Showing papers by "Samit K. Ray published in 2012"

CdS-Decorated ZnO Nanorod Heterostructures for Improved Hybrid Photovoltaic Devices

Abstract: Cadmium sulfide (CdS)-decorated zinc oxide (ZnO) nanorod heterostructures have been grown by a combination of hydrothermal and pulsed laser deposition techniques. Hybrid photovoltaic devices have been fabricated with CdS modified and unmodified ZnO nanorods blended separately with regioregular poly(3-hexylthiophene) (P3HT) polymer as the active layer. The solar cell performance has been studied as a function of ZnO concentration and the casting solvent (chlorobenzene, chloroform, and toluene) in the unmodified ZnO:P3HT devices. The power conversion efficiency is found to be enhanced with the increase of ZnO concentration up to a certain limit, and decreases at a very high concentration. The surface modification of ZnO nanorods with CdS leads to an increase in the open circuit voltage and short-circuit current, with enhanced efficiency by 300% over the unmodified ZnO:P3HT device, because of the cascaded band structure favoring charge transfer to the external circuit.

High Efficiency Si/CdS Radial Nanowire Heterojunction Photodetectors Using Etched Si Nanowire Templates

Abstract: p-Si/n-CdS radial heterojunction nanowires have been grown by pulse laser deposition of CdS on vertically aligned Si nanowires fabricated using a room temperature wafer-scale etching of p-type Si. Temperature-dependent photoluminescence characteristics have been studied in detail in the blue–green–red regions from these p-Si/n-CdS core–shell nanowires. The photocurrent spectra of the nanowire heterojunctions have been investigated at room temperature to study the spectral responsivity and detectivity of the core–shell nanowire diodes. The peak responsivity (1.37 A/W) and detectivity (4.39 × 1011 cm Hz1/2/W) at −1 V show the potential of the nanoscaled devices for the high efficiency photodetectors in the visible–near-infrared spectrum.

Nonvolatile Memristive Switching Characteristics of TiO $_{\bm 2}$ Films Embedded With Nickel Nanocrystals

Abstract: Nickel nanocrystal (Ni-NC)-embedded titanium dioxide films have been deposited for nonvolatile resistive switching memory devices. The polycrystalline behavior of the films has been observed from the X-ray diffraction spectra. Tiny isolated Ni-NCs with an average size of 4 nm are observed for the 1000 °C, 5-min annealed samples. Stable, bipolar, nonvolatile, and bistable resistive switching states are observed for the optimized annealed Ni-NC-embedded devices with a low SET and RESET voltage of 0.8 and -0.2 V, respectively. A high resistance ratio (>;10), good stability, and retention properties are observed for the nanocrystal sample. The role of thin Ni-NC layer on memory switching stability is discussed.

A photoswitchable and photoluminescent organic semiconductor based on cation-π and carboxylate-pyridinium interactions: a supramolecular approach.

Abstract: The design and discovery of novel organic electronic materials still poses a major challenge owing to their utility in fieldeffect transistors, memory devices, solar cells, lightemitting diodes, displays, and plastic electronics. The organic materials are of interest for designing electronics because of their easy fabrication, modulation capabilities, and cost-effectiveness. Several polyaromatic materials, such as tetracene, pentacene, perylene, and rubrene, were shown to be semiconductors owing to the existence of aromatic– aromatic interactions. Studies on tetrathaiafulvalene (TTF), which is a prototypical organic semiconductor, reveal the increase in charge mobility when TTF molecules are assembled through face-to-face aromatic interactions. Furthermore, TTF and tetracyanoquinodimethane (TCNQ) are known to form strong charge-transfer complexes and exhibit metallic conductance. Similarly, donor–acceptor charge-transfer (CT) complexes of TTF-imidazole with TCNQ, p-benzoquinone, and anilic acid-type acceptors exhibit a number of conducting and semiconducting materials. Despite of these potential examples based on chargetransfer interactions, very few attempts have been made to design new molecular organic semiconductors using crystal engineering strategies. Recently, the organic salt of tetraphenylethylene tetracarboxylic acid with 4,4’-bispyridyl ethylene was shown to exhibit semiconducting characteristics (meff= 0.38 cm V 1 s) owing to the presence of charge-assisted hydrogen bonding. On the other hand, photochromic materials, which can change their color and properties upon irradiation by light, are of importance for optical information storage and photonic switches. To date, many organic molecules have been shown to be semiconductors, many have been shown to be photochromic and many have been shown to be photoluminescent. To the best of our knowledge, no organic materials have been shown to contain all these three properties in combination. One of the early examples of exhibiting semiconductivity and photochromism is triphenylformazan based on cis–trans isomerization upon photoirradiation. Alternately, such materials are designed by incorporating photochromic molecules as a side chain to p-conjugated polymers. However, to the best of our knowledge, the multicomponent organic supramolecular photochromic material with switchable electrical conductivity has not been reported to date. Herein we report a crystal engineering strategy for the synthesis of supramolecular two-component organic material that exhibits reversible photochromism (yellow$green) and reversible conductivity (low conductivity$high conductivity). Carboxylic acidand pyridine-containing molecules are known to form two component materials by COOH and pyridine hydrogen-bonding interactions. These materials can be termed as either cocrystals (O H···N) or salts (N H···O ) based on the position of the proton involved in the interaction. During our ongoing crystal engineering studies on exploring the hydrogen-bonded networks of co-crystals or salts of H41 with various bis(pyridyl) derivatives, we have

