Our research focuses on measuring, understanding and manipulating heat transfer at nanometer and atomistic scales, via state-of-the-art, home built characterization tools. Knowledge of nanoscale and atomic scale heat transfer is crucial for a wide range of applications, including in thermal management of emerging electronic and optoelectronic devices, and in design of novel nanostructures with unprecedented thermal properties. For examples, in thermoelectrics, detailed understanding of nanoscale heat transfer facilitates development of novel nanomaterials for more efficient energy conversion from waste heat into useful electricity, while in photothermal therapy, knowledge of interfacial heat conduction enhances heat dissipation from gold nanoparticles to selectively induce hyperthermia in malignant cancer cells. One of the keys to advancing the frontier of nanoscale and atomic-scale heat transfer is novel characterization techniques to measure heat flow at such a small length scale, which is where our expertise lies.

Development of novel characterization tools

We focus on developing ultrafast pump-probe characterization techniques to facilitate experimental investigations of heat transport at nanometer and atomistic scales. For example, we develop an ultrafast pump-probe technique, called voltage-modulated thermoreflectance (VMTR), to directly probe the change of heat transfer across graphene interfaces under electrostatic fields. We also integrate time-resolved Raman spectroscopy and picosecond transient absorption to monitor the heating and cooling at multiple locations along ligands conjugated to gold nanorods.

Atomic-scale heat transfer across interfaces and heterojunctions

We apply the state-of-the-art characterization techniques to study heat transport across interfaces and heterojunctions. Currently, I focused on two material systems: interfaces of 2D materials (e.g., metal contacts on graphene for thermal management of graphene devices), and organic-inorganic heterojunctions (e.g., gold nanorods functionalized with organic ligands for photothermal therapy of cancers). We are also working on interfaces of a wide range of 2D materials.

Nanoscale heat transfer in crystalline materials and nanostructures

We investigate the mean-free-paths of phonons in a wide range of materials and nanostructures. We study the anisotropic heat transport in bulk black phosphorus, an important 2D material. We also accurately measured the cross-plane thermal conductivity of Si thin films, and thus demonstrated that prior first-principles calculations of Si overestimate the contribution of low-energy phonons in Si.

Developing novel nanostructures for thermal insulation and thermoelectric energy conversion

Through our expertise in nanoscale heat transport, we design novel nanostructures to meet the needs of the industry. Particularly, we design novel metal/graphene heterostructures, with ultralow thermal conductivity of < 0.1 W/m-K but with electrons contributing significantly to heat conduction. This is important for thermoelectric applications. Also, we are developing Si-based thermoelectric material that is CMOS-compatible, for seamless integration into spot cooling.