Page 199 - Science at the nanoscale

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June 5, 2009
8.3. Scanning Probe Microscopy
cantilever (cf. Fig. 8.27(d)). The back of the cantilever and a par-
allel plate forms a parallel-plate capacitor, which is sensitive to
the separation between the two plates. Figure 8.27(e) shows a de-
tection mode that makes use of optical interferometry. Here, an
optical fibre carries laser light that shines on the back of the can-
tilever. The laser light is reflected at two locations, on the back of
the cantilever and the end wall of the optical fibre. These two re-
flected beams give rise to an interference pattern that depends on
the distance between the optical fibre and cantilever, hence pro-
viding a measurement of its deflection.
The AFM has been widely used in many disciplines since the
technique is applicable to different types of samples, conducting
or not, in liquid or in air. One of the main advantages of the AFM
is that it can be used in an aqueous environment, making it par-
ticularly useful in biology. Figure 8.27(f) shows the schematic of
AFM operation in an aqueous medium. In this case, the optical
detection mode is preferred since the laser beam can readily pass
through the transparent medium. Using such a setup, one can
obtain images of biological cells in aqueous medium. Examples of
some AFM images obtained are shown in Fig. 8.28.
Since the operation of the AFM relies on the interatomic
interactions between the probing tip and the sample surface, there
are different force regimes for the operation of the AFM. This
is typically classified into the contact and non-contact modes of
operation. In the contact mode, the cantilever is positioned very
close (< a few angstroms) from the sample surface. At this range,
the interatomic force between the cantilever and sample is repul-
−8
−6
N to 10
sive, and the magnitude of the force varies from 10
N. In the non-contact mode, the cantilever is positioned at about
10 to 100 angstroms from the sample surface and measures sam- 189 ch08
ple topography with little or no contact between tip and sample.
At this range, the interatomic force between the cantilever and the
sample is attractive with a typical force magnitude of 10 −12 N. In
the non-contact mode, the cantilever is oscillated near its resonant
frequency (typically 100 to 400 kHz) with an amplitude of a few
tens of angstroms. Changes to the resonant frequency or vibration
amplitude are detected as the tip approaches the sample surface.
Since the force required are small in the non-contact mode, it is
well-suited for studies of soft or elastic samples such as biological
cells or DNA molecules.