  MoS2 monolayers exhibit excellent light absorption and large thermoelectric
power, which are, however, accompanied with very strong exciton binding energy
- resulting in complex photoresponse characteristics. We study the electrical
response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L)
devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and
monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices
to unveil the intrinsic mechanisms responsible for photocurrent generation in
these materials and junctions. Strong photoresponse modulation is obtained by
scanning the position of the laser spot, as a consequence of controlling the
relative dominance of a number of layer dependent properties, including (i)
photoelectric effect (PE), (ii) photothermoelectric effect (PTE), (iii)
excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier
recombination. The monolayer and bilayer devices show peak photoresponse when
the laser is focused at the source junction, while the peak position shifts to
the monolayer/multi-layer junction in the heterostructure devices. The
photoresponse is found to be dependent on the incoming light polarization when
the source junction is illuminated, although the polarization sensitivity
drastically reduces at the monolayer/multi-layer heterojunction. Finally, we
investigate laser position dependent transient response of photocurrent to
reveal trapping of carriers in SiO2 at the source junction is the critical
factor to determine the transient response in 2D photodetectors, and also show
that, by systematic device design, such trapping can be avoided in the
heterojunction devices, resulting in fast transient response. The insights
obtained will play an important role in designing fast 2D TMDs based
photodetector and related optoelectronic and thermoelectric devices.
