Schedule

2020 Speakers

17th December 2020

4PM GMT (UK Time)

Prof. Ignacio Franco

University of Rochester, U.S.A.

Frontiers in the Atomistic Simulation of Molecular Junctions

In this talk, I will summarize our efforts to bridge the gap between atomistic simulations and experiments in molecular junctions. First, I will introduce a non-reactive force field for metal-molecule interactions that enables the simulation of STM and AFM experiments in molecular junctions on experimentally relevant time and length scales. The force field is employed to model and atomistically understand STM experiments that measure the conductance and force of a single polyfluorene and defected graphene nanoribbon on Au(111) as a continuous function of its length. Then, I will summarize our efforts to model break-junction experiments with statistics and discuss the computational challenges in reproducing the experimental conductance histograms. Through computations, I will discuss the relative importance of possible contributing factors to the wide conductance dispersion encountered in the experiments.References:[1] Z. Li, L. Mejía, J. Marrs, H. Jeong, J. Hihath, and I. Franco “Understanding the Conductance Dispersion of Single-Molecule Junctions”, J. Phys. Chem. C (in press)
[2] Z. Li and I. Franco, “Molecular electronics: Toward the atomistic modeling of conductance histograms” J. Phys. Chem. C, 123, 9693 (2019)
[3] L. Mejía and I. Franco, “Force-conductance spectroscopy of a single-molecule reaction” Chem. Sci., 10, 3249-3256 (2019)
[4] M. Koch, Z. Li, C. Nacci, T. Kumagai, I. Franco and L. Grill, “How structural defects affect the mechanical and electrical properties of single molecular wires” Phys. Rev. Lett., 121, 047701 (2018)
[5] Z. Li, A. Tkatchenko and I. Franco, “Modeling Non-Reactive Molecule-Surface Systems on Experimentally Relevant Time and Length Scales: Dynamics and Conductance of Polyfluorene on Au(111)” J. Phys. Chem. Lett., 9, 1140 (2018)

10th December 2020

4PM GMT (UK Time)

Prof. Michael Inkpen

University of Southern California, U.S.A.

Metal-Appended Wires and Framework Fragments

Metal sites provide an intriguing handle to modulate the conductance properties of molecular junctions, through variation of the metal identity, surrounding ligand environment, or redox state. We describe scanning tunneling microscope-based break junction (STM-BJ) studies of ruthenium butenynyl complexes, conjugated carbon-based wires bearing an appended metal fragment. Here, metal coordination also influences junction conductance through modulation of C-C bond lengths in the wire. Metals also play important roles in the conductivity of extended molecular materials such as metal-organic frameworks (MOFs). We are interested in how STM-BJ studies of “framework fragments”, model molecules with framework-inspired structures, may provide insights into the electronic properties of their analogous bulk systems. We present initial STM-BJ investigations of transport across different linkage groups used in the construction of covalent-organic frameworks (COFs, metal-free analogues of MOFs), and show these groups are also capable of binding to gold.

3rd December 2020

2PM GMT (UK Time)

Dr. Shi-Xia Liu

University of Bern, Switzerland

Regulation and Control of Charge Transport through Single-Molecule Junctions

The study of single molecules trapped between two metallic electrodes provides insight into charge transport processes on the molecular level and, with the proper understanding and good control of charge flow, enables the design of smart molecular-scale junctions for diverse applications, e.g. in switches, diodes, transistors or for thermoelectric energy harvesting. Our recent work has centered on identifying the design principles to gain control over the charge flow in single molecules. Using chemical synthesis, a variety of strategies was applied to improve and facilitate conductance gating in single-molecule junctions.[1-4] In particular, connectivity-driven charge transport measurements resulted in a striking conceptual advance in comprehending the relationship between molecular structure and conductance.[5-6] Recently, we reported the first realization of quantum interference (QI) effects on the electrical conductance of massively-parallel arrays of molecules in self-assembled monolayers (SAM).[7] This breakthrough means that it is now feasible to utilize QI in the design of new functional, ultra-thin film materials and devices. In my talk, I will illustrate how to control charge transport not only in single-molecule junctions, but also in SAM-based transistors with a good understanding of QI effects manipulated by chemical modification.
[1] Angew. Chem. Int. Ed. 2017, 56, 173[2] Angew. Chem. Int. Ed. 2015, 54, 14304[3] Chem. Eur. J. 2020, 26, 5264[4] Nanoscale 2018, 10, 18131[5] J. Am. Chem. Soc. 2015, 137, 4469[6] Chem. Eur. J. 2018, 24, 4193[7] Chem 2019, 5, 474-484.

19th November 2020

2PM GMT (UK Time)

Prof. Masha Kamenetska

Boston University, U.S.A.

