Papers by Arnab Bhattacharya
Nano letters, May 21, 2017
Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with r... more Atomically thin materials such as graphene or MoS2 are of high in-plane symmetry. Crystals with reduced symmetry hold the promise for novel optoelectronic devices based on their anisotropy in current flow or light polarization. Here, we present polarization-resolved optical transmission and photoluminescence spectroscopy of excitons in 1T'-ReSe2. On reducing the crystal thickness from bulk to a monolayer, we observe a strong blue shift of the optical band gap from 1.37 to 1.50 eV. The excitons are strongly polarized with dipole vectors along different crystal directions, which persist from bulk down to monolayer thickness. The experimental results are well reproduced by ab initio calculations based on the GW-BSE approach within LDA+GdW approximation. The excitons have high binding energies of 860 meV for the monolayer and 120 meV for bulk. They are strongly confined within a single layer even for the bulk crystal. In addition, we find in our calculations a direct band gap in 1T&...
Nano letters, Dec 14, 2016
Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of gro... more Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs-Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni-Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be Ni2Ga3. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in...
Scientific Reports, 2016
We report the optimized synthesis and electrochemical characterization of a composite of few-laye... more We report the optimized synthesis and electrochemical characterization of a composite of few-layered nanostructured MoS 2 along with an electroactive metal oxide BiVO 4. In comparison to pristine BiVO 4 , and a composite of graphene/BiVO 4 , the MoS 2 /BiVO 4 nanocomposite provides impressive values of charge storage with longer discharge times and improved cycling stability. Specific capacitance values of 610 Fg −1 (170 mAhg −1) at 1 Ag −1 and 166 Fg −1 (46 mAhg −1) at 10 Ag −1 were obtained for just 2.5 wt% MoS 2 loaded BiVO 4. The results suggest that the explicitly synthesized small lateral-dimensioned MoS 2 particles provide a notable capacitive component that helps augment the specific capacitance. We discuss the optimized synthesis of monoclinic BiVO 4 , and few-layered nanostructured MoS 2. We report the discharge capacities and cycling performance of the MoS 2 /BiVO 4 nanocomposite using an aqueous electrolyte. The data obtained shows the MoS 2 /BiVO 4 nanocomposite to be a promising candidate for supercapacitor energy storage applications. The ever-increasing global energy demands have spurred increased research into energy harvesting and storage systems 1,2. The development of effective energy storage systems with high energy density as well as high power density is becoming increasingly important. Electrochemical capacitors, also termed as supercapacitors, have attracted significant interest as these devices bridge the energy density gap between conventional capacitors and batteries. Layered inorganic systems exhibit unusual properties that are technologically important. The unique mechanical, electronic, thermal and optical properties of graphene and other two-dimensional layered materials like the transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS 2) have enabled them to be utilized for various novel applications 3-6. One specific application for which these layered materials have been recently explored is in energy storage devices, such as lithium-ion batteries and supercapacitors 7,8. The recent proliferation of research into 2D layered chalcogenides is the result of their intrinsic high ionic conductivity, high surface area, inherent chemical stability (under a variety of pH conditions) and propensity for charge storage 9,10. In this work, an electroactive metal oxide BiVO 4 is combined with few-layered nanostructured (NS) MoS 2 as a capacitive component to form a hybrid structure. It should, however, be noted that over the last few years several studies that quantify the capacitive storage of MoS 2 have been reported 11-22 ; these typically use MoS 2 in conjunction with a variety of carbon allotropes, predominantly graphene (r-GO). Interestingly, as yet MoS 2 has not been coupled with a viable electrochemical component to form a hybrid supercapacitor. Table 1 shows a summary of a few different approaches; the specific capacitance values vary between 100 and 550 Fg −1 with an average of ca. 250 Fg −1. Moreover this value is highly dependent on type of MoS 2 (bulk vs. nanostructured), manner of synthesis of nanostructured MoS 2 (exfoliated vs. synthesized from precursors), current density employed and the amount of loading of MoS 2 to the carbon source. Another approach to augment the specific capacitance of electric double-layer capacitors is to incorporate a new electroactive material along with a potent capacitive component to enable high energy density supercapacitors while maintaining high power operations. It has been reported that MoS 2 suffers from low conductivity, low theoretical specific capacity and easy restacking of the sheets 23,24. Despite the aforementioned limitations of MoS 2 ,
ABSTRACT We report on the templated growth of semiconductor nanowires using nanoporous alumina fi... more ABSTRACT We report on the templated growth of semiconductor nanowires using nanoporous alumina films fabricated using a two-step anodization process in oxalic/phosphoric acid. Such membranes with hexagonally ordered nanopores have been used as templates for the synthesis of GaAs nanowires via metal-organic vapour phase epitaxy (MOVPE) using the vapour-liquid-solid (VLS) mechanism. Since the size of the gold islands that catalyse the nanowire growth is restricted by the template, the size distribution of the nanowires is narrowed compared to growth on GaAs nanowires grown in regions without the template.
