National Institute Of Materials Physics - Romania

Electron spin resonance and DC electrical investigations on the effect of chlorine doping on emeraldine base polyaniline are reported. From the experimental data it is concluded that the chlorine doping of polyanilines increases the conductivity and that the charge transport is well described by an one dimensional variable range hopping, up to a doping level of 15% Cl/N. The characteristic temperature rises as the doping level is increased. Electron spin resonance investigations revealed the contribution of polarons to the charge transport. The absence of high spin bipolarons was proved by electron spin resonance data within the experimental errors. At high doping levels, the mesoscopic nature of charge transport in polyanilines is confirmed by the Dysonian shape of the resonance lines.

Electrical conduction mechanisms in Si-SiO2 nanocomposite films were experimentally investigated and theoretically modelled. The films with silicon volume concentration varying from 0% to 100% were deposited on rectangular quartz substrates. They were prepared by co-sputtering CVD from two targets, one of silicon and one of silicon dioxide. Their middle part is formed by Si nanodots embedded in SiO2. 50 parallel aluminium electrodes were deposited, forming 49 coplanar samples, centred around x = 45% Si concentration. The microstructure investigations proved that the silicon nanodots have diameters between 4 nm (x = 23%) and 36 nm (x = 81%). The current-voltage characteristics were measured at room temperature, in the -25 divided by +25 V interval. The obtained curves are symmetric and superlinear. As the current - voltage characteristic is determined by the maximum resistance met by the carriers in their path, the experimental curve is modelled by the field-assisted tunnelling under Coulomb blockade. The used parameters are correlated with previous results obtained on nanocrystalline porous silicon. The correlation coefficient between the theoretical formula and the experimental data is greater than 99.95%.

Thallium related paramagnetic centers of Tl2+ (6s(1)) and Tl-0 (6p(1)) type were produced by low temperature x-ray irradiation in ferroelectric Rb2ZnCl4:Tl single crystals. Extensive EPR investigations have been performed in order to elucidate their intrinsic properties and sensitivity as paramagnetic probes for structural phase transitions studies. Compared to the s-type Tl2+ centers, already used in many such studies, it was found that the p-type Tl-0 centers are much more sensitive to the small variations in the local crystal field associated with these transformations. Their EPR spectra provided information about the unit cell tripling in the P2(1)cn phase, as well as the symmetry lowering and lattice dynamics in the C1c1 phase. The temperature induced, continuous changes observed in the EPR spectra of both Tl-0 and Tl2+ centers were explained by the influence of the soft modes responsible for the 74-K structural phase transition.

Structural and electrical properties of polycrystalline CdSe thin films irradiated with high-energy electrons are analyzed. The samples were prepared by vacuum deposition onto optical glass substrate. Their structure and the temperature dependence of the electrical resistance were determined, both before and after irradiation with 6 MeV electrons and fluencies up to 10(16) electronS/cm(2). There were no measurable changes in the crystalline structure of the films after irradiation. Electrical properties are controlled by a defect level of donor type, possibly a selenium vacancy, with two charge states having ionization energies of about 0.40 eV, respectively 0.22 eV. Irradiation increases significantly the concentration of those defects.

In a planar oriented sample of a calamitic nematic lyotropic system ( mixture of sodium lauryl sulphate/water/decanol), isotropic pretransitional domains appear at the nematic to isotropic transition. The domains are oblong in shape with the long axis along the orientational direction. We show experimental evidence that this oblong shape is determined by the nematic-isotropic interfacial tension anisotropy. Two uniparametric models of simple angular dependences for the interfacial tension are tested. Using the differential system obtained from the Young-Laplace condition at the nematic-isotropic interface, the domain shape can be numerically calculated for each value of the interfacial tension anisotropy. By processing the values of the transmitted light through both an isolated isotropic domain and its surrounding nematic zone, we obtain the anisotropy of the interfacial tension as the main fitting parameter. An estimation of the ratio of the extreme values for the interfacial tension is given.

