The remarkable success of silicon based microelectronics rests on a continuing decrease in the dimensions of the active devices. Eventually unavoidable physical constraints and rapidly rising manufacturing costs mean that radically different materials, technologies, and approaches must be invented to sustain the evolution of computer technology. A potential solution to this problem is the use of the self-organizing phenomena evident in biological systems to build-up or self-assemble electronic circuits and devices from nanometer size components. One such self-assembly scheme involves the casting of a monolayer film of metal clusters onto a flat solid substrate and the introduction of organic molecules that bond the clusters in the film to each other to form an electronically-conducting superlattice . Such a "linked cluster network" has many attractive properties the foremost being: 1) when the clusters have diameters smaller than approximately 5 nm, low intercluster capacitance gives rise to correlated single electron tunneling in the network even at room temperature; 2) the electronic conductance of the superlattice can be controlled by the choice of the linking molecules which act as tunnel barriers between clusters; and 3) narrow "ribbons" or "wires" of molecularly bonded clusters are mechanically stable and can span relatively large distances . We review techniques developed at Purdue for fabricating electronic nanostructures by this scheme and the current understanding of their conductance characteristics.
The two most common methods for obtaining ultra-small metal particles are the reduction of metal ions in the liquid phase and the condensation of metal atoms in the gas phase. The approach we have taken is the second, i.e. gas phase or aerosol synthesis, followed by thermal annealing of the particles to produce nanocrystals. The bare metal crystals are captured in a liquid solvent and "passivated" by adsorption of a monolayer of organic molecules on their surface facets to produce a colloidal suspension. Size-selective precipitation is then used to refine the particle size distribution and remove excess surfactant molecules. Redispersing the clusters in a solvent that is immiscible in water and casting the colloidal suspension on a water surface produces a close-packed monolayer of clusters that float at the air-water interface after the solvent evaporates. This cluster monolayer is transformed into an electronically-conducting superlattice by displacing the passivation layer around each cluster with linking molecules. Once a uniform linked film has self-assembled on the water surface, it is transformed into a patterned network on the substrate by: 1) lithographically defining areas on the substrate where the energy of adhesion of the cluster film is high and areas where the adhesion energy is low, 2) transferring the intact cluster film from the water surface to the substrate by contacting the film with the patterned substrate, and 3) removal of those portions of the film that are not strongly held to the substrate by solvent rinsing.
The above suite of techniques provide a scheme for synthesizing interesting nanoelectronic structures and devices. Our current understanding of electronic transport in such structures, obtained from experimental measurements using scanning tunneling microscopy and conductance spectroscopy and theoretical modeling using quantuum mechanics is described.