BITS Faculty Publications
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Item Modulation of 1d ZNO nanostructures for selective formaldehyde sensing: the role of surface energy, polarization, and adatom kinetics toward asymmetric growth(Elsevier, 2025-07) Hazra, Arnab; Gangopadhyay, SubhashisThe precise morphological control of highly dense, crystalline, and asymmetric one-dimensional (1D) metal oxide semiconductor nanostructures is of high practical importance for developing high-performance chemiresistive gas sensors. In this study, various 1D ZnO nanostructures, including nanobelts, nanowires, nanorods, and nanoneedles, were uniformly grown on glass substrates via the controlled thermal oxidation of thin Zn films under ambient air conditions. Comprehensive structural, chemical, optical, and electrical characterizations were conducted to investigate their properties and growth mechanism. Thermal oxidation of thin Zn films initiates only above 400 °C, whereas a transition towards asymmetric growth of 1D ZnO nanostructures starts to occur at 600 °C. Different nanoscale morphologies such as nanobelts (t = 58 nm, w = 460 nm), nanowires (d = 48 nm), nanorods (d = 280 nm), and nanoneedles (d = 85–120 nm) are obtained after thermal oxidation of [600 °C, 5h], [700 °C, 1h], [700 °C, 5h] and [800 °C, 5h], respectively. Chemiresistive gas sensing performance of these 1D nanostructures was evaluated against different volatile organic compounds (VOCs) in a static vapor mode, demonstrating exceptional sensitivity and selectivity for formaldehyde detection. The nanoneedle-based sensor exhibited superior sensitivity (36 %) with rapid response (28 s), while the nanobelt-based sensor demonstrated excellent selectivity. Notably, the nanowire-based sensor with highly porous morphology achieved an ultra-low detection limit, well below 50 ppb. The growth mechanism was explained based on surface free energy, surface polarization, and adatom diffusion kinetics. Additionally, gas sensing performance was analyzed in relation to surface-to-volume ratio, crystal defect states and crystal plane orientation. Mechanism behind controlled electron transport through the nano-junction of 1D nanostructure is also discussed. All these findings establish a growth mechanism of 1D ZnO nanostructures as promising candidates for advanced gas sensing applications.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 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.Item Surface energy and stress driven growth of extremely long and high-density ZnO nanowires using a thermal step-oxidation process(RSC, 2024-09) Panda, Sri Aurobindo; Choudhary, Sumita; Hazra, Arnab; Gangopadhyay, SubhashisFormation of highly crystalline zinc oxide (ZnO) nanowires with an extremely high aspect ratio (length = 60 μm, diameter = 50 nm) is routinely achieved by introducing an intermediate step-oxidation method during the thermal oxidation process of thin zinc (Zn) films. High-purity Zn was deposited onto clean glass substrates at room temperature using a vacuum-assisted thermal evaporation technique. Afterwards, the as-deposited Zn layers were thermally oxidized under a closed air ambient condition at different temperatures and durations. Structural, morphological, chemical, optical and electrical properties of these oxide layers were investigated using various surface characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoemission spectroscopy (XPS). It was noticed that the initial thermal oxidation of Zn films usually starts above 400 °C. Homogeneous and lateral growth of the ZnO layer is usually preferred for oxidation at a lower temperature below 500 °C. One-dimensional (1D) asymmetric growth of ZnO started to dominate thermal oxidation above 600 °C. Highly dense 1D ZnO nanowires were specifically observed after prolonged oxidation at 600 °C for 5 hours, followed by short-step oxidation at 700 °C for 30 minutes. However, direct oxidation of Zn films at 700 °C resulted in ZnO nanorod formation. The formation of ZnO nanowires using step-oxidation is explained in terms of surface free energy and compressive stress-driven Zn adatom kinetics through the grain boundaries of laterally grown ZnO seed layers. This simple thermal oxidation process using intermittent step-oxidation was found to be quite unique and very much useful to routinely grow an array of high-density ZnO nanowires. Moreover, these ZnO nanowires showed very high sensitivity and selectivity towards formaldehyde vapour sensing against