Force Measurements with the AFM

The jump to contact (2) and jump-out (5) instabilities have the consequence that not all parts of the interaction potential can be reconstructed from the measurement. Such instabilities can be avoided or suppressed by using stiffer cantilevers, but this would be at the cost of reduced sensitivity. To obtain a real force-versus-distance curve (called force curve ), and Zp have to be converted to force and distance. First, aline fit is done on the zero force region to subtract any offset in the detector signal. Then, a 

A critical point in the evaluation of AFM force curves is the determination of zero distance, meaning the point where the probe starts to touch the surface. While in the SFA, the analysis of the FECO pattern gives clear information on any gap remaining between the mica surfaces, there is no independent check for absolute distance in the AFM. This can lead to ambiguities either due to adsorbed layers or due to deformation of soft surfaces. One attempt to overcome titis limitation was the combination of AFM with TIRM [204] (see Section 3.4), where the scattering signal from a colloid probe was used for absolute determination of separation distance. 

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Soon after the invention of the AFM, it was reahzed that by taking force-versus-distance measurements, valuable information about the surfaces could be obtained [2, 3]. These measurements are usually known as force measurements. The technique of force measurements with the AFM is described in detail in the third chapter. Force measurements with the AFM were first driven by the need to reduce the total force between tip and sample in order to be able to image fragile, biological structures [4, 5], Therefore it was obligatory to understand the different components of the force. In addition, microscopists tried to understand the contrast mechanism of the AFM to interpret images correctly. Nowadays most force measurements are done by surface scientists, electrochemists, or colloidal chemists who are interested in surface forces per se. Excellent short [6] or comprehensive [7] reviews about surface force measurements with the AFM have appeared. Also an older review about surface force measurements in aqueous electrolyte exists [8], This overview focuses on electrostatic double-layer forces. 

Since the first measurements of the electrostatic double-layer force with the AFM not even 10 years ago, the instrument has become a versatile tool to measure surface forces in aqueous electrolyte. Force measurements with the AFM confirmed that with continuum theory based on the Poisson-Boltzmann equation and appKed by Debye, Hiickel, Gouy, and Chapman, the electrostatic double layer can be adequately described for distances larger than 1 to 5 nm. It is valid for all materials investigated so far without exception. It also holds for deformable interfaces such as the air-water interface and the interface between two immiscible liquids. Even the behavior at high surface potentials seems to be described by continuum theory, although some questions still have to be clarified. For close distances, often the hydration force between hydrophilic surfaces influences the interaction. Between hydrophobic surfaces with contact angles above 80°, often the hydrophobic attraction dominates the total force. 

The limits of the celebrated Derjaguin approximation for predicting forces between submicron-sized particles have been argued for some time. Now the approximation can be validated using the force data obtained for the interaction between the AFM tips on microfabri-cated cantilevers and the flat surfaces. The radius of curvature of the AFM tips is about 10 nm and provides the ideal geometry with small interaction forces. Fig. 5 shows an example for the forces measured with the graphite (HOPG) flat surfaces and the silicon nitride tips with the radius of curvature of about 7 nm in solution with different pH. 

The forces measured between the AFM tip and the PCMA coated mica surface across a 1 cmc (8.3 X 10 3 M) SDS solution are shown in Figure 12. At repulsive double-layer force dominates the long-range interaction, whereas two pronounced steps with a periodicity of 40 A are observed at small separations. We can thus conclude that the adsorbed layer is heterogeneous both parallel and perpendicular to the surface. Two other surface force techniques, the MASIF and the SFA, were used to explore this further. 

Adhesive forces were measured to analyze the interaction between Si atomic force microscope (AFM) tips and the deposited films from the force-distance measurement with the AFM. The force-distance curve is expected to elucidate the adsorption behavior of PAM on the deposited films. In the force-distance measurement, there is no interaction until the tip is close enough to be attracted to the surface. As the tip approaches to the surface, it contacts with the surface. After the contact, the tip is retracted from the surface, the cantilever is bent, and a repulsive force (positive) is measured. When the tip is being retracted, an attractive force is measured (negative). When the critical force is reached, the tip is separate from the surface and this point is called the pull-off point. Therefore, the pull-off point, which corresponds to the point of the critical force, is determined by the degree of the adhesive force between the tip and the surface. The higher the adhesive force between the tips and the films, the lower the pull-off point. Figure 6.14a shows the force-distance curves of the tips and poly Si film at pH 10 as a function of the PAM concentration. In the absence of the absorbed PAM molecule, an adhesive force was observed at approximately 20 nm of separation distance. It is of interest that there is no significant difference between the surface forces of the tip and poly Si film even with the presence of PAM. This result is almost the same for all samples, irrespective of the concentration, which means that PAM is scarcely adsorbed on poly Si film. 

