  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.
