Phase Engineering of 2D Spinel-Type Manganese Oxides

2D magnetic materials have been of interest due to their unique long-range magnetic ordering in the low-dimensional regime and potential applications in spintronics. Currently, most studies are focused on strippable van der Waals magnetic materials with layered structures, which typically suffer from a poor stability and scarce species. Spinel oxides have a good environmental stability and rich magnetic properties. However, the isotropic bonding and close-packed nonlayered crystal structure make their 2D growth challenging, let alone the phase engineering. Herein, a phase-controllable synthesis of 2D single-crystalline spinel-type oxides is reported. Using the van der Waals epitaxy strategy, the thicknesses of the obtained tetragonal and hexagonal manganese oxide (Mn3O4) nanosheets can be tuned down to 7.1 nm and one unit cell (0.7 nm), respectively. The magnetic properties of these two phases are evaluated using vibrating-sample magnetometry and first-principle calculations. Both structures exhibit a Curie temperature of 48 K. Owing to its ultrathin geometry, the Mn3O4 nanosheet exhibits a superior ultraviolet detection performance with an ultralow noise power density of 0.126 pA Hz(-1/2). This study broadens the range of 2D magnetic semiconductors and highlights their potential applications in future information devices.