Control of Charge Carriers and Band Structure in 2D Monolayer Molybdenum Disulfide via Covalent Functionalization

The fine-tuning of electro-optic properties is critical for high-performing technologies. This is now obtainable with advanced nanostructures, particularly two-dimensional (2D) monolayer materials such as molybdenum disulfide (MoS₂). Using spin-polarized periodic density functional theory (DFT), we find that the direct band gap (K → K') can be chemically tuned with covalently bound functional groups. With an electron-withdrawing group such as fluorine, we observe one occupied α and one unoccupied β band, which correspond to the addition of an α electron and a β hole, confirmed with the spin difference (Q_α - Q_β) being 1. By increasing the electron-donating behavior with the replacement of F by H and then by Me, the occupied (valence) α band shifts upward in energy relative to the Fermi energy, and the unoccupied β shifts down until they are in contact with the Fermi energy. In addition, both α and β unoccupied (conduction) bands of the MoS₂ shift down, relative to the Fermi energy, until they are in contact with the Fermi and the system can be described as metallic. The MoS₂ + F system is thus a small gap semiconductor (0.96 eV), and the MoS₂ + H and MoS₂ + Me gaps are 0.21 and 0 eV (metallic), respectively. Spin density calculations illustrate the semilocalized nature of the α spin; however, this is not formed from the radical of the functionalizing group, but rather the resulting unpaired electron is on the sulfur atom after radical abstraction to form a covalent bond with the group. Five- and six-membered heterocycles were studied and further confirm these observations. Distinct from typical functional groups such as phenyl, we find evidence for the covalent bonding of pyrrole, cyclopentadiene, and pyridine to a sulfur atom of the MoS₂ surface, from the new α and β bands in the band structure. The charge carrier nature of the 2D monolayers of functionalized MoS₂ can be further tuned with charge doping (hole or electron), such that even the metallic systems can be returned to semiconducting states, but importantly as p-type conductors. Semilocalization of the spin states and control of the band gap can be generalized to other covalently functionalized 2D materials and appears suitable for electronic applications, such as photoluminescence devices, contact-free transistors, and quantum communication.