1. For each of the monolayers obtained thus far, we compute the phonon dispersion using the finite dif-ference approach, and compute the dynamical matrix and the phonon band structure using the phonopy package. We then exclude monolayers that have imaginary modes within the Brillouin zone for strained and unstrained monolayer unit cells. The computed phonon band structures are displayed in Table in the Supplementary Information, both for the dynamically stable and unstable monolayers.