Showing papers by "Samit K. Ray published in 2010"
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TL;DR: In this article, the authors have investigated strain interaction in 10 layer QD heterostructure with varying thicknesses of combination capping (InAlGaAs and GaAs) by means of scanning transmission electron microscopy (STEM), high-resolution X-ray diffraction (HRXRD) and Raman scattering.
71 citations
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TL;DR: In this paper, the effect of postdeposition annealing on the resistance switching properties of NiO/Pt structures has been discussed and the current conduction phenomena at two resistance states have been studied.
Abstract: Unipolar nonvolatile resistive switching memory properties of pulse laser ablated nickel oxide films have been studied. Grazing incidence x-ray diffraction and electron diffraction spectra of the oxide films reveal polycrystalline nature of deposited NiO films. Cross-sectional transmission electron micrograph shows a fairly uniform oxide surface. The rms surface roughnesses of deposited oxides have been studied as a function of annealing temperature using atomic force microscopy. By applying a proper voltage bias and compliance, Pt/NiO/Pt structures exhibited unipolar resistive switching having a very low SET and RESET voltages. The OFF state resistance and SET voltage are found to increase with the increase in annealing temperature. The ratio between the two resistance states can be as high as 1000. The current conduction phenomena at two resistance states have been studied. The switching phenomena have been explained using the rupture and formation of conducting filaments. The effect of postdeposition annealing on the resistance switching properties is discussed.
49 citations
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TL;DR: In this paper, the photoluminescence transitions were observed at 10 K, below and above the optical band gap of bulk Ge/Si quantum dot, which was explained by the excitonic electric field originated due to infrared excitation at low temperatures.
Abstract: We report on the observation of intraband near infrared (∼3.1 μm) and mid infrared (∼6.2 μm) photocurrent response at room temperature using Ge/Si self-assembled quantum dots grown by molecular beam epitaxy. Due to the bimodal size distribution and SiGe intermixing, distinguishable photoluminescence transitions are observed at 10 K, below and above the optical band gap of bulk Ge. The observed redshift in photocurrent with increasing temperature has been explained by the excitonic electric field originated due to infrared excitation at low temperatures. A good correlation between the spectral photocurrent response and photoluminescence of the quantum dots has been established.
41 citations
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TL;DR: In this article, the defect characterization in 1.2 MeV Ar8+ irradiated polycrystalline ZnO has been carried out by x-ray diffraction, scanning electron microscopy (SEM) along with electrical resistivity, and photoluminescence (PL) measurements at room temperature (RT).
Abstract: Defect characterization in 1.2 MeV Ar8+ irradiated polycrystalline ZnO has been carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM) along with electrical resistivity, and photoluminescence (PL) measurements at room temperature (RT). Interestingly, irradiation with the initial fluence (1×1015 ions/cm2) changes the color of the sample from white to orange while the highest irradiation fluence (5×1016 ions/cm2) makes it dark reddish brown that appears as black. XRD study reveals no significant change in the average grain size of the samples with irradiation fluence. Increase in surface roughness due to sputtering is clearly visible in SEM with highest fluence of irradiation. RT PL spectrum of the unirradiated sample shows intense ultraviolet (UV) emission (∼3.27 eV) and less prominent defect level emissions (2–3 eV). The overall emission is largely quenched due to initial irradiation fluence. Increasing the fluence of Ar beam further, UV emission is enhanced along with prominent defect...
40 citations
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TL;DR: In this paper, the effect of thermal annealing on the photovoltaic properties of an organic solar cell based on an efficient donor/acceptor combination of pentacene / N,N′-Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) discrete heterojunctions was studied.
27 citations
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TL;DR: In this article, the authors reported the self-assembly of several micrometer long n-type organic nanoribbons with 100-200 nm width from N,N′-dioctyl-3,4,9,10-perylenedicarboximide molecules.
Abstract: One-dimensional nanostructures like nanowires, nanoribbons, or nanobelts are attractive building blocks for optoelectronic device applications. Here we report the self-assembly of several micrometer long n-type organic nanoribbons with 100–200 nm of width from N,N′-dioctyl-3,4,9,10-perylenedicarboximide molecules. Significant change in absorption spectra above 550 nm indicates strong π-π stacking along the long axis of nanoribbons. Photovoltaic properties were investigated by mixing them with two well studied donor materials. Strong photoluminescence (PL) quenching and significant decrease in PL life time surmise efficient photoinduced charge transfer and exciton dissociation between nanostructures and donor materials, which leads to a power conversion efficiency of about 1%.
