Van der Waals Epitaxy of Weyl-Semimetal T_d-WTe₂

Epitaxial growth of WTe₂ offers significant advantages, including the production of high-quality films, possible long-range in-plane ordering, and precise control over layer thicknesses. However, the mean island size of WTe₂ grown by molecular beam epitaxy (MBE) in the literature is only a few tens of nanometers, which is not suitable for the implementation of devices at large lateral scales. Here we report the growth of T_d -WTe₂ ultrathin films by MBE on monolayer (ML) graphene, reaching a mean flake size of ≃110 nm, which is, on overage, more than three times larger than previous results. WTe₂ films thicker than 5 nm have been successfully synthesized and exhibit the expected Td phase atomic structure. We rationalize the epitaxial growth of T_d-WTe₂ and propose a simple model to estimate the mean flake size as a function of growth parameters that can be applied to other transition metal dichalcogenides (TMDCs). Based on nucleation theory and the Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showing a critical coverage of 0.13 ML above which WTe₂ nucleation becomes negligible. The quality of monolayer WTe₂ films is demonstrated by electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES), which is in agreement with first-principles calculations performed on free-standing WTe₂ and previous reports. We found electron pockets at the Fermi level, indicating a n-type doping of WTe₂ with an electron density of n = 2.0 ± 0.5 × 10¹² cm^-2 for each electron pocket.