InAs nanowires (NW) are grown via the Vapour Liquid Solid (VLS) technique in a horizontal metal o... more InAs nanowires (NW) are grown via the Vapour Liquid Solid (VLS) technique in a horizontal metal organic chemical vapour deposition (MOCVD) system. Gold colloids of diameter 10 nm are used as the catalyst for NW growth. Before growth, the< 111> B oriented GaAs ...
... Rapid Research Letter. MOVPE growth and characterization of a -plane AlGaN over the entire co... more ... Rapid Research Letter. MOVPE growth and characterization of a -plane AlGaN over the entire composition range. Masihhur R. Laskar 1,* ,; Tapas Ganguli 2 ,; AA Rahman 1 ,; AP Shah 1 ,; MR Gokhale 1 ,; Arnab Bhattacharya 1. Article first published online: 21 MAY 2010. ...
Advanced Functional Materials, 2011
The rhenium-based layered dichalcogenide ReSe2 crystallizes in a distorted triclinic structure wh... more The rhenium-based layered dichalcogenide ReSe2 crystallizes in a distorted triclinic structure which results in unique, anisotropic electronic and optical properties. This, along with a weak layer-dependence of band gap has made ReSe2 a subject of intense contemporary research interest. However, there has been no agreement on the exact crystal structure of this material, or knowledge of its thermal properties like the melting point. In this work, we perform single crystal, Laue, and powder diffraction measurements on high-quality ReSe2 crystals synthesized using a modified Bridgman technique. We confirm the presence of triclinic symmetry ($P\bar{1}$ -space group) and support the view that that ReSe2 has a distorted CdCl2-type structure (rather than Cd(OH)2 as initially proposed) and obtain lattice parameter values of a = 6.5791(8) Å, b = 6.6897(10) Å, and c = 6.7013(11) Å. Further, thermal measurements on these crystals show a clear endothermic peak at around 1115 °C pointing to a melting transition, and show no other phase transitions up to 1300 °C.
Journal of Crystal Growth, 2003
This paper reports on an in-situ optical reflectance study of the development of the interface ro... more This paper reports on an in-situ optical reflectance study of the development of the interface roughness of AlGaAs/AlAs distributed Bragg reflectors during the metalorganic vapour phase epitaxy growth of visible vertical-cavity surface-emitting laser structures. We show that the surface roughness can be extracted from time-resolved UV reflectance measurements. The roughness of the surface during growth for both the AlGaAs and
Journal of Electronic Materials, 2000
Journal of Crystal Growth, 2002
In this paper we report on the optimization and control of key layer properties of visible vertic... more In this paper we report on the optimization and control of key layer properties of visible vertical cavity surface emitting laser (VCSEL) for emission at wavelengths around 650 nm by means of an optical real-time sensor allowing for both spectroscopic reflectance and reflectance anisotropy measurements. In contrast to conventional ex-situ characterization the in-situ sensor yields information on properties like thickness, composition and doping for all buried layers, especially the lower distributed Bragg reflector which has a strong influence on the final device performance. Using in-situ sensor-based optimization of the growth process 650 nm VCSEL devices with a record-high continuous-wave output power of 3 mw at room temperature have been fabricated. r
Journal of Crystal Growth, 2016
12th International Conference on Fiber Optics and Photonics, 2014
We demonstrate a simple technique to transfer CVD-grown graphene from copper and platinum substra... more We demonstrate a simple technique to transfer CVD-grown graphene from copper and platinum substrates using a soak-and-peel delamination technique utilizing only hot deionized water. The lack of chemical etchants results in cleaner CVD graphene films minimizing unintentional doping, as confirmed by Raman and electrical measurements. The process allows the reuse of substrates and hence can enable the use of oriented substrates for growth of higher quality graphene, and is an inherently inexpensive and scalable process for large-area production.
