Ferroelectric Control of the Band Structure of the Transition Metal Dichalcogenide WSe₂

Due to their great sensitivity to external stimuli, transition metal dichalchogenides (TMD) are an ideal playground for band structure engineering. The production of 2D TMD – ferroelectric heterostructures is of particular interest as theoretical investigation showed TMD could exhibit Rashba spin-splitting under moderate electric fields, paving the way for electric field control of spins. While these systems have gathered a large interest in the optoelectronic and device conception communities there is a profound need of a deeper understanding of the effect of a ferroelectric substrate on the electronic band structure.<br/>Using thin-film, oppositely-polarized BiFeO₃ substrates we show how the valence band of trilayer and monolayer tungsten diselenide (WSe₂) is modulated by the ferroelectric polarization. Using angular resolved photoelectron emission spectroscopy, we report the first direct measure of this heterostructure band properties. Specifically, we observe that in the<br/>monolayer case the effective mass at Γ in the Brillouin zone is considerably higher than in previous investigations on free-standing WSe₂ . We demonstrate this apparent effective mass change could be realistically linked to an underlying Rashba spin-splitting. Our results could prove fundamental for the further development of TMD based Ferroelectric Field Effect<br/>Transistors (FeFET) and spintronic devices.