Department of Electrical and Electronics Engineering

Permanent URI for this collectionhttp://localhost:4000/handle/123456789/1925

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Author: search.filters.author.Hazra, ArnabSubject: search.filters.subject.Physics

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    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, Subhashis
    The 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.
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    Highly selective formaldehyde sensing using ZnO nano-rods
    (AIP, 2023-03) Choudhary, Sumita; Hazra, Arnab; Gangopadhyay, Subhashis
    Early 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.
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    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, Subhashis
    Formation 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.