Mechanical Exfoliation of Select MAX Phases and Mo₄Ce₄Al₇C₃ Single Crystals to Produce MAXenes

Abstract

MXenes-2D carbides/nitrides derived from their bulk nanolamellar M_n ₊₁ AX_n 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.