Biomedical Engineering Reference
In-Depth Information
7.1.4 Friction measurement with AFM
Lateral force microscopy (LFM), or friction force microscopy, is most often applied for one
of three purposes: fundamental study of tribology of macroscopic or atomic scale systems
[442-444], determination of the friction properties of (uniform) materials [395, 445, 446]
or characterization of heterogeneous materials based on their frictional properties [447,
448]. Like some other modes of AFM, LFM allows the determination of parameters that
may also be determined by other techniques, which may be simpler to apply or easier to
quantify. However, the advantage of LFM is the ability to quantify such parameters on the
nanoscale, whether for fundamentals reasons (e.g. the study of tribology at the single atom
level), or for practical reasons (e.g. measurement of nanoscopic inclusions in a surface).
Applications include studies of monolayers, including the friction between monolayers
on the probe and on a sample surface [444, 448], friction-based discrimination of phases in
polymer blends [449, 450] and discrimination of inclusions in heterogeneous polymer
surfaces [448], and friction studies of carbide coatings for tool coatings [445, 447]. The
ability of LFM to discriminate different chemical groups means it can be used to study
phase separation in mixed monolayers [451], and is very commonly used to detect directly
deposition of features by dip-pen nanolithography or nanografting, because the contrast is
often better than in the height image [260, 452].
Practically, frictional measurements with the AFM are made by measuring friction loops.
The term 'friction loop' refers to the combination of the LFM data from the forwards and
reverse directions. Two example friction loops are shown in Figure 7.8. The actual friction
measured on the material under study is obtained by calculating the difference between
forwards and backwards scans. This calculated value is typically derived in terms of volts,
but may be converted to force with the methods described in Section 3.2.3.1. The lateral
force measured nearly always depends on the normal force applied (e.g. the set-point), and
for many materials the relationship will be linear [448], meaning that a plot of normal versus
lateral forces allows the calculation of the useful parameter
, the friction coefficient of the
material. An example of such a plot is shown in Figure 7.8.
Measuring quantitative frictional properties as described above is an important applica-
tion of LFM, but what's really unique about the technique is its ability to distinguish
frictional properties of material at the nanoscale. This means that any materials that can be
distinguished based on their frictional properties can be differentiated by the technique with
the same resolution as contact-mode AFM,
i.e
. up to atomic resolution. For example, 3 nm
resolution was seen in a semiconductor film sample consisting of InP and InGaAs regions
[455]. As well as the very high resolution, this example highlights another important aspect
of LFM for compositional mapping: many materials of very similar composition can be
distinguished. Further example are the differentiation of CdCO
3
and CaCo
3
[456], many
different mixed organic monolayer systems [334, 457-459], Si and SiO
2
[460] and many
other mixed systems [189]. Figure 7.9 shows an example of how it's possible to use lateral
force microscopy to characterize different materials in a heterogeneous surface based on
their frictional properties, in this case, filler particles in a polymer film.
7.1.5 Phase imaging to identify surface features
In the first description of phase imaging in AFM [190], It was described how the technique
could be used to generate material contrast on a wide range of materials including
