Band Gap Extraction from Individual Two-Dimensional Perovskite Nanosheets Using Valence Electron Energy Loss Spectroscopy

Abstract

Rapid progress in the synthesis of nanostructures with tailor-made morphologies necessitates adequate analytical tools to unravel their physical properties. In our study, we investigate, on the nanometer scale, the band gap of individual [TBA_xH_1–x]⁺[Ca₂Nb₃O₁₀]^? nanosheets obtained through intercalation–exfoliation of the layered bulk phase KCa₂Nb₃O₁₀ with tetra-n-butylammonium hydroxide (TBAOH) using valence electron energy loss spectroscopy (VEELS) in the scanning transmission electron microscope (STEM). The nanosheets consist of an anionically charged perovskite layer with cationic organic ligands surrounding it. Because of the hybrid nature, a careful acquisition and analysis protocol is required since the nanosheets disintegrate easily under electron beam irradiation. The VEELS data reveal a fundamental band gap of an individual freely suspended perovskite nanosheet to be 2.9 ± 0.2 eV and optically allowed transitions above 3.8 ± 0.2 eV (optical band gap). The spatial resolution of the measurements is about 9 nm, taking into account 50% of the excitations when illuminating with an incident electron beam of 1 nm diameter. Our investigations reveal that the band gap of an individual nanosheet is not changed significantly compared to the bulk phase, which is confirmed by UV–vis data. This is rationalized by the quasi-2D electronic structure of the bulk material being preserved upon delamination.