Intermittent Contact AFM

The mean tip height is a little less than the amplitude of its oscillatory motion, so the tip comes into brief intermittent contact with the sample once every cycle. At 150 kHz, each oscillation takes 6.7 /zs. For a 1 Hz line scan rate with 512 points per line, each location on the surface experiences roughly 300 of these taps. The cantilever oscillation is affected by this interaction with the surface, and the signal for the feedback loop in regular IC-AFM is the amplitude of oscillation of the cantilever (Fig. 2.9 and Fig. 3.29). Because the cantilever is relatively stiff, and the mean distance from tip to sample surface is tens of nanometers, the mean static deflection of the cantilever is negligible. 

The amplitude of the oscillation of the cantilever (with no influence from the specimen surface) depends on the cantilever material, its geometry, the damping characteristics of the medium and the magnitude of the drive signal. The material and geometry of the cantilever control the resonant frequency, a o (Eq. 3.17), and the material and its surrounding medium control the quality factor Q (Eq. 3.18). 

The oscillation can be described as a driven, damped, harmonic oscillator [101], and the amplitude. A, and phase, are given by the following equations, where a is the drive amplitude and CO is the frequency of oscillation  

Note that at resonance, when drive frequency is equal to resonant frequency (co = (0, ), the resonant amplitude, and phase, 0o, are given by simple relations  

At resonance, the amplitude increases linearly with drive amplitude and quality factor. 

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Fig. 3.44 Intermittent contact AFM of two virions in linear arrangement on graphite (reproduced with permission from [96]). The measured height of 18 nm corresponds to the true diameter, while the width (ca. 85 nm (fwhm)) is attributed to the AFM tip shape. Reproduced with permission from [96]. Copyright 2004. American Chemical Society...

The phase angle shift can be used to obtain contrast due to local differences in energy dissipation as a consequence of different surface characteristics related to materials properties. These different properties allow one to differentiate materials with different adhesion [110] or widely different Young s moduli, if these differences are related to differences in energy dissipation [111-115]. Hence the amorphous and crystalline phases in semicrystalline polymers can be clearly differentiated, as discussed in Sect. 3.2, as well as different phases in polymer blends or filled systems (see below). As an example, we show in Fig. 3.52 an intermittent contact AFM phase image of a block copolymer thin film on silicon [116]. 

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...

Intermittent contact AFM is similar to the noncontact AFM mode except that the vibrating cantilever is brought closer to the sample so that at the maximum amplitude it makes slight contact with the surface (i.e., taps the surface). The intermittent contact region is shown on Fig. 2. As with noncontact measurement, the change in cantilever oscillation amplitude is in response to variation in tip-to-sample spacing. 

Imaging In-Plane Anisotropy with Intermittent Contact AFM... 

For information on properties of cantilevers, the second edition of Sarid s book is an excellent choice [102]. Garcia and Perez [103] have reviewed the theory behind dynamic imaging modes of intermittent contact AFM (IC-AFM) [104] and noncontact AFM (NC-AFM) [105]. The influence of the tip shape in imaging has been reviewed by Villarrubia [106]. The application of SPM to polymer ultrastructure and crystallinity has been reviewed by many including the work of Reneker et al. [107], Lotz et al. [108,109], Hobbs et al. [110], and Magonov and Yerina [111]. Some of the ultrastructural studies have been conducted using hot/cold stages to study in situ crystallization [112-114]. 

Figure 5.35. Intermittent contact AFM image of 35/j.m via holes patterned in a photoBCB thin film over a copper substrate before plasma treatment to remove polymer residues (top) and after treatment (bottom). (From Meyers et al. [174], (2000) American Chemical Society used with permission.)...

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