Abstract
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 synthesized. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.
Highlights
Transition metal dichalcogenide (TMD) monolayers have emerged as a unique playground for exciton photophysics due to ∼0.5-eV-high exciton binding energies [1,2], strong light-matter interaction [3,4], optically addressable valley-contrasting spin physics caused by broken inversion symmetry [5], and large spin-orbit coupling [6]
Integrability on conventional silicon photonic technology [17,18], large-area fabrication [19,20,21], and deterministic positioning of quantum emitters [22,23,24] widen the impact of transition metal dichalcogenides (TMDs) to optoelectronics [25] and energy harvesting [26] as well as applications exploiting valleytronics [27,28], spintronics [29,30,31], and
TMD monolayers, for which rotation C2, improper rotation S3, and mirror σh symmetries are broken due to the different chalcogen atoms in the unit cell. This results in a lowering of the symmetry of the crystal to the symmorphic C31v space group (C3v point group)
Summary
Transition metal dichalcogenide (TMD) monolayers have emerged as a unique playground for exciton photophysics due to ∼0.5-eV-high exciton binding energies [1,2], strong light-matter interaction [3,4], optically addressable valley-contrasting spin physics caused by broken inversion symmetry [5], and large spin-orbit coupling [6]. New functionalities and physical phenomena appear when stacking monolayers of TMDs on top of each other, forming artificial metamaterials held together by van der Waals forces [34] Such heterostructures host long-lived, tunable dipolar interlayer [35] and trapped moiré excitons [36,37,38,39], offering a rich playground for few- and many-body phenomena [40,41,42,43,44,45,46], making them candidates for a solid-state quantum simulation platform [47]. As the experimental spectra show rich features arising beyond the calculated first-order processes, we discuss the mechanisms of higher-order and defectmediated Raman modes and assign them to the relevant experimental peaks
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