16 research outputs found
Thermoelectric properties of finite graphene antidot lattices
We present calculations of the electronic and thermal transport properties of
graphene antidot lattices with a finite length along the transport direction.
The calculations are based on a single orbital tight-binding model and the
Brenner potential. We show that both electronic and thermal transport
properties converge fast toward the bulk limit with increasing length of the
lattice: only a few repetitions (~6) of the fundamental unit cell are required
to recover the electronic band gap of the infinite lattice as a transport gap
for the finite lattice. We investigate how different antidot shapes and sizes
affect the thermoelectric properties. The resulting thermoelectric figure of
merit, ZT, can exceed 0.25, and it is highly sensitive to the atomic
arrangement of the antidot edges. Specifically, hexagonal holes with pure
zigzag edges lead to an order-of-magnitude smaller ZT as compared to pure
armchair edges. We explain this behavior as a consequence of the localization
of states, which predominantly occurs for zigzag edges, and of an increased
splitting of the electronic minibands, which reduces the power factor.Comment: 12 pages, 13 figures. Submitted to Phys. Rev.
An Investigation of the Geometrical Effects on the Thermal Conductivity of Graphene Antidot Lattices
In this work we investigate the thermal conductivity of graphene-based antidot lattices. A third nearest-neighbor tight-binding model and a forth nearest-neighbor force constant model are employed to study the electronic and phononic band structures of graphene-based antidot lattices. Ballistic transport models are used to evaluate the electronic and the thermal conductivities. Methods to reduce the thermal conductivity and to increase the thermoelectric figure of merit of such structures are studied. Our results indicate that triangular antidot lattices have the smallest thermal conductivity due to longer boundaries and the smallest distance between the neighboring dots
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.Comment: 23 pages, 9 figures, 1 tabl
Low-dimensional phonon transport effects in ultranarrow disordered graphene nanoribbons
Prospects of low-dimensional and nanostructured silicon-based thermoelectric materials: findings from theory and simulation
Silicon based low-dimensional materials receive significant attention as new generation thermoelectric materials after they have demonstrated record low thermal conductivities. Very few works to-date, however, report significant advances with regards to the power factor. In this review we examine possibilities of power factor enhancement in: (i) low-dimensional Si channels and (ii) nanocrystalline Si materials. For low-dimensional channels we use atomistic simulations and consider ultra-narrow Si nanowires and ultra-thin Si layers of feature sizes below 15 nm. Room temperature is exclusively considered. We show that, in general, low-dimensionality does not offer possibilities for power factor improvement, because although the Seebeck coefficient could slightly increase, the conductivity inevitably degrades at a much larger extend. The power factor in these channels, however, can be optimized by proper choice of geometrical parameters such as the transport orientation, confinement orientation, and confinement length scale. Our simulations show that in the case where room temperature thermal conductivities as low as κ l = 2 W/mK are achieved, the ZT figure of merit of an optimized Si low-dimensional channel could reach values around unity. For the second case of materials, we show that by making effective use of energy filtering, and taking advantage of the inhomogeneity within the nanocrystalline geometry, the underlying potential profile and dopant distribution large improvements in the thermoelectric power factor can be achieved. The paper is intended to be a review of the main findings with regards to the thermoelectric performance of nanoscale Si through our simulation work as well as through recent experimental observations