| Overview | Atomic Force Microscopy | Scanning Tunneling Microscopy | NSOM
Atomic Force Microscopy
The Atomic Force Microscope was developed
to overcome a basic drawback with STM - that it can only
image conducting or semiconducting surfaces. The AFM,
however, has the advantage of imaging almost any type
of surface, including polymers, ceramics, composites,
glass, and biological samples.
Binnig, Quate, and Gerber invented
the Atomic Force MIcroscope in 1985. Their original AFM
consisted of a diamond shard attached to a strip of gold
foil. The diamond tip contacted the surface directly,
with the interatomic van der Waals forces providing the
interaction mechanism. Detection of the cantilever’s
vertical movement was done with a second tip - an STM
placed above the cantilever.
AFM probe deflection
Today, most AFMs use a laser beam deflection
system, introduced by Meyer and Amer, where a laser is
reflected from the back of the reflective AFM lever and
onto a position-sensitive detector. AFM tips and cantilevers
are microfabricated from Si or Si3N4.
Typical tip radius is from a few to 10s of nm.
|Beam deflection system, using
a laser and photodector to measure the beam position.
Because the atomic force microscope
relies on the forces between the tip and sample, knowing
these forces is important for proper imaging. The force
is not measured directly, but calculated by measuring
the deflection of the lever, and knowing the stiffness
of the cantilever. Hook’s law gives F = -kz, where
F is the force, k is the stiffness of the lever, and z
is the distance the lever is bent.
AFM Modes of operation
Because of AFM’s versatility,
it has been applied to a large number of research topics.
The Atomic Force Microscope has also gone through many
modifications for specific application requirements.
The first and foremost mode of operation, contact mode
is widely used. As the tip is raster-scanned across
the surface, it is deflected as it moves over the surface
corrugation. In constant force mode, the tip is constantly
adjusted to maintain a constant deflection, and therefore
constant height above the surface. It is this adjustment
that is displayed as data. However, the ability to track
the surface in this manner is limited by the feedback
circuit. Sometimes the tip is allowed to scan without
this adjustment, and one measures only the deflection.
This is useful for small, high-speed atomic resolution
scans, and is known as variable-deflection mode.
Because the tip is in hard contact
with the surface, the stiffness of the lever needs to
be less that the effective spring constant holding atoms
together, which is on the order of 1 - 10 nN/nm. Most
contact mode levers have a spring constant of < 1N/m.
Lateral Force Microscopy
LFM measures frictional forces on a surface. By measuring
the “twist” of the cantilever, rather than
merely its deflection, one can qualitatively determine
areas of higher and lower friction.
Noncontact mode belongs to a family of AC modes, which
refers to the use of an oscillating cantilever. A stiff
cantilever is oscillated in the attractive regime, meaning
that the tip is quite close to the sample, but not touching
it (hence, “noncontact”). The forces between
the tip and sample are quite low, on the order of pN
(10 -12 N). The detection scheme is based
on measuring changes to the resonant frequency or amplitude
of the cantilever.
Dynamic Force / Intermittant-contact / “tapping
Commonly referred to as “tapping mode” it
is also referred to as intermittent-contact or the more
general term Dynamic Force Mode (DFM).
A stiff cantilever is oscillated closer to the sample
than in noncontact mode. Part of the oscillation extends
into the repulsive regime, so the tip intermittently
touches or “taps” the surface. Very stiff
cantilevers are typically used, as tips can get “stuck”
in the water contamination layer.
The advantage of tapping the surface is improved lateral
resolution on soft samples. Lateral forces such as drag,
common in contact mode, are virtually eliminated. For
poorly adsorbed specimens on a substrate surface the
advantage is clearly seen.
Force modulation refers to a method used to probe properties
of materials through sample/tip interactions. The tip
(or sample) is oscillated at a high frequency and pushed
into the repulsive regime. The slope of the force-distance
curve is measured which is correlated to the sample's
elasticity. The data can be acquired along with topography,
which allows comparison of both height and material
In Phase mode imaging, the phase shift of the oscillating
cantilever relative to the driving signal is measured.
This phase shift can be correlated with specific material
properties that effect the tip/sample interaction. The
phase shift can be used to differentiate areas on a
sample with such differing properties as friction, adhesion,
and viscoelasticity. The techniques is used simultaneously
with DFM mode, so topography can be measured as well.
Examples of atomic force microscope
The Nanosurf AFM systems are designed
to be easy to use, and ideal for those just getting
started with AFM. There are several models for a very
wide range of applications. Here you can find a list
of various atomic