Stoddart Mechanostereochemistry Group

 

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Nanoelectronics: From Solutions to Devices

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, 11721175) 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, 20552056) 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.

 

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