Results And Discussion

Varied molecular architectures are being explored with purposes related to nanoelectronics.2 Along with their collaborators, Tour and Reed,424 have synthesized and conducted numerous experimental characterizations of molecular electronic devices that are polyphenylene-based. By controlling the size of the polymers, the substituents on the phenylene groups, and the spacers between phenylene groups, a growing family of well-defined polymers is providing exciting opportunities for nanoelectronics. The electronic characteristics of those small molecules or polymers that have either been successfully made and tested, or proposed and studied computationally, have been argued to provide a functionally complete set of logic elements.25 In addition to the molecular "wires" that are needed for a circuit, if the circuit is to perform a simple logical function it must also possess molecular "gates". AND, OR and NOT gates form a functionally complete family of diode-based logic gates.26 Rectifying diodes are selective in the direction that current can be induced to flow through them. That is, the threshold voltage above which current will flow, strongly depends on the current direction. This "bias" effectively gives them a "one-way" characteristic regarding electron flow. Rectifying diodes can yield AND and OR gates, but not NOT gates. With applied voltage, current can also be induced to flow through resonant tunneling diodes (RTDs), but these do not have a directional bias. Instead, these devices "turn off when the current-inducing voltage exceeds a second threshold value. NOT gates can be derived from combinations of both of these types of diodes.26 Molecules 1,2 (X = S-) and 3 (Y =CH3, Z = CN) are very simple prototypes of polyphenylene-based molecular electronic device subunits that can be used in proposed diode-based digital logic circuits, such as an "adder".27 Our purpose is not to investigate the practicality, or even the feasibility, of these far-off engineering applications. Rather, we have sought to employ these prototypical devices as test cases for further development of the novel theoretical procedure that was briefly reviewed above. The reader is refered to ref. 10 for further discussion of the physical basis of the analysis. As discussed above, a synthesis of results from quantum chemistry calculations with fluid dynamics concepts and visualization techniques, yielded a large amount of data.28 We highlight key sets of data in Figs. 1 and 2. These images and other pertinent data are discussed for the individual prototypical devices below.

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