Impact of Valley Degeneracy on Thermoelectric Properties of Zigzag Graphene Nanoribbons with Staggered Sublattice Potentials and Transverse Electric Fields

TL;DR

This paper explores how valley degeneracy affects thermoelectric properties of zigzag graphene nanoribbons with staggered sublattice potentials and electric fields, revealing potential for enhanced thermoelectric performance.

Contribution

It demonstrates the influence of valley degeneracy on thermoelectric properties in ZGNRs with substrate-induced band inversion and electric field modulation, using Green's function calculations.

Findings

Valley degeneracy increases electrical conductance and power factors.

Seebeck coefficient remains stable in thermionic-assisted transport.

Power factor enhancement in ballistic transport is due to higher Seebeck coefficient at high temperatures.

Abstract

This study investigates the band inversion of flat bands in zigzag graphene nanoribbons (ZGNRs) using a tight-binding model. The band inversion results from symmetry breaking in the transverse direction, achievable through deposition on specific substrates such as separated silicon carbide or hexagonal boron nitride sheets. Upon band inversion, ZGNRs exhibit electronic structures characterized by valley degeneracy and band gap properties, which can be modulated by transverse electric fields. To explore the impact of this level degeneracy on thermoelectric properties, we employ Green's function techniques to calculate thermoelectric quantities in ZGNR segments with staggered sublattice potentials and transverse electric fields. Two carrier transport scenarios are considered: the chemical potential is positioned above and below the highest occupied molecular orbital. We analyze thermionic-assisted transport (TAT) and direct ballistic transport (DBT). Level degeneracy enhances the electric power factors of ZGNRs by increasing electrical conductance, while the Seebeck coefficient remains robust in the TAT scenario. Conversely, in DBT, the enhancement of the power factor primarily stems from improvements in the Seebeck coefficient at elevated temperatures.