Ultralow Thermal Conductivity in a Two-Dimensional Material due to Surface Enhanced Resonant Bonding

TL;DR

This study demonstrates that resonant bonding in two-dimensional materials significantly enhances anharmonic phonon interactions, leading to ultralow thermal conductivity, which is promising for thermoelectric applications.

Contribution

It reveals that resonant bonds are amplified in 2D materials due to reduced screening, resulting in lower thermal conductivity compared to bulk counterparts.

Findings

Quasi-2D Bi2PbTe4 has an ultralow thermal conductivity of 0.74 W/mK at 300K.

Resonant bonds are significantly enhanced in 2D, increasing lattice anharmonicity.

Enhanced resonant bonding in 2D reduces phonon transport, lowering thermal conductivity.

Abstract

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 due to 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 300K. 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 searching for thermal insulators in low-dimensional materials.