Spectroscopic properties of a freestanding $\mathrm{MnP}{\mathrm{S}}_3$ single 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.

Figure 1

Crystallographic bulk structure of $\mathrm{MnP}{\mathrm{S}}_3$ viewed along the $b$ axis ([010] direction). A single $\mathrm{MnP}{\mathrm{S}}_3$ layer $(\mathrm{M}{\mathrm{n}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ slab) and the vdW gap between individual $\mathrm{MnP}{\mathrm{S}}_3$ layers are also indicated.

Figure 2

Scattering geometry of an EELS experiment where the incident electron beam is characterized by energy $E_0$ and momentum $k_0$. The inelastically scattered electrons are characterized by an energy loss $\mathrm{\Delta }E$ and momentum $k^{\prime }$. The momentum transfer $q$ can be decomposed into the component parallel to the electron beam, $q\left|\right|$ and perpendicular to the electron beam $\mathit{q}\perp$.

Figure 3

$\mathrm{MnP}{\mathrm{S}}_3$ layers on $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrate. (a) Under an optical microscope. (b) Green-channel image. (c) Comparison between grayscale values across $\mathrm{MnP}{\mathrm{S}}_3$ layers and $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrate (open sphere) and calculated contrast (solid line) profiles along the $\mathrm{MnP}{\mathrm{S}}_3$ flakes and $\mathrm{Si}{\mathrm{O}}_2/\mathrm{Si}$ substrate obtained from positions 1 to 2.

Figure 4

Experimental electron-energy-loss spectra from bulk $\mathrm{MnP}{\mathrm{S}}_3$ layer (dotted curve) and a single $\mathrm{MnP}{\mathrm{S}}_3$ layer (solid curve) obtained perpendicular to the $\mathrm{MnP}{\mathrm{S}}_3$ layers. The dotted line at position 15 eV is a guide for the eye.

Figure 5

Calculated ELF parallel to the $x$ axis [100] for (a) bulk $\mathrm{MnP}{\mathrm{S}}_3$ and (b) single $\mathrm{MnP}{\mathrm{S}}_3$ layer. Calculated imaginary part of the dielectric function ${\varepsilon }_2$ (solid curve) and real part of the dielectric function ${\varepsilon }_1$ (dotted curve) for (c) bulk and (d) single-layer $\mathrm{MnP}{\mathrm{S}}_3$. The inset in (c) shows the position at which the ${\varepsilon }_1$ curve cuts the energy axis with a positive slope.

Figure 6

Spin-up (↑) and spin-down (↓) PDOS for (a) bulk $\mathrm{P}\text{−}s$ and $\mathrm{P}\text{−}p$, (b) bulk $\mathrm{S}\text{−}s$ and $\mathrm{S}\text{−}p$, (c) bulk $\mathrm{Mn}\text{−}s$ and $\mathrm{Mn}\text{−}d$, (d) single-layer $\mathrm{P}\text{−}s$ and $\mathrm{P}\text{−}p$, (e) single-layer $\mathrm{S}\text{−}s$ and $\mathrm{S}\text{−}p$, and (f) single-layer $\mathrm{Mn}\text{−}s$ and $\mathrm{Mn}\text{−}d$.

Figure 7

Indexed selected area electron diffraction from (a) bulk $\mathrm{MnP}{\mathrm{S}}_3$ obtained along the [001] crystalline direction. The diffraction pattern is indexed using the monoclinic bulk structure with space group $C2/m$. (b) Single-layer $\mathrm{MnP}{\mathrm{S}}_3$ indexed according to the trigonal structure with space group $P\text{−}31m$ (c) Momentum-resolved electron-energy-loss spectra $(q$ vs $\mathrm{\Delta }E)$ parallel to the 2 0 0 and −2 0 0 diffraction spot direction. The dotted rectangle shows the integration window width $\mathrm{\Delta }{q}_x=0.05{\AA }^{-1}$ used to obtain individual spectra at specific $q$.

Figure 8

Momentum-resolved EELS spectra in (a) bulk $\mathrm{MnP}{\mathrm{S}}_3$ and (b) single $\mathrm{MnP}{\mathrm{S}}_3$ layer for various momentum-transfer values.

Figure 9

The dispersion of the plasmon peaks for (a) bulk $\mathrm{MnP}{\mathrm{S}}_3$ and (b) single-layer $\mathrm{MnP}{\mathrm{S}}_3$. The error bars represent the uncertainty due to the integration window of the momentum $\mathrm{\Delta }{q}_x=0.05{\AA }^{-1}$ and in measuring the plasmon peak energy position. The dotted curve and dashed-dotted-dotted curves in (a) and (b) represent fitting of the dispersion curves with quadratic and square-root functions, respectively.

Figure 10

Experimental EELS spectra for (a) $\mathrm{MnP}{\mathrm{S}}_3$ single layer and (b) $\mathrm{MnP}{\mathrm{S}}_3$ bulk showing spectra at various momentum-transfer values between 0 and $0.1{\AA }^{-1}$. The effects of surface plasmons can be clearly seen between 10 and 13 eV (arrows) in the single layer. The dotted line at 15 eV is a guide for the eye.

Figure 11

Calculated probability for surface losses for $\mathrm{MnP}{\mathrm{S}}_3$ sample thickness of 10, 5, 1, and 0.7 nm.

Figure 12

Calculated scattering probabilities [energy loss $\left(E\right)$, scattering angle $\left(\theta \right)]$ at 80 kV for $\mathrm{MnP}{\mathrm{S}}_3$ sample thickness of (a) 10 and (b) 1 nm. The positions of surface and bulk losses are indicated.