Driven by applications in chemical sensing, biological imaging and material characterisation, Raman spectroscopies are attracting growing interest from a variety of scientific disciplines. The Raman effect originates from the inelastic scattering of light, and it can directly probe vibration/rotational-vibration states in molecules and materials. Despite numerous advantages over infrared spectroscopy, spontaneous Raman scattering is very weak, and consequently, a variety of enhanced Raman spectroscopic techniques have emerged. These techniques include stimulated Raman scattering and coherent anti-Stokes Raman scattering, as well as surface- and tip-enhanced Raman scattering spectroscopies. The present review provides the reader with an understanding of the fundamental physics that govern the Raman effect and its advantages, limitations and applications. The review also highlights the key experimental considerations for implementing the main experimental Raman spectroscopic techniques. The relevant data analysis methods and some of the most recent advances related to the Raman effect are finally presented. This review constitutes a practical introduction to the science of Raman spectroscopy; it also highlights recent and promising directions of future research developments.

The effect of strain on the phonon modes of monolayer and few-layer MoS${}_2$ has been investigated by observing the strain-induced shifts of the Raman-active modes. Uniaxial strain was applied to a sample of thin-layer MoS${}_2$ sandwiched between two layers of optically transparent polymer. The resulting band shifts of the $E_{2g}^1$ ($\sim$ $385.3{\mathrm{cm}}^{-1}$) and $A_{1g}$ ($\sim$ $402.4{\mathrm{cm}}^{-1}$) Raman modes were found to be small but observable. First-principles plane-wave calculations based on density functional perturbation theory were used to determine the Grüneisen parameters for the $E_{1g}$, $E_{2g}^1$, $A_{1g}$, and $A_{2u}$ modes and predict the experimentally observed band shifts for the monolayer material. The polymer–MoS${}_2$ interface is found to remain intact through several strain cycles. As an emerging 2D material with potential in future nanoelectronics, these results have important consequences for the incorporation of thin-layer MoS${}_2$ into devices.

Rhenium diselenide (ReSe₂) is a layered indirect gap semiconductor for which micromechanical cleavage can produce monolayers consisting of a plane of rhenium atoms with selenium atoms above and below. ReSe₂ is unusual among the transition-metal dichalcogenides in having a low symmetry; it is triclinic, with four formula units per unit cell, and has the bulk space group P1̅. Experimental studies of Raman scattering in monolayer, few-layer, and bulk ReSe₂ show a rich spectrum consisting of up to 16 of the 18 expected lines with good signal strength, pronounced in-plane anisotropy of the intensities, and no evidence of degradation of the sample during typical measurements. No changes in the frequencies of the Raman bands with layer thickness down to one monolayer are observed, but significant changes in relative intensity of the bands allow the determination of crystal orientation and of monolayer regions. Supporting theory includes calculations of the electronic band structure and Brillouin zone center phonon modes of bulk and monolayer ReSe₂ as well as the Raman tensors determining the scattering intensity of each mode. It is found that, as for other transition-metal dichalcogenides, Raman scattering provides a powerful diagnostic tool for studying layer thickness and also layer orientation in few-layer ReSe₂.

The rhenium and technetium diselenides and disulfides are van der Waals layered semiconductors in some respects similar to more well-known transition metal dichalcogenides (TMD) such as molybdenum sulfide. However, their symmetry is lower, consisting only of an inversion center, so that turning a layer upside-down (that is, applying a C₂ rotation about an in-plane axis) is not a symmetry operation, but reverses the sign of the angle between the two nonequivalent in-plane crystallographic axes. A given layer thus can be placed on a substrate in two symmetrically nonequivalent (but energetically similar) ways. This has consequences for the exploitation of the anisotropic properties of these materials in TMD heterostructures and is expected to lead to a new source of domain structure in large-area layer growth. We produced few-layer ReS₂ and ReSe₂ samples with controlled “up” or “down” orientations by micromechanical cleavage and we show how polarized Raman microscopy can be used to distinguish these two orientations, thus establishing Raman as an essential tool for the characterization of large-area layers.