  Recent technical progress demonstrates the possibility of stacking together
virtually any combination of atomically thin crystals of van der Waals bonded
compounds to form new types of heterostructures and interfaces. As a result,
there is the need to understand at a quantitative level how the interfacial
properties are determined by the properties of the constituent 2D materials. We
address this problem by studying the transport and optoelectronic response of
two different interfaces based on transition-metal dichalcogenide monolayers,
namely WSe2-MoSe2 and WSe2-MoS2. By exploiting the spectroscopic capabilities
of ionic liquid gated transistors, we show how the conduction and valence bands
of the individual monolayers determine the bands of the interface, and we
establish quantitatively (directly from the measurements) the energetic
alignment of the bands in the different materials as well as the magnitude of
the interfacial band gap. Photoluminescence and photocurrent measurements allow
us to conclude that the band gap of the WSe2-MoSe2 interface is direct in k
space, whereas the gap of WSe2/MoS2 is indirect. For WSe2/MoSe2, we detect the
light emitted from the decay of interlayer excitons and determine
experimentally their binding energy using the values of the interfacial band
gap extracted from transport measurements. The technique that we employed to
reach this conclusion demonstrates a rather-general strategy for characterizing
quantitatively the interfacial properties in terms of the properties of the
constituent atomic layers. The results presented here further illustrate how
van der Waals interfaces of two distinct 2D semiconducting materials are
composite systems that truly behave as artificial semiconductors, the
properties of which can be deterministically defined by the selection of the
appropriate constituent semiconducting monolayers.
