Cantilever, mechanical deflection

The most crucial component of an AFM is the cantilever. The deflection should be sufficiently large for ultra low forces (0.1 nN). Therefore, the spring constant should be as low as possible (lower than 1 N/m). On the other hand, the resonance frequency of the cantilever must be high enough (10 to 100 kHz) to minimize the sensitivity to mechanical vibrations (e.g., vibrational noise from the building —100 Hz, frequency of the corrugation signal up to a few kHz). The... 

Direct visualization of nano- and micrometer-sized objects is the most straightforward way for their analysis in surface and polymer science, biomaterial research, and biology. Rapid progress in engineering and microtechnology has led to numerous techniques that allow observation and mechanical manipulation of microscopic objects of various natures. AFM [52] is the most commonly used of these techniques. In AFM, a sample surface is mechanically scanned with a tiny probe—a sharpened stylus fixed at the end of a flexible cantilever. When the stylus interacts with the samples, the resulting force acts on the stylus and causes deflection of the cantilever. This deflection is detected via an optical lever system, that is, a laser beam reflected from the end of the... 

As shown in Figure 1, a standard AFM tip has a sharpness (i.e., tip radius) of a few nanometers and is fabricated at the free end of a microcantilever. When the AFM tip is close to a surface or in mechanical contact, forces acting on the tip translate into a mechanical deflection of the cantilever, which is detected optically using a laser beam that reflects off the back side of the cantilever (Figure 2). A position-sensitive detector (PSD) in the form of a four-quadrant diode then converts the vertical and horizontal beam deflections into voltage signals, which are continuously recorded. 

The photoisomerization of azobenzene between the cis and irons forms is well known and gels containing attached azobenzene units have been shown to respond to UV illumination by stiffening [128]. Leucocyanides are photoresponsive dyes that convert between ionized and unionized forms on irradiation with light and cause osmotic swelling and contraction of a gel [129]. Marder and co-workers developed a hydrogel that responds to UV irradiation with a keto to enol tautomerization that results in mechanical deflection of a cantilever [130]. Nitrocinnamate chemistry has been used to create a hydrogel which can be reversibly photocross-linked and photocleaved [131]. 

Weihs, T. P., Hong, S., Bravman, J. G. and Nix, W. D. (1988), Mechanical deflection of cantilever microbeams a new technique for testing the mechanical properties of thin films. Journal of Materials Research 3, 931-942. 

Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

The AFM [7] uses a sharp tip mounted at the end of a flexible cantilever to probe a number of properties of the sample, including its topographical features and its mechanical characteristics. Interaction forces, both attractive and repulsive, between atoms on the AFM tip and atoms on the sample cause deflections of the flexible cantilever (for a detailed description of the interaction forces sensed by the AFM, see Ref. [8]). These deflections are registered by a laser beam reflected off of the back of the cantilever onto a photodiode position detector (Fig. la) ... 

The AFM has a number of elements common to STM the piezoelectrc scanner for actuating the raster scan and z positioning, the feedback electronics, vibration isolation system, coarse positioning mechanism, and the computer control system. The major difference is that the tunneling tip is replaced by a mechanical tip, and the detection of the minute tunneling current is replaced by the detection of the minute deflection of the cantilever. 

Many different methods have been developed for detecting the minute deflection of the cantilever (Sarid, 1991). In this. section, we present several important ones, including vacuum tunneling (Binnig, Quate, and Gerber, 1986), mechanical resonance (Diirig, Gimzewski, and Pohl 1986), optical interferometry (Martin et al., 1988 Erlandson et al., 1988), and optical beam deflection (Meyer and Amer, 1988). 

The second assumption is based on contact mechanics models in which viscoelastic effects that might influence the instability point (pull-off) and adhesion are negligible or can be allowed for. The third assumption is based on representing the cantilever with a point mass model. Simulations using a distributed mass model indicate that ultrasonic vibration of the cantilever is relatively small and in many cases less than 0.05 of the UFM normal deflection (Hirsekorn et al. 1997). 

From mechanical measurement of the surface tension If the surface tension of a solid changes, the surface tends to shrink (if T increases) or expand (if T decreases). This leads, for instance, to the deflection of bimetallic cantilevers or a contraction of ribbons. 

Fig. 2 Schematic representation of the basic detection elements of the scanning force microscope and of the piezoelectric transducers generating the displacement modulations for purposes of dynamic mechanical measurements. The dynamic components of the tip-sample forces resulting from the normal/lateral displacement modulations are detected via the torsion/bending of the microscopic cantilever and the deflection of the laser beam reflected off the rear side of the cantilever. The positional shift of the latter is registered by means of a segmented photo-diode...

The AFM operates in an analogous fashion. A sharp tip mounted on a weak cantilever is rastered across the sample. Light interference or reflection is used to measure the deflection of the cantilever. The light signal measured as a function of sample location is used to generate a map of the surface topography. Because mechanical forces are responsible for the deflection of the probe arm, conductors as well as insulators can be studied. STM and AFM provide local real space images of the surface and are thus... 

A new alternative to solve this problem is atomic force microscopy (AFM) which is an emerging surface characterization tool in a wide variety of materials science fields. The method is relatively easy and offers a subnanometer or atomic resolution with little sample preparation required. The basic principle involved is to utilize a cantilever with a spring constant weaker than the equivalent spring between atoms. This way the sharp tip of the cantilever, which is microfabricated from silicon, silicon oxide or silicon nitride using photolithography, mechanically scans over a sample surface to image its topography. Typical lateral dimensions of the cantilever are on the order of 100 pm and the thickness on the order of 1 pm. Cantilever deflections on the order of 0.01 nm can be measured in modem atomic force microscopes. 

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