| AVS 62nd International Symposium & Exhibition | |
| Applied Surface Science | Monday Sessions |
| Session AS-MoM |
| Session: | Quantitative Surface Analysis: Obtaining Quantitative Information in the Face of Material Complexity and Morphology Influences |
| Presenter: | Cedric Powell, National Institute of Standards and Technology (NIST) |
| Authors: | C.J. Powell, National Institute of Standards and Technology (NIST) M. Chudzicki, Technical University of Vienna, Austria W.S.M. Werner, Technical University of Vienna, Austria W. Smekal, Technical University of Vienna, Austria |
| Correspondent: | Click to Email |
We used the new version of the NIST Database for the Simulation of Electron Spectra for Surface Analysis (SESSA) [1] to simulate XPS spectra of core-shell nanoparticles (NPs). Two series of simulations have been made. First, Cu 2p spectra were simulated using Al Kα x-rays for a monolayer of three types of Cu-Au NPs on an Si substrate: (a) an Au shell of variable thickness on a Cu core with diameters of 0.5 nm, 1.0 nm, 2.0 nm, 5.0 nm, and 10.0 nm; (b) a Cu shell of variable thickness on an Au core with diameters of 0.5 nm, 1.0 nm, 2.0 nm, 5.0 nm, and 10.0 nm; and (c) an Au shell of variable thickness on a 1 nm Cu shell on an Au core with diameters of 0.5 nm, 1.0 nm, 2.0 nm, 5.0 nm, and 10.0 nm. For these three morphologies, the outer-shell thickness was varied until the Cu 2p3/2 peak intensity was the same (within 2 %) as that found in our previous work with planar Cu/Au morphologies [2]. For each morphology, we performed simulations with elastic scattering switched on and off. We found that elastic-scattering effects were generally strong for the Cu-core/Au-shell and weak for the Au-core/Cu-shell NPs; intermediate elastic-scattering effects were found for the Au-core/Cu-shell/Au-shell NPs. The shell thicknesses required to give the selected Cu 2p3/2 peak intensity for the core-shell NPs were less than the corresponding film thicknesses of planar samples since Cu 2p photoelectrons can be detected from the sides and, for the smaller NPs, bottoms of the NPs.
Second, we determined effective attenuation lengths (EALs) for Cu 2p3/2 photoelectrons from NPs consisting of a Cu core of diameters 0.5 nm, 1 nm, 2 nm, 5 nm, and 10 nm and a Cu shell with thicknesses between 0.25 nm and 3.75 nm. The EAL was determined from the Cu 2p3/2 intensity from the core with no shell and from the Cu 2p3/2 intensity from the core that was attenuated by a Cu shell of specified thickness. These EALs varied systematically with both Cu-core diameter and Cu-shell thickness. While the inelastic mean free path of Cu 2p3/2 photoelectrons (at 534 eV) in Cu is 1.12 nm, the EALs for a 10 nm Cu core varied from 0.53 nm to 0.65 nm as the Cu-shell thickness increased from 0.25 nm to 3.75 nm. In contrast, the EALs for a 0.5 nm Cu core increased from 0.78 nm at a shell thickness of 0.25 nm to a maximum value of 0.86 nm for a shell thickness of 0.75 nm and then decreased to 0.81 nm for a shell thickness of 3.75 nm. These changes show the important roles of NP morphology, NP dimensions, and elastic scattering in determining shell thicknesses by XPS.
[1] http://www.nist.gov/srd/nist100.cfm .
[2] C. J. Powell, S. Tougaard, W. S. M. Werner, and W. Smekal, J. Vac. Sci. Technol. A 31, 021402 (2013).