One popular method for the fabrication of a needle tip is by electron beam induced deposition (EBID). This is usually performed by focusing an electron beam onto the tip apex and depositing a single pillar of carbon along the beam axis. However, researchers at the University of Bath, UK, are investigating other EBID techniques, which allow the fabrication of thinner needles in a shorter time, reducing the effect of drift and instability in the electron beam system and increasing the resolution attainable when the needles are used as AFM probes.

Shock-absorbing properties

The group examined the growth of free-standing carbon needles perpendicular to the electron beam axis, and successfully used the resulting structures as AFM probes for the imaging of nanoscale structures. The team was surprised to discover that these thin nanoneedle probes buckle elastically under large tip-sample forces, in the same way as carbon nanotube-tipped AFM probes.

This behaviour allows the probe to act as a "shock absorber", protecting the tip and sample from damage. However, unlike carbon nanotube probes, which require complex fabrication methods, the EBID nanoneedles are made using a single-stage deposition process.

If required, more rigid needles can also be fabricated using the EBID technique – a small increase in diameter greatly increases the force that the needle can bear before buckling, allowing tuning of the maximum tip-sample force for different applications.

Multiple applications

The study shows how robust, high-aspect ratio nanoneedle probes with small tip radii can be created by a very simple technique. These probes could have many applications in the AFM imaging of challenging samples and the investigation of the internal structure of cells.

Full details can be found in the journal Nanotechnology.