Sputtering Yields of Gold Nanoparticles by C60 Ions

The sputtering yields of nanoparticles are expected to be higher than those for traditional flat films because of the higher surface area and lower volume to dissipate the primary ion energy. In the present study, gold nanoparticles in the size range 10 to 100 nm, dispersed on a silicon wafer, are studied by secondary ion mass spectrometry (SIMS) during sputtering by 20 keV C-60(++) and by scanning electron microscopy. It is shown that early in the profile the particles melt and then resolidify. This is not mitigated by sample cooling. Profiling nanoparticles by SIMS may be very difficult without adequate heat sinking. The ways that the nanoparticle and substrate secondary ion intensities change during the profiling in a two-beam SIMS system are not simple to interpret, and so this situation is modeled in some detail. It is shown that the sputtering yield for primary ions on the axis of the nanoparticle, Y(0 degrees), can be extracted from the data. For the first time, we measure a significant increase in this sputtering yield, Y(0 degrees), for nanoparticles compared with bulk materials, in agreement with general expectations. At 9.3 nm diameter, the yield is 320 +/- 35 compared with 96 +/- 14 at 98.8 nm diameter, an increase of 3.3 times. For bulk Au films at 45 degrees incidence, the yield, Y(45 degrees), is 129 +/- 2. The sputtering yield for SiO2 films at an angle of incidence of 45 degrees may also be extracted, and this is found to be rather higher at 478 +/- 112 atoms/primary ion.