  Most of the studied two-dimensional (2D) materials have been obtained by
exfoliation of van der Waals crystals. Recently, there has been growing
interest in fabricating synthetic 2D crystals which have no layered bulk
analogues. These efforts have been focused mainly on the surface growth of
molecules in high vacuum. Here, we report an approach to making 2D crystals of
covalent solids by chemical conversion of van der Waals layers. As an example,
we use 2D indium selenide (InSe) obtained by exfoliation and converted it by
direct fluorination into indium fluoride (InF3), which has a non-layered,
rhombohedral structure and therefore cannot be possibly obtained by
exfoliation. The conversion of InSe into InF3 is found to be feasible for
thicknesses down to three layers of InSe, and the obtained stable InF3 layers
are doped with selenium. We study this new 2D material by optical, electron
transport and Raman measurements and show that it is a semiconductor with a
direct bandgap of 2.2 eV, exhibiting high optical transparency across the
visible and infrared spectral ranges. We also demonstrate the scalability of
our approach by chemical conversion of large-area, thin InSe laminates obtained
by liquid exfoliation into InF3 films. The concept of chemical conversion of
cleavable thin van der Waals crystals into covalently-bonded non-cleavable ones
opens exciting prospects for synthesizing a wide variety of novel atomically
thin covalent crystals.
