title: Evaluation of Two Methods for Determining Shell Thicknesses of Core-Shell Nanoparticles by X-ray Photoelectron Spectroscopy.
abstract: We evaluated two methods for determining shell thicknesses of core-shell nanoparticles (NPs) by X-ray photoelectron spectroscopy (XPS). One of these methods had been developed for determining thicknesses of films on a planar substrate while the other was developed specifically for NPs. Our evaluations were based on simulated Cu 2p3/2spectra from Cu-core/Cu-shell NPs with a wide range of core diameters and shell thicknesses. Copper was chosen for our tests because elastic-scattering effects for Cu 2p3/2photoelectrons excited by Al Kα X-rays are known to be strong. Elastic scattering could also be switched off in our simulations so that the two methods could be evaluated in the limit of no elastic scattering. We found that the first method, based on both core and shell photoelectron intensities, was unsatisfactory for all conditions. The second method, based on an empirical equation for NPs developed by Shard, also utilized both core and shell photoelectron intensities and was found to be satisfactory for all conditions. The average deviation between shell thicknesses derived from the Shard equation and the true values was -4.1 % when elastic scattering was switched on and -2.2 % when elastic scattering was switched off. If elastic scattering was switched on, the effective attenuation length for a Cu film on a planar substrate was the appropriate length parameter while the inelastic mean free path was the appropriate parameter when elastic scattering was switched off.
authors.affiliation: Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8370, United States.
authors.forename: C J
authors.initials: CJ
authors.lastname: Powell
authors.affiliation: Technical University of Vienna, Institute of Applied Physics, Wiedner Hauptstrasse 8-10, A-1040 Vienna, Austria.
authors.forename: W S M
authors.initials: WS
authors.lastname: Werner
authors.affiliation: National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, U.K.
authors.forename: A G
authors.initials: AG
authors.lastname: Shard
authors.affiliation: National ESCA and Surface Analysis Center for Biomedical Problems, Departments of Chemical Engineering and Bioengineering, University of Washington, Seattle, Washington 98195-1653, United States.
authors.forename: D G
authors.initials: DG
authors.lastname: Castner