  Semiconductor heterostructures are the fundamental platform for many
important device applications such as lasers, light-emitting diodes, solar
cells and high-electron-mobility transistors. Analogous to traditional
heterostructures, layered transition metal dichalcogenide (TMDC)
heterostructures can be designed and built by assembling individual
single-layers into functional multilayer structures, but in principle with
atomically sharp interfaces, no interdiffusion of atoms, digitally controlled
layered components and no lattice parameter constraints. Nonetheless, the
optoelectronic behavior of this new type of van der Waals (vdW) semiconductor
heterostructure is unknown at the single-layer limit. Specifically, it is
experimentally unknown whether the optical transitions will be spatially direct
or indirect in such hetero-bilayers. Here, we investigate artificial
semiconductor heterostructures built from single layer WSe2 and MoS2 building
blocks. We observe a large Stokes-like shift of ~100 meV between the
photoluminescence peak and the lowest absorption peak that is consistent with a
type II band alignment with spatially direct absorption but spatially indirect
emission. Notably, the photoluminescence intensity of this spatially indirect
transition is strong, suggesting strong interlayer coupling of charge carriers.
The coupling at the hetero-interface can be readily tuned by inserting
hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of
this interlayer coupling consequently provides a new degree of freedom in band
engineering and is expected to yield a new family of semiconductor
heterostructures having tunable optoelectronic properties with customized
composite layers.
