Very high thermoelectric power factor near magic angle in twisted bilayer graphene

Abstract

Recent research on twisted bilayer graphene (TBG) uncovered that its twist-angle-dependent electronic structure leads to a host of unique properties, such as superconductivity, correlated insulating states, and magnetism. The flat bands that emerge at low twist angles in TBG result in sharp features in the electronic density of states, affecting transport. Here we show that they lead to superior and tuneable thermoelectric (TE) performance. Combining an iterative Boltzmann transport equation solver and electronic structure from an exact continuum model, we calculate TE transport properties of TBG at different twist angles, carrier densities, and temperatures. Our simulations show the room-temperature TE power factor (PF) in TBG reaches 40 mW m−1 K−2, significantly higher than single-layer graphene and among the highest reported to date. The peak PF is observed near the magic angle, at a twist angle of ≈1.3∘, and near complete band filling. We elucidate that its dependence is driven by two opposing forces: the band gap between the flat and remote bands suppresses bipolar transport and improves the Seebeck coefficient but the gap decreases with twist angle; on the other hand, the Fermi velocity, which impacts conductivity, is smallest at the magic angle and rises with twist angle. We observed a further increase in the PF of TBG with decreasing temperature. The strong TE performance, along with the ability to fine-tune transport using twist angle, makes TBG an interesting candidate for future research and applications in energy conversion and thermal sensing and management.

In-plane thermoelectric properties of graphene/xBN/graphene van der Waals heterostructures

“…A potential difference of 4 mV and 39 K gradient of temperature (∆T) were observed, which gives about 99.3 µV K −1 Seebeck coefficient, a 1.51 × 10 −15 W K −2 PF, and a dimensionless figure-of-merit of 1.05 × 10 -6 for the heterostructure [25]. Twisted bilayers of graphene have also demonstrated potential for high PF [13]. However, systematic knowledge of the impact of the number of h-BN layers in the vdWHs is lacking.…”

“…Combining layers of different 2D materials provides means of utilizing the signature properties of individual materials and open new pathways for creating heterostructures with unique properties. Many researchers have focused on enhancing the TE performance of materials through reductions in lattice thermal conductivity [11,12] and improvements in the power factor (PF) (S 2 σ) [13][14][15].…”