BITS Faculty Publications
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Item Formation of all tin oxide p–n junctions (SNO–SNO2) during thermal oxidation of thin sn films(Wiley, 2024-12) Hazra, Arnab; Gangopadhyay, SubhashisMetastable stannous oxide (SnO) phase of p-type semiconductor and all tin oxides p–n junctions of SnO–SnO2 nanostructures are formed by controlled thermal oxidation of thin tin films. High purity Sn is deposited on quartz substrates using a vacuum-assisted thermal evaporation technique. Afterwards, controlled thermal oxidation at different temperatures is performed in air ambient condition (150–800 °C). Various surface characterization techniques have been employed to analyze the structure, morphology, chemistry, optical, and electronic properties of these SnOx films. P-type SnO phase is found to be thermodynamically stable at lower oxidation temperatures (250–400 °C), while n-type SnO2 phase starts to appear above 500 °C. Highly uniform and dense SnO nanospheres along with few 1D nanorods are observed after oxidation at 400 °C. Mixed oxide phases of p–n junctions with a sudden decrease in electrical conductivity is observed for 500 °C film. Significantly lower surface conductivity of mixed oxide phase indicates the formation of depletion layers between p-type SnO and n-type SnO2 nanograins. A transition from SnO layer to SnO2 layer is also observed above 600 °C. Overall, SnOx-based nanostructures would be a potential candidate for solar cells, p-channel thin film transistors, p–n junction diodes and gas sensors.Item Electron beam deposited thin titanium films and its thermal oxidation to form rutile TiO2 thin films(AIP, 2024) Gangopadhyay, SubhashisSmooth and homogeneous titanium (Ti) thin films are formed on quartz substrate using a vacuum assisted electron beam evaporation technique. Afterwards, controlled thermal oxidation of these Ti films are performed to grow a uniform titanium dioxide (TiO2) layer. Structural, morphological, chemical and optical properties of these metal and oxide layers have been investigated using various surface characterization techniques such as x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and UV-Vis spectroscopy. Formation of rutile TiO2 phase is confirmed from the XRD and Raman spectroscopy, after thermal oxidation above 400°C. SEM imaging suggests the formation of a smooth and homogeneous Ti as well as TiO2 layers which appear with a nanometer scale granular surface morphology. All findings are explained in terms of surface thermodynamics and chemical reactivity.Item Growth and Characterization of ZnO Nanostructures: Materials for CO and Ethanol Sensing(Springer, 2021) Hazra, Arnab; Choudhary, Sumita; Gangopadhyay, SubhashisControlled growth of ZnO-based nanostructures, starting from a vertical nanowall surface morphology to laterally grown highly anisotropic nanorods/wires formation has successfully been achieved by controlled thermal oxidation of thin Zn films for a temperature range of 100–700 °C. The as-grown ZnO nanorods were further used for carbon monoxide gas sensing at low temperatures (down to 150 °C) as well as ethanol vapour sensing at room temperatures. Thin films of Zn were deposited on glass and silicon substrate at room temperature, using a vacuum-assisted thermal evaporation technique. Structure, morphology and chemical property of ZnO layers were investigated using various surface characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and Raman spectroscopy. The XRD and SEM results are in very good correlation and showed vertical growth morphology of ZnO nanowall/sheet structures at a relatively lower oxidation temperature up to 400 °C. However, at higher oxidation temperature, lateral growths started to dominate over the vertical growth. Oxidation at 700 °C appeared with laterally grown one-dimensional (1D) ZnO nanowires/rods of high density. Raman spectroscopy and XPS results suggested that the vertical growth is mainly initiated by the metallic Zn film morphology, whereas the lateral growth is strongly dominated by the oxide (ZnO) formation. Finally, laterally grown ZnO nanorods could successfully sense CO gas and ethanol vapour. A drastic enhancement in CO gas sensitivity for a concentration of 230 ppm was clearly observed in dynamic gas flow mode even for a wide range of operating temperature.Item Highly selective formaldehyde sensing using ZnO nano-rods(AIP, 2023-03) Choudhary, Sumita; Hazra, Arnab; Gangopadhyay, SubhashisEarly detection of formaldehyde emission from any household materials is technologically very demanding as it can be a serious human health hazard. Even indirect inhaling of formaldehyde may cause significant harm to our eyes, skin, mouth or any other organs. Hence, fabrication of a simple and sensitive formaldehyde sensor would be of high practical importance. Within this work, formation of ZnO nano-rods by controlled thermal oxidation of vacuum deposited thin Zn films in air ambient, followed by fabrication of formaldehyde sensor operating at relatively lower operating temperature are reported. The crystal structure, surface morphology and optical properties of the as grown ZnO nano-rods have been investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Raman spectroscopy, respectively. The XRD patterns of ZnO suggested the formation of highly crystalline oxide films whereas FESEM images have revealed its nano-rods surface morphology with significantly high (length to diameter) aspect ratio. Raman spectroscopy confirms the thermal oxidation of the Zn thin films. As-grown ZnO nano-rods were then subsequently used to fabricate the chemi-resistive formaldehyde sensors. These sensors showed an extremely high formaldehyde sensing performance at a relatively lower operating temperature of 200°C. In a static measurement mode, the sensor exhibited a gas response of about 53% toward 100 ppm of formaldehyde, with a reasonable fast response and recovery time. Moreover, these ZnO nano-rod based sensors have also been tested with similar type of VOCs such as benzene, xylene, alcohols and acetone and appeared with an excellent selectivity towards formaldehyde over the other VOCs.