AFM, contact mode

For any AFM experiment, and also the interpretation of AFM data, an appreciation of the underlying fundamental AFM techniques and their opportunities and limitations is required. This statement refers in the context of this chapter to the rudimentary imaging meehanisms involved. More detailed treatments on specific aspects, including the underlying physics, have been published and the reader is referred to these reviews [13], handbooks [6], and the exaet specifications of the AFM manufacturers. 

In contact mode AFM, which is the originally reported AFM mode [1] and may serve as the general example of a typical AFM concerning its features, a sharp nanoprobe tip, mounted to a flexible cantilever (typically made of Si3N4 by chemical vapor deposition or Si by silicon etching microfabrication technology) is brought into contact with the sample surfaee. The sample (or, likewise, the tip) can be positioned in all three directions independently by appropriate transdueers. The tip, characterized by its radius of curvature Rc, may be as sharp as several nanometers typical values are 7 5-25 nm. 

IMAGING POLYMER MORPHOLOGY USING ATOMIC FORCE MICROSCOPY 

The cantilever has a spring constant of approximately 0.05-1.0 N/m and deflects up or down, if repulsive or attractive forces, respectively, are experienced by the tip. The lever behaves as a Hookean spring and by measuring the cantilever deflection (equal to its vertical displacement Az), for example, exploiting the widely used optical beam deflection technique, tip-sample interaction forces F F=k Az) can be measured (Figs. 6.2 and 6.3). 

1 Force-Displacement Curve In the so-called force-displacement experiment, the sample is brought periodically 

The previous sections showed that for an ideally rigid substrate, the operation and interpretation of contact mode AFM is straightforward. The feedback set point (see Fig. 3.22) can be chosen to correspond to any net force that keeps the tip in contact with the siurface (see Fig. 3.27). As long as the feedback system is sufficiently sensi- 

For a real surface to appear rigid, the forces on it must be small. Using a cantilever with a small spring constant k keeps the forces small. A deflection signal in Fig. 3.27 A corresponds to a smaller force in Fig. 3.27B when k is small. Contact mode cantilevers have A 1 N/m, typically 0.2 N/m. 

It would appear that small local forces can be ensured by adjusting the feedback set point to correspond to zero net force or to some point in the attractive region with net negative force. However, operation in the attractive region is usually too unstable a situation. If there is a sudden dip in the surface, the cantilever deflection increases to the point where the tip jumps out of contact before the feedback loop can correct it, and that is the end of that scan. As was pointed out above, zero net force can be a combination of strong repulsive forces at the contact point and attractive forces from the surrounding region. 

In contact mode AFM, where the normal force is the detected signal, there can also be a large contribution from lateral or shear forces. These can be significant enough so as to lead to deformation, abrasion and wear. These are generally not desired, however, this behavior has been used to probe the role of chain entanglements in polystyrene films as a function of molecular weight [119]. 

In lateral or frictional force microscopy (FFM) [120,121], lateral deflection of the cantilever is the signal of interest. The single-beam cantilever is scanned in a direction perpendicular to its long axis to enhance the twisting motion. The top to bottom signal remains under feedback control to keep the normal force constant while the right to left signal is monitored (see Fig. 3.23). 

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To minimize effects of friction and other lateral forces in the topography measurements in contact-modes AFMs and to measure topography of the soft surface, AFMs can be operated in so-called tapping mode [53,54]. It is also referred to as intermittent-contact or the more general term Dynamic Force Mode" (DFM). A stiff cantilever is oscillated closer to the sample than in the noncontact mode. Part of the oscillation extends into the repulsive regime, so the tip intermittently touches or taps" the surface. Very stiff cantilevers are typically used, as tips can get stuck" in the water contamination layer. The advantage of tapping the surface is improved lateral resolution on soft samples. Lateral forces... 

