Near-field infrared spectroscopy of monolayer ${\mathrm{MnPS}}_3$

We measured the near-field infrared response of ${\mathrm{MnPS}}_3$ 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 $450{\mathrm{cm}}^{-1}$ 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 ${\mathrm{MnPS}}_3$ 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.

Figure 1

(a) Crystal structure of monoclinic ${\mathrm{MnPS}}_3$ showing the slab thickness and interlayer distance. (b) Local structure for monolayer ${\mathrm{MnPS}}_3$ emphasizing the bonding around the Mn center and the P-P dimer [35]. (c) Close-up schematic of the near-field setup showing the cantilever tip directing light to focus on a small area of the sample. Spatial resolution is on the order of $20\times 20 {\mathrm{nm}}^2$. (d) High resolution AFM image along with the corresponding layer number, extracted from (e) the height profile along the red line in (d).

Figure 2

(a) Traditional infrared absorption (green) and Raman scattering (red) spectra at 300 K. (b) Lattice dynamics calculations of ${\mathrm{MnPS}}_3$ projected according to mode symmetry. Recall that the ungerade modes are infrared active, and the gerade symmetry modes are Raman active. The calculated frequencies are within 3% of the experimental peak positions. The calculated modes are shown with a Gaussian line shape and 2 ${\mathrm{cm}}^{-1}$ broadening. The higher peak intensity indicates near degeneracy. (c) Room temperature near-field amplitude (blue) and phase (orange) spectra for single crystalline ${\mathrm{MnPS}}_3$. We set the overall frequency scale to focus on the available near-field energy window.

Figure 3

(a) Near-field infrared response of ${\mathrm{MnPS}}_3$ single crystal compared with that of the monolayer. The latter is multiplied by a factor of 5. Corresponding lattice dynamics calculations highlight the symmetry modifications. We show both the infrared- and Raman-active modes. (b) and (c) Evolution of the near-field infrared spectra from ${\mathrm{MnPS}}_3$ single crystal ($n=\infty$) to the monolayer. The exfoliated sheets are on a gold substrate. A detailed discussion of signal-to-noise considerations is available in the Supplemental Material [48]. In (b) and (c) the spectra are shifted (by a constant amount) for clarity, the data below $n=11$ are multiplies by a factor of 5, and the well-known $\mathrm{Au}\cdots \mathrm{S}$ signature is indicated [56]. (d) Direct comparison of the predicted vibrational modes over the entire frequency range for a single sheet of ${\mathrm{MnPS}}_3$—depending on the symmetry that was imposed during the calculation ($C2/m$ vs $P\overline{3}1m$).