  Semiconductor heterostructures provide a powerful platform for the
engineering of excitons. Here we report the excitonic properties of
two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2
stacked epitaxially or non-epitaxially in the vertical direction. We find
similarly efficient interlayer relaxation and transition of excitons in both
the epitaxial and nonepitaxial heterostructures. This is manifested by a two
orders of magnitude decrease in the photoluminescence and the appearance of an
extra absorption peak at low energy region. The MoS2/WS2 heterostructures show
weak interlayer coupling and can essentially act as atomicscale heterojunctions
with the intrinsic bandstructures of the two monolayers largely preserved. They
are particularly promising for the applications that request efficient
dissociation of excitons and strong light absorption, including photovoltaics,
solar fuels, photodetectors, and optical modulators. Our results also indicate
that 2D heterostructures promise unprecedented capabilities to engineer
excitons from the atomic level without concerns of interfacial imperfection.
