Shubra Singh

Anomalous room temperature magnetoresistance in brownmillerite Ca2Fe2O5

We demonstrate the hitherto unreported field induced magnetoresistance in wet chemical synthesized bulk polycrystalline brownmillerite Ca2Fe2O5. A magnetoresistance of 12% is observed at room temperature, well below its antiferromagnetic transition temperature of 775 K. The variation in resistivity with increasing and decreasing magnetic field shows a significant memory effect. When synthesized in a nanostructured form it exhibits weak ferromagnetism due to the appearance of an Fe oxide phase, besides exhibiting a shift to a higher antiferromagnetic Neel temperature of 820 K. Spectroscopic investigations (X-ray photoelectron and Raman spectroscopy) as well as magnetic measurements have been employed to investigate the above behavior of brownmillerite Ca2Fe2O5.

Large area transparent ZnO photodetectors with Au wire network electrodes

Transparent electronics as a futuristic technology are fast growing and expanding beyond conventional optoelectronics. Here we report the fabrication of a large area, visibly transparent ultraviolet (UV) photodetector employing a laser deposited ZnO active layer in between branched Au wire networks, with the latter serving as electrodes due to being transparent in UV as well as visible regions. The Au wire networks were prepared using a crack templating method which is not only cost effective but also enables heterostructuring with simple processes. When compared to Ag contact pads, these wire network electrodes seem to enhance photocurrent collection efficiency resulting in high UV sensitivity at low response times. Importantly, the visible light transparency was as high as 80%.

Controlled nucleation and growth of nanostructures by employing surface modified GaN based layers/heterostructures as bottom layer

Controlled nucleation and growth of Zinc oxide nanorods is achieved on GaN, etched GaN and AlGaN/GaN heterostructure bottom layers grown by a metal organic chemical vapour deposition technique. The effects of the bottom crystalline layers on the structural, morphological and optical properties of the as grown ZnO nanorods have been investigated by high-resolution X-ray diffraction, scanning electron microscopy, photoluminescence and Raman measurements. HRXRD (0002) reciprocal-space mapping (RSMs) studies were performed on GaN and AlGaN/GaN layers before and after the growth of ZnO nanostructures to investigate the impact of strain upon the ZnO layer grown on GaN layers and AlGaN/GaN heterostructures. Raman intensity mapping shows the densely nucleated hexagonal pit like structures for the etched GaN layer, providing an enhanced surface area for primary nucleation suggesting that the growth species prefer to condense on locations with maximum binding energy. The increase in nucleation density for etched GaN layers also result in dense nanorods which exhibit better excitonic emission. Our studies suggest that ZnO nanostructures with improved optical and structural properties can be grown on etched-GaN as well as AlGaN/GaN heterostructures as the bottom layer. It is interesting to observe that the bottom GaN layer can be easily employed to determine the optical quality of ZnO layer.

Anomalous red emission with competition and coexistence of defect and band edge emission in photo-electrochemically active (Zn0.97Ga0.03)(O0.95N0.05) solid solution

Photo-electrochemically active nanostructured (Zn0.97Ga0.03)(O0.95N0.05) solid solution has been synthesized by a solution combustion technique for realizing photocatalytic activity under visible light. A competing free excitonic and defect bound emission is observed from the sample which was investigated by temperature dependent photoluminescence from 4 K to 300 K. The defect bound emission dominates at room temperature providing an anomalous enhanced red emission, not reported before. A broad visible emission confirms the introduction of new defect levels as an impact of solid solution formation as established later by valence band (VB) XPS spectra. VB XPS shows the top of the valence band has shifted without affecting the conduction band, thereby reducing effective band gap of the solid solution from 3.35 eV to 2.8 eV. Tuning of the bandgap is essential to facilitate its activity under visible light irradiation as demonstrated in our present work. The generation of charge carriers, their effective separation and reduced trap states has been demonstrated by photoelectrochemical measurements in order to confirm the potential of the sample for efficient photocatalytic activity. The present sample exhibits a low mean life time with reduced trap states as compared to some previously reported results.

Synthesis of GaN:ZnO solid solution by solution combustion method and characterization for photocatalytic application

GaN-ZnO solid solution has emerged as a successful and reproducible photocatalyst for overall water splitting by one-step photoexcitation, with a bandgap in visible region. When the solid solution is formed, some of the Zn and O ions are replaced by Ga and N ions respectively and there is a narrowing of bandgap which is hypothesized as due to Zn3d-N2p repulsion. The traditional method of synthesis of GaN-ZnO solid solution is by nitridation of the starting oxides under constant ammonia flow. Here we report a solution combustion technique for the synthesis of the solid solution at a temperature about 500 ° C in a muffle furnace with metal nitrates as precursors and urea as the fuel. The as prepared samples showed change in color with the increased concentration of ZnO in the solution. The structural, microstructural, morphological and optical properties of the samples were realized by Powder X ray diffraction, Scanning electron microscopy, Energy dispersive X ray analysis, Transmission electron microscopy ...

