Based on first-principles calculation using density functional theory, we study the vibrational properties and thermal expansion of mono-atomic two-dimensional honeycomb lattices: graphene, silicene, germanene and blue phosphorene. We focus on the si
milarities and differences of their properties, and try to understand them from their lattice structures. We illustrate that, from graphene to blue phosphorene, phonon bandgap develops due to large buckling-induced mixing of the in-plane and out-of-plane phonon modes. This mixing also influences their thermal properties. Using quasi-harmonic approximation, we find that all of them show negative thermal expansion at room temperature.
Low-dimensional electronic and glassy phononic transport are two important ingredients of highly-efficient thermoelectric material, from which two branches of the thermoelectric research emerge. One focuses on controlling electronic transport in the
low dimension, while the other on multiscale phonon engineering in the bulk. Recent work has benefited much from combining these two approaches, e.g., phonon engineering in low-dimensional materials. Here, we propose to employ the low-dimensional electronic structure in bulk phonon-glass crystal as an alternative way to increase the thermoelectric efficiency. Through first-principles electronic structure calculation and classical molecular dynamics simulation, we show that the $pi$-$pi$ stacking Bis-Dithienothiophene molecular crystal is a natural candidate for such an approach. This is determined by the nature of its chemical bonding. Without any optimization of the material parameter, we obtain a maximum room-temperature figure of merit, $ZT$, of $1.48$ at optimal doping, thus validating our idea.
