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STM tips


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.


(a) W(poly) probe


(b) W(100) probe


(c) W(111) probe

Fig 2: Highly oriented pryolytic graphite (HOPG) imaged under ambient conditions in MI Pico Plus STM using W(poly) probe, W(100) probe and W(111) probe.


(a) W(poly) probe


(b) W(111) probe

Fig. 3: HOPG surface imaged in RHK vacuum STM using W(poly) probe, and W(111) probe.


(a) W(poly) probe


(b) W(111) probe

Fig 4: Images of cobalt phthalocyanine formed by redox deposition on Au(111) substrate using W(poly) probe and W(111) probe.

IV Data Comparison Against Au(111) Surface in RHK vacuum STM





“The I-V data are the results of single and not averaged measurements. It is clear that the W(111) tip yields a very quiet spectrum. The I-V curve obtained with W(poly) tip breaks down at ~0.5 V because of the presence of metal oxides on the tip surface. I think that because of the orientation of the SCW, the tungsten oxide layer on the SCW tips is thinner than on the W(poly) tip.” - Ursula Mazur

Probe Mount Options

Schematic
Single crystal tungsten wire mounted on poly wire STM probe blank (unetched)

Schematic
Single crystal tungsten mounted in SS tube STM probe blank (unetched).

Single Crystal Carbide STM probes

Another option is single crystal zirconium carbide STM probes. ZrC is an extremely durable probe material . It has the same advantages of a single crystal W, with a much lower (100) plane work function, approximately 3.5 eV. The apex will be a more sensitive probe, yielding the same currents at greater tip-to-sample spacing and may be more desirable for rough surfaces.

Schematic
Etched ZrC STM probe


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Last updated: 15:49 05/02/2014

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