National Institute Of Materials Physics - Romania

The paper presents the phototransport and photoluminescence investigations on nanocrystalline porous silicon. Spectral dependence curves of the phototransport and photoluminescence were taken at room temperature. A typical phototransport curve presents several maxima and shoulders, while the photoluminescence curves present only one maximum. The temperature dependence of the phototransport was measured in the 85 - 250 K range for different wavelengths. All these curves present only one activation energy on the whole temperature interval. The experimental results are interpreted taking into account a quantum confinement model. Previous microstructure investigations proved that the studied nanocrystalline porous silicon is formed by a nanowire network. Then, the electron Hamiltonian can be split into the sum of a 1D longitudinal Bloch part and a 2D transversal part, the nanowire wall acting like a potential well. Thus, a number of discrete energy levels will appear into the band gap. Both phototransport and photoluminescence maxima are due to the transitions between these levels, corresponding to a mean nanowire diameter of 3.2 nm, in good agreement with the previous microstructure and electrical transport investigations. On the contrary, the temperature dependence of the phototransport is determined by the surface states located on the internal surface and/or the interface between nanocrystallites.

We investigate nonstationary electronic transport in noninteracting nanostructures driven by a finite bias and time-dependent signals applied at their contacts to the leads. The systems are modeled by a tight-binding Hamiltonian, and the transient currents are computed from the nonequilibrium Green-Keldysh formalism. The numerical implementation is not restricted to weak coupling to the leads and does not imply the wideband limit assumption for the spectral width of the leads. As an application of the method we study in detail the transient behavior and the charge dynamics in single and double quantum dots connected to leads by a steplike potential, but the method allows as well consideration of nonperiodic potentials or short pulses. We show that when the higher-energy levels of the isolated system are located within the bias window of the leads, the transient current approaches the steady state in a nonoscillatory smooth fashion. At moderate coupling to the leads and fixed bias the transient acquires a steplike structure, the length of the steps increasing with system size. The number of levels inside a finite bias window can be tuned by a constant gate potential. We find also that the transient behavior depends on the specific way of coupling the leads to the mesoscopic system.

We have demonstrated an electron pump with in-plane gate (IPG) transistors fabricated by focused ion beam (FIB) implantation on GaAs-A1(x)Ga(1-x)As modulation-doped heterostructure with a two-dimensional electron gas (2DEG). The maskless fabrication process does not require any alignment between the circuit elements including the sources, the drains and the gates of the IPG transistors. Although, the device is operating at low frequencies (tens of kHz), it is capable to produce a current more than one order of magnitude higher than other electron pumps based on Coulomb blockade and single electron tunneling effects. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Comparative experiments regarding electrodeposition of zinc oxide either as films on Pt and Au substrates or as microwires grown in microporous polycarbonate membranes were carried out using nitrate aqueous solutions as electrolyte. Linear sweep voltammetry and electrochemical impedance spectroscopy together with SEM and XRD techniques were used for investigation. The equivalent electrical circuits for fitting experimental results were similar to those for characterisation of metal/polymer coating systems.

The electronic and optical properties of self-assembled InN/GaN quantum dots (QDs) are investigated by means of a tight-binding model combined with configuration-interaction calculations. Tight-binding single-particle wave functions are used as a basis for computing Coulomb and dipole matrix elements. Within this framework, we analyze multiexciton emission spectra for two different sizes of a lens-shaped InN/GaN QD with wurtzite crystal structure. The impact of the symmetry of the involved electron and hole one-particle states on the optical spectra is discussed in detail. Furthermore we show how the characteristic features of the spectra can be interpreted using a simplified Hamiltonian which provides analytical results for the interacting multiexciton complexes. We predict a vanishing exciton and biexciton ground-state emission for small lens-shaped InN/GaN QDs. For larger systems we report a bright ground-state emission but with drastically reduced oscillator strengths caused by the quantum confined Stark effect.

High density (more than 90% of the theoretical density) bulk samples with relatively large size (2.0 cm in diameter) of MgB2, pristine and doped with 5 mol% SiC and B4C were obtained by the field-assisted sintering technique (FAST) from commercial MgB2 powder. The superconducting properties in the magnetic field of the doped samples show depressed upper critical field H-c2 and irreversibility field H-irr when compared with pristine MgB2. The reason for better H-c2 and H-irr in the undoped sample is not clear. Most probable is that this is due to the limited diffusion time which restricts the substitution of carbon for boron in this short-time processing method as well as due to the way grain boundaries are formed in the FAST process. However, all our samples have shown higher values of H-c2 and H-irr than the values reported previously for FAST-MgB2 samples. The data were further investigated through scaling analysis. Data suggest that both additions of SiC and B4C stabilize two well defined pinning regimes in FAST-processed MgB2. At low temperatures, the pinning occurs at the grain border whereas at high temperatures, T >= 24 K, there is mixed pinning (H <= 5 T).

Current-voltage characteristics of samples containing silicon nanodots embedded in an amorphous silicon dioxide matrix were investigated. A percolation mechanism was evidenced by the dependence on the concentration of the initial differential conductance and by the appearance of several thresholds in the I-V characteristics.

Thin films of nonstoichiometric Heusler alloys Co2MnSbxSn1-x (x = 0.2; 0.4; 0.6; 0.8) have been grown by pulsed laser deposition (double-target/double beam configuration) on Si (10 0) substrates using a KrF excimer laser (lambda = 248 nm, tau = 20 ns). The substrate temperature was held at 300 K in all experiments to prevent interface interdiffusion of the species. A comparison between the compositions of films and corresponding targets has been done through energy dispersive X-ray spectroscopy (EDS) analysis showing a very satisfactory match. Scanning electron microscopy (SEM) imaging served to investigate the morphology of the films in order to determine the size and density of droplets which may influence the optical data. Optical conductivity derived from reflectivity measurements shows absorption onsets close to 1eV, which corresponds to the onset of valence-to-conduction transitions in the minority spin bands theoretically predicted. The values of the saturation magnetisation measured at 300 K on the quaternary alloys are very close to those of ternary ones for which either half-metallic properties or high spin polarisation were theoretically predicted. (c) 2007 Elsevier B.V. All rights reserved.

We report the synthesis of well crystallized stoichiometric Er:YAG thin films by pulsed laser deposition from Er:YAG targets in low pressure oxygen, followed by a post-deposition treatment at 1400 degrees C in environmental air for 19 h. The structure and morphology of the films were examined by transmission electron microscopy and selected area electron diffraction. We demonstrated that the subsequent application of pulsed laser deposition and heat treatment resulted in the formation of well crystallized single phase cubic Er:YAG nanostructured coatings. (c) 2007 Elsevier B.V. All rights reserved.

Barium strontium titanate (BST) bulk ceramic with composition Ba0.5Sr0.5TiO3 was produced by solid-state reaction and was used as target for PLD thin film deposition. A Nd-YAG laser, working at 5-10 Hz and different wavelengths has been used. Thin films of stoichiometric BST were deposited on alumina substrate with the thickness between 400 and 500 nm. An aditional annealing was carried out at 800 degrees C for 6 h. XRD and SEM were used for sample characterization. Capacity measurements were performed versus temperature at 100 kHz in a large temperature range. A diffuse ferroelectric-paraelectric phase transition with T-C = -72 degrees C for the BST thin film was determined. A 5% tunability was measured in the transition region. (c) 2007 Elsevier B.V. All rights reserved.