This material is based upon work supported by the European Office of Aerospace Research and Development, Air Force Office of Scientific Research, Air Force Research Laboratory, under Grant No. FA8655-07-1-3087. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the European Office of Aerospace Research and Development, Air Force Office of Scientific Research or Air Force Research Laboratory. SEM observations were done at the CITIUS of the Universidad de Sevilla. J.R.-R. was funded by a Junta de Andalucía FPDI grant.