Nanoscale molecular-switch crossbar circuits

Molecular electronics offer an alternative pathway to construct nanoscale circuits in which the critical dimension is naturally associated with molecular sizes. We describe the fabrication and testing of nanoscale molecular-electronic circuits that comprise a molecular monolayer of [2]rotaxanes sandwiched between metal nanowires to form an 8 × 8 crossbar within a 1 μm2 area. The resistance at each cross point of the crossbar can be switched reversibly. By using each cross point as an active memory cell, crossbar circuits were operated as rewritable, nonvolatile memory with a density of 6.4 Gbits cm−2. By setting the resistances at specific cross points, two 4 × 4 subarrays of the crossbar were configured to be a nanoscale demultiplexer and multiplexer that were used to read memory bits in a third subarray. In order to realize functional nano-electronic circuits, researchers need to solve three problems: invent a nanoscale device that switches an electric current on or off; build a nanoscale circuit that controllably links very large numbers of these devices with each other and with external systems in order to perform memory and/or logic functions; and design an architecture that allows the circuits to communicate with other systems and operate independently of their lower-level details. At the device level, researchers in molecular electronics have achieved significant progress recently, demonstrating tunnelling junctions [1, 2], devices with negative differential resistance [3], electrically configurable switches [4–6] and transistors made from a single carbon nanotube [7, 8] or a single molecule [9, 10]. At the circuit level, devices have been connected together to separately perform basic memory [4, 5, 11] and logic [4, 6, 11–14] functions. Architectural issues are still in the early stages, but designs suitable for nano-electronic circuits have been discussed and patented [15–17]. 3 Author to whom any correspondence should be addressed. 4 Current address: Department of Chemistry,University of Southern Denmark (Odense University), Campusvej 55, DK-5230, Odense M, Denmark. To satisfy all three of the above requirements, we have proposed nanoscale circuits based on a configurable crossbar architecture to connect molecular switches [15–17] in a two-dimensional grid (as shown schematically in figure 1(a)). A crossbar has several advantages. First, the wire dimensions can be scaled continuously down to molecular sizes, while the number of wires in the crossbar can be scaled up arbitrarily to form large-scale generic circuits that can be configured for memory and/or logic applications. Second, it requires only 2N communication wires to individually address 2N nanowires with a demultiplexer [17], which allows the nano-circuit to communicate efficiently with external circuits and systems, for example, CMOS. Third, it is a reconfigurable architecture that can tolerate defective elements generated during the nanofabrication process [15]. Fourth, the simple physical structure of the crossbar makes nanoscale fabrication feasible and potentially inexpensive. Imprint lithography is a new nanoscale processing technique that can produce sub-10 nm feature sizes with high throughput and low cost [18]. In addition, imprinting may also preclude damage to sensitive circuit components, including active molecules, in contrast to the high-energy electrons 0957-4484/03/040462+07$30.00 © 2003 IOP Publishing Ltd Printed in the UK 462 Nanoscale molecular-switch crossbar circuits