Hans Kosina

Engineering enhanced thermoelectric properties in zigzag graphene nanoribbons

We theoretically investigate the thermoelectric properties of zigzag graphene nanoribbons in the presence of extended line defects, substrate impurities, and edge roughness along the nanoribbon’s length. A nearest-neighbor tight-binding model for the electronic structure and a fourth nearest-neighbor force constant model for the phonon bandstructure are used. For transport, we employ quantum mechanical non-equilibrium Green’s function simulations. Starting from the pristine zigzag nanoribbon structure that exhibits very poor thermoelectric performance, we demonstrate how after a series of engineering design steps the performance can be largely enhanced. Our results could be useful in the design of highly efficient nanostructured graphene nanoribbon–based thermoelectric devices.

A Numerical Study of Line-Edge Roughness Scattering in Graphene Nanoribbons

The role of line-edge roughness scattering on the electronic properties of graphene nanoribbons is numerically investigated. The nonequilibrium Green function formalism, along with an atomistic tight-binding model, is employed. Our results indicate that, depending on the geometrical and roughness parameters, the transport of carriers can be in the diffusive or localization regime. We extract the mean free path and the localization length, which characterize the diffusive and localization regimes, respectively. In the diffusive regime, the conductance linearly decreases with length, whereas in the localization regime, it exponentially decreases with length. However, as the localization length depends on the carrier energy, an effective transport gap in this regime can be defined. This gap is evaluated as a function of the geometrical and roughness parameters, and its impact on the device performance is discussed.

Atomistic Study of the Lattice Thermal Conductivity of Rough Graphene Nanoribbons

Following our recent study on the electronic properties of rough nanoribbons , in this paper the role of geometrical and roughness parameters on the thermal properties of armchair graphene nanoribbons is studied. Employing a fourth nearest-neighbor force constant model in conjuction with the nonequilibrium Greens function method the effect of line-edge-roughness on the lattice thermal conductivity of rough nanoribbons is investigated. The results show that a reduction of about three orders of magnitude of the thermal conductivity can occur for ribbons narrower than 10 nm. The results indicate that the diffusive thermal conductivity and the effective mean free path are directly proportional to the ribbons width and the roughness correlation length, but inversely proportional to the roughness amplitude. Based on the numerical results an analytical model for the thermal conductivity of narrow armchair graphene nanoribbons is proposed in this paper. The developed model can be used in the analysis of graphene-based nano transistors and thermoelectric devices, where the appropriate selection of geometrical and roughness parameters are essential for optimizing the thermal properties.

Numerical Study of Graphene Superlattice-Based Photodetectors

The optical properties of 1-D superlattices formed by armchair graphene nanoribbons embedded in hexagonal boron nitride superlattices (BNSLs) are studied. A set of tight-binding (TB) parameters is proposed, which gives results in excellent agreement with first-principle calculations. Based on the tight-binding model, it is demonstrated that in BNSLs, a larger bandgap opening is achieved than in hydrogen-passivated superlattices (HSL). The optical properties of BNSL and HSL are studied by employing the nonequilibrium Green function method and are verified with first-principle calculations. The role of line-edge roughness on the optical properties of such devices is carefully investigated.

An investigation of ZGNR-based transistors

Graphene, a recently discovered form of carbon, has received much attention for possible applications in nanoelectronics, due to its excellent carrier transport properties [1]. Graphene nanoribbons (GNRs) are thin strips of graphene, where the electronic properties depend on the chirality of the edge and the width of the ribbon. Zigzag GNRs (ZGNRs) show metalic behavior, whereas armchair GNRs (AGNRs) are semiconductors and their band-gap is inversely proportional to their width [2]. Therefore, narrow AGNRs have been recently suggested as a material for transistor channels. However, line edge roughness and substrate impurities can significantly degrade the ballistic transport in AGNRs, especially in narrow ribbons [3].

Hydrogen-passivated graphene antidot structures for thermoelectric applications

In this work, we present a theoretical investigation of the thermal conductivity of hydrogen-passivated graphene antidot lattices. Using a fourth nearest-neighbor force constant method, we evaluate the phonon dispersion of hydrogen-passivated graphene antidot lattices with circular, hexagonal, rectangular and triangular shapes. Ballistic transport models are used to evaluate the thermal conductivity. The calculations indicate that the thermal conductivity of hydrogen-passivated graphene antidot lattices can be one fourth of that of a pristine graphene sheet. This reduction is stronger for right-triangular and iso-triangular antidots among others, all with the same area, due to longer boundaries and the smallest distance between the neighboring dots.

An Investigation of the Geometrical Effects on the Thermal Conductivity of Graphene Antidot Lattices

In this work we investigated the thermal conductivity of graphene-based antidots. The methods to reduce the thermal conductivity and improve the thermoelectric figure of merit of graphene based devices are studied. Our results indicate that triangular antidots have the smallest thermal conductivity due to longer boundaries and the smallest distance between nearest dots.