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

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 [TBAxH1−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. ■ INTRODUCTION Since the discovery of graphene the scientific community has paid considerable attention to apply the principles of twodimensional (2D) synthesis to other families of materials. A viable approach to various classes of inorganic 2D-nanostructures has been the solution-mediated delamination of layered bulk materials down to the single sheet level. This has led to the synthesis of 2D-nanosheets based on e.g. oxides, boron nitride, metal dichalcogenides, and metal disulfides. One prominent class of such materials is derived from the Dion− Jacobson family of layered perovskites, with the prototypic member KCa2Nb3O10. 6,7 This perovskite was delaminated into 2D-nanostructures consisting of monolayer sheets for the first time about two decades ago. A systematic study of the synthesis procedure and corresponding characterization of the s h e e t s w a s m a d e b y S c h a a k a n d Ma l l o u k . 9 [TBAxH1−x] [Ca2Nb3O10] − nanosheets have a structure similar to the layered Dion−Jacobson perovskite KCa2Nb3O10 shown in Figure 1a. The K ions of KCa2Nb3O10 are chemically replaced during the intercalation process by the bulky tetra-nbutylammonium cations. This replacement is accompanied by the introduction of a large amount of water and causes a significant reduction in the interaction between adjacent [Ca2Nb3O10] − perovskite blocks to the extent that the blocks become independent of each other. In contrast to the structure Received: January 6, 2016 Revised: April 30, 2016 Published: May 2, 2016 Figure 1. KCa2Nb3O10 has a layered structure (a) with parallel planes of K ions sandwiched between perovskite blocks consisting of cornersharing NbO6 octahedra (illustrated in green with red oxygen atoms at the apex and Nb atoms in the center), filled with Ca ions on the A site positions. [TBAxH1−x] [Ca2Nb3O10] − nanosheets (b) have a structure derived from KCa2Nb3O10 where the K + ions are replaced by bulky TBA and protons for charge balance. Article