Kanchan Ulman

Passivation of surface states of α-Fe2O3(0001) surface by deposition of Ga2O3 overlayers: A density functional theory study

There is a big debate in the community regarding the role of surface states of hematite in the photoelectrochemical water splitting. Experimental studies on non-catalytic overlayers passivating the hematite surface states claim a favorable reduction in the overpotential for the water splitting reaction. As a first step towards understanding the effect of these overlayers, we have studied the system Ga2O3 overlayers on hematite (0001) surfaces using first principles computations in the PBE+U framework. Our computations suggest that stoichiometric terminations of Ga2O3 overlayers are energetically more favored than the bare surface, at ambient oxygen chemical potentials. Energetics suggest that the overlayers prefer to grow via a layer-plus-island (Stranski-Krastanov) growth mode with a critical layer thickness of 1-2 layers. Thus, a complete wetting of the hematite surface by an overlayer of gallium oxide is thermodynamically favored. We establish that the effect of deposition of the Ga2O3 overlayers on the bare hematite surface is to passivate the surface states for the stoichiometric termination. For the oxygen terminated surface which is the most stable termination under photoelectrochemical conditions, the effect of deposition of the Ga2O3 overlayer is to passivate the hole-trapping surface state.

Dielectric properties of Si3−ξGeξN4 and Si3−ξCξN4: A density functional study

Using first principles calculations, we have studied the dielectric properties of crystalline α- and β-phase silicon germanium nitrides and silicon carbon nitrides, A3−ξBξN4 (A = Si, B = Ge or C, ξ=0,1,2,3). In silicon germanium nitrides, both the high-frequency and static dielectric constants increase monotonically with increasing germanium concentration, providing a straightforward way to tune the dielectric constant of these materials. In the case of silicon carbon nitrides, the high-frequency dielectric constant increases monotonically with increasing carbon concentration, but a more complex trend is observed for the static dielectric constant, which can be understood in terms of competition between changes in the unit-cell volume and the average oscillator strength. The computed static dielectric constants of C3N4, Si3N4, and Ge3N4 are 7.13, 7.69, and 9.74, respectively.

Graphene oxide as an optimal candidate material for methane storage

Methane, the primary constituent of natural gas, binds too weakly to nanostructured carbons to meet the targets set for on-board vehicular storage to be viable. We show, using density functional theory calculations, that replacing graphene by graphene oxide increases the adsorption energy of methane by 50%. This enhancement is sufficient to achieve the optimal binding strength. In order to gain insight into the sources of this increased binding, that could also be used to formulate design principles for novel storage materials, we consider a sequence of model systems that progressively take us from graphene to graphene oxide. A careful analysis of the various contributions to the weak binding between the methane molecule and the graphene oxide shows that the enhancement has important contributions from London dispersion interactions as well as electrostatic interactions such as Debye interactions, aided by geometric curvature induced primarily by the presence of epoxy groups.

Theoretical Study of Spin Conduction in the Ni/DTE/Ni nanohybrid

The photoswitching molecule dithienylethene (DTE) is an interesting candidate for constructing optoelectronic molecular devices since it can be made to switch between a closed and an open conformation using light. We here report computations, based on density functional theory (DFT) and the non-equilibrium Green function (NEGF) method, of the spin-resolved conductance of the two DTE isomers attached to spin-polarized nickel leads. Results are compared and contrasted to those of other contact materials (nonmagnetic Ni, Ag, and Au), analyzing the physical origins of the various features in the transmission function. It was found rather surprisingly, that the two spin channels in the Ni/DTE/Ni device have almost identical I-V characteristics, despite one channel being d-dominated and the other one s-dominated. It was also observed that the Ni-based device exhibits a sustained high conductance ratio also for high bias - a property that may be of relevance in future device design. Furthermore, two computational schemes for calculating the conductance were compared and analyzed. It was found that even for very small bias the molecular orbital polarization was decisive for spin-related properties such as the spin current ratio and magneto-resistance in the Ni/DTE/Ni device.

Erratum: “Tuning spin transport properties and molecular magnetoresistance through contact geometry” [J. Chem. Phys. 140, 044716 (2014)]

Tuning spin transport properties and molecular magnetoresistance through contact geometry (vol 140, 044716, 2014)