Intermittent contact mode AFM

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...

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). 

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...

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.

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...

In addition to indentation curves, the residual imprints after the application of a defined load force can be images using intermittent contact mode AFM. This is shown below for indentation tests performed on PE, in which the size of the imprint was shown to be reduced to the level of individual lamellae (Fig. 4.23). 

Basdogan C, Varol A, Gunev I, Onm B. Numerical simulation of nano scanning in intermittent-contact mode AFM under Q control. Nanotechnology. 2008 19(7). 

Differences in, for instance, modulus can give rise to excellent materials contrast in force modulation mode AFM or intermittent contact mode AFM phase imaging [144-146]. However, since this imaging contrast is not directly related to exposed functional groups and rather depends only on mechanical properties (including different indentation depths or energy dissipation), these and related approaches will not be considered here as chemically sensitive imaging [2,147]. 

Figure 2.9. Schematic of the intermittent contact mode AFM free oscillation with free amplitude Ao far away from sample surface, and damped oscillation with set-point amplitude A,p and phase shift AO during scanning. Asp is chosen by the operator, and feedback control is used to adjust tip-sample distance such that Asp remains at constant value. The choice of Aq and Asp has great influence on tip-sample force interaction and image formation.

In contact mode AFM, k might range from 0.01 to 1.0 N/m for intermittent contact mode AFM (IC-AFM), often called tapping mode AFM (TMAFM), it ranges from 3 N/m to 500N/m. 

Figure 4.11. Intermittent contact mode AFM images of the block face of high impact polystyrene (HIPS), obtained using cryomicrotomy at -110°C. Note that the surface imperfections due to knife marks are obvious in the height image (A), whereas the phase image (B) provides contrast due to the material with less visible artifacts [133]. Comparison with a TEM image (C) of a thin section obtained by room temperature microtomy of a block face exposed to OSO4 vapor shows detail of the HIPS structure and evidence of some compression in the section. (From Li et al. [133], (2004) Taylor Francis used with permission.)...

Figure 5.55. Banded spherulites are observed in TEM of perman-ganically etched, high density copolymer exhibiting a mature banded structure (A). Intermittent contact mode AFM images of the same polymer are shown in a normal height image (B) and in a three dimensional height image (C). (From Janimak et al. [271], (2001) Elsevier used with permission.)...

Figure 5.56. Intermittent contact mode AFM height images of a microtomed surface of a commercial LDPE pellet, before and after permanganate etching (A and B, respectively), and in a phase image (C) of the spherulite. Size of images are 20/tm on a side. (From Chemoff and Magonov [84], (2003) American Chemical Society used with permission.)...

Figure 5.92. Reflected light image of a defect in cable insulation. The defect appears rougher in texture than the surrounding matrix (dotted outline) (A). Reflected light image of the thick section shown in (A) following three local thermal analysis cluster measurements using SThM (B). Intermittent contact mode AFM phase images of (C) the cross linked PE/EBA matrix and (D) the defect shown in (A). The defect is devoid of discrete EBA phases. The thermomechanical response of the heated probe in contact with the defect (solid curves) and matrix (dashed curves of [A]) are seen to melt at different temperatures (E). (From Bar and Meyers [170] used with permission of the MRS Bulletin.)...

Figure 5.67. Intermittent contact mode AFM height and phase images of CET-GRC (5 wt.% rubber, 0.1/an diameter) after cryo-polishing (images are 1 x 0.3/an). The table at the right indicates the parameters used to generate each pair of images. (From Meyers et al. [174], (2000) American Chemical Society used with permission.)...

Figure 6.6 (a) Intermittent contact mode AFM height image (top) and cross-sectional analysis along the horizontal line (bottom) of... 

Figure 6.10 (a) Low-resolution intermittent contact mode AFM height image of plasmid DNA on HOPG obtained in ambient conditions. 

Figure 6.18 Intermittent contact mode AFM height images of thin films of PS204g-f>-PtBAjj52 prior to hydrolysis and after hydrolysis in trifluoro acetic acid, respectively. Courtesy D. Tranchida, H. Schdnherr, unpublished data.

I Nanofibers The first example is a study on the organization of lamellae in electrospun fibers of a semicrystalline polymer. Poly(f-caprolactone) has been spun into submicron fibers by electrospinning [30,31]. The fiber morphology can be imaged successfully by intermittent contact mode AFM, as shown in Figure 6.19a, where a rather dense web of electrospun poly( -caprolactone) fibers can... 

Figure 6.19 Intermittent contact mode AFM phase images of electrospun poly(e-caprolactone) fibers. The white arrows highlight parts of the fiber that could not be imaged due to sidewall contact of the tip. Reproduced with permission from [32]. Copyright 2014. The Royal Society of Chemistry.

Figure 6.20 (a) Schematic of polymersome formation of poiy(styrene)-biock-poly(acrylic acid) and intermittent contact mode AFM height imageof (b) PS,5-f>-PAA,5 vesicles and (c) PEG,4-fr-PLA, vesicles deposited on glass snbsirate. Conrtesy D. Wesner, S. Handschuh-Wang, H. Schonherr, unpublished data. 

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