Nanoscale lattice dynamics in hexagonal boron nitride moire superlattices

Here, the authors investigate the lattice dynamics of twisted hexagonal boron nitride layers via nano-infrared spectroscopy, showing local and stacking-dependent variations of the optical phonon frequencies associated to the interaction with the graphite substrate. Twisted two-dimensional van der Waals (vdW) heterostructures have unlocked a new means for manipulating the properties of quantum materials. The resulting mesoscopic moire superlattices are accessible to a wide variety of scanning probes. To date, spatially-resolved techniques have prioritized electronic structure visualization, with lattice response experiments only in their infancy. Here, we therefore investigate lattice dynamics in twisted layers of hexagonal boron nitride (hBN), formed by a minute twist angle between two hBN monolayers assembled on a graphite substrate. Nano-infrared (nano-IR) spectroscopy reveals systematic variations of the in-plane optical phonon frequencies amongst the triangular domains and domain walls in the hBN moire superlattices. Our first-principles calculations unveil a local and stacking-dependent interaction with the underlying graphite, prompting symmetry-breaking between the otherwise identical neighboring moire domains of twisted hBN.

Automated summary

In this study, the lattice dynamics of twisted hexagonal boron nitride layers were investigated using nano-infrared spectroscopy, revealing local and stacking-dependent variations in optical phonon frequencies due to interaction with the graphite substrate. First-principles calculations showed a symmetry-breaking interaction with the underlying graphite in twisted hBN, leading to systematic variations in in-plane optical phonon frequencies among triangular domains and domain walls in the hBN moire superlattices.