Scanning Probe Machine Methods One Atom at a Time

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The scanning tunneling microscope (STM) and the atomic force microscope (AFM) are similar single-point probes of a surface. Both have a sharp tip which can be moved in an x-y raster scan over a 2D surface and both have a servo loop which raises the height of the tip to maintain the current (or the surface-tip force) to be constant, thereby providing a topograph of the surface. Either instrument can be imagined as capable of carrying an atom or molecule to a point on the surface and leaving it to provide one step toward fabricating a surface structure. Either can be imagined as a starting point for the "machine assembly" of a nanostructure, atom by atom. The essential ideas of the two instruments are presented in Figure 7.6 [12].

The STM is capable of atomic resolution, as is evident from many images, including those in Figure 7.7. The basic idea is that the tunneling occurs when the wave-function of an atom on the tip overlaps the wavefunction of an atom on the surface. In ideal cases these wavefunctions decay exponentially with characteristic distances on the order of the Bohr radius, around 0.1 nm. With such a short decay length, a single tip atom may carry almost all of the observed current. If the wavefunction on that atom is, e.g., a directed bonding orbital, the spatial resolution may be remarkably high, as is sometimes observed.

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Figure 7.6 Schematic diagrams of (A) scanning tunneling microscope (STM) and (B) atomic force microscope (AFM). For STM the tunnel current I is maintained constant by tip height, controlled by z-piezo. For AFM the force (spring deflection) is maintained constant [after 12].

Spring deflection sensor

Spring deflection sensor

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Figure 7.6 Schematic diagrams of (A) scanning tunneling microscope (STM) and (B) atomic force microscope (AFM). For STM the tunnel current I is maintained constant by tip height, controlled by z-piezo. For AFM the force (spring deflection) is maintained constant [after 12].

The maximum scanning and sampling rates are determined in large part by the resonant frequencies of the support structure, the tip and the xyz piezo-electric element on which it is mounted. The upper range of such frequencies in the best instruments is about 1 MHz.

The AFM (see panel (B) in Figure 7.6) is more complicated. Sensing of the force between tip and surface is by deflection of the cantilever on which the tip is mounted. The sensing in modern AFM instruments is by deflection of a light beam, focused on the upper surface of the cantilever; or by changes in the resistance of the specially designed cantilever with deflection (piezoresistance). The earliest instruments sensed the deflection of the cantilever by changes in the tunneling current at fixed bias between the tip-holder and an upper spring-deflection-sensor electrode (see panel (B)).

The force between the tip and sample is attractive at large spacing (van der Waals regime) and repulsive at small spacing (exclusion principle overlap). Atomic imaging has been reported with the AFM, but is more difficult than in the STM.

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