Controlled growth of ultrathin Mo2C superconducting crystals on liquid Cu surface

Exhibiting thickness-dependent change in the critical temperature (T-c) for the onset of superconductivity, Mo2C has emerged as an important new member in the family of two-dimensional atomic crystals. Controllable growth in terms of morphology and thickness is necessary to elucidate its intrinsic properties at the 2D limit. Here we demonstrate the chemical vapor deposition of ultrathin Mo2C crystals on liquid Cu surface where the morphology of the crystals can be controlled by tuning the carbon supersaturation. A unique staggered carbon vacancy ordering is discovered in Mo2C crystals having particular geometries. Thickness engineering of the crystal can be achieved by controlling the thickness of the Cu catalyst layer, which affords a facile route to grow ultrathin 2D samples. Ultrathin Mo2C crystals so obtained, have been characterized using aberration corrected scanning transmission electron microscopy annular dark field imaging, where the co-existence of both AA and AB stacking modes is observed. The high crystallinity of the Mo2C crystals synthesized in this work is attested by its characteristic sharp superconducting transition.