As electronics miniaturization heads towards a theoretical physical limit in the tens of nanometers, new methods of manufacturing are required to produce transistors, diodes, and other fundamental electronic components. In this vein, a new range of molecule-sized devices have been created in the laboratory, though with varying results in terms of efficiency and practicality. Now a group of researchers from Berkeley Lab and Columbia University claims to have created the highest-performing, single-molecule diode ever made, which is said to be 50 times better in performance and efficiency than anything previously produced.

Ordinary diodes are usually constructed from silicon with a p-n (positive-negative) junction created at the point of contact between a positively "doped" semiconductor (that is, one that has had its electrical properties altered with additives) and a negatively doped one. Flanked by connecting electrodes (an anode on one side and a cathode on the other), the most common function of such a diode is to permit electric current to flow in one direction only, whilst blocking current from flowing in the reverse direction. As such, a diode used in this way can be seen as a type of flow-control valve that is either "on" or "off". Technically, this one-way behavior is known as rectification as it can, for example, be used to rectify alternating current to direct current, and so these types of diodes are known as rectifiers.

This on/off – asymmetric – behavior in the nascent field of molecule-sized electronics, on the other hand, is usually achieved by the creation of molecules that chemically emulate the p-n junction. However, these synthesized molecular junctions have generally resulted in poor forward current flow capabilities and inefficient or patchy rectification. This is where the Columbia university scientists claim to have made significant improvements with their new single-molecule diode.

Diagram of the molecular junction that functions as a diode, allowing current to flow in one direction only(Credit: Columbia University)

"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," said Jeffrey Neaton, director of the Berkeley Lab’s Molecular Foundry and professor at the University of California Berkeley. "The asymmetry necessary for diode behavior originates with the different exposed electrode areas and the ionic solution. This leads to different electrostatic environments surrounding the two electrodes and superlative single-molecule device behavior."

First mooted in 1974 by Mark Ratner and Arieh Aviram, an asymmetric molecule that could act as a rectifier has been a long sought after goal, particularly as diodes form the basis of many microminiature electronic devices. Since then, a range of devices have been constructed, including single molecule diodes and transistors. Operating at this nanoscale, though, such devices may emulate their macro counterparts, but that behavior is merely a simulation; at such scales the electronic operation of these devices is governed more by quantum influences.

"Electron flow at molecular length-scales is dominated by quantum tunneling," said professor "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. This method allowed myself and Zhenfei Liu to understand the diode behavior quantitatively."

Zhenfei Liu – a postdoctoral fellow at Berkeley Lab – and professor Neaton worked with Latha Venkataraman and Luis Campos from Columbia University to create their high-performance rectifier diode using junctions prepared from symmetric molecules attached to gold electrodes. To achieve the necessary asymmetric properties required to operate as a diode, the researchers then altered the surface area of the electrodes as they were exposed to an ionic solution. As a result, a positive voltage increased the current significantly, whilst a negative voltage reduced current flow in an equally significant manner.

"The ionic solution, combined with the asymmetry in electrode areas, allows us to control the junction’s electrostatic environment simply by changing the bias polarity," said professor Neaton. "In addition to breaking symmetry, double layers formed by ionic solution also generate dipole differences at the two electrodes, which is the underlying reason behind the asymmetric shift of molecular resonance. The Columbia group’s experiments showed that with the same molecule and electrode setup, a non-ionic solution yields no rectification at all."

The combined Berkeley Lab-Columbia University research team is convinced that the way they have managed to produce a single-molecule diode sets the benchmark for future nonlinear nanoscale device tuning and development, with applications above and beyond just junctions of single-molecule components.

"We expect the understanding gained from this work to be applicable to ionic liquid gating in other contexts, and mechanisms to be generalized to devices fabricated from two-dimensional materials," said professor Neaton. "Beyond devices, these tiny molecular circuits are petri dishes for revealing and designing new routes to charge and energy flow at the nanoscale. What is exciting to me about this field is its multidisciplinary nature – the need for both physics and chemistry – and the strong beneficial coupling between experiment and theory. With the increasing level of experimental control at the single-molecule level, and improvements in theoretical understanding and computational speed and accuracy, we’re just at the tip of the iceberg with what we can understand and control at these small length scales."

The results of this research were recently published in the journal Nature Nanotechnology.

Source: Berkeley Lab

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