Raman spectrum of Janus transition metal dichalcogenide monolayers WSSe and MoSSe

Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of additional features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and composition-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temperatures, near and off resonance. By combining polarization-resolved Raman data with calculations of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quantitative assessment of the defect concentration and, thus, the crystal quality of the materials. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.

Figure 1

First-order phonons and phonon band structure of Janus TMD monolayers WSSe and MoSSe. (a) Schematic representation of the atomic vibrations at the center $\mathrm{\Gamma }$ of the Brillouin Zone. Transition metal, selenium, and sulfur atoms are identified by gray, orange, and yellow, respectively. DFPT calculations of the phonon band structure of WSSe [(b), left panel] and MoSSe [(c), left panel]. Each energy dispersion diagram shows three acoustic (ZA, TA, LA) and six optical (${\mathrm{ZO}}_1$, ${\mathrm{ZO}}_2$, ${\mathrm{TO}}_1$, ${\mathrm{TO}}_2$, ${\mathrm{LO}}_1$, ${\mathrm{LO}}_2$) phonon branches. The corresponding PhDOS are shown in (b) and (c) (right panels). Vibrational modes are labeled at the high-symmetry points $\mathrm{\Gamma }$ ($E^{1,2}$, $A_1^{1,2}$), $\mathbf{K}$(${}^{1}E^{1,2,3},{}^{2}E^{1,2,3}$, $A^{1,2,3}$), and $\mathbf{M}$ ($A^{\prime 1,2,3}$, $A^{\prime \prime 1,2,3}$).

Figure 2

Raman spectra of Janus TMD monolayers and first-order phonon modes. (a) Room temperature and (b) 10-K Raman spectra of WSSe at ${\lambda }_{\mathrm{ex}}=532\mathrm{n}\mathrm{m}$ (green curves) and ${\lambda }_{\mathrm{ex}}=633\mathrm{n}\mathrm{m}$ (red curves). Blue and gray arrows indicate first-order and defect-activated Raman modes, respectively. (c) PhDOS of WSSe, with the calculated positions of the first-order Raman modes identified by dashed blue lines. (d), (e) Corresponding Raman spectra and (f) PhDOS with predicted first-order Raman modes of MoSSe.

Figure 3

Polarization-resolved, first-order Raman modes. Raman spectra of Janus monolayers (a) WSSe and (b) MoSSe for $0^{\circ }$, ${30}^{\circ }$, ${60}^{\circ }$, and ${90}^{\circ }$ angle between excitation and detection polarization, with experimental data shown as dots and their respective fits as solid curves; $E$ and $A_1$ Raman modes are plotted in blue and red, respectively. The spectra and fits are scaled with respect to the $A_1^1$ mode for clarity. Fits of the neighboring peaks are represented in gray. Polar plots of the integrated peak intensities of the complete polarization-resolved data set collected from (c) WSSe and (d) MoSSe as a function of the angle between excitation and detection polarization (dots), and their respective fits (solid curves). $E$ and $A_1$ modes are represented in blue and red, respectively. The spectra are taken at room temperature and ${\lambda }_{\mathrm{ex}}=532\mathrm{n}\mathrm{m}$.

Figure 4

Higher-order Raman processes in Janus WSSe and MoSSe monolayers. Raman spectra of (a) WSSe and (c) MoSSe at ${\lambda }_{\mathrm{ex}}=532\mathrm{n}\mathrm{m}$ (green curves) and ${\lambda }_{\mathrm{ex}}=633\mathrm{n}\mathrm{m}$ (red curves). Blue-shaded arrows indicate Raman peaks corresponding to double-resonant processes with mode assignments following Table 2. Defect-assisted processes are indicated by dark gray. For comparison, light gray denotes first-order modes. All measurements are taken at 10 K. Black arrows indicate the Raman signal of the substrate. PhDOS of double-resonant phonon modes of (b) WSSe and (d) MoSSe. Dotted lines indicate energies of double-resonant phonons that correspond to observed Raman peaks indicated by the same color.

Figure 5

Groups and subgroups for the high-symmetry points of the Brillouin zone of Janus TMDs.

Figure 6

Photoluminescence spectra of Janus WSSe and MoSSe monolayers at 10 and 300 K. The laser excitation wavelength ${\lambda }_{\mathrm{ex}}=633\mathrm{n}\mathrm{m}$ is close to the $A$ exciton resonance in WSSe (dark blue) and to the $B$ exciton resonance in MoSSe (dark green).

Figure 7

Complete set of the polarization-resolved spectra of Janus TMD monolayers. Specific peaks are rescaled as indicated in the figure for clarity.