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invented by Binnig, Quate and Gerber in 1986. Besides imaging it is also one of the foremost tools for the manipulation of matter at the nanoscale.
The AFM consists of a cantilever with a sharp tip at its end. The tip is brought into close proximity of a sample surface. The force between the tip and the sample leads to a deflection of the cantilever according to Hooke's law. Typically, the deflection is measured using a laser spot reflected from the top of the cantilever.
If the tip were scanned at constant height, there would be a risk that the tip would collide with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to keep the force between the tip and the sample constant. This can be achieved by mounting the sample on a piezoelectric crystal.
The tip is then scanned across the sample surface and the vertical displacement s necessary to maintain a constant force on the tip is recorded. The resulting map of s(x,y) represents the topography of the sample.
Over the years several modes of operation have been developed for the AFM. The primary modes of operation are contact mode and dynamic mode. In the contact mode operation, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection. In the dynamic mode, the cantilever is externally oscillated close to its resonance frequency. The oscillation gets modified by the tip-sample interaction forces; these changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics. The dynamic mode generates lower lateral forces on the sample and is widely used to image biological samples.
Schemes for dynamic mode operation include frequency modulation and the more common amplitude modulation. In frequency modulation, changes in the frequency of modulation provide information about a sample's characteristics. In amplitudeAmplitude is a nonnegative scalar measure of a wave's magnitude of oscillation. In the following diagram, the distance y is the amplitude of the wave. Sometimes that distance is called the "peak amplitude", distinguishing it from another concept of amplit modulation (better known as intermittent contact or tapping mode), changes in the oscillation amplitude yield topographic information about the sample. Additionally, changes in the phaseThe phase of a wave relates the position of a feature, typically a peak or a trough of the waveform, to that same feature in another part of the waveform (or, which amounts to the same, on a second waveform). The phase may be measured as a time, distance, of oscillation under tapping mode can be used to discriminate between different types of materials on the surface.
The AFM has several advantages over the electron microscopeThe electron microscope is a microscope that can magnify very small details with high resolving power due to the use of electrons rather than light to scatter off material, magnifying at levels up to 500,000 times. History The first electron microscope wa. Unlike the electron microscope which provides a two-dimensional projection or a two-dimensional image of a sample, the AFM provides a true three-dimensional surface profile. Additionally, samples viewed by an AFM do not require any special treatment that would actually destroy the sample and prevent its reuse. While an electron microscope needs an expensive vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also environment for proper operation, the AFM can work perfectly well in an ambient or even liquid environment.
The main disadvantage that the AFM has compared to the electron microscope is the image size. The electron microscope can show an area on the order of millimetreA millimetre ( American spelling: millimeter , symbol mm is an SI unit of length that is equal to one thousandth of a metre. 1 mm is equal to: about 0. 03937 inches 0. 1 cm See 1 E-3 m for comparisons. The level of rainfall is also reported as millimeterss by millimetres and a depth of fieldIn film and photography, the depth of field is the distance in front of and behind the subject which appears to be in focus. For any given lens setting, there is only one distance at which a subject is in focus, but focus falls off gradually on either sid on the order of millimetres. The AFM can only show a maximum height on the order of micrometreA micrometre ( American spelling: micrometer , symbol m is an SI unit of length. It is defined as one millionth of a metre (1×10−6 m), equivalent to one thousandth of a millimetre. The symbol µ ( Unicode character U+00B5; HTML µ) is the " micrs and a maximum area of around 100 by 100 micrometres.
See also: scanning tunneling microscope, scanning voltage microscopy
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