General Extensions. - Bader applied ideas of AIM to the atomic force microscope (AFM). In a quantum system, the force exerted on the tip is the Ehrenfest force, a force that is balanced by the pressure exerted on every element of its surface, as determined by the quantum stress tensor. The surface separating the tip from the sample is an IAS. Thus the force measured in the AFM is exerted on a surface determined by the boundaries separating the atoms in the tip from those in the sample, and its response is a consequence of the atomic form of matter. This approach is contrasted with literature results that equate it to the Hellmann-Feynman forces exerted on the nuclei of the atoms in the tip. 

Butt H-J, Cappella B, Kappl M. Force measurements with the atomic force microscope Technique, interpretation and applications. Surface Science Reports. 2005 59(1-6) 1-152. Roduit C. "AFM figures", www.freesbi.ch,. Creative Commons Attribution. 2010. 

AFM force measurements can be done with a range of surfaces. Conventional AFM tips, however, should not be used for quantitative interfacial force measurements because quantitative comparison with theoretical predictions requires that the radius of the approaching probe be much greater than the separation distance. The latter point has often been overlooked in surface force measurements with standard AFM tips. The coUoid-probe AFM method > > is appropriate for quantitative surface force measurements. A colloidal (usually silica) microsphere attached to the end of the AFM cantilever provides a well-defined, mathematically tractable sphere-vs-flat geometry for the scaling of forces, and allows the use of different colloid materials, or the surface modification of colloid spheres, for investigating interactions between surfaces with various physicochemical properties. 

Noel O, Awada H et al (2006) Force curve measurements with the AFM application to the in situ determination of grafted silicon-wafer surface energies. J Adhes... 

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. 

Regarding the spatial aspects of the enzymatic degradation of CA-g-PLLA, a surface characterization [30] was carried out for melt-molded films by atomic force microscopy (AFM) and attenuated total-reflection Fourier-transform infrared spectroscopy (ATR-FTIR) before and after the hydrolysis test with proteinase K. As exemplified in Fig. 3 for a copolymer of MS = 22, the AFM study showed that hydrolysis for a few weeks caused a transformation of the original smooth surface of the test specimen (Fig. 3a) into a more undulated surface with a number of protuberances of 50-300 nm in height and less than a few micrometers in width (Fig. 3b). The ATR-FTIR measurements proved a selective release of lactyl units in the surface region of the hydrolyzed films, and the absorption intensity data monitored as a function of time was explicable in accordance with the AFM result. 

Figure 8.3 shows the typical force-distance curve for a K-carrageenan film. From the slope of the curve, where the AFM tip is in contact with the film surface. Young s modulus of the K-carrageenan film can be estimated by using the Hertz model with the proper measurement of the AFM tip radius using scanning electron microscopy (SEM) and estimated value of Poisson s ratio based on the characteristics of film surface, which is around 1.4 MPa. 

Post-CMP topography is commonly evaluated by profilometry and atomic force microscopy (AFM). Both techniques are suitable for dishing measurements with the latter having superior resolution. Additionally, when equipped with an electrical measurement system, AFM is capable of detecting excessive nitride erosion with nanometer resolution (Fig. 12.15) [30]. Except for process control, the detailed dishing and erosion AFM data can be successfully implemented for calibration of CMP simulation tools. 

The -propanol adsorption from the gas phase significantly decreases the adhesion force between the silicon oxide surfaces measured with AFM. Fig. 7 illustrates the adhesion force measured with a single tip (2.2 N/m) at various partial pressures of n-propanol. The adhesion force decreases 40% compared with the dry Ar case upon initial introduction of n-propanol partial pressure. This reduction is not as drastic upon further increase of the n-propanol partial pressure. A sudden change in adhesion with only 10% partial pressure indicates that a few monolayer thick n-propanol film, as shown in Fig. 4, is sufficient to reduce the adhesion between the silicon oxide surfaces. This behavior is in sharp contrast to the relationship between the adhesion force and the water adsorption isotherm. In the case of water, the adhesion force increases several fold when the relative humidity increases from zero to... 

Fig. 7 Adhesion force measured with AFM as a function of n-propanol partial pressure. The data are normalized to the dry Ar case, which is measured before the n-propanol exposure. Data are acquired with a single tip (2.2 N/m).

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