Stoddart Mechanostereochemistry Group

 

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Stoddart Mechanostereochemistry Group
 

nanoelectronics

 

Collaboration with Prof. James R. Heath (California Institute of Technology)


"Bottom-up" approaches to the fabrication of nanometer-scale electronic devices are likely to become increasingly important to overcome the fundamental limitations of "top-down" lithographic techniques. In the context of this rapidly changing agenda, the use of molecules as the passive and active components in devices promises ultimate scalability, minimal power consumption, and low fabrication costs. Over the past few years, we have tapped into the advances in supramolecular chemistry1 and template-directed synthesis2,3 to come up with a successful design of electrochemically switchable bistable\]atenanes4 and\]otaxanes.5,6 These molecular switches, based on mechanically interlocked and movable components, have been fabricated into nanowire crossbar arrays, producing working, defect-tolerant memory devices of unprecedented densities.7-10

The bistable, redox-switchable\]otaxanes that have been utilized in molecular electronic devices (MEDs) consist of a tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+), with its two π-electron deficient bipyridinium units, encircling a linear rod component containing π-election rich tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) units and terminated by bulky stoppers at each end (Figure 1). Under standard conditions, the CBPQT4+ ring in the bistable\]otaxane 14+ exhibits a 9:1 preference for encircling the TTF rather than the DNP unit. The TTF-encircled translational isomer of the bistable\]otaxane is therefore the ground-state co-conformation (GSCC) and the DNP-encircled isomer is the metastable-state co-conformation (MSCC).11



The first two oxidation processes carried out on 14+ remove electrons from the TTF π-system (green), resulting in the sequential formation of the TTF radical cation (TTF+.) and dication (TTF2+), respectively. Coulombic repulsion between the TTF+. and CBPQT4+ induces the ring to move to the DNP site. When TTF+. or TTF2+ is reduced back to the neutral TTF form, the molecule remains in the MSCC for a period of time (ranging from < 1s in solution to ca. 1h in crossbar devices) determined by the energy barrier (ΔG) separating the two co-conformations. The GSCC exhibits a ten-fold lower tunneling conductivity relative to the MSCC, allowing the two molecular states to serve as the "open" and "closed" states of the switch, forming the basis for using these molecules in information storage.



A 4.5 kbit memory was fabricated with a bistable\]otaxane assembled between bottom Si and top Ti/Al nanowires.10 Each junction of the circuit is 10 nm x 40 nm in width, affording an area that includes ~300 molecules per junction. This first demonstration of a working ultra-dense memory indicates that these devices may be successfully scaled to junctions containing a very small number of molecules. Recent efforts in the Heath group have been focused on the fabrication and testing of a 160-kbit memory at 1011 bits/cm2, a density which correlates to the 2028 node of the International Technology Roadmap for Semiconductors (ITRS) published in 2005.



One of the fundamental goals of molecular electronics is to develop the ability to manipulate device properties predictably through chemical synthesis and solution-state characterization. We undertook a major effort to correlate the switching mechanism of bistable\]otaxanes in solution with their behavior in other more device-relevant environments - for example, in self-assembled monolayers (SAMs),13 in viscous polymer gels,14 and in molecular memory devices.11 These investigations confirmed convincingly that a universal switching mechanism is operative across all environments. The kinetics of the switching process slow down roughly 10,000 times as the molecules are operated in more condensed phases, a phenomenon which allows memory storage half-lives of ~1 h. These important demonstrations link easily measurable molecular properties with device behavior, a feedback loop that we are currently using to design new molecules.

Current research efforts in the Stoddart group in the area of molecular electronics include the design of molecules for use in non-volatile molecular memory devices and the development of entirely new electrochemically switchable materials appropriate for reconfigurable molecular logic.


References:


  1. Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, Germany, 1995.

  2. Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philip, D.; Pietraszkiewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 193-218.

  3. Stoddart, J. F.; Tseng, H.-R. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4797-4800.

  4. Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Raymo, F. M.; Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2000, 65, 1924-1936.

  5. Tseng, H.-R.; Vignon, S. A.; Celestre, P. C.; Perkins, J.; Jeppesen, J. O.; Di Fabio, A.; Ballardini, R.; Gandolfi, M. T.; Venturi, M.; Balzani, V.; Stoddart, J. F. Chem. Eur. J. 2004, 10, 155-172.

  6. Bissell, R. A.; Cordova, E.; Kaifer, A. E.; Stoddart, J. F. Nature 1994, 369, 133-137.

  7. Collier, C. P.; Jeppesen, J. O.; Luo, Y.; Perkins, J.; Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2001, 123, 12632-12641.

  8. Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172-1175.

  9. Diehl, M. R.; Steuerman, D. W.; Tseng, H.-R.; Vignon, S. A.; Star, A.; Celestre, P. C.; Stoddart, J. F.; Heath, J. R. Chemphyschem 2003, 4, 1335-1339.

  10. Beckman, R.; Beverly, K.; Boukai, A.; Bunimovich, Y.; Choi, J. W.; DeIonno, E.; Green, J.; Johnston-Halperin, E.; Luo, Y.; Sheriff, B.; Stoddart, J. F.; Heath, J. R. Faraday Discuss. 2006, 131, 9-22.

  11. Choi, J. W.; Flood, A. H.; Steuerman, D. W.; Nygaard, S.; Braunschweig, A. B.; Moonen, N. N. P.; Laursen, B. W.; Luo, Y.; DeIonno, E.; Peters, A. J.; Jeppesen, J. O.; Xu, K.; Stoddart, J. F.; Heath, J. R. Chem. Eur. J. 2006, 12, 261-279.

  12. Luo, Y.; Collier, C. P.; Jeppesen, J. O.; Nielsen, K. A.; DeIonno, E.; Ho, G.; Perkins, J.; Tseng, H.-R.; Yamamoto, T.; Stoddart, J. F.; Heath, J. R. Chemphyschem 2002, 3, 519-525.

  13. Tseng, H.-R.; Wu, D. M.; Fang, N. X. L.; Zhang, X.; Stoddart, J. F. Chemphyschem 2004, 5, 111-116.

  14. Steuerman, D. W.; Tseng, H.-R.; Peters, A. J.; Flood, A. H.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Heath, J. R. Angew. Chem. Int. Ed. 2004, 43, 6486-6491.

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