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Two-Dimensional Covalent Crystals by Chemical Conversion of Thin van der Waals Materials
journal contribution
posted on 2019-08-23, 20:17 authored by Vishnu Sreepal, Mehmet Yagmurcukardes, Kalangi S. Vasu, Daniel J. Kelly, Sarah F. R. Taylor, Vasyl G. Kravets, Zakhar Kudrynskyi, Zakhar D. Kovalyuk, Amalia Patanè, Alexander N. Grigorenko, Sarah J. Haigh, Christopher Hardacre, Laurence Eaves, Hasan Sahin, Andre K. Geim, Francois M. Peeters, Rahul R. NairMost 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 used 2D indium selenide
(InSe) obtained by exfoliation and converted it by direct fluorination
into indium fluoride (InF3), which has a nonlayered, rhombohedral
structure and therefore cannot possibly be 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 noncleavable ones opens exciting prospects for synthesizing
a wide variety of novel atomically thin covalent crystals.