Ajay Kushwaha

Defect induced high photocurrent in solution grown vertically aligned ZnO nanowire array films

Highly oriented and homogeneously distributed single crystalline zinc oxide nanowires (NWs) are fabricated on amorphous glass substrates using soft solution growth approach. The nanowire films and sol-gel grown ZnO films are devised and tested for UV light detection applying four-probe conductivity measurements. As-grown ZnO NWs film device demonstrates three orders enhancement (sensitivity = 440) in conductivity at room temperature under an illumination of 365 nm UV light, while the sol-gel based thick film reveals two orders of enhancement in device conductance. A clear correlation of conductivity and photoluminescence measurements suggest that surface oxygen vacancies (singly charged/Vo+) which render higher green defect luminescence intensity (IG/IUV = 1.8) in ZnO NWs leads to poor dark conductance and higher photo-conductance. Post growth annealing of nanowire arrays either in air (IG/IUV = 0.85) or oxygen ambience (IG/IUV = 0.38) results in reduction of green defects and corresponding suppression of...

Influence of Cu doping on the structural, photoluminescence and formaldehyde sensing properties of SnO2 nanoparticles

In this paper we report Cu doping induced modifications in the structural, photoluminescence and gas sensing behaviour of SnO2 nanoparticles. Our results show that crystallinity is reduced upon Cu doping. The PL emissions observed in the visible region are attributed to the defect levels arising due to oxygen vacancies. The 1.5 at% Cu-doped SnO2 shows the selective high response (∼80%) to 50 ppm concentration of formaldehyde over methanol, ethanol, propanol-2-ol, acetone and n-butylacetate at 200 °C. The sensing mechanism has been explained based on chemisorption of oxygen on the SnO2 surface and the subsequent reaction between the adsorbed oxygen species and the formaldehyde molecules.

Role of defect states in magnetic and electrical properties of ZnO nanowires

The perfect crystalline nature along with a defect ridden surface controls the electrical and magnetic properties of ZnO nanowires. Herein, a soft chemical approach is presented to grow ZnO nanowires in powder as well as highly oriented nanowire film form. Photoluminescence measurements reveal high surface defects in as-grown nanowire and post growth annealing treatment in argon and oxygen atmosphere reduces intensity of defect emissions. Magnetic measurements illustrate the ferromagnetic nature of submicron sized zinc oxide (ZnO) nanorods arising due to singly charged oxygen vacancies. Nanowires show diamagnetic behavior when annealed at higher temperature in oxygen while argon annealing does not affect the magnetic behavior. In an analogous manner, we also investigated the effect of surface defects on electrical properties and correlated electrical conductivity with a responsible defect state.

Rapid thermal annealing assisted stability and efficiency enhancement in a sputter deposited CuO photocathode

We designed a stable and efficient CuO based photocathode by tuning the crystallinity and surface morphology of films by rapid thermal treatment. The role of the annealing temperature on film crystallinity, optical absorption and grain size is studied. The impact of these parameters upon the photocatalytic water splitting performance of CuO films is investigated. We observed that a higher annealing temperature improves the film crystallinity and increases the grain size of CuO film, which significantly enhance the photocurrent generation capability. Rapid thermal annealing at 550 °C is found the best temperature to achieve the highest PEC performance. The thickness of the CuO photocathodes is also optimized and we observed that 550 nm thick films results in the highest photocurrent of 1.68 mA cm−2. Our optimized CuO photocathode has shown better stability against photo-corrosion and a 30% decrease in the initial value of photocurrent is measured after 15 min, while a 60% decrease in the photocurrent is noticed in case of the as-deposited film.

Spray-deposited nanocrystalline WO3 thin films prepared using tungsten hexachloride dissolved in N-N dimethylformamide and influence of in doping on their structural, optical and electrical properties

Undoped and In-doped nanocrystalline tungsten oxide (WO3) thin films were prepared by chemical spray pyrolysis using tungsten hexachloride (WCl6) dissolved in N-N dimethylformamide as the host precursor solution and indium chloride (InCl3) as the source of dopant. XRD analyses confirm the monoclinic phase of the prepared films with the predominance of triplet (002), (020) and (200) in the spectra. On indium doping, the crystallinity of the films decreases and becomes minimum at 1.5 at. % doping. EDX analyses confirm the incorporation of In dopants into the WO3 lattice network. SEM micrographs show nonspherical grains over the surface and the average grain size decreases with higher In doping. AFM images of the films exhibit large nicely separated conical columnar grains (except in 1 at. %) throughout the surface with coalescence of some columnar grains at few places. UV-visible measurements reveal that the optical transmittance of the 1 at. % In-doped film increases significantly throughout the wavelength range 300–800 nm relative to that of the undoped film. Room temperature photoluminescence spectra show pronounced enhancement in the peak intensity of NBE emission on In doping. Electrical conductivity has been found to increase on In doping.

