BITS Faculty Publications
Permanent URI for this communityhttp://localhost:4000/handle/123456789/1867
Browse
44 results
Search Results
Item Strain-mediated rapid growth of vertically oriented 2D MoSe₂: Insights into the growth mechanism(Elsevier, 2025-11) Gangopadhyay, SubhashisHorizontally aligned transition metal dichalcogenides (TMDCs) are well-suited for charge transport applications, while vertically oriented TMDCs are advantageous for high surface area applications such as catalysis, water splitting, and energy storage. However, the mechanism governing these structural and morphological transition remains unclear. This study provides a comprehensive growth time profile and demonstrates that parameters such as strain, distribution of grain boundaries, randomness and interlayer distance plays the critical role in driving the desirable morphological evolution. Here, we investigate the growth dynamics of MoSe₂ thin films synthesized via chemical vapor deposition (CVD), with a focus on understanding and optimizing the horizontal-to-vertical transition as a function of growth time. Scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM) were used to trace the stacking and structural order, while Raman spectroscopy and photoluminescence spectroscopy (PL) were used to investigate the variation of stacking and optical order. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical environment of the films, and field-effect transistors (FET) measurements were used to assess electrical properties such as mobility and surface carrier density. To support the experimental findings, a computational multilayer stacking framework was developed to project desirable randomness in a controlled manner through the variation of interlayer distance and simulating extent of strain mediation through wide range of unstrained, compressive, tensile and even highly diffusive strain states. This model helps to establish the relationship between the interlayer distortion and randomness with the optical asymmetry, providing insights into strain-mediated widely distributed direct to indirect optical transitions. This can further serve as an optical marker especially for these highly randomized directional vertical oriented flakes. Overall, this study presents a fundamental understanding of strain-induced morphological transitions in MoSe₂ thin films and offers a framework for tracing and tuning the optical and electronic properties in anisotropic 2D materials.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 Synthesis and characterization of Co3O4 spinel nanowall: understanding the growth mechanism and properties(IOP, 2024) Gangopadhyay, SubhashisFormation of spinel tricobalt tetraoxide (Co3O4) nanostructures through a controlled thermal oxidation process is discussed here. Thin films of high purity cobalt (Co) were deposited on glass/quartz substrates using an electron beam (E-beam) evaporation technique. Thermal oxidation of the as-deposited Co thin films was carried out at various oxidation temperatures (400 °C to 600 °C) for different durations (5 h to 15 h) to grow various oxide nanostructures. Different surface characterizations techniques were used to investigate the structure, chemistry and electronic properties of the as-grown cobalt oxide nanostructures. x-ray diffraction analysis revealed the presence of the CoO phase along with the Co3O4 phases at relatively lower oxidation temperature. However, the Co3O4 phase becomes more predominant for longer oxidation durations at higher oxidation temperatures. Field emission scanning electron microscopy analysis showed a surface morphological transition from nanowalls to nanograins with an increase in the oxidation temperature. The surface electrical conductivity of the oxidized Co films is also increased for higher oxidation temperature and/or duration mainly due to the oxide phase purity and larger particle sizes. Ultraviolet–visible spectroscopy indicated two distinct optical energy bandgaps, which effectively decreased with an increase in the oxidation temperature and duration. Raman spectroscopy identified five different Raman-active modes corresponding to the Co3O4 phase, with the F2g mode dominating at higher temperatures. All these findings provide clear insights into the structural, electrical, chemical and optical properties of cobalt oxide thin films. Moreover, it provides a mechanism on how to grow 2D nanowalls morphology of Co3O4 films which can further be used in energy, sensor or catalytic applications.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 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 Two-step growth of InGaN quantum dots and application to light emitters(Wiley, 2007-06) Gangopadhyay, SubhashisA two-step growth method for creating InGaN quantum dots (QDs) was developed by using a combination of an InxGa1–xN nucleation layer (NL) without island structures and an InyGa1–yN formation layer (FL) with an indium content lower than that of the InxGa1–xN NL. The realization of QDs was confirmed by micro-photoluminescence (μ-PL) measurements only for the sample with both the InxGa1–xN NL and the InyGa1–yN FL. The spectral position of the QD ensemble recombination was controlled mainly by the deposition time of the InxGa1–xN NL. Green (∼520 nm) and amber (∼600 nm) LEDs with the QD layers grown by the two-step growth method as the active region were also fabricated and compared with that having InGaN QW layers, reported previously. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Item Purpose-led Publishing logo. Evolution of Ge nanoislands on Si(110)-'16 × 2' surface under thermal annealing studied using STM(IOP, 2009-10) Gangopadhyay, SubhashisThe initial nucleation of Ge nanoclusters on Si(110) at room temperature (RT), annealing-induced surface roughening and the evolution of three-dimensional Ge nanoislands have been investigated using scanning tunneling microscopy (STM). A few monolayers (ML) of Ge deposited at room temperature lead to the formation of Ge clusters which are homogeneously distributed across the surface. The stripe-like patterns, characteristic of the Si(110)-'16 × 2' surface reconstruction are also retained. Increasing annealing temperatures, however, lead to significant surface diffusion and thus, disruption of the underlying '16 × 2' reconstruction. The annealing-induced removal of the stripe structures (originated from '16 × 2' reconstruction) starts at approximately 300 °C, whereas the terrace structures of Si(110) are thermally stable up to 500 °C. At approximately 650 °C, shallow Ge islands of pyramidal shape with (15,17,1) side facets start to form. Annealing at even higher temperatures enhances Ge island formation. Our findings are explained in terms of partial dewetting of the metastable Ge wetting layer (WL) (formed at room temperature) as well as partial relaxation of lattice strain through three-dimensional (3D) island growth.Item Thin Cu film resistivity using four probe techniques: Effect of film thickness and geometrical shapes(AIP, 2018-05) Gangopadhyay, SubhashisPrecise measurement of electrical sheet resistance and resistivity of metallic thin Cu films may play a significant role in temperature sensing by means of resistivity changes which can further act as a safety measure of various electronic devices during their operation. Four point probes resistivity measurement is a useful approach as it successfully excludes the contact resistance between the probes and film surface of the sample. Although, the resistivity of bulk samples at a particular temperature mostly depends on its materialistic property, however, it may significantly differ in the case of thin films, where the shape and thickness of the sample can significantly influence on it. Depending on the ratio of the film thickness to probe spacing, samples are usually classified in two segments such as (i) thick films or (ii) thin films. Accordingly, the geometric correction factors G can be related to the sample resistivity r, which has been calculated here for thin Cu films of thickness up to few 100 nm. In this study, various rectangular shapes of thin Cu films have been used to determine the shape induced geometric correction factors G. An expressions for G have been obtained as a function of film thickness t versus the probe spacing s. Using these expressions, the correction factors have been plotted separately for each cases as a function of (a) film thickness for fixed linear probe spacing and (b) probe distance from the edge of the film surface for particular thickness. Finally, we compare the experimental results of thin Cu films of various rectangular geometries with the theoretical reported results.Item Role of Different States of Solubilized Water on Solvation Dynamics and Rotational Relaxation of Coumarin 490 in Reverse Micelles of Gemini Surfactants, Water/12-s-12.2Br– (s = 5, 6, 8)/n-Propanol/Cyclohexane(ACS, 2020-03) Gangopadhyay, SubhashisThe present study demonstrates how the different states of solubilized water viz. quaternary ammonium headgroup-bound, bulklike, counterion-bound, and free water in reverse micelles of a series of cationic gemini surfactants, water/12-s-12 (s = 5, 6, 8).2Br–/n-propanol/cyclohexane, control the solvation dynamics and rotational relaxation of Coumarin 490 (C-490) and microenvironment of the reverse micelles. The relative number of solubilized water molecules of a given state per surfactant molecule decides major and minor components. A rapid increase in the number of bulklike water molecules per surfactant molecule as compared to the slow increase in the number of each of headgroup- and counterion-bound water molecules per surfactant molecule with increasing water content (Wo) in a given reverse micellar system is responsible for the increase in the rate of solvation and rotational relaxation of C-490. The increase in the number of counterion-bound water molecules per surfactant molecule and the concomitant decrease in the number of bulklike water molecules per surfactant molecule with increasing spacer chain length of gemini surfactants at a given Wo are ascribed to the slower rates of both solvation and rotational relaxation. Relative abundances of different states of water have a role on the microenvironment of the reverse micelles as well. Thus, a comprehensive effect of different states of water on dynamics in complex biomimicking systems has been presented here.