Conductance of Molecular Junctions With Transition Metal Centers Wired via 5-Membered Rings

Conductance of single molecules is exquisitely sensitive to the atomic arrangement and chemical nature of the interface. I will describe how we leverage this property of conductance to develop tools for mapping the Angstrom-scale features of single molecule junctions. I will then apply these experimental approaches to explore conductance and geometry of transition metal centers wired in single molecule junctions via 5-membered rings in different configurations, including Group VIII metallocenes and in situ assembled imidazole-linked coordination complexes.

12th November 2020

2PM GMT (UK Time)

Dr. Andrea Vezzoli

University of Liverpool, United Kingdom

Mechanoresistive Molecular Junctions

Molecular junctions that show electrical response to mechanical stress such as stretching or compression are nanoelectromechanical devices where quantum mechanical effect can be exploited to attain high efficiency. We reported in 2017 the first strategy of chemical control of such effects.[1] In our devices, based on substituted 4,4’-bipyridyls, we were able to control the magnitude of the electromechanical response (or completely turning it OFF) by adding different substituents on the molecular wire, exploiting side-groups that could interact with the electrode upon compression of the junction. More recently, we reported on a more general strategy, based on the use of hemilabile ligands as molecular termini, that can transition between a monodentate to a chelate/bidentate electrode binding configuration as the junction is compressed. [2] These devices can be driven at very high frequencies (up to 10 kHz), with no sign of fatigue or deterioration in the electrical response. DFT calculations demonstrate that the above effects are quantomechanical in origin, arising from greater electronic coupling of the molecules to the electrodes in the compressed state, resulting in broadened orbital resonances, more efficient transport channels and significantly greater conductance.
All these phenomena are however dependent on the molecule-electrode interface, and therefore difficult to translate into functional, large area devices. We focussed our efforts on finding a system that has the mechanoresistive moiety embedded in the molecular backbone, and we found an analogue of benzil that fulfilled our expectations.[3] In this compound, mechanical compression of the junction triggers a conformational change to a structure where a through-space charge transport pathway effectively shortcuts a non-conjugated portion of the molecular wire. Noise density analysis demonstrate the presence of through-space tunnelling in the compressed junction, thus highlighting a new strategy for the introduction of mechanosensitive behaviour in single-molecule devices.
In all cases, junction size modulation of only 3-4 Å resulted in conductance changes of orders of magnitude, making these devices exquisitely sensitive nanoelectromechanical systems.

[1] Angewandte Chemie Int. Ed. 2017 DOI: 10.1002/anie.201709419[2] Angewandte Chemie Int. Ed. 2019 DOI: 10.1002/anie.201906400 [3] Nano Letters 2020 DOI: 10.1021/acs.nanolett.0c02815

5th November 2020

4PM GMT (UK Time)

Prof. Kun Wang

Mississippi State University, U.S.A.

Mapping plasmonic hot carriers via single molecule transport measurements

The generation of hot-carriers in plasmonic nanostructures, via plasmon decay, is of great current interest as hot-carriers hold promise for the development of a variety of novel technologies, including plasmon-driven photocatalysis, bandgap-free photodetection and photothermal therapy.[1] A key aspect that is central to the design and development of these applications is knowledge of the energy distributions of the generated hot-carriers under steady-state conditions. However, direct experimental elucidation of steady-state energy distributions of hot-carriers in plasmonic nanostructures, which is key for systematically advancing and rationally engineering the aforementioned technologies, has not been possible to date.In this talk, I will present our recent work in developing a novel scanning probe-based approach that overcomes this outstanding challenge and show for the first time that transport measurements from single molecule junctions, created by trapping suitably chosen single molecules between an ultra-thin gold film supporting surface plasmon polaritons and a scanning probe tip, can enable quantification of plasmonic hot-carrier energy distributions.[2] Our work reveals that Landau damping is the dominant physical mechanism of hot-carrier generation in nanoscale systems with strong confinement. The approaches developed in this work will enable fundamental insights into hot-carrier generation processes that are critical for future hot-carrier assisted technologies.
References:
[1]. ML Brongersma, NJ Halas, P Nordlander, Nature Nanotechnology 10, 25–34 (2015)[2]. H Reddy, K Wang, Z Kudyshev, L Zhu, S Yan, A Vezzoli, SJ Higgins, V Gavini, A Boltasseva, P Reddy, VM Shalaev, E Meyhofer, Science 369, 423-426 (2020)

29th October 2020

10AM GMT (UK Time)

Prof. Paul Low

UWA, Australia

The curious case of conductive carbon: from models to molecular junctions.

