Imaging probes contact mode

FIGURE 6.2 Diagrams of different AFM operating modes. (A) Contact mode and (B) dynamic mode for topographic imaging. (C) Force spectroscopy mode for interaction probing. Reprinted with permission from Liu and Wang (2010). 

The lateral force microscope (LFM) is a modification of the standard contact mode SFM [87-90]. In addition to the normal forces, the friction forces exerted on the probe are measured via torsion of the cantilever (Fig. 5). This mode is sometimes called friction force microscopy . LFM can be used in combination with topographic imaging as it shows changes in material as well as enhanced contrast on sharp edges (Fig. 9). In addition to morphology, it provides information on the friction and wear properties (Sect. 3.4). 

The so-called Force Modulation Mode (FMM) was introduced in 1991 as a modification of contact-mode SFM [123] and has since found a wide range of applications in heterogeneous polymer systems [124-129]. The FMM imaging is performed by z-modulating the probe (or sample) at a low frequency of about 5 to 20 kHz and an amplitude greater than 10 nm. The frequency is sig-... 

Fig. 13. NSOM fluorescence (left) and force (right) images of 50 nm fluorescent latex spheres in a thin film. The fluorescence image exhibits high resolution while the small spring constant of the probe allowed the force image to be taken in contact mode without damaging the NSOM aperture.

Fig. 15 shows the simultaneously measured NSOM fluorescence, topography, and deflection of a viable HASM cell under buffered conditions [28]. These images were collected in contact mode using a hybrid NSOM/AFM probe. The fluorescence signal maps the location of adrenergic receptors in the cellular membrane, which have been fluorescently labeled with prazosin BODIPY-FL. These measurements show that the hybrid NSOM/AFM probes are capable of... 

Hence it can be concluded that CM-AFM is in many cases a poor choice for imaging the surface morphology of amorphous polymers. Intermittent contact modes, such as tapping mode, are less invasive, since here lateral forces are virtually eliminated and the cantilever-tip assembly is oscillated at relatively high frequencies (on the order of 100 kHz in air). Despite these facts, it must be ensured in any experiment that the interaction of the scanned probe and the surface did not lead to a modification of the surface. This can be conveniently done by imaging both a previously scanned and previously unscanned area by zooming out. 

Fig. 3.38 Contact mode AFM height image of egg PC vesicles adsorbed on glass captured with an imaging force of 30 pN left) and 50 pN right). The halo in the fast scan direction right to left) indicates that the tip can no longer track the surface features accurately, when imaging force and noise of the deflection signal become comparable ( 30 pN in this case). When the imaging forces are increased to 50 pN, the surface is tracked better. The asymmetry of the features can be explained by tip convolution effects (asymmetry of the probe tip) [87]...

Compositional contrast, as well as modulus, can also be assessed by laterally resolved imaging of normal or lateral forces in the contact mode AFM. In the corresponding mapping [121] (see also Chap. 4), force-displacement curves are recorded for each pixel. Subsequently, the pull-off forces, as a measure for adhesion, and the indentation part of the loading curves, to extract/fit the elastic modulus, are evaluated for each pixel. In particular for adhesion mapping, the use of chemically functionalized AFM probe tips [122], has been shown to be a suitable approach to map chemical composition and functional group distributions down to the sub-50 nm scale [123]. The mapping of adhesion, friction, and surface mechanical properties will be treated in more detail in Chap. 4. 

Atomic force microscope (AFM). Sample solutions at 100 ng/ml or less were cemented onto mica and imaged in a model Nanoscope Ilia scanning probe microscope with TESP cantilevers (Veeco/Digital Instruments, Santa Barbara, CA) operated in the intermittent contact mode on an atomic force microscope. 

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