There has been much interest in improving the aspect ratio of AFM probes by fabricating needle-like structures on the probe tip to allow accurate profiling of samples with high, steep features. In addition, these so-called "nanoneedle" probes have recently shown great promise as tools for investigating biological cells, and are capable of penetrating the cell at a local site without damaging its overall structure. Many groups have used nanoneedle-like structures for the microinjection of biomolecules into cells, including for the injection of DNA into cell nuclei. These "nanosurgical" methods represent an entirely new way to study the structure of cells.
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.
About the author
This work was carried out by a group in the Department of Physics at the University of Bath, UK, supported by EPSRC. The group is led by Dr Sergey Gordeev who is a senior lecturer at Bath, and whose current research focuses on nanomanipulation, nanoscale electronic devices and nanobiotechnology. The nanoneedle fabrication and testing using AFM was carried out by James Beard, a PhD student in Dr Gordeev's group, who is currently investigating the application of EBID-modified probes to the "nanosurgery" of biological cells.
Source: nanotechweb.org
