A Linear Relationship between the Charge Transfer Amount and Level Alignment in Molecule/Two-Dimensional Adsorption Systems

We systematically study the adsorption of tetrathiafulvalene (TTF), tetracyanoquinodimethane (TCNQ), and tetracyanoethylene (TCNE) on a variety of two-dimensional (2D) monolayers with weak van der Waals (vdW) interactions based on density functional theory. We confirm that TTF can act as an effective donor when its highest occupied molecular orbital (HOMO) level is higher than the conduction band minimum (CBM) state of 2D materials, while TCNQ and TCNE can act as effective acceptors when their lowest unoccupied molecular orbital (LUMO) levels are lower than the valence band maximum (VBM) state of 2D materials. Moreover, our calculations reveal a linear relationship between the charge transfer amount and level alignment between the molecule and 2D monolayer. In other words, the charge transfer is linearly dependent on the energy difference between the HOMO level and 2D CBM state for the donor molecule or the energy difference between the LUMO level and 2D VBM state for the acceptor molecule. The linear relationship indicates that the charge transfer is insensitive to the local binding environments due to the weak vdW interaction. However, the linear relationship cannot be applied to atoms or molecules that are chemisorbed on 2D materials.