Design of an efficient coherent multi-site single-molecule rectifier

Abstract

aWe propose the design of a single-molecule diode with a rectification ratio exceeding a million. The employed mechanism is based on coherent resonant charge transport across a molecule that consists of four conjugated sites coupled by non-conjugated bridges. Using density functional theory calculations, we rationalize the design of the molecule and demonstrate the crucial role of aligning the sites at a specific voltage. Rectification ratios are calculated for a series of chemical substituents and demonstrate that with careful molecular design, high rectification ratios can be achieved. Finally, we comment on the shortcomings of our approach, how further improvements can be obtained and discuss some of the experimental challenges.Molecular diodes have been attracting much attention recently, both in self-assembled monolayers 1-7 and on the single-molecule scale. [8][9][10][11][12][13][14][15][16] Even though the rectifier has been the seminal device which launched the field of single-molecule electronics, 17 up to date, their performance have many shortcomings. In particular, their rectification ratios (RR) are orders of magnitude lower than typical semiconducting rectifiers. The highest RR reported experimentally on a single-molecule junction 15 is about 600, and is no match for a semiconducting diode where ratios exceeding 10 6 are common. In that study, a molecular backbone was considered which consisted of two identical, weakly-coupled conjugated parts. In such a system, current flows coherently when both halves are on resonance, i.e., the sites are aligned with each other within the bias window. The alignment of the two sites can be modified by applying a bias voltage and/or by chemical substituents. With proper design, one can align the sites at zero bias, while misaligning them at finite bias, yielding negative differential conductance. 18 One can also align the sites at a finite bias, 15,19-22 and achieve single-molecule rectification.Based on a similar principle, here we investigate theoretically how the rectification ratio of single-molecule diodes can be significantly enhanced by the addition of more sites in series, as shown in as presented in Fig. 1b. Low RR are primarily caused by a high reverse current. Adding more sites in series leads to a strong suppression of the reverse current, thereby drastically improving the RR. However, the addition of sites comes at the cost of an increased molecular complexity and number of model parameters. In this study, we first focus on the design of a molecule which behaves as having four sites in series. For this purpose, we use density functional theory (DFT) calculations. We then investigate how the molecule can be engineered by the means of chemical substitution to align the sites at a particular bias voltage. We eventually show that the molecule designed with four sites represents a drastic improvement compared to a two-site molecule, with RR approaching 10 6 , competitive with siliconbased diodes.1 Design guidelines Fig. 1 depicts the concept of a rectifier...

Single-molecule functionality in electronic components based on orbital resonances

“…The IV is highly asymmetric and shows a current peak at a voltage of 1.1 V. The high asymmetry becomes even more clear from the rectification ratio, shown in the inset, reaching over 1 Â 10 6 . To the best of our knowledge, this rectification ratio is the highest predicted in literature for a single-molecule junction, 101 exceeding even that of state-of-the-art self-assembled monolayers. 79 Importantly, our mechanism is internal to the molecule, and does not rely on Coulomb interaction with the electrodes.…”

“…With the symmetric and asymmetric two-site model being verified both theoretically as well as experimentally, we now proceed to the theoretical investigation of molecule 4, which possesses four sites in series. Following the guidelines described in Section 5, molecule 4 was proposed, 101 with ethylene groups containing only two carbon atoms used as sites. The molecule also has linkers consisting of 4 C-sp 3 for an optimized inter-site coupling.…”

“…Similar to the two-site case, the on-site energy can be manipulated using electronegative substituents. 101 With the proper choice of side groups, the sites can be aligned at a particular bias voltage. This scenario is displayed in Fig.…”

Single-molecule quantum-transport phenomena in break junctions

“…However, before molecular electronic components can be fabricated, the single-molecule building blocks need to be characterized in junctions to understand their electronic behaviour. To this end, tremendous advances have been reported and reviewed 3,4,5,6,7,8,9 ; for example, single-molecule transistors, 10 switches 11 and diodes 12,13,14,15 have been demonstrated. Nevertheless, it is still difficult to experimentally identify the physical mechanisms behind these functionalities or how single molecules arrange in an electronic junction; therefore, predictions based on existing models for molecular device functionality are often unreliable.…”

Electrical and spin switches in single‐molecule junctions

“…The initially ultimate goal of molecular electronics is to build integrated circuits for the next generation of super‐fast computer and ultrahigh density information storage. Currently, many functional single‐molecule electronic components, including molecular wires, molecular switches, molecular rectifiers, molecular transistors, and sensors, have been realized. In addition, many novel physical phenomena, such as quantum interference, Coulomb blockade, and Kondo effect, were discovered in the process of investigating charge transport within molecular junctions.…”