Catalyst-free deposition of few layer graphene on c-plane sapphire substrates by drop casting technique

A simple and easy method of synthesis and catalyst-free deposition of few layer graphene (FLG) on non-conducting sapphire substrates still remains a challenge. This work involves preparation of FLG by liquid exfoliation method and deposition of FLG onto chemically etched sapphire substrate with and without catalyst by drop casting technique. Field emission scanning electron microscopy (FESEM) was used to observe presence of FLG on sample surfaces and its energy dispersive X-ray (EDX) spectra reveals absence of other trace impurity. Atomic force microscopy (AFM) shows FLG within and along the sidewalls of the dislocation site for catalyst free sample. Fourier transform infrared spectroscopy confirmed the dispersion of FLG in solvent. Raman spectroscopy revealed prominent graphene peak positions corresponding to D, G and 2D bands. Also, diamond like carbon, fullerene and additional Raman modes have been observed using Raman spectroscopy excited at higher wavelength. FESEM, EDX and AFM results prove better FLG dispersions on catalyst-free sample in comparison to sample with catalyst. This method of FLG deposition will be highly suitable for optoelectronic applications for developing cost-efficient device with optimum performance.

Electrochemical performance of brownmillerite calcium ferrite for application as supercapacitor

In the past few years with increase in population and global warming the requirement to store energy from various sources has increased. Recent research have been focused on oxide materials as energy storage device due to their unique structure and interesting properties. Perovskites materials such as La0.5Sr0.5CoO3-δ and SrRuO3 shows capacitive behavior. In previous studies perovskite with different A and B sites have been studied. In this report we have chosen Ca2Fe2O5 perovskite material due to its interesting properties, structure and morphology which changes on introducing cobalt as dopant material. Further we investigated the performance of cobalt doping on the electrochemical behavior of Ca2Fe2-xCoxO5 (x=0, 0.01 and 0.03) using electrochemical characterization such as Cyclic voltammetry (CV), Galvanostatic charge-discharge (GCD) and Electrochemical impedance spectra (EIS).

Synthesis and characterization of polycrystalline brownmillerite cobalt doped Ca2Fe2O5

Brownmillerite compounds with general formula A2BB’O5 (BB’ = Mn, Al, Fe, Co) have attracted attention in wide range of applications such as in solid oxide fuel cell, oxygen separation membrane and photocatalysis. Brownmillerite compounds have unique structure with alternate layers of BO6 octahedral layers and BO4 tetrahedral layers. Presence of dopants like Co in place of Fe increases oxygen vacancies. In the present work we have synthesized polycrystalline Ca2Fe2O5 and Ca2Fe1-xCoxO5 (x = 0.01, 0.03) by citrate combustion route. The as prepared samples were characterized by XRD using PANalytical X’Pert System, DRS (Diffuse reflectance spectroscopy) and SEM (Scanning electron microscopy).

Effect of Al-mole fraction in AlxGa1−xN grown by MOCVD

AlGaN/AlN layers were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The AlxGa1−xN layer composition was varied from 15% to 25%. The crystalline quality, thickness and aluminum (Al) composition of AlGaN were determined using high resolution X-ray diffraction (HRXRD). The growth rate decreases on increasing Al composition. Reciprocal space mapping (RSM) was used to estimate the strain and relaxation between AlGaN and AlN. The optical properties of AlGaN layers were investigated by room temperature Photoluminescence (PL). The AlGaN peak shifts towards lower wavelength with Al composition. The surface morphology of AlGaN was studied by atomic force microscopy (AFM). Root mean square (RMS) roughness values were found to be increased in AlGaN layers with composition.

Effect of varying Ga content in ZnO:GaN solid solution synthesized by solution combustion technique for photocatalytic applications

ZnO:GaN (oxy)nitride solid solution has been established as the most efficient non-oxide photocatalyst for water splitting under visible irradiation with one step photoexcitation and also boasts a band gap tunability from 2.8 eV to 2.5 eV[1]. The solid solution of GaN in ZnO is formed by the intersubstitution of few of Zn/O ions by Ga/N ions, and this results in the introduction of new defect levels above the valence band which narrows the effective band gap enabling activity under visible region of spectra. In this work, we report the synthesis of ZnO:GaN solid solution by a solution combustion technique where metal nitrates and urea are used as precursors. The Zn/Ga ratio was varied from 16 to 1 in the precursors. The as synthesized samples were characterized as phase pure by X-ray diffraction, where the wurtzite structure was retained up to Zn/Ga ratio of 5. The Diffuse reflectance spectroscopy studies revealed that as the Ga content in the solid solution increases there is a reduction in band gap, fro...