Ultra-low lattice thermal conductivity and promising thermoelectric figure of merit in borophene via chlorination

Monolayer boron-based materials are of current interests due to its polymorphism. Herein, motivated by the recent experimental synthesis of semiconducting hydrogenated αʹ-borophene and the regulation of the physical properties in layered materials by surface functionalization, we study the thermal and electronic properties of αʹ-borophene with three different types of gas functional groups (H, F, and Cl) based on first-principles and Boltzmann transport theory. It is found that αʹ-borophene can be well stabilized by fluorination and chlorination and maintain the semiconductor nature. More interestingly, when hydrogen is replaced with fluorine or chlorine, the lattice thermal conductivity changes from 24.3 to 5.2 or 0.73 W/(m·K) along armchair direction at 300 K, exhibiting a huge reduction by two orders of magnitude. The main reason is the decrease of both phonon group velocities and acoustic phonon relaxation time resulting from the strong phonon mode softening due to the weaken B–B bond strength and heavier atomic mass of fluorine and chlorine. Consequently, the chlorinated αʹ-borophene exhibits a high thermoelectric figure of merit ~ 2 at 300 K along armchair direction. Our study illustrates the importance of the modulation of transport properties by gas functional groups, which may promote the thermoelectric application of boron-based materials.