Conversion of non-van der Waals VO 2 solid to 2D ferromagnet by CO 2 -induced phase engineering

Two-dimensional (2D) ferromagnetic semiconductors that combine ferromagnetic order with desirable physical attributes could find transformative applications in atomically-thin magneto-optical and magnetoelectric devices. The mainstream strategies of creating magnetic moments in 2D materials are introducing charge carriers. Here we introduce a CO 2 -induced phase engineering strategy that achieves 2D ferromagnet via the transformation of non-van der Waals (non-vdW) VO 2 solid to 2D defective structure with identified metastable phases. Our approach requires only exposing the structure to supercritical CO 2 liquid that is able to first infiltrate and swell the material at the molecular scale, and then “plasticize” VO 2 solid at the architectural scale to form a 2D defective network that ‘lock’ the metastable phases into a new topological structure, which would lead to a significantly enhanced ferromagnetic response. We attribute the phase transformation to the CO 2 pressure-induced selective cleavage of covalent bond.