Physics

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Shiv Nadar University

To modulate the electronic and optical properties of graphene oxide via controlled deoxidation, a proper understanding of the role of the individual functional group in determining these properties is required. We, therefore, have performed ab initio density functional theory based calculations to study the electronic and optical properties of model structures of graphene oxide with different coverages and compositions. In particular, we considered various concentrations of major functional groups like epoxides, hydroxyls, and carbonyls, which mainly consititute the graphene oxide and the reduced graphene oxide. Our calculated electron energy loss spectra (EELS) demonstrate the π plasmon peak to be less sensitive, while π + θ plasmon is found to have a significant blue shift of about 1.0-3.0 eV, when the concentration of epoxy and hydroxyl functional groups in graphene oxide vary from 25% to 75%. However, the increase in carbonyl groups in the center of the graphene sheet creates holes, which lead to the red shift of the EELS. In the case of 37.5% of oxygen-to-carbon ratio, we find the π plasmon peak to be shifted by roughly 1.0 eV as compared to that of the pristine graphene. Our results agree well with the experimental findings which suggest a blue shift in the EELS of graphene oxide and an absorption feature due to a π electron transition of the carbonyl groups at a lower energy than that of epoxy and hydroxyl groups. We also show that the increase in the width of the hole created by the carbonyl groups significantly decreases the optical gap and opens the band gap, and thus, we argue that reduced graphene oxide with mostly carbonyl groups could be a useful material for developing tunable opto-electronic nanodevices. © 2011 American Chemical Society.

(SoNS) School of Natural Sciences

World University root node

ACS Nano

Modulating optical properties of graphene oxide: Role of prominent functional groups

Useful electronic, magnetic, and optical properties have been proposed and observed in thin films of Ti1-x Mx O2 (M=Ta,Nb,V). In this work, we have studied phase formation for films of Ti 1-x Tax O2 prepared by pulsed laser deposition. We show that substitutional Ta in TiO2 results in a different material system in terms of its electronic properties. Moss-Burstein shift is ruled out by comparing the electrical transport data of anatase and rutile TiO2. Vegard's law fit to the blueshift data and the high energy optical reflectivity studies confirm the formation of an alloy with a distinct band structure. © 2011 American Institute of Physics.

Fulltext

Applied Physics Letters

Multifunctional Ti1-x Tax O2: Ta doping or alloying?

We consider the possibility that the vacuum energy density of the MSSM (Minimal Supersymmetric Standard Model) flat direction condensate involving the Higgses H1 and H2 is responsible for inflation. We also discuss how the finely tuned Higgs potential at high vacuum expectation values can realize cosmologically flat direction along which it can generate the observed density perturbations and after the end of inflation - the coherent oscillations of the Higgses reheat the universe with all the observed degrees of freedom, without causing any problem for the electroweak phase transition. {\textcopyright} 2011 IOP Publishing Ltd and SISSA.

Journal of Cosmology and Astroparticle Physics

Tuned MSSM Higgses as an inflaton

We show that a five-dimensional Universal Extra Dimension model, compactified on a line segment, is consistently formulated even when the gauge symmetry is broken solely by non-zero Dirichlet boundary conditions on a bulk Higgs field, without any quartic interaction. We find that the longitudinal W+W- elastic scattering amplitude, under the absence of the Higgs zero mode, is unitarized by exchange of infinite towers of KK Higgs bosons. Resultant amplitude scales linearly with the scattering energy ∝ √S, exhibiting five-dimensional nature. A tree-level partial-wave unitarity condition is satisfied up to 6.7 (5.7) TeV for the KK scale mKK = 430 (500) GeV, favored by the electroweak data within 90% CL. © Springer-Verlag / Società Italiana di Fisica 2011.

European Physical Journal C

Unitarity in Dirichlet Higgs model

In order to realize Si as a negative electrode material in commercial Li-ion batteries, it is important to understand the mixing mechanism of Li and Si, and stress evolution during lithiation in Si negative electrode of Li-ion batteries. Available experiments mainly provide the diffusivity of Li in Si as an averaged property, neglecting information regarding diffusivity of Si. However, if Si can diffuse as fast as Li, the stress generated during Li diffusion can be reduced. We, therefore, studied the diffusivity of Li as well as Si atoms in the Si-anode of Li-ion battery using an ab initio molecular dynamics-based methodology. The electrochemical insertion of Li into crystalline Si prompts a crystalline-to-amorphous phase transition. We considered this situation and thus examined the diffusion kinetics of Li and Si atoms in both crystalline and amorphous Si. We find that Li diffuses faster in amorphous Si as compared to crystalline Si, while Si remains relatively immobile in both cases and generates stresses during lithiation. To further understand the mixing mechanism and to relate the structure with electrochemical mixing, we analyzed the evolution of the structure during lithiation and studied the mechanism of breaking of Si-Si network by Li. We find that Li atoms break the Si rings and chains and create ephemeral structures such as stars and boomerangs, which eventually transform to Si-Si dumbbells and isolated Si atoms in the LiSi phase. Our results are found to be in agreement with the available experimental data and provide insights into the mixing mechanism of Li and Si in Si negative electrode of Li-ion batteries. © 2011 American Chemical Society.

