Strongly enhanced thermoelectric properties are predicted for graphene nanoribbons (GNRs) with optimized pattern. By means of nonequilibrium Green's function atomistic simulation of electron and phonon transport, we analyze the thermal and electrical properties of perfect GNRs as a function of their width and their edge orientation to identify a strategy likely to degrade the thermal conductance while retaining high electronic conductance and thermopower. An effect of resonant tunneling of electrons is detected in mixed GNRs consisting of alternate zigzag and armchair sections. To fully benefit from this effect and from strongly reduced phonon thermal conductance, a structure with armchair and zigzag sections of different widths is proposed. It is shown to provide a high thermoelectric factor of merit ZT exceeding unity at room temperature.

The thermoelectric properties of graphene and graphene nanostructures have recently attracted significant attention from the physics and engineering communities. In fundamental physics, the analysis of Seebeck and Nernst effects is very useful in elucidating some details of the electronic band structure of graphene that cannot be probed by conductance measurements alone, due in particular to the ambipolar nature of this gapless material. For applications in thermoelectric energy conversion, graphene has two major disadvantages. It is gapless, which leads to a small Seebeck coefficient due to the opposite contributions of electrons and holes, and it is an excellent thermal conductor. The thermoelectric figure of merit ZT of a two-dimensional (2D) graphene sheet is thus very limited. However, many works have demonstrated recently that appropriate nanostructuring and bandgap engineering of graphene can concomitantly strongly reduce the lattice thermal conductance and enhance the Seebeck coefficient without dramatically degrading the electronic conductance. Hence, in various graphene nanostructures, ZT has been predicted to be high enough to make them attractive for energy conversion. In this article, we review the main results obtained experimentally and theoretically on the thermoelectric properties of graphene and its nanostructures, emphasizing the physical effects that govern these properties. Beyond pure graphene structures, we discuss also the thermoelectric properties of some hybrid graphene structures, as graphane, layered carbon allotropes such as graphynes and graphdiynes, and graphene/hexagonal boron nitride heterostructures which offer new opportunities. Finally, we briefly review the recent activities on other atomically thin 2D semiconductors with finite bandgap, i.e. dichalcogenides and phosphorene, which have attracted great attention for various kinds of applications, including thermoelectrics.

In this work, we investigate thermoelectric properties of junctions consisting of two partially overlapped graphene sheets coupled to each other in the cross-plane direction. It is shown that because of the weak van-der Waals interactions between graphene layers, the phonon conductance in these junctions is strongly reduced, compared to that of single graphene layer structures, while their electrical performance is weakly affected. By exploiting this effect, we demonstrate that the thermoelectric figure of merit can reach values higher than 1 at room temperature in junctions made of gapped graphene materials, for instance, graphene nanoribbons and graphene nanomeshes. The dependence of thermoelectric properties on the junction length is also discussed. This theoretical study hence suggests an efficient way to enhance thermoelectric efficiency of graphene devices.PACS numbers: xx.xx.xx, yy.yy.yy, zz.zz.zzThe thermoelectric effect enables direct conversion of a temperature difference into an electric voltage and vice versa, and provides a viable route for electrical power generation from waste heat. The efficiency of thermoelectric conversion is determined by the dimensionless figure of merit, ZT , which is given bywhere G e is the electrical conductance, S is the Seebeck coefficient, and κ e,p is the thermal conductance contributed by charged carriers and lattice vibrations (phonons), respectively. For conventional materials, these transport coefficients are not independent and it is usually difficult to greatly improve their thermoelectric performance. In principle, to achieve a high ZT , it is simultaneously needed to suppress thermal conductance while keeping G e and S less affected. Some efficient approaches [1,2] have been suggested to guide thermoelectrics studies. They are mainly based on the use of low dimensional materials and/or nanostructuring as, for instance, thin films [3], quantum dot supperlattices [4], and silicon nanowires [5,6]. Graphene, a 2D mono-layer material, is expected to become one of the next generation electronic materials because of its outstanding properties such as high electron mobility [7] and high thermal conductivity [8,9]. Interestingly, the two above-mentioned approaches can be naturally combined in graphene nanostructures for better thermoelectric applications. For achieving large ZT in graphene systems, two important disadvantages have to be overcome: (i) S is too small due to the gapless character of graphene and (ii) κ p is too high. Many studies to improve thermoelectric properties of graphene with different strategies of * E-mail: hung@iop.vast.ac.vn nanostructuring have been suggested. In particular, it has been shown that the Seebeck effect can be significantly enhanced in graphene nanostructures having finite energy gaps such as graphene armchair nanoribbons (GNRs) [10], graphene nano-hole (nanomesh, i.e. GNM) lattices [11], hybrid graphene/boron nitride structures [12], graphene nanoribbons with a nanopore array [13], graphene nanoribbons consisting of alternate zig...

In this paper we report on the possibility to use particle-based Monte Carlo techniques to incorporate all relevant quantum effects in the simulation of semiconductor nanotransistors. Starting from the conventional Monte Carlo approach within the semi-classical Boltzmann approximation, we develop a multi-subband description of transport to include quantization in ultra-thin body devices. This technique is then extended to the particle simulation of quantum transport within the Wigner formulation. This new simulator includes all expected quantum effects in nano-transistors and all relevant scattering mechanisms which are taken into account the same way as in Boltzmann simulation. This work is illustrated by analyzing the device operation and performance of multi-gate nano-transistors in a convenient range of channel lengths and thicknesses to separate the influence of all relevant effects: significant quantization effects occurs for thickness smaller than 5 nm and wave mechanical transport effects manifest themselves for channel length smaller than 10 nm. We also show that scattering mechanisms still have an important influence in nanoscaled double-gate transistors, both in the intrinsic part of the channel and in the resistive lateral extensions.

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A new two-dimensional self-consistent Monte Carlo simulator including multi sub-band transport in 2D electron gas is described and applied to thin-film SOI double gate MOSFETs. This approach takes into account both out of equilibrium transport and quantization effects. Our method allows us to significantly improve microscopic insight into the operation of deep sub-100 nm CMOS devices. We compare and analyse the results obtained with and without quantization effects for a 15 nm long DGMOS transistor.

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The enhancement of thermoelectric figure of merit ZT requires to either increase the power factor or reduce the phonon conductance, or even both. In graphene, the high phonon thermal conductivity is the main factor limiting the thermoelectric conversion. The common strategy to enhance ZT is therefore to introduce phonon scatterers to suppress the phonon conductance while retaining high electrical conductance and Seebeck coefficient. Although thermoelectric performance is eventually enhanced, all studies based on this strategy show a significant reduction of the electrical conductance. In this study we demonstrate that appropriate sources of disorder, including isotopes and vacancies at lowest electron density positions, can be used as phonon scatterers to reduce the phonon conductance in graphene ribbons without degrading the electrical conductance, particularly in the low-energy region which is the most important range for device operation. By means of atomistic calculations we show that the natural electronic properties of graphene ribbons can be fully preserved while their thermoelectric efficiency is strongly enhanced. For ribbons of width M = 5 dimer lines, room-temperature ZT is enhanced from less than 0.26 to more than 2.5. This study is likely to set the milestones of a new generation of nano-devices with dual electronic/thermoelectric functionalities.