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
2
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
2
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.