Force modulation imaging

Force modulation imaging is the dynamic contact mode that identifies and maps differences in surface stiffness or elasticity. These techniques use a variety of surface properties to differentiate among materials where topographical differences are small or not measurable. 

Figure 5.21 Cross-section of carbon fibers in an epoxy matrix (a) topographic image and (b) force modulation image. The image width is 32 //m. (Reproduced with permission from R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge University Press, Cambridge, UK. 1994 Cambridge University Press.)...

Figure 8 Height mode topographical (A) and force modulation (B) images of colloidal hydrogel nanoparticles immobilized on a silica substrate. Scan domain is 12 pm x 12 pm. Note that in the topographical image darker areas represent lower regions, whereas in the force modulation image the darker areas represent greater surface stiffness.

Figure 5.12 Phase contrast and force modulation images of a Matrimid membrane 14/10 (14% of Matrimid, 10% of (i-BuOH, in a 50 50 mixture of THF/CBL) evaporated during 40s.

A natural extension of contact mode imaging is to operate in a mode sensitive to material properties, force modulation imaging [131]. The setup for this is similar to contact mode, with three differences ... 

Force modulation imaging requires careful identification of the appropriate contact resonance. The frequency spectrum of the combined piezo stack and cantilever is generally contaminated with system resonances. It is important to identify the fundamental resonance that will attenuate on contact with the surface. In force modulation imaging, the tip is still subjected to lateral forces and this can cause problems. 

Maivald, P, Butt, H.J., Gould, S.A., Prater, C.B., Drake, B., Gurley, J.A., Elings, VB. and Hansma, P.K.. Using force modulation to image surface elasticities with the atomic force microscope. Nanotechnology, 2, 103-106 (1991). 

Asif, S.A.S., Wahl, K.J., Colton, R.J. and Warren, 0.1.., Quantitative imaging of nanoscale mechanical properties using hybrid nanoindentation and force modulation. J. Appl. Phys., 90(3), 1192-1200 (2001). 

Fig. 7 (a) Catechol derivatized tetracenes self-assemble on metal oxide surfaces such as aluminum oxide, (b) Schematic and (c) scanning electron micrographs of FET structures fabricated with a 5-nm aluminum oxide layer on top of a 5-nm thermally oxidized Si wafer to allow self-assembly of the derivatized tetracene between sub-100 nm Au source and drain electrodes, (d) /d-Eds characteristics of the assembled tetracene monolayer FET for a 40 nm channel length showing hole modulation and (inset) an atomic force microscope image of the FET channel... 

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. 44. The SFM amplitude (a) and force modulation (b) maps of a cryogenic faced impact copolymer (ICP) composed of a polypropylene (PP) matrix with high ethylene (60 wt. %) ethylene-propylene copolymer (EP). Crystalline polyethylene (PE) phases are seen in the EP domains, which are surrounded by the PP matrix. Modulus contrast in the force modulation (drive amplitude 100 mV) image associated with the three polymers the stiff PP matrix is dark, the soft EP domains are light. The crystalline PE regions have modulus between the PP and the EP,thus an intermediate shade of grey is observed for the PE domains [128]...

For pulsed force mode imaging of the PP sample, a modulation frequency in the range of 800 Hz is selected. The cantilever oscillations are adjusted such that the adhesive interactions between tip and surface are overcome, as monitored using an oscilloscope. The modulation must clearly show the snap-off of the tip. Next the four markers are set as shown in Fig. 3.22. 

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)). 

Sasaki, S., Monimoto, M., Haga, H., Kawabata, K., Ito, E., Ushiki, T., Abe, K., and Sambongi, T., Elastic properties of living fibroblasts as imaged using force modulation mode in atomic force microscopy. Arch. Histol. Cytol. 61, 57-63 (1998). 

FIGURE 6.1 The processed SPIP image of nanocomposite butadiene—styrene mbber/ nanoshungite, obtained by force modulation method, and mechanical characteristics of stmctural components. 

FIGURE 6.2 The images, obtained in the force modulation regime, for nanocomposites, filled with technical carbon (a) nanoshungite, (b) microshungite, and (c) corresponding to them fractal dimensions rff. ... 

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