Composition dependent lattice dynamics in MoSxSe(2–x) alloys

Abstract

We report on room temperature, polarization-resolved Raman scattering measurements on layered crystals of the series MoSxSe(2–x) (0 ≤ x ≤ 2) grown by chemical vapor transport technique. The results reveal two distinct sets of features related to the E2g1 and A1g modes of pure members of series. As composition x changes, the in-plane E2g1 mode shows two-mode behavior, whereas the out-of-plane A1g mode presents more complex evolution. The MoSe2-like branch reveals the splitting associated with the altering arrangement of S and Se atoms around Mo and the resulting changes in the dipole moment of the molecule. The X-ray diffraction measurements confirm that the samples are single-phase materials of 2H-type structure over the entire range of the sulfide composition x, while the scanning transmission electron microscopy imaging reveals a random arrangement of the S and Se atoms. Modified random-element-isodisplacement model is adopted to predict the behavior of the individual modes in the alloys. The model successfully confirms the two-mode behavior exhibited by the MoSxSe(2–x) series.

One-pot growth of two-dimensional lateral heterostructures via sequential edge-epitaxy

“…24,25 Figures 4a and 4b show the optical image and the composite PL map, respectively, of a threejunction lateral heterostructure composed of monolayered alloys (MoS2(1-x)Se2x -WS2(1-x)Se2x -MoS2(1-x)Se2x -WS2(1-x)Se2x), These were grown in a sequence of 12-0.3-3-0.7 min; via a similar carrier gas switching procedure using a mixture of MoS2 and WSe2 as the solid source. The sequential morphology observed in this case is close to conformal, indicating that the growth rate for each material in the heterostructure is isotropic (Extended data Fig.…”

Photoluminescence Studies of Excitonic Complexes in Atomically Thin Mo(SySe1-y)2 Alloys

“…Among mixed Se-S branch, we can identify at least three components, shifting to higher frequency upon increasing y. They are well resolved for 0.1 < y < 0.2, when the 2Se 2 + 1SeS, 2Se 2 + 1S 2 or 1Se 2 + 2SeS arrangements are the most probable [33][34][35][36]. By contrast, Se-Se branch shifts to lower frequency.…”

“…In MoS 2 the A 1 phonon energy of 51 meV [33] strongly exceeds those in MoSe 2 . For that reason incorporation of sulfur atoms into MoSe 2 host lattice results in significant increase of the A 1 phonon energy [36]. We found that the high temperature trion to exciton PL intensity ratio (I T /I X ) increase with sulfur mole content.…”

“…Confinement to a single layer Coulomb and reduced dielectric screening yields also to strong binding energy of charged excitons in TMDCs [30][31][32][33]. In selenides MoSe 2 and WSe 2 binding energy of an additional electron or hole to a neutral exciton is equal to 30 meV and is comparable with energies of optical phonons [33][34][35][36] which leads to strong coupling of the exciton and trion by exchange of an optical phonon [37]. In MoSe 2 the A 1 optical phonon energy, 29.8 meV [33], resonances with the trion binding energy equal 30 meV [33].…”

“…1, the A 1 (Se-Se) and A * (Se-S) phonons evolve in a different manner as a function increasing S content y. Based on their intensities, we have divided them into two groups [33][34][35][36]. The first group, at lower frequencies, corresponds to the Se-Se configuration, whereas the second group at higher frequencies is likely related to the Se-S vibration [33][34][35][36].…”