First-Principle Study of Bandgap Engineering and Optical Properties of Monolayer WSe2 in Second Near-Infrared Windows

Abstract

Fluorescence imaging in the second near-infrared II (NIR-II) window is opening up new possibilities in bioimaging due to its low scattering rate within the tissue. The integration of 2D materials with NIR-II fluorescence will enable the development of multifunctional imaging probes. However, there are very few 2D materials that can fluoresce in the NIR-II range. Monolayer WSe2 is a potential 2D material, but its photoluminescence (PL) around 790 nm is still far from the NIR-II range due to its bandgap of 1.54 eV. In this study, it is investigated the electronic structures, dielectric functions, and PL spectra for Te, I, and Cr-doped monolayer WSe2, as well as W and S vacant monolayer WSe2. Most of the defected monolayer WSe2 remain semiconductors, except for a few configurations exhibiting metallic properties after making vacancies. Among the monolayer WSe2 under investigation, the Cr-doped WSe2 performs the best, exhibiting a strong PL peak in NIR-II with a decreased bandgap around 1.0 eV. As increasing Cr concentration, the peak shifts further toward the red end of the spectrum due to an enhancement of p–d transition. The results provide a useful guideline for material synthesis applied in NIR-II bioimaging and other biophysics.