Thickness-Dependent Spintronic Terahertz Emission in Molecular Beam Epitaxy-Grown PtTe2: From Semiconductor to Type-II Dirac Semimetal

Spintronic THz emitters are the most practical broadband sources, but their performance is constrained by the spin Hall conductivity of the conversion layer. We show that PtTe2, a type II Dirac semimetal, overcomes this limitation through thickness-driven electronic phase control. Growing PtTe2 films from 1 to 20 monolayers (MLs) by molecular beam epitaxy, we find that THz emission mirrors the electronic phase diagram: it remains weak in the semiconducting single layer, turns on sharply at the semimetal transition near 2 ML, and reaches a maximum at 10 ML. When compared to a reference Pt emitter, this thickness dependence cannot be explained by an intrinsic inverse spin Hall effect but points to a spin-to-charge conversion dominated by the inverse Rashba-Edelstein effect involving topological surface states and Rashba spin splitting. First-principles calculations reproduce this picture quantitatively. These results establish the thickness engineering of van der Waals semimetals as a practical strategy for optimizing spintronic THz emitters and devices.