Atomic force microscopy piezoelectric scanners

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.

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. 

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 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.)...

J Akila, SS Wadhwa. Correction for nonlinear behavior of piezoelectric tube scanners used in scanning tunneling and atomic force microscopy. Rev Sci Instrum 66 2517-2519, 1995. 

Recendy, surface structure studies have been aided by the development of scanning tunneling microscopy (STM) and atomic force microscopy (AFM). In STM, a very sharp metallic tip is brought into close proximity of a conducting sample through the use of piezoelectric scanners (Figure 3). 

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. 

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