AFM is not a black box: Quiz for the casual user

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It still amazes me to hear how many AFM users are still missing many of the basics of AFM operation.  I interact heavily with users from a wide variety of backgrounds at conferences and workshops.  Certainly researchers who work focuses on AFM as their main method are knowledgeable and have ample resources in the lab typically in terms of other graduate students or postdocs. 

But the users I worry most about are in labs that are using AFM as one characterization tool, among many, to image topography or material properties, or measure nanomechanical properties.  These could be materials, chemistry, biology, or engineering labs that are using AFM to help their research, but not as the main focus of their research. Typically these users have inherited the AFM, or are using the AFM that has been sitting around but unused in the lab meaning that there was no opportunity for meaningful training or to ask questions.  I can see from their questions that they lack a basic understanding of how the AFM works, what parameters are the “important” ones for operation, and then how to optimize the parameters.  As a result, these users struggle (and rightly so!) to obtain reproducible and understandable images of their materials. The end-result?  Poor results, serious frustration, and then a bad reputation for the AFM.

Here is the trouble for the “casual” AFM  user in a lab where there are no dedicated resources to maintaining and training on this microscope:   The AFM is not a black box where you can just push a few buttons, adjust some parameters, and get your beautiful 3D image of your surface or measure a quantitative adhesion with confidence.  Being a physical chemist, my favorite black box instrument is a UV-VIS.  In this simple, albeit incredibly important and useful analytical instrument, you literally stick your sample into the (gray or white?) box, select your wavelength range of interest and hit “scan” to generate your absorbance (or transmission) vs. wavelength plot. It just doesn’t get any easier.   AFM is NOT a UV/VIS!

Unless you image the exact same kind of sample all the time, there is no “protocol” or canned set of parameters.  In fact there are about half a dozen AFM parameters (for the “basic” modes) that you have to optimize routinely when you set up an image.  AFM is still a young enough technique that a basic understanding of the tip-sample interaction, and then the parameters that control different aspects of it, are critical to effective operation of your instrument.   If you are a casual user, can you answer the following questions:

How do you choose the cantilever for your experiment? (do you understand the differences between the various cantilevers?)

What are the feedback parameters in the different modes?

What is the difference between flattening and planefitting?

How do optimize the setpoint and free air amplitude in tapping mode?

How do I interpret phase? (did you know that interpretation of phase is vendor-dependent?)

How can you calibrate your cantilever?

What is a feedback loop?  How can I control it?

Do you recognize the artifact(s) in the AFM images below?

                                    Artifact 1

                                Artifact 2

If you are having trouble with answering the above, you should either read a book or take a course!  3 good AFM books are Atomic force microscopy by Peter Eaton and Paul West (for beginners, published in 2010 so a bit old but still good), Atomic force microscopy by Greg Haugstad (for beginners/intermediate users, published in 2012 by Wiley), and the first half my book Scanning Probe Microscopy in Industrial Applications (for beginners/intermediates, published in 2013 by Wiley).

Better yet, I highly recommend a course with a hands-on component. There is nothing like playing with the instrument and doing labs with an instructor on hand to go through questions and implement the concepts you learn in lecture.   Hooke College of Applied Sciences runs excellent microscopy courses and will be running a hands-on SPM course Nov 6-8 at its site in Westmont IL.  I am also assessing interest for an “Intro to SPM” course at MIT this fall.  Email classes@surfacechar.com if you are interested in learning more about SPM courses for improved and more intelligent (and hopefully more frustration-free) operation of your instrument!

Dalia Yablon, Ph.D.

SurfaceChar LLC

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