Cantilever holders

Figure 2,33 Schematic representation of an AFM electrochemical cell and its mode of operation. (I) photodiode, (2) electrolyte solution inlet/outlet, (3) spring clip, (4) cantilever holder, (5) glass cell body, (6) O ring, (7) sample, (8) r, v, z translator, (9) mirror and (10) tip. After Manne et at.

The surface deformation could be reduced even further with intermittent contact SFM. Tapping mode imaging in liquids has been described by several groups [185-191]. The main focus has been put on biological systems such as DNA, cells, chromosomes and proteins. However, it turned out to be rather tricky to perform the measurements. The resonant frequencies were usually 2-5 times lower than in air and the resonant peak gets strongly dampened and broadened [ 192,193]. Because of acoustic excitation of the cantilever holder and the body of fluid, the spectrum can be superimposed by other resonance s which not sensitive to the surface approach and cannot be used for the feedback control [185]. 

Better control of the cantilever oscillation in liquid environment can be achieved when the cantilever is oscillated directly by an external force. This idea was implemented by the so-called Magnetic-Alternative-Current Mode (MAC Mode) [194]. A magnetic cantilever is driven by an external magnetic field which is generated by a solenoid placed beneath the sample. The direct excitation of the cantilever avoids unwanted resonance s from the cantilever holder, the fluid body, and the sample itself. Furthermore, the improved signal-to-noise ratio allows smaller oscillation amplitudes and set point ratios Asp/Af closer to 1. Both factors result in a significant reduction in the energy deposited into the sample,... 

The cantilevers chip was mounted in a standard cantilever holder and was coated with 27 nm thick Cr to ensure good electrical conductivity. Aligning the probe array parallel to the sample surface was achieved using the internal laser signal feedback control system of the... 

Fig. 2.1 Photographs of the essential components of a sample scanning AFM (a) scanner base, (b) cantilever holder, (c) optical head, and (d) scanner of a typical scanned sample AFM...

Mounting of the optical head/cantilever holder assembly to the scanner... 

Fig. 2.2 Stepwise mounting of the cantilever into the cantilever holder (details see text)...

To open the brass clip of the cantilever holder, the holder is pressed against the table. Then the chip is slid carefully under the clip and the load is released. One should gently push the chip to the end of the mold, preferably in plane-parallel contact with the sidewall, to avoid possible movement at a later stage. 

Fig. 2.5 Essential steps of mounting the optical head (a) Mounting of the optical head and securing of the springs (b) optical head mounted on the scanner (c) insertion of cantilever holder into optical head (d) fixation of cantilever holder...

Fig. 2.6 (a) Insertion of cantilever holder (b) premounted cantilever holder (the holder has been fixed by tightening the corresponding screws, compare Fig. 2.5d) (c) mounting of cantilever holder/optical head assembly onto the scanner... 

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). 

Standard CM-AFM set-up A CM cantilever with a spring constant ofca. 0.3 N/m, or preferably smaller, is inserted into the cantilever holder, the laser is aligned, and the photodiode is moved such that the laser spot is centered. The alignment of the laser must be carried out carefully in order to ensure that the spot is located very close to the end of the cantilever. This can be verified by studying the shape of the reflected laser beam using a piece of paper (Chap. 2). 

The typical design of a liquid cell for an AFM with stationary cantilever-tip assembly is shown in Fig. 3.34. A transparent holder made, e.g., of quartz or PMMA replaces the conventional cantilever holder. The liquid fills the indicated volume between the holder and the sample surface, thus, the entire cantilever-tip assembly, including the cantilever chip, is immersed in the liquid. There are two ways of operation First, it is possible to seal off the mentioned volume by means of a flexible rubber ring (Fig. 3.34) in many cases, it is possible for, e.g., aqueous solution to work without the rubber rings (Fig. 3.37) [82-84],... 

For the FMM AFM experiments, rectangular Si cantilevers (225 pm long, 30 pm wide, approx, force constant of 0.4 to 1 N/m) are inserted into the FMM cantilever holder. The cantilever is tuned as described in the manual of the AFM. Typical excitation frequencies for these cantilevers using the integrated bimorph are 11 kHz. Subsequently, the AFM is adjusted according to the standard contact mode AFM protocol (see Sect. 3.2.2). 

We use a standard tapping/intermittent contact mode set-up, which is assembled as described in Sect. 3.2.1. After inserting the TM cantilever into the cantilever holder, the optical head is mounted, the laser is aligned and the resonance peak is analyzed. The operation frequency and amplitude are adjusted to v at 0.85 Ac and 100 nm, respectively. Prior to engaging, the phase signal is zeroed. The crude and fine engagement procedures are carried out as described in Chap. 2 and Sect. 3.2. After the successful engagement the operation point must be carefully adjusted to maximize the contrast between the EPR and PP phases present (compare... 

The cantilevers are set into vibration by a piezoelectric cantilever holder driven by an external alternating current (ac) voltage. The cantilevers, however, also resonate in response to ambient conditions such as room temperature or acoustic noise without requiring any external power. Since our detection concept was based on cantilever bending mode, we did not excite the cantilever. However, the Brownian motion frequency was determined using a spectrum analyzer as a diagnostic tool for cantilever integrity. 

The cantilever excitation in liquid is more complicated than in air or vacuum. As described before, the cantilever vibration is t3q)ically excited by the piezo actuator integrated in the cantilever holder. 

Recently, another approach has been proposed by Asakawa et al Instead of using an acoustic wave, they used the deformation of a flexure hinge prepared in the cantilever holder to drive a cantilever. To illustrate the effect of the flexure drive mechanism, they made two cantilever holders with the same structure but made of different materials. One of them is made of SS316, and the other is made of polyetheretherketone (PEEK). The former is too hard to be deformed by the impulsive force generated by the vibration of the piezo actuator, while the latter is compliant enough to be deformed by that. This difference is illustrated in the simulation results obtained by the finite element method (Fig. 18.3a,b). 

Figure 18.3 The 2D distribution of vibration amplitude in the simplifled model of the flexure hinge made of (a) SS316 and (b] PEEK simulated by the finite element method. Amplitude and phase curves obtained in water by the cantilever holder with the holder body made of (c, e] SS316 and (d, f] PEEK. The cantilever was driven with an excitation signal having an amplitude of 50, 100, or 150 mV. Note that the measured phase curves in (e] and (f] appear as a single curve due to their small dependence on the excitation amplitude. Abbreviation-. 2D, two dimensional.

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