  Transition metal dichalcogenides (TMDs) are ideal systems for two-dimensional
(2D) optoelectronic applications, owing to their strong light-matter
interaction and various band gap energies. New techniques to modify the
crystallographic phase of TMDs have recently been discovered, allowing the
creation of lateral heterostructures and the design of all-2D circuitry. Thus
far, the potential benefits of phase-engineered TMD devices for optoelectronic
applications are still largely unexplored. The dominant mechanisms involved in
the photocurrent generation in these systems remain unclear, hindering further
development of new all-2D optoelectronic devices. Here, we fabricate locally
phase-engineered MoTe$_{2}$ optoelectronic devices, creating a metal (1T')
semiconductor (2H) lateral junction and unveil the main mechanisms at play for
photocurrent generation. We find that the photocurrent originates from the
1T'-2H junction, with a maximum at the 2H MoTe$_{2}$ side of the junction. This
observation, together with the non-linear IV-curve, indicates that the
photovoltaic effect plays a major role on the photon-to-charge current
conversion in these systems. Additionally, the 1T'-2H MoTe$_{2}$ heterojunction
device exhibits a fast optoelectronic response over a wavelength range of 700
nm to 1100 nm, with a rise and fall times of 113 $\mu$s and 110 $\mu$s, two
orders of magnitude faster when compared to a directly contacted 2H MoTe$_{2}$
device. These results show the potential of local phase-engineering for all-2D
optoelectronic circuitry.
