National Institute Of Materials Physics - Romania

Publications

918 articles found

851. C-60 aggregates in pyrrolidine and N-methyl-2-pyrrolidinone evidenced by surface-enhanced Raman scaterring spectra

Authors: Baibarac, M; Preda, N; Mihut, L; Velula, T; Baltog, I; Mevellec, JY; Lefrant, S

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 417, 587, DOI: 10.1080/15421400490478434

At room temperature, soluted C-60 in pyrrolidine and N-methyl-2-pyrrolidinone associates slowly in stable aggregates. The process originates in the charge transfer from the solvent molecules as electron donors towards fullerene. In the aggregate phase the intermolecular interactions lead to a large reduction in parent I-h C-60 symmetry such that a modified phonon spectrum is observed. Using the surface-enhanced Raman scattering we demonstrate that the Raman range below 800 cm(-1) is most diagnostic for aggregate assignment. In particular, the strong decrease of the radial breathing mode and appearance of new bands at 110 and 93 cm(-1) due to the interball interactions must be noticed.

852. Photoluminescence studies on [C60] fullerene solutions in N-methyl-2-pyrrolidinone and pyrrolidine

Authors: Mihut, L; Preda, N; Baibarac, M; Baltog, I; Lefrant, S; Wery, J

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 415, 21, DOI: 10.1080/15421400490478209

Photoluminescence (PL) spectra of C-60 solutions in N-methyl-2-pyrrolidinone (NMP) show a drastic increase of the PL intensity if the solutions are stored longer periods of time. Two distinct spectral components, one in the red (1.92-2.00 eV) and another in the green (2.18-2.3 eV) were identified in the whole PL emission. Their weights depend on C-60 concentration and on storage time. The two emissions come from two successive processes running with different kinetics. Charge transfer, leading to (C-60, NMP) loose molecular complexes and a slow aggregation process of these entities seems to originate the red and green emission, respectively. Similar PL properties are found when pyrrolidine (P) is used as solvent.

853. Electrochemical deposition of PBSe1-xTex nanorod arrays using ion track etched membranes as template

Authors: Sima, M; Enculescu, I; Visan, T; Spohr, R; Trautmann, C

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 418, 755, DOI: 10.1080/15421400490478885

Lead chalcogenides (PbS, PbSe, PbTe) are narrow band gap semiconductors which have been studied in the field of IR detection and thermoelectric devices. The template method is a general approach for synthesizing nanomaterials within the pores of membranes. The membranes employed contain cylindrical pores with monodisperse diameters, and corresponding cylindrical nanostructures are obtained. The aim of the present study was to prepare PbSe1-xTex nanorod arrays using electrodeposition. The process was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The resulted PbSe1-xTex nanostructures were characterized using scanning electron microscopy (SEM) as well as energy dispersive X-ray analysis (EDAX).

854. Emission properties of T-a-centres in KCl

Authors: Topa, V; Tsuboi, T; Polosan, S

Published: AUG-DEC 2004, RADIATION MEASUREMENTS, 38, 734, DOI: 10.1016/j.radmeas.2004.01.021

Electrolytic colouration at about 600degreesC and 350 V/cm, for KCl crystal containing Pb2+ and for KCl:Pb2+ crystals co-doped with Li+, Na+ and Rb+ has been undertaken. Several absorption bands were observed in both doped and co-doped crystals in the visible-UV region. Excitation into these bands gives rise to the same 0.86 eV emission band except for the Li+-co-doped crystal which gives rise to a 0.80 eV emission band. These absorption bands are due to the same T-a-centre related to Pb-. The observed infrared emission intensity of the crystal with F-centres is higher than of without F-centres. (C) 2004 Elsevier Ltd. All rights reserved.

855. Electronic transport in nanosystems

Authors: Wulf, U; Racec, ER; Racec, PN; Aldea, A

Published: DEC 15 2003, MATERIALS SCIENCE & ENGINEERING C-BIOMIMETIC AND SUPRAMOLECULAR SYSTEMS, 23, 681, DOI: 10.1016/j.msec.2003.09.080

We demonstrate the application of the Landauer-Buttiker formalism (LBF) to MIS (metal-insulator-semiconductor)-type quantum nanostructures and analyze their static and dynamical response properties. For the time-dependent problem, we employ the random phase approximation (RPA) and calculate the irreducible polarization from the one-particle scattering wave functions in the Hartree approximation. For their calculation, we employ the R-matrix approach which we have developed over the past years. We find in the low frequency limit a reasonable agreement with the static Hartree calculations and experimental capacitance data. (C) 2003 Published by Elsevier B.V.

