We present temperature-dependent resonance Raman measurements on monolayer WS for the temperature range 4-295 K using excitation photon energies from 1.9 to 2.15 eV in ∼7 meV steps. These are analyzed to determine the resonance profiles of five previously assigned phonon based Raman peaks (A', E', 2ZA, LA, 2LA) and a previously unassigned peak at 485 cm whose possible attributions are discussed. The resonance profiles obtained are fitted to a perturbation theory derived model and it is shown that both excitons and trions are required to explain the profiles. The model is used to separate the contribution of exciton-exciton, trion-trion, and exciton-trion scattering to each of the Raman peaks at 4 K. This separation allows the ratios of the rates of scattering involving the A' and E' phonons for each of the three types of scattering to be determined. The explanation of the multiphonon Raman peaks requires the coupling of bright excitons and trions to large wavevector dark states. The fitting of the resonance Raman profiles for these Raman peaks demonstrates scattering of bright excitons to bright trions via these large wavevector dark states.

We present a high-resolution resonance Raman study of hBN encapsulated MoSe2 and WSe2 monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe2 A2s and MoSe2 A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates for k = 0 optical phonons are comparable for both 1s and 2s states despite differences in the envelope functions. We also observe multiple new dispersive Raman peaks including a peak at the WSe2 A2s resonance that demonstrates non-linear dispersion and peak-splitting behavior that suggests the dispersion relations for dark excitonic states at energies near the 2s state are extremely complex.

Hybridisation of electronic bands of two-dimensional materials, assembled into twistronic heterostructures, enables one to tune their optoelectronic properties by selecting conditions for resonant interlayer hybridisation. Resonant interlayer hybridisation qualitatively modifies the excitons in such heterostructures, transforming these optically active modes into superposition states of interlayer and intralayer excitons. For MoSe2/WSe2 heterostructures, strong hybridization of both single particle and excitonic states can occur via single particle tunnelling. Here we use resonance Raman scattering to provide direct evidence for the hybridisation of excitons in twistronic MoSe2/WSe2 structures, by observing scattering of specific excitons by phonons in both WSe2 and MoSe2. We also demonstrate that resonance Raman scattering spectroscopy opens up a wide range of possibilities for quantifying the layer composition of the superposition states of the exciton and the interlayer hybridisation parameters in heterostructures of two-dimensional materials.

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This paper briefly describes how nanowires with diameters corresponding to 1 to 5 atoms can be produced by melting a range of inorganic solids in the presence of carbon nanotubes. These nanowires are extreme in the sense that they are the limit of miniaturization of nanowires and their behavior is not always a simple extrapolation of the behavior of larger nanowires as their diameter decreases. The paper then describes the methods required to obtain Raman spectra from extreme nanowires and the fact that due to the van Hove singularities that 1D systems exhibit in their optical density of states, that determining the correct choice of photon excitation energy is critical. It describes the techniques required to determine the photon energy dependence of the resonances observed in Raman spectroscopy of 1D systems and in particular how to obtain measurements of Raman cross-sections with better than 8% noise and measure the variation in the resonance as a function of sample temperature. The paper describes the importance of ensuring that the Raman scattering is linearly proportional to the intensity of the laser excitation intensity. It also describes how to use the polarization dependence of the Raman scattering to separate Raman scattering of the encapsulated 1D systems from those of other extraneous components in any sample.