Quasiparticle electronic structure and optical spectra of single-layer and bilayer PdSe2: Proximity and defect-induced band gap renormalization

Abstract

The fundamental properties of recently synthesized single- and bilayer PdSe2 are investigated using accurate many-body perturbation GW theory to quantitatively examine their electronic structure and explain the insufficiency of previously reported experimental and theoretical results. Including electron-hole interactions responsible for exciton formation, we solve the Bethe-Salpeter equation on top of the GW0 approximation to predict the optical properties. The fundamental quasiparticle band gaps of single- and bilayer PdSe2 are 2.55 and 1.89 eV respectively. The optical gap of monolayer PdSe2 reduces significantly due to a large excitonic binding energy of 0.65 eV comparable to that of MoSe2, while an increase of the layer number decreases the excitonic binding energy to 0.25 eV in bilayer PdSe2. The giant band gap renormalization of ~36-38% in BL PdSe2/graphene heterostructure has a high impact on the construction of PdSe2-based devices and explains the experimentally observed band gap. The small value of the experimental optical gap of SL PdSe2 (1.3 eV) can be explained by the presence of Se vacancies, which can drop the Tauc-estimated optical gap to ~1.32 eV. The absorption spectra of both mono- and bilayer PdSe2 cover a wide region of photon energy demonstrating promising application in solar cells and detectors. These findings provide a basis for a deeper understanding of the physical properties of PdSe2 and PdSe2-based heterostructures.