Helicity-Resolved Raman Scattering of MoS₂, MoSe₂, WS₂, and WSe₂ Atomic Layers

Abstract

The two-fold valley degeneracy in two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) (Mo,W)(S,Se) 2 is suitable for "valleytronics", the storage and manipulation of information utilizing the valley degree of freedom. The conservation of luminescent photon helicity in these 2D crystal monolayers has been widely regarded as a benchmark indicator for charge carrier valley polarization. Here we perform helicityresolved Raman scattering of the TMDC atomic layers. In drastic contrast to luminescence, the dominant first-order zone-center Raman bands, including the low energy breathing and shear modes as well as the higher energy optical phonons, are found to either maintain or completely switch the helicity of incident photons. These experimental observations, in addition to providing a useful tool for characterization of TMDC atomic layers, shed new light on the connection between photon helicity and valley polarization. KeywordsValleytronics, transition metal dichalcogenide, pseudospin, Raman scattering, shear mode, breathing mode 3 Manuscript textSince the discovery of graphene, 1 the mechanical, electronic, chemical and optical properties of various two-dimensional (2D) materials as well as their heterostructures have been widely investigated. [2][3][4][5] A prominent example is the semiconducting transition metal dichalcogenides (TMDCs) that exhibit rich physical phenomena, including indirect to direct bandgap transition, 6, 7 large exciton and trion binding energy, [8][9][10][11] strong photoluminescence and electroluminescence, 7, 12-14 superior transistor performance with large on-off ratio [15][16][17] and reasonably high mobility, 5, 18, 19 and perhaps most strikingly, the capability to address the valley degree of freedom. [20][21][22][23][24] Manipulation of valley polarized carriers excited by circularly polarized light has led to recent observation of the valley Hall effect 25 that opens up potential for applications in 'valleytronics' envisioned before in graphene. 26, 27 Here we apply circularly polarized light to excite electrons in TMDC atomic layers and measure the helicity of the photons emitted after they are inelastically scattered by phonons. We discovered that while some phonons maintain helicity from incident to emitted photon, others can switch it completely. Our results can be explained by the symmetry of participating lattice vibrations in the Raman scattering process. The helicity selection rules provide clean Raman spectra and prove to be a powerful tool for resolving phonon mode assignment and characterization. Importantly, the helicity of Raman scattered photons is independent of layer number and excitation laser wavelength, drastically different from observations in valley pumping of TMDC with helicity resolved luminescence. Our experiments consequently provide new insights into the relation between the photon helicity and valleytronics in semiconducting TMDCs. 4Layered TMDC materials have a graphite-like structure with each graphene sheet replaced wi...

Apparent breakdown of Raman selection rule at valley exciton resonances in monolayerMoS2

“…We note that the behavior we observe did not seem to appear prominently in the helicity-resolved Raman scattering measurements performed in Ref. [23], which may be due to a lower concentration of defects in the samples used by Ref. [23].…”

“…Raman spectroscopy has been used extensively on 2D materials, such as graphene and TMDC's, for studying thicknessdependent properties [13][14][15][16][17][18][19][20] and probing strain effects [21,22]. However, few Raman studies have examined systematically the polarization selection rule for TMDCs with circularly polarized excitation and detection [23]. Here, we perform polarization-resolved Raman spectroscopy on monolayers of MoS 2 with excitation wavelengths both on and off valley exciton resonance.…”

“…[23], which may be due to a lower concentration of defects in the samples used by Ref. [23]. Now, we consider the helicity of the valley excitons to address the polarization dependence we observe.…”

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

“…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].…”

“…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].…”