(Cu, C)Ba2Ca3Cu4Ox superconductor with addition of y mol LiF has been synthesized by a high-pressure method. For the same synthesis conditions it was found that (almost) single-phase Cu, C-1234 samples can be synthesized for y(LiF) = 0-0.1 if the amount of z mol AgO oxidizer is increased linearly from Z(AgO) = 0.45 to 0.73 and for y(LiF) = 0 - 1-0.2 if ZAgO = 0.73 = constant. Transport measurements (rho(T) and room-temperature Seebeck coefficient) have shown that these samples are overdoped: LiF is an effective addition for synthesis of overdoped Cu, C-1234 with a controlled level of carriers. LiF addition continuously decreases T-c. The critical point at y(LiF) = 0.1 is discussed as the solubility limit of LiF and/or the point where the doping mechanism changes. It is proposed that the reason is the reaction of extra Li with C and O to form Li2CO3, inducing a lower concentration of C in Cu, C-1234/LiF crystals, and at the same time a possible substitution of Li not only for the Cu site but also for the Ca site, resulting in formation of a higher amount of residual Ca0.828CuO2 (for y(LiF) > 0.1). LiF induces the formation of a liquid phase and acts as a flux promoting the formation of Cu, C-12(n - 1)n with n greater than or equal to 4. LiF modifies to some degree the grain growth from a 3D to a 2D type (thinner platelike grains have been observed in the LiF added samples).

The principal of a semiconductor particle detector is briefly outlined and applications in several fields of fundamental and applied physics mentioned. Starting from the basic physics motivation in High Energy Physics (HEP) research, the further discussion then focuses on the use of these devices in present and future HEP experiments outlining especially the requirements for precise measurements of elementary particle tracks, a task ideally fulfilled by segmented silicon detectors. These developments have also initiated further applications in different fields as e.g. material analysis, medical imaging and space missions. In future HEP-experiments such devices will however face an unprecedented challenge imposed by extremely intense radiation fields. Basics of hadron and lepton induced radiation damage in silicon are described together with their implications on the detector quality. The following part of the report is then dealing with several approaches to improve the radiation tolerance of silicon detectors. Defect engineering of the semiconductor material, modified detector geometries and specific operational conditions are outlined. First results toward a complete understanding of detector degradation as caused by point and cluster defects are given and these promising successes may finally pave the way for developments of superior devices.

The complex structure fullerene(C-60)-nanotubule has been built from special plastic units and relaxed by a computer Monte-Carlo - Metropolis procedure. The purpose was to show that interconnection fullerene-nanotubule is possible from the energetical point of view. The results open the way towards the assessing the possibility to create nano-conductors and connections between different parts of nano-devices, using nano-carbon configurations filled by metal atoms.

CdTe nanowires were electrodeposited in ion track membranes with pore diameters in the range 100-2000 nm. Acidic and basic baths were tested for obtaining materials with good stoichiometry. The cyclic voltammograms were compared for the case of deposition on a carbon rotating disc and on the porous membrane substrates, the differences being attributed to the additional resistance induced by the pores. For larger pores a tendency of growing hollow structures was observed.

Samples of FC3-xMnxO4 (x=0-0.6) were studied below the Verwey transition (T=122, 77, and 4.2 K) by Fe-57 Mossbauer spectroscopy. Considering that Mn2+ substitutes for Fe2+ on octahedral sites, we inferred the existence of five subspectra at all temperatures, one for Fe3+ ions on tetrahedral sites, two for Fe3+ ions on octahedral sites, and two for Fe2+ ions on octahedral sites. The hyperfine magnetic fields of one Fe3+ sublattice and two Fe2+ sublattices decrease slightly with increasing the amount x of Mn substitutions. The results are consistent with the existence of localized octahedral Fe3+ and Fe2+ spin states below the Verwey temperature. (C) 2003 Elsevier B.V. All rights reserved.