Epitaxial Synthesis of a Vertically Aligned Two-Dimensional van der Waals Crystal: (110)-Oriented SnO

During the explosion of research on two-dimensional (2D) van der Waals crystals over the past couple decades, synthesis has steadily advanced to include epitaxial films but always with the low-energy plane of the 2D material parallel to the surface of the substrate. Here, we report epitaxial synthesis of a 2D van der Waals crystal, SnO, in which the layers are aligned perpendicular to the surface. This accomplishment is particularly important for SnO, a top candidate p -type transparent conducting oxide, because the crystallographic direction with the highest hole mobility lies in the plane of the film where it could be utilized in a transistor. We find that the discontinuous nanowire morphology of the films prevents fabrication of useful devices, but nonetheless, this discovery represents a milestone in the fields of 2D materials synthesis and p -type oxides.