Poly-ynes and other linear ‘all-carbon’ (or at least ‘carbon-rich’) compounds have inspired interest of synthetic chemists, materials scientists, physics and device engineers for decades. Once seen as models for the 1D linear allotrope of carbon, carbyne, poly-ynes have become recognised as research objects in their own right, although taming these reactive species is usually only possible in the presence of suitable end-caps that provide the necessary kinetic stability to allow their (safe) handing.This presentation will summarise some aspects of our studies of poly-ynes over a period that is likely longer than should be considered sensible. Nevertheless, this provides opportunity to touch base with synthetic methods, metal complexes and mixed-valence complexes featuring poly-carbon bridges that serve as misleading models for molecular junctions, and our endeavours carried out in collaboration with colleagues from across Europe to prepare molecular junctions based on poly-ynes and true all-carbon chains.
References:

A. Moneo, A. González-Orive, S. Bock, M. Fenero, I.L. Herrer, D.C. Milan, M. Lorenzoni, R.J. Nichols, P. Cea, F. Perez-Murano, P.J. Low, S. Martin, Towards molecular electronic devices based on ‘all-carbon’ wires, Nanoscale, 2018, 10, 14128-14138

D.C. Milan, O.A. Al-Owaedi, M.-C. Oerthel, S. Marques-Gonzalez, R.J. Brooke, M.R. Bryce, P. Cea, J. Derrer, S.J. Higgins, C. Lambert, P.J. Low, D.Z. Manrique, S. Martin, R.J. Nichols, W. Schwazacher, V.M. Garcia-Suarez, Solvent dependence of the single molecule conductance of oligoyne-based molecular wires, J. Phys. Chem. C, 2016, 120, 15666-15674

S. Gückel, J.B.G. Gluyas, S. El-Tarhuni, A.N. Sobolev, M.W. Whiteley, J.-F. Halet, C. Lapinte, M. Kaupp, P.J. Low, Iron versus Ruthenium: clarifying the electronic differences between prototypical mixed-valence organometallic butadiyndiyl-bridged molecular wires, Organometallics, 2018, 37, 1432-1445

S. Bock, P.J. Low, A safe and simple synthesis of 1,4-bis(trimethylsilyl)buta-1,3-diyne, Aust. J. Chem., 2017, 71, 307-310

22nd October 2020

10AM BST (UK Time)

Prof. Gemma Solomon

University of Copenhagen, Denmark

Better than nothing? The Search for Quantum Interference Based Single-Molecule Insulators.

While there has been significant focus on making high-conductance molecular wires, it is equally challenging to make extremely low conductance systems. Here we present some of our efforts to find highly insulating molecules. We have found the first molecule with clear suppression of the single-molecule conductance due to σ-interference in the form of a functionalized bicyclo[2.2.2]octasilane¹. The interference effects in this system are so significant that our calculations show the central unit is more insulating than a vacuum gap of the same dimensions. Through an extensive investigation of a family of cyclic and bicyclic silanes² we show that their transport properties can largely be understood by considering these otherwise complex molecules as constrained linear systems. From a high-throughput screening study³ varying the constituent atoms between carbon, silicon, and germanium, we know that majority of the molecules in the bicyclo[2.2.2]octane class are likely highly insulating. Finally, we have recently discovered that substituents play a major role in controlling interference and side-groups previously thought to be unimportant can be critical for the appearance of significant destructive interference⁴. References:

Garner, M. H.; Li, H.; Chen, Y.; Su, T. A.; Shangguan, Z.; Paley, D. W.; Liu, T.; Ng, F.; Li, H.; Xiao, S.; Nuckolls, C.; Venkataraman, L.; Solomon, G.C. Comprehensive suppression of single-molecule conductance using destructive σ-interference Nature 2018, 558, 415-419
Li, H.; Garner, M. H.; Shangguan, Z.; Chen, Y.; Zheng, Q.; Su, T.; Neupane, M.; Liu, T.; Steigerwald, M.; Ng, F.; Nuckolls, C.; Xiao, S.; Solomon, G.C.; Venkataraman, L. Large Variations in Single Molecule Conductance of Cyclic and Bicyclic Silanes J. Am. Chem. Soc., 2018, 140 (44), 15080–15088
Garner, M.H.; Koerstz, M.; Jensen, J.H.; Solomon, G.C.; The Bicyclo[2.2.2]octane Motif: A Class of Saturated Group 14 Quantum Interference Based Single-molecule Insulators J. Phys. Chem. Lett., 2018 9(24), 6941-6947
Garner, M.H.; Li, H.; Neupane, M.; Zou,Q.; Liu,T.; Su, T. A.; Shangguan, Z.; Paley, D. W.; Ng, F.; Xiao, S.; Nuckolls,C.; Venkataraman,L.; Solomon, G. C. J. Am. Chem. Soc. 2019, 141(39), 15471-15476

15th October 2020

10AM BST (UK Time)

Prof. Wenjing Hong

Xiamen University, China

Counting the number of molecules using single-molecule break-junction techniques.

