AFM modes - Atomic Force Microscopy - page 57

Table 3.1. Nomenclature of some oscillating probe AFM modes.

Detection Amplitude

Low High

Amplitude Rarely used Intermittent Contact AFM

(IC-AFM), also known as
AC-AFM or Tapping

Phase Non-contact AFM (NC-AFM), Rarely used

also known as close-contact AFM

Typically an AFM designed for use in air or liquid has electronics that can measure
changes in vibrational amplitude or phase at a preselected frequency. So the instrument
operator can choose to use either of these for feedback. In combination with large or small
amplitudes, there are four types of oscillating experiment available to most AFM users,
which are shown in Table 3.1.
It should be stressed that the two possible conditions described as 'rarely used' in Table
3.1 are not unusable, just that they are not commonly applied. Phase detection is usually
used with small amplitudes (close-contact AFM), due to somewhat higher sensitivity, and
amplitude detection is usually used with large amplitudes (intermittent-contact AFM), but
these are not the only possible imaging methods. Optimal imaging conditions are some-
times difficult to establish, and it may be necessary to try different amplitudes and
detection schemes to find the ideal conditions.
An alternative to amplitude or phase detection is frequency-modulation detection (FM-
AFM), typically used in ultra-high vacuum conditions (UHV-AFM). FM-AFM is typically
applied with small oscillation amplitudes in the non-contact regime. Typically FM-AFM
is carried out with a phase-locked loop device. This technique is unavailable to most AFM
users due to the need for additional equipment, so it is not covered in detail in this topic.
However, it has been described in detail [106, 107], and compared with the amplitude
modulation (AM-AFM) techniques we discuss here elsewhere [108].

3.1.2.1 Non-contact mode/close-contact mode
One of the great advantages of oscillating modes in AFM is that they can decrease the
size of tip-sample forces, while maintaining high sensitivity to the sample topography.
To achieve non-contact AFM, the tip must be close enough to the sample surface to
achieve this high sensitivity, without passing into the repulsive regime used for contact-
mode AFM. Non-contact AFM is therefore carried out in the attractive regime, as shown
in Figure 3.7.
By using a highly stiff cantilever and monitoring the dynamic effects of the attractive
force (i.e. the change in the oscillation) in this regime, it is possible to maintain the
cantilever very close to the surface without jumping to the repulsive regime. It is possible
to observe changes in the oscillation amplitude and phase in this regime. These effects are
caused by a change in the cantilever resonant frequency which is in turn caused by forces
from the surface (normally attractive van der Waals forces) acting on the tip. The resonant
frequency far from the surface, c p where c is a function of the ø 0 is given by ø 0 ¼