Mechanical Exfoliation of Select MAX Phases and Mo4 Ce4 Al7 C3 Single Crystals to Produce MAXenes.

MXenes-2D carbides/nitrides derived from their bulk nanolamellar Mn +1 AXn phase (MAX) counterparts-are, for the most part, obtained by chemical etching. Despite the fact that the MA bonds in the MAX phases are not weak, in this work it is demonstrated that relatively large MAX single crystals can be mechanically exfoliated using the adhesive tape method to produce flakes whose thickness can be reduced down to half a unit cell. The exfoliated flakes, transferred onto SiO2 /Si substrates, are analyzed using electric force microscopy (EFM). No appreciable variation in EFM signal with flake thickness is found. EFM contrast between the flakes and SiO2 not only depends on the contact surface potential, but also on the local capacitance. The contribution of the latter can be used to show the metallic character-confirmed by four-contact resistivity measurements-of even the thinnest of flakes. Because the A-layers are preserved, strictly speaking MXenes are not dealt with in this work, but rather MAXenes. This is important in the case where the "A" layers contain magnetic elements such as Mo4 Ce4 Al7 C3 , whose structure is a derivative of the MAX structure.

Elementary processes governing V2AlC chemical etching in HF.

The literature on MXenes, an important class of 2D materials discovered in 2011, is now abundant. Yet, the lack of well-defined structures, with definite crystal orientations, has so far hindered our capability to identify some key aspects ruling MXene's chemical exfoliation from their parent MAX phase. Herein the chemical exfoliation of V2AlC is studied by using well-defined square pillars with lateral sizes from 7 µm up to 500 µm, processed from centimeter-sized V2AlC single crystals. The MXene conversion kinetics are assessed with µm spatial resolution by combining Raman spectroscopy with scanning electron and optical microscopies. HF penetration, and the loss of the Al species, take place through the edges. At room temperature, and on a reasonable time scale, no etching can takes place by HF penetration through the basal planes, viz. normal to the basal planes. In defect-free pillars, etching through the edges is isotropic. Initially the etching rate is linear with a rate of 2.2 ± 0.3 µm h-1 at 25 °C. At a distance of ≈45 µm, the etching rate is greatly diminished.