K.N. Sood

Large area fabrication of vertical silicon nanowire arrays by silver-assisted single-step chemical etching and their formation kinetics

Vertically aligned silicon nanowire (SiNW) arrays have been fabricated over a large area using a silver-assisted single-step electroless wet chemical etching (EWCE) method, which involves the etching of silicon wafers in aqueous hydrofluoric acid (HF) and silver nitrate (AgNO3) solution. A comprehensive systematic investigation on the influence of different parameters, such as the etching time (up to 15 h), solution temperature (10-80u2009°C), AgNO3 (5-200 mM) and HF (2-22 M) concentrations, and properties of the multi-crystalline silicon (mc-Si) wafers, is presented to establish a relationship of these parameters with the SiNW morphology. A linear dependence of the NW length on the etch time is obtained even at higher temperature (10-50u2009°C). The activation energy for the formation of SiNWs on Si(100) has been found to be equal to ∼0.51 eV . It has been shown for the first time that the surface area of the Si wafer exposed to the etching solution is an important parameter in determining the etching kinetics in the single-step process. Our results establish that single-step EWCE offers a wide range of parameters by means of which high quality vertical SiNWs can be produced in a very simple and controlled manner. A mechanism for explaining the influence of various parameters on the evolution of the NW structure is discussed. Furthermore, the SiNW arrays have extremely low reflectance (as low as <3% for Si(100) NWs and <12% for mc-Si NWs) compared to ∼35% for the polished surface in the 350-1000 nm wavelength range. The remarkably low reflection surface of SiNW arrays has great potential for use as an effective light absorber material in novel photovoltaic architectures, and other optoelectronic and photonic devices.

Surface distribution studies and improved photoluminescence characteristics of silica coated ZnS:Mn nanophosphor layers

A simple methodology is presented for dispersing the silica-capped ZnS:Mn nanophosphors with controlled particle density as two-dimensional (2D) layers. Si wafer and borosilicate glass slides were taken as substrates for this method. Size-controlled ZnS:Mn nanoparticles were capped with silica for their surface passivation, growth retardation, and stabilization against environmental attacks. By varying the molar concentration of ZnS:Mn nanoparticles in silica sol, a controlled particle density on the substrates has been achieved. The morphological investigations of the layers showed that the surface distribution of the nanophosphor particles within the active area is uniform for a given molar concentration. The room-temperature photoluminescence (PL) studies of these layers showed a broad and intense peak at ∼593nm indicating a blueshift of about 7 nm compared to their conventional bulk counterparts due to quantum size effect. However, annealing of the layers improved the PL intensity by several orders at...

Flux line motion in superconducting (YBa2Cu3O7−d)1−x/(SiO2)x composite systems in high magnetic fields

Superconducting (YBa2Cu3O7−d)1−x/(SiO2)x (i.e. Y123/S) composites with x = 0, 0.05, 0.1, 0.15 and 0.2 were prepared by solid state synthesis. Detailed investigations of structural and electrical transport and magnetization in applied magnetic fields (B = 0–9xa0T) were carried out. The typical double-step superconducting transitions observed both resistively (B = 0) and magnetically (B = 1xa0mT) reveal the granular nature of all the samples. The XRD, SEM and superconducting transitions show that addition of SiO2 mainly influences the inter-grain regions of the samples, leaving the pure Y123 phase in intra-grain regions practically intact. This leads to an interesting behaviour of M(T,B) and resistivity ρ(T,B) in the samples. The field-cooled (FC) magnetization M(T) measured at B = 1xa0mT shows a dip in the diamagnetic transition at a characteristic temperature that coincides with the onset of the second step in the zero-field-cooled (ZFC) M(T) measured in the same field. In the presence of B≥1xa0T, the broadening of the superconducting transition revealed by ρ(T) below the first step is found to be field-dependent, whereas below the second step at lower temperatures the broadening becomes field-independent. The width (ΔM) of the high field hysteresis loops (M–B) measured isothermally in the superconducting state initially varies as a power law of B. Depending on SiO2 content and temperature, the power law further deviates into either a shallow peak or a monotonically decreasing curve. We show that these results and a host of other considerations point towards an interaction of vortices in inter-xa0and intra-grain regions.

Surface modified ZnO nanoparticles: structure, photophysics, and its optoelectronic application

Nanostructured ZnO has been synthesized by wet chemical route. Poly(vinylpyrrolidone) is used for stabilization and surface passivation of synthesized nanoparticles, thus tailoring the growth of ZnO at nanoscale. Structural characterization using X-ray diffraction, scanning electron microscopy, high-resolution transmission electron micoroscopy and Fourier transformed infrared spectroscopy of the synthesized nanoparticles confirm the evolution of nanocrystalline ZnO prevailing in hexagonal wurtzite phase. UV–Vis and photoluminescence spectroscopy studies show blue shift phenomenon in the synthesized nanoparticle in contrast to the bulk ZnO furnishing evidence in support of quantum size effect. The nanocomposites of ZnO and poly [9,9-dioctylfluorenyl-2,7-diyl] are prepared and characterized to investigate its luminescent and spectral emission effects. The nanocomposites are then incorporated in light-emitting diodes, and influence of ZnO on the device performance has been explored via electroluminescence, current density evaluation, and corresponding CIE coordinates calculation.

Study of microstructural changes in MgB2 thin film superconductors irradiated with 200 MeV 107Ag ions

MgB2 superconductor thin films, prepared by pulsed laser deposition (PLD) and electron beam evaporation (EBE), have been investigated by scanning electron microscopy (SEM) before and after 200 MeV 107Ag ion irradiation. The severe degradation of superconducting properties in irradiated PLD film and the absence of the same in EBE film correlates with the observed changes in their microstructures. The micrographs of the PLD film show an overall smoothing, flow pattern and reduction in size of the bigger agglomerates after irradiation. On the other hand, the microstructure of the EBE film does not show any significant change after irradiation. The flow pattern observed in the PLD film can be understood on the basis of the viscoelastic model for irradiation induced shear flow in amorphous solids. The observed degradation of the PLD film may thus be attributed primarily to its disordered nature under irradiation, whereas the radiation hardness of the EBE film may be due to its higher crystallinity.