An experimental study on thermoelectric transport at van der Waals interfaces

Abstract

When two planar atomic or molecular layers are brought to sub-nanometer proximity, they form a van der Waals interface because the van der Waals force forms the dominant attractive force between them. The van der Waals interfaces provide a novel platform for exploring various fundamental physical phenomena in light-matter interaction, electronphonon coupling, lattice strain engineering, charge-transfer dynamics and so on. New functionalities can also be engineered in homogeneous/heterogeneous interfaces, which is further enriched by the interlayer hybridization of electron wavefunctions. This has led to realization of many rich and novel electronic phases like Mott-insulator and superconductivity. The layer-hybridization and inter-layer electron-phonon interaction directly determine in electrical transport across the van der Waals interface through coupling of electrons to the interlayer breathing modes, but their impact on the thermal and thermoelectric properties remain unexplored. The motivation of this thesis is to understand the physical mechanism for thermoelectric transport across the sub-nanometer gap created at the van der Waals interface of two layers of graphene. We form atomically clean van der Waals interface at the junction between two independently contacted graphene layers, called twisted bilayer graphene, in a eld-e ect geometry. The crystallographic orientation of the participating layers was varied to tune the interlayer electronic hybridization. Independent electrical contacts allowed us to investigate both electrical and thermoelectric transport across the van der Waals junction as function of doping. To obtain thermopower or Seebeck coe cient (S) of the junction, we have employed Joule heating in one of the graphene layers using sinusoidal current and the 2nd harmonic voltage is measured between the two layers. The temperature di erence ( T) is measured graphene in-plane resistance thermometry. We show that for large twist angle stacking, i.e. lattice mis-orientation angle larger than about 4 degrees, the cross-plane Seebeck coe cient, which is the ratio of 2nd harmonic voltage and T is driven by an e ective interlayer phonon drag. The cross-plane thermo-voltage, which shows non-monotonic behaviour with respect to both temperature and number density, is originated through scattering of charge carriers by the out-of-plane layer breathing (ZO0/ZA2) phonon modes. The resulting Seebeck coe cient shows signi cant deviation from the expected Landauer- Buttiker formalism in the context of coherent transport in conventional tunnel junctions. At small twist angle, however, interlayer hybridization of electron wavefunctions comes into play and vertical transport is driven by momentum conserving coherent tunnelling. We show that in presence of strong interlayer coupling the thermoelectric transport can be described by the semiclassical Mott relation. Finally, exploiting the decoupling of charge and heat at large lattice mis-orientation, we estimate that it is possible to achieve thermoelectric gure-of-merit, or the ZT factor ( S2T= ) as large as 1 at room temperature, surpassing most common bulk thermoelectric materials around room temperature. In summary, the contrasting nature of the thermoelectric transport for small and large rotational stacking provides tunability of coherence motion of charge carriers through atomically-layered hybrids which can be manifested in engineering new phases of thermoelectricity in van der Waals epitaxy.