Download Abstract NanoCars and Molecular Electronics

(A) NanoCars
(B) Constructing a Computer from Molecular Components
James M. Tour, Ph.D.
Chao Professor of Chemistry
Professor of Computer Science
Professor of Mechanical Engineering and Materials Science
Rice University
Center for Nanoscale Science and Technology, MS 222
6100 Main Street, Houston, Texas 77005 USA
Email: [email protected], Web: http://www.jmtour.com
(A) The thrust to design single-molecule-sized nanoscale machines with controlled mechanical
motion has yielded a variety of molecular machinery resembling macroscopic motors, switches,
shuttles, turnstiles, gears, bearings, and elevators. In most cases, however, the nanomachines
have been operated and observed spectroscopically as ensembles of molecules in solution phase.
On the other hand, molecules such as benzene, porphyrins, fullerene-C60, a cyclodextrin
necklace, molecular landers, and altitudinal molecular rotors have been treated as individual
molecules on solid surfaces using a scanning tunneling microscope (STM). Most observed
tangential motion of these latter systems has been attributed to a stick-slip movement. The
exception, C60, has demonstrated a rolling motion across various metal and semiconductor
surfaces due to its highly symmetric, closed cage structure. With the hope of directing future
bottom-up fabrication through bulk external stimuli, such as electric fields, on nanometer-sized
transporters, we sought to study controlled molecular motion on surfaces through the rational
design of surface-capable molecular structures called nanocars: nanometer-sized vehicles that are
each a single molecule possessing four wheels (spherical C60 units) connected through four
independently-rotating axles (alkynes) to a chassis (an oligo(phenylene ethynylene) (OPE)
moiety). The observed movement of the nanocars was not stick-slip action, but instead directed
rolling motion perpendicular to the axles. Additionally, the alkynyl axles posses a very low
barrier to rotation allowing the translational motion of these molecules to be governed by the
interaction of the fullerene wheels with the substrate. The studies here underscore the ability to
control directionality of motion in molecular-sized nanostructures through precise molecular
synthesis. Use of spherical wheels based on fullerene-C60 and freely rotating axles based on
alkynes permit directed nanoscale rolling of a molecular structure. Further studies are
concentrating on electric field-induced motion of nanocars and nanotrains, and use of nanotrucks
for assisted small molecule transport across surfaces. This work is presently unpublished,
however it is submitted for publication.
(B) Research efforts directed toward constructing a molecular computer will be described in the
context of recent developments in nanotechnology. Routes will be outlined from the synthesis of
the basic building blocks such as wires and alligator clips, to the assembly of the processing
functional blocks. Specific achievements include: (1) isolation of single molecules in alkane
thiolate self-assembled monolayers and addressing them with an STM probe, (2) single
molecule conductance measurements using a mechanically controllable break junction, (3) 30
nm bundles, approximately 1000 molecules, of precisely tailored molecular structures showing
negative differential resistance with peak-to-valley responses far exceeding those for solid state
devices, (4) dynamic random access memories (DRAMs) constructed from 1000 molecule units
that possess 15 minute information hold times at room temperature, (5) demonstration of singlemolecule switching events, (6) direct molecular attachment to Si and GaAs via Si-C and As-C
bonds, respectively, with no oxide layer, (7) silicon-molecule-nanotube-based arrays that provide
high-yielding electronic memory storage devices (8) initial assemblies and programming of
molecular CPUs in a NanoCell configuration that show room temperature electronic memory
with days or electronic hold time, and the programming of molecular logic gates. See: (a)
Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.;
Allara, D. L; Tour, J. M. “Direct Covalent Grafting of Conjugated Molecules onto Si, GaAs, and
Pd Surfaces from Aryldiazonium Salts,” J. Am. Chem Soc. 2004, 126, 370-378. (b) Tour, J. M.;
Cheng, L.; Nackashi, D. P.; Yao, Y.; Flatt, A. K.; St. Angelo, S. K.; Mallouk, T. E.; Franzon, P.
D. “NanoCell Electronic Memories,” J. Am. Chem Soc. 2003, 125, 13279-13283. (c) Blum, A.
S.; Kushmerick, J. G.; Long, D. P.; Patterson, C. M.; Yang, J. C.; Henderson, J. C.; Yao, Y.;
Tour, J. M.; Shashidar, R.; Ratna, B. R. "Molecularly Inherent Voltage-Controlled Conductance
Switching," Nature Mater. 2005, 4, 167-172. (d) Husband, C. P.; Husband, S. M.; Daniels, J. S.;
Tour, J. M. “Logic and Memory with Nanocell Circuits,” IEEE Trans. Elect. Dev. 2003, 50,
1865-1975. (e) Tour, J. M. “Molecular Electronics: Commercial Insights, Chemistry, Devices,
Architecture and Programming,” ISBN 981-238-296-0 and 981-238-341-7 (pbk), World
Scientific, Teaneck, NJ, 2003. (f) Yu, L. H.; Keane, Z. K.; Ciszek, J. W.; Cheng, L.; Stewart, M.
P.; Tour, J. M.; Natelson, D. “Inelastic Electron Tunnelling Via Molecular Vibrations in SingleMolecule Transistors,” Phys. Rev. Lett. 2004, 92, 266802.