Depth Profiling and Melting of Nanoparticles in Secondary Ion Mass Spectrometry (SIMS)

Issues encountered in the depth profiling analysis, by secondary ion mass spectrometry (SIMS), of structured nanopartides based on Stober silica are studied. The main issue concerns melting under the impact of the primary ions designed to sputter the sample for the profiling. The nanoparticles chosen are deliberately large to minimize the effects of heating. The nanoparticles used are either of a core of Au encased in a shell of SiO2 or a core of SiO2 with a thin layer of very small Ag nanoparticles, also encased in SiO2, both the final particles being similar to 200 nm in diameter and being characterized and designed for optical purposes. The SIMS depth profiles are conducted with Ar-n(+) and C-60(+) cluster primary ions that generate very high sputtering yields and so may be expected to have a low energy deposition per unit of layer removal and per emitted signal ion. They are also conducted with ultralow energy Cs+ and O-2(+) primary ions that have an intrinsically low energy deposition per impacting ion. In all cases, the interpretation of the SIMS depth profiles shows that melting occurs, although it is likely that with ultralow energy Cs+ this effect is minimized. The Au core may be profiled without melting. These results are confirmed by scanning electron microscope studies and show that meaningful SIMS depth profiling of nanoparticles may only be possible if an efficient heat sinking method is available.