Substrate pre-sputtering for layer-by-layer van der Waals epitaxy of 2D materials

Two-dimensional transition metal chalcogenides, with their atomically layered structure, favorable electronic and mechanical properties, and often strong spin–orbit coupling, are ideal systems for fundamental studies and for applications ranging from spintronics to optoelectronics. Their bottom–up synthesis via epitaxial techniques such as molecular-beam epitaxy (MBE) has, however, proved challenging. Here, we develop a simple substrate pre-treatment process utilizing exposure to a low-energy noble gas plasma. We show how this dramatically enhances nucleation of an MBE-grown epilayer atop, and through this, realize a true layer-by-layer growth mode. We further demonstrate the possibility of tuning the resulting growth dynamics via control of the species and dose of the plasma exposure.