  Energy band realignment at the interfaces between materials in
heterostructures can give rise to unique electronic characteristics and
non-trivial low-dimensional charge states. In a homojunction of monolayer and
multilayer MoS$_2$, the thickness-dependent band structure implies the
possibility of band realignment and a new interface charge state with
properties distinct from the isolated layers. In this report, we probe the
interface charge state using scanning photocurrent microscopy and
gate-dependent transport with source-drain bias applied along the interface.
Enhanced photoresponse observed at the interface is attributed to band bending.
The effective conductivity of a material with a monolayer-multilayer interface
of MoS$_2$ is demonstrated to be higher than that of independent monolayers or
multilayers of MoS$_2$. A classic heterostructure model is constructed to
interpret the electrical properties at the interface. Our work reveals that the
band engineering at the transition metal dichalcogenides monolayer/multilayer
interfaces can enhance the longitudinal conductance and field-effect mobility
of the composite monolayer and multilayer devices.
