Faculty Publications
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Item Investigation of charge transport in Vanadyl-phthalocyanine with molybdenum trioxide as a buffer layer: Impedance spectroscopic analysis(Elsevier Ltd, 2015) Raveendra Kiran, M.R.; Ulla, H.; Krishnamanohara; Satyanarayan, M.N.; Umesh, G.Charge transport in organic materials is one of the intrinsic properties, which governs the device performance. In this paper, we report the fabrication and electrical characterization of two diodes ITO/VOPc/MoO3/Al and ITO/VOPc/Al. We investigate the electrical conduction of Vanadyl phthalocyanine (VOPc) in both the devices and also the effect of MoO3 as a buffer layer. Improvement of current density through the device is estimated using current density - voltage characteristics and capacitance - voltage characteristics. Space charge limited current (SCLC) conduction with an exponential trap distribution is observed from Impedance measurements. The dominant hopping charge transport is discussed based on ac conductivity measurements and by adopting Correlated barrier hopping (CBH) model. © 2015 Elsevier B.V. All rights reserved.Item Effect of deposition rate on the charge transport in Vanadyl-phthalocyanine thin films(Elsevier Ltd, 2017) Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.We report fabrication of Vanadyl phthalocyanine (VOPc) based diodes with different deposition rates (0.1, 1 and 5 Å/s) in hole only device configuration: ITO/MoO3/VOPc/MoO3/Al. The dc and ac electrical conductivity of Vanadyl phthalocyanine based devices is investigated by employing Impedance spectroscopy measurements. The frequency dependence of conductivity indicates that the dominant mechanism for charge transport is the hopping type. Further, the dependence of conductivity on temperature and bias voltage clearly indicates that the hopping mechanism is described by the correlated barrier hopping (CBH) model. The thin layer (3 nm) of MoO3 in our devices is seen to enhance the electrical conductivity. J-V measurements indicate that the current density J as well as the charge carrier mobility are higher for the devices fabricated at a relatively lower deposition rate (0.1 Å/s). Our results suggest that the VOPC films deposited at lower rates are more appropriate for the optoelectronic device applications. © 2016 Elsevier B.V.Item Melt quenched vanadium oxide embedded in graphene oxide sheets as composite electrodes for amperometric dopamine sensing and lithium ion battery applications(Elsevier B.V., 2017) Moolayadukkam, M.; Shenoy, S.; Sridharan, K.; Kufian, D.; Arof, A.K.; Nagaraja, H.S.Electrochemical sensors and lithium-ion batteries are two important topics in electrochemistry that have attracted much attention owing to their extensive applications in enzyme-free biosensors and portable electronic devices. Herein, we report a simple hydrothermal approach for synthesizing composites of melt quenched vanadium oxide embedded on graphene oxide of equal proportion (MVGO50) for the fabrication of electrodes for nonenzymatic amperometic dopamine sensor and lithium-ion battery applications. The sensing performance of MVGO50 electrodes through chronoamperometry studies in 0.1 M PBS solution (at pH 7) over a wide range of dopamine concentration exhibited a highest sensitivity of 25.02 ?A mM ?1 cm ?2 with the lowest detection limit of 0.07 ?M. In addition, the selective sensing capability of MVGO50 was also tested through chronoamperometry studies by the addition of a very small concentration of dopamine (10 ?M) in the presence of a fairly higher concentration of uric acid (10 mM) as the interfering species. Furthermore, the reversible lithium cycling properties of MVGO50 are evaluated by galvanostatic charge-discharge cycling studies. MVGO50 electrodes exhibited enhanced rate capacity of up to 200 mAhg ?1 at a current of 0.1C rate and remained stable during cycling. These results indicate that MVGO composites are potential candidates for electrochemical device applications. © 2017 Elsevier B.V.Item Optoelectronic properties of hybrid diodes based on vanadyl-phthalocyanine and zinc oxide(Academic Press, 2017) Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.We report an investigation of the optoelectronic properties of a hybrid p-n diode device fabricated using ZnO film prepared by sol-gel technique on which a VOPc organic film is deposited by vacuum evaporation. The charge transport properties of devices having the configurations ITO/ZnO/Al and ITO/ZnO/VOPc/MoO3/Al were investigated at different annealing temperatures (150 °C, 250 °C, 350 °C and 