AVS 56th International Symposium & Exhibition
    Applied Surface Science Thursday Sessions
       Session AS-ThA

Paper AS-ThA11
Organic Depth Profiling of a Model Binary System: The Demonstration of Charge Transfer between Secondary Species

Thursday, November 12, 2009, 5:20 pm, Room C2

Session: Chemical State Depth Profiling
Presenter: A.G. Shard, National Physical Laboratory, UK
Authors: A.G. Shard, National Physical Laboratory, UK
A. Rafati, University of Nottingham, UK
J.L.S. Lee, National Physical Laboratory, UK
M.R. Alexander, University of Nottingham, UK
M.C. Davies, University of Nottingham, UK
Correspondent: Click to Email
In recent years, it has been demonstrated that cluster ion beams may be used to ablate some materials, particularly organic materials, without the significant accumulation of damage. For such materials it is therefore possible to use cluster ion beam sputtering in conjunction with a surface analytical technique, such as SIMS or XPS, to obtain depth profiles and three-dimensional images of the distribution of species in the near-surface region. For SIMS organic depth profiling to find wide acceptance as an analytical tool it is important that it is able to measure physically meaningful quantities, such as the local concentration of a species within a blend. Building upon our recent advances in the understanding of organic depth profile using model multilayers[1], we have investigated a model miscible binary mixture of codeine and poly(lactide) using both SIMS and XPS as analytical techniques.

We show that these samples possess an overlayer of almost pure poly(lactide), which fortuitously allows the direct comparison of different samples in terms of secondary ion yield behaviour. By comparing between data obtained from samples with different concentrations, it is found that secondary ion intensities do not scale linearly with composition. However, it is possible to relate secondary ion intensities to local concentrations for a binary system. The dependence of secondary ion yield on composition is described in terms of a model based on the kinetically limited transfer of charge between secondary ions and secondary neutrals. In this case, secondary ions from codeine, which contain a basic amine group, have an enhanced yield when dilute in poly(lactide). The enhancement diminishes with increasing concentration. The suppression in secondary ion yield for ions arising from poly(lactide) has the same functional form as the enhancement for secondary ions from codeine. The model is found to describe the data very well and has some wider implications for the interpretation of SIMS data from organic systems.

[1] A.G. Shard, F.M. Green, P.J. Brewer, M.P. Seah, I.S. Gilmore, Journal of Physical Chemistry B 112 (2008) 2596-2605.