Strained hexagonal boron nitride: Phonon shift and Grüneisen parameter

Owing to its large electric band gap, two-dimensional (2D) hexagonal boron nitride (hBN) is a very promising candidate for making atomically thin flexible devices. Further, its high mechanical strength makes it suitable as a reinforcing material in composites. It is thus of great interest to study the strain-induced phonon shift of mechanically exfoliated hBN of various thicknesses under tension. It is found that splitting of the ${E}_{2g}$ phonon to two subpeaks due to the lifting of degeneracy under uniaxial loading occurs at low strain level with average shift rates of $\ensuremath{-}8.4\phantom{\rule{0.28em}{0ex}}(1.0)\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/%$ and $\ensuremath{-}25.2\phantom{\rule{0.28em}{0ex}}(1.5)\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}/%$ for the ${E}_{2g}^{+}$ and ${E}_{2g}^{\ensuremath{-}}$, respectively. These large redshifts of the ${E}_{2g}$ modes with strain are close to the values obtained from graphene and confirm the high stiffness of 2D hBN. Furthermore, the measured ${E}_{2g}$ mode Gr\"uneisen parameter is about 1.88 (2), in agreement with the values obtained by classical molecular dynamics (MD) and ab initio MD simulations. The shift rates are similar for hBN with thickness of two to four layers, which is in contrast with graphene, indicating the effective load transfer between the hBN layers. The present results provide useful insights for the use of hBN in electronic devices, 2D heterostructures, and also as a reinforcing agent in composite materials.