One of the main goals of organic and molecular electronics is to relate the performance and electronic function of devices to the chemical structure and intermolecular interactions of the organic component inside them, which can take the form of an organic thin film, a self-assembled monolayer or a single molecule. This goal is difficult to achieve because organic and molecular electronic devices are complex physical-organic systems that consist of at least two electrodes, an organic component and two (different) organic/inorganic interfaces. Singling out the contribution of each of these components remains challenging. So far, strong π-π interactions have mainly been considered for the rational design and optimization of the performances of organic electronic devices, and weaker intermolecular interactions have largely been ignored. Here, we show experimentally that subtle changes in the intermolecular van der Waals interactions in the active component of a molecular diode dramatically impact the performance of the device. In particular, we observe an odd-even effect as the number of alkyl units is varied in a ferrocene-alkanethiolate self-assembled monolayer. As a result of a more favourable van der Waals interaction, junctions made from an odd number of alkyl units have a lower packing energy (by ∼0.4-0.6 kcal mol(-1)), rectify currents 10 times more efficiently, give a 10% higher yield in working devices, and can be made two to three times more reproducibly than junctions made from an even number of alkyl units.

A challenge in molecular electronics is to control the strength of the molecule-electrode coupling to optimize device performance. Here we show that non-covalent contacts between the active molecular component (in this case, ferrocenyl of a ferrocenyl-alkanethiol self-assembled monolayer (SAM)) and the electrodes allow for robust coupling with minimal energy broadening of the molecular level, precisely what is required to maximize the rectification ratio of a molecular diode. In contrast, strong chemisorbed contacts through the ferrocenyl result in large energy broadening, leakage currents and poor device performance. By gradually shifting the ferrocenyl from the top to the bottom of the SAM, we map the shape of the electrostatic potential profile across the molecules and we are able to control the direction of rectification by tuning the ferrocenyl-electrode coupling parameters. Our demonstrated control of the molecule-electrode coupling is important for rational design of materials that rely on charge transport across organic-inorganic interfaces.

We studied the electrochemical behavior of self-assembled monolayers (SAMs) of n-alkanethiolates with ferrocenyl (Fc) termini on gold (S­(CH2) n Fc, n = 0–15) in relation to their supramolecular structures (characterized by photoemission spectroscopy (PES) supported by molecular dynamics (MD) simulations) to elucidate the origin of nonideal features commonly observed in cyclic voltammograms (CVs) of these SAMs by systematically changing n from 0 to 15. For all SAMs the CV data are dominated by a main peak for Fc units directly in contact with the electrolyte solution and interacting with neighboring Fc units. A second peak is observed for SAMs with n ≥ 2 ascribed to partially buried Fc units as a result of lattice strain due to the different sizes of the lattices of the Fc units and the sulfur atoms. For thick SAMs with strong molecule–molecule interactions, the strain is large, resulting in a third peak due to Fc units that are well shielded from the electrolyte by other Fc units. Our results do not agree with widely used explanations to account for peak splitting involving isolated Fc units vs clustered Fc units, disordered vs ordered domains, or back bending (which requires the formation of Gauche defects). In contrast, we show that the peak splitting and peak broadening is an inherent property of densely packed SC n Fc SAMs with Fc units in different electrochemical environments to release the buildup of strain. On the basis of our results we propose a simple model to explain all the nonideal features observed in CV data. In addition, we show that this model can also explain the abnormal shapes of CV recorded on S­(CH2)11Fc SAMs formed on very rough and defective electrodes or derived from the corresponding disulfide, i.e., (S­(CH2)11Fc)2.

Layer-by-layer-stacked chemical vapour deposition (CVD) graphene films find applications as transparent and conductive electrodes in solar cells, organic light-emitting diodes and touch panels. Common to lamellar-type systems with anisotropic electron delocalization, the planeto-plane (vertical) conductivity in such systems is several orders lower than its in-plane conductivity. The poor electronic coupling between the planes is due to the presence of transfer process organic residues and trapped air pocket in wrinkles. Here we show the plane-to-plane tunnelling conductivity of stacked CVD graphene layers can be improved significantly by inserting 1-pyrenebutyric acid N-hydroxysuccinimide ester between the graphene layers. The six orders of magnitude increase in plane-to-plane conductivity is due to hole doping, orbital hybridization, planarization and the exclusion of polymer residues. Our results highlight the importance of interfacial modification for enhancing the performance of LBL-stacked CVD graphene films, which should be applicable to other types of stacked two-dimensional films.

Understanding the charge transport properties in general of different molecular components in a self-assembled monolayer (SAM) is of importance for the rational design of SAM molecular structures for molecular electronics. In this study, we study an important aspect of the charge transport properties, i.e. the charge transfer (CT) dynamics between the active molecular component (in this case, the ferrocenyl moieties of a ferrocenyl-n-alkanethiol SAM) and the electrode using synchrotron-based core-hole clock (CHC) spectroscopy. The characteristic CT times are found to depend strongly on the character of the ferrocenyl-derived molecular orbitals (MOs) which mediate the CT process. Furthermore, by systemically shifting the position of the ferrocenyl moiety in the SAM, it is found that the CT characteristics of the ferrocenyl MOs display distinct dependence on its distance to the electrode. These results demonstrate experimentally that the efficiency and rate of charge transport through the molecular backbone can be modulated by resonant injection of charge carriers into specific MOs.