Molecular electronics holds promise to explore the intrinsic properties of molecules, not only in discovering new physical and chemical phenomena but also in the inspiration of designing various molecular devices. In the main steam of molecular electronics, a series of progress has been made by the qualitative analysis of electrical properties such as their single-molecule conductance. In recent years, we extended the application of single-molecule techniques to the quantitative analysis of the molecular physical-chemical process by counting the number of molecules. Interestingly, we found that the single-molecule technique offers more insights beyond the ensemble techniques such as NMR. In this talk, I will share our recent effort towards quantitative analysis of the reaction rate[1], adsorption free energy[2], isomerization[3], the movement of molecules[4], and even the assembly of molecules at the scale of single or several molecules.
[1] Huang, X.#; Tang, C.#; Li, J.#; Chen, L. C.#; Zheng, J.; Zhang, P.; Le, J.; Li, L.; Li, X.; Liu, J.*; Yang, Y.; Shi, J.; Chen, Z.; Bai, M.; Zhang, H. L.; Xia, H.; Cheng, J.*; Tian, Z.; Hong, W.*, Electric-field-induced selective catalysis of single-molecule reaction, Science Advances, 2019, 5, eaaw3072 [2] Zhan, C.#; Wang, G.#; Zhang, XG.; Li, ZH.; Wei, JY.; Si, Y.; Yang Y *; Hong, W.*; Tian, Z., Single-molecule measurement of adsorption free energy at the solid-liquid interface, Angewandte Chemie International Editions, 2019, 58, 14534-14538 [3] Tang, C.#;Tang, Y.#, Ye, Y.; Yan, Z.; Chen, Z.; Chen, L.; Zhang, L.; Liu, J.; Shi, J.; Xia, H.*; Hong, W.*, Identifying the conformational isomers of single-molecule cyclohexane at room temperature, Chem, 2020, DOI:10.1016/j.chempr.2020.07.024[4] Zhan, C.#; Wang, G.#; Yi, J.#; Wei, J.; Li, Z.; Chen, Z.; Shi, J.; Yang, Y.*; Hong, W.*; Tian, Z.*, Single-molecule plasmonic optical trapping, Matter, 2020, DOI:10.1016/j.matt.2020.07.019[5] unpublished results

8th October 2020

2PM BST (UK Time)

Prof. Ryan Chiechi

University of Groningen, Netherlands

Robust and Air-Stable Molecular Electronics

The majority of work in large-area molecular tunneling junctions leverages the self-assembly of thiols on coinage metals to form single-molecule-thick layers onto which a top-contact is applied. These self-assembled monolayers comprise metal thiolate bonds, which are unstable to ambient conditions, necessitating the complexity and overhead of encapsulation if they are to find technological applications. In this talk I will describe the use of non-covalent fullerene-gold interactions to form self-assembled monolayers and bilayers. These interactions are stronger than metal-thiolate bonds, but are stable for at least one month in ambient conditions. Self-assembly is driven by interactions between glycol ethers, leading to bilayers that share all of the useful properties of thiol-monolayers, including self-assembly and exchange, but that are both mechanically robust and stable to ambient conditions. [https://doi.org/10.1038/s41563-019-0587-x] I will discuss how these properties can be used to reconstitute junctions in operando by demonstrating the reversal of the rectification of tunneling currents in arrays of junctions incorporated into microfludic channels.

1st October 2020

Dr. Ganna (Anya) Gryn'ova

Heidelberg Institute for Theoretical Studies, Germany

Crossing Electronic Bridges: Computational Chemistry of Molecular Junctions

Molecular junctions are nanoscale electronic devices that consist of a (organic) molecule bridging the gap between two conducting electrodes (typically, gold). They represent a powerful tool for studying the intimate details of the electron and heat transport through molecules, their reactions and interactions with external electromagnetic fields and mechanic forces, interface phenomena, etc. In this talk I will discuss our recent in silico research efforts towards a deeper understanding of the role of molecular structure in such electronic junctions. These include rationalising and predicting the crossover from n- to p-type transport in oligothiophene-based wires, as well as paralleling the dimer molecular junctions with their organic semiconductor crystal counterparts leading to the design criteria for more conductive assemblies. I will also highlight the role of molecular topology and illustrate how it can be used to achieve unexpectedly high and persistent conductance in fully saturated hydrocarbon wires. Finally, if time allows, our on-going investigation into the role of aromaticity, particularly the s-aromaticity, in the molecular junctions properties will be briefly discussed.

