What’s in a name? The scanning probe world by any other name might be less confusing
The jargon in the scanning probe world has become really confusing, especially as it is condensed mostly into 3-4 letter acronyms, where the final letter is usually “m” for microscopy. Let’s take the overall name of the field – is it atomic force microscopy (AFM) or scanning probe microscopy (SPM)? Does it matter? Is there a difference?
Back in the day when this field started in the mid 1980’s, the term atomic force microscopy was really meant to describe an instrument that measured tiny mechanical forces between the tip and probe. So the early modes of contact mode, where the tip is dragged across the surface, or tapping mode, where the tip is oscillated at a resonance frequency and “taps” along the surface, really were appropriate modes to describe an atomic force microscope. But scientists quickly recognized that the AFM instrument could be used for so much more than just measuring these tiny forces on the surface. For example, the probe could be used to write features in nanolithography experiments. The probe could also be used to move molecules or other features around on the surface. The probe could even be replaced with an optical fiber to facilitate interaction of light with the sample. So the term of scanning probe microscopy, of which AFM was a subset, started to be used to describe the more general capabilities that the instrument had to offer. Not an unreasonable distinction.
Thirty years later however, the terms of atomic force microscopy and scanning probe microscopy have become synonymous with each other and are used completely interchangeably. But what I see students find even more confusing is that the world of scanning probe microscopy (or should I say atomic force microscopy?) has become an alphabet soup of jargon of modes. I just sat down and created this list of almost 3 dozen AFM modes off the top of my head, putting them all under the proverbial AFM umbrella.
This list is not exhaustive, but it certainly can be confusing. As a fun Sunday morning exercise instead of the crossword puzzle, see how many of the modes you can guess! (feel free to email me for help!)
What is even more confusing is that the exactly same imaging mode will have different names depending on the instrument vendor. Some of this is because of trademark issues, and some just good old fashioned creative fun on the part of the vendor. But let’s take the example of one of the most common AFM imaging modes that is ubiquitous today: tapping mode. Tapping mode was the original name assigned to his mode back in the early 1990’s by Digital Instruments when it was invented because it describes the motion of the cantilever: cantilever oscillating at resonance and “tapping” along the surface enabling a gentle interaction with the surface and also access to new kinds of tip-sample interactions resulting in new information. There was a trademark issue with the name and so every vendor calls “tapping mode” a different name on its instrument. Here is the list of names for tapping mode that I have been able to amass so far from different vendors: intermittent contact mode, AC mode, vibrating mode, amplitude modulation mode.
So where does this leave us? Bon appétit on your bowl of alphabet soup, and thank goodness for Wikipedia to help us decipher it all.
Dalia Yablon, Ph.D.