Browsing by Author "Gangopadhyay, Subhashis"

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1-D TiO2 Nanorods Array-Based Parallel Electrode Sensor for Selective and Stable Detection of Organic Vapors
(IEEE, 2020-01) Hazra, Arnab; Gangopadhyay, Subhashis
A 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.
100 year anniversary Measurement Science and Technology logo. Purpose-led Publishing logo. A reverse electrochemical floating-layer technique of SPM tip preparation
(IOP, 2000) Gangopadhyay, Subhashis
An experimental set-up involving a combination of reverse electrochemical etching and the floating-layer technique of tip preparation is presented. In the model only dc bias is used, avoiding the shortcomings of ac bias. The collection of two tips in a single etching process and mechanical auto-breaking of the circuit are emphasized. This cost and time effective, easy in-laboratory approach yields two sharp tips without paying attention to switching off the circuit. The nature of the tip shape arising from the floating-layer technique has been explained in terms of the electrical field distribution between the electrodes. The addition of some new information together with a few established facts will enrich and enlighten the art of tip preparation.
Adsorbate induced self-ordering of germanium nanoislands on Si(113)
(IOP, 2007-10) Gangopadhyay, Subhashis
The impact of Ga preadsorption on the spatial correlation of nanoscale three-dimensional (3D) Ge-islands has been investigated by low-energy electron microscopy and low-energy electron diffraction. Submonolayer Ga adsorption leads to the formation of a 2D chemical nanopattern, since the Ga-terminated (2×2) domains exclusively decorate the step edges of the Si(113) substrate. Subsequent Ge growth on such a partially Ga-covered surface results in Ge 3D islands with an increased density as compared to Ge growth on clean Si(113). However, no pronounced alignment of the Ge islands is observed. Completely different results are obtained for Ga saturation coverage, which results in the formation of (112) and (115) facets regularly arranged with a periodicity of about 40 nm. Upon Ge deposition, Ge islands are formed at a high density of about 1.3×1010 cm−2. These islands are well ordered as they align at the substrate facets. Moreover, the facet array induces a reversal of the Ge islands' shape anisotropy as compared to growth on planar Si(113) substrates.
Ag2S/Ag Heterostructure: A Promising Electrocatalyst for the Hydrogen Evolution Reaction
(ACS, 2017) Basu, Mrinmoyee; Pande, Surojit; Gangopadhyay, Subhashis
Different metal chalcogenides, being a potential candidate for hydrogen evolution catalysts, have attracted enormous attention in the field of water splitting. In the present study, Ag2S/Ag is revealed as an efficient catalyst for hydrogen evolution. When a sacrificial template of the CuS nanostructure is used, Ag2S/Ag heterostructures are synthesized following a simple wet-chemical technique. Two different routes, wet chemical and hydrothermal, are followed to modulate the morphology of the CuS templates from flower ball to wirelike structures, which subsequently results in the formation of Ag2S nanostructure. Finally, the Ag layer is deposited on Ag2S with the help of a photoreduction technique. The unique heterostructure of Ag2S/Ag shows efficient catalytic activity in the H2 evolution reaction. A Ag2S/Ag wire can successfully generate a 10 mA/cm2 current density at a −0.199 V potential. Ag2S/Ag contains the micronanostructure where nanoplates of Ag2S/Ag assemble to give rise to microstructures such as flower balls and wire.
Alignment of Ge Nanoislands on Si(111) by Ga-Induced Substrate Self-Patterning
(APS, 2007-02) Gangopadhyay, Subhashis
A novel mechanism is described which enables the selective formation of three-dimensional Ge islands. Submonolayer adsorption of Ga on Si(111) at high temperature leads to a self-organized two-dimensional pattern formation by separation of the 7 7 substrate and Ga=Si 111 - 3 p 3 p R30 domains. The latter evolve at step edges and domain boundaries of the initial substrate reconstruction. Subsequent Ge deposition results in the growth of 3D islands which are aligned at the boundaries between bare and Ga-covered domains. This result is explained in terms of preferential nucleation conditions due to a modulation of the surface chemical potential.
