Hydrogenation driven ultra-low lattice thermal conductivity in β ₁₂ borophene

Abstract

Borophene gathered large interest owing to its polymorphism and intriguing properties such as Dirac point, inherent metallicity, etc. but oxidation limits its capabilities. Hydrogenated borophene was recently synthesized experimentally to harness its applications. Motivated by experimental work, in this paper, using first-principles calculations and Boltzmann transport theory, we study the freestanding β12 borophene nanosheet doped and functionalized with hydrogen (H), lithium (Li), beryllium (Be), and carbon (C) atoms at different β12 lattice sites. Amongst all possible configurations, we screen two stable candidates, pristine and hydrogenated β12 borophene nanosheets. Both nanosheets possess dynamic and mechanical stability while the hydrogenated sheet has different anisotropic metallicity compared to pristine sheet leading to enhancement in brittle behaviour. Electronic structure calculations reveal that both nanosheets host dirac cones, while hydrogenation leads to shift and enhancement in tilt of the dirac cones. Further hydrogenation leads to the appearance of additional fermi pockets in the fermi surface. Transport calculations reveals that the lattice thermal conductivity changes from 12.51 to 0.22 Wm-1K-1 (along armchair direction) and from 4.42 to 0.07 Wm-1K-1 (along zigzag direction) upon hydrogenation at room temperature (300 K), demonstrating a large reduction by two orders of magnitude. Such reduction is mainly attributed to decreased phonon mean free path and relaxation time alongwith the enhanced phonon scattering rates stemming from high frequency phonon flat modes in hydrogenated nanosheet. Comparatively larger weighted phase space leads to increased anharmonic scattering in hydrogenated nanosheet contributing to ultra-low lattice thermal conductivity. Consequently, hydrogenated β12 nanosheet exhibits a comparatively higher thermoelectric figure of merit (∼0.75) at room temperature along armchair direction. Our study demonstrates the effects of functionalization on transport properties of freestanding β12 borophene nanosheets which can be utilized to enhance the thermoelectric performance in two-dimensional systems and expand the applications of boron-based two-dimensional materials.