Yunshan's paper on Low thermal conductivity in Single Crystalline Porous Silicon Nanowires publi

We measure the porosity and thermal conductivity of electrolessly-etched single-crystalline silicon nanowires by means of an electron-beam heating technique. Upon irradiating the nanowire with a focused electron beam, the power absorbed by the nanowire scales with the cross-sectional area, thus allowing us to ascertain the material porosity. Such porous silicon nanowires exhibit extremely low thermal conductivity (as low as 0.33 W/m-K at 300K for 43% porosity), even lower than that of amorphous silicon.

See the paper here!

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...

Anomalously low electronic thermal conductivity in metallic vanadium dioxide (S. Lee et. al. Science

Decoupling charge and heat transport

In metals, electrons carry both charge and heat. As a consequence, electrical conductivity and the electronic contribution to the thermal conductivity are typically proportional to each other. Lee et al. found a large violation of this so-called Wiedemann-Franz law near the insulator-metal transition in vanadium dioxide nanobeams. In the metallic phase, the electronic contribution to thermal conductivity was much smaller than what would be expected from the Wiedemann-Franz law. The results can be explained in terms of independent propagation of charge and heat in a strongly correlated system.

Thermal conductivity of VO2

Phonon thermal conductivity

Breakdown of the WF law

In electrically conductive solids, the Wiedemann-Franz law requires the electronic contribution to thermal conductivity to be proportional to electrical conductivity. Violations of the Wiedemann-Franz law are typically an indication of unconventional quasiparticle dynamics, such as inelastic scattering, or hydrodynamic collective motion of charge carriers, typically pronounced only at cryogenic temperatures. We report an order-of-magnitude breakdown of the Wiedemann-Franz law at high temperatures ranging from 240 to 340 kelvin in metallic vanadium dioxide in the vicinity of its metal-insulator transition. Different from previously established mechanisms, the unusually low electronic thermal conductivity is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently.