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

In this work, we have measured the thermoelectric transport across the van der Waals gap formed in twisted bilayer graphene (tBLG). We show that the thermoelectric properties are entirely determined by the out of plane ZO’/ZA₂ phonons which can manifest as an effective “interlayer phonon drag” through inter-layer electron-phonon scattering. The gate-voltage and temperature-dependence of Seebeck coefficient differs dramatically from the expected Landauer-Buttiker formalism in conventional tunnel junctions, along with a violation of Mott formula, which indicate possible signature of lattice anisotropy on thermoelectric transport. The power factor-temperature (PFT) product calculated from measured Seebeck coefficient and cross-plane resistance, can be as high as 0.3 W/mK which, together with the possible low inter-layer thermal conductance due to phonon filtering in cross-plane transport may lead to a high figure of merit thermoelectric system.