Au/TiO2 Nanotubes/Ti-Based Solid-State Vapor Sensor: Efficient Sensing in Resistive and Capacitive Modes
(IEEE, 2018-05) Hazra, Arnab; Gangopadhyay, Subhashis
Fabrication 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.
Biocompatible carbon dots derived from κ-carrageenan and phenyl boronic acid for dual modality sensing platform of sugar and its anti-diabetic drug release behavior
(Elsevier, 2019-07) Gangopadhyay, Subhashis
Detection of sugar by enzymatic assay has been suffering from costly, time-taking, instable and denaturation of glucose oxidase. Recently, chemosensors that have affinity towards boronate became the hot topic in the domain of monosaccharide detection. In this work, a novel strategy was addressed to fabricate carbon dots (C-dots) from linear sulfated polysaccharides κ- carrageenan and phenyl boronic acid for nonenzymatic monosaccharide (glucose) detection. The boronic acid group anchored C-dots surface can form assembly by covalently bonded with the cis-diol moiety of the glucose which caused fluorescence quenching of the C-dots. The inert surface nature of the luminescent C-dots enables them to sense as low as 1.7 μM glucose without the interference of other biomolecules. The proposed sensing system was successfully applied for assay of glucose in blood serum. Interestingly, these C-dots were used as a nano vehicle for delivery of anti-diabetic drug Metformin. Good biocompatibility results were found with MTT and hemolysis assay. Owing to its simplicity and effectiveness, the as-prepared C-dots offered great promise for blood sugar diagnosis and treatment.
C60 submonolayers on the Si(1 1 1)-(7 × 7) surface: Does a mixture of physisorbed and chemisorbed states exist?
(Elsevier, 200-09) Gangopadhyay, Subhashis
We have carried out a combined X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy(UPS), and scanning tunnelling microscopy (STM) study of the C60-Si(1 1 1) interaction where the XPS/UPS spectrometer and STM are integrated on a single UHV system. This enables a direct comparison of the XPS/UPS spectra with the STM data and eliminates any uncertainty in C60 coverage measurements. X-ray standing wave measurements and density functional theory calculations have been used to support and interpret the results of the XPS/UPS/STM experiments. Our data conclusively rule out models of C60 adsorption which involve a mixture of physisorbed and chemisorbed molecules [K. Sakamoto, et al., Phys. Rev. B 60 (1999) 2579]. Instead, we find that all molecules, up to 1 monolayer coverage, bond to the surface via Si–C bonds which are predominantly of covalent character.
Cleaning and growth morphology of GaN and InGaN surfaces
(Wiley, 2011-05) Gangopadhyay, Subhashis
The structure and chemistry of clean GaN surfaces and InGaN thin films and nanostructures grown by metal organic vapour pressure epitaxy (MOVPE) has been studied by means of X-ray photoemission spectroscopy, low-energy electron diffraction as well as scanning tunneling microscopy (STM) and transmission electron microscopy. Thermal annealing strongly improves the cleanliness of samples after dry nitrogen transfer and related exposure to residual oxygen. Nitrogen plasma assisted cleaning is shown to successfully further remove carbon contaminations, while Ga deposition with subsequent desorption to is shown to be superior for an enhanced reduction of surface oxygen. Using STM, the surface morphology has been studied in dependence on major growth parameters at various stages of InGaN MOVPE growth. The formation of nano-islands is reported for different growth conditions. By means of micro-photoluminescence measurements, we find samples to show strong photoluminescence from quantum-dot-like structures, however, the corresponding growth front is found to be rather flat throughout InGaN deposition. This leads to the conclusion that the formation of quantum dots does not proceed in a Stranski–Krastanov-like fashion but most likely during overgrowth.
