National Institute Of Materials Physics - Romania

This paper discusses the quantum confinement effects in nanometric structures that form low dimensional systems. In such systems, each surface/interface acts like a potential barrier, i.e. the wall of a quantum well, generating new energy levels. These levels are computed in a model that uses the approximation of the infinite rectangular quantum wells. The model is adapted for 2D, 1D and 0D systems, respectively. Different applications are discussed. The differences between the model results and the experimental data are proved to be of the same order of magnitude as the differences between the levels computed within the frame of infinite and finite quantum well approximations.

Optical and electrical behavior of Giant Magnetoresistivc (GMR)/Tunneling Magnetoresistive (TMR) multilayer thin films were controlled adjusting thermionic vacuum arc plasma parameters. Smooth, dense and adherent films were prepared using optimized plasma conditions. A new setup for simultaneous depositions of pure materials was made, having inside the vacuum chamber three simultaneous discharges. To obtain the discharge all of the tungsten filaments of the thermionic vacuum arc guns were heated by a 45-55 A current. The emitted electrons were focused on a tungsten coated carbon crucible anode, filled with Cu, and the others crucible anodes filed with MgO, Co and Ag, respectively. The anode applied voltages for Cu, MgO Co and Ag were up to 1500 V, 1700V 1800V and 820V respectively. An optimization of the geometrical parameters as the distance between the cathode and the anode, and of the external parameters as the discharge voltage and the heating current of tungsten current filament was performed. Structural and morphological properties of the prepared films were analyzed by scanning electron microscopy (SEM). The behavior of the obtained films was first analyzed in a Magneto-Optical Kerr Effect (MOKE) experiment. The electrical resistance behavior of the prepared films was determined in a magnetic field which values varied from 0.4 T to 0.4 T. The important changes in the values of the electrical resistance of the films were observed and correlated to the plasma conditions.

The wettability of 18 dental impression materials, commercially available, during their working time was investigated using the contact angle method, as a good wettability is desired to obtain a high fidelity impression. Polyetheric materials, A-silicones with added surfactants and C-silicones were found to have a good wettability. Alginate was found to be the most hydrophilic material, but this quality decreases in time. In the paper an explanation for the time dependence of the contact angle is advanced.

The two crystallographic modifications of AuSe, alpha-AuSe and beta-AuSe, were studied by Au-197 Mossbauer spectroscopy. Despite the different crystal structures, the Mossbauer spectra of the two phases are nearly the same, and reveal the presence of two gold sites with Mossbauer parameters typical for Au-I and Au-III. The Mossbauer-Lamb factor of the Au-I is less than half of that of the Au-III. Calculations using the WIEN2k program reproduce the experimental results quite well and also yield the signs of the electric field gradients.

Carbon nanotubes are attractive materials with numerous possibilities of applications in various domains. Nevertheless, pertinent characterization studies are required in order to insure reproducibility in preparation, handling and incorporation in devices. Several spectroscopic tools have been developed so far, such as transmission electron microscopy and near-field electrical microscopy. Optical techniques have also been improved with the use of Raman Scattering, Surface Enhanced Raman Scattering (SERS) and near-field optical spectroscopy (SNOM). These non-destructive techniques have been used for nanotubes after synthesis and exploited to follow the modifications of their spectroscopic features when they are functionalized or embedded in host matrices. In this paper, we show that, by using SERS conditions, a new effect is observed: a "single-beam pumped'' Coherent anti-Stokes Raman Scattering (CARS). We demonstrate that under a tight-focusing of the excitation light, this emission, resulting from a wave mixing process between the incident laser light (omega(l)) and Stokes Raman light (omega(s)), is generated by a Surface Enhanced Raman Scattering (SERS) mechanism. Since abnormal anti-Stokes/Stokes intensity ratios are also observed for carbon nanotubes in powders, we have investigated in details their behaviour as a function of several parameters which include the excitation wavelengths, the laser power, the sample support, as well as the presence of nanotubes in isolated or in bundled form. It appears that resonance phenomena explain the anti-Stokes/Stokes intensity ratios in the case of powders and that CARS is only observed when SERS conditions are used. In addition, we have investigated other materials in which anomalies are also observed. This is the case of poly(bithiophene) (PBTh) electrochemically polymerized on carbon nanotubes for which its main Raman line at 1450 cm(-1) is enhanced, this enhancement being presumably generated by the plasmon excitation of metallic tubes. In the case of PEDOT, the analysis of the anti-Stokes branch provides additional information on the functionalization of carbon nanotubes with this polymer.

