Department of Physics
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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 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 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.Item Formation and morphology of InGaN nanoislands on GaN(0001)(AIP, 2007-04) Gangopadhyay, SubhashisThe morphology and density of InGaN nanoislands can be controlled by the choice of proper growth conditions for metal organic vapor phase epitaxy. Scanning tunneling microscopy has been used to investigate the dependence of InGaN island morphology on the growth parameters. A heterogeneous nucleation of large InGaN islands with a complex structure is observed after growth at in conjunction with a high In partial pressure. For and low In partial pressure, however, the homogeneous nucleation of small islands of sizes suitable for three-dimensional quantum confinement is found, with very high densities of . The influence of the growth temperature and the In partial pressure is discussed in terms of thermally activated diffusion and surface mobility.Item Growth and formation of InGaN and GaN nano-structures studied by STM(JSTAGE, 2006) Gangopadhyay, SubhashisGrowth and morphology of metal organic vapour phase epitaxy (MOVPE) deposited InGaN nano-islands and molecular beam epitaxy (MBE) grown GaN films on GaN(0001) template layers on sapphire substrates have been investigated using scanning tunneling microscopy. For MOVPE InGaN growth, the nucleation of self-organized nano-structures can be achieved by a careful choice of the growth temperature, the In partial pressure, the growth rate and V/III flux ratio. For growth at 650°C, large spiral disc-like islands are found, preferentially nucleating at GaN substrate defects. At 600°C, islands of smaller average size are observed. Lowering the In flux at this temperature, a homogeneous nucleation of small quantum dot like islands with a density of 1012/cm2 is found. For homoepitaxial MBE growth of thin GaN layers on GaN templates, a layer-by-layer growth mode is observed for Ga rich growth conditions. For growth at 750°C, an atomically resolved 4×4 surface reconstruction with a high defect density is found in the initial growth stage. However, subsequent growth at 790°C leads to the formation of one dimensional nanoclusters of about 3 nm lateral spacing. For GaN growth at a lower Ga-flux, a rougher surface morphology and three dimensional growth is observed. Independent on the Ga flux, one-dimensional nanostructures appear after prolonged growth at higher temperature, which are attribute to the impact of ions emerging from the N-plasma.Item Influence of substrate domain boundaries on surface reconstructions of Ga/Si(1 1 1)(Elsevier, 2004-03) Gangopadhyay, SubhashisThe temperature induced phase transition of the Ga/Si(1 1 1) surface for submonolayer Ga coverages has been monitored by variable temperature scanning tunneling microscopy. After room temperature deposition of about 1/3 monolayer of Ga on Si(1 1 1), the Si(1 1 1) surface is mostly covered with Ga-induced magic clusters in a 7 × 7 like arrangement whereas at the domain boundary regions of the former Si(1 1 1)-7 × 7 reconstruction an increased density of excess Ga islands is found. The magic clusters are stable against annealing up to 350 °C. At this temperature, however, the Ga-islands coalesce and a mixture of and 6.3 × 6.3 structures is formed at the domain boundary regions. At an annealing temperature of 400 °C a phase transformation of the structure to 6.3 × 6.3 is found at the domain boundary region opposite to the usual thermal phase sequence. This can be explained in terms of an interplay of surface kinetics and surface stress.