25 citations
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TL;DR: The growth mechanisms of the formation of the nanostructures have also been discussed in this article, where X-ray diffraction patterns showed that the nanowires and nanobelts are highly crystalline with tetragonal rutile phase.
Abstract: SnO2 nanowires and nanobelts have been grown by the thermal evaporation of Sn powders. The growth of nanowires and nanobelts has been investigated at different temperatures (750–1000°C). The field emission scanning electron microscopic and transmission electron microscopic studies revealed the growth of nanowires and nano-belts at different growth temperatures. The growth mechanisms of the formation of the nanostructures have also been discussed. X-ray diffraction patterns showed that the nanowires and nanobelts are highly crystalline with tetragonal rutile phase. UV-visible absorption spectrum showed the bulk bandgap value (∼ 3–6 eV) of SnO2. Photoluminescence spectra demonstrated a Stokes-shifted emission in the wavelength range 558–588 nm. The Raman and Fourier transform infrared spectra revealed the formation of stoichiometric SnO2 at different growth temperatures.
25 citations
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TL;DR: In this article, the authors used copper phthalocyanine (CuPc) nanorods as donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as acceptor material.
Abstract: We have fabricated efficient organic photovoltaic cells using copper phthalocyanine (CuPc) nanorods as donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as the acceptor material. Highly dense randomly oriented and vertically aligned nanorods with diameters of about 30?50?nm have been achieved from vacuum-deposited CuPc films by simple surface solvent treatment. X-ray diffraction confirms the polycrystalline nature of the CuPc nanorods. Significantly improved cell performance was observed with the change in shape and orientations of the nanorods. Maximum power conversion efficiency of 2.57% was obtained from the vertically aligned nanorods, which is a result of an increment in the donor?acceptor interface area and efficient photogenerated charge carrier transports.
24 citations
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22 Feb 2010-Journal of Environmental Science and Health Part A-toxic\/hazardous Substances & Environmental Engineering
TL;DR: A kinetic model is proposed for the oxidation of 1,4-dioxane by Fenton reagent and this model is applied to calculate the kinetic rate constant for the reaction between hydroxyl radicals and 1, 4 dioxane.
Abstract: Kinetic studies of oxidative degradation of 1,4-dioxane present in aqueous solutions in the concentration range between 2.27 x 10(-3) mol/L (200 ppm) and 5.68 x 10(-3) mol/L (500 ppm) were conducted in batch rector. This analysis considers the hydroxyl radicals ((*)OH) produced by Fenton reagent as the main species responsible for the degradation process. 1,4-dioxane was degraded to the extent of 97 to 99% after 30 min of reaction time. The pH of the reaction medium decreased substantially during the experiment. In all cases, most of 1,4-dixane degradation occurred during the initial 5 min of reaction. A kinetic model is proposed for the oxidation of 1,4-dioxane by Fenton reagent and this model is applied to calculate the kinetic rate constant for the reaction between hydroxyl radicals and 1,4 dioxane. The reaction rate constant for the reaction of 1,4-dioxane with (*)OH radicals was evaluated and the value is 2.25 x 10(8)/M-s.
16 citations
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TL;DR: In this paper, the formation of spherical shape Ge nanocrystals of 4-6 nm diameters for 800 °C and 6-9 nm for 900 Ã −C annealed samples was confirmed by X-ray photoelectron spectroscopy.
Abstract: Ge nanocrystals embedded in HfO2 matrices were prepared by rf magnetron sputtering technique. Transmission electron micrographs reveal the formation of spherical shape Ge nanocrystals of 4–6 nm diameters for 800 °C and 6–9 nm for 900 °C annealed samples. X-ray photoelectron spectroscopy confirms the formation of surface oxidized Ge nanocrystals. Embedded Ge nanocrystals show strong photoluminescence peaks in visible and ultraviolet region even at room temperature. Spectral analysis suggests that emission in 1.58 and 3.18 eV bands originate from TΣ(TΠ)→S0, and TΠ′→S0 optical transitions in GeO color centers, respectively, and those in the range 2.0–3.0 eV are related to Ge/O defects at the interface of the oxidized nanocrystals. Temperature dependent photoluminescence study has revealed additional fine structures with lowering of temperature, the origin of which is attributed to the strong coupling of electronic excitations with local vibration of germanium oxides at the surface.
