. In a thin sample the feedback is provided both by multiple scattering of light on the particles and light reflection from the sample surface. It is found that the lasing spectra peak wavelength can be varied within 13nm by varying sample thickness in a range 0.1 -1.65mm. A method of continuos laser tuning by wedge-like sample shift with respect to the pump beam is proposed. The presented results are aimed at the development of a new type of microlaser.

Abstract: Fabrication of surface-relief microstructures in GaSb for application in mid-infrared optoelectronic devices is described. Photo- and e-beam lithography was used to define patterns on GaSb surfaces. Ar/O2 sputter etching and RIE in BCl3-based plasma were applied to transfer preshaped master into the GaSb substrate. Circular microlenses with an aspect ratio (height to diameter) 0.4/10 µm and circular gratings with 0.4 µm linewidth / 1 µm period and 1.7 µm depth have been demonstrated.

Abstract: Yttria-doped zirconia nanopowders have been obtained using the hydrothermal procedure starting from soluble inorganic salts. The mechanisms and kinetics of the process have been studied to obtain high purity powders with a crystalline size range of 4 to 22nm and specific surface near 200 m2/g. These powders have been have been used to obtain membranes with controlled thickness and with densities over 95% of the theoretical value by employing the tape casting technique using organic binders, dispersants and surfactants. The influence of the additives and sintering regime on the density and microstructure of membranes has been studied. The ionic conductivity of the materials was investigated and modelled. Different types of ruthenate pastes were used to obtain thick resistive films on the zirconia membranes and interactions between the substrate and membranes were studied. Finally the gauge characteristics of the device and possibilities for applications as mechanical pressure sensors with high sensitivity are discussed.

Abstract: We present the results of time-resolved photoluminescence (TRPL) and optically detected microwave resonance (ODMR) spectroscopy investigations of semiconductor quantum dots and quantum wells. The ODMR spectra of InAs/GaAs QDs were detected via modulation of the total intensity of the QDs emission induced by 95 GHz microwave excitation and exciton fine structure was studied. Very long life times (up to 10 ns) of photoexcited carriers were observed in this system using TRPL at low temperatures and excitation intensities promising higher responsitivity of such QDs for quantum dot infrared photodetector development. The effects of proton and alpha particles irradiation on carrier dynamics were investigated on different InGaAs/GaAs, InAlAs/AlGaAs and GaAs/AlGaAs QD and QW systems. The obtained results demonstrated that carrier lifetimes in the QDs are much less affected by proton irradiation than that in QWs. A strong influence of irradiation on the PL intensity was observed in multiple QWs after high-energy alpha particles irradiation.

Abstract: The paper describes the use of an in-situ microscopy technique, which combines transmission electron microscopy (TEM) with scanning probe microscopy (SPM), to investigate the electrical and mechanical properties of individual silicon and germanium nanowires. Additionally, the formation of ordered arrays of size-monodisperse silicon and germanium nanowires within mesoporous silica powders and thin films using a supercritical fluid inclusion phase technique is described. In particular, we demonstrate ultra high-density arrays of germanium nanowires, up to 2 x 1012 wires per square centimetre. These matric embedded nano-composite materials display unique optical properties such as intense room temperature ultraviolet and visible photoluminescence.

Abstract: The thermodynamic properties of the ABO3 perovskite-type compounds are strongly affected by the A- and B- site substitution and the oxygen nonstoichiometry. The paper presents the thermodynamic properties of some doped nano- and microstructured lanthanum manganites of general formula La1-xMxMn1-yMeyO3 (where M=Sr, Ca; Me=Al; x=0.3;0.33, y=0;0.05) which have been studied within the temperature range of 873-1273 K in a reducing atmosphere (10-6 Pa). The relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase and the equilibrium partial pressures of oxygen have been obtained by using a solid electrolyte galvanic cell method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with measurements of the electromotive force (EMF). The focus of the research was to emphasise modifications of the thermodynamic properties connected with the nanocrystalline state. The results obtained are discussed in relation to significant changes in the overall concentration of defects.

Abstract: The microstructural characterization of r.f. magnetron sputtered ZnO thin films deposited on 6H-SiC is presented with a comprehensive investigation of their properties as a function of annealing temperature and film thickness. These structures, with some modifications, are utilised as Schottky diode hydrogen gas sensors and Surface Acoustic Wave (SAW) devices.

Abstract: A sol-gel method of preparation doped ZnO varistor nanomaterials is described, The influences of doped ZnO nanomaterials for varistor microstructure and electrical properties (nonlinear coefficient α, breakdown voltage V1mA , dielectric constant ε, and dielectric loss tan δ) are investigated. Compared with the conventional mixed oxide technique, varistor ceramic of prepared by nanometer materials showed a more homogeneous microstructure, smaller grain sizes, higher densities and excellent electrical properties.