Piezoelectric scanners

Depth-sensing nanoindentation is one of the primary tools for nanomechanical mechanical properties measurements. Major advantages to this technique over AFM include (1) simultaneous measurement of force and displacement (2) perpendicular tip-sample approach and (3) well-modeled mechanics for dynamic measurements. Also, the ability to quantitatively infer contact area during force-displacement measurements provides a very useful approach to explore adhesion mechanics and models. Disadvantages relative to AFM include lower force resolution, as well as far lower spatial resolution, both from the larger tip radii employed and a lack of sample positioning and imaging capabilities provided by piezoelectric scanners. 

The basic idea of AFM is to use a sharp tip scanning over the surface of a sample while sensing the interaction between the tip and the sample (Dufrene, 2008b). The tip with a flexible cantilever (in some AFM models the sample) is mounted on a piezoelectric scanner which can move... 

Figure 7.14 Experimental set-up for atomic force microscopy. The sample is mounted on a piezoelectric scanner and can be positioned with a precision better than 0.01 nm in the x, y, and z direction. The tip is mounted on a flexible arm the cantilever. When the tip is attracted or repelled by the sample, the deflection of the cantilever/tip assembly is measured as follows. A laser beam is focussed at the end of the cantilever and reflected to two photodiodes, numbered 1 and 2. If the tip bends towards the surface, photodiode 2 receives more light than 1, and the difference in intensity between 1 and 2 is a measure of the deflection of the cantilever and thus of the force between the sample and the tip. With four photodiodes, one can also measure the sideways deflection of the tip, for example at an edge on the sample surface.

The heart of STM is a piezoelectric scanner, sometimes called piezodrive or simply piezo. In this chapter, we provide a brief summary of the basic physics of piezoelectricity and piezoelectric ceramics relevant to the applications in STM. Three major types of piezodrives, the tripod, the bimorph, and the mbe, are analyzed in detail. 

In the early years of STM instrumentation, tripod piezoelectric scanners were the predominant choice, as shown in Fig. 9.6. The displacements along the X, y, and z directions are actuated by three independent PZT transducers. Each of them is made of a rectangular piece of PZT, metallized on two sides. Those three PZT transducers are often called x piezo, y piezo, and z piezo, respectively. By applying a voltage on the two metallized surfaces of a piezo, for example, the x piezo, the displacement is... 

From Fig. 9.16, we obtain c/33 1.05 kN, a value consistent with the value listed in the catalog (1.27 kN). The value might be somewhat lower than the true value because the bonding of the tube ends is not perfectly rigid. If one end of the tube is free, or both ends are free, the deformation pattern varies significantly at the end(s). The net end effect is to reduce the value of the double piezoelectric response. Even if the end-bonding condition is unknown, an accurate measurement of the temperature or time variation of the piezoelectric constant can still be achieved. In other words, if the piezoelectric scanner is calibrated by a direct mechanical measurement or by the scale of images at one temperature, then its variation can be precisely determined by the electrical measurements based on double piezoelectric responses. 

Chen, C. J. (1992a). In situ testing and calibration of tube piezoelectric scanners. Ultramicroscopy, 42-44, 1653-1658. 

See Lead zirconate titanate ceramics Quality number 219 Quantum transmission 59 Reciprocal space 123, 353 Reciprocity principle 88 Reconstruction 14, 327 Au(lll) 327 DAS model 16 Si(lll)-2X1 14 Recursion relations 352 Repulsive atomic force 185, 192 Resonance frequency 234, 241 piezoelectric scanners 234 vibration isolation system 241 Resonance interactions 171, 177 and tunneling 177 Resonance theory of the chemical bond 172... 

Trigonal symmetry 137 Tube piezoelectric scanner 224—235 deflection 226... 

Scanning of an SFM probe across the investigated surface is achieved by means of a piezoelectric scanner which moves the sample (or the tip) to generate a scan... 

In an STM experiment the dendrimer sample deposited on a conductive substrate (e.g. highly ordered pyrolytic graphite (HOPG)) is scanned line-by-line with a fine conductive microscopy tip. Depending upon the measuring mode, a piezoelectric scanner moves either the microscopy tip over the sample surface or the sample under the fixed tip. The microscopy tip approaches the sample... 

