Surface-confined two-dimensional mass transport and crystal growth on monolayer materials

Conventional vapour deposition or epitaxial growth of two-dimensional (2D) materials and heterostructures is conducted in a large chamber in which masses transport from the source to the substrate. Here we report a chamber-free, on-chip approach for growing 2D crystalline structures directly in a nanoscale surface-confined 2D space. The method is based on the surprising discovery of the rapid, long-distance, non-Fickian transport of a uniform layer of atomically thin palladium on a monolayer crystal of tungsten ditelluride at temperatures well below the known melting points of all the materials involved. The nanoconfined growth realizes the controlled formation of a stable 2D crystalline material, Pd7WTe2, when the monolayer seed is either free-standing or fully encapsulated in a van der Waals stack. The approach is generalizable and compatible with nanodevice fabrication, promising to greatly expand the library of 2D materials and their functionalities. The unexpected phenomenon of rapid, long-distance transport of an ultrathin and uniform metal film on two-dimensional crystals is reported at temperatures well below the melting points of all of the materials involved. The effect is generalizable and may offer possibilities in confined space chemistry, as well as in two-dimensional crystal growth and devices.