Optical and electrical properties of undoped and doped Ge nanocrystals.

Abstract: Size-dependent photoluminescence characteristics from Ge nanocrystals embedded in different oxide matrices have been studied to demonstrate the light emission in the visible wavelength from quantum-confined charge carriers. On the other hand, the energy transfer mechanism between Er ions and Ge nanocrystals has been exploited to exhibit the emission in the optical fiber communication wavelength range. A broad visible electroluminescence, attributed to electron hole recombination of injected carriers in Ge nanocrystals, has been achieved. Nonvolatile flash-memory devices using Ge nanocrystal floating gates with different tunneling oxides including SiO2, Al2O3, HfO2, and variable oxide thickness [VARIOT] tunnel barrier have been fabricated. An improved charge storage characteristic with enhanced retention time has been achieved for the devices using VARIOT oxide floating gate.

CdSe quantum dots-poly(3-hexylthiophene) nanocomposite sensors for selective chloroform vapor detection at room temperature

Abstract: Olive oil capped CdSe quantum dots (QDs) of average size ∼6 nm have been grown by a green chemical route synthesis for the fabrication of nanocomposite organic vapor sensing devices. A highly selective, room temperature chloroform vapor sensor has been fabricated using capped CdSe QDs and conducting polymer [poly(3-hexylthiophene)] nanocomposites. The nanocomposite sensor has been tested with the choloroform vapor of concentration varying from 100-1200 ppm at room temperature using different bias voltages. The recovery time of the sensor has been found to be improved on illumination with a monochromatic light of 600 nm, due to the photo-induced enhancement of charge transfer in nanocomposites.

Optical and bio-sensing characteristics of ZnO nanotubes grown by hydrothermal method.

Abstract: ZnO nanostructures were fabricated on copper substrates by hydrothermal method at an optimized growth temperature of -95 degrees C. Structural properties were investigated by field emission scanning electron and transmission electron microscopy. Distinct morphologies were found to be formed at different growth times. The formation of nanotubes mainly involved the initial nucleation followed by the growth of nanorods at 95 degrees C, and then with the increase of dissolution time at room temperature, the preferential chemical dissolution of the metastable Zn-rich [0001] polar surfaces resulted in removing the atoms from the surfaces, thus leading to the thinning of the wall of the nanostructures. Completely hollow ZnO nanotubes could be obtained at a high dissolution time. The room temperature photoluminescence and optical absorption properties of ZnO nanotubes have been studied as a function of dissolution time. The efficacy of ZnO nanotubes for glucose sensing applications has been studied.

Effect of Fungicides and Insecticides on Growth and Enzyme Activity of Four Cyanobacteria

Abstract: Cyanobacterial populations introduced into crop fields as biofertilizer become non-target organisms for the pesticides and fungicides applied in the field Effect of four commonly used pesticides viz Bagalol, Mancozeb (fungicides), Thiodan and Phorate (insecticides) was studied on growth and different enzymes of four cyanobacterial species viz Nostoc ellipsosporum, Scytonema simplex, Tolypothrix tenuis, and Westiellopsis prolifica EC 50 concentration of each pesticide was determined for all cyanobacteria Bagalol and Thiodan were found to be the most toxic Both the fungicides and insecticides inhibited the activity of nitrogenase and glutamine synthetase (GS) at EC 50 concentration in all the four species studied Bagalol incurred maximum inhibition of nitrogenase and GS activity on N ellipsosporum and S simplex while Thiodan and Phorate had maximum effect on T tenuis, and W prolifica Mancozeb had lesser effect on all the above enzymes One catabolic enzyme of carbohydrate metabolism, isocitrate dehydrogenase (ICDH) and one anabolic enzyme isocitrate lyase (ICL), which is related to glyoxylate pathway as well as gluconeogenesis, were also assayed Cell free extracts of cyanobacteria treated with pesticides for 7 days show a drastic reduction of ICDH activity ICL activity was induced in the organisms when treated with pesticides

