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include selenols (-SeH), phosphines (R
3
P), carboxylates (-CO
2
) and pyradines
[19]. We developed a method for direct assembly of aryl groups on silicon
(n-doped, p-doped, undoped, single crystal, or polycrystalline) and gallium
arsenide using aryl diazonium salts [20].
One can also construct molecules that have a preference for a given surface by
judicious choice of the alligator clip. This surface-selective SAM formation could
be a critical factor in determining the usefulness of SAMs for device placement, as
heterogeneity in device patterns must be attained. In other words, an array of all
AND logic gates would be useless. The heterogeneity is best if it is programmable
in its design pattern. Using crystalline substrates that are composed of different
atom types and yet are regular in their periodicity could provide a method for the
predictable arrangement of surface molecules, but programmability to the array
architecture would then be limited to one's ability to form tailored substrate
crystals.
11.2.2. Molecular Bundle Switches
Reed developed a test-bed method, called the nanopore, to make reliable
measurements on groups of molecules [21]. The nanopore consists of a small
(30-50-nm diameter) surface of evaporated metal, most often gold or palladium,
on which a SAM of the molecular wires or devices with
1000 molecules is
permitted to form. An upper metal (usually gold or titanium) contact is then
evaporated onto the top of the SAM layer, forming a metal-SAM-metal sandwich
through which I(V) measurements are recorded (Figure 11.2). Using such a small
area for the SAM (
B
1000 molecules), we can potentially form defect-free SAMs
because the entire area is smaller than the typical defect density of a SAM. This
would eliminate electrical shorts that can occur when a larger, e.g., microscale,
SAM is used.
We hypothesized that the conductivity of these OPE molecular scale wires and
devices arises from electron transfer through the
p
-orbital backbone that extends
over the entire molecule. When the phenyl rings of the OPEs are planar, the
p
-orbital overlap within the molecules is continuous. Thus transfer over the entire
molecule is achieved: Electrons can freely flow between the two metal contacts,
leading to maximum conductivity. But if the phenyl rings become perpendicular
with respect to each other, the
p
-orbitals between the phenylene rings likewise
become perpendicular. The discontinuity of the
p
-orbital network in the perpen-
dicular arrangement minimizes electron transport through the molecular system,
leading to greatly reduced conductivity [22].
We devised a method that would permit altering the degree of a molecule's
p
-orbital overlap through the use of a third electrode (gate). Thus molecules
with orthogonal net dipoles could be controlled by use of a third electrode in
the nanopore to modulate the conformation, and hence the current through the
system. However, since nanopore devices with an electrode perpendicular to the
SAM axis had not yet been fabricated, we simply began with the control
B
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