Figure 4.7. Contact-mode AFM surface and line-profile scans of a 148-nm A1PO film on Si after a 1000 °C anneal for 5 min. [Reproduced with permission. Meyers, S. T. Anderson, J. T. Hong, D. Hung, C. M. Wager, J. F. Keszler, D. A. 2007. Solution processed aluminum oxide phosphate thin-film dielectrics. Chem. Mater. 19 4023 1029. Copyright 2007 American Chemical Society.]...

Figure 5.36 Pseudo-three-dimensional perspective of right-handed (top) and left-handed (bottom) C4 phosphonate (42) helices obtained with underwater contact-mode AFM. Reprinted with permission from Ref. 145. Copyright 1998 by the American Chemical Society.

Figure 15. Contact mode AFM height images of PE foils before (HDPE and LDPE) and after 400 s modification in Ar plasma (HDPE/400, LDPE/400). [70],...

Contact Mode AFM is the so-called traditional mode of AFM. Topography contours of solids can be obtained in air and fluids. 

In the contact mode the tip scans the sample in close contact with the surface. The force on the tip is repulsive with a mean value of 10 N. This force is set by pushing the cantilever against the sample surface with a piezoelectric positioning element. In contact mode AFM the deflection of the cantilever is sensed and compared in a DC feedback amplifier to some desired value of deflection. If the measured deflection is different from the desired value, the feedback amplifier applies a voltage to the piezo to raise or lower the sample relative to the cantilever in order to restore the desired value of deflection. The voltage that the feedback amplifier applies to the piezo is a measure of the height of features on the sample surface. It is displayed as a function of the lateral position of the sample. 

Pioneering contact mode AFM studies by Meiners et ai. (1995) show that chemical sensitivity at the surface of thin polymer films can be achieved by measuring the microscopic friction and stiffness for glassy block copolymers. This provides invaluable information to complement topography on the nature of the block at the surface. 

FIGURE 3.16 Example of high-resolution AFM imaging of a biological surface. Contact mode AFM image of Aquaporin-Z membrane protein crystals showing their surface structure with <1 nm resolution. Scale bar, 10 nm. 

Kuwahara, Y. (1999) Muscovite surface structure imaged by fluid contact mode AFM. Physics and Chemistry of Minerals, 26, 198-205. 

FIGURE 5. (a) Contact-mode AFM image showing the boundary of Au film on a mica substrate and (b) the z-profile. Reproduced from ref 24. Copyright 2005 American Chemical Society. 

Fig. 4 The effect of the AFM tip on the nucleation of PEO crystallization in contact-mode AFM scanning. The micrograph exhibits a defection contact-mode image [79]...

Fig. 68 Binding of ferrocenes to SAM siufaces via quadrupolar hydrogen bonds (ure-idopyrimidines). The contact-mode AFM images reveal the enhanced thickness of the layer due to the adsorption process. Reprinted with permission from [236]...

The most common operating modes of AFM are contact, noncontact, and tapping, which are self-explanatory in their manner of interrogation of the surface. In contact-mode AFM, there is a repulsive force between the sample and tip (ca. 10 N) the piezoelectric response of the cantilever either raises or lowers the tip to maintain a constant force. Similarly as STM, the best resolution will be obtained under UHV conditions. That is, in an ambient environment, adsorbed... 

Fig. 13 Non-contact mode AFM images of SLNs at scan ranges of (A) 50 pm (B) 25 pm and (C) 5 pm. (With permission from Ref. . )...

Due to the finite force of contact, a contact-mode AFM (C-AFM) can mechanically modify a surface, particularly a soft surface. This leads to creation of a pattern on the substrate. However, in this review we do not discuss mechanical nano-patterning. 

The anodic oxide pattern of 100 nm dimension grown by SPM-based techniques can be used as a mask that can be transferred to a substrate below. An anodized alumina mask (produced by contact mode AFM on sputtered A1 film) has been used to transfer pattern on Si by using a combination of wet and reactive ion etching. Anodic oxidation creates a protruding oxide pattern that can be etched. The pattern can form a positive or negative mask depending on what is etched, the aluminum or the oxides. In Figure 21.14 we show an example of a nanopattern (an array of points) created by anodic oxidation on a surface of aluminum. 

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