Characterization of surface and layered films with cluster secondary ion mass spectrometry
Cluster secondary ion mass spectrometry (SIMS) analyses of layer-by-layer thin films were performed to investigate the depth/volume of SI emission and accuracy of the SI signal. The thin-layered samples were assembled by alternate adsorption of polyethylenimine (PEI), polystyrene sulfonate (PSS), polydiallyldimethylammonium chloride (PDDA) and clay nanoparticles. The films have controlled 3-D structure to test the depth of secondary ion (SI) emission and evaluate planar homogeneity. The SI emission depth is ~ 6-9 nm with 136 keV Au400 4+ (340 eV/atom) and 26 keV C60
- (433 eV/atom) projectile impacts. The diameter of the SI emission area is ~ 15 nm by assuming a semispherical emission volume. The SI yields oscillate with the alternation of the compositions of the topmost layers, which was observed with small cluster projectiles (CsICs+ and Au3 +) as well as with the large cluster projectiles (C60
and Au400 4+). The SI signals of C- and CH- are enhanced in the presence of metal atoms in the expanding plume. Recoiled C60 projectile fragments (m/z=12, 13, 36) are observed in the SI mass spectra. Caution must be taken when monitoring the yields of such carbon cluster ions from organic surfaces because their yields don?t reflect the true surface concentration. The Au400 4+ projectile impacts produce abundant co-emission. The correlation coefficient between the co-emitted SIs can be used to evaluate the planar homogeneity. The results show that the PSS layer is more uniform than the clay layers. The effect of alkali metal ion implantation on the nature and abundance of SI emission was investigated on Cs+ or Na+ implanted glycine samples. The alkali metal implantation induces surface damage and decreases the glycine molecular ion yields. Glycine molecular ions and fragment ions (CN-, CNO-) are emitted from different depths and locations of the emission volume. The same implanted glycine sample analyzed with different cluster projectiles (Au400 4+ and C60 +) shows different trends in the yields of molecular and fragment ions, which suggest a different mechanism of SI emission with different projectile impacts. The Na+ beam induces more surface damage compared with the Cs+ at equal impact energy.