Sample preparation
Fingerprints were deposited on two non-porous surfaces (microscope glass slides and aluminum sheet), and also on a porous surface (white photocopier paper) for comparison purposes. White paper is a porous surface and 1,2-indanedione is a very good fingerprint reagent on it. The glass surfaces were cleaned by sequentially washing in acetone, trichloroethane, ethanol and de-ionized water to remove all greasy materials and chance fingerprints, while fresh window’s frame aluminum were used. Both surfaces were finally cleaned by blowing dry nitrogen to remove dust particles. In each case, the finger was washed with commercial handwash soap and dried just before depositing the fingerprint. Care was taken to just touch the surface without imposing much contact force so as to create fingerprints as if by chance contact. This was to prevent leaving behind large amounts of fingerprint residues in order to test the sensitivity of the methods. The samples were left at room temperature for two days to up to a week to allow them to dry. Before any physical or chemical treatment, the fingerprint substrates (glass, window’s frame aluminum and paper) were then cut into two pieces (left and right). On one side, typically the left side, a columnar thin film of CaF2 or SiO2 was deposited at large oblique angles by e-beam evaporation process. The deposition process is detailed in Section 2.2