Shard, A G; Rafati, A*; Ogaki, R*; Lee, J L S*; Hutton, S*; Mishra, G*; Davies, M C*; Alexander, M R* (2009) Organic depth profiling of a binary system: the compositional effect on secondary ion yield and a model for charge transfer during secondary ion emission. J. Phys. Chem. B, 113 (34). pp. 11574-11582.

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Abstract

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, 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. In this paper we investigate a model system of a miscible binary mixture of codeine and poly(lactide). We show that there is a strong surface enrichment of poly(lactide), which fortuitously allows the direct comparison of different samples in terms of secondary ion yield behaviour. We demonstrate that 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 using a model of the kinetically limited transfer of charge between secondary ions and secondary neutrals. Application of the model to pure materials under the assumption that only highly fragmented secondary ions are initially produced and interact with unfragmented secondary neutrals leads to the prediction that high molecular mass quasi-molecular ions have intensities proportional to the square of the total secondary ion yield. This relationship has been independently observed in other work.

Item Type:	Article
Subjects:	Nanoscience Nanoscience > Surface and Nanoanalysis
Last Modified:	02 Feb 2018 13:15
URI:	http://eprintspublications.npl.co.uk/id/eprint/4470

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