Editing Atomic force microscopy (section)

==Principles==
{{Multiple image|direction=vertical|align=right|image1=AFM (used) cantilever in Scanning Electron Microscope, magnification 1000x.JPG|image2=AFM (used) cantilever in Scanning Electron Microscope, magnification 3000x.JPG|width=130|caption1=Electron micrograph of a used AFM cantilever. Image width ~100 micrometers|caption2=Electron micrograph of a used AFM cantilever. Image width ~30 micrometers}}
The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically [[silicon]] or [[silicon nitride]] with a tip [[radius of curvature (applications)|radius of curvature]] on the order of nanometers. When the tip is brought into proximity of a sample surface, [[force]]s between the tip and the sample lead to a deflection of the cantilever according to [[Hooke's law]].<ref>{{cite journal|last=Cappella |first=B |author2=Dietler, G |journal=[[Surface Science Reports]] |year=1999 |volume=34 |issue=1–3 |pages=1–104 |doi=10.1016/S0167-5729(99)00003-5 |url=http://www.see.ed.ac.uk/~vkoutsos/Force-distance%20curves%20by%20atomic%20force%20microscopy.pdf |bibcode=1999SurSR..34....1C |title=Force-distance curves by atomic force microscopy |url-status=dead |archive-url=https://web.archive.org/web/20121203031934/http://www.see.ed.ac.uk/~vkoutsos/Force-distance%20curves%20by%20atomic%20force%20microscopy.pdf |archive-date=2012-12-03 }}</ref> Depending on the situation, forces that are measured in AFM include mechanical contact force, [[van der Waals force]]s, [[Capillarity|capillary forces]], [[chemical bond]]ing, [[Coulomb's law|electrostatic forces]], magnetic forces (see [[magnetic force microscope]], MFM), [[Casimir effect|Casimir forces]], [[solvation|solvation forces]], etc. Along with force, additional quantities may simultaneously be measured through the use of specialized types of probes (see [[scanning thermal microscopy]], [[scanning joule expansion microscopy]], [[photothermal microspectroscopy]], etc.).

[[File:AFMimageRoughGlass20x20.JPG|thumb|upright=1.15|Atomic force microscope topographical scan of a glass surface. The micro and nano-scale features of the glass can be observed, portraying the roughness of the material. The image space is (x,y,z) = (20&nbsp;μm&nbsp;× 20&nbsp;μm&nbsp;× 420&nbsp;nm).]]
The AFM can be operated in a number of modes, depending on the application.  In general, possible imaging modes are divided into static (also called ''contact'') modes and a variety of dynamic (non-contact or "tapping") modes where the cantilever is vibrated or oscillated at a given frequency.<ref name="BinnigQuate1986" />

===Imaging modes===
AFM operation is usually described as one of three modes, according to the nature of the tip motion: contact mode, also called static mode (as opposed to the other two modes, which are called dynamic modes); tapping mode, also called intermittent contact, AC mode, or vibrating mode, or, after the detection mechanism, amplitude modulation AFM; and non-contact mode, or, again after the detection mechanism, frequency modulation AFM.

Despite the nomenclature, repulsive contact can occur or be avoided both in amplitude modulation AFM and frequency modulation AFM, depending on the settings.{{citation needed|date=February 2016}}

====Contact mode====
In contact mode, the tip is "dragged" across the surface of the sample and the contours of the surface are measured either using the deflection of the cantilever directly or, more commonly, using the feedback signal required to keep the cantilever at a constant position. Because the measurement of a static signal is prone to noise and drift, low stiffness cantilevers (i.e. cantilevers with a low spring constant, k) are used to achieve a large enough deflection signal while keeping the interaction force low. Close to the surface of the sample, attractive forces can be quite strong, causing the tip to "snap-in" to the surface. Thus, contact mode AFM is almost always done at a depth where the overall force is repulsive, that is, in firm "contact" with the solid surface.

====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.

====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.