National Institute Of Materials Physics - Romania

A detailed description of the tunneling processes, within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Buttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot.

Arrays of US: Mn2+ : Cu+ micro- and nanowires grown in polycarbonate ion-track templates exhibit photoluminescence in the spectral domain ranging from 500 to 800 mn at room temperature. A comparison with similar US and CdS:Mn2+ wire arrays is presented. The individual contributions to the emission spectra of Cu+ and Mn2+ ions in the US matrix are explained using their energy level schemes. Also SEM, EDX and EPR data are given or these wires. (c) 2005 WILEY-VCH Verlag GmbH A Co. KGaA, Weinheim.

The behavior of two semi-interpenetrating polymer networks (S-IPNs) based on polyurethane (PU) and epoxy maleate of bisphenol A (EMBA) under exposure to UV radiation for long period of times (up to 200 h) was investigated. The networks obtained by the specific mixture under certain thermal treatment included in their structures crosslinked EMBA and linear PU. Advanced photooxidative degradation took place for long time of UV irradiation (over 160 h, especially), with almost a total destruction of the raw structures. The important modification of both the surface and of the morphology of the tested samples with the time of irradiation was also analyzed. Simultaneously, as compared to results obtained for short time of UV radiations (up to 10 h), where the mechanical properties of the tested samples improved, long time of UV radiations led to deterioration of many of the mechanical properties. (C) 2004 Elsevier B.V. All rights reserved.

Surface-enhanced Raman scattering (SERS) was used to investigate C-60 self-assembling in solvents like pyrrolidine (Py) and Nmethyl-2-pyrrolidinone (NMP) as well as in binary mixtures of o-dichlorobenzene (DCB)/acetonitrile (ACN) and DCB/NMP. For a correct evaluation of the modifications of Raman spectra induced by the C-60 aggregation, we have also presented the variations due to the measuring method, i.e., the signal dependence of the metallic support type and the surface roughness. The interaction between C-60 and the Au substrate, appearing as a chemical component in SERS generation, is mainly evidenced by a band at similar to342cm(-1). In the aggregated phase, the intermolecular interactions lead to a reduction in the parent I-h C-60 symmetry as observed by a modified phonon spectrum. As a general feature, the spectral range below 800cm(-1) is the most diagnostic for the aggregate assignment, the main indicative being the disappearance of the Raman bands associated to the radial vibration modes. SERS measurements have revealed two stages in the self-assembling of C-60 in NMP. In the beginning, charge-transfer molecular complexes that associate slowly in stable aggregates are formed by the binding of an NMP molecule to the C-60 cage. These complexes are noticed in the SERS spectrum by the replacement of the original H-g(l) band at similar to269cm(-1) with two others at similar to255 and similar to246cm(-1). In the aggregated phase, when using NMP and P as a solvent, the Raman spectrum reveals new bands that appear around 94 and 110-118cm(-1), which are associated with the interball interactions. In a DCB/ACN solvent mixture, the self-assembling process is driven by weak van der Waals type forces and resembles a precipitation, yielding C60 clusters of different size. (C) 2004 Published by Elsevier Ltd.

Manganese and copper doped CdS nanowires were obtained by the template technique in a two step process. In the first step metallic alloy CdMnCu nano and microwires were electrodeposited in ion track membranes with pore diameters in the range 150 nm-2 mum. In the following step the resulted nanowires were anodized in a sodium sulphide alkaline solution. Electron microscopy results show participation of the whole metallic wire in the anodization process; manganese and copper doped CdS wires are obtained The composition of micro and nanowires was determined by energy dispersive X-ray analysis.

A binary telluride (CdTe) as well as a ternary telluride (PbSeTe) were electrodeposited in the pores of a track-etched polycarbonate membrane, resulting nanowires and nanotubes. Either acid or alkaline solutions were used for preparing CdTe compound, whereas the ternary PbSe1-xTex compound was deposited from an acid solution, only. The investigation of electrode mechanisms was performed by cyclic voltammetry. The nanostructures were visualized using scanning electron microscopy (SEM).

Polycarbonate porous membranes were prepared by etching the ion tracks contained in foils irradiated in the fluence range of 1 ion/sample to 10(8) ions/cm(2). The membranes were used as templates for the preparation of tubes employing auto-catalytic copper deposition. A three-step process was used: pre-activation, activation and copper deposition. The deposition bath employed formaldehyde as reducing agent and tartrate as complexing agent. Copper nucleation was studied in order to optimize the activation steps thus allowing the preparation of thin tubes. The external diameters of the tubes ranged from 300 to 2000 nm. The inner diameters varied as a function of the time-of-exposure to the electroless bath. The length of the tubes was 30 mum, corresponding to the thickness of the foils. (C) 2004 Elsevier B.V. All rights reserved.

New magnetic composite materials, ribbon samples, with nominal compositions FexCu1-x (x=0.2; 0.5; 0.8) were obtained by the melt spinning technique. Various information was provided via X-ray diffraction, Mossbauer and magnetic measurements. The structural aspects: phase composition, size and texture effects, have been corroborated with magnetic properties. Finally, the possibility to choose the adequate composition and the suitable processing in order to obtain desired anisotropic magnetic materials were presented.

We study the transport properties of coherently coupled quantum dots in the quantum Hall regime within the Landauer-Buttiker formalism which captures and explains the experimentally observed features in terms of the spectral properties of the coupled dot system. The subpeak structure of the transmittance spectrum and the charging stability diagrams are obtained and discussed. The role of the intradot and interdot Coulomb interaction are pointed out. We show the subpeak evolution with the magnetic field and predict a specific oscillatory behavior of the Hall resistance in strong magnetic field which can be experimentally tested.

We review some aspects concerning the disorder effects on the spectral properties of 2D electronic systems with relevance for the transport and orbital magnetism in perpendicular magnetic field. The lattice model and random Anderson disorder are used. The discussion is carried out separately for periodic and hard wall boundary conditions. The degree of localization is evidenced by the calculation of the inverse participation number of the eigenstates of the Hamiltonian and a striking asymmetry of the localization effect of states in the Landau band is reported. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.