Quantification of Surface Reactivity and Step-Selective Etching Chemistry on Single-Crystal BiOI(001)

Abstract

To bridge the gap between the cleanliness of a freshly cleaved surface of 2D BiOI and that available from a purely chemical-etching means, we subjected single-crystal BiOI to a series of surface treatments and quantified the resulting chemical states and electronic properties. Vapor transport syntheses included both physical vapor transport from singlesource BiOI, as well as chemical vapor transport from Bi 2 O 3 + BiI 3 and from Bi + I 2 + Bi 2 O 3 . Surface treatments included tape cleaving, rinsing in water, sonication in acetone, an aqueous HF etch, and a sequential HF etch with subsequent sonication in acetone. X-ray diffraction, XRD, and X-ray photoelectron spectroscopy, XPS, probed the resulting bulk crystalline species and interfacial chemical states, respectively. In comparison with overlayer models of idealized oxide-terminated or iodide-terminated BiOI, angle-resolved XPS elucidated surface terminations as a function of each treatment. Ultraviolet photoelectron spectroscopy, UPS, established work-function, and Fermi-level energies for each treatment. Data reveal that HF etching yields interfacial BiI 3 at BiOI steps that is subsequently removed with acetone sonication. UPS establishes n-type behavior for the vapor-transport-synthesized BiOI, and surface work function and Fermi level shifts for each chemical treatment under study. We discuss the implications for processing BiOI nanofilms for energy-conversion applications.

Interfacial States, Energetics, and Atmospheric Stability of Large-Grain Antifluorite Cs2TiBr6

“…The x -axis intercepts of each green-dashed regression mark the energy difference between the binding energy of the valence band maximum and the sample Fermi level or E F – E VBM . The red-dashed regression to the x -axis reveals secondary-electron photoemission cutoff energies, E SEC , that determine sample work functions, ϕ = E He I – E SEC , where E He I = hν = 21.218 eV, and when E F = 0 eV based on recent calibrations …”

“…The red-dashed regression to the x-axis reveals secondary-electron photoemission cutoff energies, E SEC , that determine sample work functions, ϕ = E He I − E SEC , where E He I = hν = 21.218 eV, and when E F = 0 eV based on recent calibrations. 22 The nascent sample in Figure 5A demonstrates a work function of ϕ =3.35 eV with a significant density of states and photoelectron counts at higher binding energies (or lower kinetic energies) than the secondary-electron-cutoff feature. Such features typically indicate surface contamination.…”

“…A PHI 5600 multitechnique system acquired all X-ray and ultraviolet photoelectron spectra as described previously. − The apparent rapid oxidation of the material necessitated a vacuum suitcase (Transfer Engineering and Manufacturing, Inc., Fremont, CA) for air-free transfer of puck-mounted Cs 2 TiBr 6 samples from a recirculating glovebox to the instrument. Peak quantification utilized an in-house-developed LabVIEW program based on published spectral shapes and corrected for instrument-specific sensitivity factors , and background energy loss functions. − For ultraviolet photoelectron spectroscopy, a gas discharge lamp (UVS 40A2, Prevac, Rogów, Poland) generated the He I spectroscopic line, E He I = hν = 21.218 eV.…”