Physics

<p>Shiv Nadar University is a comprehensive, multidisciplinary, research-focused, and student-centric University offering a full range of academic programs at the undergraduate, postgraduate and doctoral level. With state-of-the-art infrastructure, the University comprises of academic wings, sophisticated labs, international standard sports facilities, amphitheaters, auditorium, conference rooms and smart classrooms. The University&rsquo;s goal is to become internationally recognized for the quality of its research and creative endeavors and their applicability to improving quality of life, generating new insights and expanding the boundaries of human knowledge creativity. Committed to excellence in teaching, research and service, the University aims to serve the higher education needs of India and the world beyond.</p>

Shiv Nadar University

The phase behavior of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the strongly binding counterion p-toluidine hydrochloride (PTHC) has been examined using small-angle X-ray diffraction and polarizing microscopy. A hexagonal-to-lamellar transition on varying the PTHC to SDS molar ratio (α) occurs through a nematic phase of rodlike micelles (N C) → isotropic (I) → nematic of disklike micelles (N D) at a fixed surfactant concentration (φ). The lamellar phase is found to coexist with an isotropic phase (I′) over a large region of the phase diagram. Deuterium nuclear magnetic resonance investigations of the phase behavior at φ = 0.4 confirm the transition from NC to N D on varying α. The viscoelastic and flow behaviors of the different phases were examined. A decrease in the steady shear viscosity across the different phases with increasing a suggests a decrease in the aspect ratio of the micellar aggregates. From the transient shear stress response of the NC and ND nematic phases in step shear experiments, they were characterized to be tumbling and flow aligning, respectively. Our studies reveal that by tuning the morphology of the surfactant micelles strongly binding counterions modify the phase behavior and rheological properties of concentrated surfactant solutions. © 2009 American Chemical Society.

(SoNS) School of Natural Sciences

World University root node

Langmuir

Re-entrant phase behavior of a concentrated anionic surfactant system with strongly binding counterions

We report on the effects of 2 MeV He+-ion irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe He+-ion irradiation leads to mixing across the interfaces [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)] × 6/Si multilayers. In addition, we observe Co-Cr-Pt phase formation at the highest fluence of 5.5 × 1016 ions cm-2. This is accompanied by an enhancement in the coercivity. Such enhancement in the coercivity is attributed to inhomogeneous alloying and a possible mixing-induced strain. High-resolution transmission electron microscopy confirms the formation of CoCrPt ternary alloy phase. These findings are explained in the light of ion beam induced recoil mixing and ionization events. © 2009 Elsevier B.V. All rights reserved.

Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms

Modification of the magnetic and the structural properties of Pt/Cr/Co multilayers by He+-ion irradiation

The Journal of chemical physics

Large-scale correlated study of excited state absorptions in naphthalene and anthracene

Using combined electroluminescence (EL) and photoluminescence (PL) studies we establish that the energy transfer process from the Er3+ to the Ge-related oxygen-deficiency centers (GeODCs) plays the key role in enhancing the 404 nm EL intensity in Ge-rich SiO2. Er doping induced structural modification does not appear to be relevant, which is deduced from the 404 nm PL quenching with increasing Er concentration, implying a gradual loss of GeODCs. In contrast to PL, the 404 nm EL intensity increases by 0.3% Er doping followed by a gradual decrease in intensity for higher Er concentrations, which is described in terms of a competition between the energy transfer process and the gradual segregation of Er due to the destruction of GeODCs with Er doping. This fact is further discussed in the light of ab initio molecular orbital calculations. © 2009 American Institute of Physics.

Fulltext

Journal of Applied Physics

The role of Ge-related oxygen-deficiency centers in controlling the blue-violet photo- and electroluminescence in Ge-rich SiO2 via Er doping

Using combined microstructural and electroluminescence (EL) investigations of the Er-doped Ge-rich SiO2 layers, it is established that the Ge-related oxygen-deficiency centers (GeODCs), which are associated with the 407 nm light emission, are situated at the Ge nanocrystal/ SiO2 interface. Electrically driven energy transfer from the Er3+ to GeODCs causes an increase in the 407 nm EL intensity. It reaches a maximum before quenching with increasing Er concentration due to the crystalline-to-amorphous transition of Ge nanocrystals. Ge concentration dependent quenching of the maximum EL intensity and the peak shifting toward higher Er concentration are discussed in terms of the reduction of the surface-to-volume ratio with increasing nanocrystal size. © 2009 American Institute of Physics.

