Publications

The electronic structure of polycrystalline C-60 thin films has been investigated using surface photovoltage spectroscopy (SPS) and conductivity measurements. The films show n-type semiconductivity with an activation energy of approximate to 0.8 eV as found from the temperature dependence of the conductivity at high temperatures. The electronic structure emerging from our SPS results comprises a 1.6 eV photoconduction gap, a mobility gap of about 2.25 eV and two gap states, a donor and an acceptor, at 0.35 and 0.8 eV, respectively, below the photoconduction edge. The proposed model positions the Fermi level between these two localised levels. Thus, the high-temperature conductivity is due to electrons excited across the photoconduction gap.

A new structure is proposed for the PbS based photoresistors. This structure consists from a thin film of PbS (about 1 mu m) chemically deposited on a dielectric layer (SiO2 or Si3N4 Of about 2000 Angstrom) grown, using different methods, on a single crystal Si wafer. The drain and the source contacts are the two gold electrodes evaporated on the surface of the PbS film. The resulting active area is of 6x6 mm(2) and is subjected to an incident light with the wavelength in the domain 0.8 - 3 mu m (at room temperature). The gate contact is the aluminum electrode evaporated on the back side of the Si substrate. Spectral distribution measurements were performed for different voltages applied on the gate electrode. It was found that the shape of the spectral distribution remains the same but the amplitude of the signal depends on the value and on the polarity of the applied voltage. An improvement of about 40 % was obtained for the PbS photoconductive signal compared with the situation when the gate electrode is in air. The new structure offer the possibility to improve the photoconductive signal generated by the PbS film. This improvement is due to a field effect.

Orientational properties of rubbed thin polymer layers with vinylic backbone and different side groups are presented. Some of these polymers lead to the alignment of LC molecules parallel to the rubbing direction whereas other polymers impose a perpendicular orientation. The stability of the obtained alignment as function of the temperature was tested.

The paper presents two original methods for obtaining electrical mins up to the transition point in quartz crystal: a) thermal treatment in a temperature gradient of 10-15 degree/cm at about 500 degrees C and b) applying an electric field of 1-4kV/cm at 500 degrees C. The twinning time dependence on the temperature. the dynamic of twin domain formation as a function of directions. thickness, temperature gradient, for +5X resonator plates are discussed. The influence of the Dauphine twins on the performances of piezoelectric resonators: resonant frequency, dynamic inductance and equivalent dynamic capacity was previously reported [1]. This paper presents the influence of twinning area on the frequency spectrum of a resonator plate of +5X-cut and the changing of the frequency-temperature characteristic when the resonator plate becomes twinned.

Temperature induced surface transitions in liquid crystal hybrid cells with asymmetrical anchoring, planar to one surface and tilted to the other, are experimentally studied. The results are theoretically explained by an elastic theory and the resulting values for the effective splay-bend elastic constant for a given liquid crystal, as well as the parameters characterising the anchoring on the boundary surface with tilted anchoring are compared with data from previous experiments, with good agreement.

The paper describes some results concerning technology and behaviour of X-and gamma-ray N+PP+ silicon defectors used in physics research, industrial and medical radiography and non-destructive testing. Devices manufactured under this technology proved to be stable alter an exposure in high intensity the dose range of 10 krad-5 Mrad. Nuclear radiation resistance was studied by irradiation with Co-60 gamma source (1.17, 1.33 MeV) at dose rates of 59 krad/hour and 570 krad/hour.

X-ray diffraction and transmission experiments as well as hardness measurements on virgin and ultra violet (UV)-irradiated amorphous films in the system GexAs40-xS60 have been performed. The W-light determines the plm softening for x19. The structure remains amorphous but the distance characteristic to medium range order (MRO) shows variations as a function of composition and irradiation time. The main effect of the UV light is the release of a significant amount of sulphur.

Samples consisting of Ni0.8Zn0.2Fe2O4 nanoparticles, both interconnected or dispersed in a matrix, were prepared by high energy ball milling, starting from bulk material previously synthesised by the conventional ceramic method at 1250 degrees C under O-2 flux. Samples with different grain size were performed. The as-milled samples were characterised by chemical analysis, X-ray diffraction and transmission electron microscopy (TEM). The nondispersed samples consist of grains with a mean size of 100 nm and a large size distribution. The dispersed powder in silica shows individual particles almost spherical with size ranging from 8 up to 50 nm. A good agreement was obtained between X-ray and TEM results. Because the as-milled powder showed after 200 hours of milling a high impurity degree, due to the stainless steel vial and balls, an original method was used to separate the phases. Magnetic measurements and Mossbauer spectra were performed.