24th September 2020

Dr. Linda A. Zotti

University of Seville, Spain

Electron transport through single proteins, peptides and amino acids

Proteins have proven to be promising candidates for molecular electronics, showing in some cases much higher conductance than one would naively expect from their size. In particular, the blue-copper azurin extracted from Pseudomonas aeruginosa has been the subject of many experimental studies, although the exact transport mechanism is still under debate. Furthermore, amongst the numerous interesting properties revealed by this system, it was observed that the gate-dependence can be considerably affected by insertion of mutations [1]. Here I will present our efforts towards understanding the origin of such interesting effects from a theoretical perspective, analyzing both the electronic structure and the geometrical arrangement [2,3]. In addition, I will discuss results obtained on the conductance of individual heptapeptides [4] and amino acids [5], which are the building blocks of proteins, as well on the electronic properties of junctions based on Cytochrome C [6].
[1] M. P. Ruiz; A. C. Aragonès; N. Camarero; J. G. Vilhena; M. Ortega; L. A. Zotti; R. Pérez; J. C. Cuevas; P. Gorostiza; I. Díez-Pérez, “Bioengineering a Single-Protein Junction”, J. Am. Chem. Soc. 139- 43, 15337 (2017)[2] C. Romero-Muñiz, M. Ortega, J. G. Vilhena, I. Díez Pérez, J. C. Cuevas,a; R. Pérez and L. A. Zotti, “Ab-initio Electronic Structure Calculations of Entire Blue Copper Azurins”, Phys. Chem. Chem. Phys., 20, 30392 (2018). [3] Romero-Muñiz, C., Ortega, M., Vilhena, J. G., Diéz-Pérez, I., Cuevas, J. C., Pérez, R., & Zotti, L. A., “Mechanical Deformation and Electronic Structure of a Blue Copper Azurin in a Solid-State Junction”, Biomolecules, 9(9), 506 (2019)[4] L. A. Zotti; J. C. Cuevas, “Electron transport through homopeptides: are they really good conductors?”, ACS omega. 3, 3778 (2018).[5] Zotti, L. A., Bednarz, B., Hurtado-Gallego, J., Cabosart, D., Rubio-Bollinger, G., Agrait, N., & van der Zant, H. S. “Can one define the conductance of amino acids?”, Biomolecules, 9(10), 580 (2019)[6] Fereiro, J.A., Kayser, B., Romero‐Muñiz, C., Vilan, A., Dolgikh, D.A., Chertkova, R.V., Cuevas, J.C., Zotti, L.A., Pecht, I., Sheves, M. and Cahen, D., “A Solid‐State Protein Junction Serves as a Bias‐Induced Current Switch”, Angewandte Chemie International Edition, 58(34), 11852 (2019).

17th September 2020

Prof. Richard Nichols

University of Liverpool, UK

Oligoporphyrin Single-Molecule Electronics

Measurement of the electrical properties of porphyrin single molecular wires sandwiched between metal contacts is now an experimental reality and such methods have contributed to understanding charge flow through and the current-voltage response of porphyrin wires. We have exploited scanning-tunneling-microscopy-based methods such as the STM break junction method and the non-contact I(s) technique (I=current and s=distance between STM tip and substrate) for achieving this feat. In both these techniques single molecule junctions are formed by bringing a gold STM tip into contact or very close to the gold substrate surface. As the tip is rapidly retracted the conductance of the porphyrin molecule junction or the complete current-voltage response can be recorded. When combined with statistical analysis this provides a strong platform for investigating the electrical properties of porphyrin junctions. This presentation will review some of our findings on the electrical properties of porphyrin single molecule wires as well as presenting our latest data recorded using the STM break junction method.
Topics to be discussed include:

Single molecule junction formation mechanisms and evolution during junction stretching.
The influence of contacting groups on porphyrin electrical junctions.
Long range electron transport in porphyrin oligomers, their current-voltage response and unusual voltage dependent length decay of conductance.
Mechanisms of charge transport in porphyrin single molecule wires.
The mechanochemical properties of 5,15-diaryl porphyrins with thiol end groups.

References to our work on porphyrin single molecule electronics:

