Department of Physics

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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.
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.
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.
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 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 formation of InGaN and GaN nano-structures studied by STM
(JSTAGE, 2006) Gangopadhyay, Subhashis
Growth 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.
Heteroatom doped blue luminescent carbon dots as a nano-probe for targeted cell labeling and anticancer drug delivery vehicle
(Elsevier, 2019-11) Gangopadhyay, Subhashis
Herein, a simple and efficient method to fabricate nitrogen and sulphur doped photoluminescent carbon dots (C-dots) has been judiciously developed for targeting cancer cells. Advancement in newfangled imaging tool for cancer cells is a vital and highly trustworthy strategy for cancer therapy. The fluorescent C-dots prepared from κ-carrageenan and folic acid can serve as an efficient cargo for cancer cell labeling expressing the folate receptor on their surface. The prepared C-dots unveiled good water solubility, excellent photostability, and biocompatibility. The C-dots played the role of nano-vehicle for anticancer drug capecitabine under different pH environments. Also, C-dots were applied as invisible marker and finger print recovery applications. The folate receptor in C-dots led to amazing targetability of cancer cells and which embraces a great promise in biomedical studies.
Imaging and manipulation of the Si(100) surface by small-amplitude NC-AFM at zero and very low applied bias
(IOP, 2012-02) Gangopadhyay, Subhashis
We use a noncontact atomic force microscope in the qPlus configuration to investigate the structure and influence of defects on the Si(100) surface. By applying millivolt biases, simultaneous tunnel current data is acquired, providing information about the electronic properties of the surface at biases often inaccessible during conventional STM imaging, and highlighting the difference between the contrast observed in NC-AFM and tunnel current images. We also show how NC-AFM (in the absence of tunnel current) can be used to manipulate both the clean c(4 × 2) surface and dopant-related defects.
Influence of substrate domain boundaries on surface reconstructions of Ga/Si(1 1 1)
(Elsevier, 2004-03) Gangopadhyay, Subhashis
The 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.
Initial stage of silicon nitride nucleation on Si(111) by rf plasma-assisted growth
(JSTAGE, 2006) Gangopadhyay, Subhashis
The nucleation of silicon nitride films on Si(111) using a radio frequency nitrogen plasma source has been investigated by scanning tunneling microscopy. The initial nucleation of Si3N4 is always observed at the steps, i.e., either at the step-edges of the initial Si(111) surface or at the step edges of vacancy islands (etch pits) formed on the terrace areas. With increasing nitridation temperature the nitrified patches become larger with lower density and show a triangular shape. After post annealing the triangular nucleation patches at the step-edges disappear and free-standing Si3N4 islands are observed with a hexagonal shape. Nitridation at high temperatures or post-annealing improves the crystalline quality of the nitride films and an atomically resolved honeycomb-like ”8×8” surface reconstruction is observed in STM for thin Si3N4 films grown at 850°C.
Interfacial interactions at Au/Si3N4/Si(111)and Ni/Si3N4/Si(111) structures with ultrathin nitride films
(AIP, 2004-06) Gangopadhyay, Subhashis
Synchrotron photoemission spectromicroscopy has been used to study the interfacial interactions, metal diffusivity, and electronic barriers of Au and Ni contacts on ultrathin silicon nitride films. The interface was found to be nonreactive, and only in the case of a very thin nitride film and elevated temperatures, Si can segregate from the Si(111) substrate and interact with Au. In the case of structures, Ni diffusion and degradation of the lattice are evidenced even at room temperature and strongly enhanced at elevated temperatures, leading to formation of a Ni silicide interlayer.
Mg and Si dopant incorporation and segregation in GaN
(Wiley, 2011-03) Gangopadhyay, Subhashis
The surface segregation of Mg and Si dopant species and theiratomic incorporation sites in GaN films grown by metal-organicvapour phase epitaxy (MOVPE) on sapphire (0001) substrateshave been analysed by X-ray photoemission spectro-micro-scopy and X-ray standing waves (XSW). As revealed byspectro-microscopy, both Mg and Si tend to segregate to thesurface. In case of Mg, an enhanced surface dopant concen-tration is found even after sputter-removal of several tens ofnanometres, which confirms a segregation mechanism pro-posed earlier [S. Figge et al., Appl. Phys. Lett.81, 4748 (2002)].Si doping has been found to result in the formation of facettedgrooves in the GaN films. The surface silicon concentration atthe facets is determined to be about 2.5 times higher ascompared to the planar (0001) surface by micro-spectroscopy.XSW results show that with increasing Mg dopant concen-tration, non-substitutional lattice sites are progressivelyoccupied. The results can quantitatively be explained by Mgatoms incorporated in an anti-bixbyite-like structure and isrelated to inversion domain boundaries. For Si doping, noevidence is found for the occupation of non-substitutional sites.However, a decrease in crystal quality is observed withincreasing Si concentration