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Hello All,
I am using a Multimode AFM (SN: 438EX) in conjunction with Version 4 of the Nanoscope software. I have been trying to get this AFM to image in Tapping Mode without contacting the surface for some time now. What is the best way to manipulate the AFM controller to image this way? How can I tell if the cantilever is not contacting the surface? I realize these are very difficult questions to answer, but I would greatly appreciate any and all input.
Thank you!
Hi Benjamin,
That is a loaded question indeed.
There is no switch that lets you operate an AFM in one or the other way but rather a mix of control settings. Your MM with the NS4 controller uses AM detection to control oscillating modes of which Tapping Mode operation is arguably the most useful and popular as judged e.g. by the number of publications using TappingMode AFM. TappingMode will allow the AFM tip to penetrate the contamination layer present on virtually any surface in ambient condition, touch or "tap" the surface and then pull back so that the tip gets completly out of the contamination layer again. Because of the tap, TappingMode is able not only to generate accurate information about surface topography but also detect some mechanical properties that can e.g. be visualized in whats often rerfered to as "Phase Imaging". In "non-contact" the tip would not be allowed to touch the surface. Without touching one can obviously not expect to get useful information from the "Phase" with regards to sample complience. To get close to the "non-conact" condition one has to use rather small tip amplitudes. Whereas tradtional Tapping Mode operation typically uses 10nm or more, you want to try using 2-3 nm of tip amplitude. You also want to choose a very stiff cantilever. The reasoning here is that you want the restoring force of your cantilever to be greater that the external force exerted on it. If this is not the case, your tip wil experience what is known as "jump to contact" and is often seen in force vs distance curves in air.
How do you know you are in non-contact? Maybe look at it this way: How close do two atoms have to be together to be considered part of the bulk? If the atoms are separated far enough they are significantly different than the bulk. By bringing them together the interaction will become euqal to the interaction of atoms within the bulk of the material. If the interaction is stronger than the bulk some changes such as plastic deformation can happen. Such change can be taken as proof for tip-surface contact.
Hope that helps,
Stefan
In the common amplitude-modulation AFM, where a feedback circuit is raising and lowering the Z scanner to maintain constant cantilever oscillation amplitude (reduced from free amplitude), the more standard terminology (in the scientific literature, though not necessarily the vendor literature) is attractive versus (net) repulsive regimes. For a fairly comprehensive treatment circa 2001 see Ricardo Garcia's review article, Surf. Sci. Rep. v47 p197 (2002). Finer understandings have been ongoing throughout the past decade, mainly found in physics journals such as PRB, PRL and others.
Attractive regime may or may not be true non-contact on soft materials. On rigid high-energy surfaces one may be dipping into a water layer. In any case, there are many instances where phase imaging in the attractive regime actually gives very revealing materials contrast. This is an empirical fact. Contrast may derive from differences in Hamaker constant or surface potential, for example.
Hi Roger, Angus here:
Your description of the action of the feedback loop is valid for almost all modes, e.g. contact, Tapping (AM or FM modulated). My point was that i) the definition of true non-contact is very ill defined and ii) can not give very useful material information if that requires e.g. some deformation. Van der Walls interaction extends some nanometers so above the surface . So as long as the tip is not in the retarded regime one would expect that a tip can measure an interaction based on these forces. That is even more true for longer range electrical and magnetic forces. That is as a matter of fact the reason that one uses LiftMode operation to clearly visualize these long-range interactions and separates them this way from other (topographic) interactions.
When talking to Ben we figured out that his current application is to image biological samples. One can of course attempt to image these samples in "true non-contact", whatever that maybe, or simply use Tapping Mode operation that has proven itself in thousands of peer reviewed publications. Another very interesting alternative is of course PeakForce Tapping as this allows not only direct force control, which neiter TappingMode nor True non-contact allow but additionally provides clearly separated information about sample deformation, modulus, adhesion, and energy dissipation.
Best, Stefan