Decoration of Pd and Pt nanoparticles on a carbon nitride (C3N4) surface for nitro-compounds reduction and hydrogen evolution reaction
(RSC, 2017) Basu, Mrinmoyee; Gangopadhyay, Subhashis; Pande, Surojit
Herein, we propose the synthesis of Pd and Pt monometallic nanoparticles on a carbon nitride (C3N4) surface for the reduction of nitro compounds as well as for electrocatalysis. For the synthesis of C3N4/Pd and C3N4/Pt, metal ions were initially adsorbed on the C3N4 surface and then subsequently reduced by NaBH4. The as-synthesized heterostructures were authenticated by different characterization techniques: UV-vis, PXRD, XPS, TEM, FESEM, and EDS. Decorations of monometallic NPs on C3N4 not only improved the reduction efficiency of nitro-compounds but also enhanced the electrocatalytic activity in the hydrogen evolution reaction. C3N4/Pt proved to be an efficient electrocatalyst as it requires a potential of −0.339 V to attain a current density of 10 mA cm−2; whereas, C3N4/Pd requires −0.371 V to reach a current density of 10 mA cm−2vs. Ag/AgCl. Both C3N4/Pd and Pt heterostructures are better than bare C3N4, which needs −0.596 V to achieve a current density of 10 mA cm−2vs. Ag/AgCl. On the other hand, C3N4/Pd showed a better performance in nitro-compound reduction compared to C3N4/Pt and bare C3N4. The kinetic study reveals that the rate constant using a C3N4/Pd catalyst is 6.7 × 10−1 min−1 for p-nitroaniline reduction, which is 101 times higher compared to bare C3N4 and 4.7 times higher in comparison to C3N4/Pt.
Development of gold nanoparticle-fungal hybrid based heterogeneous interface for catalytic applications
(Elsiever, 2015-08) Panwar, Jitendra; Gangopadhyay, Subhashis
Unsupported and free gold nanoparticles (Au NPs) represent great potential in the field of catalysis. However, shortcomings like agglomeration and loss of the precious catalyst has encouraged the development of supported Au NPs as catalyst with increased activity, selectivity, ease of separation from the reaction mixture and recyclability. The present work demonstrates an eco-friendly, rapid and facile synthesis of catalytically active bio-supported Au NPs using a soil fungus, Aspergillus japonicus AJP01. The dual role of the fungal isolate in synthesis as well as immobilization of Au NPs is the remarkable feature of the study. The fungus successfully reduced Au(III) into Au NPs containing principally Au(0) with a small percentage of Au(I) as revealed by X-ray photoelectron spectroscopy. The particles were spherical in shape and well distributed on fungal mycelia with size ranging predominantly between 15 and 20 nm. The as-synthesized nanoparticle-fungal hybrid was found to be highly efficient in catalyzing sodium borohydride mediated reduction reactions of 4-nitrophenol and hexacyanoferrate(III). The versatility of the bionanocatalyst was further demonstrated by catalyzing the A3 coupling reactions for the synthesis of propargylamines.
Dual doped biocompatible multicolor luminescent carbon dots for bio labeling, UV-active marker and fluorescent polymer composite
(Wiley, 2018-07) Gangopadhyay, Subhashis
We report on metal–non-metal doped carbon dots with very high photoluminescent properties in solution. Magnesium doping to tamarind extract associated with nitrogen-doping is for the first time reported here which also produce very high quantum yield. Our aim is to develop such dual doped carbon dots which can also serve living cell imaging with easy permeation towards cells and show non-cytotoxic attributes. More importantly, the chemical signatures of the carbon dots unveiled in this work can support their easy solubilization into water; even in sub-ambient temperature. The cytotoxicity assay proves the almost negligible cytotoxic effect against human cell lines. Moreover, the use of carbon dots in UV-active marker and polymer composites are also performed which gave clear distinguishable features of fluorescent nanoparticles. Hitherto, the carbon dots can be commercially prepared without adopting any rigorous methods and also can be used as non-photo-bleachable biomarkers of living cells.
Electron beam deposited thin titanium films and its thermal oxidation to form rutile TiO2 thin films
(AIP, 2024) Gangopadhyay, Subhashis
Smooth 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.
Formation and morphology of InGaN nanoislands on GaN(0001)
(AIP, 2007-04) Gangopadhyay, Subhashis
The 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.
Formation of all tin oxide p–n junctions (SNO–SNO2) during thermal oxidation of thin sn films
(Wiley, 2024-12) Hazra, Arnab; Gangopadhyay, Subhashis
Metastable 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.
