Controllable oxidation of h-BN monolayer on Ir(111) studied by core-level spectroscopies

► Stepwise oxidation of a single nearly free-standing h-BN monolayer by atomic O. ► Oxidation results in a gradual substitution of N by O in the h-BN lattice. ► Three different types of the B atom environment (BN3 − xOx with x = 1, 2, 3). ► New π resonances in the B K-edge NEXAFS are due to the oxidation, not N vacancies. ► Oxidation occurs very differently for isostructural h-BN and graphene monolayers.