“Using a single symmetric molecule, an ionic solution and two gold electrodes of dramatically different exposed surface areas, we were able to create a diode that resulted in a rectification ratio, the ratio of forward to reverse current at fixed voltage, in excess of 200, which is a record for single-molecule devices,” says Jeff Neaton, Director of the Molecular Foundry. “The asymmetry necessary for diode behavior originates with the different exposed electrode areas and the ionic solution,” he says. “This leads to different electrostatic environments surrounding the 2 electrodes and superlative single-molecule device behavior.”
With “smaller and faster” as the driving mantra of the electronics industry, single-molecule devices represent the ultimate limit in electronic miniaturization.
>>A typical diode consists of a silicon p-n junction b/n a pair of electrodes (anode, cathode) that serves as the “valve” of an electrical circuit, directing the flow of current by allowing it to pass through in only “forward” direction. Asymmetry of a p-n junction presents the electrons with an “on/off” transport environment. “Electron flow at molecular length-scales is dominated by quantum tunneling…The efficiency of the tunneling process depends intimately on the degree of alignment of the molecule’s discrete energy levels with the electrode’s continuous spectrum. In a molecular rectifier, this alignment is enhanced for positive voltage, leading to an increase in tunneling, and is reduced for negative voltage. At the Molecular Foundry we developed an approach to accurately compute energy-level alignment and tunneling probability in single-molecule junctions.”
Neaton, Liu etc fabricated a high-performing rectifier from junctions made of symmetric molecules with molecular resonance in nearly perfect alignment with the Fermi electron energy levels of the gold electrodes. Symmetry was broken by a substantial difference in the size of the area on each gold electrode that was exposed to the ionic solution. Owing to asymmetric electrode area, the ionic solution, and junction energy level alignment, a +ve voltage increases current substantially; a-ve voltage suppresses it equally significantly. The new approach to a single-molecule diode provides a general route for tuning nonlinear nanoscale-device phenomena that could be applied to systems beyond single-molecule junctions and two-terminal devices. http://newscenter.lbl.gov/2015/07/29/high-performance-single-molecule-diode/
Schematic of the molecular junction created using asymmetric area electrodes which functions as a diode, allowing current to flow in one direction only. Credit: Berkeley Lab and Columbia University
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