  Transition metal dichalcogenides (TMDs) exist in various crystal structures
with semiconducting, semi-metallic, and metallic properties. The dynamic
control of these phases is of immediate interest for next generation
electronics such as phase change memories. Of the binary Mo and W-based TMDs,
MoTe2 is attractive for electronic applications because it has the lowest
energy difference (40 meV) between the semiconducting (2H) and semi-metallic
(1T') phases, allowing for MoTe2 phase change by electrostatic doping. Here we
report phase change between the 2H and 1T' polymorphs of MoTe2 in thicknesses
ranging from the monolayer case to effective bulk (73nm) using an ionic liquid
electrolyte at room temperature and in air. We find consistent evidence of a
partially reversible 2H-1T' transition using in-situ Raman spectroscopy where
the phase change occurs in the top-most layers of the MoTe2 flake. We find a
thickness-dependent transition voltage where higher voltages are necessary to
drive the phase change for thicker flakes. We also show evidence of
electrochemical activity during the gating process by observation of Te metal
deposition. This finding suggests the formation of Te vacancies which have been
reported to lower the energy difference between the 2H and 1T' phase,
potentially aiding the phase change process. Our discovery that the phase
change can be achieved on the surface layer of bulk materials reveals that this
electrochemical mechanism does not require isolation of a single layer and the
effect may be more broadly applicable than previously thought.
