Ultralow thermal conductivity in a two-dimensional material due to surface-enhanced resonant bonding

Crystalline materials with ultralow thermal conductivity are highly desirable for thermoelectric applications. Many known crystalline materials with low thermal conductivity, including PbTe and Bi₂Te₃, possess a special kind of chemical bond called ‘resonant bond’. Resonant bonds consist of superposition of degenerate bonding configurations that leads to structural instability, anomalous long-range interatomic interaction, and soft optical phonons. These factors contribute to large lattice anharmonicity and strong phonon-phonon scattering, which result in low thermal conductivity. In this work, we use first principles simulation to investigate the effect of resonant bonding in two dimensions (2D), where resonant bonds are in proximity to the surface. We find that the long-range interatomic interaction due to resonant bonding becomes more prominent in 2D because of reduced screening of the atomic displacement–induced charge density distortion. To demonstrate this effect, we analyze the phonon properties of quasi-2D Bi₂PbTe₄ with an ultralow thermal conductivity of 0.74 W/mK at 300 K. By comparing the interatomic force constants of quasi-2D Bi₂PbTe₄ and its bulk counterpart and the properties of resonant bonds near the surface and in the bulk, we conclude that resonant bonds are significantly enhanced in reduced dimensions and are more effective in reducing the lattice thermal conductivity. Our results will provide new clues to search for thermal insulators in low-dimensional materials.