Defect-engineered blue-violet electroluminescence from Ge nanocrystal rich SiO2 layers by Er doping

Efficient white electroluminescence (EL) is obtained from metal-oxynitride-oxide-silicon (MONOS) structures with a silicon-rich SiO 2 -SiON interface as an active layer. Samples containing only silicon show white EL under a constant current excitation of 500 μA. The EL spectra consist of three main bands associated with the neutral oxygen vacancy at 430 nm, Eδ′ centers at about 550 nm, and nonbridging oxygen hole centers peaking at 650 nm. After Gd co-doping into the SiO2:Si-nc layer, the light emitted from the MONOS structure observed by the naked eye is clearly white, but the constant current needed for the EL excitation is reduced five times. A fourfold increase in the external EL power efficiency after Gd co-doping (up to 3× 10-4 %) is observed. © 2009 The Electrochemical Society.

Electrochemical and Solid-State Letters

White electroluminescence from a gadolinium-doped si-nanocluster-enriched SiO2 -SiON interface region

We present the microstructural evolution in Ge-rich SiO2 doped with Er under different fabrication conditions. At sufficiently high Er contents and annealing temperatures, the Er2 O3 and Er2 Ge2 O7 phases are eventually formed, leading to an electroluminescence quenching of Ge-related oxygen-deficiency centers. The correlation between the microstructure and electroluminescence is discussed based on: (i) an Er doping dependent fragmentation/amorphization of Ge nanocrystals, (ii) a temperature dependent Ge diffusion toward the Si/ SiO2 interface, and (iii) the formation of different Er phases. © 2009 The American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Probing the impact of microstructure on the electroluminescence properties of Ge-nanocrystal enriched Er-doped SiO2 layers

Temperature-dependent photoluminescence of Ge-rich SiO2 in the presence or absence of Er shows a crossover between defect-related (15-150 K) and Er-related (150-295 K) emission within 1525 and 1440 nm. The origin of the near-infrared defect-related bands is discussed in the light of recombination of localized excitons in luminescence centers at the Ge cluster/ SiO2 interface. Time-resolved photoluminescence further enables us to illustrate the observed 1.53μm Er emission above 150 K in terms of a phonon-assisted nonradiative energy-transfer process from the luminescence centers to the Er3+ ions. © 2009 The American Physical Society.

Temperature dependence of the crossover between the near-infrared Er and defect-related photoluminescence bands of Ge-rich Er-doped SiO2 layers

The composition, electronic structure and molecular orientation of a self-assembled monolayer (SAM) of benzenethiolate on the Cu(1 0 0) surface have been investigated by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). An ordered benzenethiolate SAM with c(2 × 6) superstructure has been obtained by the reaction of diphenyldisulfide with the Cu(1 0 0) surface at room temperature. S 2p and C 1s XP spectra show S-Cu bond formation without significant thiolate decomposition. The strong dichroic signal of C K-edge NEXAFS upon varying polarization angle indicates upstanding of the adsorbed molecules with the benzene ring tilted by 20° from the surface normal. © 2009 Elsevier B.V. All rights reserved.

Journal of Electron Spectroscopy and Related Phenomena

Characterization of benzenethiolate self-assembled monolayer on Cu(1 0 0) by XPS and NEXAFS

We develop a systematic approach to construct novel completely solvable rational potentials. Second-order supersymmetric quantum mechanics dictates the latter to be isospectral to some well-studied quantum systems. PT symmetry may facilitate reconciling our approach to the requirement that the rationally extended potentials be singularity free. Some examples are shown. © Indian Academy of Sciences.