Defect Engineering of Ultrathin WO₃ Nanosheets: Implications for Nonlinear Optoelectronic Devices

Ultrathin two-dimensional (2D) metal oxides have recently emerged as members of the 2D family with broad use in the catalytic field and energy storage techniques. However, investigations on their optoelectronic properties, nonlinear optical properties in particular, remain largely elusive. Defect (site) engineering had been a powerful tool to tailor semiconductor band gaps for their catalytic use, while, herein, it was carried out to expand the third-order nonlinear response of ultrathin 2D transition-metal oxide WO3 to the infrared region. By engineering the oxygen vacancy, WO3 demonstrated an indirect band gap adjustable from 2.33 eV (WO3) to 1.54 eV (D-WO3) with enhanced nonlinear saturable absorption extending from the visible to the near-infrared range, which have promising use in mode-locking, laser beam shaping, and ultrafast photonics. Transient absorption techniques revealed rapid fs-to-ps carrier dynamics followed by slower exciton bleaching on the μs time scale for the oxygen vacancy-rich metal oxides upon photoexcitation, which accounted for the origin of their strong and broad nonlinear optical responses. The strategy provided not only insights into the underlying photophysics of the 2D metal oxides but also fresh ammunition to bring out their remaining potential into full play, such as the fabrication of optoelectronic devices for nonlinear optics.