Apr 3, 2012
Coaxial Kelvin probe sharpens work-function images
Kelvin probe force microscopy (KPFM) is a powerful atomic force microscope (AFM)-based technique for measuring the local work function of a sample, a property that is useful for investigating semiconductors, designing solar cells and sensing biomolecules. There are two main challenges in performing high-quality KPFM measurements. First, the long range of the electrostatic interaction provides a restriction on the spatial resolution, and second, it is difficult to separate the desired work function data from sample topography. Both problems stem from the fact that while the interaction of interest is the electrical force between the sharp tip of an AFM probe and a nearby substrate, in reality there are confounding electrostatic contributions from the macroscopic cone and cantilever of the probe.
Reporting their results in the journal Nanotechnology, researchers in the US (K A Brown, K J Satzinger and R M Westervelt) have proposed and demonstrated a method for performing KPFM with enhanced spatial resolution and improved resistance to topographical artifacts. The technique involves the use of coaxial AFM probes that are electrically shielded everywhere apart from at the very tip. The nanoscale electrode resembles the end of a coaxial cable.
In the set-up, the spatial resolution of the probe is determined entirely by the size of the electrodes at the tip. This allows the users to tailor the technique and potentially offers a route to ultra-high resolution. Topographical artifacts are avoided because the tip-sample interaction changes very little with respect to the tip-sample separation.
Factor of five improvement
To demonstrate their method, the scientists created coaxial AFM probes with 50 nm thick insulating layers and calibrated the probes to measure the true work function despite the presence of the shell electrode. The configuration showed a factor of five improvement in spatial resolution over an unshielded probe.
Not only does this technique offer a way to improve the resolution and fidelity of KPFM measurements, but coaxial probes may improve any AFM measurement in which electrostatic interactions are important.
The group reported its work in the journal Nanotechnology.
About the author
Dr Keith A Brown conducted this research as an applied physics PhD student in the Harvard School of Engineering and Applied Science, supported in part by a National Defense Science and Engineering Graduate Fellowship. Currently, he is a postdoctoral associate at the International Institute for Nanotechnology at Northwestern University. Kevin J Satzinger is an undergraduate student in physics at Truman State University who participated in this work in the Research Experience for Undergraduates program at Harvard University under NSF grant ECCS-0821565. Robert M Westervelt is a Mallinckrodt professor of applied physics and physics at Harvard University. This work is part of an initiative in the Westervelt research group to develop scanning-probe based tools to image and manipulate materials at the nanoscale. It was supported in part by the Department of Energy under grant DE-FG02-07ER46422.