Editing Atomic force microscopy (section)

==Overview==
[[Image:Atomic force microscope by Zureks.jpg|thumb|right|An atomic force microscope on the left with controlling computer on the right]]
Atomic force microscopy<ref>{{cite news |title=Measuring and Analyzing Force-Distance Curves with Atomic Force Microscopy |url=https://www.afmworkshop.com/images/news2019/01/Measuring-and-understanding-force-distance-curves-v2.pdf |agency=afmworkshop.com}}</ref> (AFM) gathers information by "feeling" or "touching" the surface with a mechanical probe. [[Piezoelectric]] elements that facilitate tiny but accurate and precise movements on (electronic) command enable precise scanning. Despite the name, the Atomic Force Microscope does not use the [[nuclear force]].

===Abilities and spatial resolution===
[[File:Atomic Force Microscope.ogv|thumb|Atomic Force Microscope]]
The AFM has three major abilities: force measurement, topographic imaging, and manipulation.

In force measurement, AFMs can be used to measure the forces between the probe and the sample as a function of their mutual separation. This can be applied to perform [[force spectroscopy]], to measure the mechanical properties of the sample, such as the sample's [[Young's modulus]], a measure of stiffness.

For imaging, the reaction of the probe to the forces that the sample imposes on it can be used to form an image of the three-dimensional shape (topography) of a sample surface at a high resolution. This is achieved by [[raster scan]]ning the position of the sample with respect to the tip and recording the height of the probe that corresponds to a constant probe-sample interaction {{xref|(see {{section link||Topographic image}} for more)}}. The surface topography is commonly displayed as a [[pseudocolor]] plot.

Although the initial publication about atomic force microscopy by Binnig, Quate and Gerber in 1986 speculated about the possibility of achieving atomic resolution, profound experimental challenges needed to be overcome before atomic resolution of defects and step edges in ambient (liquid) conditions was demonstrated in 1993 by Ohnesorge and Binnig.<ref>{{cite journal|last=Ohnesorge|first=Frank|title=True atomic resolution by atomic force microscopy through repulsive and attractive forces|journal=Science|date=4 June 1993|volume=260|issue=5113|pages=1451–6|doi=10.1126/science.260.5113.1451|pmid=17739801|bibcode=1993Sci...260.1451O|s2cid=27528518}}</ref> True atomic resolution of the silicon 7x7 surface had to wait a little longer before it was shown by Giessibl.<ref>{{cite journal|last=Giessibl|first=Franz|title=Atomic Resolution of the Silicon (111)-(7x7) Surface by Atomic Force Microscopy|journal=Science|date=6 January 1995|volume=267|issue=5194|pages=68–71|doi=10.1126/science.267.5194.68|pmid=17840059|bibcode=1995Sci...267...68G |s2cid=20978364|url=https://epub.uni-regensburg.de/33828/1/Atomic%20Resolution%20of%20the%20Silicon%20%28111%29-%287x7%29%20Surface%20by.pdf}}</ref> Subatomic resolution (i.e. the ability to resolve structural details within the electron density of a single atom) has also been achieved by AFM. 

In manipulation, the forces between tip and sample can also be used to change the properties of the sample in a controlled way. Examples of this include atomic manipulation, [[scanning probe lithography]] and local stimulation of cells.

Simultaneous with the acquisition of topographical images, other properties of the sample can be measured locally and displayed as an image, often with similarly high resolution. Examples of such properties are mechanical properties like stiffness or adhesion strength and electrical properties such as conductivity or surface potential.<ref>{{cite video|title=Atomic Force Microscopy for electrical characterization |url=https://www.youtube.com/watch?v=O8ZvU0vZsYE |work=www.youtube.com/user/MINATEC}}</ref> In fact, the majority of SPM  techniques are extensions of AFM that use this modality.<ref>{{cite news |title=Atomic Force Microscopy Research involving the study of Neglected Tropical Diseases |url=https://www.afmworkshop.com/newsletter/269-atomic-force-microscopy-research-involving-the-study-of-neglected-tropical-diseases |work=www.afmworkshop.com}}</ref>

===Other microscopy technologies===
The major difference between atomic force microscopy and competing technologies such as optical microscopy and [[Electron microscope|electron microscopy]] is that AFM does not use lenses or beam irradiation.  Therefore, it does not suffer from a limitation in spatial resolution due to diffraction and aberration, and preparing a space for guiding the beam (by creating a vacuum) and staining the sample are not necessary.

