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.
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
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