Dielectric Function of Silicon Nanoclusters: Role of Hydrogen

Electronic and optical properties of small silicon quantum dots having 3 to 44 atoms per dot with and without surface passivation are investigated by computer simulation using the pseudo-potential approach.An empirical pseudo-potential Hamiltonian,a plane-wave basis expansion and a basic tetrahedral structure with undistorted local bonding configurations are used.The structures of the quantum dots are relaxed and optimized before and after hydrogen passivation.It is found that the gap increases more for a hydrogenated surface than the unpassivated one.Thus,both quantum confinement and surface passivation determine the optical and electronic properties of Si quantum dots.Visible luminescence is probably due to the radiative recombination of electrons and holes in the quantum-confined nanostructures.The effect of passivation of the surface dangling bonds by hydrogen atoms and the role of surface states on the gap energy is also examined.The results for the density of states,the dielectric function,the frequency dependent optical absorption cross section,the extinction coefficient and the static dielectric constants of the size are presented.The importance of the confinement and the role of surface passivation on the optical effects are discussed.

Selective microwave absorption in Nd3+ substituted barium ferrite composites

Microwave(MW)frequency based wireless communications and electronic devices became prospective due to several ramifications.To meet this need,a series of neodymium ions(Nd3+)substituted barium ferrite composites with composition(20)BaO:(80-x)Fe2 O3:(x)Nd2 O3(0≤x≤3 mol%)was prepared at1100℃using solid-state reaction method.We evaluated the effect of various Nd3+ions contents on the surface morphology,structure,and magnetic properties of the as-synthesized barium ferrite composites.Meanwhile,microwave reflection loss,complex permittivity and permeability were determined using the transmission/reflection line method in the X-band(8-12 GHz).SEM image of the composites shows that the surface morphology consists of rough and porous microstructures.XRD patterns of the un-doped composites reveal the existence of BaFe12O19(hexagonal)and Fe21.333O32(tetragonal)crystalline phases.Furthermore,a new hexagonal crystalline phase of Ba6 Nd2 Fe4 O15 with the crystallite sizes between 15 and 67 nm is observed due to Nd3+ions substitution in the composite.The saturation magnetization of the composite containing 2 mol%of Nd3+does not exhibit any significant alteration compared to the one devoid of Nd3+.The complex relative permitivity and permeability of the achieved composites enriched in Ba6 Nd2 Fe4 O15 and BaFe2 O4 phases disclose significant MW frequency dependence.The composites also display selective MW absorption in the X-band which could be useful for diverse applications.

A functional approach toward xerogel immobilization for encapsulation biocompatibility of Rhizobium toward biosensor

Sol-gel derived silica has tremendous applications as a biocompatible scaffold for the immobilization of cells. The use of xerogel as a matrix in the blueprint of biosensors is an appealing proposition due to several inimitable characteristics of xerogels, primarily because of their high porous nature, amendable pore size, and exceptionally large internal surface area. Morphological （X-Ray Diffraction and Thermogravimmetric Analysis） and optical （Fourier Transform Infrared and UV-Vis absorption） studies of the silica matrices with entrapped Rhizobial （Rz） structure of the biomaterial has been made. Temporal and concentration dependent studies were conducted for impregnated samples; it showed that the response time for the new biosensor for determining the concentration of Rz is less than 20 min. In this work, first time a novel avenue to create a generic approach for the fabrication of biosensor has been created.

Optical behavior of self-assembled high-density Ge nanoislands embedded in SiO_2

The radio frequency magnetron sputtering method is used to prepare well-dispersed pyramidal-shaped Ge nanoislands embedded in amorphous SiO2 sublayers of various thicknesses. The estimated size and number density of Ge nanoislands in SiO2 sublayer thicknesses beyond 30 nm are approximately 15 nm and 1011 cm-2, respectively. Atomic force microscopy （AFM） reveals root mean square （RMS） roughness sensitivity as the SiO2 sublayer thickness varies from 30 to 40 nm. The formation of nanoislands with high aspect ratios is attributed to the higher rate of surface reactions between Ge adatoms and nucleated Ge islands than reactions associated with SiO2 and Ge. The Ge nanoisland polyorientation on SiO2 （50-nm thickness） is revealed by X-ray diffraction （XRD） patterns. Photoluminescence （PL） peaks of 2.9 and 1.65 eV observed at room temperature （RT） are attributed to the radiative recombination of electrons and holes from the Ge nanoislands/SiO2 and Si02/Si interfaces, respectively. The mean island sizes are determined by fitting the experimental Raman profile to two models, namely, the phonon confinement model and the size distribution combined with phonon confinement model. The latter model yields the best fit to the experimental data. We confirm that SiO2 matrix thickness variations play a significant role in the formation of Ge nanoislands mediated via the minimization of interfacial and strain energies. OCIS codes： 250.5230, 170.5660.