Revealing the Role of Anchoring Groups in the Electrical Conduction Through Single‐Molecule Junctions

A combined experimental and theoretical study is presented revealing the influence of metal–molecule coupling on electronic transport through single‐molecule junctions. Transport experiments through tolane molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups are performed. By fitting the experimental current–voltage characteristics to a single‐level tunneling model, we extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal–molecule coupling. The values found for these parameters are rationalized with the help of density‐functional‐theory‐based transport calculations. In particular, these calculations show that the anchoring groups determine the junction conductance by controlling not only the strength of the coupling to the metal but also the position of the relevant molecular energy levels.     

A Detailed Experimental and Theoretical Study into the Properties of C60 Dumbbell Junctions

A combined experimental and theoretical investigation is carried out into the electrical transport across a fullerene dumbbell one‐molecule junction. The newly designed molecule comprises two C₆₀s connected to a fluorene backbone via cyclopropyl groups. It is wired between gold electrodes under ambient conditions by pressing the tip of a scanning tunnelling microscope (STM) onto one of the C₆₀ groups. The STM allows us to identify a single molecule before the junction is formed through imaging, which means unambiguously that only one molecule is wired. Once lifted, the same molecule could be wired many times as it was strongly fixed to the tip, and a high conductance state close to 10^?2 G₀ is found. The results also suggest that the relative conductance fluctuations are low as a result of the low mobility of the molecule. Theoretical analysis indicates that the molecule is connected directly to one electrode through the central fluorene, and that to bind it to the gold fully it has to be pushed through a layer of adsorbates naturally present in the experiment.     

High-fidelity formation of a molecular-junction device using a thickness-controlled bilayer architecture

The device yield of molecular junctions has become a major issue for the practical application of molecular electronics based on a crossbar system of a metal-molecule-metal (MMM) junction. As the thickness of self-assembled monolayers (SAMs) is typically 1-2 nm, it is difficult to avoid electrical shorts due to the penetration of top metal particles into the SAMs. A simple and effective strategy for the creation of a reliable molecular junction using a thickness-controlled bilayer with a bifunctional heterostructure is presented. In the MMM device, the Au adlayer on the molecular layer is spontaneously formed with deposition of the top Au metals and the sandwiched molecular layer maintains the quality of the SAMs. This method greatly reduces electrical shorts by preventing the diffusion of the top metal electrode and offsetting the surface roughness of the bottom metal electrode, resulting in a device yield of more than 90%.     

Charge Transport through Ferrocene 1,1′‐Diamine Single‐Molecule Junctions

The charge transport through ferrocene 1,1′‐diamine (FDA) molecules between gold electrodes is investigated using the mechanically controllable break junction technique combined with a theoretical framework of density functional theory simulations to understand the physics of these molecular junctions. The characteristic conductances of the molecule are measured at low bias as well as current–voltage (IV) characteristics. By fitting the IV characteristics to the single‐level model, the values for the position of the molecular level, mainly responsible for the transport, and its coupling to the leads, are obtained. The influence of the binding sites, molecular conformation, and electrode distance are systematically studied from a theoretical perspective. While a strong dependence of conductance on the adsorption geometry is found, the decrease of conductance as a function of electrode distance arises mainly from a decrease of coupling strength of the molecular electronic orbitals through a reduced overlap and, to a lesser extent, from a shift of their alignment with respect to the Fermi energy.     

Molecular Switch Controlled by Pulsed Bias Voltages

Recent experiments have shown that the current–voltage characteristics (I–V) of BPDN‐DT (bipyridyl‐dinitro oligophenylene‐ethynylene dithiol) can be switched in a very controlled manner between “on” and “off” traces by applying a pulse in a bias voltage, V_bias. Here, the polaron formation energies are calculated to check a frequently held belief, namely, that the polaron formation can explain the observed bistability. These results are not consistent with such a mechanism. Instead, a conformational reorientation is proposed. The molecule carries an intrinsic dipole moment, which couples to V_bias. Ramping V_bias exerts a force on the dipole that can reorient (“rotate”) the molecule from the ground state (“off”) into a metastable configuration (“on”) and back. By elaborated electronic structure calculations, a specific path for this rotation is identified through the molecule's conformational phase space. It is shown that this path has sufficiently high barriers to inhibit thermal instability but that the molecule can still be switched in the voltage range of the junction stability. The theoretical I–Vs qualitatively reproduce the key experimental observations. A proposal for the experimental verification of the alternative mechanism of conductance switching is presented.     

Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

Molecular electronics seeks to incorporate molecular components as functional elements in electronic devices. There are numerous strategies reported to date for the fabrication, design, and characterization of such devices, but a broadly accepted example showing structure‐dependent conductance behavior has not yet emerged. This progress report focuses on experimental methods for making both single‐molecule and ensemble molecular junctions, and highlights key results from these efforts. Based on some general objectives of the field, particular experiments are presented to show progress in several important areas, and also to define those areas that still need attention. Some of the variable behavior of ostensibly similar junctions reported in the literature is attributable to differences in the way the junctions are fabricated. These differences are due, in part, to the multitude of methods for supporting the molecular layer on the substrate, including methods that utilize physical adsorption and covalent bonds, and to the numerous strategies for making top contacts. After discussing recent experimental progress in molecular electronics, an assessment of the current state of the field is presented, along with a proposed road map that can be used to assess progress in the future.     

Charge Transport Through a Cardan‐Joint Molecule

The charge transport through a single ruthenium atom clamped by two terpyridine hinges is investigated, both experimentally and theoretically. The metal‐bis(terpyridyl) core is equipped with rigid, conjugated linkers of para‐acetyl‐mercapto phenylacetylene to establish electrical contact in a two‐terminal configuration using Au electrodes. The structure of the [Ru^II( L )₂](PF₆)₂ molecule is determined using single‐crystal X‐ray crystallography, which yields good agreement with calculations based on density functional theory (DFT). By means of the mechanically controllable break‐junction technique, current–voltage (I–V), characteristics of [Ru^II( L )₂](PF₆)₂ are acquired on a single‐molecule level under ultra‐high vacuum (UHV) conditions at various temperatures. These results are compared to ab initio transport calculations based on DFT. The simulations show that the cardan‐joint structural element of the molecule controls the magnitude of the current. Moreover, the fluctuations in the cardan angle leave the positions of steps in the I–V curve largely invariant. As a consequence, the experimental I–V characteristics exhibit lowest‐unoccupied‐molecular‐orbit‐based conductance peaks at particular voltages, which are also found to be temperature independent.     

A Single‐Molecular AND Gate Operated with Two Orthogonal Switching Mechanisms

Single‐molecular electronics is a potential solution to nanoscale electronic devices. While simple functional single‐molecule devices such as diodes, switches, and wires are well studied, complex single‐molecular systems with multiple functional units are rarely investigated. Here, a single‐molecule AND logic gate is constructed from a proton‐switchable edge‐on gated pyridinoparacyclophane unit with a light‐switchable diarylethene unit. The AND gate can be controlled orthogonally by light and protonation and produce desired electrical output at room temperature. The AND gate shows high conductivity when treated with UV light and in the neutral state, and low conductivity when treated either with visible light or acid. A conductance difference of 7.3 is observed for the switching from the highest conducting state to second‐highest conducting state and a conductance ratio of 94 is observed between the most and least conducting states. The orthogonality of the two stimuli is further demonstrated by UV–vis, NMR, and density function theory calculations. This is a demonstration of concept of constructing a complex single‐molecule electronic device from two coupled functional units.     

Fe–Co magnetic nanoclusters by density functional theory calculations

ABSTRACT

We present density functional theory results referring to the structural, electronic and magnetic properties of 13, 55, 147 and 309 Fe–Co (magnetic–magnetic) icosahedral nanoclusters (ICO) comparing with our previous results on Fe–Cu (magnetic–nonmagnetic). It came out that the Fe atoms always favour the edge surface sites exhibiting higher average magnetic moment (MM) for Fe and FeCo ICOs than FeCu while the local Fe MM is greater for FeCu₁₂ and Fe₆Cu₄₉ ones. This is due to the strong hybridisation of Fe 3d–Co 3d states, while in Fe–Cu the Fe spin down states are restricted close to fermi without been affected by the corresponding Cu states. These results could be used for the design of environmental sustainable smart magnetic alloys.

This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.