Van der Waals epitaxy of Weyl-semimetal Td-WTe_2

Epitaxial growth of WTe_2 offers significant advantages, including the production of high-qualityfilms, possible long range in-plane ordering and precise control over layer thicknesses. However,the mean island size of WTe_2 grown by molecular beam epitaxy (MBE) in litterature is only a fewtens of nanometers, which is not suitable for an implementation of devices at large lateral scales.Here we report the growth of Td-WTe_2 ultrathin films by MBE on monolayer (ML) graphenereaching a mean flake size of ≅110nm, which is, on overage, more than three time larger thanprevious results. WTe_2 films thicker than 5nm have been successfully synthesized and exhibit theexpected Td-phase atomic structure. We rationalize epitaxial growth of Td-WTe_2 and propose asimple model to estimate the mean flake size as a function of growth parameters that can be appliedto other transition metal dichalcogenides (TMDCs). Based on nucleation theory and Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showinga critical coverage of 0.13ML above which WTe_2 nucleation becomes negligible. The quality ofmonolayer WTe_2 films is demonstrated from electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES) in agreement with first-principle calculationsperformed on free-standing WTe_2 and previous reports.