The atomic arrangements can be easily characterized in the group’s field ion microscope, but it turns out to be a remarkable experimental challenge to keep these tips clean (and therefore atomically defined), even in ultra-high vacuum. A protocol, which the scientists have named the “force field”, was developed to prevent the atoms at the end of the tips from reacting with the exceedingly few gas molecules left in the vacuum chamber – a necessary first step in using these tips to create a nano-junction where the position of all the atoms is known.

Electron overlap

Next, the researchers used a scanning tunnelling microscope to bring these atomically defined tips within a few Angstroms of well defined surfaces until the electrons of the tips and samples overlapped enough to measure a few pico-amperes of current that tunnelled quantum mechanically between them. The current they measured was very unstable when the tip was approached to a clean gold surface: these instabilities were attributed to the rearrangement of the nano-junction by mobile gold adatoms that were transferred from the gold sample to the tip. The transferred atoms were then visible in the field ion microscope image of the tip apex.

On the other hand, when the scientists approached their atomically defined tips to silicon, the tunnelling current was very stable, and the tip showed an identical atomic structure in the field ion microscope. This demonstrated that the choice of substrate is an important factor for keeping the tip apex clean.

The newly developed “force field” protocol and these pioneering experiments show that tips that are atomically characterized in a field ion microscope can be used as atomically defined probes for scanning tunnelling microscopy and atomic force microscopy experiments. Atomically defined tips are valuable to our understanding of contrast formation, the relationship between forces and currents, and theoretical modelling in scanning probe microscopy experiments.

Ultimately, the researchers will use these atomically characterized tungsten wires to study contact formation to individual molecules where a knowledge of the exact atomic structure is necessary to understand how electrons flow through the metal-molecule connection.

Additional information can be found in the journal Nanotechnology.