Thermal Transport in a 2D Amorphous Material

Amorphous materials such as glass have captured human imagination for thousands of years and continue to be of immense value for diverse technologies including advanced manufacturing, high-performance electronics, and thermal barrier coatings, in light of their intriguing structures and fascinating properties. In particular, recent years have witnessed substantial progress in terms of fabrication, characterization, and simulation. For example, the atomic structures of glassy solids (e.g., silica and high-entropy alloy) have finally been directly observed ever since the continuous-random-network (CRN) model was developed in 1932. Two-dimensional (2D) crystals proved revolutionary soon after graphene was discovered in 2004, however, 2D amorphous materials only became accessible in 2020 and remain largely unexplored. In particular, the thermophysical properties of amorphous materials are of great interest upon transition from 3D to 2D.<br/>In this work, by systematically measuring and simulating thermal transport in 2D amorphous carbon, we attempt to address one important scientific question: how would thermal transport in amorphous materials vary upon transition from 3D to 2D? First, multiple monolayers were assembled into a set of vdW stacks for cross-plane thermal measurement using the laser pump-probe technique of frequency-domain thermoreflectance. Subsequently, for in-plane transport, the microbridge method was employed with the monolayer amorphous carbon samples suspended across pairs of custom-fabricated heating and sensing micro-islands. We observed a cross-plane down to 0.079 ± 0.012 Wm^-1K^-1 at room temperature which is one of the lowest values reported to date, and a remarkable high in-plane up to 5.47 ± 0.32 Wm^-1K^-1 which is a few times larger than what is predicted by conventional wisdom for 3D amorphous carbon with similar sp² fraction.<br/>In order to understand these unusual observations, we then performed systematic molecular dynamics simulations which highlighted the role of disorder and dimensionality in both directions. The ultralow cross-plane thermal conductivity originates from the synergistic interplay between the cross-plane structural disorder and the weak interlayer vdW interaction characteristic of 2D materials. For the unusual high in-plane thermal conductivity, the crucial role of the low-frequency propagating modes in MAC is corroborated by their notably higher vibrational density of states and participation ratio. These distinct characteristics can be traced back to the 2D nature of MAC which significantly promotes out-of-plane vibrations. Amorphous materials at the 2D limit open up new avenues for understanding and manipulating heat at the atomic scale. We expect the unusual thermal properties of 2D MAC combined with its chemical stability, mechanical strength, and electrical tunability to be uniquely beneficial to various thermal management and energy harvesting applications.