IBM Almaden Research Center

Title: Molecule Cascades: Nanometer-Scale Architectures That Compute

Abstract: The scanning tunneling microscope (STM) can be used to build
atomically-precise structures and investigate their physical and
functional properties. I will present a new class of nanometer-scale
structures, "molecule cascades," that are both instructive -- they
enable detailed studies of adsorbate motion, and functional -- they
do computation.

Carbon monoxide molecules were arranged in atomically-precise
configurations in which the motion of one molecule causes the
subsequent motion of another, and so on in a cascade of motion similar
to a row of toppling dominoes. Isotopically pure cascades
were assembled on a Cu(111) surface using a low temperature STM.
The hopping rate of CO molecules in cascades was found to be
independent of temperature below 6K and exhibit a pronounced
isotope effect, hallmarks of a quantum tunneling process. At
higher temperatures we observed a thermally-activated hopping rate with
an anomalously low Arrhenius prefactor that we interpret as tunneling
from excited vibrational states. We present a cascade-based computation
scheme which has all of the devices and interconnects required for the
one-time computation of an arbitrary logic function. Logic gates and
other devices were implemented by engineered arrangements of molecules
at the intersections of cascades. We demonstrate a 3-input sorter
that uses several AND gates and OR gates, and the crossover and fan-out
units needed to connect them.

I will compare our circuits with CMOS technology and motivate why we
are interested in implementing spin-based cascade computation in
nanometer-scale structures.

Work done in collaboration with Christopher Lutz, Andreas Heinrich and Jay
Gupta of the IBM Almaden Research Center