Intermittent contact mode

The dynamic imaging mode can be further classified into two subcategories intermittent contact mode (also known as tapping mode) and noncontact mode. In both techniques, the AFM tip is attached to the end of an oscillating cantilever. For the intermittent contact technique, the cantilever is vibrated near its resonance frequency. The amplitude of the oscillation is typically 100-200 nm with the tip intermittently contacting... 

There are several other operation modes, such as the intermittent contact mode, (Fig. 6c) where the cantilever oscillates and the tip records the repulsive contact with the surface at the lowest point of the oscillation, and the force modulation mode (Fig. 6d) where the tip does not leave the surface at all during the oscillation cycle. 

Near-field scanning techniques are relative newcomers, and the basis for their interpretation is less well established. However, AFM has opened up new perspectives for morphological studies, particularly given that excessive surface damage in soft specimens can be avoided by use of non-contact or intermittent contact modes. Its sensitivity to surface topography nevertheless makes AFM prone to artefacts when used to observe surfaces prepared by microtoming, and its effective depth of field is limited compared with SEM. On the other hand, if lamellar surfaces can be prepared such that the surface relief (or hardness, friction variations) is representative of the bulk texture, very striking detail can be recorded at the nanometre scale in deformed polyolefins [11]. 

A tapping mode (also called intermittent contact mode) it is a non linear resonance mode. In this case, the oscillation amplitude is larger and the mean position of the tip is closer to the surface. The tip almost touches the surface at each oscillation. In this mode, friction can be avoided as well as the sample deformation and wear. Adhesion is also avoided thanks to the extremely short time of contact . The height of the sample is generally controlled so that the oscillation amplitude remains constant. The phase shift of the oscillation is then characteristic of the system dissipation, which is very useful for characterizing viscoelastic materials. 

This mode is an intermittent contact mode in which the cantilever oscillates and brings the tip to lightly touch the surface to provide high resolution and then lifts the tip off the surface to avoid surface damage as illustrated in Figure 5.14. As the oscillating cantilever begins... 

Fig. 2.49 Overview on artefacts in TM-AFM (a) TM AFM height image of a scan during which the tip temporarily lost contact with the sample surface (setpoint free oscillation amplitude) panel (b) displays the bistability effect in intermittent contact mode AFM...

The TM-AFM set up and procedure do not differ from the details discussed above in hands-on example 1 (Standard tapping intermittent contact mode set-up). Care must be taken to operate the AFM with a, minimum free amplitude (A0) in order to minimize the flattening of the observed macromolecules due to the interaction with the probe tip. 

Amorphous polymers can be imaged by AFM in both the glassy state and in the melt. Owing to the occurrence of plastic deformation, CM-AFM is inferior to intermittent contact mode measurements. Using the latter approach, polymers can be imaged in the molten state. 

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. 3.33 Intermittent contact mode AFM height images acquired in air of purified K-carrageenan (left) and r-carrageenan (right) diluted in water and prepared by aerosol spray deposition (scale bars 200 nm). Reproduced with permission from [78]. Copyright 1999. Elsevier...

As mentioned, in AFM studies of biopolymers the use of a suitable liquid cell is indispensable in many cases. On the one hand, biopolymers or even living cells may be studied in vitro under natural conditions (pH, temperature, salt, etc.) and variations of these conditions is often possible during the experiment [71-74], on the other hand excessive normal and lateral forces can be reduced to a minimum, which still allows one to image and study the biopolymeric samples non-invasively [79-81], Hence we will first provide an introduction to the use of the mentioned liquid cells and then treat contact mode AFM and intermittent contact mode AFM operation under liquid. The procedures and operation principles discussed can of course be readily extended to the study of non-biological polymers (see e.g. Chap. 4). 

For CM-AFM, soft cantilevers with spring constants between 0.02 N/m and 0.3 N/m are frequently used in liquids, while for intermittent contact modes various possibilities exist. Many researchers have successfully employed contact mode cantilevers (triangular and preferably single beam) with a spring constant between... 

The standard TM-AFM procedure discussed in Sect. 3.2 is used (Standard tapping intermittent contact mode set-up). In order to obtain high-resolution images in air, high-aspect-ratio Si-tips with a radius of 10 nm or less is advised (resonance frequency 300-350 kHz). The engagement procedure is carried out as described in Sect. 3.2. Similar to all other experiments the scan size is increased stepwise, while the gains are optimized as described. 

Fig. 3.52 Left (a) Schematic of intermittent contact mode AFM and phase imaging right (b) intermittent contact AFM phase image of a 30 nm thin block copolymer films on silicon [(poly(isoprene)-b-poly(ferrocenyl dimethylsilane), 29 kg/mol/15 kg/mol], which displays a in-plane worm-like surface pattern of poly(ferrocenyl dimethylsilane) in a matrix of poly (isoprene). From the 2D FFT analysis (inset) an average repeat period of 33 nm was estimated. Reprinted with permission from [116]. Copyright 2000. American Chemical Society...

We use a standard tapping/intermittent contact mode set-up, which is started as described in Sect. 3.2.1. The procedures used for imaging are identical to those mentioned for hands-on example 24 except for a, lower amplitude A0. 

Intermittent contact mode phase imaging is, similar to force modulation and pulsed, force mode, sensitive to differences in materials properties. In Fig. 3.67 (a), the stiffness difference between glass and a polymer-based matrix gives rise to excellent image contrast. In addition, the different components of the polymer blend can be recognized (compare schematic in Fig. 3.67 (b)). 

Fig. 3.71 Intermittent contact mode AFM height of (a) an un-stretched blown Him of hexene linear low density polyethylene and (b) a film that was placed under stress (in the vertical direction) leading to an extension of 250%. Image dimensions are 20 x 20 pm2. Reprinted with permission of John Wiley Sons, Inc. from [149]. Copyright 2002. John Wiley Sons, Inc.

We use a standard tapping/intermittent contact mode set-up, which is assembled as described in Sect. 3.2.1. After inserting the TM cantilever into the cantilever holder, the optical head is mounted, the laser is aligned and the resonance peak is analyzed. The operation frequency and amplitude are adjusted to v at 0.85 Ac and 100 nm, respectively. Prior to engaging, the phase signal is zeroed. The crude and fine engagement procedures are carried out as described in Chap. 2 and Sect. 3.2. After the successful engagement the operation point must be carefully adjusted to maximize the contrast between the EPR and PP phases present (compare... 

In the intermittent contact mode phase images shown in Fig. 3.74 a clear contrast between the constituent phases can be observed both for the compression-molded hmsPP (panel a), as well as for the blown film (in the machine and transverse direction in panels band c, respectively). The soft EPR phase appears with dark phase contrast indicating that a substantial amount of energy is dissipated in this phase compared to the PP phase. The EPR particles measure 1 pm or smaller in diameter dispersed in the polypropylene matrix. The horizontal lines and the occasional vertical streaks in the images are artifacts of microtoming. 

Fig. 3.74 Intermittent contact mode AFM phase images obtained with an amplitude setpoint ratio of 0.5 on (a) compression molded high melt strength polypropylene and microtomed sections of hmsPP blown film in the (b) machine direction (MD), and (c) the transverse direction (TD). Reproduced with permission from [152]. Copyright 2002. Elsevier...

Fig. 3.76 Intermittent contact mode AFM phase images of (a) undeformed and (b-d) uniaxially deformed film of SBS. Reprinted with permission from [156]. Copyright 2002. Wiley-VCH...

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