G. Sedghi, K. Sawada, L. J. Esdaile, M. Hoffmann, H. L. Anderson, D. Bethell, W. Haiss, S. J. Higgins, and R. J. Nichols
Single Molecule Conductance of Porphyrin Wires with Ultralow Attenuation.
Journal of the American Chemical Society 130, 8582 (2008).
G. Sedghi, V.M. Garcia-Suarez, L.J. Esdaile, H.L. Anderson, C.J. Lambert, S. Martin, D. Bethell, S.J. Higgins, M. Elliott, N. Bennett, J.E. Macdonald and R.J. Nichols
Long-range electron tunnelling in oligoporphyrin molecular wires.
Nature Nanotechnology, 6, 517-523, (2011).
G. Sedghi, L.J. Esdaile, H.L. Anderson, S. Martin, D. Bethell, S.J. Higgins and R.J. Nichols
Comparison of the Conductance of Three Types of Porphyrin-Based Molecular Wires: beta,meso,beta-Fused Tapes, meso-Butadiyne-Linked and Twisted meso-meso Linked Oligomers.
Advanced Materials, 24, 653-, (2012).
Leary, E.; Roche, C.; Jiang, H. W.; Grace, I.; Gonzalez, M. T.; Rubio-Bollinger, G.; Romero-Muniz, C.; Xiong, Y. Y.; Al-Galiby, Q.; Noori, M.; Lebedeva, M. A.; Porfyrakis, K.; Agrait, N.; Hodgson, A.; Higgins, S. J.; Lambert, C. J.; Anderson, H. L.; Nichols, R. J.
Detecting Mechanochemical Atropisomerization within an STM Break Junction.
Journal of the American Chemical Society 2018, 140 (2), 710-718.
Leary, E.; Limburg, B.; Alanazy, A.; Sangtarash, S.; Grace, I.; Swada, K.; Esdaile, L. J.; Noori, M.; Gonzalez, M. T.; Rubio-Bollinger, G.; Sadeghi, H.; Hodgson, A.; Agrdit, N.; Higgins, S. J.; Lambert, C. J.; Anderson, H. L.; Nichols, R. J.
Bias-Driven Conductance Increase with Length in Porphyrin Tapes.
Journal of the American Chemical Society 2018, 140 (40), 12877-12883.
Leary, E.; Kastlunger, G.; Limburg, B.; Rincón-García, L.; Hurtado Gallego, J.; González, M. T.; Rubio-Bollinger, G.; Agrait, N.; Higgins, S. J.; Anderson, H. L.; Stadler, R.; Nichols, R. J.
Long-Lived Charged States of Single Porphyrin-Tape Junctions Under Ambient Conditions. Submitted 2020.

10th September 2020

Prof. Tim Albrecht

University of Birmingham, UK

When charge transport data are a worm – a transfer learning approach for unsupervised data classification

Advanced data analysis methodologies, and in particular dimensionality reduction techniques, are now used more and more widely in the single-molecule charge transport community. They allow for comprehensive exploration of large datasets, where data display significant variance and sometimes contain (unknown) sub-populations. To this end, unsupervised approaches, which do not rely on class labels or pre-defined expectations can be advantageous. Multi-Parameter Vector Classification (MPVC) is one example and PCA-based methods have also been employed in this context.[1,2,3]
We have recently shown how Transfer Learning may be employed to identify and quantify hidden features in single-molecule charge transport data.[3] Using open-access neural networks such as AlexNet, trained on millions of seemingly unrelated image data, feature recognition then does not require network training with application-specific data. Instead, the network recognises features in the input that it had learned in other contexts and, for example, identifies different shapes in conductance-distance traces as images of different worm species. Thus, our results show how Deep Learning methodologies can readily be employed for unsupervised data classification,[4] even if the amount of problem-specific, ‘own’ data is limited. [1] M Lemmer, MS Inkpen, K Kornysheva, NJ Long, T Albrecht, “Unsupervised vector-based classification of single-molecule charge transport data”, Nat. Comm. 2016, 7, 12922.[2] T Albrecht, G Slabaugh, E Alonso, SMMR Al-Arif, “Deep learning for single-molecule science”, Nanotechnology 2017, 28 (42), 423001.[3] A Vladyka, T Albrecht, “Unsupervised classification of single-molecule data with autoencoders and transfer learning”, Mach. Learn.: Sci. Technol. 2020, 1, 035013.[4] P Yasini, S Shepard, T Albrecht, M Smeu, E Borguet, “Combined Impact of Denticity and Orientation on Molecular-Scale Charge Transport”, J. Phys. Chem. C 2020, 124, 17, 9460–9469.

3rd September 2020

Prof. Colin Lambert

Lancaster University, UK

How to deal with noisy neighbours: Molecular-scale quantum interference for beginners

Have you ever been disturbed by noisy neighbours? Instead of becoming annoyed, if you agree to cooperate, then you could discover the main concepts governing quantum interference in molecules. A wave is a wave is a wave and the wave patterns generated by the neighbour’s music have many properties in common with de Broglie waves of electrons. In this talk, I will demonstrate that if neighbours agree to measure carefully the wave amplitudes appearing in each other’s apartments, they would understand why the electrical conductance of molecules decays with length and why molecules with parallel conductance paths behave non-classically.

