Polarized Raman spectroscopy at the edges of exfoliated GeS and GeSe crystals

Germanium sulfide (GeS) and germanium selenide (GeSe) are layered crystals which structure bears a strong resemblance to that of black phosphorus (BP). As recently observed and reported, BP exhibits atomic rearrangements at crystal terminations, which lead to the activation of Raman modes that are otherwise forbidden by polarization selection rules. Considering the similarities in the crystalline structure of BP, GeS, and GeSe, the same behavior would be expected at the edges of these crystals. In this work, the Raman modes GeS and GeSe edges were experimentally studied using polarized Raman spectroscopy. We show, at the edges, the appearance of modes that are symmetry-forbidden. By carrying out DFT calculations, we conclude that the anomalous behavior is a consequence of edge atomic rearrangements. Such rearrangements, therefore, appear to be a general feature in lamellar crystals with the crystalline structure of BP.