Faculty Publications
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Item Unintentionally doped homoepitaxial InSb films have been grown by liquid phase epitaxy employing ramp cooling and step cooling growth modes. The effect of growth temperature, degree of supercooling and growth duration on the surface morphology and crystallinity were investigated. The major surface features of the grown film like terracing, inclusions, meniscus lines, etc are presented step-by-step and a variety of methods devised to overcome such undesirable features are described in sufficient detail. The optimization of growth parameters have led to the growth of smooth and continuous films. From the detailed morphological, X-ray diffraction, scanning electron microscopic and Raman studies, a correlation between the surface morphology and crystallinity has been established.(Indian Academy of Sciences, Influence of growth parameters on the surface morphology and crystallinity of InSb epilayers grown by liquid phase epitaxy) Udayashankar, N.K.; Bhat, H.L.2003Item Effect of heat treatment on pitting corrosion resistance of 6061 Al/SiCP composite coated by the cerium oxide film in 3.5 N NaCl solution(2011) Rajasekaran, S.; Udayashankar, N.K.; Nayak, J.One of the main drawbacks of 6061 Al/SiCP composite is its poor pitting corrosion resistance in the aggressive environment containing chloride ions, such as seawater, for example. The present article deals with the investigations of effects of aging on the corrosion behavior of 6061 Al/SiCP composite and of the heat treatment on the pitting corrosion resistance of 6061 Al/SiCP composite coated by cerium oxide prepared by chemical bath technique. Potentiodynamic polarization test was used to study the corrosion behavior of cerium oxide coatings in 3. 5N NaCl solution. The microstructure of cerium oxide was examined by scanning electron microscopy (SEM) and the formed phases were identified by X-ray diffraction (XRD). The pitting corrosion resistance of the cerium oxide coating was found to be improved after heat treatment at 300°C for 30 min. © 2011 Allerton Press, Inc.Item Fabrication and high-temperature structural characterization study of porous anodic alumina membranes(2012) Choudhari, K.S.; Sudheendra, P.; Udayashankar, N.K.Porous anodic alumina (PAA) membranes with highly ordered array of nanopores were prepared by twostep anodization process. Studies on structural and thermal properties and the thermal stability of the prepared PAA membranes were carried out. Investigation using scanning electron microscopy, atomic force microscopy, X-ray diffraction, thermal analysis and infrared spectroscopy was performed on the prepared PAA membranes at room temperature and in the temperature range 600-1,400 °C. The as-prepared PAA membranes revealed the amorphous nature. Polycrystalline PAA membranes were obtained by annealing carried out at different temperatures. Annealing study confirmed that the heat treatment transformed the amorphous PAA membranes to their crystalline phases, namely, ?-alumina at about 870 °C and then to ?-alumina around 1,250 °C. © Springer Science+Business Media, LLC 2011.Item Memory type switching behavior of ternary Ge20Te80-xSnx (0 x 4) chalcogenide compounds(Institute of Physics Publishing michael.roberts@iop.org, 2016) Fernandes, B.J.; Sridharan, K.; Pumlianmunga, P.; Ramesh, K.; Udayashankar, N.K.Chalcogenide compounds have gained huge research interest recently owing to their capability to transform from an amorphous to a crystalline phase with varying electrical properties. Such materials can be applied in building a new class of memories, such as phase-change memory and programmable metallization cells. Here we report the memory type electrical switching behavior of a ternary chalcogenide compound synthesized by doping Tin (Sn) in a germanium-telluride (Ge20Te80) host matrix, which yielded a composition of Ge20Te80-xSnx (0 x 4). Results indicate a remarkable decrease in the threshold switching voltage (V T) from 140 to 61 V when the Sn concentration was increased stepwise, which is attributed to the domination of the metallicity factor leading to reduced amorphous network connectivity and rigidity. Variation in the threshold switching voltage (V T) was noticed even when the sample