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 (10-80 °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 (10-50 °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.

Comparative study of leaching of silver nanoparticles from fabric and effective effluent treatment.

Nano silver (Ag(n)) is employed as an active antimicrobial agent, but the environmental impact of Ag(n) released from commercial products is unknown. The quantity of nanomaterial released from consumer products during use should be determined to assess the environmental risks of advancement of nanotechnology. This work investigated the amount of silver released from three different types of fabric into water during washing. Three different types of fabric were loaded with chemically synthesized Ag nanoparticles and washed repeatedly under simulated washing conditions. Variable leaching rates among fabric types suggest that the manufacturing process may control the release of silver reaching the waste water treatment plants. In an attempt to recover the Ag(n) for reutilization and to save it from polluting water, the effluents from the wash were efficiently treated with bacterial strains. This treatment was based on biosorption and was very efficient for the elimination of silver nanoparticles in the wash water. The process ensured the recovery of the Ag(n) leached into the effluent for reutilization, thus preventing environmental repercussions.

Novel nanostructures and optical properties of silver doped sodium phosphate thin films.

(50-x)Na2O-50P2O5-xAgCl (x = 0 to 15 mol%) glasses in bulk form were synthesized using melt-quenching technique. Thin films of these silver doped sodium phosphate glasses were deposited by thermal evaporation process. The influence of deposition of these glasses in thin film form and subsequent annealing at 600 degrees C on the structure and optical properties such as transmittance, reflectance, refractive index and band gap have been investigated in detail. X-ray diffraction studies of the as-deposited films show the films to be amorphous whereas annealed films show existence of orthorhombic and monoclinic phase of NaPO3 along with crystalline cubic phase of AgCl in doped glasses. Structural investigations of these annealed films show unique morphologies (needle-like and granular) at nano-scale. Both as-fabricated and annealed films are poor reflectors but show high transmittance in the entire spectral region under consideration, which is a direct consequence of particle size effects. Indirect band gap narrowing and variation in refractive index upon annealing is consistent with nanostructural transformations in these samples.

Effect of variable pressure on growth and photoluminescence of ZnO nanostructures.

Thin films constituted of equiaxed and one dimensional nanostructures of ZnO via metal-catalyst-free vapor phase were grown using a simplistic thermal evaporation technique under two different pressure conditions approximately of the order of 10(-1) and 10(-3) torr, respectively. ZnO deposited at low vacuum (approximately 10(-1) torr) exhibited the formation of nanograins of variable size between 60 to 180 nm. In contrast the film grown at high vacuum (approximately 10(-3) torr) resulted the nanowired type morphology with a random networking, generally distributed with equiaxed grains of film microstructure. The diameter of maximum number of these nanowires lies between 45 to 65 nm. The films grown at low vacuum has shown almost equal composition of Zn and O while the film grown at high vacuum has shown lower content of O. The nanowires formed under limited O (high vacuum: approximately 10(-3) torr) signifies the role of O vacancies during growth. It has been postulated that presumably under high vacuum deposition, initially formed ZnO transforms to ZnOx (x < 1) through creation of O vacancies due to limited presence of O. Subsequently ZnOx acts as self-catalyst and heterogeneous nuclei are responsible for the formation of nanowired type morphology. The effect of different microstructures has been correlated and discussed to understand the photoluminescence characteristics obtained on these films.

Nanostructured mesoporous tungsten oxide films with fast kinetics for electrochromic smart windows.

A potential driven self-assembly of sodium dodecyl sulfate/tungsten oxide aggregates at the electrolyte-electrode interface followed by template extraction and annealing yielded mesoporous thin films of electrochromic tungsten oxide (WO(3)). Electron microscopy images revealed that the films are characterized by a hitherto unreported hybrid structure comprising nanoparticles and nanorods with a tetragonal crystalline phase of WO(3) with the measured lattice parameters: a = 0.53 nm and c = 0.37 nm. In addition to pentagonal voids characteristic of the tetragonal WO(3) phase at the lattice scale, open channels and pores of 5-10 nm in diameter lie between the nanoparticles, which cumulatively promote rapid charge transport through the film. This resulted in colouration efficiency (η(max)∼90 cm(2) C(-1) at λ = 900 nm) and switching kinetics (colouration time =  3 s and bleaching time =  2 s for a 50% change in transmittance) higher and faster than previously reported values for mesoporous WO(3) films. Repetitive cycling between the clear and blue states has no deleterious effect on the electrochromic performance of the film, which is suggestive of its potential as a cathode in practical electrochromic windows.