Pratibha Dev

Determining the appropriate exchange-correlation functional for time-dependent density functional theory studies of charge-transfer excitations in organic dyes

UV-Vis spectra are calculated using time-dependent density functional theory for several organic dyes--4-(N, N-dimethylamino) benzonitrile, alizarin, squaraine, polyene-linker dyes, oligothiophene-containing coumarin dyes (NKX series) and triphenylamine-donor dyes. Most of these dyes (except, for the first two) or their derivatives are considered to be promising organic dyes for dye-sensitized solar cells. An accurate description of the photophysics of such dyes is imperative for understanding and creating better dyes. To this end, we studied the dyes within several approximations to the exchange-correlation functional. The chosen functionals--PBE, M06L, B3LYP, M06, CAM-B3LYP, and wB97--represent the various classes of approximations that are currently being used to study material properties. From amongst the six approximations studied here, CAM-B3LYP outperformed the others in its description of charge-transfer excitations in most (though, not all) of the dyes. This study shows why it is difficult to choose a particular functional a priori, especially when starting out with a new dye for solar cell application. A possible way to judge the fitness of an approximation is used in this work and it is shown to provide a good quantitative guideline for subsequent research in this field.

Van der Waals screening by single-layer graphene and molybdenum disulfide

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Spin coherence and echo modulation of the silicon vacancy in4H−SiCat room temperature

The silicon vacancy in silicon carbide is a strong emergent candidate for applications in quantum information processing and sensing. We perform room temperature optically-detected magnetic resonance and spin echo measurements on an ensemble of vacancies and find the properties depend strongly on magnetic field. The spin echo decay time varies from less than 10

Prediction of a multicenter-bonded solid boron hydride for hydrogen storage

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Silicon vacancy center in4H-SiC: Electronic structure and spin-photon interfaces

s at low fields to 80

Spin-photon entanglement interfaces in silicon carbide defect centers

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Defect-Induced intrinsic magnetism in wide-gap III nitrides.

s at 68 mT, and a strong field-dependent spin echo modulation is also observed. The modulation is attributed to the interaction with nuclear spins and is well-described by a theoretical model.

Unconventional magnetism in semiconductors: Role of localized acceptor states

An ideal material for on-board hydrogen storage must release hydrogen at practical temperature and pressure and also regenerate efficiently under similarly gentle conditions. Therefore, thermodynamically, the hydride material must lie within a narrow range near the hydrogenation/dehydrogenation phase boundary. Materials involving only conventional bonding mechanisms are unlikely to meet these requirements. In contrast, materials containing certain frustrated bonding are designed to be on the verge of frustration-induced phase transition, and they may be better suited for hydrogen storage. Here we propose a novel layered solid boron hydride and show its potential for hydrogen storage. The absence of soft phonon modes confirms the dynamical stability of the structure. Charging the structure significantly softens hydrogen-related phonon modes. Boron-related phonons, in contrast, are either hardened or not significantly affected by electron doping. These results suggest that electrochemical charging may facilitate hydrogen release while the underlying boron network remains intact for subsequent rehydrogenation.

Defect-induced magnetism in nitride and oxide nanowires: Surface effects and quantum confinement

Defects in silicon carbide are of intense and increasing interest for quantum-based applications due to this materials properties and technological maturity. We calculate the multi-particle symmetry adapted wave functions of the negatively charged silicon vacancy defect in hexagonal silicon carbide via use of group theory and density functional theory and find the effects of spin-orbit and spin-spin interactions on these states. Although we focused on

Nitrogen-Doped Graphene and Twisted Bilayer Graphene via Hyperthermal Ion Implantation with Depth Control

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