Abstract In this work, we present measurements of angle-resolved polarized Raman spectra of single-layer (1L) and bulk ReSe2 recorded with excitation energies of 1.92 eV (647.1 nm) and 2.34 eV (530.8 nm). The Raman tensors for all modes were obtained by fitting simultaneously the angular dependence of the parallel (I‖) and crossed (I⊥) polarized intensities. We observed that the tensor elements are, in general, complex numbers, and their magnitudes and phases depend on both the dimensionality of the sample (1L or bulk) and the excitation energy. Results are discussed by considering the intrinsic contribution of a single layer to the tensor elements and the macroscopic contribution coming from the stacking of several layers. We show that the different behaviour of angle-resolved polarized Raman spectra for different excitation energies is due to the resonant Raman effect, which affects both the real and imaginary parts of the Raman tensor elements. Our work highlights the importance of understanding the fundamental physics of low symmetry 2D materials that can be used to fabricate devices sensitive to the direction of light polarization or electrical current.

We investigate theoretically, through of first-principles calculations, the effect of the application of large in-plane uniaxial stress on single-layer of MoS2, MoSe2, and MoSSe alloys. For stress applied along the zigzag (zz) direction, we predict an anomalous behavior near the point fracture. This behavior is characterized by the reorientation of the MoS2 structure along the applied stress from zz to armchair due to the formation of transient square-lattice regions in the crystal, with an apparent crystal rotation of 30 degrees. After reorientation, a large plastic deformation remains after the stress is removed. This behavior is also observed in MoSe2 and in MoSSe alloys. This phenomenon is observed both in stress-constrained geometry optimizations and in ab initio molecular dynamics simulations at finite temperature and applied stress.

Raman spectroscopy has been extensively used to probe disorder in graphene and other carbon-related materials, and disorder-induced (DI) Raman bands are prominent even for low defect densities. The DI bands in MoS2 have been studied in the last years, but a multiple excitation study using laser excitation energies near the excitonic energies was still lacking. In this work, we investigate the low-frequency defect-induced Raman bands in MoS2 coming from the acoustic phonon branches near the Brillouin zone edge using samples produced by mechanical exfoliation and chemical vapor deposition, recorded with different laser excitation energies close to the resonance with the excitonic transitions, and measured at different temperatures, from 100 K to 400 K. Our results show that the defect-induced Raman processes are affected by both excitation energy and temperature. We find that the temperature of measurement affects the linear dependence between the intensities of the DI peaks and the defect concentration. In particular, we observed that the ratio of intensities of the DI longitudinal acoustic (LA) and transversal acoustic (TA) modes with respect to the first-order E′ mode is about the same for the two different samples when results are corrected by the defect density. We show in this work that the largest intensity of the DI peaks occurs for laser energies in the resonance with the excitonic transitions. Finally, we introduce a general expression that provides the parameters for the quantification of defects in MoS2 samples based on the intensity of the DI Raman bands, measured at different laser energies across the excitonic transitions.

The double-resonance (DR) Raman process is a signature of all sp2 carbon material and provide fundamental information of the electronic structure and phonon dispersion in graphene, carbon nanotubes and different graphite-type materials. We have performed in this work the study of different DR Raman bands of rhombohedral graphite using eight different excitation laser energies and obtained the dispersion of the different DR features by changing the laser energy. Results are compared with those of Bernal graphite and shows that rhombohedral graphite exhibit a richer DR Raman spectrum. For example, the 2D band of rhombohedral graphite is broader and composed by several maxima that exhibit different dispersive behavior. The occurrence of more DR conditions in rhombohedral graphite is ascribed to the fact that the volume of its Brillouin zone (BZ) is twice the volume of the Bernal BZ, allowing thus more channels for the resonance condition. The spectra of the intervalley TO-LA band of rhombohedral graphite, around 2450 cm−1, is also broader and richer in features compared to that of Bernal graphite. Results and analysis of the spectral region 1700-1850 cm−1, where different intravalley processes involving acoustic and optical phonons occurs, are also presented.

When two periodic two-dimensional structures are superposed, any mismatch rotation angle between the layers generates a Moiré pattern superlattice, whose size depends on the twisting angle θ. If the layers are composed by different materials, this effect is also dependent on the lattice parameters of each layer. Moiré superlattices are commonly observed in bilayer graphene, where the mismatch angle between layers can be produced by growing twisted bilayer graphene (TBG) samples by CVD or folding the monolayer back upon itself. In TBG, it was shown that the coupling between the Dirac cones of the two layers gives rise to van Hove singularities (vHs) in the density of electronic states, whose energies vary with θ. The understanding of the behavior of electrons and their interactions with phonons in atomically thin heterostructures is crucial for the engineering of novel 2D devices. Raman spectroscopy has been often used to characterize twisted bilayer graphene and graphene heterostructures. Here, we review the main important effects in the Raman spectra of TBG discussing firstly the appearance of new peaks in the spectra associated with phonons with wavevectors within the interior of the Brillouin zone of graphene corresponding to the reciprocal unit vectors of the Moiré superlattice, and that are folded to the center of the reduced Brillouin Zone (BZ) becoming Raman active. Another important effect is the giant enhancement of G band intensity of TBG that occurs only in a narrow range of laser excitation energies and for a given twisting angle. Results show that the vHs in the density of states is not only related to the folding of the commensurate BZ, but mainly associated with the Moiré pattern that does not necessarily have a translational symmetry. Finally, we show that there are two different resonance mechanisms that activate the appearance of the extra peaks: the intralayer and interlayer electron–phonon processes, involving electrons of the same layer or from different layers, respectively. Both effects are observed for twisted bilayer graphene, but Raman spectroscopy can also be used to probe the intralayer process in any kind of graphene-based heterostructure, like in the graphene/h-BN junctions.

Abstract The Raman spectra of graphene-based matter exhibit a set of defect/disorder-induced bands. The D band, which exhibits a strong dispersion up to  50 cm−1/eV, comes from transverse optical phonons around K or K′ in the first Brillouin zone and involves an intervalley double resonance (DR) Raman process. In the present work, resonant Raman scattering (lines ranging from 1.58 to 3.81 eV) is used to study the unusual behavior of the one-phonon Raman band of a carbonaceous material (anthracene-based carbon which is one of the graphitizable carbons) upon its secondary carbonization stage (450°C–1000°C). While the G band appears to be nondispersive, the D band exhibits a change in both position and intensity. Its dispersion progressively rises from  6 cm−1/eV to values close to what is usually observed in defected graphene-based systems when anthracene-based carbon becomes almost pure. This evolution appears to be correlated with a release of hydrogen (fixed on the edges of polyaromatic layers) questioning their role in changing the D band resonance conditions.