  Different two-dimensional materials, when combined together to form
heterostructures, can exhibit exciting properties that do not exist in
individual components. Therefore, intensive research efforts have been devoted
to their fabrication and characterization. Previously, vertical and in-plane
two-dimensional heterostructures have been formed by mechanical stacking and
chemical vapor deposition. Here we report a new material system that can form
in-plane p-n junctions by thermal conversion of n-type SnSe2 to p-type SnSe.
Through scanning tunneling microscopy and density functional theory studies, we
find that these two distinctively different lattices can form atomically sharp
interfaces and have a type II to nearly type III band alignment. We also
demonstrate that this method can be used to create micron sized in-plane p-n
junctions at predefined locations. These findings pave the way for further
exploration of the intriguing properties of the SnSe2-SnSe heterostructure.
