Close-packed planar arrays of nanometer-diameter metal clusters that are covalently linked to each other by rigid, double-ended organic molecules have been self-assembled. Gold nanocrystals, each encapsulated by a monolayer of alkyl thiol molecules, were cast from a colloidal solution onto a flat substrate to form a close-packed cluster monolayer. Organic interconnects (aryl dithiols or aryl di-isonitriles) displaced the alkyl thiol molecules and covalently linked adjacent clusters in the monolayer to form a two-dimensional superlattice of metal quantum dots coupled by uniform tunnel junctions. Electrical conductance through such a superlattice of 3.7-nanometer-diameter gold clusters, deposited on a SiO 2 substrate in the gap between two gold contacts and linked by an aryl di-isonitrile [1,4-di(4-isocyanophenylethynyl)-2-ethylbenzene], exhibited nonlinear Coulomb charging behavior.

The formation and characterization of nanometer-size, ohmic contacts to n-type GaAs substrates are described. The nanocontacts are formed between a single-crystalline, nanometer-size Au cluster and a GaAs structure capped with layer of low-temperature-grown GaAs ͑LTG:GaAs͒. An organic monolayer of xylyl dithiol (p-xylene-␣,␣Ј-dithiol; C 8 H 10 S 2 ͒ provides mechanical and electronic tethering of the Au cluster to the LTG:GaAs surface. The I(V) data of the Au cluster/xylyl dithiol/ GaAs show ohmic contact behavior with good repeatability between various clusters distributed across the surface. The specific contact resistance is determined to be 1ϫ10 Ϫ6 ⍀ cm 2. Current densities above 1ϫ10 6 A/cm 2 have been observed.

The self-assembly of well-characterized, nanometer-size Au clusters into ordered monolayer arrays spanning several microns has been achieved. Techniques to insert molecular wires to link adjacent clusters in the self-assembled array have also been developed. ‘‘Unit cell’’ nanostructures formed from individual Au clusters supported on a self-assembled monolayer film of the double-ended thiol molecule p-xylene-α,α′- dithiol show evidence for reproducible single electron effects at room temperature when studied by scanning tunneling microscopy. From these measurements, estimates for the electrical resistance of a single molecule can be obtained. The experimental values for this resistance are in reasonable agreement with theoretical calculations using the Landauer approach.

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Three-terminal vertical quantum structures using a self-aligned sidewall gating technique have been developed. Resonant tunneling transistors with physical widths of about 0.7 μm demonstrate pinch-off of the resonant peak at room temperature. Comparable gating characteristics for forward and reverse drain-source biases indicate that the gating action is vertically uniform, making this topology suitable for the fabrication of low-dimensional structures and study of multiple quantum well devices.