Applied Physics Technologies has extensive experience in the fabrication of single-crystal tungsten wire, and its use in electron emission sources. Our single crystal wire is produced in a zone refining process that ensures both a very high level of purity and a specific axial orientation. Our mount design minimizes the length of single crystal wire (SCW) required, thus keeping costs low.
Resolution can be improved with atomically sharp tips, but at the expense of tip lifetime. By using single crystal probe materials, it may be possible to achieve comparable resolution with less sharp tips, thereby increasing probe lifetime.
Currently, STM tips are etched from drawn tungsten wire which has a natural (110) orientation on axis. This (110) plane has the highest work function at approximately 5.25 eV. Lower work functions (210) and (310) planes, which have an approximate work function at 4.3eV, are located in two-fold symmetry nearby. This results in a low sensitivity tip with potential for unwanted emission from the low work function planes obscuring resolution.
Emission from a point above the probe apex can cause 'shadow images' such as the one next to the real image of the structure shown in Fig. 1. This STM image was supplied by Laura Ruppalt and Professor Joseph Lyding at the University of Illinois.
Fig 1: High resolution STM image of a carbon nanotube on GaAs(110) using a standard STM probe
Performance comparisons of single crystal and polycrystalline tungsten tips were
made at Washington State University by Prof. Ursula Mazur and associates. They
“found that the SCW tips were consistently superior to the W(poly) tips
providing higher quality images than the W(poly) tips.” The following pictures
are from their work.