Identifying Degenerate Shear and Layer Breathing Modes in Few-Layer 1T-PtS2 by Circularly Polarized Raman Spectroscopy.

Quasi-covalent interlayer bonding in 1T-PtS2 gives rise to pronounced thickness-dependent optical and electronic properties, yet its impact on vibrational properties remains poorly understood. In particular, low-frequency interlayer vibrations probe directional restoring forces, but the shear and breathing components have not been independently identified in 1T-PtS2. In this work, nearly degenerate in-plane shear modes and out-of-plane layer-breathing modes in few-layer 1T-PtS2 are resolved using circularly polarized Raman spectroscopy. Helicity selection rules separate E-symmetry in-plane vibrations from A-symmetry out-of-plane vibrations, enabling independent extraction of shear and breathing force constants with a linear chain model. The extracted values are nearly identical, in sharp contrast to conventional van der Waals layered materials, where the breathing response is typically much stiffer than the shear response. The same helicity-resolved spectra further clarify the Davydov splitting of high-frequency intralayer modes and reveal additional E-symmetry features tentatively assigned to second-order combination modes. These findings show that strong interlayer bonding in 1T-PtS2 underlies a nearly isotropic interlayer force landscape, providing a vibrational basis for its thickness-tunable optoelectronic and phononic functionality.