Hidden Direct Bandgap of Bi 2 O 2 Se by Se Vacancy and Enhanced Direct Bandgap of Bismuth Oxide Overlayer.

The Bi 2 O 2 Se surfaces are well-known to possess 50% Se vacancies, yet they have shown no in-gap states within the indirect bandgap (∼0.8 eV). We have found that the hidden in-gap states arising from the Se vacancies in a 2 × 1 pattern induce a reduced direct bandgap (∼0.5 eV). Such a reduced direct bandgap is responsible for the high electron mobility of Bi 2 O 2 Se. Moreover, the Bi oxide overlayers of the Bi thin films, formed through air exposure and annealing, unexpectedly exhibit a large direct bandgap (∼2.1 eV). The simplified fabrication of Bi oxide overlayers provides promise for improving Bi 2 O 2 Se electronic devices and enhancing photocatalytic activity.