Tailored Synthesis and Band-Structure Modulation in Nitrogen-Doped Perovskite Oxide Nanosheets

Two-dimensional (2D) inorganic materials provide a powerful platform for electronic-structure engineering through precise control of the composition and crystal structure. While cation substitution has been widely exploited in oxide nanosheets, anion engineering remains far less developed, particularly in molecularly thin oxynitride systems with controlled nitrogen doping. Here, we report a generalizable route to nitrogen-doped perovskite oxide nanosheets that overcomes long-standing challenges associated with nitridation and structural instability. Using Dion-Jacobson (DJ)-type perovskite oxynitrides, RbSr₂(Nb_1-xTa_x)₃O_10-yN_y, as a model platform, we demonstrate that the combination of cation substitution and nitrogen doping enables systematic modulation of both composition and electronic band structure in 2D perovskites. DJ-type perovskite oxynitrides with substantial nitrogen incorporation can be obtained via an unexpected transformation from pseudo-Ruddlesden-Popper-type phases, induced by alkali metal salt-assisted nitridation followed by simple aqueous treatment, without altering the anion composition. These oxynitrides are subsequently exfoliated into single-layer nanosheets that preserve the perovskite framework and the designed cation stoichiometry. Direct determination of both valence and conduction band edges by combined ultraviolet and inverse photoelectron spectroscopy reveals composition-dependent, nonmonotonic band alignment behavior that cannot be resolved by indirect optical or electrochemical approaches. This work establishes an integrated materials and characterization framework for the rational electronic-structure design in 2D oxynitride nanosheets.