Research Talks

Day 2 (24/05/2022)

Introduction - 10:00 BST

Enrique Escorihuela - 10:10 BST

Nanofabrication of large-area scaffolding architectures for molecular electronics

The properties of advanced materials are closely related to the internal organization of their structures. In this context, the development of the forthcoming generation of molecular optoelectronic devices will require accurate control over the orientation and spatial distribution of functional molecules. Although unimolecular electronics is already a reality, the difficulty of precisely controlling the geometry in the molecule-metal junctions remains a problem. For these reasons, manufacturing multiple unimolecular devices in parallel in the same system is one of the long-standing challenges of this field. In this work, we have developed a versatile, simple, and easy-to-implement strategy to fabricate molecular scaffoldings composed of millions of unimolecular devices in parallel, following a "layer-by-layer" methodology. The procedure combines alternately self-assembled layers of a metalized porphyrin derivative and layers of functional molecules that act as molecular wires. These perpendicularly bind to the metal center of the porphyrin, maintaining a stoichiometric ratio of 1:1 between layers through electrical contact and spaced from each other. This novel strategy allows modulation of the final structure of the scaffold by varying the functional molecule according to the desired application. This methodology makes it possible to extend to a wide variety of compounds and, thereby, towards the fabrication of more complex devices with other potential nanotechnological applications.

Scheme of the layer-by-layer procedure to build the supramolecular scaffolding device.

Zhixin Chen - 10:30 BST

Phase-Coherent Charge Transport through a Porphyrin Nanoribbon-Graphene Junction

Understanding quantum coherence in single-molecule junctions (SMJs) allows the rational design of molecular-scale devices and materials with desirable functions. However to realise these designs, it is necessary to study the hierarchy of quantum interference phenomena that are possible within a given junction and to manipulate these via electric or magnetic fields and temperature. Here we implement such control in a SMJ formed from a porphyrin nanoribbon connected to graphene electrodes and demonstrate that a wide spectrum of quantum coherence can be observed in a single 8 nm-long porphyrin nanoribbon junction, including the Fabry-Pérot interference, Aharonov-Bohm effect, tuneable Kondo and Fano resonances. Our results provide direct experimental demonstration for quantum coherence in single-molecule junctions, which would be a milestone for molecular electronics and quantum-coherent nanoscience.

a Schematic representation of phase-coherent electron transport through a graphene-Ni-FP8 porphyrin nanoribbon-graphene junction. The purple shaded indicates the HOMO of the nanoribbon. b Device architecture with electronic circuit and (c) false-colour scanning electron microscope (SEM) image of the device. d Differential conductance (G = dIsd/dVsd) as a function of bias voltage (Vsd) and gate voltage (Vg) for the device. The N–1 charge state is highlighted by grey diamond.

Ya-Hao Wang - 10:50 BST

Probing Molecular Adsorption and Electrochemical Processes on Au(111) with STM Break-Junction Technique

A molecular-level understanding of the interfacial process is fundamentally important in sensors, catalysis, energy harvesting and storage, biochemistry, and molecular electronics. Achieving this level of characterization requires techniques that can probe the interfacial molecular structures, especially distinguishing single-molecule adsorption to avoid averaging effects. However, this remains a big challenge. Recently, STM breaking junction (STM-BJ) technique has recently been developed to insight into adsorbate–surface interaction and adsorbate orientation/configuration based on the statistical analysis of characteristic single-molecule conductance peaks and values.1-3 This enables STM with chemical resolution to probe interfacial process and molecular adsorption on single-crystal model surfaces.

Therefore, our research group has recently employed the STM-BJ to probe interfacial electronic effect upon tuning adsorption geometries of bipyridine molecules and electron transport at atomically-flat Au(111). It is interestingly found that both 1-butyl-3-methylimidazolium (BMI) cation-containing ionic liquids and co-assembled 1-ethylimidazole (EIM) on Au (111) can stabilize a vertical orientation through σ-bond interaction of pyridine’s nitrogen atom; resulting in a uniform configuration for these single-molecule junction. In addition, the acid-base chemistry of carboxylic acid-based molecules at Au (111) is also examined, proposing a prototype of single-molecule pH sensor. Further, the electrochemical process of bipyridine and carboxylic acid-based molecules at Au(111)/ electrolyte interfaces are investigated by STM-BJ, which proves the electrochemically-induced radical cation of bipyridine in BMIPF6 ionic liquid and reversible gate of molecular contact of carboxylic molecules in aqueous solution. These provide a new class of electrochemically-activated molecular switches with giant on/off ratio.

