AVS 51st International Symposium
    Applied Surface Science Wednesday Sessions
       Session AS+BI-WeA

Paper AS+BI-WeA4
Model Calculations for the Quantification of XPS-Results; Application to Self Assembled Monolayers on Gold

Wednesday, November 17, 2004, 3:00 pm, Room 210A

Session: Biological Applications of Surface Analysis
Presenter: C. Van der Marel, Philips Research, The Netherlands
Authors: C. Van der Marel, Philips Research, The Netherlands
J.H.M. Snijders, Philips Research, The Netherlands
H.R. Stapert, Philips Research, The Netherlands
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XPS-analysis is widely applied for the characterization of surfaces and multilayers of thin films. In order to obtain quantitative results, the XPS peak areas generally are divided by appropriate sensitivity factors and normalized to 100 at% to obtain the apparent concentrations. Within the model developed by us, the sample is assumed to consist of a substrate on top of which a number of homogeneous layers are present. Starting from the apparent concentrations, the model calculation provides the thickness, the number of sulphur atoms per surface area and the composition of all layers. The proposed method requires only one measurement at one measuring angle to obtain these results. The method has been verified for a large series of self assembled monolayers made of mercaptoalkyl-polyethylene glycol compounds with various chain lengths on gold-plated Si. The influence of deposition time upon the obtained SAM-layers was examined; also SAM-layers deposited from mixtures of molecules with different chain length were investigated. RBS analysis was applied to determine in an independent way the amount of sulphur at the gold surface (expressed in number of Sulphur atoms per unit area); the RBS results correlated well with the XPS data. Also XPS-results obtained from mixtures of mercaptodeca-triethylene glycol and biotinylated thiols were analyzed in this way. The method resembles the calculation method proposed recently by Petrovykh et al@footnote 1@. Yet, in the latter method the analysis is restricted to only one layer on top of a substrate. Moreover, ion-etching is required to obtain a reference signal of the gold substrate; the consequence is, that elastic scattering of Au4f-electrons in the top layer is partly neglected in Ref.@footnote 1@. @FootnoteText@ @footnote 1@ D. Y. Petrovykh, H. Kimura-Suda, M.J. Tarlov and L.J. Whitman, Langmuir 20 (2004) 429.