thickness and temperature were altered, confirming that the memory switching process is of thermal origin. Investigations using x-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the formation of a crystalline channel that acts as the conduction path between the two electrodes in the switched region. Structural and morphological studies indicated that Sn metal remained as a micro inclusion in the matrix and hardly contributed to the rigid amorphous network formation in Ge20Te80-xSnx. Memory type electrical switching observed in these ternary chalcogenide compounds synthesized herein can be explored further for the fabrication of phase-change memory devices. © 2016 IOP Publishing Ltd.Item The effective role of alkali earth/alkali ratio on formation HCA nano particles for soda lime phospho silicate glass system(Elsevier Inc, 2017) Kiran, P.; Ramakrishna, V.; Udayashankar, N.K.; Shashikala, H.D.In the present work, we report on the effect of the CaO/Na2O ratio on Non-Bridging Oxygen/Bridging Oxygen (NBO/BO) ratio for sol-gel synthesised 58SiO2-(38-x)CaO-xNa2O-4P2O5 glasses and establish the correlation between Hydroxy Carbonated Apatite layer (HCA) forming ability and the dissolution behaviour in simulated body fluid (SBF) solution. Thermal stabilities were calculated as 221, 135, 153 and 77 °C respectively. It is inferred that thermal stabilities varied nonlinearly with CaO/Na2O ratio. Similarly NBO/BO ratios were obtained using Raman spectroscopic analysis as 0.6293, 0.7917, 0.2264 and 0.3513, respectively. All samples were soaked in the SBF solution for 7 days. The calculated weight losses of these samples were 55.32, 69.13, 18.09 and 20.08 for the corresponding NBO/BO ratios. The decrease in CaO/ Na2O ratio led to nonlinear variation of the NBO/BO ratios. Consequently the non linear variation in NBO/BO ratio led to the nonlinear variation of HCA forming ability of SBF treated samples. © 2017 The AuthorsItem Influence of annealing temperature on the structural, morphological, mechanical and surface properties of near equiatomic NiTi thin films(Elsevier Ltd, 2017) Reddy, B.; Udayashankar, N.K.In this work, we report our studies on near equiatomic NiTi thin films grown on silicon (1 0 0) substrates deposited without intentional heating of the substrate by DC magnetron sputtering technique with separate elemental targets Ni and Ti. The films were annealed for one hour at various temperatures, i.e. 350, 450, 550 and 650 °C. X-ray diffraction (XRD) studies revealed that the degree of crystallinity increases up to 550 °C but decreases at 650 °C. The surface roughness value Rq, Ra and Rmax have been obtained by Atomic Force Microscopy (AFM). The hardness and elastic modulus values increase up to 550 °C and decreases at 650 °C. High Resolution X-ray Photoelectron Spectroscopy (HR-XPS) studies revealed that the annealed near equiatomic NiTi thin films have a strong tendency to form TiO2 (metal oxide) layer onto the film surface. This is due to the higher thermodynamic reactivity of Ti by leaving Ni-rich matrix trapped behind, most likely in the sub layers of NiTi along with Titanium carbide (Ti-C) and Silicon carbide (Si-C) precipitates. © 2017 Elsevier LtdItem Synthesis, structural, optical and electrical (DC) properties of a semiorganic Thiourea Barium Chloride (TBC) single crystal(Elsevier GmbH journals@elsevier.com, 2017) Mahendra, K.; D'Souza, A.; Udayashankar, N.K.In the present study, TBC single crystals were synthesized using solution evaporation method. The structural analysis of TBC crystal was studied using Powder X-ray diffraction. Optical properties of TBC crystals were carried out using UV–vis, FTIR, Raman and photoluminescence (PL) spectra. The crystal shows absorbance maxima at 209 nm and transparent in the visible region 300–900 nm. The various optical constants such as refractive index, reflectance, speed of light, extinction coefficient, electrical susceptibility, dielectric constant, optical and electrical conductivity were evaluated. Thus results revealed that all these optical constants shows a strong dependence on optical absorption coefficient. Using single oscillator model (Wemple ?Didomenico), lattice dielectric constant and the ratio of free charge carrier to their effective mass were evaluated. The PL study of