Every two years the Kavli Foundation awards three large scientific prizes, in astrophysics, neuroscience, and nanoscience. This year's nanoscience prize goes to Gerd Binnig, Christoph Gerber, and Cal Quate, for the invention and development of the atomic force microscope (AFM).
The AFM is a great example of one of those inventions that seems elegant and simple, yet could only come into being after the stage had been set through the development of several other enabling technologies. (My former faculty colleague Prof. Cyrus Mody does an excellent job telling this story in his book, which I heartily recommend.)
The atomic force microscope idea is very simple in concept. Take a very sharp stylus on a flexible cantilevered arm, and scan it in a controlled way over a surface. If the stylus tip is actually in contact with the sample surface, changes in surface topography will be detectable through the deflection of the cantilever, which can be measured optically (e.g. deflection of a laser) or by other means (e.g., changes in the electrical resistance of the cantilever as it is strained). This is basically an extrapolation to the very small scale of the profilometer. Alternately, you don't need the tip to be in hard contact with the surface - it just needs to get close enough to detect the short-range forces between the tip atoms and the surface. Oscillating the cantilever/tip up and down at or near its tuning-fork-like mechanical resonance can give you benefits in terms of detection sensitivity. Unlike STM, AFM has the benefit of working on insulating surfaces.
To implement this requires a number of building blocks: fabrication of tips with nm-scale sharpness; precise (nm-scale or better) control at the nanoscale of the tip position relative to the sample; computerized data acquisition to map out the tip response as a function of tip position. These are similar to the necessary requirements for scanning tunneling microscopy, and it is no coincidence that Binnig was associated with STM as well. Widespread adoption of AFM (as discussed in Mody's book) required these building blocks to be widely available.
AFM has turned out to be incredibly versatile. These devices can be used to measure extremely tiny local forces. Once you know the topography, you can withdraw the tip a little, scan back over the surface and measure longer-ranged forces (electrostatics, magnetic forces if you have a magnetic tip). Lateral deflection of the tip can tell you about frictional interactions between the tip and the sample. A conducting tip may be used as a local potentiometer, or as a scanning "gate" electrode. Functionalizing the tip and high frequency techniques have enabled AFM to image surfaces and even molecular orbitals with better-than-atomic resolution. AFM has been an incredible enabling technology with utility far beyond the original vision of its pioneers. That's exactly the kind of achievement that big prizes are meant to recognize.
2 comments:
I am not sure "molecular orbitals" is the best way to describe the images of molecules obtained by AFM. The origin of the chemical-structure-resolution in AFM (as well as in STHM) is still under discussion, but one very successful model to explain and simulate these images is based only on the atomic coordinates of the measured molecule. A flexible probe on the tip (such as a CO-molecule) then follows the Lennnard-Jones potential that is given by the molecular atoms.
Further refinements taking into account the electrostatic potential can be made, too. Such simulations then go beyond using just the atomic positions as an input. Still, I wouldn't say that these images resemble molecular orbitals.
Anon, fair point. I was being sloppy. I had in mind this paper and this one.
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