BioPHYSICAL chemistry

CHAPTER 10 PARTICLE IN A BOX AND TUNNELING 213

position is determined sensitively by measuring
the deflection of a laser beam off a cantilever that
contains the tip (Figure 10.13). The deflection is
magnified over 1000-fold, allowing a near-atomic
resolution to be achieved. The extent of the deflec-
tion provides a measure of the force. Unlike the
stylus system of a record player, the probe is con-
trolled by the use of a feedback loop that not
only allows the measurement of the force but also
regulates it to allow the acquisition of accurate
measurements at very low forces.
In addition to probing the surface of a material, the technique can also be
used to measure the conductivity of an electron through a single molecule.
A monolayer of molecules is assembled on a gold surface linked through
thiol groups at the end of each molecule (Figure 10.14). An approximate
vertical orientation arises from the choice of molecules that prefer not
to bind together. Electrical contact is made between the probe tip and
the monolayer, and the current is measured as a function of the applied
voltage. In a set of experiments on 1,8-octanedithiol, the measurements
revealed only a limited number of distinct curves that were modeled as
arising from quantization of the current, allowing the resistance of a
single molecule to be determined at 900 MΩ(Cui et al. 2001).

Probe

Laser

radiation

Cantilever

Surface

Figure 10.13

AFM is similar to

STM in being able

to measure at the

atomic level except

that the probe works

with a cantilever

to probe the force

between the tip and

surface, which may

be noncoducting

as no current is

involved.

 20

 10

0

10

20

30

 30

 40

1.0 0.5 0.0 0.5 1.0

Current (nA)

Tip bias (V)

1I(V)

2I(V)

3I(V)

4I(V)

5I(V)

i

Figure 10.14A tip is brought into contact with a layer of 1,8-
octanedithiol [HS(CH 2 ) 8 SH] molecules that are attached to the gold surface
though the thiol groups. The electrical current is measured as a function of
the applied voltage, which yields the conductivity for a single molecule.
Modified from Cui et al. (2001).