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.
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