The field of molecular electronics is
teeming with results, rationalizations and speculations from many research
laboratories. At the heart of this field is the drive to construct molecular
electronic devices that exhibit and take advantage of the rich set of molecular
properties which are under modular chemical control through contemporary
synthesis. This challenge is being met with increasing frequency, ranging from
the simplest molecular electronic devices that include resistive tunnel
junctions and rectifiers to active devices, including two- and three-terminal
electronic switches. In a long-standing collaboration, since 1999, Stoddart (CNSI
& UCLA) and Heath (Caltech) have developed molecular memory devices that have
withstood extensive scientific scrutiny.
In 2000, the UCLA and Caltech groups
reported (Science 2000, 289, 1172–1175) that a two-terminal
molecular switch tunnel junction (MSTJ) device containing a bistable catenane
sandwiched between silicon and metallic electrodes exhibits a memory effect.
When biased at +2 V the device is switched ON. Whereas, following a –2 V bias,
the device is returned to its OFF state. The ON state has a finite,
temperature-dependent lifetime of about 10 minutes, i.e., it is metastable.
Subsequently, amphiphilic bistable rotaxanes were reported (ChemPhysChem,
2002, 3, 519-525) to switch by a related mechanism.
The explanation advanced to account for
these observations is based on a cycle of voltage-controlled, mechanical
movements between two isomers that occur within the bistable molecules in the
device. This mechanism agrees qualitatively with that derived from
solution-phase measurements but with significant quantitative differences. For
example, the relaxation of the ON to OFF state within the MSTJ is much slower
than that observed in solution. Subsequently, similar results for bistable
rotaxane-based MSTJs were reported. The hypothesis was that, although the
confined environment of the MSTJ impacts the molecular switching cycle, the
overall mechanism is the same. Thus, variable temperature measurements were
performed on the cycle of switching bistable molecules in solution, in
self-assembled monolayers, in a polymer matrix, and in MSTJs (Appl. Phys. A
2005, 80, 1197-1209). Within these environments, we recorded
the lifetime of the metastable state increases from ~0.1 second to several
minutes as the molecules are confined to smaller spaces.
The UCLA and Caltech groups reported
recently (Science 2004, 306, 2055–2056) their findings in
support of the role the molecules play in their memory devices. As practitioners
of molecular electronics, they believe the field will be best able to provide
practical support to the traditional electronics industry when its development
is based on sound scientific conclusions that have been tried and tested at
every step. The most important early applications will emerge when molecules are
integrated in hybrid fashion with existing technologies. When this development
happens, the opportunities for bringing the science to maturity will be obvious.