MOLECULAR COMPUTING: INTEGRATION OF MOLECULES FOR NANOCOMPUTING - Bio-Inspired and Nanoscale Integrated Computing - page 304

mask, and the I(V) curve was determined. The data, when compared to controls,
indicated that the molecules were acting as solid-state molecular switches [28, 29].
Small logic circuits [31] and, recently, large-scale memory units [32] have been
demonstrated. Unlike the nanopore, Stoddart and Heath leveraged the nanoscale
crosspoint formed between two orthogonal nanowires. In the most recent work
[32], about 100 [2]rotaxane molecules were estimated to be sandwiched in the
crosspoint of two nanowires.
A challenge to computing based on molecular switches is the impedance
mismatch between the molecule and the metal contact, leading to a resistance
barrier [16, 33-35]. Neither the selenium or tellurium alligator clip significantly
reduced the barrier height [35]. But the use of an isonitrile as the contact between
the organic molecular scale wire and a palladium probe would significantly reduce
the conduction barrier, and would allow an increase in the conductivity of the
molecular scale wires. Therefore molecular scale devices with isonitrile attachment
moieties were synthesized [33, 36, 37].
11.3. CIRCUIT AND ARCHITECTURES IN MOLECULAR COMPUTING
Molecular computing requires not only molecular switches but also solutions to
integrate them into microscale functional circuits. Two popular approaches have
been taken. The first is based on quantum cellular automata (QCA) and related
electrostatic information transfers [38-40]. It relies on electrostatic field repulsions
to transport information throughout the circuitry. One major benefit of the QCA
or electrostatics approach is that energy consumption is diminutive because only a
few or fractions of an electron are used for each bit of information. Unfortunately,
molecular QCA is based on the interconnection of individual molecules. While
implementations of small QCA circuits have been reported, none are based on
molecular quantum dots, due to our inability to position and interconnect
individual molecules. The second approach is based on the crossbar of single-
walled carbon nanotubes (SWNT) [41] or synthetic nanowires [42, 43] that
sandwich molecular bundles as switching devices.
Many have investigated the integration of molecular circuits into large-scale
computing systems. In particular, numerous architectural solutions have been
proposed to combine the strength of conventional MOSFETs and molecular
units. An orthogonal approach is to employ conventional lithography-based top-
down approach to fabricate microscale patterns for large-scale integration while
employing bottom-up nanoscale fabrication methods to fill microscale units with
programmable molecular switches.
11.3.1. Quantum Cellular Automata (QCA)
and Electrostatics Architectures
In the QCA approach toward molecular computing systems, four quantum
dots in a square array are placed in a cell such that electrons are able to tunnel