Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar+ bombardment

• Published in: Applied surface science Vol. 252; no. 22; pp. 7895 - 7903
• Main Authors: WEAVER, Jason F, HAGELIN-WEAVER, Helena A. E, HOFLUND, Gar B, SALAITA, Ghaleb N
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Science 15.09.2006
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Electron and ion emission by liquids and solids; impact phenomena; Electron energy loss spectra; Electron energy loss spectroscopy; Exact sciences and technology; Impact phenomena (including electron spectra and sputtering); Physics; Conduction bands; Chemisorption; Oxygen; Inorganic compounds; Cr2O3; Interband transitions; Transition element compounds; Ion beam; Cr metal; Surface plasmons; ELS; Surface states; Sputtering; Surface structure; Energy-level transitions; EEL spectroscopy; Transition elements; Chromium oxides; XPS; Charge transfer; Surface layers; Bilayers; X-ray photoelectron spectra
• DOI: 10.1016/j.apsusc.2005.09.059
• ISSN: 0169-4332

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr2O3 before and after surface reduction by 2keV Ar+ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr2O3 exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4eV. The ELS feature at 4.0eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8eV decreases after Ar+ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr2+ states on the surface, which in turn increases the intensity of the 4.0eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr2O3 surface exhibit features characteristic of both Cr0 and Cr2O3, indicating that Ar+ sputtering reduces Cr2O3. The fact that neither the surface- nor the bulk-plasmon features of Cr0 can be observed in the ELS spectra obtained from sputtered Cr2O3 while the loss features due to Cr0 interband transitions are clearly present indicates that Cr0 atoms form small clusters lacking a bulk metallic nature during Ar+ bombardment of Cr2O3.