TBC crystals shows two emission peaks (419 nm ?S, 441 nm- Ba) in blue region. The DC resistivity and conductivity of the crystal was investigated in the temperature range 26 °C–115 °C. © 2017 Elsevier GmbHItem Enhanced structural, optical, thermal, mechanical and electrical properties by a noval approach (nanoparticle doping) on ferroelectric triglycine sulphate single crystal(Springer Verlag service@springer.de, 2019) Mahendra, K.; Kumar, H.K.T.; Udayashankar, N.K.The pristine and AgNP-doped TGS crystals are studied using powder X-ray diffraction, density measurements, solubility studies, UV–Vis analysis, photoluminescence spectroscopic analysis, thermal gravimetric analysis, differential thermal analysis, differential scanning calorimetry, Vicker’s hardness measurements, I–V and impedance measurements. From the XRD studies, TGS crystal formation is confirmed and by further analyzing X-ray diffraction data it is noticed that the incorporation of nanoparticles induce stress in the lattice of TGS crystal, which, in turn, lead to shift in peak positions. Crystal solubility and density values are increased after doping process. From the emission spectrum it is evident that emission intensity increases as the doping concentration increased. The melting point and mechanical hardness of the crystals also showed improvement after AgNP doping. Band gap calculated by Tauc’s relation is found to decrease with increase in doping concentration. Further, electrical studies demonstrated that crystal conductivity is improved as the doping concentration increases. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.Item Modulations of physio-chemical and electronic properties of metalorganic KHO single crystals through Co(OH)2 nanoparticles doping(Springer New York LLC barbara.b.bertram@gsk.com, 2019) Mahendra, K.; Bhat, K.S.; Nagaraja, H.S.; Udayashankar, N.K.Semiorganic crystals play vital role in the design and development of optical devices. In this context, we report the synthesis of metalorganic potassium hydrogen oxalate oxalic acid dihydrate (KHO) single crystals using slow solvent evaporation technique. Further, Co(OH)2 nanoparticles pre-synthesized using hydrothermal method are doped to KHO single crystals. Doping of Co(OH)2 nanoparticles in the crystal system were confirmed through XRD, EDAX and PL measurements. The XRD measurements indicate a shift in the peak positions and variation in the overall intensities. On the other hand, PL measurements also indicate enhanced emission peaks confirming the successful doping of Co(OH)2 nanoparticles in the KHO system. As the effect of doping, the declination in the optical bandgap and improvement of electronic conductivity were also observed. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.Item Role of soaking time on the phase evolution of Cu2ZnSnS4 polycrystals synthesized using melting route for photovoltaic applications(Elsevier Ltd, 2019) Choudhari, N.J.; Raviprakash, Y.; Fernandes, B.J.; Udayashankar, N.K.Cu2ZnSnS4(CZTS) is an emerging quaternary semiconductor material to use as absorber layer for solar cells due its suitable band gap, high absorption coefficient, earth abundancy and less toxic nature. This work provides a comprehensive insight into the phase evolution of CZTS synthesized at a relatively lower process time. In this study, CZTS bulk polycrystals were synthesized using elemental pre cursors via melting route. The influence of soaking time on the structural, compositional and optical properties were investigated using XRD, EDS, Raman, DRS, PL and XPS measurements. XRD pattern revealed a highly crystalline tetragonal structure corresponding to kesterite phase. EDS mapping were performed over a large area of the sample revealed homogeneous distribution and near stoichiometric composition for the sample soaked for 14 h (S14). Raman spectra confirmed the existence of single phase CZTS without any secondary and ternary phases for S14. Diffuse reflectance spectroscopy gave a band gap value in the range 1.34–1.39 eV. PL analysis revealed that asymmetric band shape and higher energy shift is the characteristics of radiative transitions which are influenced by fluctuating potentials. XPS studies confirmed the oxidation states as Cu(I), Zn(II), Sn(IV) and S(II). © 2019 Elsevier B.V.