Photophysics of resonantly and non-resonantly excited erbium doped Ge nanowires

Abstract: We have fabricated Er doped germanium nanowires of different diameters by pulsed laser deposition and chemical methods. Er induced photoluminescence emission due to the intra-4f 4I13/2→4I15/2 transition of Er energy levels at 1.53 µm has been achieved at room temperature using both resonant (980 nm) and non-resonant (325 nm) excitation of Er ions. The observed 1.53 µm photoluminescence signal upon non-resonant 325 nm excitation is attributed to the Ge related oxygen deficiency centers surrounding the Ge core. For direct excitation, the infrared photoluminescence characteristics have been studied as a function of Er concentration, photon flux, and diameter of the nanowires. The Er related emission signal is found to be enhanced with increase in Er concentration, pump flux of 980 nm, and the nanowire diameter. The time resolved characteristics of the Er induced emission peak have been studied as a function of the pump flux as well as the diameter of the Ge nanowires.

Photoluminescence and positron annihilation spectroscopic investigation on a H+ irradiated ZnO single crystal

Abstract: Low temperature photoluminescence and room temperature positron annihilation spectroscopy have been employed to investigate the defects incorporated by 6?MeV H+ ions in a hydrothermally grown ZnO single crystal Prior to irradiation, the emission from donor bound excitons is at 3378?eV (10?K) The irradiation creates an intense and narrow emission at 3368?eV (10?K) The intensity of this peak is nearly four times that of the dominant near band edge peak of the pristine crystal The characteristic features of the 3368?eV emission indicate its origin as a ?hydrogen at oxygen vacancy? type defect The positron annihilation lifetime measurement reveals a single component lifetime spectrum for both the unirradiated (164???1?ps) and irradiated crystal (175???1?ps) It reflects the fact that the positron lifetime and intensity of the new irradiation driven defect species are a little higher compared to those in the unirradiated crystal However, the estimated defect concentration, even considering the high dynamic defect annihilation rate in ZnO, comes out to be ?4???1017?cm?3 (using SRIM software) This is a very high defect concentration compared to the defect sensitivity of positron annihilation spectroscopy A probable reason is the partial filling of the incorporated vacancies (positron traps), which in ZnO are zinc vacancies The positron lifetime of ?175?ps (in irradiated ZnO) is consistent with recent theoretical calculations for partially hydrogen-filled zinc vacancies in ZnO Passivation of oxygen vacancies by hydrogen is also reflected in the photoluminescence results A possible reason for such vacancy filling (at both Zn and O sites) due to irradiation has also been discussed

Cadmium Sulfide Nanostructures for Photovoltaic Devices

Abstract: Cadmium sulfide (CdS) nanostructures have been found to be attractive for possible applications in semiconductor lasers, light emitting diodes, photovoltaic cells, display devices and biological sensing. The present paper reviews our work on the potential of CdS based nanostructures and composites for photovoltaic and photosensitive devices. The realization of a hybrid photovoltaic device has been demonstrated by the dispersion of multi-walled carbon nanotube (MWCNT)–CdS nanocomposites in a conducting polymer [poly(3-hexylthiophene)] (P3HT) matrix. An improved photovoltaic effect has been observed in MWCNT–CdS/P3HT composite over control P3HT polymeric device. An improved photocurrent spectrum with broadband response (visible to near-infrared region) has been demonstrated in the Ge/CdS core–shell heterojunction nanowires, making them useful candidates for nanowire heterojunction solar cells.

Record Resistance Ratio and Bipolar/Unipolar Resistive Switching Characteristics of Memory Device Using Germanium Oxide Solid Electrolyte

Abstract: The bipolar and unipolar resistive switching characteristics of a memory device using a Cu filament in a new Cu/GeOx/W structure under low-voltage operation ( 100 µA. This memory device has excellent uniformity in SET/RESET voltages, low resistance state/high resistance state (LRS/HRS), long read endurance of >1×105 cycles, and good data retention of >1×104 s with high resistance ratios of >105 in the bipolar mode and >109 in the unipolar mode.

Comparative study of mangrove litter nitrogen cycling to the adjacent estuary through modelling in pristine and reclaimed islands of Sundarban mangrove ecosystem, India

Abstract: Sundarban mangrove ecosystem is consisted of many islands both virgin and reclaimed in nature. A seven compartment model featuring different forms of nitrogen in soil and water is applied on virgin island in order to compare for estimating several parameters with reclaimed island. The physical, chemical factors and other rate parameters are retained same as in reclaimed island model, published before. The study is carried out for over two years, primary and secondary data are used in the model belong to same time scale for both the islands. Loss rate of soil organic nitrogen as humic acid and fulvic acids is very sensitive parameter for both the systems. Model results depict that virgin island is detritus based system governed by leaching and microbial processes.