This dissertation discusses design aspects, materials growth issues, device fabrication and chara... more This dissertation discusses design aspects, materials growth issues, device fabrication and characterization of high-power phase locked arrays of diode lasers and describes a series of experiments with progressive improvements in output power and beam quality from the first high-power 1.55mum-wavelength near-resonant antiguided arrays. Also presented are results and discussion of the first above-threshold analysis of the effect of element width on the modal behaviour of resonant antiguided arrays. Two appendices give experimental details of the MOCVD growth of InGaAsP on InP and the fabrication procedures for the broad-area type and self-aligned array devices. The best results obtained from strained-layer quaternary-quantum-well InGaAsP/InP 1.55mum-emitting near-resonant antiguided arrays include 2.5W front-facet peak-power in a 2.6sp° wide beam (6×diffraction-limit, D.L.), with 1.2W in the central lobe, from 40-element, 250mum aperture devices. The width of the central lobe remains constant from 4 to 15×threshold. Devices with improved temperature characteristics and closer to the resonance condition provide 1W power in a beam 1.15sp° wide (2.7×D.L.), with 61% of the energy in the central lobe; and 1.75W peak-power in a 1.5sp° wide beam (3.5×D.L.) at heatsink temperatures between 15sp°C and 45sp°C.
We fabricate and study Indium Arsenide (InAs) nanowire electromechanical resonators, in field eff... more We fabricate and study Indium Arsenide (InAs) nanowire electromechanical resonators, in field effect transistor (FET) geometry, which allows us to tune the carrier density and tension in the wire at electromechanical resonance by tuning the dc gate voltage. At temperatures below 5K, quality factor (Q) of these resonators is ˜10000, two orders of magnitude larger than at room temperature, and
Scientific reports, 2016
Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or sili... more Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice-matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS2 and MoS2 by metalorganic vapour phase epitaxy. Structural and optical characterization show that strain-free, single-crystal islands of GaN are obtained on the underlying chalcogenide flakes. We obtain strong near-band-edge emission from these layers, and analyse their temperature-dependent photoluminescence properties. We also report a proof-of-concept demonstration of large-area growth of GaN on CVD MoS2. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth.
LEOS '95. IEEE Lasers and Electro-Optics Society 1995 Annual Meeting. 8th Annual Meeting. Conference Proceedings, 1995
High-power narrow-beam single-mode ARROW-type semiconductor diode lasers. [Proceedings of SPIE 26... more High-power narrow-beam single-mode ARROW-type semiconductor diode lasers. [Proceedings of SPIE 2682, 154 (1996)]. Luke J. Mawst, Arnab Bhattacharya, Mike Nesnidal, James G. Lopez. Abstract. Antiresonant reflecting ...
Anatomy & Cell Biology, 2012
The effects of sodium arsenite exposure on the hepatic maturation period of cellular and function... more The effects of sodium arsenite exposure on the hepatic maturation period of cellular and functional reorganization in developing rat livers were evaluated. Animals received intraperitoneal injections of sodium arsenite (1.5 mg/kg body weight) or distilled water on days 9 to 28 after birth. On day 29, the animals were sacrificed either by cervical dislocation or by perfusion fixation. The perfusion fixed liver tissue was processed for paraffin embedding, sectioning and hematoxylin and eosin staining. The fresh liver tissue was processed for cryo-sectioning followed by Sudan Black B staining and for biochemical estimation of reduced glutathione. Microscopic observation revealed comparable preserved hepatic lobular patterns and distributions of uninucleate and binucleate hepatocytes in the control and the experimental groups. The mean nuclear area and diameter of the hepatocytes was increased in the experimental group. Lipid droplet distribution pattern in Sudan Black B stained sections revealed higher staining intensity towards the centrilobular area in both groups. Semiquantitative estimation of staining intensity showed lower mean gray values in zone 3 than in zones 2 and 1 (suggestive of the setting in of the adult pattern) in both groups. The reduced glutathione levels in the liver tissue and the altered nuclear size of the hepatocytes in the experimental group suggested the impairment of morphological and biochemical processes induced by arsenic exposure during the postnatal period.
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Papers by Arnab Bhattacharya