Origin and Suppression of Beam Damage-Induced Oxygen-K Edge Artifact from γ-Al2O3 using Cryo-EELS

Henry O Ayoola; Cheng-Han Li; Stephen D House; Cecile S Bonifacio; Kim Kisslinger; Joerg Jinschek; Wissam A Saidi; Judith C Yang

doi:10.1016/j.ultramic.2020.113127

Origin and Suppression of Beam Damage-Induced Oxygen-K Edge Artifact from γ-Al₂O₃ using Cryo-EELS

Ultramicroscopy. 2020 Dec:219:113127. doi: 10.1016/j.ultramic.2020.113127. Epub 2020 Oct 1.

Authors

Henry O Ayoola¹, Cheng-Han Li², Stephen D House³, Cecile S Bonifacio¹, Kim Kisslinger⁴, Joerg Jinschek², Wissam A Saidi⁵, Judith C Yang⁶

Affiliations

¹ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh PA 15261, USA.
² Department of Materials Science and Engineering, The Ohio State University, Columbus OH 43210, USA.
³ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh PA 15261, USA; Environmental TEM Catalysis Consortium (ECC), University of Pittsburgh, Pittsburgh PA 15261, USA.
⁴ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton NY 11973, USA.
⁵ Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh PA 15261, USA.
⁶ Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh PA 15261, USA; Environmental TEM Catalysis Consortium (ECC), University of Pittsburgh, Pittsburgh PA 15261, USA; Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA 15260, USA. Electronic address: judyyang@pitt.edu.

PMID: 33059174
DOI: 10.1016/j.ultramic.2020.113127

Abstract

Gamma-alumina (γ-Al₂O₃), like other low-Z oxides, is readily damaged when exposed to an electron beam. This typically results in the formation of a characteristic pre-edge peak in the oxygen-K edge of electron energy-loss spectra (EELS) acquired during or after the damage process. This artifact can mask the presence of intrinsic O-K edge fine structure that would reveal chemical properties of the material; therefore, its suppression is key. In this work, we systematically investigate the conditions that give rise to the damage-induced O-K pre-edge peak and show that it can be effectively suppressed by performing EELS experiments at cryogenic (cryo) temperatures. Prolonged exposure of γ-Al₂O₃ to a focused electron beam results in a hole bored through the sample; this was used as a reproducible beam damage condition. O-K edge EELS spectra were collected from a single-crystal γ-Al₂O₃ sample both during and after focused electron beam hole drilling, and at room and cryo temperatures, using a monochromated scanning transmission electron microscope (STEM). The characteristic 531 eV pre-edge peak visible in the room temperature EELS spectra was completely suppressed in the cryo-EELS spectra, even in the presence of a visible drilled hole. We then correlated these experimental observations with multiple-scattering EELS simulations to determine the likely atomistic origin of the damage-induced O-K pre-edge peak. The findings indicate that the pre-edge peak is caused primarily by the presence of surface O-O bonds formed during beam damage, and that operating at cryo temperature suppresses the formation of surface O-O bonds, thus preventing formation of the O-K pre-edge peak. Additionally, Al-L_2,3 edge EELS spectra revealed Al loss primarily from tetrahedral sites during hole drilling.

Keywords: Beam damage; EELS; STEM; cryo-EELS; γ-Al(2)O(3).