National Institute Of Materials Physics - Romania

Publications

873 articles found

811. 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).

812. 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.

813. 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.

814. 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)].

815. 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.

816. Silver nanoclusters in potassium halides obtained from Ag--ions by electron detachment

Authors: Enculescu, M; Topa, V; Vasile, E

Published: MAY 2002, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 191, 436, DOI: 10.1016/S0168-583X(02)00587-6

We studied by means of optical absorption and transmission electron microscopy the formation of Ag nanoclusters from Ag--ions in three different potassium halides (KCl, KI and KBr). By stepwise UV illumination at a suitable temperature, the electrons of Ag--ions were detached and the controlled formation and growth of Ag nanoclusters from Ago-atoms take place. At the same time, the detached electrons are captured by K+-ions, proved by the appearance of K-colloid bands. The kinetics of the conversion Ag- + hv + U --> Ag nanoclusters was followed by monitoring the Ag--ions and Ag nanoclusters absorption. The temperatures of illumination are different, depending on the matrix in which Ag nanoclusters are formed (220 degreesC for KCl, 160 degreesC for KBr and 140 degreesC for KI). By optical absorption and TEM we proved that the nanoclusters of Ag obtained by our method are separated, spherical and ovoidal, with average diameters between 7.5 and 50 nm. (C) 2002 Elsevier Science B.V. All rights reserved.

817. A model for structures growth by sodium electrodiffusion in quartz crystals

Authors: Enculescu, I; Iliescu, B; Enculescu, M; Covalcica, I

Published: 2002, CRYSTAL RESEARCH AND TECHNOLOGY, 37, 874, DOI: 10.1002/1521-4079(200208)37:8<868::AID-CRAT868>3.0.CO;2-J

Dendritic structures were grown inside the quartz crystal by electrodiffusion of sodium ions from a NaCl layer at the anode. The electric field was applied along the optical axis Z and the structures were grown in the xy plane. An experimental arrangement using a pointed cathode and a graphite plate anode was employed. X ray diffraction analysis shows that the structures' chemical composition is a mixture of sodium silicates. To explain the structures' growth a model is proposed. The model is a bidimensional one. The transport of the particles along the Z axis structural channels by the electric field was taken into account by introducing a probability of generating the particles in the growth plane as a function of the distance to the cathode. The computer generated results and the experimental ones were compared and a good correlation was found.

818. Fractal patterns formed by thermal treatment in alkali halide crystals

Authors: Enculescu, M; Enculescu, I; Topa, V; Vasile, E

Published: NOV-DEC 2002, PHYSICA B-CONDENSED MATTER, 324, 392, DOI: 10.1016/S0921-4526(02)01451-5

We have succeeded in growing nanocrystals of Ag structured like fractals in several solid matrices (NaG, KBr and KCl). These are shown to be diffusion-limited aggregates when fractal dimensions were evaluated. Depending upon the crystal in which the fractal structures were obtained, the fractal dimensions of the Ag nanoclusters were found to be between 1.73 and 1.81. (C) 2002 Elsevier Science B.V. All rights reserved.

819. Lead nanocrystals in KCl matrix

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

Published: MAR 2001, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 3, 140, DOI:

820. Non steady-state thermoelectric conduction

Authors: Apostol, M; Nedelcu, M

Published: MAR 2001, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 3, 132, DOI:

It is shown that the figure of merit of a thermoelectric couple can be appreciably improved by using a pulsed current transferred from the thermoelectric circuit. In the frame of this theory un unusual drop of temperature in a thermoelectric circuit is possible. An ultrafast process of thermoelectric conduction is described, whose increased performance is realized by minimizing the effects of thermal dissipation.