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.