Editing Atomic force microscopy (section)

====Tapping mode====
[[Image:Single-Molecule-Under-Water-AFM-Tapping-Mode.jpg|thumb|upright=1.15|Single polymer chains (0.4 nm thick) recorded in a tapping mode under aqueous media with different pH.<ref name=roiter>{{cite journal |doi=10.1021/ja0558239|date=Nov 2005|author1=Roiter, Y |author2=Minko, S |title=AFM single molecule experiments at the solid-liquid interface: in situ conformation of adsorbed flexible polyelectrolyte chains |volume=127|issue=45|pages=15688–9|issn=0002-7863|pmid=16277495|journal=[[Journal of the American Chemical Society]]}}</ref>]]
In ambient conditions, most samples develop a liquid meniscus layer.  Because of this, keeping the probe tip close enough to the sample for short-range forces to become detectable while preventing the tip from sticking to the surface presents a major problem for contact mode in ambient conditions.  Dynamic contact mode (also called intermittent contact, AC mode or tapping mode) was developed to bypass this problem.<ref>{{cite journal|doi=10.1016/0167-2584(93)90906-Y|title=Fractured polymer/silica fiber surface studied by tapping mode atomic force microscopy |vauthors=Zhong Q, Inniss D, Kjoller K, Elings V |year=1993 |journal=[[Surface Science Reports|Surface Science Letters]] |volume=290|issue=1|pages=L688–L692 |bibcode=1993SurSL.290L.688Z}}</ref> Nowadays, tapping mode is the most frequently used AFM mode when operating in ambient conditions or in liquids.

In ''tapping mode'', the cantilever is driven to oscillate up and down at or near its resonance frequency. This oscillation is commonly achieved with a small piezo element in the cantilever holder, but other possibilities include an AC magnetic field (with magnetic cantilevers), piezoelectric cantilevers, or periodic heating with a modulated laser beam. The amplitude of this oscillation usually varies from several nm to  200&nbsp;nm. In tapping mode, the frequency and amplitude of the driving signal are kept constant, leading to a constant amplitude of the cantilever oscillation as long as there is no drift or interaction with the surface. The interaction of forces acting on the cantilever when the tip comes close to the surface, [[van der Waals force]]s, [[dipole–dipole interaction]]s, [[electrostatic force]]s, etc. cause the amplitude of the cantilever's oscillation to change (usually decrease) as the tip gets closer to the sample. This amplitude is used as the parameter that goes into the [[Servomechanism|electronic servo]] that controls the height of the cantilever above the sample. The servo adjusts the height to maintain a set cantilever oscillation amplitude as the cantilever is scanned over the sample. A ''tapping AFM'' image is therefore produced by imaging the force of the intermittent contacts of the tip with the sample surface.<ref name="Geisse 2009 40–45">{{cite journal|last=Geisse|first=Nicholas A.|title=AFM and Combined Optical Techniques|journal=[[Materials Today]]|date=July–August 2009|volume=12|issue=7–8|pages=40–45|doi=10.1016/S1369-7021(09)70201-9|doi-access=free}}</ref>

Although the peak forces applied during the contacting part of the oscillation can be much higher than typically used in contact mode, tapping mode generally lessens the damage done to the surface and the tip compared to the amount done in contact mode. This can be explained by the short duration of the applied force, and because the lateral forces between tip and sample are significantly lower in tapping mode over contact mode.
Tapping mode imaging is gentle enough even for the visualization of supported [[Lipid bilayer#Characterization methods|lipid bilayers]] or adsorbed single polymer molecules (for instance, 0.4&nbsp;nm thick chains of synthetic [[polyelectrolyte]]s) under liquid medium. With proper scanning parameters, the conformation of [[Single-molecule experiment|single molecules]] can remain unchanged for hours,<ref name=roiter/> and even single molecular motors can be imaged while moving.

When operating in tapping mode, the phase of the cantilever's oscillation with respect to the driving signal can be recorded as well. This signal channel contains information about the energy dissipated by the cantilever in each oscillation cycle. Samples that contain regions of varying stiffness or with different adhesion properties can give a contrast in this channel that is not visible in the topographic image. Extracting the sample's material properties in a quantitative manner from phase images, however, is often not feasible.