Ultra-low thermal conductivity and super-slow hot-carrier thermalization induced by a huge phononic gap in multifunctional nanoscale boron pnictides

Recent synthesis of thin films of boron monophosphide (BP) motivates us to report herewith the lattice thermal conductivity calculated in nanoscale boron pnictides. The lattice thermal conductivity in BX (X = P, As, Sb) monolayers are calculated to be 51.3, 20.7 and 5.4 Wm−1K−1 respectively at 300 K. The ultra-low lattice thermal conductivity in BSb, which is comparable to that in SnSe, is attributed to low elastic and bond stiffness, low Debye temperature, small group velocity and large mode Gruneisen parameter near zone center. Phonon group velocities in BSb are comparable to that in SnSe and Bi2Te3. Up to 50% reduction in lattice thermal conductivity is noticed at 300 K upon shortening the phonon mean free path, which is realizable, e.g., in 1D nanoribbons, or via defects, vacancies, nanoengineering, etc. A comparative study based on different models to compute the lattice thermal conductivity will provide useful insights into the experimentally measured ones. The widest ever phononic gap (436 & 438 cm−1) is found in BAs and BSb monolayers, where the transverse and longitudinal optical phonons meet the criterion for the prevention of Klemens decay and hence, they can be gainfully exploited in hot-carrier solar cells. BX (X = P, As, Sb) monolayers show a direct bandgap and piezoelectricity. Nanoscale boron pnictides are promising materials in futuristic thermoelectrics, bolometers, third-generation solar cells and nanopiezotronics.