Modelling the impact of mangroves on fish population dynamics of Hooghly - Matla estuarine system, West Bengal, India

Abstract: Breakdown of mangrove litter, adjacent to Hooghly-Matla estuarine ecosystem, plays a major role in regulating the fish population of the estuary. For last several decades, there has been a gradual decline in fish population due to over exploitation of resource but the exact cause of this decrease in production is still not known. To know this cause, an eleven compartment holistic model of nutrient source to estuary and grazing food chain is constructed. Model results depict the role of mangroves in regulating fish population dynamics of the estuary and predation rate of carnivorous fish on detritivorous fish is sensitive parameter in maintaining the balance between grazing food chain and detritus food chain.

Growth and low-temperature photoluminescence properties of hybrid ZnO-SnO2 nanobelts

Abstract: Hybrid ZnO–SnO2 nanobelts were synthesized through a simple thermal evaporation technique without using any catalyst. Detailed microstructural investigation showed that the nanobelts possessed doped/alloyed wurtzite (ZnO) and rutile (SnO2) structures. The diameter and length of the nanobelts were in the ranges 100–200 and 20–40 µm, respectively. Low-temperature photoluminescence properties of the hybrid ZnO–SnO2 nanostructured aggregate revealed a red shift of near-band-edge emission peaks of ZnO with increasing temperature. The method of synthesis offers a convenient and effective technique of producing hybrid ZnO–SnO2 nanobelts for gas sensing in the large quantity.

A Study on Bacterial Disease of Betelvine in West Bengal, India

Abstract: Betelvine (Piper betle L.) is an important cash crop in West Bengal. This crop is commonly affected by stem rot and leaf spot disease caused by two different genera of bacteria- Xanthomonas axonopodis pv. betlicola and Pseudomonas betle. Two bacterial pathogens enter into the host through stomata, hydathode and injury. Both the bacteria, at any portion, of the vine stem form prominent dark brown lesions. Surface of such lesion becomes sticky in humid condition. Occasionally small brown cracked lesions are found on the stem. Such lesion is formed due to infection of X. a. pv. betlicola. On the leaf, small to large, circular to irregular and/or angular brown colored spots and marginal leaf blight symptoms are produced by both the bacteria. All types of spots are surrounded by yellow halo or the halo is present in between brown and green tissue. At the under side of the leaf, the brown lesion is encircled by a water soaked zone or water soaked area, which is found in between brown lesion and green tissue in marginal blight. In addition, yellow colony forming bacterium, X. a. pv. betlicola produces very small brown spot surrounded by prominent yellow halo but without water soaked zone. Frequently both the bacteria have been detected from same leaf spot or stem lesion. Association of these bacteria increases disease severity. Initially the bacteria colonize in parenchyma tissue and later move into vascular tissue. After entry into the vascular tissue X. a. pv. betlicola becomes systemic, produces no further lesion but causes sudden wilting of vine. Such situation causes much damage of the plantation in West Bengal. Pre-inoculation treatment with streptomycin and oxy-tetracycline hydrochloride greatly reduces lesion expansion.

Preferential ordering of self-assembled Ge islands on focused ion-beam patterned Si(100)

Abstract: We report the growth of Ge islands on Si (001) substrates with lithographically defined two-dimensionally periodic pits using focused ion-beam patterning and molecular beam epitaxy. The formation of circularly ordered Ge islands has been achieved by means of nonuniform strain field around the periphery of the holes due to ion bombardment. Lateral ordering of the Ge islands have been controlled by both the pit size and pit separation. Preferential growth at the pit sites has also been achieved by using appropriate pattern shape and size.

Record Resistance Ratio and Bipolar/Unipolar Resistive Switching Characteristics of Memory Device Using Germanium Oxide Solid Electrolyte (Special Issue : Solid State Devices and Materials (2))

Abstract: The bipolar and unipolar resistive switching characteristics of a memory device using a Cu filament in a new Cu/GeOx/W structure under low-voltage operation ( 100 µA. This memory device has excellent uniformity in SET/RESET voltages, low resistance state/high resistance state (LRS/HRS), long read endurance of >1×105 cycles, and good data retention of >1×104 s with high resistance ratios of >105 in the bipolar mode and >109 in the unipolar mode.

Optical characteristics of pulsed laser deposited Ba0.8Sr0.2TiO3 thin films grown on fused quartz substrate

Abstract: Thin films of Ba0.8Sr0.2TiO3 (BST) have been deposited on fused quartz substrate by pulsed laser ablation technique at different substrate temperatures. X-Ray diffraction analysis clearly showed the formation of a perovskite structure at substrate temperature 6000 C. An amorphous nature was found for film deposited below 5000 C, and their inferior crystallinity suggested the formation of an incomplete perovskite phase. Optical band gap calculated from UV-visible transmittance spectra decreases with the increasing substrate temperatures. The infrared (IR) spectra with dominant absorption bands due to Ti-O stretching and Ti-O bending suggest the formation of a single-phase perovskite BST film in pulsed laser deposition.