There are several types of scanning microscopy including SPM (which includes AFM, [[scanning tunneling microscope|scanning tunneling microscopy]] (STM) and [[near-field scanning optical microscope]] (SNOM/NSOM), [[STED microscopy]] (STED), and [[scanning electron microscopy]] and [[electrochemical AFM]], EC-AFM). Although SNOM and STED use [[visible light|visible]], [[infrared]] or even [[Terahertz radiation|terahertz]] light to illuminate the sample, their resolution is not constrained by the diffraction limit.

===Configuration===
Fig. 3 shows an AFM, which typically consists of the following features.<ref name="Biningpat">[https://patents.google.com/patent/US4724318 Patent US4724318 – Atomic force microscope and method for imaging surfaces with atomic resolution]</ref> Numbers in parentheses correspond to numbered features in Fig. 3. Coordinate directions are defined by the coordinate system (0).
[[File:AFM conf.jpg|thumb|upright=1.15|'''Fig. 3:''' Typical configuration of an AFM.<br />
'''(1)''': Cantilever,
'''(2)''': Support for cantilever,
'''(3)''': Piezoelectric element (to oscillate cantilever at its eigen frequency),
'''(4)''': Tip (Fixed to open end of a cantilever, acts as the probe),
'''(5)''': Detector of deflection and motion of the cantilever,
'''(6)''': Sample to be measured by AFM,
'''(7)''': xyz drive, (moves sample (6) and stage (8) in x, y, and z directions with respect to a tip apex (4)), and
'''(8)''': Stage.]]

The small spring-like [[cantilever]] (1) is carried by the support (2). Optionally, a piezoelectric element (typically made of a ceramic material) (3) oscillates  the cantilever (1). The sharp tip (4) is fixed to the free end of the cantilever (1). The detector (5) records the deflection and motion of the cantilever (1). The sample (6) is mounted on the sample stage (8).  An xyz drive (7) permits to displace the sample (6) and the sample stage (8) in x, y, and z directions with respect to the tip apex (4). Although Fig. 3 shows the drive attached to the sample, the drive can also be attached to the tip, or independent drives can be attached to both, since it is the relative displacement of the sample and tip that needs to be controlled. Controllers and plotter are not shown in Fig. 3.

According to the configuration described above, the interaction between tip and sample, which can be an atomic-scale phenomenon, is transduced into changes of the motion of cantilever, which is a macro-scale phenomenon. Several different aspects of the cantilever motion can be used to quantify the interaction between the tip and sample, most commonly the value of the deflection, the amplitude of an imposed oscillation of the cantilever, or the shift in resonance frequency of the cantilever (see section Imaging Modes).

====Detector====
The detector (5) of AFM measures the deflection (displacement with respect to the equilibrium position) of the cantilever and converts it into an electrical signal. The intensity of this signal will be proportional to the displacement of the cantilever.

Various methods of detection can be used, e.g. interferometry, optical levers, the piezoelectric method, and STM-based detectors (see section "AFM cantilever deflection measurement").

====Image formation====
''This section applies specifically to imaging in {{section link||Contact mode}}. For other imaging modes, the process is similar, except that "deflection" should be replaced by the appropriate feedback variable.''

When using the AFM to image a sample, the tip is brought into contact with the sample, and the sample is raster scanned along an x–y grid. Most commonly, an electronic feedback loop is employed to keep the probe-sample force constant during scanning. This feedback loop has the cantilever deflection as input, and its output controls the distance along the z axis between the probe support (2 in fig. 3) and the sample support (8 in fig 3). As long as the tip remains in contact with the sample, and the sample is scanned in the x–y plane, height variations in the sample will change the deflection of the cantilever. The feedback then adjusts the height of the probe support so that the deflection is restored to a user-defined value (the setpoint). A properly adjusted feedback loop adjusts the support-sample separation continuously during the scanning motion, such that the deflection remains approximately constant. In this situation, the feedback output equals the sample surface topography to within a small error.

Historically, a different operation method has been used, in which the sample-probe support distance is kept constant and not controlled by a feedback ([[Servomechanism|servo mechanism]]). In this mode, usually referred to as "constant-height mode", the deflection of the cantilever is recorded as a function of the sample x–y position. As long as the tip is in contact with the sample, the deflection then corresponds to surface topography. This method is now less commonly used because the forces between tip and sample are not controlled, which can lead to forces high enough to damage the tip or the sample.{{Citation needed|date=March 2022}} It is, however, common practice to record the deflection even when scanning in constant force mode, with feedback. This reveals the small tracking error of the feedback, and can sometimes reveal features that the feedback was not able to adjust for.

