Spectroscopic properties of a freestandingMnPS3single layer

Quasi-two-dimensional manganese thiophosphate, $\mathrm{MnP}{\mathrm{S}}_{3}$, is interesting due to its two-dimensional antiferromagnetic structure observed at low temperatures as well as possible technological applications. While the spectroscopic properties of bulk $\mathrm{MnP}{\mathrm{S}}_{3}$ structures have been extensively studied, the spectroscopic characteristics of freestanding $\mathrm{MnP}{\mathrm{S}}_{3}$ layers are yet to be explored. We present an experimental study on the spectroscopic properties of a freestanding $\mathrm{MnP}{\mathrm{S}}_{3}$ single layer obtained through exfoliation from bulk $\mathrm{MnP}{\mathrm{S}}_{3}$. We find that the position of the main peak in the electron-energy-loss spectrum (EELS) is shifted from approximately 19 eV in bulk $\mathrm{MnP}{\mathrm{S}}_{3}$ to 15 eV in single $\mathrm{MnP}{\mathrm{S}}_{3}$ layer. Theoretical calculations show that this peak corresponds to a volume plasmon in bulk $\mathrm{MnP}{\mathrm{S}}_{3}$ and a damped plasmon peak in single-layer $\mathrm{MnP}{\mathrm{S}}_{3}$. In addition, the dispersion of this peak was investigated using momentum-resolved electron-energy-loss spectroscopy. The peak dispersion for the single-layer $\mathrm{MnP}{\mathrm{S}}_{3}$ displays a square-root behavior characteristic of a two-dimensional plasmon. We show that EELS spectra provide both a means to identify single-layer $\mathrm{MnP}{\mathrm{S}}_{3}$ from bulk structures and also show the effects of low dimensionality on the electronic excitations.