Biomedical Engineering Reference
In-Depth Information
size of the tip, physical interactions between the tip and the sample cause
changes in the tip position during the raster-scanning motion.
11-14,108
This can
only happen if the intermolecular interaction between the probe tip and
the sample surface is operated in a hard-core repulsion regime.
109,110
The
refl ected signal of a laser beam from the back of a probe tip is detected by
a quadrant photodetector, which becomes a voltage output. The interac-
tions between the probe tip and the surface features lead to changes of the
probe tip position and hence the voltage output, which is then fed into a
feedback loop. Since the mechanical movement of the tubular piezo in 3D
(three-dimensions) is quantitatively related to the detected signals, three-
dimensional topography of the sample can be generated. Typical resolution
on the
x
-
y
plane can easily reach 1-2 nm per pixel and up to sub-angstrom
per pixel in the
z
direction.
11
6.11.2 Modes of operation
Contact mode
In contact mode operation, the tip/surface interaction is kept in the hard-
core repulsion regime in order to maintain close contact.
11-14
The applied
force from the probe tip is kept constant.
109,110
An optical scheme allows the
amplifi cation of very small topographic changes due to the physical inter-
action between the tip and the surface into detectable voltage output. An
advantage of contact mode is that adhesion force can be easily obtained
from the force plot as a product of the spring constant and the pulling dis-
tance. Contact mode imaging has routinely achieved molecular and even
atomic level resolution.
16,18,72,111
However, a disadvantage of contact mode is
that the shear force applied by the tip to the surface can be suffi ciently large
and disruptive to samples that are soft or loosely adsorbed to the surface.
The shear force is on the order of nanoNewtons (nN) and is strong enough
to deform surface-bound globular protein or even sweep it away.
112
Tapping mode
In tapping mode imaging, the tip is driven to oscillate vertically at high
frequency (e.g. 8-12 kHz in fl uid and ≥ 200 kHz in air) during
x-y
raster-
scanning.
113-115
The root-mean-square (RMS) amplitude of the probe
oscillation at the operating frequency is attenuated during tip/surface inter-
action. The attenuation is referenced to a preset control signal and used
in a feedback loop that controls the sample position. A three-dimensional
topographic map of the sample is generated by recording the sample posi-
tion required to keep a constant amplitude attenuation. An advantage of
using tapping mode is that negligible lateral forces are registered during
