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. [Figure not available: see fulltext.].