Indium Nitride Nanometric-Objects on c-Sapphire Grown by Plasma-Assisted Molecular Beam Epitaxy

The indium nitride (InN)-based nanometric-objects were grown directly on a c-sapphire substrate by using plasma-assisted molecular beam epitaxy (PAMBE) at different substrate temperatures. High resolution X-ray diffraction (HRXRD) reveals the InN (0002) reflection and full width at half maximum (FWHM) found to be decreased with increasing the growth temperature. The size, height and density of the grown nanometric-objects studied by scanning electron microscopy (SEM) has remarkable differences, evidencing the decisive role of substrate temperature. Photoluminescence (PL) studies revealed that the emission energy is shifted towards the higher side from the bulk value, i.e., a blue shift in the PL spectra was observed. The temperature dependence of the PL peak position shows an ``S-shaped'' emission energy shift, which can be attributed to the localization of carriers in the nanometric-objects.

A Study of Nanostructures of Thin Films in B–C–N System Produced by Pulsed Laser Deposition and Nitrogen Ion-Beam-Assisted Pulsed Laser Deposition

We report the synthesis of thin films of B–C–N and C–N deposited by N+ ion-beam-assisted pulsed laser deposition (IBPLD) technique on glass substrates at different temperatures. We compare these films with the thin films of boron carbide synthesized by pulsed laser deposition without the assistance of ion-beam. Electron diffraction experiments in the transmission electron microscope shows that the vapor quenched regions of all films deposited at room temperature are amorphous. In addition, shown for the first time is the evidence of laser melting and subsequent rapid solidification of B4C melt in the form of micrometer- and submicrometer-size round particulates on the respective films. It is possible to amorphize B4C melt droplets of submicrometer sizes. Solidification morphologies of micrometer-size droplets show dispersion of nanocrystallites of B4C in amorphous matrix within the droplets. We were unable to synthesize cubic carbon nitride using the current technique. However, the formation of nanocrystalline turbostratic carbo- and boron carbo-nitrides were possible by IBPLD on substrate at elevated temperature and not at room temperature. Turbostraticity relaxes the lattice spacings locally in the nanometric hexagonal graphite in C–N film deposited at 600 °C leading to large broadening of diffraction rings.

Effect of irradiation on the microstructure and mechanical behavior of nanocrystalline nickel

The effect of 4.0 MeV proton irradiation on the microstructure and mechanical properties of nanocrystalline (nc) nickel was investigated. The irradiation damage induced in the sample was of the order of 0.004 dpa. Transmission electron microscopy of irradiated samples indicated the presence of dislocation loops within the grains. An increase in hardness and strain-rate sensitivity (m) of nc-Ni with irradiation was noted. The rate-controlling deformation mechanism in irradiated nc-Ni was identified to be interaction of dislocations with irradiation-induced defects. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Substrate nitridation induced modulations in transport properties of wurtzite GaN/p-Si (100) heterojunctions grown by molecular beam epitaxy

Phase pure wurtzite GaN films were grown on Si (100) substrates by introducing a silicon nitride layer followed by low temperature GaN growth as buffer layers. GaN films grown directly on Si (100) were found to be phase mixtured, containing both cubic (beta) and hexagonal (alpha) modifications. The x-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy studies reveal that the significant enhancement in the structural as well as in the optical properties of GaN films grown with silicon nitride buffer layer grown at 800 degrees C when compared to the samples grown in the absence of silicon nitride buffer layer and with silicon nitride buffer layer grown at 600 degrees C. Core-level photoelectron spectroscopy of Si(x)N(y) layers reveals the sources for superior qualities of GaN epilayers grown with the high temperature substrate nitridation process. The discussion has been carried out on the typical inverted rectification behavior exhibited by n-GaN/p-Si heterojunctions. Considerable modulation in the transport mechanism was observed with the nitridation conditions. The heterojunction fabricated with the sample of substrate nitridation at high temperature exhibited superior rectifying nature with reduced trap concentrations. Lowest ideality factors (similar to 1.5) were observed in the heterojunctions grown with high temperature substrate nitridation which is attributed to the recombination tunneling at the space charge region transport mechanism at lower voltages and at higher voltages space charge limited current conduction is the dominating transport mechanism. Whereas, thermally generated carrier tunneling and recombination tunneling are the dominating transport mechanisms in the heterojunctions grown without substrate nitridation and low temperature substrate nitridation, respectively. (C) 2011 American Institute of Physics. [doi:10.1063/1.3658867]

Gray track formation in KTiOPO4 by swift ion irradiation

Potassium titanyl phosphate single crystals were irradiated with 48 MeV lithium ions at fluences varying from 5×1012 to 1016 ions/cm2. The defects created in the crystal have been characterized using x-ray rocking curve measurements, optical transmittance, and photoluminescence spectroscopy. From x-ray rocking curve studies, the full width at half maximum for the irradiated samples was observed to increase, indicating lattice strain caused by the energetic ions. Optical transparency of these samples was found to decrease upon irradiation. The irradiated samples exhibited a broadband luminescence in the 700–900 nm region, for fluences above 5×1013 ions/cm2. The results indicate that ion-beam-induced optical effects in KTiOPO4 single crystals are very similar to the ones obtained for crystals with “gray tracks,” which are attributed to the electronic transitions in the Ti3+ levels.

