Keywords:

Mechanical exfoliation, MAXene flakes, surface characterization, electrical characterization, electronic properties

Abstract

Another class of lamellar compounds named MAX phases, where M is an early transition metal, A belongs to group 13-16 of the periodic table and X is C or N, has globally aroused interest owing to a particular combination of ceramic and metal properties. Their layered structure displays stronger inter-layer (metallic) bonds than van der Waals and since 2011, when the first 2D counterparts, named MXenes, were produced by chemically etching of the parent MAX phases, no other delamination technique has been applied so far.

In this thesis we introduce a new way to delaminate MAX phases down to the ultimate thinness, via mechanical exfoliation. While such a process has been initially developed for van der Waals layered compounds, the strong inter-layer bonds in MAX phases seems unfavorable a priori. Our study was focused on Cr₂AlC, V₂AlC, Ti₂SnC MAX phases and the Mo₄Ce₄Al₇C₃ phase, for which we demonstrated that modifications to the transfer recipe known for graphite/graphene lead to remarkable results : flakes with large lateral dimensions and homogeneous thickness down to monolayer could be isolated. The isolation of few-monolayers thick flakes on SiO₂/Si substrates paved the way for their surface and electrical properties characterization. Besides, Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM) measurements of the surface of cleaved Cr₂AlC single crystals in ultra-high vacuum conditions revealed that clusters of Al atoms remain on the cleaved surface. Electrostatic Force and Kelvin Probe Force Microscopies (EFM and KPFM) provide clear evidence that flakes are metallic down to monolayer thickness. The same conclusion is deduced both from resistivity measurements as a function of temperature and I-V curves.

Finally, a kinetics study of MAX-to-MXene conversion based on the HF etching of a well-defined structure comprised of square-size V₂AlC pillars indicated that HF penetration takes place at facets perpendicular to the basal planes of the single crystal.

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