  Tungsten ditelluride (WTe2) is a layered material that exhibits excellent
magnetoresistance and thermoelectric behaviors, which are deeply related with
its distorted orthorhombic phase that may critically affect the lattice
dynamics. Here, for the first time, we present comprehensive characterization
of the Raman spectroscopic behavior of WTe2 from bulk to monolayer using
experimental and computational methods. We discover that mono and bi-layer WTe2
can be easily identified by Raman spectroscopy since double or single Raman
modes that are observed in higher-layer WTe2 are substantially suppressed in
the monolayer and bilayer WTe2, respectively. In addition, different from
hexagonal metal dichalcogenides, the frequency of in-plane mode of WTe2 remains
almost constant as the layer number decreases, while the other Raman modes
consistently blueshift. First-principles calculation validates the experiments
and reveals that the negligible shift of the mode is attributed to the lattice
vibration along the tungsten chains that make WTe2 structurally
one-dimensional.
