Low-frequency Raman signature of Ag-intercalated MoS₂: A first-principles study.

Two-dimensional layered materials (2DMs) are promising candidates for novel electronic devices as their electronic properties can be tuned e.g. by controlling their layer number. Precise characterization of 2DMs' is crucial for exact property control. Raman spectroscopy is a nondestructive technique that can identify even small structural/electronic changes.
Recent advances in Raman spectroscopy hardware made resolution of low-frequency (LF) Raman response in 2DMs possible. This response has a low intensity compared to the high-frequency (HF) Raman signature and is located close to the strong Rayleigh line making the detection challenging. However, as LF interlayer modes are rigid motions of each layer as a whole unit within the 2DM with restoring forces governed by the weak interlayer interactions, they are more sensitive to structural parameters than their HF intralayer counterparts (Liang L. et al., ACS Nano Article ASAP, DOI:10.1021/acsnano.7b06551).
Motivated by experiment, we present a Density Functional Theory based study of LF Raman modes of few-layer MoS₂ intercalated with silver. We predict a noticeable red shift of LF modes with increasing Ag concentration. This shift is confirmed experimentally and can be used for confirmation of successful silver diffusion into MoS₂.