Nano Letters

The mixing mechanism during lithiation of Si negative electrode in Li-ion batteries: An Ab initio molecular dynamics study

In this work, we report a novel donor-acceptor based solution processable low band gap polymer semiconductor, PDPP-TNT, synthesized via Suzuki coupling using condensed diketopyrrolopyrrole (DPP) as an acceptor moiety with a fused naphthalene donor building block in the polymer backbone. This polymer exhibits p-channel charge transport characteristics when used as the active semiconductor in organic thin-film transistor (OTFT) devices. The hole mobilities of 0.65 cm2 V-1 s-1 and 0.98 cm2 V -1 s-1 are achieved respectively in bottom gate and dual gate OTFT devices with on/off ratios in the range of 105 to 10 7. Additionally, due to its appropriate HOMO (5.29 eV) energy level and optimum optical band gap (1.50 eV), PDPP-TNT is a promising candidate for organic photovoltaic (OPV) applications. When this polymer semiconductor is used as a donor and PC71BM as an acceptor in OPV devices, high power conversion efficiencies (PCE) of 4.7% are obtained. Such high mobility values in OTFTs and high PCE in OPV make PDPP-TNT a very promising polymer semiconductor for a wide range of applications in organic electronics. © 2011 The Royal Society of Chemistry.

Energy and Environmental Science

High mobility organic thin film transistor and efficient photovoltaic devices using versatile donor-acceptor polymer semiconductor by molecular design

Zinc oxide-organic semiconductor lateral heterostructure based field-effect bipolar transistors (FEBTs) having heterointerfaces approximately midway between the source and drain electrodes are fabricated and characterized. These hybrid FEBTs comprise zinc oxide (ZnO) and p -channel organic semiconductors [Pentacene and α -sexithiophene (6T)] supporting electron transport and hole transport on either side of the heterojunction, respectively. Current flow in the transistor channel is established as a result of carrier injection across the heterointerface followed by recombination. In steady state, such devices possess significant populations of holes and electrons in the transistor channel and operate in bipolar mode. © 2011 American Institute of Physics.

Electrical characteristics of zinc oxide-organic semiconductor lateral heterostructure based hybrid field-effect bipolar transistors

Electroluminescence in SiO2 layers can be created by Ge implantation and a subsequent heat treatment, leading to the formation of Ge nano-particles inside the SiO2. An additional implantation of Er, connected with a further annealing, can lead to an improvement of the luminescent properties. However, the intensity of electroluminescence was found to decrease drastically after exceeding an optimum concentration of the Er doping. Slow positron implantation spectroscopy (SPIS), both in single (DB) and coincidence (CDB) Doppler broadening mode, was applied to probe processes at a microscopic level which might have an impact on the optical response. It shows that the increasing intensity of the electro-luminescence is connected with a crystalline structure of the SiO2 covering the nano-particles and also with the improved reverse energy transfer process between Er and Ge.

Materials Science Forum

Structural characterisation of Er implanted, Ge-rich SiO2 layers using slow positron implantation spectroscopy

Two anomalous bias dependent resistive peaks induced by the SrTiO 3 structural phase transitions at 55 and 110 K were observed in a LaAlO3/SrTiO3 and Nb:SrTiO3 rectifying junction when the LaAlO3/SrTiO3 was depleted under reverse bias. At these transition temperatures, the barrier between LaAlO3/ SrTiO3 and Nb:SrTiO3 showed abrupt changes in the tunneling energy under forward bias. The peak at 110 K was an insulator-metal phase transition while the peak at 55 K was a metal-insulator one. We propose that the phase transitions of the SrTiO3 substrate influence the charge transfer to the LaAlO3/SrTiO3 layer, giving rise to these anomalous resistive peaks. © 2011 American Institute of Physics.

Metal-insulator transition at a depleted LaAlO3/SrTiO 3 interface: Evidence for charge transfer induced by SrTiO 3 phase transitions

Quantum entanglement plays a crucial role in quantum information, quantum teleportation and quantum computation. The information about the entanglement content between subsystems of the composite system is encoded in the Schmidt eigenvalues. We derive here closed expressions for the spectral density of Schmidt eigenvalues for all three invariant classes of random matrix ensembles. We also obtain exact results for average von Neumann entropy. We find that maximum average entanglement is achieved if the system belongs to the symplectic invariant class. © 2011 IOP Publishing Ltd.