Size dependent bandgap of molecular beam epitaxy grown InN quantum dots measured by scanning tunneling spectroscopy

InN quantum dots (QDs) were grown on Si (111) by epitaxial Stranski-Krastanow growth mode using plasma-assisted molecular beam epitaxy. Single-crystalline wurtzite structure of InN QDs was verified by the x-ray diffraction and transmission electron microscopy. Scanning tunneling microscopy has been used to probe the structural aspects of QDs. A surface bandgap of InN QDs was estimated from scanning tunneling spectroscopy (STS) I-V curves and found that it is strongly dependent on the size of QDs. The observed size-dependent STS bandgap energy shifts with diameter and height were theoretical explained based on an effective mass approximation with finite-depth square-well potential model.

Reactive biased target ion beam deposited W-DLC nanocomposite thin films - Microstructure and its mechanical properties

Tungsten incorporated diamond like carbon (W-DLC) nanocomposite thin films with variable fractions of tungsten were deposited by using reactive biased target ion beam deposition technique. The influence of tungsten incorporation on the microstructure, surface topography, mechanical and tribological properties of the DLC were studied using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy. Atomic force microscope (AFM), transmission electron microscopy (TEM), nano-indentation and nano-scratch tests. The amount of W in films gets increases with increasing target bias voltage and most of the incorporated W reacts with carbon to form WC nanoclusters. Using TEM and FFT pattern, it was found that spherical shaped WC nanoclusters were uniformly dispersed in the DLC matrix and attains hexagonal (W2C) crystalline structure at higher W concentration. On the other hand, the incorporation of tungsten led to increase the formation of C-sp(2) hybridized bonding in DLC network and which is reflected in the hardness and elastic modulus of W-DLC films. Moreover, W-DLC films show very low friction coefficient and increased adhesion to the substrate than the DLC film, which could be closely related to its unique nanostructure of the W incorporated thin films. (C) 2011 Elsevier B.V. All rights reserved.

The irradiation of 1:1 mixture of ammonia:carbon dioxide ice at 30 K using 1 kev electrons

In this Letter the results of an experimental investigation of 1 keV electron irradiation of a 1:1 ice mixture of NH3:CO2 at 30 K was made under ultrahigh vacuum (10(-9) mbar) conditions. Molecular products formed within the ice were detected and monitored using FTIR spectroscopy. The formation of ammonium ions (NH4+), cyanate ions (OCN-), CO was observed leading to the synthesis of ammonium carbamate (NH4NH2CO2). The consequences of these results for prebiotic chemistry in the interstellar medium and star forming regions are discussed. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.

Structural Characterization and Ultraviolet Photoresponse of GaN Nanodots Grown by Molecular Beam Epitaxy

The present work explores the electrical transport and UV photoresponse properties of GaN nanodots (NDs) grown by molecular beam epitaxy (MBE). Single-crystalline wurtzite structure of GaN NDs is verified by X-ray diffraction and transmission electron microscopy (TEM). The interdigitated electrode pattern was created and current-voltage (I-V) characteristics of GaN NDs were studied in a metal-semiconductor-metal configuration. Dark I-V characteristics of lateral grown GaN NDs obeyed the Frenkel-Poole emission model, and the UV response of the device was stable and reproducible with on/off. The responsivity of the detectors is found to be 330 A/W with an external quantum efficiency of 1100%. (C) 2012 The Japan Society of Applied Physics

The irradiation of pure CH3OH and 1:1 mixture of NH3:CH3OH ices at 30 K using low energy electrons

The results of an experimental investigation of 1 keV electron irradiation of ices (deposited at 30 K) of (i) pure methanol and (ii) of a 1:1 mixture of NH3:CH3OH are reported. Molecular products formed within the ice were detected and monitored using FTIR spectroscopy. The products observed were methyl formate (H3COHCO), methane (CH4), hydroxymethyl (CH2OH), formamide (HCONH2), formic acid (HCOOH), formaldehyde (H2CO), formyl radical (HCO), cyanate ion (OCN-), isocyanic acid (HNCO), carbon monoxide (CO) and carbon dioxide (CO2). The consequences of these results for prebiotic chemistry in the interstellar medium and star forming regions are discussed. Crown Copyright (C) 2012 Published by Elsevier B. V. All rights reserved.

