Enhanced thermoelectric figure of merit in vertical graphene junctions

Abstract

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...

Spin and Charge Caloritronics in Bilayer Graphene Flakes with Magnetic Contacts

“…We choose the ribbons to be semiconducting, because they yield better values of the thermoelectric properties than the metallic counterparts [31], for which a fine tuning of their geometry is needed to increase their thermoelectric response [9]. We have verified that certainly is more interesting to focus in semiconducting flakes.…”

“…It is a consequence of the increase in the electronic conductance with T and the independence of the phonon thermal conductance for T ≥ 300 K in these structures (see Ref. [31]). In the P configuration, Z c T has a dual behavior with T as a function of the chemical potential.…”

“…In Ref. [31] the same geometry (AC12) for different flake lengths is investigated, albeit without including the spin dependence. In their Fig.…”

“…Such systems were studied before due to their remarkable transport properties; if the ribbons are metallic, a series of resonances and antiresonances appear related to the length of the bilayer region [29,30]. If the ribbons are semiconducting, they present important advantages due to the reduction of the thermal conductance in roughly an order of magnitude, yielding an enhanced figure of merit [31]. This is due to the fact that only van der Waals forces couple the bilayer central part, so the strong graphene σ-bond lattice is interrupted in the overlapping region, hindering phonon transport.…”

Seebeck Coefficient of a Single van der Waals Junction in Twisted Bilayer Graphene

“…These phonons are also expected to determine thermal and thermoelectric transport across the interface [10][11][12][13][14][15]. In fact, since the in-plane transverse and longitudinal phonons are effectively filtered out from contributing to cross-plane transport because they do not substantially alter the tunneling matrix elements, theoretical calculations predict enhanced cross-plane thermoelectric properties in van der Waals heterojunctions, including high ZT factors at room temperature [12].…”