Thushari Jayasekera

Thermoelectric properties of graphene nanoribbons, junctions and superlattices.

Using model interaction Hamiltonians for both electrons and phonons and Greens function formalism for ballistic transport, we have studied the thermal conductance and the thermoelectric properties of graphene nanoribbons (GNR), GNR junctions and periodic superlattices. Among our findings we have established the role that interfaces play in determining the thermoelectric response of GNR systems both across single junctions and in periodic superlattices. In general, increasing the number of interfaces in a single GNR system increases the peak ZT values that are thus maximized in a periodic superlattice. Moreover, we proved that the thermoelectric behavior is largely controlled by the width of the narrower component of the junction. Finally, we have demonstrated that chevron-type GNRs recently synthesized should display superior thermoelectric properties.

Tunable Electronics in Large-Area Atomic Layers of Boron-Nitrogen―Carbon

We report on the low-temperature electrical transport properties of large area boron and nitrogen codoped graphene layers (BNC). The temperature dependence of resistivity (5 K < T < 400 K) of BNC layers show semiconducting nature and display a band gap which increases with B and N content, in sharp contrast to large area graphene layers, which shows metallic behavior. Our investigations show that the amount of B dominates the semiconducting nature of the BNC layers. This experimental observations agree with the density functional theory (DFT) calculations performed on BNC structures similar in composition to the experimentally measured samples. In addition, the temperature dependence of the electrical conductivity of these samples displays two regimes: at higher temperatures, the doped samples display an Arrhenius-like temperature dependence thus indicating a well-defined band gap. At the lowest temperatures, the temperature dependence of the conductivity deviates from activated behavior and displays a conduction mechanism consistent with Motts two-dimensional (2D) variable range hopping (2D-VRH). The ability to tune the electronic properties of thin layers of BNC by simply varying the concentration of B and N will provide a tremendous boost for obtaining materials with tunable electronic properties relevant to applications in solid state electronics.

First-principles calculation of thermal transport in metal/graphene systems

Thermal properties in the metal/graphene (Gr) systems are analyzed by using an atomistic phonon transport model based on Landauer formalism and first-principles calculations. The specific structures under investigation include chemisorbed Ni(111)/Gr, physisorbed Cu(111)/Gr and Au(111)/Gr, as well as Pd(111)/Gr with intermediate characteristics. Calculated results illustrate a strong dependence of thermal transfer on the details of interfacial microstructures. In particular, it is shown that the chemisorbed case provides a generally smaller interfacial thermal resistance than the physisorbed one due to the stronger bonding. However, our calculation also indicates that the weakly chemisorbed interface of Pd/Gr may be an exception, with the largest thermal resistance among the considered. Further examination of the electrostatic potential and interatomic force constants reveals that the mixed bonding force between the Pd and C atoms results in incomplete hybridization of Pd and graphene orbital states at the junction, leading effectively to two phonon interfaces and a larger than expected thermal resistance. Comparison with available experimental data shows good agreement. The result clearly suggests the feasibility of phonon engineering for thermal property optimization at the interface.

Transport in multiterminal graphene nanodevices.

We study the transport properties of multiterminal graphene nanodevices using the Landauer-Buttiker approach and the tight binding model. We consider a four-terminal device made at the crossing of a zigzag and armchair nanoribbons and two types of T-junction devices. The transport properties of graphene multiterminal devices are highly sensitive to the details of the junction region. Thus the properties are drastically different from those on the armchair and zigzag counterparts. In the cross-junction device, we see a conductance dip in the armchair lead associated with a conductance peak in the zigzag lead. We find that this effect is enhanced in a T-junction device with one armchair sidearm.

Phonon engineering in nanostructures: Controlling interfacial thermal resistance in multilayer-graphene/dielectric heterojunctions

Article discussing phonon engineering in nanostructures and controlling interfacial thermal resistance in multilayer-graphene/dielectric heterojunctions.

Charge transfer equilibria in ambient-exposed epitaxial graphene on (0001) 6 H-SiC

The transport properties of electronic materials have been long interpreted independently from both the underlying bulk-like behavior of the substrate or the influence of ambient gases. This is no longer the case for ultra-thin graphene whose properties are dominated by the interfaces between the active material and its surroundings. Here, we show that the graphene interactions with its environments are critical for the electrostatic and electrochemical equilibrium of the active device layers and their transport properties. Based on the prototypical case of epitaxial graphene on (0001¯) 6 H-SiC and using a combination of in-situ thermoelectric power and resistance measurements and simulations from first principles, we demonstrate that the cooperative occurrence of an electrochemically mediated charge transfer from the graphene to air, combined with the peculiar electronic structure of the graphene/SiC interface, explains the wide variation of measured conductivity and charge carrier type found in prior re...

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.

Electronic and Structural Properties of Turbostratic Epitaxial Graphene on the 6H-SiC (000-1) Surface

We propose an atomistic model to study the interface properties of mis-oriented (turbostratic) epitaxial graphene on SiC (000-1) surface. Using calculations from first principles, we compare the energetics, and structural/electronic properties of AB and turbostratic stacking sequences within a model based on the Si adatom surface reconstruction. Our calculations show that the systems with AB and turbostratic sequences are very close in energy, demonstrating the possibility of the observation of Moire patterns in epitaxial graphene on the C-face of SiC. The two-dimensional electron gas behavior is preserved in the epitaxial turbostratic graphene systems. However, there are deviations from the ideal turbostratic epitaxial graphene.