  Layered inorganic materials, such as the transition metal dichalcogenides
(TMDs), have attracted much attention due to their exceptional electronic and
optical properties. Reliable synthesis and characterization of these materials
must be developed if these properties are to be exploited. Herein, we present
low-frequency Raman analysis of MoS2, MoSe2, WSe2 and WS2 grown by chemical
vapour deposition (CVD). Raman spectra are acquired over large areas allowing
changes in the position and intensity of the shear and layer-breathing modes to
be visualized in maps. This allows detailed characterization of mono- and
few-layered TMDs which is complementary to well-established (high-frequency)
Raman and photoluminescence spectroscopy. This study presents a major stepping
stone in fundamental understanding of layered materials as mapping the
low-frequency modes allows the quality, symmetry, stacking configuration and
layer number of 2D materials to be probed over large areas. In addition, we
report on anomalous resonance effects in the low-frequency region of the WS2
Raman spectrum.
