Introduction to Nanotechnology

54 METHODS OF MEASURING PROPERTIES
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‘FFgwe 3.17. Micrograph 01 3-nm-diameter (30-A) gold particles on a carbon subslrale taken
with a scanning electrofi microscope. [From J. D. Verhoeven, chapter in Whan (1986). p. 497.1
The varying magnetic fields in thc coil pairs 1; ,f; and j j,j; produce the smaIler
deflections from points 1 to 1’ to I” shown in thc detail A insct. Thus the clectron
beam scans repeatedly from left to right across the sample in a raster pattern that
eventually covers tlic cntire r x I’ fnrne area on the sarnpIe. Figure 3.17 shows 3-nm
gold particles on a carbon substratc rcsolved by an SEM.
A scanning tunneling microscope utilizes a wire with a very fine point. This fine
point is posirively charged and acts as a pmhc wkn It is lowered 60 a distance of
about 1 nm above thc surface undcr study. Electrons at individual surface atonis are
attracted to the positive charge or the probe wire and jump (tunnel) up to it, thereby
generating a weak clccrric current. The probe wire is scanned back and forth across
the surface in a raskr partern. in cithes a constarif-height modc. or a constant-current
mode. in the arrangements sketched in Fig. 3.1H. In the constant current mode a
feedback loop mainrains a consttlnt probe height above the sample surfxc profile.
and thc up/down pmbe variations are recordcd. This mode of operation aswnes a
constant tunneling harrier across the surface. In Ihe constant-probe-height rnode the
tip is constantly changing its distance from the surface. and this is reflected in
variations of the recorded tunneling current as the probe scans. The feedback loop
establishes the initinl pro& height, and is thcn turned off during the scan. The
scanning pmbe probides a mapping of the distrihution of thc atoms on the surface.
Thc STM often employs a pie7oclectric Tripod scanner, and an early design of this
scanner, built by Binning and Rohrer. is depicted in Fiy. 3. E9. A piezoclcctric is a
material in which an applied voltage elicits a mechanical rcspnse, and thc reverse.
Applicd voltages inducc piezo transducers to move the scanning prnhc (or the
sarnplc) in nanornctcr increinenls along the x. y or z dircctions indicated on the arms
(3) of the 3canner in the figure. The initial senin& is accomplished with thc aid of a
stepper motor and micrometer screws. The tunneling currcnt. which varies expo-