23rd July 2020

Dr. Jan Mol

Queen Mary University of London, UK

Understanding resonant charge transport through weakly coupled single-molecule junctions

Electron-transfer reactions are ubiquitous in chemistry, however, there are still gaps in the fundamental understanding of electron transfer at the molecular level, particularly the degree to which the nuclear dynamics that accompany the process straddle the quantum-classical boundary. We use graphene-based single-molecule transistors [1] to study the mechanism of electron transfer over a wide range of temperatures – from 3 K to 77 K – at the level of an individual molecule. Charge transport through molecular junctions is often described either as a purely coherent or a purely classical phenomenon, and described using the Landauer formalism or Marcus theory, respectively. In our experiments, however, observe a simultaneous breakdown of quantum coherent Landauer and semi-classical Marcus theory. We propose a theoretical model based on generalised quantum master equation [2], where we derive an expression for current through a molecular junction modelled as a single electronic level coupled to a collection of thermalised vibrational modes, and demonstrate that it quantitatively describes the experimental data. We show that nuclear tunnelling enhances the rates of low-energy electron transfer, and demonstrate that the rates are sensitive to both the outer and inner-sphere environmental interactions. We find that the nuclear dynamics accompanying electron transfer must be treated quantum mechanically as the quantitative validity of Marcus theory is expected to occur at temperatures exceeding 298 K [3]. [1] Limburg et al., Adv. Funct. Mater.1803629 (2018)[2] Sowa et al., J. Chem. Phys. 149, 154112 (2018)[3] Thomas et al., Nat. Commun. 10, 4628 (2019)

16th July 2020

Dr. Andrea Droghetti

Trinity College Dublin, Ireland

Electronic structure and transport properties of hybrid molecular junctions

New functionalities can be tailored in molecular devices by specifically designing the interfaces between the electrodes and the molecules. I will present several prominent examples realized experimentally and that I contributed to model by means of Density Functional Theory-based calculations.
In the first place, I will describe hybrid graphite (graphene)/molecule/Au junctions [1, 2]. The functionalization of the graphite (graphene) electrode with molecules attached via a C-C direct bond results in a room temperature mechanically stable and precisely-defined contact. The charge transport properties are dictated by the asymmetry of the transmission function around the Fermi energy induced by the molecule–graphite (graphene) bond. In particular, this leads to electrical rectification. Furthermore, it also affects the thermoelectric properties [3]. We predict that the room-temperature thermopower in these systems can be an order of magnitude larger than any value reported so far in experiments with symmetric Au/molecule/Au junctions. Finally, we will extend our study to hybrid Ni/graphene/molecule/Ni junctions in order to address also the spin-transport properties [4]. These molecular spin-valves show a (low-temperature) magnetoresistance, whose sign depends on the bias voltage applied across the junctions. This property has never been reported in nanoscale spin-valve devices and is caused by the interplay between the Ni/graphene, the molecule/Ni and the graphene/molecule contacts.
In summary, hybrid junctions with molecules attached to graphite (graphene) represent new multifunctional molecular devices with a large mechanical stability and where one can tune the electronic, thermoelectric and spin transport characteristics.
[1] A.V. Rudnev, V. Kaliginedi, A. Droghetti, H. Ozawa, A. Kuzume, M. Haga, P. Broekmann and I. Rungger, Science Advances 3, e1602297 (2017).
[2] G. Seber, A.V. Rudnev, A Droghetti et al., Chemistry-A European Journal 23, 1415 (2017).
[3] A. Droghetti and I. Rungger, Phys. Chem. Chem. Phys. 22, 1466 (2020).
[4] C. Barraud, A. Droghetti et al., in preparation.

9th July 2020

Dr. Benjamin Robinson

University of Lancaster, UK

Thermoelectric properties of ultrathin organic films and molecular scale junctions

Here I will report on our recent progress in the design, fabrication and characterisation of thermoelectric SAMs. This includes a new reproducible and non-destructive method for probing the electrical and thermoelectric properties of small assemblies (10 – 10³) of thiol-terminated molecules arranged within a SAM on gold and demonstrate the successful retention of the single-molecule electrical conductivity and Seebeck values. We have used a modified thermal-electric force microscopy approach, which integrates the conductive-probe atomic force microscope with a sample positioned on a temperature controlled heater, operating with a probe-sample peak-force feedback that interactively limits the normal force across the molecular junctions.