Intermission - 11:10 BST

Xinkai Qiu - 11:20 BST

Photosystem I: From Ensemble Junctions to Functional Devices

Photosystem I (PSI) is a functional nano-device that is produced by and readily isolated from photosynthetic organisms. It is a trimeric membrane protein complex that converts photon energy into separated electron/hole pairs to generate chemical reduction potentials as part of natural photosynthesis. Due to the difference in hydrophobicity on the periphery, PSI can be induced to self-assemble on surfaces with a preferred orientation; in particular, linkers with hydrophilic terminal groups form strong hydrogen bonds with the hydrophilic luminal or stromal surface of PSI trimers, positioning either the P700 reaction center or the FB iron-sulfur complex next to the substrate. We used self-assembled monolayers (SAMs) of thiols on Au and peptides engineered by phage display on tin-doped indium oxide (ITO) to orient PSI with near-perfect selectivity. This high degree of anisotropy translates into the temperature-independent rectification of tunneling current that depends on the absolute orientation of PSI complexes. Moreover, the degree and direction of orientation can be related to the magnitude and polarity of rectification empirically in conductive probe atomic force microscopy (CP-AFM) and large-area junctions comprising eutectic Ga-In (EGaIn) electrodes, giving rise to soft biophotovoltaic devices based on these assemblies of oriented PSI on Au. Recently, we introduced the self-assemblies of fullerene (C60) derivatives on to PSI junctions to i) mitigate the series resistance created by the tunneling barrier between the PSI and the electrode, and ii) amplify the rectification on those junctions for practical application, e.g., logic circuits. In this system, resistors and diodes exhibit efficient charge-transport over a distance of approximately 10 nm. We have surveyed several possible mechanisms for this phenomenon and will share in detail during the talk.

Leopoldo Mejia - 11:40 BST

Microscopic Theory, Analysis, and Interpretation of Conductance Histograms in Molecular Junctions

We develop a rigorous microscopic theory for the conductance dispersion encountered in molecular electronics break-junction experiments by merging the theory of force-spectroscopy with molecular conductance. These experiments are widely used to investigate fundamental physics and chemistry at the nanoscale. However, they exhibit a broad conductance dispersion that, while statistically reproducible, limits their utility to resolve single-molecule events. To capture the conductance histograms, we propose a microscopic model of the junction evolution under the driving of external mechanical forces and combine it with the statistics of junction rupture and formation. Our formulation is based on the hypothesis that the dispersion in the conductance is dominated by conductance changes as the junction is mechanically manipulated while the shape of the histogram is determined by the stochastic nature of junction rupture and formation. The procedure yields analytical equations for the conductance distribution in terms of parameters that describe the free-energy profile of the system, the intrinsic properties of the molecule, and the mechanical manipulation of the junction. Our theory can be used to capture the conductance histogram of benchmark break-junction experiments and augment the information content that can be extracted from them. Further, the predicted behavior with respect to physical parameters can be used to design experiments with narrower conductance distribution and to test the range of validity of the hypothesis.

Boyuan Zhang - 12:00 BST

Short Molecular Insulator Based on Quantum Interference

Designing highly insulating sub-nanometer molecules is difficult because tunneling conductance increases exponentially with decreasing molecular length. This challenge is further enhanced by the fact that most molecules cannot achieve full conductance suppression with destructive quantum interference. Here, we present results for a series of small saturated heterocyclic alkanes where we show that conductance is suppressed due to destructive interference. Using the STM-BJ technique and density functional theory calculations, we confirm that their single-molecule junction conductance is lower than analogous alkanes of similar length. We rationalize the suppression of conductance in the junctions through analysis of the computed ballistic current density. We find there are highly symmetric ring currents, which reverse direction at the antiresonance in the Landauer transmission near the Fermi energy. This pattern has not been seen in earlier studies of larger bicyclic systems exhibiting interference effects and constitutes clear-cut evidence of destructive -interference. The finding of heterocyclic alkanes with destructive quantum interference charts a pathway for chemical design of short molecular insulators using organic molecules.

Concluding Remarks - 12:20 BST

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