Raman detection of hidden phonons assisted by atomic point defects in a two-dimensional semimetal

Defects usually have an important role in tailoring various properties of two-dimensional (2D) materials. However, optical detection of defects, especially single-atom point defects, is very challenging in 2D layers. Here, we report our systematic studies on the Raman-activated defect vibrational modes in 2D semimetallic material by combining Raman spectroscopy, density functional theory (DFT) calculation and scanning tunneling microscopy (STM). We observed three common Raman-active vibrational modes located at 95 (A1g2), 228 (A1g1), and 304 cm⁻¹ (B1g1) in ZrSiTe few-layers, consistent with our theoretical calculations. Moreover, a pronounced mode sitting at 131.7 cm⁻¹ was found in the ZrSiTe monolayer. This mode fades out quickly in the bilayer (2L) and eventually disappears in 4L. The high-resolution STM images and DFT calculations suggest this mode to be an intralayer shear mode at the Brillouin zone boundary which is activated by atomic point defects, and STM-based inelastic tunneling spectrum further confirms the existence of such a defect mode. The appearance of such ‘forbidden’ modes in Raman spectra may pave an avenue for the optical characterization of single-atom point defects in metallic 2D layers.