Origin of contact polarity at metal-2D transition metal dichalcogenide interfaces

Using state-of-the-art ab initio GW many-body perturbation theory calculations, we show that monolayer MoS2 on Au is a p-type contact, in contrast to the vast majority of theoretical predictions using density functional theory. The predominantly n-type behaviour observed experimentally for MoS2/Au junctions can be attributed to the presence of sulfur vacancies, which pin the Fermi level. GW calculations on WSe2/Au junctions likewise predict p-type contacts for pristine WSe2 and n-type contacts for junctions with selenium vacancies. Experimentally, WSe2/metal junctions are predominantly p-type or ambipolar, with p-type junctions being observed for selenium-deficient WSe2, suggesting that selenium vacancies are not effective in pinning the Fermi level for WSe2/metal junctions. We rationalize these apparently contradictory results by noting that selenium vacancies in WSe2 are readily passivated by oxygen atoms. Taken together, our state-of-the-art calculations clearly elucidate the relation between contact polarity and atomic structure. We show that non-local exchange and correlation effects are critical for determining the energy level alignment and even the contact polarity (in the case of MoS2 on Au). We further reconcile a large body of experimental literature on TMDC/metal contact polarities by consideration of the defect chemistry.

Automated summary

Using advanced calculations, we have investigated the contact properties of monolayer MoS2 and WSe2 on Au, revealing results that contradict previous theoretical predictions. The presence of sulfur and selenium vacancies plays a crucial role in pinning the Fermi level, and selenium vacancies in WSe2 are easily passivated by oxygen atoms.