A novel catalyst-free synthesis of vertically aligned silicon nanowire–carbon nanotube heterojunction arrays for high performance electron field emitters

A novel, catalyst-free strategy for the direct synthesis of vertically aligned silicon nanowire-carbon nanotube (SiNW-CNT) heterojunction arrays is presented. Such a heterojunction with the junction area in the nanoscale displays enhanced field emission characteristics at low turn-on field, with a nearly three times increase in the field enhancement factor.

Impact of molybdenum out diffusion and interface quality on the performance of sputter grown CZTS based solar cells

We have investigated the impact of Cu2ZnSnS4-Molybdenum (Mo) interface quality on the performance of sputter-grown Cu2ZnSnS4 (CZTS) solar cell. Thin film CZTS was deposited by sputter deposition technique using stoichiometry quaternary CZTS target. Formation of molybdenum sulphide (MoSx) interfacial layer is observed in sputter grown CZTS films after sulphurization. Thickness of MoSx layer is found ~142 nm when CZTS layer (550 nm thick) is sulphurized at 600 °C. Thickness of MoSx layer significantly increased to ~240 nm in case of thicker CZTS layer (650 nm) under similar sulphurization condition. We also observe that high temperature (600 °C) annealing suppress the elemental impurities (Cu, Zn, Sn) at interfacial layer. The amount of out-diffused Mo significantly varies with the change in sulphurization temperature. The out-diffused Mo into CZTS layer and reconstructed interfacial layer remarkably decreases series resistance and increases shunt resistance of the solar cell. The overall efficiency of the solar cell is improved by nearly five times when 600 °C sulphurized CZTS layer is applied in place of 500 °C sulphurized layer. Molybdenum and sulphur diffusion reconstruct the interface layer during heat treatment and play the major role in charge carrier dynamics of a photovoltaic device.

pH-DEPENDENT SYNTHESIS OF STABILIZED GOLD NANOPARTICLES USING ASCORBIC ACID

The stabilization of gold nanoparticles solution synthesized using ascorbic acid at room temperature is examined in detail. It is found that gold nanoparticles (AuNPs) synthesized using ascorbic acid have a narrow size distribution (31 ± 5, 36 ± 6, and 40 ± 5 nm) and can be readily stabilized just by adjusting the initial pH conditions of the reaction solutions. Whilst the initial pH strongly affects the stability of particles, it has no effect on the size of stabilized particles. The particles are nearly spherical having size distribution effectively in the range of 30 nm to 40 nm. Through time dependent UV-Vis spectra studies we also hypothesize the pH dependent stabilization mechanism wherein a layer of adsorbed ionic complex over AuNPs slows down the aggregation of AuNPs. The information obtained in this study can be used to design in-situ controlled nanoparticle synthesis system especially within the biological cells. Moreover, the method is green and biologically acceptable as well.

Defect controlled water splitting characteristics of gold nanoparticle functionalized ZnO nanowire films

Functionalization of ZnO nanowires by size controlled gold nanoparticles is demonstrated through sputtering of a gold layer followed by annealing at different temperature. Rationally designed sputtered Au nanoparticle–ZnO nanowire (AuNP–ZnO) exhibits high photocatalytic activity for water splitting. The tailored structure shows an enhanced photocurrent of 300 μA cm−2 under visible light illumination. Additionally, the efficiency of 0.13% at a low bias of 0.45 V vs. Ag–AgCl indicates an efficient charge separation and collection without a conducting ITO/FTO base. The AuNPs tune the visible light absorption and utilize the SPR of the Au nanoparticle to boost the hot electron injection from Au nanoparticles to the ZnO conduction band. Moreover, AuNPs also assist in the suppression of surface defects of ZnO NWs (or hole traps) significantly and enhance the water splitting performance of the material under visible light.

Burr Formation and Surface Quality in High Speed Micromilling of Titanium Alloy (Ti6Al4V)

Titanium and Ti alloys are popular materials used in aviation and biomedical field due to their excellent strengthto-weight ratio and corrosion resistance properties. Micromilling is a common mechanical machining process used in the production of microscale features. The microtool has very low stiffness and even small forces can lead to catastrophic tool failure. High speed micromachining can be used to address the issue because of lower chip loads at higher rotational speeds. Consequently, high speed micromilling can be used for micromachining of hard metals/alloys which are difficult to accomplish at lower speeds. Now days high speed micromilling is gaining popularity due to its high material removal rate and good surface finish. In many cases, the machined product does not need an additional finishing process. However, the burr formation in the mechanical machining process is the most important problem which becomes more critical for a microscale feature. Removal of micro-size burr is much more difficult than its macro counterpart. The current work is focused on the characterization of the burr formation in high speed micromilling. Influence of various process parameters, viz., spindle speed, feed rate, depth of cut, tool diameter and number of flutes of the micromilling tool has been analyzed on the burr size and on the quality of the machined surface via measuring the surface roughness.