2nd July 2020

Dr. Hatef Sadeghi

University of Warwick, UK

Quantum and Phonon Interference Enhanced Molecular-Scale Thermoelectricity

There is a worldwide race to find materials with high thermoelectric (TE) efficiency to convert waste heat in consumer electronics and server farms to useful energy. Despite several decades of development, the state-of-the-art TE materials are not sufficiently efficient to deliver viable technology platform for energy harvesting from consumers electronics or on-chip cooling of CMOS-based devices.
The efficiency of a TE material is defined by a dimensionless figure of merit ZT = S^2GT/κ, where S is the Seebeck coefficient, G is the electrical conductance, T is temperature and κ = κel + κph is the thermal conductance due to electrons and phonons, respectively. Therefore low-κ, high-G and high-S materials are needed. This is constrained by the interdependency of G, S and κ. Consequently, the world record ZT is about unity at room temperature in inorganic materials which are toxic and their global supply is limited. To develop high-performance TE devices, simultaneous engineering of electron and phonon transport through nanostructured TE materials is needed. In molecular scale junctions, electrons behave phase coherently and can mediate long-range phase-coherent tunneling even at room temperature.
This creates the possibility of engineering quantum interference in these junctions for thermoelectricity. In this talk, I will discuss strategies to improve the efficiency of molecular scale TE materials. This includes utilising quantum interference to enhance electrical conductance and Seebeck coefficient and phonon interference to suppress thermal conductance in molecular scale junctions.

25th June 2020

Dr. Albert C. Aragones

Max Planck Institute for Polymer Research, Germany

New Spectroscopic Tools for Single-Molecule Junctions

Biological charge transport (CT) is the key step in many basic cellular processes such as respiration or photosynthesis and nature has developed highly specialized molecular building blocks to achieve it with unprecedented efficiency. Understanding the mechanisms behind biological CT is key to elucidate the changes in its regimes caused by specific structural variations of the associated molecular machinery. Such knowledge will ultimately lead us to tailor its electrical properties and exploit them as high performance bioelectronic devices with a wide variety of applications in organic electronics, sensing, biomanufacturing etc. To investigate CT in single-molecule bioelectronic devices, we exploit Scanning Tunnelling Microscopy-based approaches in the break-junction mode (STM-BJ) [1,2] under electrochemical control (EC-STM). It allows the trapping of individual molecules in a junction to characterize their main electrical signatures. [3]The first block of this contribution will present novel light-induced tunneling transport studies carried out with Azurines molecules (blue copper proteins) under electrochemical control. Evident effects over the electron transport mechanism have been demonstrated by employing laser illumination in resonance with the Ligand-to-Metal Charge Transfer (LMCT) transition of the Azurine molecules. We will end this contribution by showing the work in progress on the development of a new hybrid platform with spectro-electrical single-molecule detection capabilities under ambient conditions. It aims to explore several key structural and physicochemical aspects that remain unknown during the single-molecule electrical contact formation. This is possible thanks to the operando capabilities of the hybrid platform to work in the near-field Raman between the two STM electrodes (TERS),[4]a high ultrasensitive non-destructive spectroscopic method with molecular resolution. The new platform opens the gates to obtain detailed insights into the molecular junction structure by simultaneously capturing the current during the spontaneous formation of a molecular junction, i.e. the evolution of spectro-electrochemical characteristics of the junction. [1] B. Xu and N. J. Tao, Science, 301, 5637, 221 (2003).[2] A. C. Aragonès et al., Nature, 531, 7592, 88 (2016).[3] M. P. Ruiz et al., J. Am. Chem. Soc., 139, 15337 (2017).[4] N. Martín Sabanés et al., Anal. Chem., 88, 7108 (2016).

18th June 2020

Dr. Matteo Palma

Queen Mary University of London, UK

Tuning the coupling of proteins to carbon nanotube nanoscale systems and devices

A central challenge in nanobiotechnology is the bottom-up assembly of platforms capable of monitoring and exploiting biomolecular interactions with single-molecule control; this in turn can allow the development of novel bioelectronics interfaces.
In this regard, we developed different platforms so as to couple single-walled carbon nanotubes (SWCNTs) electronic output to biomolecular function, and allow for single-molecule and nanoscale studies to be performed. In particular, we assembled and investigated static and dynamic organic-inorganic heterostructures consisting of single Quantum Dots interfaced to individual CNT hybrids via DNA linkers,[1] and stimuli-responsive DNA-CNT junctions.[2] Here I will report the site-specific coupling of single proteins to individual CNTs with single-molecule control, confirming the importance of bioengineering optimal protein attachment sites to achieve direct protein−nanotube communication and bridging.[3]
Additionally, I will discuss the extension of this rationale to the fabrication of bioelectronic devices with engineered protein interfacing; this allowed us to control the local electrostatic surface presented within the Debye length in CNT-based transistors, and thus modulate the conductance gating effect upon sensing protein targets. [4] [1] Advanced Science, 2018, 5, 1800597[2] Chemistry of Materials, 2019, 31, 1537-1542[3] Journal of the American Chemical Society, 2017, 139, 17834-17840[4] Submitted, 2020

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