Electromigrated nanogaps have shown great promise for use in molecular scale electronics. We have fabricated nanogaps on free-standing transparent SiNx membranes which permit the use of transmission electron microscopy (TEM) to image the gaps. The electrodes are formed by extending a recently developed controlled electromigration procedure and yield a nanogap with approximately 5 nm separation clear of any apparent debris. The gaps are stable, on the order of hours as measured by TEM, but over time (months) relax to about 20 nm separation determined by the surface energy of the Au electrodes. A major benefit of electromigrated nanogaps on SiNx membranes is that the junction pinches in away from residual metal left from the Au deposition which could act as a parasitic conductance path. This work has implications to the design of clean metallic electrodes for use in nanoscale devices where the precise geometry of the electrode is important.

We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions.This advance will make feasible single molecule measurements of macromolecules with tertiary and quaternary structures that do not survive the liquid-helium temperatures at which electromigration is typically performed.A second advance is that it yields gaps of desired tunnelling resistance, as opposed to the random formation at liquid-helium temperatures.Nanogap formation occurs through three regimes: First it evolves through a bulk-neck regime where electromigration is triggered at constant temperature, then to a few-atom regime characterized by conductance quantum plateaus and jumps, and finally to a tunnelling regime across the nanogap once the conductance falls below the conductance quantum.

The photoconductivity of nanorods self-assembled from meso-tetrakis(4-sulfonatophenyl)porphine is described. The nanorods are insulating in the dark. Upon illumination with 488 nm light, the nanorods become photoconductive, exhibiting a rapid turn on/off (<100 ms) of the current when the light is turned on/off. This photoconductivity grows over hundreds of seconds with light exposure and decays slowly when the light is off. The nanorods can be trained via an applied bias to exhibit a short-circuit photocurrent (with corresponding open-circuit photovoltage) that flows in the direction opposite that of the training bias. A qualitative model is proposed, in which conduction occurs through the tightly coupled LUMOs of close-packed porphyrin molecules.

Aggregates of the diacid form of tetrakis(4-sulfonatophenyl)porphine (TPPS), formed in acidic aqueous solutions, were deposited onto hydrophilic (mica) and hydrophobic (polystyrene and graphite) substrates and imaged using atomic force microscopy (AFM). The AFM images revealed the aggregates were either individual rods with a diameter of 3.8 +/- 0.3 mn and a length of 0.77 +/- 0.43 mum or larger structures composed of bundles of individual rods. This study demonstrates that aggregates of the diacid form of TPPS are remarkably straight nanorods with a well-defined height. UV/vis spectroscopy and dynamic light-scattering measurements show that the aggregates form in solution and not on the surface of the substrate, with nucleation of the aggregates being the rate-limiting step, followed by rapid growth at the ends of the rods.