Paper HI+NS-ThM1
Generation of Hydrogen Beams using Single Atom and Trimer Nanotips

Thursday, November 10, 2016, 8:00 am, Room 104A

Hydrogen ion beams have been discussed as useful for scanning ion microscopy (SIM) due to their low mass and low sputtering rates. However, hydrogen ion beams are known to occur as mixtures of H⁺, H₂⁺ and H₃⁺ depending on the electric field strength. 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 the field strength dependence, tip shape, and apex termination with specific hydrogen ion creation is therefore critical to prepare pure hydrogen ion beams of a single species. We employed W and Ir to prepare atomically sharp nanotips with various atomic arrangements at the very apex to compare the ratios of H⁺, H₂⁺ and H₃⁺.

The experimental setup included a custom field ion microscope (FIM) operating in ultrahigh vacuum (base pressure <5x10^-11 Torr). The tip was mounted on a heating loop wire for degassing and could was cooled with a liquid helium flow cryostat. Nanotips were prepared from tungsten single crystal W(111) wire and polycrystalline Ir wire using the field assisted chemical etching method. A magnetic field of ~1T was generated using two permanent magnets mounted between the extractor and the micro-channel plate.

The hydrogen beam composition from a single atom W(111) and Ir nanotips at different applied tip voltages was recorded and analyzed. At low voltages the H₂⁺ beam dominates. As the voltage is increased, H₃⁺ is also observed until it dominates at larger voltages. In this manner, a particular species can be selected depending on the operating voltage. Furthermore, comparing the hydrogen beam composition between W(111) single atom tip and trimer structure reveal important differences. For trimer nanotip, H₂⁺ becomes a significant species and equals the H₃⁺ current. However, in the case SAT, H₃⁺ becomes the only contribution to ion current at higher voltages resulting in pure H₃⁺ beam suitable for imaging.

Relative ratios of H⁺, H₂⁺ and H₃⁺ were studied as a function of tip material (tungsten and iridium), applied voltage, and tip apex structure (single atom and trimer nanotips). We have determined that the tip structure and apex termination for both tungsten and iridium nanotips play important roles in the production of hydrogen ion beams. It has been found that single atom tip at high tip voltages produces nearly pure H₃⁺ beam.