Tapping mode deflection

Since the first AFM applications, researchers have examined so-called force curves. In the contact mode, these are deflection-versus-distance (DvZ) curves, as seen in Figure 20.2a. Initially, DvZ curves were employed to check whether a particular deflection set point used for imaging corresponds to a net repulsive or net attractive force [25]. This curve can also be obtained in tapping mode... 

A third mode, which has recently become the standard for work on surfaces that are easily damaged, is in essence a hybrid between contact and non-contact modes, and is sometimes called the tapping mode. In this case the cantilever is brought into oscillation such that the tip just touches the surface at the maximum deflection towards the sample. When the oscillating cantilever approaches the maximum deflection, it starts to feel the surface and the oscillation becomes damped, which is... 

Figure 32 shows the continuous AFM deflection images of growing crystals in a (P[(S)-LA]) thin film with a thickness of about 70 nm obtained isother-mally at 165 °C using tapping-mode AFM. After 5 min, some concave areas are detected on the amorphous surface of the thin film (c.f., Fig. 32a). In the concave area, edge-on lamellae emerge and grow with time. Many S-shaped and bow-shaped structures form on the surface after 38 min, as shown in...

In AFM, a sharp scanned probe collects localized information. Typically, the proximal probe and the sample surface are in contact and the corresponding data are collected, while probe and sample are mutually displaced (Fig. 1.1). Among the various modes utilized, contact mode (CM) AFM and intermittent contact (tapping) mode are most prominent. In CM-AFM, the deflection of a cantilever to which the tip is attached is monitored, while in intermittent CM the tip—cantilever assembly is resonating and the amplitude of the cantilever is monitored to extract the surface profile. 

The prime differences among the different AFM modes, such as CM (discussed above) and intermittent CM, as elucidated in the following section, are the feedback parameters and the choice of the cantilever. For intermittent contact (tapping) mode AFM, a stiff cantilever (k typically 10—50 N/m) with a resonance frequency of 100—400 kHz is chosen. The cantilever, which is inserted in an identical manner as for CM into the cantilever holder, is excited to vibrate by an integrated piezo actuator. Instead of deflection (contact force), the amplitude of the forced oscillating lever is detected, analyzed, and utilized in the feedback loop (Fig. 2.20). 

The movement of a tip which is made of ceramic or some other conductor material moves on the smface. AAfhen the tip positioned at the end of cantilever it is attracted to or pushed away from the samples surface, it deflects the cantilever beam - and a laser measures the deflection. The AFM then produces a visible profile of the little hills and valleys that make up the samples surface more informative for studying a three-dimensional image. There are three different type of scan modes 1. contact mode, 2. non-contact mode and finally 3. tapping mode. 

Static deflection AFM can be used to measure local mechanical properties of polymer surfaces, but only after consideration of the relative stiffness of the cantilever and the surface under study. Cantilevers with stiffness in excess of 50 N/m are necessary to indent materials with a bulk modulus in excess of 1 GPa (10 N/m ). Soft levers with a spring constant less than 1 N/m are sufficient to indent elastomers. Conventional staining techniques used in electron microscopy provide a viable way to harden unsaturated, hydrocarbon elastomers for imaging with soft cantilevers. Alternatively, low bulk modulus polymers (E < 1 MPa or 10 N/m ) require resonant imaging techniques such as Tapping Mode for direct imaging. 

AFM generally operates in three different modes contact, noncontact, and tapping mode. In the contact (also called static) mode, the tip is in contact with the sample surface. The force between the tip and the surface is kept constant during scanning by maintaining a constant deflection, which is used as the feedback signal. 

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