Physical vapor deposition of 2D Van der Waals materials: a review

A strong resurgence of interest in vdW solid processing has accompanied two-dimensional (2D) materials research in the past decade. 'Two dimensional' is terminology generally reserved for solids comprised of 5 molecular layers or less. As materials approach this ultimate thickness limit, unique properties emerge. vdW solids are well-suited for 2D materials research and applications as the anisotropic weak inter-layer bonding facilitates separation of bulk crystals into molecular constituents. Despite the great effort fueling the 2D materials revolution, electronics and other devices that truly capitalize on the benefits afforded by 2D van der Waals materials are not yet commercially available. The widespread use of 2D vdW solids is primarily inhibited by the lack of reliable, large-area synthesis approaches. Chemical vapor deposition is the primary means by which large area 2D TMD films are grown. This approach, however, has serious shortcomings, such as a lack of repeatable growth rates and kinetics, as well as the requirement for high processing temperatures (typically > 650 degrees C). Physical vapor deposition (PVD) constitutes a family of synthesis processes with inherent qualities enabling large-scale 2D vdW materials processing. With no fundamental limitations on size, uniformity over large areas (> 1 m(2)) for diverse materials, PVD has been demonstrated on thicker vdW films for decades. PVD processes are often characterized by the presence of energetic particles with kinetic energies that serve as an additional knob, along with process temperature and pressure, for control of growth kinetics. Ideally, a process would enable utilization of functional and mechanical properties of 2D vdW solids via synthesis directly on flexible polymer substrates at suitable temperatures. This objective is straightforward with PVD-based approaches. Here we review PVD techniques applied to the best-known vdW solids including graphene, transition metal dichalcogenides (such as MoS2), and boron nitride. Key points from decades of prior studies are summarized with the objective of applying this knowledge to hasten process development for future applications of large-area 2D vdW solids in advanced optical and electronic device applications meeting the needs of humankind beyond our current capabilities.