National Institute Of Materials Physics - Romania

We report on structure-property relationships in Pr-doped CeO2 and ZrO2 using X-ray diffraction (XRD), Raman, UV to Vis Diffuse Reflectance (DR-UV/Vis), X-ray Photoelectron (XPS), and luminescence (PL) spectroscopies. Both 3+ and 4+ valence states of Pr are evidenced, irrespective of the host and calcination temperature, T (T = 500 and 1000 degrees C) with consequences on absorption, surface, vibrational and luminescence properties. Only zirconia represents a suitable host for Pr3+ luminescence. The distinct trivalent Pr centers and their excitation mechanism are identified in relation to the tetragonal and monoclinic phases of ZrO2. A near-infrared to visible up-conversion (UPC) emission of Pr3+ is observed upon excitation at 959 nm which occurs, most probably, via a two-photon excited state process. By using a multi-wavelength, time-gated excitation, the UPC process is established as phase selective, i.e. only Pr3+ located in the monoclinic sites of the mixed phase, monoclinic and tetragonal ZrO2 (T = 1000 degrees C) contribute to the UPC emission. We believe that, besides the local symmetry, a key role in phase selective UPC is played by the presence of Pr3+ low-lying 4f 5d levels. To the best of our knowledge, this is the first report of phase selective up-conversion emission in a lanthanide doped multi-phase host.

The aim of this study was to synthetize new porous nanoparticles based on methyltrimethoxysilane coated hydroxyapatite (MTHAp) for lead removal form aqueous solutions. The morphological and compositional analysis of MTHAp were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). Removal experiments of Pb2+ ions were carried out in aqueous solutions with controlled concentration of Pb2+ at a fixed pH value of 3 and 5 respectively. After the removal experiment of Pb2+ ions from solutions, porous hydroxyapatite nanoparticles were transformed into PbMTHAp_3 and PbMTHAp_5 via the adsorption of Pb2+ ions followed by a cation exchange reaction. The X-ray diffraction spectra of PbMTHAp_3 and PbMTHAp_5 revealed that the powders, after removal of the Pb2+ ions, were a mixture of Ca2.5Pb7.5(PO4)(6) (OH) 2, Pb2Ca4(PO4)(2)(SiO4), and Ca-10(PO4)(6)(OH)(2). Our results demonstrate that the porous hydroxyapatite nanoparticles can be used as an adsorbent for removing Pb2+ ions from aqueous solutions.

Vortex pinning on natural and artificial defects is essential for large scale applications of superconducting materials. One of the most promising solutions for the creation of efficient pinning structures is to combine the strong pinning supplied by columnar defects (with the radius of the order of the superconducting coherence length) and the presence of random quenched disorder, which inhibits the detrimental vortex kink formation. A strong pinning is revealed by high values of the vortex activation energy in the magnetic relaxation process. We present a critical analysis of the interpretation of the relaxation data at long- and short time scales, by extracting the so called normalized vortex-creep activation energy. This allowed us to find the actual temperature interval for the characteristic vortex excitations in YBa2Cu3O7 films with embedded BaZrO3 nanorods 9preferentially oriented along the c axis), and to unambiguously determine the characteristic vortex pinning energy. The observed drastic change of magnetic relaxation at short time scales (attained in standard AC measurements) is attributed to a large contribution of the pinning enhanced viscosity to the vortex hopping activation energy.

Cladding waveguides have been realized in Nd:YAG by direct writing with a femtosecond-laser beam. A classical method that inscribes many tracks around the waveguide circumference with step-by-step translations of the laser medium, and a new technique in which the laser medium is moved on a helical trajectory and that delivers waveguides with well-defined walls were employed. Laser emission on the 1.06 mu m F-4(3/2)-> I-4(11/2) transition and at 1.3 mu m on the F-4(3/2)-> I-4(13/2) line was obtained under the pump with a fiber-coupled diode laser. Thus, laser pulses at 1.06 mu m with energy of 1.3 mJ for the pump at 807 nm with pulses of 12.5-mJ energy were recorded from a circular waveguide of 100-mu m diameter that was inscribed in a 5-mm long, 0.7-at.% Nd: YAG single crystal by the classical translation technique. A similar waveguide that was realized in a 5-mm long, 1.1-at.% Nd: YAG ceramic increased the 1.06-mu m laser pulse energy to 2.15 mJ for the pump pulses of 13.1-mJ energy. Furthermore, a circular waveguide of 100-mu m diameter that was inscribed in the Nd: YAG ceramic by the helical-movement method yielded pulses at 1.06 mu m with increased maximum energy of 3.2 mJ; the overall optical-to-optical efficiency was 0.24, and the laser operated with a slope efficiency of 0.29. The same device outputted laser pulses at 1.3 mu m with energy of 1.15 mJ.

