Force spectroscopy - introduction
|
Information about the sample is also available from measuring
the changes while the separation from the surface is varied
at a single point, rather than by scanning the lateral position
of the tip. In this mode the base of the cantilever is moved
in the vertical direction towards the surface using the piezo
and then retracted again. During the motion, the deflection
of the cantilever and other signals, such as the amplitude or
phase in dynamic AFM modes, can be measured. This is usually
called force spectroscopy.
|
|
|
|
The basic force spectroscopy
curves can be understood by thinking about the example of a cantilever
in air approaching a hard, incompressible surface such as glass
or mica. As the cantilever approaches the surface, initially the
forces are too small to give a measurable deflection of the cantilever,
and the cantilever remains in its undisturbed position. At some
point, the attractive forces (usually Van der Waals, and capillary
forces) overcome the cantilever spring constant and the tip jumps
into contact with the surface. Once the tip is in contact with
the sample, it remains on the surface as the separation between
the base and the sample decreases further, causing a deflection
of the tip and an increase in the repulsive contact force.
As
the cantilever is retracted from the surface, often the tip remains
in contact with the surface due to some adhesion and the cantilever
is deflected downwards. At some point the force from the cantilever
will be enough to overcome the adhesion, and the tip will break
free.
|
 |
|
Many interesting samples are not hard and incompressible, and
a more general pair of approach and retract curves will include
sample compression, hysteresis and more complex adhesion between
the tip and surface. In liquid, there may not be an obvious
snap to contact in the approach curves even over a hard surface
such as mica. Over a soft, compressible sample in liquid, the
force curve often shows a gradual increase in force, without
the sharp onset of the interactions seen in air. It is often
difficult to define a single point where the tip and sample
come into "contact", since the initial compression
of the surface causes very little deflection of the cantilever.
The gradient of the repulsive contact region changes as the
sample is indented and the apparent stiffness may change as
the structure is compressed. For thin samples on a hard surface,
the linear repulsive contact regime may be seen at large deflections,
as the tip may indent the sample enough to feel the supporting
surface below. The contact area will change as the tip indents
a soft surface, so the actual interactions involved in compression
can be hard to quantify, and different points within the region
will experience different levels of compression.
|
 |
|
When the tip is retracted from the surface, there is often
a hysteresis seen, if the sample is not perfectly elastic, and
many different adhesion responses can be observed. In some cases,
the cantilever pulls the tip free in stages, such as when there
are long molecules in the sample or on the tip. Extendable contacts
are made between the tip and sample, so that as the base of
the cantilever retracts, the tip is deflected down towards the
sample until the force is strong enough to break the contacts.
Different molecules or parts of the sample may adhere and each
part may be broken separately, or together. These situations
produce a variety of adhesion events, and successive force curves
can show very different responses.
|
 |
|
|
|