Different phases of boron nitride (BN) nano-structures are synthesized from an hBN ceramic target immersed in acetone, by ablation with a high power pulsed Nd: YAG laser. Transmission electron microscopy (TEM) and electron diffraction (ED) are used to identify the morphology and structure of the prepared colloidal suspensions. It is revealed that by varying solely a single experimental parameter, i.e. the laser pulse fluency, a large variety of BN nanostructures can be produced: nanotubes, very thin graphene-like foils, nano-curls and nanoparticles, all with the hexagonal graphite-like hBN structure, as well as high pressure BN phases: orthorhombic explosion E-BN nano-rods, or cubic diamond-like cBN nano-particles.

Calcium phosphate ceramic powders were synthesized via sol-gel method. The powders were sintered at 600 and 1000 degrees C and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron microscopy (SEM and TEM), elemental microanalysis (EDS), infrared spectroscopy (FTIR), and thermal analysis (TGA and DTA). The XRD analysis revealed a well crystallized hydroxyapatite (HAp) structure at all temperatures. At 1000 degrees C a small amount of CaO (about 1.2 %) was detected. The average crystallite size increased from 19 nm at 80 degrees C to about 125 nm at 1000 degrees C. FTIR spectra showed the presence of various PO43- and OH- groups present in the powders. In order to verify the biocompatibility of HAP powders with hFOB 1.19 osteoblasts cells, an 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test was applied. It was noticed that the ceramics obtained at 1000 degrees C, increased the cell viability with 10%, 40% and 55% after 6, 12 respectively 24 h, whereas that obtained at 600 degrees C was less effective. Our results proved the high biocompatibility of calcium phosphate ceramic powders obtained by sol-gel and sintered at 1000 degrees C.

Tin chalcogenides SnX2 and SnX, where X = S, Se and Te present a particularly interest for their electronic properties and applications in gas sensors. The state of tin in these materials is important for understanding of the sensing effect and improvement of the sensor performances. Mossbauer spectroscopy is a widely used technique for the analysis of the local electronic structure or chemical bonding in solids. In this paper we applied Mossbauer technique for the investigation of bulk and thin films of SnSe2 chalcogenide. The films of SnSe2 chalcogenide were obtained by the methods: PLD ("Pulsed laser deposition") and PED ("Pulsed electron deposition"). Mossbauer measurements were performed by transmission (TMS), respectively conversion electron spectroscopy (CEMS). By CEMS spectroscopy surfaces, coatings and thin films containing Sn can be studied on substrates and to various depths up to 1000 nanometers.

The precipitation of calcium carbonate by liquid-liquid reaction usually provides a mixture of polymorphs: rhombohedra calcite, orthorhombic aragonite with whiskers like particles and hexagonal vaterite with crystallites agglomerated as spheres. Calcite is the stable polymorph at ambient conditions. Recently the interest for aragonite, the high pressure polymorph is increasing because new fields, like biomaterials, are developing. We investigated the possibility of developing an aragonite type calcium carbonate with uniform particle size and submicron dimensions by precipitation in liquid-liquid reaction using double-jet precipitation method in the presence of ethanol, at the temperature of 50 degrees C, with a flow rate of the reagents of 10 ml/min. Two agitation modes were used: magnetic stirrer and an ultrasonic device. The ultrasonic field was applied either at the beginning of the reaction period or at the end of it. Intensity of the ultrasounds was also varied from 50 % to 100 %. All samples were examined by FTIR spectroscopy, scanning electron microscopy, and particle size analyzer. FTIR spectra proved that calcium carbonate synthesized in these conditions is almost pure aragonite. As example, the SEM image shows uniform product structure with fine rods with 2 divided by 3 mu m lengths and submicron diameters.

We present the fabrication and electric properties of MgB2 ceramic samples doped with nanosized spheres, 4-8 nm, of graphite with a metallic core. The samples were prepared using the spark plasma sintering technique. The size of the additive is comparable to the superconducting coherence length. The short processing time limits the diffusion of the carbon while keeping the core intact. Therefore, in addition to the doping with carbon, the metallic core, which has the size smaller than the superconducting coherence length, create pinning centers which might improve the dissipationless electric transport. The results are analyzed in the framework of different pinning models.