Biomedical Engineering Reference
In-Depth Information
Fig. 3.8. Possible non-contact imaging conditions under ambient conditions, with a sample covered
in a contamination layer. Such a layer exists on most samples in air. In the first case on the left, the
probe oscillates above the contamination layer. In the second case; right, the probe oscillates within
the contamination layer.
contact in ambient conditions. However, with care, it can replace intermittent contact in
nearly all applications, and often gives better, and more consistent results due to lower tip
wear. One of the limiting factors for non-contact mode in air is the contamination layer
present on most surfaces under ambient conditions. In general, the presence of this layer
means that the probe-surface interaction forces are governed by the capillary forces
between the probe and the contamination layer. For non-contact AFM, The probe may be
vibrated in two different distinct regimes as it is scanned across the surface, see Figure 3.8.
In the first regime, the probe is oscillated above the surface of the contamination layer.
The vibration amplitude must be very small and a very stiff probe must be used. The
images of the surface contamination layer are typically unrepresentative of the substrate
topography and appear to have low resolution. This is because the contamination fills in
the nanostructures at the surface. However, in some cases this technique allows the
determination of the location or shape of liquid droplets on the samples' surface, which
may be desirable [112, 113]. In the second regime the probe is scanned inside the
contamination layer [110]. This technique, sometimes called 'near contact', requires
great care to achieve. Again, the cantilever must be stiff so that the tip does not jump
to the surface from the capillary forces caused by the contamination layer, and very small
vibration amplitudes must be used. However, high-resolution images may be measured in
this regime. Non-contact AFM fully immersed in liquid is also possible [114], and
delicate samples such as DNA molecules or other biological samples have been imaged
by in this way, and such molecules may suffer less distortion when imaged like this than
when imaged by intermittent-contact mode [114-116].
Using ultra-high vacuum (UHV) conditions, FM detection has advantages over ampli-
tude or phase detection [117] and FM detection is widely used for UHV non-contact AFM.
Some amazing results have been shown for frequency-modulation based non-contact
AFM in ultra-high vacuum, including true atomic resolution [118, 119]. For instance,
the Morita group have shown true atomic resolution in a number of systems with this
technique [106, 118, 120-122]. The system must be very stable for operation to be reliable
without the risk of jump-to-contact. An example image showing true atomic resolution by
NC-AFM is shown in Figure 3.9. Figure 3.9 also shows a rare example of using NC-AFM
to identify atoms on a surface. Force spectroscopy is described further in Section 3.2.1,
with respect to using force spectroscopy in contact mode. But in this example, unusual due
to the measurement of force curves in FM-AFM mode, force spectroscopy was used to
