  We study the interface exciton at lateral type II heterojunctions of
monolayer transition metal dichalcogenides (TMDs), where the electron and hole
prefer to stay at complementary sides of the junction. We find that the 1D
interface exciton has giant binding energy in the same order as 2D excitons in
pristine monolayer TMDs although the effective radius (electron-hole
seperation) of interface exciton is much larger than that of 2D excitons. The
binding energy, exciton radius and optical dipole strongly depends on the band
offset at the junction. The inter-valley coupling induced by the electron-hole
Coulomb exchange interaction and the quantum confinement effect at interface of
a closed triangular shape are also investigated. Small triangles realize 0D
quantum dot confinement of excitons, and we find a transition from
non-degenerate ground state to degenerate ones when the size of the triangle
varies. Our findings may facilitate the implementation of the optoelectronic
devices based on the lateral heterojunction structures in monolayer
semiconductors.
