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“By contrast, our nanowire probe tips have a calibration lifetime about ten times longer than any commercial tip. We see no visible wear after performing tens of scans, whereas platinum deforms, losing resolution and calibration, after five to ten scans.”
In a series of twelve scans, the platinum tip radius changed from ~ 50 nm to ~150 nm. The nanowire, however, retained its original dimensions. Moreover, the GaN tips exhibited improved sensitivity and reduced uncertainty compared to a commercial platinum tip.
NSMM can produce very detailed imaging of the local density of positive and negative charge carriers inside a nanostructure - information of great practical significance to microdevice fabricators - and scientists from PML’s Electromagnetics Division have made notable progress in the technique.
They believe that the use of nanowire probes, in conjunction with the recent arrival of a brand-new, custom-built, four-probe NSMM instrument, will reveal new aspects of nanostructure composition and performance.
A single GaN nanowire is removed from a “forest” of wires grown by MBE. Inset: The nanowire is being placed into a hole drilled in an AFM probe. Both images are false-coloured for clarity
The researchers tested their tip against a silicon tip, a platinum tip, and an uncoated GaN nanowire, each of which was scanned across an array of microcapacitors of different sizes. The coated nanowire proved about twice as sensitive as the platinum probe, and four times as sensitive as the others, with superior mechanical performance.
“That can be extremely important for s the next generation of advanced electronic and optoelectronic devices,” Bertness says. At present only a few GaN probes can be made at once, but the team is at work on developing ideas for producing them in wafer-scale quantities.
The new, four-probe NSMM instrument has four tips, permitting simultaneous comparisons of materials. The probes are enclosed in an ultra-high vacuum chamber to minimise interference and contamination
For example, in biological materials, it could locate the attachment of chemical agents or particles that are bound to a cell, and aid in the study of protein dynamics.
Deploying a nanowire as a probe tip sounds deceptively simple. The researchers obtain a conventional AFM cantilever and probe, remove the existing tip, and use a focused ion beam to drill a hole about 5 µm deep in the tip mount.
Then, using a minuscule manipulator, they break off a single nanowire from a “forest” of them grown by MBE, insert the wire into the hole, and weld it in place. Finally, the wire is coated with thin layers of 20nm thick titanium and 200nm thick aluminium in order to conduct the microwave signal all the way to the end of the tip and back.
At the same time, the researchers are preparing to test a new technology for which they were awarded a patent in July, 2013. This regards using the nanowire tip as a light source by doping it so that it functions as an LED. Optical radiation can serve to excite the sample in a different way from the microwave signal, and scientists are already using lasers to illuminate nanoscale samples during AFM scans.
“The problem with that approach,” says NSMM researcher Pavel Kabos of the Advanced High-Frequency Devices Program in PML’s Electromagnetics Division, “is that the laser has to shine in from the side. As a result, you get cast shadows and significant uncertainty as to exactly what area is being illuminated. And, of course, the laser and its mounting take up a great deal of space.
Illuminating an NSMM sample with a conventional laser brings June 2014
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