  Semiconductor heterostructures are fundamental building blocks for many
important device applications. The emergence of two-dimensional semiconductors
opens up a new realm for creating heterostructures. As the bandgaps of
transition metal dichalcogenides thin films have sensitive layer dependence, it
is natural to create lateral heterojunctions using the same materials with
different thicknesses. Using scanning tunneling microscopy and spectroscopy,
here we show the real space image of electronic structures across the
bilayer-monolayer interface in MoSe2 and WSe2. Most bilayer-monolayer
heterojunctions are found to have a zigzag-orientated interface, and the band
alignment of such atomically sharp heterojunctions is of type-I with a
well-defined interface mode which acts as a narrower-gap quantum wire. The
ability to utilize such commonly existing thickness terrace as lateral
heterojunctions is a crucial addition to the tool set for device applications
based on atomically thin transition metal dichalcogenides, with the advantage
of easy and flexible implementation.