Figure 2 shows a schematic of a typical AFM instrument that consists of a cantilever-mounted tip, a Piezoelectric scanner, four position-sensitive photo detectors, a laser diode and a control unit. The process of operation of an AFM is relatively simple. The beam from the laser is directed against the back of the cantilever beam onto the quadrants of the photo detector. As the tip is moved across a sample, this causes the cantilever beam to bend or be twisted in manner that is proportional to the interaction force. This bending or twisting of the cantilever causes the position of the laser on the photo detector to be altered. The deflection of the cantilever beam can then be converted into a 3D topographical image of the sample surface (Gaboriaud and Dufrene, 2007 Kuznetsova et al., 2007 Lim et al., 2006). 

Initially this technique was intended purely as a high-resolution imaging device, down to the sub-nanometre level (Alessandrini and Facci, 2005). However, it was soon adapted for measuring the interactive forces in a process known as force spectroscopy. In this process, the sample is moved towards the tip and then retracted, with the vertical displacement of the Piezoelectric scanner being recorded. This produces voltage data recorded by the photo detector as a function of the displacement of the Piezoelectric stage. A force curve can be produced from this which provides information about the interactions between the tip of the probe and the sample. This force data can be interpreted to... 

Atomic force microscopy (AFM) has become a standard technique to image with high resolution the topography of surfaces. It enables one to see nanoscopic surface features while the electrode is under potential control. This powerful probe microscopy operates by measuring the force between the probe and the samples (56,57). The probe consists of a sharp tip (made of silicon or silicon nitride) attached to a force-sensitive cantilever. The tip scans across the surface (by a piezoelectric scanner), and the cantilever deflects in response to force interactions between the tip and the substrate. Such deflection is monitored by bouncing a laser beam off it onto a photodetector. The measured force is attributed to repulsion generated by the overlap of the electron cloud at the probe tip with the electron cloud of surface atoms. 

The next radical improvement in scanned probe microscopies was the invention of the atomic force microscope (AFM) in 1986 by Binnig, Quate,92 and Gerber93 [29]. The X- and Y-piezoelectric scanners were kept, but the atomically sharp conducting tip was replaced by a sharp (but not atomically sharp ) Si cantilever (Fig. 11.42), with a mirror glued to its back. 

Atomic Force Microscopy Atomic force microscopy is a direct descendant of STM and was first described in 1986 [254], The basic principle behind AFM is straightforward. An atomically sharp tip extending down from the end of a cantilever is scanned over the sample surface using a piezoelectric scanner. Built-in feedback mechanisms enable the tip to be maintained above the sample surface either at constant force (which allows height information to be obtained) or at constant height (to enable force information to be obtained). The detection system is usually optical whereby the upper surface of the cantilever is reflective, upon which a laser is focused which then reflects off into a dual-element photodiode, according to the motion of the cantilever as the tip is scanned across the sample surface. The tip is usually constructed from silicon or silicon nitride, and more recently carbon nanotubes have been used as very effective and highly sensitive tips. 

Figure 3 (a) Scatmiag tunneling microscope depicting tip and piezoelectric scanner, (b) With the tip in close proximity to the surface, tunnehng between tip and surface electronic states can occur... 

Figure 5.25 Image distortion caused by nonlinearity of the piezoelectric scanner (a) regular grid and (b) distorted image of grid. (Reproduced with permission from PC. Braga and D. Ricci (eds), Atomic Force Microscopy, Humana Press. 2004 Humana Press.)...

The maximum scan size that can be achieved with a particular piezoelectric scanner depends upon the length of the scanner tube, the diameter of the tube, its wall thickness, the strain coefficients of the piezoelectric ceramic and the applied voltage. The sensitivity of the piezo depends on temperature its maximum scan range is approximately reduced by a factor 5-6 by cooling the piezo material from room temperature to liquid helium temperature (4.2 K). The process of calibration is described in Tutorial 3. 

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