Giant Thermoelectric Effect in Graphene-Based Topological Insulators with Heavy Adatoms and Nanopores

Abstract

Designing thermoelectric materials with high figure of merit ZT = S 2 GT /κ requires fulfilling three often irreconcilable conditions, i.e., the high electrical conductance G, small thermal conductance κ and high Seebeck coefficient S. Nanostructuring is one of the promising ways to achieve this goal as it can substantially suppress lattice contribution to κ. However, it may also unfavorably influence the electronic transport in an uncontrollable way. Here we theoretically demonstrate that this issue can be ideally solved by fabricating graphene nanoribbons with heavy adatoms and nanopores. These systems, acting as a two-dimensional topological insulator with robust helical edge states carrying electrical current, yield a highly optimized power factor S 2 G per helical conducting channel. Concurrently, their array of nanopores impedes the lattice thermal conduction through the bulk. Using quantum transport simulations coupled with first-principles electronic and phononic band structure calculations, the thermoelectric figure of merit is found to reach its maximum ZT 3 at T 40 K. This paves a way to design high-ZT materials by exploiting the nontrivial topology of electronic states through nanostructuring.PACS numbers: 73.50. Lw, 03.65.Vf, 85.80.Fi Thermoelectrics [1-3] transform temperature gradients into electric voltage and vice versa. Although a plethora of thermoelectric energy harvesting and cooling applications has been envisioned, their usage is presently limited by their small efficiency. This is due to the fact that increasing thermoelectric figure of meritrequires careful trade-off between electrical conductance G, the Seebeck coefficient S, and the thermal conductance κ el + κ ph . The total thermal conductance has contributions from both electrons κ el and phonons (i.e., lattice vibrations) κ ph . ZT quantifies the maximum efficiency of a thermoelectric cycle conversion in the linearresponse regime where a small voltage ∆V = −S∆T exactly cancels the current induced by the small temperature difference ∆T = T H − T C at average operating temperature T = (T H + T C )/2. The values approaching ZT → ∞ would ensure Carnot efficiency as the theoretical limit for a heat engine operating between a hot T H and a cold T C temperature. However, ZT of realistic devices is limited by irreversible energy losses via Joule heat and thermal conduction, so that a pragmatic goal is to achieve ZT 3 with low parasitic losses and stability over a broad temperature range [2, 3]. The major directions to increase ZT have been focused on either materials with high power factor S 2 G, such as doped narrow-gap semiconductors; or on minimizing κ ph by enhanced phonon scattering in different frequency ranges, such as through nanostructuring [1,2]. Although nanostructuring has progressed rapidly over the past decade [1,2], it typically affects bulk electronic states of conventional materials in unfavorable way for thermoelectricity. Thus, the recently discovered topological insulator (TI) materials [4,5] are of particular intere...

Thermoelectric effects and topological insulators

“…A general discussion on TE properties of TIs has been made [33] and some related discussions can be found in elsewhere [34,[46][47][48][49] . Next we show an example study of a specific material, fluorinated stanene [50] .…”

Model spin-orbit coupling Hamiltonians for graphene systems

“…28,[52][53][54][55][56][57][58][59][60][61][62][63][64] In this paper we present a detailed symmetry analysis focusing on effective SOC Hamiltonians in a way that is complementary to Refs. [40,56,65].…”

Enhancing the thermoelectric figure of merit in engineered graphene nanoribbons

“…Carbon-based materials show a wide range of thermal properties from about 0.01 W·mK −1 in amorphous carbon to above 2,000 W·mK −1 at room temperature in graphene [1,[16][17][18][19] and even higher in few layer graphene [20]. This means that 2D graphene and its multilayer counterparts are useful for thermal management applications [21].…”