Chien-Cheng Chang

Anisotropic thermal conductivity of MoS2 nanoribbons: Chirality and edge effects

Previous studies of the thermal transport in MoS2 are limited to the 0° (zigzag) and 30° (armchair) chiralities. We investigate the anisotropic thermal transport properties of MoS2 nanoribbons with various crystal chiralities by employing the full-band phonon dispersion relations obtained from first-principle calculations. The ribbons with chiralities other than 0° and 30° always have lower thermal conductivity, yet a local maximum at 19.1°. In addition, the thermal conductivity can be further decreased by increasing the edge roughness due to the largely degraded longitudinal phonons. These findings suggest possibilities of obtaining a higher thermoelectric efficiency in MoS2 nanoribbons.

Low-temperature grown graphene films by using molecular beam epitaxy

Complete graphene film is prepared by depositing carbon atoms directly on Cu foils in a molecular beam epitaxy chamber at 300 °C. The Raman spectrum of the film has indicated that high-quality few-layer graphene is obtained. With back-gated transistor architecture, the characteristic current modulation of graphene transistors is observed. Following the similar growth procedure, graphitization is observed at room temperature, which is consistent with the molecular dynamics simulations of graphene growth.

An analytical model for calculating thermal properties of two-dimensional nanomaterials

Most previous analytical theories for microscale heat transfer are limited to low temperatures (≤100 K). We present simple yet general analytical formulae which reveal the essential features of the thermal properties of two-dimensional nanomaterials in a wide range of temperatures by full coverage of guided wave and bulk wave modes. In particular, we are able to handle the intermediate ballistic-diffusive regime. As an illustration, the formulae are applied on graphene to obtain its specific heat, thermal conductance, and thermal conductivity. The predictions are remarkably consistent with existing theories and experiments.