Near-field infrared spectroscopy of monolayer MnPS3

We measured the near-field infrared response of MnPS3 in bulk, few-, and single-layer form and compared our findings with traditional far-field vibrational spectroscopies, a symmetry analysis, and first-principles lattice dynamics calculations. Trends in the B-u mode near 450 cm(-1) are striking, with the disappearance of this structure in the thinnest sheets. Combined with the amplified response of the activated A(g) mode and analysis of the A(u) + B-u features, we find that symmetry is unexpectedly increased in few- and single-sheet MnPS3 due to a restoration of the threefold axes of rotation. Monoclinicity in this system is therefore a consequence of the long-range stacking pattern and temperature rather than local structure.