National Institute Of Materials Physics - Romania

This paper presents experiments to develop SAW structures with operating frequencies in the GHz range. The experiments were developed on AlN and GaN thin films with nano-metric lines for the IDT. The nanolithography was performed at IMT with the new purchased VEGA 5136 LM SEM from TESCAN and ELPHY Plus EBL from Raith GmbH. There were obtained some very good quality inerdigitated Ti/Au structures (with 30 digits and 29 interdigits 300nm wide, 220nm high and and 200 mu m long). Lift-off techniques to remove the undesired metal were used On wafer microwave measurements of the SAW structure (performed at IMT on the new purchased Anritsu vector analyzer equipped with Suss Microtec probes), have demonstrated its functionality. It was evidenced a pronounced resonance at about 2.8GHz.

A theoretical study of resonant tunneling in multilayered heterostructures is presented based on an exact solution of the Schrodinger equation under the application of a constant electric field and a uniform magnetic field. By use of the transfer matrix approach, the transmissivity of the structure is determined as a function of the incident electron energy. The results show good agreement with other existing models as well as with the bound-state energies. The solutions obtained are exact and can be generated easily by using MathCad. Based on these calculations, a new class of resonant tunneling superlattice devices can be designed.

Titanium dioxide thin films prepared by sol-gel method and deposited on p-Si (111), n-GaAs (100) and glass have been investigated by Raman spectrometry and photoluminescence analysis. The films were annealed in the temperature range (500-800)degrees C in air. Raman scattering studies reveal the presence of Raman active modes in the calcinated TiO2 films related to anatase phase of titania. The photo luminescence spectrum presents prominent red shifted peaks (e.g. at 400 nm for TiO2/glass), where the maximum is related to the presence in titania films of defect levels corresponding to oxygen vacancies.

Last years the ternary stabilised zirconia systems attract a considerable attention as a source of materials for structural ceramics as well as for modern fuel cells. The simultaneous influence of a pair of cations (twinned) such as: Mg2+-Ca2+ and Sr2+-Ca2+ on zirconia powders stabilisation process in 1550-1750 degrees C temperature range. Samples for investigations were performed by classical ceramic method. Comparative microstructure investigations were made by SEM on sample surfaces. The XRD, FTIR analysis and EDS spectra for the considered samples are in good agreement with the determined properties of the studied compositions. (c) 2006 Elsevier Ltd. All rights reserved.

We describe our low spectral resolution Raman system that allows us to directly appreciate the quality of YBa2Cu3O7-delta (YBCO) films, obtained by pulsed laser deposition on LaAlO3 substrate. We identify the basic features of YBCO films' spectral fingerprints, validated by high spectral resolution analysis, described in previous reports. This fast and non-destructive valuable analytical tool functions at a convenient time and environment after deposition.

Pure and strontium-doped lanthanum manganite nanopowders were obtained by solution combustion method using metal nitrates as oxidants and different organic compounds as fuel (alpha-alanine, glycine, urea and semioxamazide). Different molar ratios, fuel/oxidant, were chosen to study the effect of the fuel content on the phase formation and powder properties. The complex precursors isolated before ignition of the system, were characterized by FT-IR spectroscopy and thermal analysis. After ignition, the as-synthesized powders were calcined at different temperatures for obtaining single phases. Lanthanum manganite powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), specific surface area and magnetic measurements. (c) 2007 Elsevier Ltd. All rights reserved.

In this paper, we recall the main characteristics of Raman spectra of single-walled carbon nanotubes. We illustrate our studies by results obtained on HiPCO samples on one hand, and "arc electric" samples on the other hand by using two different excitation wavelengths. We focus on the peculiarity of observing separately semi-conducting and metallic tubes. We emphasize also the role played by the radial breathing modes whose frequency depends on the tube diameter and on the need to take into account the interactions between tubes when they are in bundles in order to determine accurately the tube diameter distribution. Finally we demonstrate briefly the evidence of using the SERB (surface enhanced Raman scattering) spectroscopy to study reactions at interfaces or between components in nanotubes/conjugated polymers composites.

We have synthesized polymer particles of the type alternated copolymers DVB-maleic anhydride. The dimensions of the obtained particles have been determined by electron microscopy and are in the range 1-2 mu m. These particles have been mixed by stirring and ultrasound with nematic liquid crystals ZLI 1221 (Merck), in a concentration of 5% b.w. (polymer/LC).

The high exciton binding energy and band gap energy of ZnO thin films open the prospect of fabricating semiconductor lasers in the ultraviolet spectral range. A prerequisite for laser diode fabrication is highly p-doped ZnO which was not reproducibly obtained up to now. Without intentional doping ZnO exhibits n-type conduction. ZnO thin films have been obtained by radio-frequency assisted pulsed laser deposition. A metallic Zn target was used for ablation in an oxygen and nitrogen RF discharge. The electrical and morphological properties of the films grown on Si were studied by Atomic Force Microscopy (AFM), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), optical absorption and Hall Effect measurements for different ratios between the nitrogen and oxygen content. The AFM images of the as grown ZnO films reveal high quality surfaces with low values for the surface roughness and a sharp distribution of grains sizes as an effect of the RF discharge. The XRD patterns for all samples exhibit only (002) and (004) peaks indicating that the c-axis is always oriented normal to the substrate surface. The films present p-type conductivity with different carrier concentration and mobility depending on the nitrogen/oxygen ratio. (c) 2007 Elsevier Ltd. All rights reserved.

Synthesis of CaCO3 was performed in ultrasonic field in order to improve its properties. The evolution of the precipitation reaction in ultrasonic field provided a solid phase with aragonite morphology slightly impurified with vaterite. Particles dimensions were diminished, the solid phase presenting an important fraction of nanometer size particles.