856. Copper nanowires electrodeposited in etched single-ion track templates

Authors: Enculescu, I; Siwy, Z; Dobrev, D; Trautmann, C; Toimil-Molares, ME; Neumann, R; Hjort, K; Westerberg, L; Spohr, R

Published: NOV 2003, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 77, 755, DOI: 10.1007/s00339-003-2216-3

The replication of single-ion track templates opens up the possibility of accessing electrical properties of nanowires without using lithographic techniques. Polycarbonate foils of 30-mum thickness were irradiated with single swift heavy ions (e.g. An 11.4 or Xe 8.3 MeV/nucleon). By controlled one-sided etching of the damage trail caused by the ion, templates containing a single conical pore were prepared. The narrow pore openings had diameters down to 25 nm and opening angles up to 2degrees. By electrochemical deposition of copper, single conical wires were obtained. The electric current recorded during electrodeposition reflects the geometry of the pore. The wires were provided with electrical contacts. Current-voltage measurements confirmed that their resistance was ohmic. The wires could withstand a maximum current density above 10(8) A/cm(2).

857. Isolated nanocrystals of indium in KCl crystals

Authors: Polosan, S; Apostol, E; Vasile, E; Topa, V

Published: SEP 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 704, DOI:

Metal nanoparticles embedded in KCl crystals exhibit different forms and distributions, depending on the type of metal. The nanoparticles can be efficiently studied by optical absorption on these structures, calculated from Mie theory and comparison with Transmission Electron Mmicroscopy (TEM) measurements. The TEM images provide the detailed characteristics of the structures under study. Metal crystalline particle obtained by thermal treatments of negative indiurn ions are spherical. The particle size distribution was determined, the standard deviation was found 1.46, and the main statistical characteristics were calculated.

858. Magnetic properties of gamma-Fe2O3 nanoparticles

Authors: Predoi, D; Kuncser, V; Nogues, M; Tronc, E; Jolive, JP; Filoti, G; Schinteie, G

Published: MAR 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 216, DOI:

The gamma-Fe2O3 nanoparticles with different surface states are investigated by X-ray Diffraction, Electron Microscopy, Differential Thermal Analysis and magnetic: measurements in low applied field. The influence of the adsorbed species (H2O and sulphate complexes) on the inter-particle magnetic interactions is analysed in relation to both the magnetic states of the surface and the inter-particle distance.

859. Orbital magnetization of single and double quantum dots in a tight-binding model

Authors: Aldea, A; Moldoveanu, V; Nita, M; Manolescu, A; Gudmundsson, V; Tanatar, B

Published: JAN 15 2003, PHYSICAL REVIEW B, 67, DOI: 10.1103/PhysRevB.67.035324

We calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb interaction and by electron tunneling. The electronic states of the quantum dots are calculated in a tight-binding model, and the magnetization is discussed in relation to the energy spectrum and to the edge and bulk states. We identify effects of chirality of the electronic orbits and of the anticrossing of the energy levels when the magnetic field is varied. We also consider the effects of detuning the energy spectra of the quantum dots by an external gate potential. We compare our results with the recent experiments of Oosterkamp [Phys. Rev. Lett. 80, 4951 (1998)].

860. Negatively charged Pb- ion produced by electrolytical colouration of KCl crystals containing Li+, Na+ and Rb+ ions

Authors: Tsuboi, T; Polosan, S; Topa, V

Published: AUG 5 2002, JOURNAL OF PHYSICS-CONDENSED MATTER, 14, 7272, DOI: 10.1088/0953-8984/14/30/315

Electrolytic colouration has been undertaken for not only a KCl crystal containing Pb2+ ions but also KCl crystals containing alkali impurity ions (Li+, Na+ and Rb+) besides Pb2+ ions. Absorption spectra of these electrolytically coloured crystals have been studied, together with the emission spectra. Similar spectra were observed among these crystals. Eight weak absorption bands are observed in the 290-700 nm region, while five intense bands are observed in the 200-290 nm region. It is suggested-that these bands arise from Pb- ions with tetragonal crystal field symmetry. The energy-level assignment for these absorption bands is made using the energy-level diagram of the isoelectronic Bi-0 atom with C-4v crystal field symmetry. It is suggested that an off-centre position effect of the Li+ ion is observed in the kCl crystal codoped with Li+ ions.. Broad emission bands at 1530 and 1580 nm have been. observed in electrolytically coloured KCl containing Pb2+ ions and KCl containing Li+ and Pb2+ ions, irrespectively.