Ambient Bistable Single Dipole Switching in a Molecular Monolayer

Switchable molecular electrocatalysis

We demonstrate a switchable electrocatalysis mechanism modulated by hydrogen bonding interactions in ligand geometries. By manipulating these geometries, specific electrochemical processes at a single catalytic site can be selectively and precisely activated or deactivated. The α geometry enhances dioxygen electroreduction (ORR) while inhibiting protium redox processes, with the opposite effect seen in the β geometry. Intramolecular hydrogen bonding in the α geometry boosts electron density at the catalytic center, facilitating a shift of ORR to a 4-electron pathway. Conversely, the β geometry promotes a 2-electron ORR and facilitates electrocatalytic hydrogen evolution through an extensive proton charge assembly; offering a paradigm shift to conventional electrocatalytic principles. The expectations that ligand geometry induced electron density modulations in the catalytic metal centre would have a comparable impact on both ORR and HER has been questioned due to the contrasting reactivity exhibited by α-geometry and β-geometry molecules. This further emphasizes the complex and intriguing nature of the roles played by ligands in molecular electrocatalysis.

Supramolecular Memristor Based on Bistable [2]Catenanes: Toward High-Density and Non-Volatile Memory Devices

The ever-increasing demand for data storage and neuromorphic computing calls for innovative, high-density solutions, such as resistive random-access memory (RRAM). However, the integration of resistive switching and rectification at the nanoscale remains a formidable challenge. In this study, we introduce a bistable [2]catenane-based supramolecular junction that simultaneously functions as a resistive switch and a diode. All supramolecular junctions are highly stable and reproducible over thousands of resistive switching cycles, because the nano-confinement of two mechanically interlocked rings can stabilize the radical states of pyridinium moieties under ambient conditions. The successful realization of supramolecular junctions in functionality with a thickness of approximately 2 nm presents a promising avenue for the development of molecule-scale based RRAM for a better solution to high density and energy efficiency.

STM studies on porphyrins and phthalocyanines at the liquid/solid interface for molecular-scale electronics

Porphyrins and phthalocyanines are promising candidates for single-molecule electronics. Among the many characterization tools, scanning tunneling microscopy (STM) represents a very powerful one to gain insight into the electronic properties at the molecular level, by correlating the charge transport behaviours of π-conjugated molecules with ultrahigh resolution imaging. In view of the sophistication of molecular self-assembly in the presence of a solution phase, in this frontier, we focus on STM studies on porphyrins and phthalocyanines at the liquid/solid interface, placing emphasis on the electronic and magnetic properties, as well as the switching behaviour of surface-confined or surface-anchored molecules. Furthermore, we have also addressed the topics of potential that can be exploited in this area.

Dipolar Noise in Fluorinated Molecular Wires

We demonstrate a strategy to directly map and quantify the effects of dipole formation on electrical transports and noises in the self-assembled monolayers (SAMs) of molecular wires. In this method, the SAM patterns of fluorinated molecules with dipole moments were prepared on conducting substrates, and a conducting probe in contact-mode atomic force microscopy was utilized to map currents and noises through the probe on the molecular patterns. The maps were analyzed to extract the characteristic parameters of dipolar noises in SAMs, and the results were compared with those of hydrogenated molecular patterns without dipole moments. At rather low bias conditions, the fluorinated molecular junctions exhibited a tunneling conduction and a resistance value comparable to that of the hydrogenated molecules with a six-times-longer length, which was attributed to stronger dipoles formation in fluorinated molecules. Interestingly, conductance (G) in different regions of fluorinated molecular patterns exhibited a strong correlation with a noise power spectral density of S_I/I² like S_I/I² ∝ G^-2, which can be explained by enhanced barrier fluctuations produced by the dipoles of fluorinated molecules. Furthermore, we observed that the noise power spectral density of fluorinated molecules showed an anomalous frequency (f) dependence like S_I/I² ∝ 1/f^1.7, possibly due to the slowing down of the tunneling of carriers from increased barrier fluctuations. In rather high bias conditions, conductions in both hydrogenated and fluorinated molecules showed a transition from tunneling to thermionic charge transports. Our results provide important insights into the effects of dipoles on mesoscopic transport and resistance-fluctuation in molecules and could have a significant impact on the fundamental understanding and applications in this area.

TiO x /Pt3Ti(111) surface-directed formation of electronically responsive supramolecular assemblies of tungsten oxide clusters

Highly ordered titanium oxide films grown on a Pt₃Ti(111) alloy surface were utilized for the controlled immobilization and tip-induced electric field-triggered electronic manipulation of nanoscopic W₃O₉ clusters. Depending on the operating conditions, two different stable oxide phases, z'-TiO _x and w'-TiO _x , were produced. These phases show a strong effect on the adsorption characteristics and reactivity of W₃O₉ clusters, which are formed as a result of thermal evaporation of WO₃ powder on the complex TiO _x /Pt₃Ti(111) surfaces under ultra-high vacuum conditions. The physisorbed tritungsten nano-oxides were found as isolated single units located on the metallic attraction points or as supramolecular self-assemblies with a W₃O₉-capped hexagonal scaffold of W₃O₉ units. By applying scanning tunneling microscopy to the W₃O₉-(W₃O₉)₆ structures, individual units underwent a tip-induced reduction to W₃O₈. At elevated temperatures, agglomeration and growth of large WO₃ islands, which thickness is strongly limited to a maximum of two unit cells, were observed. The findings boost progress toward template-directed nucleation, growth, networking, and charge state manipulation of functional molecular nanostructures on surfaces using operando techniques.

Reliable Computational Prediction of the Supramolecular Ordering of Complex Molecules under Electrochemical Conditions

We propose a computationally lean, two-stage approach that reliably predicts self-assembly behavior of complex charged molecules on metallic surfaces under electrochemical conditions. Stage one uses ab initio simulations to provide reference data for the energies (evaluated for archetypical configurations) to fit the parameters of a conceptually much simpler and computationally less expensive force field of the molecules: classical, spherical particles, representing the respective atomic entities; a flat and perfectly conducting wall represents the metallic surface. Stage two feeds the energies that emerge from this force field into highly efficient and reliable optimization techniques to identify via energy minimization the ordered ground-state configurations of the molecules. We demonstrate the power of our approach by successfully reproducing, on a semiquantitative level, the intricate supramolecular ordering observed experimentally for PQP+ and ClO4- molecules at an Au(111)-electrolyte interface, including the formation of open-porous, self-host-guest, and stratified bilayer phases as a function of the electric field at the solid-liquid interface. We also discuss the role of the perchlorate ions in the self-assembly process, whose positions could not be identified in the related experimental investigations.