450 °C) by Impedance Spectroscopy (IS). The structural, morphological, optical and electrical properties were also studied at different annealing temperatures. The parameters related to the ITO/ZnO and ZnO/VOPc interfaces such as ideality factor (n), barrier height (q?B) and rectification ratio (RR) of the diodes were determined from current density-voltage (J-V) characteristics. IS measurements suggest that the large photocurrent generated is due to the decrease in bulk resistance of the device on account of the generation of electron-hole pairs in the organic active layer when exposed to light. The RR and the photocurrent responsivity (Rph) values obtained from the J-V characteristics compare well with those obtained from the IS measurements. It was observed that the absolute value of Rph (470 mA/W) for the p-n diode with ZnO annealed at 350 °C is high compared to that of diodes with different ZnO annealing temperatures. These values also agree well with the values obtained for p-n diodes of other phthalocyanines. Our studies clearly demonstrate that a p-n diode with ZnO film annealed at 350 °C exhibits much better optoelectronic characteristics on account of increased grain size, improved charge injection due to the reduction of barrier height and hence higher (up to 5 orders) charge carrier mobility. © 2017 Elsevier LtdItem Effect of annealing-temperature-assisted phase evolution on conductivity of solution combustion processed calcium vanadium oxide films(Springer, 2018) Manjunath, G.; Vardhan, R.V.; Salian, A.; Jagannatha, R.; Kedia, M.; Mandal, S.In thiswork, the effect of annealing temperature on the conductivity of solution-combustion-synthesized calcium vanadium oxide (CVO) films was studied. Conductivity was tailored by the appearance of the phases like CaVO3, CaV2O5 and Ca2V2O7 as a function of annealing temperature; CaVO3 and CaV2O5 are responsible for high conductivity, whereas V5+ presence in Ca2V2O7 contributes towards dielectric nature. Evolution of phases of CVO was identified through X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. A detailed conductivity measurement as a function of annealing temperature helps us to identify the decreasing trend of conductivity with increasing temperature up to 400°C; beyond this it behaves like an insulator. There was a stable conductivity while aging the films in ambient for a few days. This study revealed safe application temperature domain of CVO, and a clear correlation of electrical conductivity with the in-depth structural-compositional-morphological study. © Indian Academy of Sciences.Item Optoelectronic properties of hybrid diodes based on vanadyl- phthalocyanine and zinc oxide nanorods thin films(Elsevier B.V., 2019) Raveendra Kiran, M.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.Herein, we report the optoelectronic properties of hybrid diodes fabricated using vanadyl phthalocyanine (VOPc) and zinc oxide nanorods (ZNR) with the configuration: ITO/ZNR/VOPc/MoO3/Al. Vertically aligned ZnO nanorods were grown using a simple aqueous solution (AS) method as a function of growth temperature. The correlation between the morphology of ZNR films and the optoelectronic properties of the ZNR/VOPc hybrid devices was investigated. The results show that the hybrid diodes with ZNR films grown at 120 °C offer the best optoelectronic properties. The higher photocurrent responsivity, Rph, (16.28 A/W) was achieved for devices with ZNR films grown at 120 °C. This value is 25 times higher than the Rph value obtained for the devices made with ZnO nanoparticle films that were reported earlier. © 2019Item Observation of resistance switching in Vanadyl-phthalocyanine thin films(Elsevier Ltd, 2020) Raveendra Kiran, M.R.; Ulla, H.; Krishnamanohara; Satyanarayan, M.N.; Umesh, G.Herein, we report the first observation of Negative differential resistance (NDR) associated with resistance switching in Vanadyl-phthalocyanine (VOPc) based devices with the configuration: ITO/F4TCNQ/VOPc/MoO3/Al. It was observed that the devices were initially at low resistance ON state (LS) and were switched to high resistance OFF state (HS) at sufficient applied bias. The NDR behaviour was observed during the initial sweep for each device (often referred to as the writing process). The ON/OFF state transition was attributed to the formation and neutralization of interface dipoles at the ITO/VOPc interface. Finally, the observed non-volatile RS switching behaviour was demonstrated employing impedance spectroscopic studies. This study opens up the potential applications of VOPc Resistance Switching devices in