Non-covalent Functionalization of Pristine and Defective WSe2 by Electron Donor and Acceptor Molecules

Two-dimensional transition-metal dichalcogenide (2D-TMD) monolayers have recently attracted growing interest, thanks to their excellent optoelectronic properties, especially a moderate direct band gap in the visible spectral range and an extremely strong light–matter interaction. Herein, by means of density functional theory (DFT) and ab initio molecular dynamics (AIMD), we systematically investigate the chemical doping of the WSe2 monolayer upon non-covalent attachment of electron donor and acceptor molecules, namely, fullerenes (C20, C26, and C60), tetrathiafulvalene (TTF), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). Our results confirm that the physisorbed molecules stack on WSe2 via a weak van der Waals interaction; this precludes any significant damage in the basal-plane structure of the monolayer. In turn, the modifications in the carrier density in the WSe2 monolayer due to organic dopants result in a change of the work function by up to 0.41 eV. By performing AIMD calculations, we show that the effect is more pronounced upon increasing the coverage density of physisorbed TTF and TCNQ molecules. The impact of Se-vacancy (VSe) defects on the electronic properties of the WSe2 monolayer and thermodynamic stability of physisorption (including the molecular density) is also considered. Interestingly, the molecules demonstrate an ability to modulate the degree of spatial localization of VSe trap states. Moreover, the shift of the Fermi level upon molecular adsorption also enables further stabilization of charged defect states associated to a VSe vacancy. The pronounced effect of molecular adsorption on the VSe defect behavior in WSe2 might open the door for potential engineering of defect states in 2D TMDs.