Hironori Moritani

Hydrogen Ion Beams from Nanostructured Gas Field Ion Sources

Hydrogen ion beams have been discussed as useful for scanning ion microscopy because of hydrogen’s low mass and low sputtering rates. Hydrogen ion beams have been reported from various nanotips including pure iridium tips and noble metal covered tungsten tips.[1, 2] However, hydrogen ion beams are known to occur as mixtures of H, H2 + and H3 + depending on the electric field strength.[3] There is some evidence that various tip orientations contribute differently to the ratios of the ions and also that site specific regions also affect the gas species but it has not been clearly determined. Understanding the relationship between tip shape and apex termination with specific hydrogen ion creation is important in order to prepare pure hydrogen ion beams of a single species. This would be beneficial to future applications related to hydrogen ion beam production using gas field ion sources.

Selective production of hydrogen ion species at atomically designed nanotips

Hydrogen scanning ion microscopy systems rely on nanotip gas field ion sources to generate the hydrogen ion beam. The exact structure of the nanotip and the applied electric field are shown to be important. It is demonstrated that hydrogen ion beams are found to occur as mixtures of H+, H2+ and H3+ depending on the electric field strength and the nanotip structure. Various nanotips were prepared, including single atom tips (SATs), trimers and other nano-structured tips to compare the contents of hydrogen ion beams. It was found that single atom tips produce primarily H2+ at low operating voltages, but as the voltage is increased, H3+ dominates. For the trimer case, H2+ becomes a significant species and equals the H3+ current but H3+ can be isolated at higher voltages. For the hexamer tip structure, H2+ almost completely dominates with little H3+ being produced. H+ is only observed in small quantities for all tip structures until a high voltage regime, where apex atom resolution is not observed. Comparisons W SATs and Ir SATs showed similar H3+/H2+ product ratios indicating the nanotip structure plays a key role in the catalytic formation of H3+. Temperature affects are also discussed and operating parameters for single species ion beams are discussed.

Reactive Gas Ion Beam Generation Using Single Atom W(111) Gas Field Ion Sources

The scanning ion microscopy is gaining momentum as it provides several key advantages over scanning electron microscopy: (i) enhanced depth of focus, (ii) improved surface and element sensitivity, (iii) better lateral resolution, and (iv) nanomachining and milling. It uses different ions to achieve these tasks ranging from inert gases like helium and neon for imaging and ion milling. Other gases such as argon, nitrogen, and oxygen have potential for further sputtering and etching. It is therefore crucial that gas field ion sources provide necessary robustness and stability for range of various gases.