  Heterojunctions between three-dimensional (3D) semiconductors with different
bandgaps are the basis of modern light-emitting diodes, diode lasers, and
high-speed transistors. Creating analogous heterojunctions between different
two-dimensional (2D) semiconductors would enable band engineering within the 2D
plane and open up new realms in materials science, device physics and
engineering. Here we demonstrate that seamless high-quality in-plane
heterojunctions can be grown between the 2D monolayer semiconductors MoSe2 and
WSe2. The junctions, grown by lateral hetero-epitaxy using physical vapor
transport, are visible in an optical microscope and show enhanced
photoluminescence. Atomically resolved transmission electron microscopy reveals
that their structure is an undistorted honeycomb lattice in which substitution
of one transition metal by another occurs across the interface. The growth of
such lateral junctions will allow new device functionalities, such as in-plane
transistors and diodes, to be integrated within a single atomically thin layer.