The AFM signals, such as sample height or cantilever deflection, are recorded on a computer during the x–y scan. They are plotted in a [[pseudocolor]] image, in which each pixel represents an x–y position on the sample, and the color represents the recorded signal.

[[File:Schematics of Topographic image forming.jpg|thumb|upright=1.15|'''Fig. 5:''' Topographic image forming by AFM.<br />
'''(1)''': Tip apex,
'''(2)''': Sample surface,
'''(3)''': Z-orbit of Tip apex,
'''(4)''': Cantilever.
]]

===History===
The AFM was invented by IBM scientists in 1985.<ref>{{Cite journal |doi = 10.1103/PhysRevLett.56.930|title = Atomic Force Microscope|year = 1986|last1 = Binnig|first1 = G.|last2 = Quate|first2 = C. F.|last3 = Gerber|first3 = Ch.|journal = Physical Review Letters|volume = 56|issue = 9|pages = 930–933|pmid = 10033323|bibcode = 1986PhRvL..56..930B|doi-access = free}}</ref> The precursor to the AFM, the [[scanning tunneling microscope]] (STM), was developed by [[Gerd Binnig]] and [[Heinrich Rohrer]] in the early 1980s at [[IBM Research – Zurich]], a development that earned them the 1986 [[Nobel Prize for Physics]]. Binnig invented<ref name="Biningpat" /> the atomic force microscope and the first experimental implementation was made by Binnig, [[Calvin Quate|Quate]] and [[Christoph Gerber|Gerber]] in 1986.<ref name="BinnigQuate1986" />

The first commercially available atomic force microscope was introduced in 1989. The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the [[nanometre|nanoscale]].

===Applications===
The AFM has been applied to problems in a wide range of disciplines of the natural sciences, including [[solid-state physics]], [[semiconductor]] science and technology, [[molecular engineering]], [[polymer chemistry]] and [[Polymer physics|physics]], [[Surface science|surface chemistry]], [[molecular biology]], [[cell biology]], and [[medicine]].

Applications in the field of solid state physics include (a) the identification of atoms at a surface, (b) the evaluation of interactions between a specific atom and its neighboring atoms, and (c) the study of changes in physical properties arising from changes in an atomic arrangement through atomic manipulation.

In molecular biology, AFM can be used to study the structure and mechanical properties of protein complexes and assemblies. For example, AFM has been used to image [[microtubules]] and measure their stiffness.

In cellular biology, AFM can be used to attempt to distinguish cancer cells and normal cells based on a hardness of cells, and to evaluate interactions between a specific cell and its neighboring cells in a competitive culture system. AFM can also be used to indent cells, to study how they regulate the stiffness or shape of the cell membrane or wall.

In some variations, [[electric potential]]s can also be scanned using conducting cantilevers. In more advanced versions, [[Electric current|current]]s can be passed through the tip to probe the [[electrical conductivity]] or transport of the underlying surface, but this is a challenging task with few research groups reporting consistent data (as of 2004).<ref name="Lang et. al.">{{cite journal|last=Lang|first=K.M.|author2=D. A. Hite|author3=R. W. Simmonds|author4=R. McDermott|author5=D. P. Pappas|author6=John M. Martinis|title=Conducting atomic force microscopy for nanoscale tunnel barrier characterization|journal=[[Review of Scientific Instruments]]|year=2004|volume=75|pages=2726–2731|doi=10.1063/1.1777388|url=http://rsi.aip.org/resource/1/rsinak/v75/i8/p2726_s1|bibcode=2004RScI...75.2726L|issue=8|url-status=dead|archive-url=https://archive.today/20130223113907/http://rsi.aip.org/resource/1/rsinak/v75/i8/p2726_s1|archive-date=2013-02-23|url-access=subscription}}</ref>
AFM techniques such as [[conductive atomic force microscopy]] (C-AFM) and [[Kelvin probe force microscopy]] (KPFM) are increasingly used in [[solid-state battery]] research to analyze local conductivity variations, interfacial potential changes, and degradation mechanisms at the nanoscale.