Tuning optical properties of monolayer WS2 for near-infrared-II photothermal therapy: First-principles study

Photothermal therapy (PTT) is a new therapeutic cancer treatment strategy that offers many advantages to traditional cancer treatments, such as minimal invasiveness, higher specificity, and precise temporal selectivity. Optical imaging in the second near-infrared region (NIR-II, 1000-1350nm) has prospective applications in PTT and is superior to the first near-infrared region (650-1000nm). However, it proves challenging to find NIR-II materials. To resolve this, multiple elements were tested in substitutional doping to tune the bandgap of monolayer Tungsten Disulfide (WS₂). Two-dimensional materials have long proved to offer various properties, with WS₂ predicted to have ideal adjustable optical properties. Density functional theory calculations revealed the band structure and optical spectra of the different doped monolayer WS₂. The bandgap of monolayer WS₂ was successfully tailored to activate absorption in the NIR-II region. Results show that Zirconium doped WS₂ had a bandgap of 0.04 eV and Titanium doped WS₂ had a bandgap of 0.06 eV, which was ideal for NIR-II. Doping with Zirconium or Titanium exhibits strong absorption in the NIR-II range with absorption coefficients of 18830 and 13314 cm^-1, respectively, which is very competitive for PTT. This demonstrates the potential of these materials as NIR-II photothermal agents.