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

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

doi:10.1021/acs.nanolett.9b02700

Abstract

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.

Highlights

Chemical modification of materials has proved to be a powerful tool for obtaining novel materials with desired and often unusual properties.[1−6] Following the exfoliation of graphene,[7] the family of two-dimensional (2D) materials was populated either by direct exfoliation of layered bulk crystals[7−10] or by epitaxial growth techniques.[11−14] the concept of using an existing 2D material as an atomic scaffold for synthesizing novel 2D materials has been demonstrated by hydrogenated and fluorinated graphene, called graphane[5] and fluorographene,[6] respectively
We successfully demonstrate the validity of this approach by reporting the chemical conversion of three or more layers of indium selenide (InSe) into covalent InF3 thin films by fluorination of layered 2D crystals of InSe
Exfoliated InSe flakes and bulk InSe crystals were fluorinated by direct exposure to Xenon difluoride (XeF2) at elevated temperature using a method reported earlier[6]

Summary

Nano Letters

InSe, commercial InF3, and pristine bulk InSe. (h) HAADF STEM image region in Figure 1h, showing {102} and {104} planes in k-space. The. Tauc plot analysis (Figure 2b bottom inset; see Methods) reveals that the Se-doped InF3 samples obtained by fluorination of both InSe flakes and bulk crystals have a direct bandgap with Eg ≈ 2.2 eV.[3] This value is significantly larger than the energy 2EA = 1.4 eV derived from the electrical measurements, suggesting that the chemical potential is pinned on midgap impurity states. Calculated Raman spectrum of InF3 and Se doped InF3, atomic force microscopy image of thin InF3 crystals, Raman spectrum of fluorinated InSe over an extended range of wavenumbers, selected area electron diffraction and X-ray diffraction pattern of fluorinated InSe, XPS and Raman spectra from InF3 with different Se doping levels, optical characterization of InF3, phonon dispersion curve for thin InF3 structures, bandstructure of InF3, characterization of liquid exfoliation of InSe nanosheets, and large area InF3 film (PDF).

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