security and data protection applications. © 2020 Elsevier B.V.Item Supercapacitor activity studies of a unique triangular oxo-vanadate-bisphosphonate composite with activated carbon(Springer Science and Business Media Deutschland GmbH, 2022) Thakre, D.; Anandan Vannathan, A.; Banerjee, A.; Mal, S.S.Electrochemical studies have been performed on a hybrid inorganic–organic oxo-vanadate (IV)-bisphosphonate assembly, viz. (NH4)2[H6(VIVO)3(O){O3P-C(OH)(CH2-4-C5NH4)-PO3}3]•26H2O [V3-BP]. The compound has a triangular shaped structure consisting of reduced vanadium (IV) with 1-hydroxo-2-(4-pyridyl) ethylidene-1,1-bisphosphonic acid. Composites of this compound were prepared with activated carbon (AC), viz. AC-V3-BP, and extensively characterized using various analytical techniques (e.g., FT-IR, TGA, P-XRD, FESEM, EDX, and BET) to study the interaction of V3-BP with the activated carbon support. Electrochemical properties of AC-V3-BP were studied, vis-à-vis its supercapacitor behavior, with the electrochemical performance of the composite AC-V3-BP material tested in 0.1 (M) H2SO4 aqueous electrolytic solution. The composite exhibits a specific capacitance of 140.8 F g−1 with a specific energy of 19.56 W kg−1. With such interesting capacitive properties, electrochemical cells prepared with this material were observed to light up red and blue LED bulbs for 90 and 20 s, respectively, suggesting a substantial power density of the materials. The respective electrode prepared from the composite was also observed to retain 79.23% of the electrode stability after 4500 cycles. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.Item Growth optimization and DFT investigation of doping effect on properties of VS2 monolayer crystals(Springer Science and Business Media Deutschland GmbH, 2023) Yadav, A.K.; Patel, C.; Kiran, G.; Singh, R.; Singh, A.K.; Garg, V.; Mukherjee, S.; Pandey, S.K.The vanadium disulfide (VS2) material, a prominent member of the two-dimensional materials family, has great potential to bridge the performance gap between current performance and contemporary energy storage device needs. Here, we report the optimization of the growth temperature of VS2 monolayer crystals using a chemical vapor deposition system. It is also found the crystal size increases with the increase of growth temperature up to 770 °C. Further increasing of growth temperature resulted in a reduction of crystal size. The atomic force microscopy measurement demonstrated the growth of monolayer thick VS2 crystal. Raman spectra revealed the formation of H-phase monolayer high-quality VS2 crystals. To understand the precise impact of doping on electronic properties, the substitutional doping of VS2 monolayer with chromium, molybdenum, and tungsten was also examined using density functional theory. The VS2 monolayer exhibits an indirect energy band gap that decreases after chromium doping of the VS2 lattice and vanishes after molybdenum and tungsten doping. Finally, it is found that tungsten-doped VS2 monolayer exhibits strong metallic character and other exceptional properties, making it suitable for electrodes of various energy storage devices. Graphical abstract: [Figure not available: see fulltext.]. © 2023, The Author(s), under exclusive licence to EDP Sciences, SIF and Springer-Verlag GmbH Germany, part of Springer Nature.Item Effect of Introducing Defects and Doping on Different Properties of Monolayer MoS2(John Wiley and Sons Inc, 2023) Prajakta, K.; Vinturaj, V.P.; Singh, R.; Garg, V.; Pandey, S.K.; Pandey, S.K.Herein, the comprehensive study of different properties of undoped MoS2, MoS2 lattice with sulfur (S) and, molybdenum (Mo) vacancy, and MoS2 with substitutional doping of niobium (Nb), vanadium (V), and zinc (Zn) atoms is done. The density functional theory (DFT) is used and the electronic properties like density of states, band structure, electron density, and optical properties like dielectric function, optical conductivity, and refractive index are studied. It is observed that undoped MoS2 monolayer shows direct bandgap semiconductor characteristics with a bandgap of around 1.79 eV. P-type characteristics are observed for Nb-, V-, and Zn-doped MoS2 lattices. The real part and imaginary parts of all optical parameters along x and z directions for different MoS2 supercells are found to be anisotropic in nature up to a photon energy of almost 11 eV and thereafter they show nearly isotropic nature. Finally, it is found that the obtained properties of MoS2 monolayer as per literature are suitable for next-generation MOSFET application. © 2023 Wiley-VCH GmbH.