1：Experimental Design and Method Selection:

The study involved the synthesis of layered perovskite oxynitride LiLaTa2O

2：15N57 with Dion-Jacobson phase, followed by its exfoliation into colloidal suspension through proton-exchange reaction and intercalation of ethylamine and tetrabutylammonium ion. Sample Selection and Data Sources:

The oxynitride-type perovskite was synthesized by the ammonolysis of the pristine perovskite oxide, LiLaTa2O

3：List of Experimental Equipment and Materials:

X-ray di?raction (XRD) patterns were collected by a powder X-ray di?ractometer (Bruker D2 phaser). Transmission electron microscope (TEM) studies were performed with a Hitachi H-

4：UV–vis spectra were obtained by UV–vis spectrometer (Thermo Scienti?c Evolution 200). Atomic force microscopy (AFM, Bruker-Nano N8 NEOS) was used. X-ray absorption spectroscopy (XAS) experiments were carried out with the extended X-ray absorption ?ne structure (EXAFS) facility installed at beamline 8 C in the Pohang Accelerator Laboratory, Korea. Experimental Procedures and Operational Workflow:

76 Sequential ion-exchange reactions of LLTA with proton, ethylammonium and tetrabutylammonium were used for the exfoliation of LLTA into colloidal nanosheets.

5：Data Analysis Methods:

XAS data analyses such as X-ray absorption near edge structure (XANES) and EXAFS were performed by standard procedure using UWXAFS package.