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 x H1–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.

Photocatalytic Nanosheet Lithography: Photolithography based on Organically Modified Photoactive 2D Nanosheets

“…TBA x H 1Àx Ca 2 Nb 3 O 10 nanosheets were obtained from KCa 2 Nb 3 O 10 in at wo-step process as described previously (see Experimental Details,and Table S1 and Figure S1 in the Supporting Information). [7] Theo rganically modified nanosheets exhibit ah eight of 2.6 nm ( Figure 1a), which is in agreement with the XRD measurements (see below) and other data from the literature. [7] Then anosheets were spincoated into optically homogenous films on silicon substrates as depicted in Figure 1b.N ote that the color of the Fabry-PØrot thin films originates from interference,because the film has an optical thickness in the range of half of the wavelength of visible light, [13] according to Equation…”

Momentum-Resolved Dielectric Response of Free-Standing Mono-, Bi-, and Trilayer Black Phosphorus

“…The EELS spectra in the range between 0 eV and 4 eV of the corresponding layers (inset of Fig 1d) are used to evaluate excitation threshold energies of 1.9±0.1 eV, 1.4±0.1 eV and 1.1±0.1 eV for 1, 2 and 3 layer BP, respectively using the tangential method, which is a standard analysis method applied to semiconductors. [19][20][21][22][23][24][25] As shown in the Supporting Information, this empirical method leads to reproductible values of the threshold (see in particular Figure S6).…”

“…[8][9][10][11][12] Distances are in Bohr, energies in eV. References in the Table are [14,15,16,20,21].…”

Mechanochemical synthesis of the lead-free double perovskite Cs2[AgIn]Br6 and its optical properties

“…While our finding confirms the general conclusion of prior theoretical work that the compound should be stable, we find an optical bandgap of E g =2.36 eV from diffuse reflectance measurements (figure 2). The optical bandgap was obtained by treating the diffuse reflectance data with the Kubelka-Munk function F(R)= (1−R) 2 /2R (where R is the reflectance of the sample) to obtain a calculated absorption combined with a Taucplot treatment, as successfully applied on halide semiconductors previously [26]. Plotting the data as for a direct forbidden transition with [F(R)·hν] 2/3 gives a very good linear fit over a wide region above the optical bandgap.…”