Faculty Publications
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Item The effect of substrate temperature on the structural, optical and electrical properties of vacuum deposited ZnTe thin films(2009) Rao, G.K.; Bangera, K.V.; Shivakumar, G.K.The present paper reports the effect of substrate temperature on the structural, optical and electrical properties of vacuum deposited zinc telluride (ZnTe) thin films. X-ray diffraction (XRD) analysis of the films, deposited on glass substrates, revealed that they have cubic structure with strong (111) texture. Room temperature deposits are tellurium rich and an increase in the substrate temperature up to 553 °K results in stoichiometric films. Electrical conductivity has been observed to increase with the increase in substrate temperature, accompanied by increase in the carrier concentration and the mobility of the carriers. The optical bandgap energy and the thermal activation energy of the films have also been evaluated. © 2009 Elsevier Ltd. All rights reserved.Item Electrical switching behavior of bulk Si15Te85-xSbx chalcogenide glasses - A study of compositional dependence(2010) Lokesh, R.; Udayashankar, N.K.; Asokan, S.Studies on the electrical switching behavior of melt quenched bulk Si15Te85-xSbx glasses have been undertaken in the composition range (1 ? x ? 10), in order to understand the effect of Sb addition on the electrical switching behavior of Si15Te85-x base glass. It has been observed that all the Si15Te85-xSbx glasses studied exhibit a smooth memory type switching. Further, the switching voltages are found to decrease almost linearly with Sb content, which indicates that the metallicity of the dopant plays a dominant role in this system compared to network connectivity/rigidity. The thickness dependence of switching voltage (Vth) indicates a clear thermal origin for the switching mechanism. The temperature variation of switching voltages reveals that the Si15Te85-xSbx glasses studied have a moderate thermal stability. © 2009 Elsevier B.V. All rights reserved.Item The p-type doping of vacuum deposited ZnTe thin films with bismuth by a new technique of using nano-spheres(Elsevier Ltd, 2010) Rao, K.G.; Shivakumar, G.K.; Kasturi, V.B.The present paper reports the successful doping of vacuum evaporated zinc telluride (ZnTe) thin films with bismuth by a new technique of using nano-spheres. The discontinuous films of bismuth (the dopant material), containing bismuth in the form of nano-spheres, were prepared by vacuum evaporation and the ZnTe films were then deposited on top of them. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were used to ascertain the formation of discontinuous bismuth films and the proper diffusion of bismuth in ZnTe films, respectively. After doping, the carrier concentration of the ZnTe films was found to increase by an order of the magnitude. The electrical conductivity also improved significantly. The photoconductivity and photo-response properties of the doped films were also analysed. © 2010 Elsevier B.V. All rights reserved.Item Porous nickel telluride nanostructures as bifunctional electrocatalyst towards hydrogen and oxygen evolution reaction(Elsevier Ltd, 2017) Bhat, K.S.; Barshilia, H.C.; Nagaraja, H.S.Electrochemical water splitting technology has attracted researchers for the development of next generation fuels. Herein, we report the synthesis of nanostructured porous hollow nickel telluride nanosheets and their use as bifunctional electrocatalyst towards hydrogen and oxygen evolution reaction, anticipating an enhanced performance owing to their 2D sheet like morphology, conductivity, porous nature providing larger catalytic surface for water splitting reaction. In this regard, nickel telluride nanostructures were synthesized via an anion-exchange-reaction between pre-synthesized nickel hydroxide hexagonal nanosheets and tellurium ions under hydrothermal conditions. The as-synthesized nanostructures were characterized for structural, morphological and compositional properties using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Nickel telluride modified electrodes were tested as bifunctional electrocatalyst under acidic and alkaline conditions, through linear sweep voltammetry and constant current chronopotentiometry methods. The modified electrodes revealed an onset potential of ?422 mV and 87.4 mV dec?1 Tafel slope towards HER and overpotential of 679 mV and 151 mV dec?1 Tafel slope towards OER. The lower onset potentials are complimented with excellent electrocatalytic stability. © 2017 Hydrogen Energy Publications LLCItem Porous cobalt chalcogenide nanostructures as high performance pseudo-capacitor electrodes(Elsevier Ltd, 2017) Bhat, K.S.; Shenoy, S.; Nagaraja, H.S.; Sridharan, K.Electrochemical supercapacitor is an essential technology that is pivotal for the development of reliable energy storage devices. Herein, we report the fabrication of supercapacitor electrodes using nanostructured porous cobalt chalcogenide (CoTe2 and CoSe2) electrodes, anticipating an enhanced performance owing to their higher contact area with electrolyte and large pore volume enabling shorter diffusion paths for ion exchange. In this regard, we synthesized CoTe2 and CoSe2 nanostructures via an anion-exchange-reaction between pre-synthesized Co(OH)2 hexagonal nanosheets and chalcogen (tellurium and selenium) ions under hydrothermal conditions. Structural, morphological and compositional properties of the as-synthesized materials are examined using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. Pseudo-capacitive properties of CoTe2 and CoSe2 nanostructures as working electrodes are studied through cyclic voltammetry and galvanostatic charge-discharge methods using an electrochemical workstation. CoSe2 electrode delivered a specific capacitance of 951 F g?1 at a scan rate of 5 mV s?1, which surprisingly is almost three times higher in comparison to CoTe2 electrode (360 F g?1). Both CoTe2 and CoSe2 electrodes exhibited good capacitance retention capability for 2500 CV cycles. The superior electrochemical performance of the nanoporous CoSe2 electrode indicate their applicability for high-performance energy storage device applications. © 2017 Elsevier LtdItem Electrical switching in Si20Te80 ? xBix (0 ? x ? 