Controlling heat flow is a key challenge for applications ranging from thermal management in electronics to energy systems. However, progress has generally been limited by slow response times and low tunability in thermal conductance. In this work, we demonstrate an electronically gated solid-state thermal switch using self-assembled molecular junctions to achieve excellent performance at room temperature. In this three-terminal device, heat flow is continuously and reversibly modulated by an electric field through carefully controlled chemical bonding and charge distributions within the molecular interface. The devices exhibit ultrahigh switching speeds above 1 megahertz, on/off ratios in thermal conductance greater than 13 times, and can be switched more than 1 million times. Density functional theory calculations are applied to understand the electron density distribution of the chemical bond at the interface tuned by applied electric field. We anticipate that these advances will generate opportunities in molecular engineering for thermal management systems and thermal circuit design.

[1] Electrically gated molecular thermal switch, Science 382, 585-589 (2023). [link]

The development of materials with high thermal conductivity is key to effective thermal management. Ab initio calculations, grounded in quantum mechanical principles without the need for measured parameters or semi-empirical models, serve as a powerful tool for predicting material properties. I applied ab initio approach to predict material thermal conductivity, thereby guiding experimentalists in the synthesis and characterization of high thermal conductivity materials.

The investigation of high thermal conductivity materials presents considerable challenges: (i) the temperature gradient can easily drive phonons far away from equilibrium due to weak phonon scatterings, and (ii) higher order anharmonicity may effectively reduce the thermal conductivity. 

To capture these unconventional physics, I have developed computational codes for calculating lattice thermal conductivity, accounting for (i) high non-equilibrium by solving phonon Boltzmann transport equations with full scattering matrix, and (ii) the four-phonon scattering processes resulting from higher-order anharmonicity.

[1] Anomalous thermal transport under high pressure in boron arsenide, Nature 612, 459-464 (2022). [link]

[2] Ab initio determination of ultrahigh thermal conductivity in ternary compounds, Physical Review B 103, L041203 (2021). [link]

[3] Experimental observation of high thermal conductivity in boron arsenide, Science 361, 575-578 (2018). [link]

As the dimensions of microelectronic devices continue to decrease, the impact of size on thermal transport becomes increasingly critical. When the characteristic size is smaller than phonon mean free path (travelling distance between two consecutive phonon scatterings), phonons transport in a ballistic manner, similar to thermal radiation, which fails the classical heat diffusion theory.

To accurately solve multi-scale phonon transport, I have developed a Monte Carlo simulation code that solves the three-dimensional, mode-dependent Boltzmann transport equation. Especially, I incorporated phonon-phonon interactions from ab initio calculations, which led to ab initio Monte Carlo simulation that extends beyond the conventional relaxation time approximation.

This research offers a comprehensive theoretical view on the size effect in thermal transport, revealing the distinct physics of non-diffusive heat transport in microelectronics. It also enables exploration of highly non-equilibrium transport and novel phonon physics, such as phonon convection in solids.

[1] Ab initio investigations on hydrodynamic phonon transport: From diffusion to convection, International Journal of Heat and Mass Transfer 220, 124988 (2024). [link]

[2] Integration of boron arsenide cooling substrates into gallium nitride devices, Nature Electronics 4, 416-423 (2021). [link]

[3] Thermal properties and phonon spectral characterization of synthetic boron phosphide for high thermal conductivity applications, Nano Letters 17, 7507-7514 (2017). [link]

This research focuses on the phonon properties of ultra-low thermal conductivity materials, with a particular emphasis on SnSe, a high-performance thermoelectric material. Through interdisciplinary collaboration, we combine theory and experiments to explore the strong phonon softening in SnSe.

Our theoretical study suggests a significant phonon renormalization due to strong higher-order anharmonicity. Upon comparing with experimental results, it becomes clear that the commonly employed quasi-harmonic model is inadequate in describing the strong phonon softening observed in SnSe. Instead, based on quantum field theory, I employed self-consistent phonon theory to capture the phonon renormalization due to four-phonon interactions, aligning well with the experimental thermophysical properties of SnSe.

Our investigation uncovers an exceptionally strong intrinsic anharmonicity in SnSe, leading to significant phonon renormalization near room temperature. This work offers fundamental insights for refining modern calculation methods in phonon transport.

[1] Intrinsic low thermal conductivity and phonon renormalization due to strong anharmonicity of single-crystal tin selenide, Nano Letters 19, 4941–4948 (2019). [link]