few other VOCs.Item Au/TiO2 Nanotubes/Ti-Based Solid-State Vapor Sensor: Efficient Sensing in Resistive and Capacitive Modes(IEEE, 2018-05) Hazra, Arnab; Gangopadhyay, SubhashisFabrication of TiO 2 nanotubes-based solid-state vapor sensor (Au/TiO 2 nanotubes/Ti) and its performance analysis for both resistive- and capacitive-mode sensing mechanisms are discussed here. Highly ordered TiO 2 nanotubes array has been synthesized by the electrochemical anodization technique. Structure and morphology of the as-grown TiO 2 nanotubes have been characterized using X-ray diffraction and field-emission scanning electron microscopy. X-ray photoelectron spectroscopy has been used to study the chemical states of the TiO 2 nanotubes. The sensor device has successfully been tested for ethanol vapor. The effect of temperature, pressure, and reducing ambient (i.e., partial pressure of ethanol vapor) has been studied using the impedance analysis method. The resistive and capacitive components of the impedance were measured individually. The sensor resistance decreased by 93.38%, whereas the capacitance increased by 28789.95% after an exposure to 1000 ppm of ethanol. Both the resistive and capacitive sensing performance of Au/TiO 2 nanotubes/Ti device have been correlated with the proposed circuit model to achieve an improvised vapor sensing.Item 1-D TiO2 Nanorods Array-Based Parallel Electrode Sensor for Selective and Stable Detection of Organic Vapors(IEEE, 2020-01) Hazra, Arnab; Gangopadhyay, SubhashisA solid-state vapor sensor in parallel electrode configuration was fabricated by employing 1-D TiO 2 nanorods as a sensing layer. Highly ordered and oriented TiO 2 nanorods were synthesized on a Ti substrate by using hydrothermal method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were used to characterize the TiO 2 nanorods/Ti samples. The developed Au/TiO 2 nanorods/Ti type parallel electrodes sensor demonstrated the potential of integrated operations of both resistive and capacitive changes towards various concentrations (50-300 ppm) of volatile organic compounds (VOCs) like methanol, ethanol, 2-propanol, acetone and benzene at 50 °C. The resistive response magnitude of the sensor was found to be increased from 13 % to 87% while the capacitive response magnitude of the sensor was increased from 32 % to 200%, as methanol concentration was increased from 50 ppm to 300 ppm. However, the use of both modes enhances the selectivity performance of the sensor as the resistive mode exhibited better selectivity for a lower concentration of VOCs and the capacitive mode for higher concentration of VOCs. Moreover, the sensor showed a very good stability because of low operating temperature (50°C) as well as rutile (major) phase of TiO 2 nanorods.Item Optimized Resistive Switching in TiO2 Nanotubes by Modulation of Oxygen Vacancy Through Chemical Reduction(IEEE, 2020-05) Hazra, Arnab; Gangopadhyay, SubhashisThe resistive switching behavior of 1-D TiO 2 nanotube-based resistive random access memory (ReRAM) is discussed in this article. Highly oriented TiO 2 nanotubes were synthesized by anodic oxidation method on a Ti substrate, which was used as the bottom electrode. To modulate the oxygen vacancy (VO) in TiO 2 nanotubes, hydrazine hydrate reduction was employed in the temperature range 60 °C-100 °C. After charactering the morphological, elemental, and crystallographic properties, the level of reduction in different TiO 2 nanotubes array was estimated by Raman, photoluminescence, and X-ray photoelectron spectroscopies. Au/TiO 2 nanotubes/Ti devices were fabricated by using TiO 2 nanotubes with various levels of reductions where thin and porous Au top electrode was used to make the resistive switching faster. TiO 2 Nanotubes array, reduced at 80 °C showed promising resistive switching performance with SET/RESET voltages of 2 V/1.8 V, R OFF /R ON of 19 at a read voltage of 0.5 V (25 °C) and stable endurance behavior after the 100th cycle. Interestingly, reduction temperature at 60 °C and 100°C, offered degraded resistive switching within the same voltage range. All the devices showed electroforming free bipolar resistive switching. Efforts were devoted to establish the role of V O and its tuning to improve the resistive switching behavior in 1-D TiO 2 nanotubes. This article systematically showcases the efficacy of 1-D metal oxide for potential ReRAM application and establishes an easy but efficient approach to improve the resistive switching by modulating oxygen vacancy in it.