Publications

Multifrequency electron paramagnetic resonance (EPR) and high resolution transmission electron microscopy (HRTEM) investigations were performed on small (2 nm) cubic ZnS nanocrystals (quantum dots-QDs) doped with 0.2% mol Mn2+, self-assembled into a mesoporous structure. The EPR data analysis shows that the substitutional Mn2+ ions are localized at Zn2+ sites subjected to a local axial lattice distortion, resulting in the observed zero-field-splitting parameter vertical bar D vertical bar = 41 x 10(-4) cm(-1). The local distortion is attributed to the presence in the second shell of ligands of a stacking fault or twin, which alters the normal stacking sequence of the cubic structure. The HRTEM results confirm the presence of such extended planar defects in a large percentage of the investigated QDs, which makes possible the proposed substitutional Mn2+ impurity ions localization model. Based on these results it is suggested that the high doping levels of Mn2+ ions observed in cubic ZnS and possible in other II-VI semiconductor QDs prepared at low temperatures can be explained by the assistance of the extended lattice defects in the impurities incorporation.

Reaction of the preformed cluster [Mn6O2(Piv)(10)(4-Me-py)(2.5)(PivH)(1.5)] with Nd(NO3)(3)center dot 6H(2)O, N-butyldiethanolamine (bdeaH(2)) and ferrocene-1,1'-dicarboxylic acid (fcdcH(2)) resulted in the formation of the first 3d-4f complex incorporating organometallic ferrocene [Mn4Nd4(OH)(4)(fcdc)(2)(Piv)(8)(bdea)(4)]center dot H2O; we report the X-ray structure and preliminary magnetic studies of this high-nuclearity cluster. (C) 2009 Elsevier Ltd. All rights reserved.

This work deals with the influence of neutron and proton induced cluster related defects on the properties of n-type silicon detectors. Defect concentrations were obtained by means of Deep Level Transient Spectroscopy (DLTS) and Thermally Stimulated Current (TSC) technique, while the full depletion voltage and the reverse current were extracted from capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The annealing behaviour of the reverse current can be correlated with the annealing of the cluster related defect levels labeled E4a and E4b by making use of their bistability. This bistability was characterised by isochronal and isothermal annealing studies and it was found that the development with increasing annealing temperature is similar to that of divacancies. This supports the assumption that E4a and E4b are vacancy related defects. In addition we observe an influence of the disordered regions on the shape and height of the DLTS or TSC signals corresponding to point defects like the vacancy-oxygen complex. (C) 2009 Elsevier B.V. All rights reserved.

We examined the electrical and optical properties of a new material containing polymer-clay nano-composite dispersed in a nematic liquid. The clay (Cloisite-type) was modified by copolymerization of maleic anhydride and divinyl benzene, using azobisisobutytironitrile as the initiator. The final polymer-clay nano-composite has an intercalated structure according to XRD patterns. We measured the thermally stimulated depolarization currents and determined the activation energies. Simultaneously we measured the optical transmission and we studied the influence of the previously applied polarizing electric fields.

Pt-ZnO-Pb(Zr0.2Ti0.8)O-3-Pt (PZT-ZnO) heterostructures were fabricated by using a sol-gel process. Capacitance-voltage measurements performed on a wide temperature range (20-450 K) have revealed the presence of a hysteresis that undergo a change of direction from clockwise at temperatures below 350 K to counter-clockwise at higher temperatures. In the first case, the hysteresis is produced by charge injection, similar to the case of classical metal-oxide-semiconductor capacitors. In the last case, the hysteresis is the fingerprint of polarization reversal, as reported for metal-ferroelectric-semiconductor (MFS) structures based on n-Si. The memory window at 450 K is about 6 V. This result suggests that PZT-ZnO MFS heterostructures can be used for memory devices working at elevated temperatures, in which the ZnO plays the role of the semiconductor.

We report on the vortex dynamics of a single crystal of the noncentrosymmetric heavy-fermion superconductor CePt3Si. Decays of the remnant magnetization display a logarithmic time dependence with rates that follow the temperature dependence expected from the Kim-Anderson theory. The creep rates are lower than observed in any other centrosymmetric superconductor and are not caused by high critical currents. On the contrary, the critical current in CePt3Si is considerably lower than in other superconductors with strong vortex pinning indicating that an alternative impediment on the flux-line motion might be at work in this superconductor.

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa2Cu3Ox (PrBCO) nano-dots and layers of YBa2Cu3O7-delta (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 rim, and total thickness between 1.13 and 5.31 mu m. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J(c) at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J(c) only in high fields. We have also investigated the frequency dependence of J(c) from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nanodots, both isotropic and c-axis correlated. (C) 2009 Elsevier B.V. All rights reserved.

YBa2Cu3O7/PrBa2Cu3O7 (YBCO/PBCO) superlattices with a different ratio of the superconducting and insulating layer thicknesses were prepared by high pressure dc sputtering. The vortex-creep activation energy U-o was determined by analyzing the in-plane resistive transition of 200 mu m, wide bridges with the external magnetic field B oriented along the c axis. It was found that U-o is proportional to the thickness of the YBCO layers, and does only weakly depend on the PBCO layer thickness, when the latter exceeds two unit cells. We observed a change in the variation of U-o with the current I in the specimen: U-o exhibits a plateau in the low-I region, then decreases significantly with increasing I. This behaviour is explained in terms of a crossover plastic vortex creep - elastic (collective) creep induced by the transport current. (C) 2009 Elsevier B.V. All rights reserved.

Intercalation, i.e. the insertion of guest species in a crystalline layered structure, is an efficient route for generating new materials with novel properties. Thin films and crystalline powder of BiI3 layered semiconductor were intercalated by exposure to ammonia vapors at room temperature. The intercalated compound was studied by thermo-gravimetric analysis, differential scanning calorimetry, X-ray diffraction, UV-vis optical absorption, FTIR spectroscopy and Raman scattering. After exposure of BiI3 to ammonia the formation of a new phase, BiI3(NH3)(3.83), was evidenced by thermal analysis. The intercalation process leads to a blue shift of the BiI3 Optical absorption edge by 0.5 eV. The appearance of new Raman lines at 135 and 353 cm(-1) in the Raman spectrum of intercalated BiI3 is considered as an evidence of the chemical interaction between the ammonia molecules and BiI3 lattice. (C) 2009 Elsevier Ltd. All rights reserved.