Dimensionality and Thermoelectrics in 2D MoS2: Hong Kuan et. al. in JAP, 121, 204303 (2017)

It has been an experimental challenge to show that electronic confinement is actually better for thermoelectrics. In our work published in Physical Review B (Jan 2017), we showed that high powerfactors are indeed possible in semiconducting 2D MoS2. In that work, we didn't analyze in detail what the exact benefits of 2D over 3D are.

In this work, Hong Kuan presents that two-dimensional (2D) bilayer molybdenum disulfide (MoS2) does indeed exhibit an enhanced Seebeck coefficient over its three-dimensional (3D) counterpart arising from dimensionality confinement. In this work, he extensively studies the Seebeck coefficient, S, the electrical conductivity, σ, and the thermoelectric powerfactor, S2σ of 2D monolayer and bilayer MoS2 using theoretical Boltzmann Transport Equation calculations and compares the results to well-characterized experimental data. We concluded that dimensional confinement indeed gives a Seebeck coefficient by up to ∼50% larger in 2D bilayer MoS2 over 3D MoS2 under similar doping concentrations because of the discretization of density of states. We also consider electrical conductivity with various energy-dependent scattering rates considering charged-impurities and acoustic phonon mediated scattering, and comment on a theoretical comparison of the powerfactor to the best-case scenario for 3D MoS2. More details can be found here.

Hopefully in the future, we can measure highly doped bulk (3D) MoS2 samples that can corroborate our estimations...

Thermal Interface Resistance of 2D Materials (Liu Yi et. al. in press, Scientific Reports)

The interface thermal conductance (G) at the MoS2/h-BN interface is measured by Raman spectroscopy, and the room-temperature value is (17.0±0.4) MW.m-2K-1. For comparison, G between graphene and h-BN is also measured, with a value of (52.2±2.1) MW.m-2K-1. Non-equilibrium Green’s function (NEGF) calculations, from which the phonon transmission spectrum can be obtained, show that the lower G at the MoS2/h-BN interface is due to the weaker cross-plane transmission of phonon modes compared to graphene/h-BN. This study demonstrates that the MoS2/h-BN interface limits cross-plane heat dissipation, and thereby could impact the design and applications of 2D devices while considering critical thermal management.

2D_interface_expt

2D_theory

High Thermoelectric Powerfactor in 2D Crystals of Molybdenum Disulfide (K. Hippalgaonkar et. al. in

The quest for high-efficiency heat-to-electricity conversion has been one of the major driving forces towards renewable energy production for the future. Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity, while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, the thermoelectric powerfactor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to high powerfactor due to the unique density of states (DOS) of confined electrons and holes. 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to such confinement effects, but especially due to their large effective masses and valley degeneracies. Here we report a powerfactor of MoS2 as large as 8.5 mWm−1K−2 at room temperature, which is amongst the highest measured in traditional, gapped thermoelectric materials. To obtain these high powerfactors, we perform thermoelectric measurements on few-layer MoS2 in the metallic regime, which allows us to access the 2D DOS near the conduction band edge and exploit the effect of 2D confinement on electron scattering rates, which result in a large Seebeck coefficient. The demonstrated high, electronically modulated powerfactor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.