Application of Helium Ion Microscopy to surface science problems

Helium Ion Microscopy (HIM) is well known for its high lateral resolution and unique nanomachining capabilities. In addition it is a very surface sensitive technique and therefore ideally suited to answer scientific questions in surface and interface science. I will give a brief introduction of the technique followed by a selection of problems related to surface and interface science.
The high surface sensitivity of HIM allowed us to measure the thickness of thin carbon layers present on gold nanorods. On the other hand one can use back scattered helium (BSHe) particles to reveal buried interfaces such as the diffusion front of a Pd2Si layer covered by more than 100 nm of SiO2.
The orientation of a sample can be determined using channeling. I will show that with a simple geometrical model channeling directions can be predicted with sufficient accuracy to align the He beam parallel to low index directions. Exploiting channeling into a crystalline sample the background from the substrate can be suppressed, thus enhancing the surface sensitivity even further. This has been used in a recent study of the surface reconstruction observed in the case of a few ML of Ag deposited on Pt(111). Based on a change of the work function of 25meV across the atomically flat terraces we can distinguish Pt rich from Pt poor areas and visualize the single atomic layer high steps between the terraces. Utilizing channeling/dechanneling and the exceptional surface sensitivity of the HIM we can measure the periodicity of the hcp/fcc pattern formed in the 2 ML thick Ag alloy layer. A periodicity of 6.65 nm along the <-1-12> surface direction has been measured. In terms of crystallography a hcp domain is obtained through a lateral displacement of a part of the outermost layer by 1/√3 of a nearest neighbor spacing along <-1-12>.