LAUSR

To advance fundamental understanding, and ultimate application, of transition-metal dichalcogenide (TMD) monolayers, it is essential to develop capabilities for the synthesis of high-quality single-layer samples. Molecular beam epitaxy (MBE), a leading technique for the fabrication of the highest-quality epitaxial films of conventional semiconductors has, however, typically yielded only small grain sizes and sub-optimal morphologies when applied to the van der Waals growth of monolayer TMDs. Here, we present a systematic study on the influence of adatom mobility, growth rate, and metal:chalcogen flux on the growth of NbSe2, VSe2 and TiSe2 using MBE. Through this, we identify the key drivers and influence of the adatom kinetics that control the epitaxial growth of TMDs, realising four distinct morphologies of the as-grown compounds. We use this to determine optimised growth conditions for the fabrication of high-quality monolayers, ultimately realising the largest grain sizes of monolayer TMDs that have been achieved to date via MBE growth.