Formation of Monolayer Graphene by Annealing Sacrificial Nickel Thin Films
(ACS, 2009-09) Gangopadhyay, Subhashis
Graphene films have been formed by annealing Ni thin films at 800 °C under vacuum conditions. The Ni thin films are deposited on Si/SiO2 and, following annealing, have a polycrystalline morphology with grain sizes on the order of 1 μm. Following growth, the Ni is removed by etching, and the graphene is transferred as a single continuous layer onto a separate surface. The fraction of monolayer graphene is investigated using optical and electron microscopy and Raman spectroscopy and is shown to be >75%.
Graphitic-carbon nitride support for the synthesis of shape-dependent ZnO and their application in visible light photocatalysts
(RSC, 2015-09) Pande, Surojit; Gangopadhyay, Subhashis
Shape-dependent synthesis of ZnO has been developed on the surface of g-C3N4 following a simple and reproducible strategy. Initially, graphitic-carbon nitride (g-C3N4) was synthesized by pyrolysis of urea which was further used to grow ZnO nanostructures via refluxing conditions. Different hydrolyzing agents, such as hexamethylenetetramine (HMT) and ammonia were used to synthesize dumbbell and cone structures, respectively. Apart from hydrolyzing agents, cetyltrimethylammoniumbromide (CTAB) was also used as a growth controlling agent. Structural, morphological and optical characterizations of the as-synthesized materials were performed by using FESEM, TEM, XRD, XPS, UV-vis etc. techniques. After successful synthesis, the as-synthesized heterostructures were explored as visible light driven photocatalysts towards organic pollutant (methylene blue and phenol) degradation. The photocatalytic performances of bare ZnO, dumbbell and cone structures of g-C3N4/ZnO as well as g-C3N4, have been examined thoroughly. Photocatalytic results revealed that g-C3N4/ZnO heterostructures exhibit a higher efficiency under the illumination of visible light as compared to pure g-C3N4. Superior photodegradation activity of the g-C3N4/ZnO heterostructure originated from the synergistic effect and high charge separation at the interface of g-C3N4 and ZnO has also been discussed.
Growth and characterization of epitaxially stabilized ceria(001) nanostructures on Ru(0001)
(RSC, 2016-05) Gangopadhyay, Subhashis
We have studied (001) surface terminated cerium oxide nanoparticles grown on a ruthenium substrate using physical vapor deposition. Their morphology, shape, crystal structure, and chemical state are determined by low-energy electron microscopy and micro-diffraction, scanning probe microscopy, and synchrotron-based X-ray absorption spectroscopy. Square islands are identified as CeO2 nanocrystals exhibiting a (001) oriented top facet of varying size; they have a height of about 7 to 10 nm and a side length between about 50 and 500 nm, and are terminated with a p(2 × 2) surface reconstruction. Micro-illumination electron diffraction reveals the existence of a coincidence lattice at the interface to the ruthenium substrate. The orientation of the side facets of the rod-like particles is identified as (111); the square particles are most likely of cuboidal shape, exhibiting (100) oriented side facets. The square and needle-like islands are predominantly found at step bunches and may be grown exclusively at temperatures exceeding 1000 °C.
Growth and characterization of single phase Cu2O by thermal oxidation of thin copper films
(AIP, 2016-04) Gangopadhyay, Subhashis
We report a simple and efficient technique to form high quality single phase cuprous oxide films on glass substrate using thermal evaporation of thin copper films followed by controlled thermal oxidation in air ambient. Crystallographic analysis and oxide phase determination, as well as grain size distribution have been studied using X-ray diffraction (XRD) method, while scanning electron microscopy (SEM) has been utilized to investigate the surface morphology of the as grown oxide films. The formation of various copper oxide phases is found to be highly sensitive to the oxidation temperature and a crystalline, single phase cuprous oxide film can be achieved for oxidation temperatures between 250°C to 320°C. Cu2O film surface appeared in a faceted morphology in SEM imaging and a direct band gap of about 2.1 eV has been observed in UV-visible spectroscopy. X-ray photoelectron spectroscopy (XPS) confirmed a single oxide phase formation. Finally, a growth mechanism of the oxide film has also been discussed.
Growth and Characterization of ZnO Nanostructures: Materials for CO and Ethanol Sensing
(Springer, 2021) Hazra, Arnab; Choudhary, Sumita; Gangopadhyay, Subhashis
Controlled 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.