Beam polarization effects in the radiative production of lightest neutralinos in e(+)e(-) collisions in supersymmetric grand unified models

We study the production of the lightest neutralinos in the process e(+)e(-) -> chi(0)(1)chi(0)(1)gamma in supersymmetric grand unified models for the International Linear Collider energies with longitudinally polarized beams. We consider cases where the standard model gauge group is unified into the grand unified gauge groups SU(5), or SO(10). We have carried out a comprehensive study of this process in the SU(5) and SO(10) grand unified theories which includes the QED radiative corrections. We compare and contrast the dependence of the signal cross section on the grand unified gauge group, and on the different representations of the grand unified gauge group, when the electron and positron beams are longitudinally polarized. To assess the feasibility of experimentally observing the radiative production process, we have also considered in detail the background to this process coming from the radiative neutrino production process e(+)e(-)-> nu(nu) over bar gamma with longitudinally polarized electron and positron beams. In addition we have also considered the supersymmetric background coming from the radiative production of scalar neutrinos in the process e(+)e(-) -> (nu) over tilde(nu) over tilde*gamma with longitudinally polarized beams. The process can be a major background to the radiative production of neutralinos when the scalar neutrinos decay invisibly.

Effects of growth temperature on nonpolar a-plane InN grown by molecular beam epitaxy

Nonpolar a-plane InN films were grown on r-plane sapphire substrate by plasma assisted molecular beam epitaxy with GaN underlayer. Effect of growth temperature on structural, morphological, and optical properties has been studied. The growth of nonpolar a-plane (1 1 -2 0) orientation was confirmed by high resolution X-ray diffraction study. The film grown at 500 degrees C shows better crystallinity with the rocking curve FWHM 0.67 degrees and 0.85 degrees along 0 0 0 1] and 1 - 1 0 0] directions, respectively. Scanning electron micrograph shows formation of Indium droplets at higher growth temperature. Room temperature absorption spectra show growth temperature dependent band gap variation from 0.74-0.81 eV, consistent with the expected Burstein-Moss effect. The rectifying behaviour of the I-V curve indicates the existence of Schottky barrier at the InN and GaN interface. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A transmission electron microscopy and X-ray photoelectron spectroscopy study of annealing induced gamma-phase nucleation, clustering, and interfacial dynamics in reactively sputtered amorphous alumina thin films

Pure alpha-Al2O3 exhibits a very high degree of thermodynamical stability among all metal oxides and forms an inert oxide scale in a range of structural alloys at high temperatures. We report that amorphous Al2O3 thin films sputter deposited over crystalline Si instead show a surprisingly active interface. On annealing, crystallization begins with nuclei of a phase closely resembling gamma-Alumina forming almost randomly in an amorphous matrix, and with increasing frequency near the substrate/film interface. This nucleation is marked by the signature appearance of sharp (400) and (440) reflections and the formation of a diffuse diffraction halo with an outer maximal radius of approximate to 0.23 nm enveloping the direct beam. The microstructure then evolves by a cluster-coalescence growth mechanism suggestive of swift nucleation and sluggish diffusional kinetics, while locally the Al ions redistribute slowly from chemisorbed and tetrahedral sites to higher anion coordinated sites. Chemical state plots constructed from XPS data and simple calculations of the diffraction patterns from hypothetically distorted lattices suggest that the true origins of the diffuse diffraction halo are probably related to a complex change in the electronic structure spurred by the a-gamma transformation rather than pure structural disorder. Concurrent to crystallization within the film, a substantially thick interfacial reaction zone also builds up at the film/substrate interface with the excess Al acting as a cationic source. (C) 2015 AIP Publishing LLC.

High photoconductive combustion synthesized TiO2 derived nanobelts for photocatalytic water purification under solar irradiation

Drinking water scarcity is a major issue that needs to be addressed seriously. Water needs to be purified from organic pollutants and bacterial contamination. In this study, sunlight driven photocatalysis for the degradation of dyes and bacterial inactivation has been conducted over TiO2 nanoparticles (CST) and TiO2 nanobelts (CSTNB). TiO2 nanoparticles were synthesized by a solution combustion process using ascorbic acid as a fuel. Acid etched TiO2 nanobelts (CSTNB) were synthesized using combustion synthesized TiO2 as a novel precursor. The mechanism of formation of TiO2 nanobelts was hypothesized. The antibacterial activity of combustion synthesized TiO2 and acid etched TiO2 nanobelts were evaluated against Escherichia coli and compared against commercial TiO2. Various characterization studies like X-ray diffraction analysis, BET surface area analysis, diffused reflectance measurements were performed. Microscopic structures and high resolution images were analyzed using scanning electron microscopy, transmission electron microscopy. The extent of photo-stability and reusability of the catalyst was evaluated by conducting repeated cycles of photo degradation experiments and was compared to the commercial grade TiO2. The reactive radical species responsible for high photocatalytic and antibacterial activity has been determined by performing multiple scavenger reactions. The excellent charge transfer mechanism, high generation of hydroxyl and hole radicals resulted in enhanced photocatalytic activity of the acid etched TiO2 nanobelts compared to commercial TiO2 and nanobelts made from commercial TiO2.