Editing Atomic force microscopy (section)

====Non-contact mode====

In [[non-contact atomic force microscopy]] mode, the tip of the cantilever does not contact the sample surface. The cantilever is instead oscillated at either its [[Resonance|resonant frequency]] (frequency modulation) or just above (amplitude modulation) where the amplitude of oscillation is typically a few nanometers (<10&nbsp;nm) down to a few picometers.<ref>{{cite journal|last=Gross|first=L.|author2=Mohn, F.|author3= Moll, N.|author4= Liljeroth, P.|author5= Meyer, G.|s2cid=9346745|title=The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy|journal=[[Science (journal)|Science]]|date=27 August 2009|volume=325|issue=5944|pages=1110–1114|doi=10.1126/science.1176210|bibcode = 2009Sci...325.1110G|pmid=19713523 }}</ref> The [[van der Waals forces]], which are strongest from 1&nbsp;nm to 10&nbsp;nm above the surface, or any other long-range force that extends above the surface acts to decrease the resonance frequency of the cantilever. This decrease in resonant frequency combined with the feedback loop system maintains a constant oscillation amplitude or frequency by adjusting the average tip-to-sample distance. Measuring the tip-to-sample distance at each (x,y) data point allows the scanning software to construct a topographic image of the sample surface.

Non-contact mode AFM does not suffer from tip or sample degradation effects that are sometimes observed after taking numerous scans with contact AFM. This makes non-contact AFM preferable to contact AFM for measuring soft samples, e.g. biological samples and organic thin film. In the case of rigid samples, contact and non-contact images may look the same. However, if a few monolayers of [[adsorbed]] fluid are lying on the surface of a rigid sample, the images may look quite different. An AFM operating in contact mode will penetrate the liquid layer to image the underlying surface, whereas in non-contact mode an AFM will oscillate above the adsorbed fluid layer to image both the liquid and surface.

Schemes for dynamic mode operation include [[frequency modulation]] where a [[phase-locked loop]] is used to track the cantilever's resonance frequency and the more common [[amplitude modulation]] with a [[PID controller|servo loop]] in place to keep the cantilever excitation to a defined amplitude.  In frequency modulation, changes in the oscillation frequency provide information about tip-sample interactions.  Frequency can be measured with very high sensitivity and thus the frequency modulation mode allows for the use of very stiff cantilevers.  Stiff cantilevers provide stability very close to the surface and, as a result, this technique was the first AFM technique to provide true atomic resolution in [[ultra-high vacuum]] conditions.<ref>{{cite journal|doi=10.1103/RevModPhys.75.949|title=Advances in atomic force microscopy|year=2003|author=Giessibl, Franz J.|journal=[[Reviews of Modern Physics]]|volume=75|pages=949–983|bibcode=2003RvMP...75..949G|arxiv = cond-mat/0305119|issue=3 |s2cid=18924292}}</ref>

In [[amplitude]] modulation, changes in the oscillation amplitude or phase provide the feedback signal for imaging.  In amplitude modulation, changes in the [[phase (waves)|phase]] of oscillation can be used to discriminate between different types of materials on the surface.  Amplitude modulation can be operated either in the non-contact or in the intermittent contact regime. In dynamic contact mode, the cantilever is oscillated such that the separation distance between the cantilever tip and the sample surface is modulated.

[[Amplitude]] modulation has also been used in the non-contact regime to image with atomic resolution by using very stiff cantilevers and small amplitudes in an ultra-high vacuum environment.