Electronic and Near-Infrared-II Optical Properties of I-Doped Monolayer MoTe: A First-Principles Study.

Near-infrared-II (NIR-II, 1000-1700 nm) fluorescence imaging is widely used for in vivo biological imaging. With the unique electronic structures and capability of band-gap engineering, two-dimensional (2D) materials can be potential candidates for NIR-II imaging. Herein, a theoretical investigation of the electronic structure and optical properties of iodine (I)-doped monolayer MoTe systems with different doping concentrations is carried out through simulations to explore their NIR optical properties. The results suggest that the emergence of impurity levels due to I doping effectively reduces the bandwidth of I-doped monolayer MoTe systems, and the bandwidth decreases with the increase in the I doping concentration. Although the I and Mo atoms possess clear covalent-bonding features according to the charge density difference, impurity levels induced by the strong hybridization between the I 5p and Mo 4d orbitals cross the Fermi level, making the doped systems exhibit metallic behavior. In addition, with the increase in the I doping concentration, the energy required for electron transition from valence bands to impurity levels gradually decreases, which can be linked to the enhancement of the optical absorption in the red-shifted NIR-II region. Meanwhile, with a higher I doping concentration, the emission spectra, which are the product of the absorption spectra and quasi-Fermi distributions for electrons and holes, can be enhanced in the NIR-II window.