Hossein Karamitaheri

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.

Geometrical effects on the thermoelectric properties of ballistic graphene antidot lattices

The thermoelectric properties of graphene-based antidot lattices are theoretically investigated. A third nearest-neighbor tight-binding model and a fourth nearest-neighbor force constant model are employed to study the electronic and phononic band structures of graphene antidot lattices with circular, rectangular, hexagonal, and triangular antidot shapes. Ballistic transport models are used to evaluate transport coefficients. Methods to reduce the thermal conductance and to increase the thermoelectric power factor of such structures are studied. Our results indicate that triangular antidot lattices have the smallest thermal conductance due to longer boundaries and the smallest distance between the neighboring antidots. Among them, iso-triangular antidot lattices have also a large power factor and as a result a large figure of merit.

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.

Ballistic phonon transport in ultra-thin silicon layers: Effects of confinement and orientation

We investigate the effect of confinement and orientation on the phonon transport properties of ultra-thin silicon layers of thicknesses between 1 nm-16 nm. We employ the modified valence force field method to model the lattice dynamics and the ballistic Landauer transport formalism to calculate the thermal conductance. We consider the major thin layer surface orientations {100}, {110}, {111}, and {112}. For every surface orientation, we study thermal conductance as a function of the transport direction within the corresponding surface plane. We find that the ballistic thermal conductance in the thin layers is anisotropic, with the {110}/ channels exhibiting the highest and the {112}/ channels the lowest thermal conductance with a ratio of about two. We find that in the case of the {110} and {112} surfaces, different transport orientations can result in ~50% anisotropy in thermal conductance. The thermal conductance of different transport orientations in the {100} and {111} layers, on the other hand, is mostly isotropic. These observations are invariant under different temperatures and layer thicknesses. We show that this behavior originates from the differences in the phonon group velocities, whereas the phonon density of states is very similar for all the thin layers examined. We finally show how the phonon velocities can be understood from the phonon spectrum of each channel. Our findings could be useful in the design of the thermal properties of ultra-thin Si layers for thermoelectric and thermal management applications.

Study of thermal properties of graphene-based structures using the force constant method

The thermal properties of graphene-based materials are theoretically investigated. The fourth-nearest neighbor force constant method for phonon properties is used in conjunction with both the Landauer ballistic and the non-equilibrium Green’s function techniques for transport. Ballistic phonon transport is investigated for different structures including graphene, graphene antidot lattices, and graphene nanoribbons. We demonstrate that this particular methodology is suitable for robust and efficient investigation of phonon transport in graphene-based devices. This methodology is especially useful for investigations of thermoelectric and heat transport applications.

Investigation of quantum conductance in semiconductor single-wall carbon nanotubes: Effect of strain and impurity

In this paper the effect of strain and impurity on the quantum conductance of semiconducting carbon nanotubes (CNTs) have been studied by ab-initio calculations. The effect of strain and impurity on the CNT conducting behavior and physical characteristics, like density of states (DOS), band structure, and atomic local density of state (LDOS), is considered and discussed separately and simultaneously. Our results show that the quantum conductance of semiconductor CNTs is increased by compression strain, elongation strain, and replacing nitrogen and boron doping in its structure. The amount of increasing in the conductance depends on the type of strain and impurity. Conductance of CNT can be increased even more in the presence of both strain and impurity, consequently semiconducting CNT can show metallic properties. This can open the study on the possibility of changing the semiconducting/metallic properties of CNTs along its length and the use of semiconductor CNTs in interconnects.

Stable Semi-analytical Method for Analysis of Plasmonic Propagation on Periodically Patterned Metal Plates

The need for antennas with improved characteristics for communication and radar applications has resulted in an ever-increasing demand for research in the field of high impedance surfaces, which can work as an artificial magnetic conductor. One method in fabrication of these surfaces is formation of a metamaterial by patterning a metallic surface in the shape of space filling curves (e.g. Hilbert or Peanu Curves). In this paper, we present a novel semi-analytical solution to the problem of plasmonic propagation on these surfaces. The method is based on a previously presented Greens function formalism, which has been reported in an earlier paper of ours. We have modified and improved the method for analysis of periodic structures with a large number of spatial harmonics, and used different methods to get the necessary stabilization. Here propagating modes of different structures and their corresponding frequencies are calculated, and the possibility of frequency gap formation and stability of the method are investigated.

Phonon transport effects in one-dimensional width-modulated graphene nanoribbons

We investigate the thermal conductance of one-dimensional periodic width-modulated graphene nanoribbons using lattice dynamics for the phonon spectrum and the Landauer formalism for phonon transport. We conduct a full investigation considering all relevant geometrical features, i.e., the various lengths and widths of the narrow and wide regions that form the channel. In all cases that we examine, we find that width-modulation suppresses the thermal conductance at values even up to ∼70% below those of the corresponding uniform narrow nanoribbon. We show that this can be explained by the fact that the phonon spectrum of the width-modulated channels acquires less dispersive bands with lower group velocities and several narrow bandgaps, which reduce the phonon transmission function significantly. The largest degradation in thermal conductance is determined by the geometry of the narrow regions. The geometry of the wider regions also influences thermal conductance, although modestly. Our results add to the ongoing efforts in understanding the details of phonon transport at the nanoscale, and our conclusions are generic and could also apply to other one-dimensional channel materials.

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.