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Atomic Force Microscop (AFM)
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History and Definitions in Scanning Probe Microscopy (SPM)
History
Scanning Tunneling Microscope (STM)
Developed in 1982 by Binning, Rohrer, Gerber, and Weibel at IBM in
Zurich, Switzerland.
Binning and Rohrer won the Nobel Prize in Physics for this invention
in 1986.
Atomic Force Microscope (AFM)
Developed in 1986 by Binning, Quate, and Gerber as a collaboration
between IBM and Stanford University.
Definitions
Scanning Probe Microscopy (SPM): Consists of a family of
microscopy forms where a sharp probe is scanned across a surface
The two primary forms of SPM consist of:
Scanning Tunneling Microscopy (STM)
Atomic Force Microscopy (AFM) (also called Scanning Force
Microscopy (SFM))
There are 3 primary modes of AFM:
Contact Mode AFM
Non-contact Mode AFM
TappingMode™ AFM
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The atomic force microscope (AFM) or scanning
force microscope (SFM) is a very high-resolution type
of scanning probe microscopy, with demonstrated
resolution of fractions of a nanometer, more than 1000
times better than the optical diffraction limit.
The information is gathered by "feeling" the surface with
a mechanical probe.
Piezoelectric elements that facilitate tiny but accurate
and precise movements on (electronic) command enable
the very precise scanning.
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 on the order of nanometers.
Typically, the deflection is measured using a laser spot
reflected from the top surface of the cantilever into an
array of photodiodes.
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Atomic Force Microscope
mirror
laser beam
photodiode
fluid cell
SPM tip
fluid in
fluid out
tipholder
sample
piezo
translator
O-ring
x,y,z piezo translator
sample
motor
control
in air and in buffer solutions
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Basic SPM Components
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Atomic Force Microscope
laser
photodiode
cantilever
z
piezo
x
cantilever tip
y
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Contact mode AFM operates by scanning a tip attached to the end of a
cantilever across the sample surface while monitoring the change in
cantilever deflection with a split photodiode detector.
The tip contacts the surface through the adsorbed fluid layer on the sample
surface.
A feedback loop maintains a constant deflection between the cantilever and
the sample by vertically moving the scanner at each (x,y) data point to
maintain a "setpoint" deflection.
By maintaining a constant cantilever deflection, the force between the tip
and the sample remains constant.
The force is calculated from Hooke's Law: where
F = Force
k = spring constant
x = cantilever deflection.
Force constants usually range from 0.01 to 1.0 N/m
The distance the scanner moves vertically at each (x,y) data point is stored
by the computer to form the topographic image of the sample surface.
Operation can take place in ambient and liquid environments.
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Contact Mode AFM
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Tapping Mode AFM operates by scanning a tip attached to the end
of an oscillating cantilever across the sample surface.
The cantilever is oscillated at or near its resonance frequency with
an amplitude ranging typically from 20nm to 100nm. The frequency
of oscillation can be at or on either side of the resonant frequency.
The tip lightly “taps” on the sample surface during scanning,
contacting the surface at the bottom of its swing.
The feedback loop maintains a constant oscillation amplitude.
The vertical position of the scanner at each (x,y) data point in order
to maintain a constant "setpoint “ amplitude is stored by the
computer to form the topographic image of the sample surface.
By maintaining a constant oscillation amplitude, a constant tipsample interaction is maintained during imaging.
Operation can take place in ambient and liquid environments. In
liquid, the oscillation need not be at the cantilever resonance.
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TappingMode™ AFM
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Contact Mode AFM
Advantages:
High scan speeds
Contact mode AFM is the only AFM technique which can obtain
"atomic resolution" images.
Rough samples with extreme changes in vertical topography can
sometimes be scanned more easily in contact mode.
Disadvantages:
Lateral (shear) forces can distort features in the image.
The forces normal to the tip-sample interaction can be high
Tapping Mode AFM
Advantages:
Higher resolution on most samples (1 nm to 5 nm).
Lower forces and less damage to soft samples imaged in air.
Disadvantages:
Slightly slower scan speed than contact mode AFM.
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Advantages and disadvantages
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The AFM has several advantages over the scanning electron
microscope (SEM). 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 AFM do not require any special
treatments (such as metal/carbon coatings) that would irreversibly
change or damage the sample. While an electron microscope needs
an expensive vacuum environment for proper operation, most AFM
modes can work perfectly well in ambient air or even a liquid
environment. This makes it possible to study biological
macromolecules and even living organisms. In principle, AFM can
provide higher resolution than SEM. It has been shown to give true
atomic resolution in ultra-high vacuum (UHV) and, more recently, in
liquid environments.
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A disadvantage of AFM compared with the scanning electron microscope
(SEM) is the image size. The SEM can image an area on the order of
millimetres by millimetres with a depth of field on the order of millimetres.
The AFM can only image a maximum height on the order of 10-20
micrometres and a maximum scanning area of around 150 by 150
micrometres.
Another inconvenience is that an incorrect choice of tip for the required
resolution can lead to image artifacts. Traditionally the AFM could not scan
images as fast as an SEM, requiring several minutes for a typical scan,
while a SEM is capable of scanning at near real-time
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