W. Richard Bowen and Nidal Hilal 4

0 1. BAsIC PRINCIPLEs OF ATOMIC FORCE MICROsCOPy
.4.2 Intermittent Contact (Tapping) mode
In order to overcome the limitations of contact mode imaging as mentioned
earlier, the intermittent, or tapping, mode of imaging was developed
[53–55]. Here the cantilever is allowed to oscillate at a value close
to its resonant frequency. When the oscillations occur close to a sample
surface, the probe will repeatedly engage and disengage with the surface,
restricting the amplitude of oscillation. As the surface is scanned,
the oscillatory amplitude of the cantilever will change as it encounters
differing topography. By using a feedback mechanism to alter the
z-height of the piezocrystal and maintain a constant amplitude, an image
of the surface topography may be obtained in a similar manner as with
contact mode imaging. In this way as the probe is scanned across the surface,
lateral forces are greatly reduced compared with the contact mode.
When using tapping mode in air, capillary forces due to thin layers of
adsorbed water on surfaces, as well as any other adhesive forces which
may be present, have to be overcome. If the restoring force of the cantilever
due to its deflection is insufficient to overcome adhesion between
the probe and the surface, then the probe will be dragged along the surface
in an inadvertent contact mode. As a result, for this mode in air the
spring constants of AFM cantilevers are by necessity several orders of
magnitude greater than those used for either tapping mode in liquid or
contact mode (typically in the range of 0.01–2 Nm �1 for contact mode to
20–75 Nm �1 for tapping in air).
As surfaces with different mechanical and adhesive properties are
scanned, the frequency of oscillation will change, causing a shift in the
phase signal between the drive frequency and the frequency with which
the cantilever is actually oscillating [56, 57]. This phenomenon has been
used to produce phase images alongside topographic images, which are
able to show changes in the material properties of the surfaces being
investigated. However, whilst the qualitative data provided by the phase
images are useful, it is difficult to extract quantitative information from
them because they are a complex result of a number of parameters including
adhesion, scan speed, load force, topography and the material, especially
elastic, properties of the sample and probe [57, 58].
.4.3 non-Contact mode
In non-contact mode imaging, the cantilever is again oscillated as in
intermittent contact mode, but at much smaller amplitude. As the probe
approaches the sample surface, long-range interactions, such as van der
Waals and electrostatic forces, occur between atoms in the probe and the
sample. This causes a detectable shift in the frequency of the cantilever’s