  Tungsten ditelluride (WTe2), a layered transition-metal dichalcogenide (TMD),
has recently demonstrated an extremely large magnetoresistance effect, which is
unique among TMDs. This fascinating feature seems to be correlated with its
special electronic structure. Here, we report the observation of 6 Raman peaks
corresponding to the A_2^4, A_1^9, A_1^8, A_1^6, A_1^5 and A_1^2 phonons, from
the 33 Raman-active modes predicted for WTe2. This provides direct evidence to
distinguish the space group of WTe2 from that of other TMDs. Moreover, the
Raman evolution of WTe2 from bulk to monolayer is clearly revealed. It is
interesting to find that the A_2^4 mode, centered at ~109.8 cm-1, is forbidden
in a monolayer, which may be attributable to the transition of the point group
from C2v (bulk) to C2h (monolayer). Our work characterizes all observed Raman
peaks in the bulk and few-layer samples and provides a route to study the
physical properties of two-dimensional WTe2.
