  Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition
metal dichalcogenide heterostructures with potential applications in
optoelectronics. Critical for carrier and exciton transport is the quality of
the two materials that constitute the monolayer and the nature of the lateral
heterojunction. Important details of the optical properties were inaccessible
in as-grown heterostructure samples due to large inhomogeneous broadening of
the optical transitions. Here we perform optical spectroscopy at T = 4 K and
also at 300 K to access the optical transitions in CVD grown MoSe2-WSe2 lateral
heterostructures that are transferred from the growth-substrate and are
encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman
spectroscopy reveal considerably narrowed optical transition linewidth similar
to high quality exfoliated monolayers. In high-resolution transmission electron
microscopy (HRTEM) we find near-atomically sharp junctions with a typical
extent of 3nm for the covalently bonded MoSe2-WSe2. In PL imaging experiments
we find effective excitonic diffusion length that are longer for WSe2 than for
MoSe2 at low T=4 K, whereas at 300 K this trend is reversed.
