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.

In a first-principles study based on density-functional theory and many-body perturbation theory, we investigate the electronic properties and the optical excitations of ZrS2 and HfS2 monolayers and their van der Waals heterostructure. Both materials have an indirect quasi-particle band gap, which amounts to about 2.8 eV in ZrS2 and to 2.6 eV in HfS2. In both systems the valence-band maximum is at Γ and the conduction-band minimum at M. Spin-orbit coupling induces a splitting of about 100 meV at the Γ point in the valence band, while it does not affect the conduction band. The optical absorption spectra are dominated by excitonic peaks, with binding energies between 0.6 eV and 0.8 eV. The ZrS2/HfS2 heterobilayer exhibits a peculiar type-I level alignment with a large degree of hybridization between the two monolayers in the valence band, while the conduction bands retain either ZrS2 or HfS2 character, respectively. As a consequence, both the electron and the hole components of the first exciton are localized in the ZrS2 monolayer with non-vanishing probability of finding the hole also in the HfS2 sheet.