MOLECULAR COMPUTING: INTEGRATION OF MOLECULES FOR NANOCOMPUTING - Bio-Inspired and Nanoscale Integrated Computing - page 298

later [18]. Numerous molecules with sophisticated looks are now accessible to
researchers wherein function is built into the molecule, 10 23 units at a time.
Significant research progress has been made to orient 10 14 molecules/cm 2 in a
predefined addressable array on a chip. The atomic order of a surface coupled
with molecular packing requirements can give rise to thermodynamically driven
self-assembly over large surfaces. The gold-thiolate (R-S-Au) system is the most
commonly studied self-assembled monolayer (SAM) [19]. The assembly takes
place within seconds or minutes. Sometimes 24-hour periods are used for the
assembly to permit dense packing of otherwise slow-to-assemble molecules.
Crystalline ordering of the system can occur in domains ranging to hundreds of
square nanometers. The gold-thiolate bond is quite robust relative to typical
metal-molecule bonds that can be formed at ambient temperature. A SAM also
slows the inherent surface reconstruction, a phenomenon that often causes rapid
randomization of surface atoms. Figure 11.1 illustrates the self-assembly process.
Molecules are constructed that bear some functional component, such as the
nitroaromatic as a memory storage unit. These are randomized in solution in
which a gold substrate is dipped. In a matter of a few seconds to minutes, the
molecules faithfully arrange on the substrate surface. The aromatic thiolates have
a tilt angle of approximately 20

1

relative to the surface normal, while alkanethio-

late are tilted approximately 30 to the normal. Few molecules stand perpendi-

cular to the surface; the tilt is a function of the hybridization state of the sulfur and
the intermolecular packing requirements.
Besides gold, there are numerous other surfaces that have been employed in
SAM construction, including other conductors such as copper, silver, and
palladium; semiconductors such as silicon, gallium arsenide, and cadmium sulfide;
and insulators such as silicon oxide. Other alligator clips that have been studied

1

=

NO 2

SH

Synthesis Solution Self-Assembly

Figure 11.1. The self-assembly process wherein molecules that have programmed
function built into their structure then self-order on a surface upon dipping the
surface into a solution of the molecules. Once the surface is removed and rinsed,
it will bear a monolayer of molecules that possesses long-range order.