3) chalcogenide glassy alloys(Elsevier B.V., 2018) Fernandes, B.J.; Ramesh, K.; Udayashankar, N.K.Chalcogenide glasses have attained enormous research interest due to their importance in finding electronic memories. Here we report electrical switching and thermal crystallization behavior of Si20Te80 ? xBix (0 ? x ? 3) glasses. We observe a significant decrease in the threshold voltage (VT) and the thermal stability (?T), indicating that in Si20Te80 ? xBix glasses, the resistivity of the additive element Bi plays a dominant role over network connectivity/rigidity. The variation of VT with respect to thickness and temperature of the sample indicates that the memory switching observed in Si20Te80 ? xBix glasses is influenced by the thermally induced transitions (thermal mechanism). Scanning electron microscopy (SEM) studies on pre-switched and post switched samples reveal the morphological changes on the surface of the sample, and serve as an experimental evidence for the formation of the crystalline filament between two electrodes during switching. Furthermore, the decrease in ?T values indicates that the Si-Te glasses become de-vitrifiable more easily with the addition of Bi, influencing the decrease of VT. Structural evaluation like thermal devitrification studies and morphological changes elucidate the restricted glass formability of the studied glass system. © 2018 Elsevier B.V.Item Electronic structure engineering of tin telluride through co-doping of bismuth and indium for high performance thermoelectrics: A synergistic effect leading to a record high room temperature ZT in tin telluride(Royal Society of Chemistry, 2019) Shenoy, U.S.; Bhat, D.K.The ever increasing demand for alternative clean energy sources has led to intense research towards the optimization of thermoelectric performance of known systems. In this work, we engineer the electronic structure of SnTe by co-doping it with Bi and In. The co-doping not only results in the formation of two different resonance states and a reduced valence band offset, as in the case of previously reported co-doped SnTe, but also leads to opening of the band gap, which otherwise was closed in the case of Bi and In doped SnTe configurations, leading to suppression of bipolar diffusion. The synergistic action of all these effects leads to an increased Seebeck co-efficient throughout the temperature range and a ZTmax of ?1.32 at 840 K. This strategy of co-doping two different resonant dopants resulted in a record high room temperature ZT of ?0.25 at 300 K for SnTe based materials. This work suggests that appropriate combination of dopants to engineer the electronic structure of a material can lead to unpredictable results. © 2019 The Royal Society of Chemistry.Item Enhanced thermoelectric power of Al and Sb doped In 2 Te 3 thin films(Elsevier Ltd, 2019) Vallem, V.; Bangera, K.V.; G.k, S.Aluminium and antimony are used as dopants for In 2 Te 3 to study their influence on the thermoelectric power of the films. Both aluminium and antimony are expected to replace indium in the film and contribute to the structural, electrical and thermoelectric behaviour of indium telluride. It is observed that addition of both Al and Sb dopants induced an additional phase of free Te. The electrical conductivity of In 2 Te 3 films is observed to reduce with Al doping and increase with Sb doping when compared with the electrical conductivity of un-doped films. The thermoelectric power is found to be maximum for 2.1% Al doped and 1% Sb doped films. Moreover, the thermoelectric power factor of In 2 Te 3 films is found to be enhanced 3.1 times for 2.1% doping of Al and 8.7 times for 1% Sb doping. © 2019 Elsevier LtdItem Synthesis of single-phase stoichiometric InTe thin films for opto-electronic applications(Academic Press, 2019) Vallem, V.; Bangera, K.V.; G.k, S.Mono-phased and stoichiometric InTe thin films were successfully prepared using vacuum evaporation technique. A systematic variation in substrate temperature and annealing temperature along with annealing duration resulted stoichiometric and single phase InTe films. The annealing treatment of as-deposited films resulted in the structural transformation from mixed phase of In 2 Te 3 and InTe to mono-phased InTe. The electrical conductivity of stoichiometric single phase films was found to be 15.612 ? ?1 cm ?1 . The optical band gap of stoichiometric InTe films was found to be 1.42 eV and absorption coefficient of the films was of the order of 10 6 cm ?1 . Electrical properties of mono-phased films accompanied with optical properties such as direct band gap and absorption coefficient makes them suitable for optoelectronic devices. © 2019 Elsevier LtdItem Performance evaluation of molybdenum dichalcogenide (MoX2; X= S, Se, Te) nanostructures for hydrogen evolution reaction(Elsevier Ltd, 2019) Bhat, K.S.; Nagaraja, H.S.Hydrogen evolution reaction (HER) using transition metal dichalcogenides (TMDs) have gained interest owing to their low-cost, abundancy and predominant conductivity. However, forthright comparisons of transition metal chalcogenides for HER are scarcely conducted. In this work, we report the synthesis of series of molybdenum chalcogenide nanostructures MoX2 (X = S, Se, Te) via a facile hydrothermal method. Used as an electrocatalyst for HER, MoS2 nanograins, MoSe2 nanoflowers and MoTe2 nanotubes could afford the benchmark current densities of 10 mA cm?2 at the overpotentials of ?173 mV, ?208 mV and ?283 mV with the measured Tafel slope values of 109.81 mV dec?1, 65.92 mV dec?1 and 102.06 mV dec?1, respectively. Besides other factors influencing HER, the role of electronic conductivity in HER of these molybdenum dichalcogenides are elucidated. In addition, the presented molybdenum dichalcogenides in this work are also complimented with robustness as determined from high-current density stability measurements. © 2019 Hydrogen Energy Publications LLC