Self-organization and size effects in amorphous silicon have been investigated by modelling of the structure at nanoscale. The size effect related to the disorder in silicon is treated by the free energy balance in nanometric clusters using valence force field theory. The computed structural and energetical parameters of three continuous random network (CRN) models of amorphous silicon with 2,052, 156 and 155 atoms are compared with the experimental values. In order to show the importance of the interfaces between different a-Si clusters, two networks of 200 and 205 atoms were modelled separately and then linked using an amorphous and a crystalline interface. Also the voids in the a-Si clusters are investigated.

The objective of this study was to obtain nanocomposites based on SWCNTs functionalized with carboxyl groups and doxorubicin (DOX) as a chemotherapeutic drug through covalent bonds formed by carboxyl groups from SWCNTs and amino groups from DOX. The formation of these nanocomposites was proved by using different characterization methods like Fourier transform Infrared spectroscopy and X-ray photoelectron spectroscopy (XPS). Also thermogravimetrical analysis was employed to study the thermal behavior of our nanocomposites. X-ray diffraction and Raman spectroscopy revealed that the surface was modified by the covalent bonding of DOX to SWCNTs. The in vitro drug release was studied by using UV-VIS Spectroscopy.

Luminescent polymer fibers were obtained by electrospinning solutions of 8% (in ethanol) polyvinylpyrrolidone (PVP) doped with three different dyes (coumarin 6, rhodamine 6G and sulforhodamine 101). Using the same parameters for the electrospinning process, nanofibers with diameters between 200 and 800 nm and different sizes and distributions of the beads were obtained as proven by scanning electron microscopy (SEM). We assessed the dependence of their emissive properties (intensity and wavelength) on the type of dye using photoluminescence (PL) spectra for the same concentration of the dopand dye (10(-3)M). Moreover, employing 4 different concentrations for coumarin 6 and rhodamine 6G (from 10(-3) to 10(-6) M) we evaluated the dependence with the concentration of the dye on the emissive properties of the electrospun dye-doped PVP nanofibers.

This work reports on the use of X-ray photoelectron spectroscopy to quantify band bending at ferroelectric free surfaces and at their interfaces with metals. Surfaces exhibiting out-of-plane ferroelectric polarization are characterized by a band bending, due to the formation of a dipole layer at the surface, composed by the uncompensated polarization charges (due to ionic displacement) and to the depolarization charge sheet of opposite sign, composed by mobile charge carriers, which migrate near surface, owing to the depolarization electric field. To this surface band bending due to out-of-plane polarization states, metal-semiconductor Schottky barriers must be considered additionally when ferroelectrics are covered by metal layers. It is found that the net band bending is not always an algebraic sum of the two effects discussed above, since sometimes the metal is able to provide additional charge carriers, which are able to fully compensate the surface charge of the ferroelectric, up to the vanishing of the ferroelectric band bending. The two cases which will be discussed in more detail are Au and Cu deposited by molecular beam epitaxy on PbZr0.2Ti0.8O3(001) single crystal thin layers, prepared by pulsed laser deposition. Gold forms unconnected nanoparticles, and their effect on the band bending is the apparition of a Schottky band bending additional to the band bending due to the out-of-plane polarization. Copper, starting with a given thickness, forms continuous metal layers connected to the ground of the system, and provide electrons in sufficient quantity to compensate the band bending due to the out-of-plane polarization.

We report on the charge storage properties of trilayer structures consisting in sputtered gate HfO2/co-sputtered Ge-HfO2 layer/rapid thermal tunneling SiO2 oxide. Investigations of transmission electron microscopy and X-ray diffraction evidence the formation of HfO2 with mixed structure of monoclinic and tetragonal in the annealed structures. Capacitance-voltage (C-V) characteristics were measured on Al/HfO2/Ge-HfO2/SiO2/Si/Al metal-oxide-semiconductor capacitors based on as-deposited and annealed structures. Large C-V hysteresis is observed for the as-deposited structures and is controlled by traps present in oxide and interface. The annealing yields a C-V hysteresis with smaller memory window being due to injected charges in Ge nanocrystals.

Electron paramagnetic resonance (EPR) spectroscopy has been extensively employed to investigate the presence, localization, distribution and interaction with the host crystalline lattice of the paramagnetic point defects (intrinsic defects and transition metal ions) in semiconductors. The retrieval of such information for nanostructured semiconductors is considerably more difficult, due to the high disorder level in such systems, reflected in broad, featureless EPR spectra. We show here how, with proper adjustments of the EPR experiments and accurate numerical analysis of the resulting spectra, it was possible to obtain more accurate information regarding the localization and structure of various Mn2+ centers in ZnS and ZnO semiconductor nanoparticles (NPs). This lead to the observation and investigation of size related effects such as the presence of the extended lattice defect assisted incorporation of impurities in small (similar to 3 nm) cubic ZnS NPs, the dominant size induced lattice disorder observed for ZnO NPs, independent of the synthesis procedures, or the three steps decomposition of the epsilon-Zn(OH)(2) disordered shell of ZnS NPs with formation of new oxy-hydrated zinc compounds. These effects can be used to synthesize semiconductor nanoparticles with controlled size distribution, doping level and functionalized surfaces for specific technological applications.