Hansika Sirikumara

Symmetry induced semimetal-semiconductor transition in doped graphene

Substitutional chemical doping is one way of introducing an electronic bandgap in otherwise semimetallic graphene. A small change in dopant arrangement can convert graphene from a semiconducting to a semimetallic state. Based on ab initio Density Functional Theory calculations, we discuss the electron structure of BN-doped graphene with Bravais and non-Bravais lattice-type defect patterns, identifying semiconducting/semimetallic configurations. Semimetallic behavior of graphene with non-Bravais lattice-type defect patterns can be explained by a phase cancellation in the scattering amplitude. Our investigation reveals for the first time that the symmetry of defect islands and the periodicity of defect modulation limit the phase cancellation which controls the semimetal-semiconductor transition in doped graphene.

Tunable indirect-direct transition of few-layer SnSe via interface engineering

Tin selenide (SnSe) is one of the best thermoelectric materials reported to date. The possibility of growing few-layer SnSe helped boost the interest in this long-known, earth abundant material. Pristine SnSe in bulk, mono- and few-layer forms are reported to have indirect electronic bandgaps. Possible indirect-direct transition in SnSe is attractive for its optoelectronic-related applications. Based on the results from first principles density functional theory calculations, we carefully analyzed electronic band structures of bulk, and bilayer SnSe with various interlayer stackings. We report the possible stacking-dependent indirect-direct transition of bilayer SnSe. By further analysis, our results reveal that it is the directionality of interlayer interactions that determine the critical features of their electronic band structures. In fact, by engineering the interface stacking between layers, it is possible to achieve few-layer SnSe with direct electronic band gap. This study provides fundamental insights to design few-layer SnSe and SnSe heterostructures for electronic/optoelectronic applications, where the interface geometry plays a fundamental role in device performance.

Recent advances in investigations of the electronic and optoelectronic properties of group III, IV, and V selenide based binary layered compounds

This review article presents a comprehensive update on the recent research trends, advancement and future outlook of selected layered selenide based binary compounds featuring elements from group III, IV, and V of the periodic table. Due to their highly anisotropic structure as well as their availability in mono, few- and multi-layer form, these compounds constitute a perfect playground where a variety of possibilities in structural variation as well as functionalities are expected. This potentially gives rise to a library of unique and fascinating 2D selenide based systems. These systems appear to demonstrate some spectacular variety of fundamental physics as well as indicate that some of these systems can be beneficial for several niche applications directly or indirectly resulting from their electrical and optical properties. As such, a description of recent investigations pertaining to some of the key electrical and optical properties of a few chosen binary selenide based compounds such as indium selenide, tin selenide, gallium selenide, germanium selenide and bismuth selenide is described. A final note on immediate research needs and directions in developing these materials systems for future applications is discussed.

Buffer-eliminated, charge-neutral epitaxial graphene on oxidized 4H-SiC (0001) surface

Buffer-eliminated, charge-neutral epitaxial graphene (EG) is important to enhance its potential in device applications. Using the first principles Density Functional Theory calculations, we investigated the effect of oxidation on the electronic and structural properties of EG on 4H-SiC (0001) surface. Our investigation reveals that the buffer layer decouples from the substrate in the presence of both silicate and silicon oxy-nitride at the interface, and the resultant monolayer EG is charge-neutral in both cases. The interface at 4H-SiC/silicate/EG is characterized by surface dangling electrons, which opens up another route for further engineering EG on 4H-SiC. Dangling electron-free 4H-SiC/silicon oxy-nitride/EG is ideal for achieving charge-neutral EG.

Lattice vibrational properties of Si/Ge core-shell nanowires for thermoelectric applications

Using first principles Density Functional Theory (DFT) calculations, we have studied the structural and lattice vibrational properties of [111]-oriented Si/Ge core-shell nanowires. Our results show that the fundamental atomicity of the underlying lattice is important for an accurate explanation of phonon frequencies. The detailed analysis shows that thermal conductance due to selective phonon modes of Si/Ge coreshell nanowires can be suppressed by engineering the ratio of core/shell atoms, as well as the detailed atomistic configuration. In particular, our results reveal that heavier shell atoms in Si/Ge core-shell nanowires reduce thermal conductivity, increasing their thermoelectric figure of merit.