  Two-dimensional semiconductors - atomic layers of materials with covalent
intra-layer bonding and weak (van der Waals or quadrupole) coupling between the
layers - are a new class of materials with great potential for optoelectronic
applications. Among those, a special position is now being taken by
post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been
found that the band gap in 2D crystals of InSe more than doubles in the
monolayer compared to thick multilayer crystals, while the high mobility of
conduction band electrons is promoted by their light in-plane mass. Here, we
use Raman and PL measurements of encapsulated few layer samples, coupled with
accurate atomic force and transmission electron microscope structural
characterisation to reveal new optical properties of atomically thin GaSe
preserved by hBN encapsulation. The band gaps we observe complement the
spectral range provided by InSe films, so that optical activity of these two
almost lattice-matched PTMC films and their heterostructures densely cover the
spectrum of photons from violet to infrared. We demonstrate the realisation of
the latter by the first observation of interlayer excitonic photoluminescence
in few-layer InSe-GaSe heterostructures. The spatially indirect transition is
direct in k-space and therefore is bright, while its energy can be tuned in a
broad range by the number of layers.
