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Two-Dimensional Covalent Crystals by Chemical Conversion of Thin van der Waals Materials

Sreepal, Vishnu; Yagmurcukardes, Mehmet; Vasu, Kalangi S.; Kelly, Daniel J.; Taylor, Sarah F. R.; Kravets, Vasyl G.; Kudrynskyi, Zakhar; Kovalyuk, Zakhar D.; Patanè, Amalia; Grigorenko, Alexander N.; Haigh, Sarah J.; Hardacre, Christopher; Eaves, Laurence; Sahin, Hasan; Geim, Andre K.; Peeters, Francois M.; Nair, Rahul R.


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. Nair


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 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.

Journal Article Type Article
Publication Date Aug 20, 2019
Journal Nano Letters
Print ISSN 1530-6984
Electronic ISSN 1530-6992
Publisher American Chemical Society
Peer Reviewed Peer Reviewed
APA6 Citation Sreepal, V., Yagmurcukardes, M., Vasu, K. S., Kelly, D. J., Taylor, S. F. R., Kravets, V. G., …Nair, R. R. (2019). Two-Dimensional Covalent Crystals by Chemical Conversion of Thin van der Waals Materials. Nano Letters, doi:10.1021/acs.nanolett.9b02700
Keywords Indium selenide; fluorination; indium fluoride; 2D covalent crystal; van der Waals materials
Publisher URL
Additional Information This document is the Accepted Manuscript of a Published Work that appeared in final form in ACS Nano Letters, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see