Abstract: Stoichiometric mercury-telluride (HgTe) nanocrystals, free from any foreign impurities, have been prepared by an electrochemical technique. Transmission electron microscopy and selected area diffraction studies confirm that the size of HgTe nanocrystals with cubic phase range from 4 – 7 nm and are preferentially oriented. Optical absorption studies indicate that the band gap of HgTe nanocrystals is 2.14 eV. Room temperature photoluminescence measurements show band edge and trap dominated luminescence in the visible regime. Micro-Raman scattering analysis of the HgTe nanocrystals sample reveals the 1LO Raman vibrational mode is at 142.5 cm-1suggesting a shift of 6 cm-1 from it’s bulk value and confirms the crystalline size to be small.
Abstract: We study (through computer simulation) the variation of the band gap as a function of sizes and shapes of small Silicon (Si) dots using pseudo-potential approach. We have used empirical pseudo-potential Hamiltonian and a plane wave basis expansion and a basic tetrahedral structure. It is found that the gap decreases for increasing dot size. Furthermore, the band gap increases as much as
0.13eV on passivation the surface of the dot with hydrogen. So both quantum confinement and surface passivation determine the optical and electronic properties of Si quantum dots. Visible luminescence is probably due to 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 as well as on the HOMO-LUMO states has also been examined. We have investigated the entire energy spectrum starting from the very low lying ground state to the very high lying excited states for silicon dots having 5, 18, 17 and 18 atoms. The results for the size dependence of the HOMO-LUMO gap and the wave functions for the bonding-antibonding states are presented and the importance of the confinement and the role of hydrogen
passivation on the confinement are also discussed.
Abstract: A quantitative approach to determination of depth profiles of optical properties of
Si-implanted SiO2 films based on spectroscopic ellipsometry (SE) is presented. From the SE measurements, the depth profiles of the complex refractive index of SiO2 films containing Si nanocrystals (Si-nc) are obtained with an effective medium approximation (EMA) in the wavelength range of 400-1200nm. The optical profiles obtained imply the existence of a wave-guide in the Si-doped SiO2 films.
Abstract: Boron-doped (B-doped) silicon nanowires (SiNWS) have been prepared and characterized by Raman scattering and photoluminescence (PL). B-doped SiNWS were grown by plasma enhanced chemical vapor deposition (PECVD), using diborane (B2H6) as the dopant gas. Raman spectra show a band at 480cm-1,which is attributed to amorphous silicon. Photoluminescence at room
temperature exhibits three distinct emission peaks at 1.34ev,1.42ev,1.47ev. Possible reason for these is suggested.
PACS: 36.40._c; 81.15.Gh; 81.20._n
Abstract: Copper ferrite nanoparticles are prepared by sol-gel synthesis using polyacrylic acid
(PAA) as chelating agent. The crystal structure and surface morphology are studied by XRD and SEM techniques. Microwave dielectric parameters such as real and imaginary part of complex permittivity and a.c. conductivity are determined in the frequency range 2247 - 2970 MHz by microwave cavity perturbation technique. Microwave magnetic parameters such as the real and imaginary part of complex permeability are also determined.
Abstract: A series of experiments conducted on a lead-free eutectic solder (Sn-3.5%Ag) have shown that addition of trace amounts of nanometer-sized particles does have an influence on mechanical properties of materials. In this study, three different types of nanoparticles (copper, nickel and iron) were chosen as the reinforcing candidate. For each particulate reinforcement the reflow process was performed under identical cooling conditions. Addition of trace amounts of nano-particles alters the kinetics governing solidification of the composite solder paste while concurrently exerting an influence on microstructural development, particularly the formation and presence of second phases in the solidified end product. The nano-sized powder particle-reinforced composite solder revealed an increase in microhardness compared to the unreinforced monolithic counterpart.
Abstract: Magnesium based composite with 2.5 weight percentage of nano-sized Al2O3 particulates reinforcement was fabricated using powder metallurgy technique. Al2O3 particulates with an equivalent size of 50-nm were used as reinforcement. Microstructural characterization of the materials revealed recrystallization and grain refinement due to the presence of nano-Al2O3. Mechanical properties characterization revealed that the presence of nano-Al2O3 particulates as
reinforcement lead to a significant increase in hardness, elastic modulus, 0.2% yield strength and UTS and ductility of pure magnesium. The results revealed that the specific tensile properties of these materials are superior when compared to high strength magnesium alloy AZ91 reinforced with much higher weight percentage of SiC. An attempt is made in the present study to correlate the effect of nano-sized Al2O3 particulates as reinforcement with the microstructural and mechanical
properties of magnesium.
Abstract: In the present study, elemental Ni powder was mechanically milled (MMed) for 10 hours to reduce the grain (crystalline) size in the nano-range (<100nm). The mechanically milled powder (10h-MMed) was consolidated by die-cold compaction and was further hot extruded at high temperatures to maintain a crystallite size within the nano range. Further, the specimen was tested by a novel free-free type suspended beam arrangement, coupled with circle-fit approach to
determine damping characteristics. To vary the resonant frequency of the suspended beam, end masses with different weights were added. The characterization results revealed that the nano-size grains exhibit increased damping compared to a coarse-grained sample, under similar vibration frequency. Results also show that the damping capacity of both nano and coarse grained samples decreases with an increase in frequency of vibration. Particular emphasis was placed to correlate the damping capacity with the process induced residual stresses present in the samples.
Abstract: In this study, composites based on Al-Mg/Al2O3 formulation were fabricated using an innovative solidification route followed by hot extrusion. The studies clearly indicate an increase in retention of nanometric alumina with an increase in magnesium content. Microstructural characterization studies of the extruded composite samples displayed fairly uniform distribution of reinforcement phases and minimal porosity. Results of mechanical properties characterization showed that a cumulative increase in magnesium and nanometric alumina content led to an increase in elastic modulus, 0.2% YS and UTS while the ductility of the composite was adversely affected. A comparison between monolithic Al-3.8Mg and Al-3.4Mg/1.4Al2O3 revealed that the presence of nanometric alumina play a dominant role in realizing a significant increase in elastic modulus, 0.2%YS and UTS of the composites.