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Tunneling current

STM - Constant Current Mode (Feedback On)

Tunneling current

Figure 3.18. Constant-height (top sketch) and constant-current (bottom sketch) imaging mode of a scanning tunneling microscope. [From T. Bayburt, J. Carlson, B. Godfrey, M. Shank-Retziaff, and S. G. SHgar, in Nalwa (2000), Vol. 5, Chapter 12, p. 641.]

nential ly with the probe-surface atom separation, depends on the nature of the probe tip and the composition of the sample surface. From a quantum-mechanical point of view, fee current depends on the dangling bond state of the tip apex atom and on the orbital states of the surface atoms.

Figure 3.19. Scanning mechanism for scanning tunneling microscope, showing (1) the piezoelectric baseplate, (2) the three feet of the baseplate and (3) the piezoelectric tripod scanner holding the probe tip that points toward the sample. (From R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge Univ. Press, Cambridge, UK, 1994, p. 81.)

Figure 3.19. Scanning mechanism for scanning tunneling microscope, showing (1) the piezoelectric baseplate, (2) the three feet of the baseplate and (3) the piezoelectric tripod scanner holding the probe tip that points toward the sample. (From R. Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge Univ. Press, Cambridge, UK, 1994, p. 81.)

The third technique in wide use for nanostructure surface studies is atomic force microscopy, and Fig. 3.20 presents a diagram of a typical atomic force microscope (AFM). The fundamental difference between the STM and die AFM is that die former monitors the electric tunneling current between the surface and fee probe tip

Figure 3.20. Sketch of an atomic force microscope (AFM) showing the cantilever arm provided witti a probe tip that traverses the sample surface through the action of the piezoelectric scanner. The upper figure shows an interference deflection sensor, and the lower enlarged view of the cantilever and tip is provided with a laser beam deflection sensor. The sensors monitor the probe tip elevations upward from the surface during the scan.

Figure 3.20. Sketch of an atomic force microscope (AFM) showing the cantilever arm provided witti a probe tip that traverses the sample surface through the action of the piezoelectric scanner. The upper figure shows an interference deflection sensor, and the lower enlarged view of the cantilever and tip is provided with a laser beam deflection sensor. The sensors monitor the probe tip elevations upward from the surface during the scan.

and the latter monitors the force exerted between the surface and the probe tip. The AFM, like the STM, has two modes of operation. The AFM can operate in a close contact mode in which the core-to-core repulsive forces with the surface dominate, or in a greater separation "noncontact" mode in which the relevant force is the gradient of the vim der Waals potential. As in the STM case, a piezoelectric scanner is used. The vertical motions of the tip during the scanning may be monitored by the interference pattern of a light beam from an optical fiber, as shown in the upper diagram of the figure, or by the reflection of a laser beam, as shown in the enlarged view of the probe tip in the low«- diagram of the figure. The atomic force microscope is sensitive to the vertical component of the surface forces. A related but more versatile device called a friction force microscope, also sometimes referred to as a lateral force microscope, simultaneously measures both normal and lateral forces of the surface on the tip.

All three of these scanning microscopes can provide information on the topography and defect structure of a surface over distances close to the atomic scale. Figure 3.21 shows a three-dimensional rendering of an AFM image of chromium deposited on a surface of Si02. The surface was prepared by the laser-focused deposition of atomic chromium in the presence of a Gaussian standing wave , that reproduced the observed regular array of peaks and valleys on the surface. When the laser-focused chromium deposition was carried out in the presence of two plane waves displaced by 90° relative to each other, the two-dimensional arrangement AFM image shown in Fig. 3.22 was obtained. Note that the separation between the peaks, 212.78 nm, is the same in both images. The peak heights are higher (13 nm) in the two-dimensional array (8 nm) than in the linear one.

Figur« 3.21. Three-dimensional rendering of an AFM image of nanostructure formed by laser focused atomic Cr deposition in a Gaussian standing wave on an Si02 surface. [From J. J. McClelland, R. Gupta, Z. J. Jabbour, and R. L. Celotta, Aust J. Phys. 49, 555 (1996).]
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