54
METHODS OF MEASURING PROPERTIES
P‘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 i n 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 w k n 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 i s reflected in
variations o f 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 o f 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) o f the 3canner in the figure. The initial senin& is accomplished with thc aid o f a
stepper motor and micrometer screws. The tunneling currcnt. which varies expo-
METHODS OF MEASURING PROPERTIES
P‘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 i n 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 w k n 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 i s reflected in
variations o f 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 o f 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) o f the 3canner in the figure. The initial senin& is accomplished with thc aid o f a
stepper motor and micrometer screws. The tunneling currcnt. which varies expo-