12 citations
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TL;DR: ZnO flower-like nanostructures were grown on Ge (100) substrate, by a modified chemical vapor condensation technique of zinc acetate dihydrate at 300 °C, without using any catalyst.
Abstract: ZnO flower-like nanostructures were grown on Ge (100) substrate, by a modified chemical vapor condensation technique of zinc acetate dihydrate at 300 °C, without using any catalyst. These self-organized three-dimensional nanostructures were composed of hierarchical arrangement of ZnO nanorods of diameter ~50 nm around a common nucleus and were distributed uniformly over the entire substrate surface. Evolution study of these structures indicates that the growth begins with a two-dimensional planar arrangement of (0001)-oriented ZnO nanorods. With increasing growth time, the expanding adjacent two-dimensional growth fronts approach each other, followed by which, the formation of three-dimensional flower-like structures evolve. Surface diffusion mechanism seems to play an important role in forming these nanostructures, which has been discussed in detail. Elaborate electron microscopic (SEM, TEM) techniques have been used to investigate the growth characteristics of the flower structures. The photoluminescence measurements showed pure free excitonic transition centered at about 3.249 eV with full width at half-maximum of about 141 meV at 300 K, which blue shifted to 3.361 eV at 10 K with corresponding half width of 7 meV with no defect-related bandgap peak due to relatively low growth temperature. The optical emission area was imaged through a cathodoluminescence technique. Scanning electron micrograph of a typical ZnO nanorod flower structure grown at 300 °C on Ge (100).
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TL;DR: Self-assembled Ge x Si 1− x islands were grown on Si substrates by solid source molecular beam epitaxy and the electrical properties of the islands were studied through photoexcited I – V characteristics and current imaging using conducting mode atomic force microscopy.
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TL;DR: In this article, the effect of the presence of oxygen on the film deposition rate and mechanism and the physical properties of the films, particularly the step coverage properties (conformality), are discussed in detail.
Abstract: Silicon dioxide films deposited from tetraethylorthosilicate (TEOS) using plasma-enhanced chemical vapour deposition (PECVD) are reviewed. The effect of the presence of oxygen on the film deposition rate and mechanism and the physical properties of the films, particularly the step coverage properties (conformality), are discussed in detail. Structural characterisation of the films has been carried out via etch rate measurements, infrared transmission spectroscopy, X-ray photoelectron spectroscopy (XPS) and Auger and secondary ion mass spectroscopy (SIMS) analysis. Electrical properties, i.e. resistivity, breakdown strength, fixed oxide charge density, interface state density and trapping behaviour, have been evaluated using metal-oxide-semiconductor (MOS) structures fabricated using the deposited oxides. Films deposited by microwave plasma-enhanced decomposition of TEOS in the presence of oxygen have been found to be comparable with standard silane-based low-pressure chemical vapour deposition (LPCVD) and PECVD oxides. It has been shown that films deposited on thin native oxides grown by either in situ plasma oxidation or low-temperature thermal oxidation exhibit excellent electrical properties.
05 Feb 2010
TL;DR: In this article, the growth and characteristics of Ge-CdS core-shell nanowire radial heterojunctions and memory characteristics were analyzed for organic light emitting diodes, photovoltaic cells and flat panel displays.
Abstract: Germanium nanowires are attractive due to their higher carrier mobilities and lower effective masses and compatibility with Si-based CMOS devices. Semiconductor heterojunctions grown on one dimensional nanowire templates have been demonstrated to be useful for photoelectrochemical cell, light-emitting diodes, electrochromic devices, and sensor systems. This core-shell nanowire heterojunction possesses a unique advantage of high surface area-to-volume ratio with carrier separation taking place in the junction in radial direction within one diffusion length of minority carriers. These nanowires sandwiched/embedded in polymers are also attractive for applications in organic light emitting diodes, photovoltaic cells, flat panel displays and in organic field effect transistors. In addition, the memory devices using semiconductor/organic heterostructures make them useful for the realization of flexible and low cost polymeric electronic and optoelectronic devices. We shall present the growth and characteristics of Ge-CdS core-shell nanowire radial heterojunctions and memory characteristics of Ge/GeO 2 core-shell nanowires sandwiched between a tunnel silicon oxide and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) polymer.