Lateral force microscope

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). 

Complementary to the indentation experiments, the lateral force microscope has been introduced to measure lateral forces exerted on the tip [388]. Somewhat latter, different modifications of contact-mode SFM have been developed to investigate indentation and wear in thin films [ 115]. On the lateral force measurement, one has to distinguish between operation at very small lateral displacements of the cantilever when the shear forces dominate in the net force, and a... 

The atomic force microscope (AFM), (Aktary et al., 2001 Binnig et al. 1986 Warren et al., 1998) and the lateral force microscope (LFM), (Mate et al., 1987) are valuable tools for characterizing the forces involved between surfaces in contact, both lubricated and unlubricated. A particular strength of AFM/LFM is... 

Table 5.8. Comparison of some tribological parameters in nano- (lateral force microscope, (LFM)) and macro-scale...

The concept of the STM has initiated, besides the AFM, a huge family of scanning probe microscopes (SPM), including lateral force microscopes and scanning near field optical microscopes. All these instruments are built up in a very similar way and differ only in the detected interaction between the probe and the sample. We start our discussion with a summary of the common essential parts of all SPMs. 

Figure 5.12 Detection of distortion of the cantilever due to lateral force between tip and sample in the lateral force microscope. (Reproduced with permission from P.C. Braga and D. Ricci (eds), Atomic Force Microscopy, Humana Press. 2004 Humana Press.)...

The harmonic drive approach which is basic in rheological experiments was first implemented by Colchero et al. (25) for an AFM. The basic idea couples a lock-in technique with the lateral force microscope and has been essentially used for measuring the lateral contact stiffness (26,27). If the tip is modulated periodically in successive back and forth scans the lateral force signal is periodic too. The lock-in technique is particularly appropriate to analyze a signal in reference to another signal of the same frequency. 

Meyer G and Amer N M 1990 Simultaneous measurement of lateral and normal forces with an optical-beam-deflection atomic force microscope Appl. Phys. Lett. 57 2089... 

Mazeran, P.E. and Loubet, J.L., Normal and lateral modulation with a scanning force microscope, an analysis implication in quantitative elastic and friction imaging. Tribal. Lett., 7(4), 199-212(1999). 

A very similar technique is atomic force microscope (AFM) [38] where the force between the tip and the surface is measured. The interaction is usually much less localized and the lateral resolution with polymers is mostly of the order of 0.5 nm or worse. In some cases of polymer crystals atomic resolution is reported [39], The big advantage for polymers is, however, that non-conducting surfaces can be investigated. Chemical recognition by the use of specific tips is possible and by dynamic techniques a distinction between forces of different types (van der Waals, electrostatic, magnetic etc.) can be made. The resolution of AFM does not, at this moment, reach the atomic resolution of STM and, in particular, defects and localized structures on the atomic scale are difficult to see by AFM. The technique, however, will be developed further and one can expect a large potential for polymer applications. 

Mayer, G. and Amer, N. M., Simultaneous Measurement of Lateral and Normal Forces with an Optical-Beam-Deflection Atomic Force Microscope, AppZ. Phys. Lett., Vol. 57, 1990, pp. 2089-2091. 

Recent developments have allowed atomic force microscopic (AFM) studies to follow the course of spherulite development and the internal lamellar structures as the spherulite evolves [206-209]. The major steps in spherulite formation were followed by AFM for poly(bisphenol) A octane ether [210,211] and more recently, as seen in the example of Figure 12 for a propylene 1-hexene copolymer [212] with 20 mol% comonomer. Accommodation of significant content of 1-hexene in the lattice allows formation and propagation of sheaf-like lamellar structure in this copolymer. The onset of sheave formation is clearly discerned in the micrographs of Figure 12 after crystallization for 10 h. Branching and development of the sheave are shown at later times. The direct observation of sheave and spherulitic formation by AFM supports the major features that have been deduced from transmission electron and optical microscopy. The fibrous internal spherulite structure could be directly observed by AFM. 

Fig. 13.2. Addition of piezoelectric transducers to an atomic force microscope for acoustically excited probe microscopy. The forces acting between the tip and the sample are measured by the vertical and lateral deflections of the cantilever...

The atomic force microscope (AFM) and its closely allied device, the lateral... 

In atomic force microscopy (AFM), the sharp tip of a microscopic probe attached to a flexible cantilever is drawn across an uneven surface such as a membrane (Fig. 1). Electrostatic and van der Waals interactions between the tip and the sample produce a force that moves the probe up and down (in the z dimension) as it encounters hills and valleys in the sample. A laser beam reflected from the cantilever detects motions of as little as 1 A. In one type of atomic force microscope, the force on the probe is held constant (relative to a standard force, on the order of piconewtons) by a feedback circuit that causes the platform holding the sample to rise or fall to keep the force constant. A series of scans in the x and y dimensions (the plane of the membrane) yields a three-dimensional contour map of the surface with resolution near the atomic scale—0.1 nm in the vertical dimension, 0.5 to 1.0 nm in the lateral dimensions. The membrane rafts shown in Figure ll-20b were visualized by this technique. 

In 1987 Mate et al. [468] used, for the first time, an atomic force microscope (AFM) to measure friction forces on the nanometer scale (review Ref. [469]). This technique became known as friction force microscopy (FFM) or lateral force microscopy (LFM). To measure friction forces with the AFM, the fast scan direction of the sample is chosen perpendicular to the direction of the cantilever. Friction between the tip and the sample causes the flexible cantilever to twist (Fig. 11.7). This torsion of the cantilever is measured by using a reflected beam of light and a position-sensitive detector in the form of a quadrant arrangement of photodiodes. This new method made it possible for the first time to study friction and lubrication on the nanometer scale. 

Figure 11.7 Working principle of the lateral or friction force microscope (LFM or FFM).

Figure 11.9 Friction force microscope pictures (a, b) of a graphite(OOOl) surface as obtained experimentally with FFM and results of simulations (c, d) of the stick-slip friction using a two-dimensional equivalent of the Tomlinson model. The friction force parallel to the scan direction (a, c) and the lateral force perpendicular to the scan direction (b, d) are shown. The scan size is 20 Ax 20 A. Pictures taken from Ref. [481] with kind permission from R. Wiesendanger.

Fig. 20. Different surface morphologies illustrate limitations in the nanoscopic resolution of the scanning force microscope. a - two rigid spheres, b - two rigid spikes, c - two soft spheres. While Az is the lower limit for the dimple to be resolved by the tip of radius R, d corresponds to the lateral resolution of the SFM tip...

Complementary to the SFA experiments, SFM techniques enabled direct, non-destructive and non-contact measurement of forces which can be as small as 1 pN. Compared to other probes such as optical tweezers, surface force balance and osmotic stress [378-380], the scanning force microscope has an advantage due to its ability in local force measurements on heterogeneous and rough surfaces with excellent spatial resolution [381]. Thus, a force-distance dependence measured from a small surface area provides a microscopic basis for understanding the macroscopic interfacial properties. Furthermore, lateral mapping... 

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