Periodic Reporting for period 2 - HOPELEC (Harnessing Hopping in Molecular-Scale Electronics: Approaching New Devices 'One Step at a Time')
Periodo di rendicontazione: 2021-11-01 al 2022-10-31
In a seminal study, Aviram and Ratner proposed that a molecule comprising electron-rich (donor) and electron-poor (acceptor) regions could conduct current better in one direction than the other and so function as a diode or rectifier. Subsequent experimental works have shown that molecules can indeed function as diodes, switches and simple wires, with ongoing efforts to improve functionality. These molecular-electronic systems have the potential to address emerging paradigms in computing at scales smaller than that which can be achieved with conventional materials and with much lower power consumption. However, despite these promising properties, molecular-electronics struggles to realize technologically relevant systems due to the complexity of working on the molecular scale and the complexity of molecule-electrode interactions. This proposal aims to study intramolecular hopping processes that bypass the latter problem in two laboratories; the first focuses on fundamental, single-molecule properties and the second focuses on device-relevant molecular-electronic platforms. The combination of the two-lab approach and the exploitation of intramolecular processes is meant to yield oscillatory molecular-electronic circuits in device-relevant platforms.
To exploit and improve our understanding of hopping processes on the nanoscale, the specific objectives of this project are to: (i) identify different classes of redox-active complexes that are stable under the experimental conditions used to probe their charge transport properties; (ii) develop methods to covalently link these individual complexes together into multi-site molecular systems; (iii) synthesize and demonstrate the first single-molecule current oscillator by exploiting intramolecular charge hopping in a junction (Fig 1a); (iv) prepare linear multi-site systems comprising a redox-potential gradient as hopping-based current rectifiers (Fig 1b); (v) compare charge transport through the same molecules at the single-molecule scale using STM-BJ and in large-area measurements using EGaIn. My previous experience studying electron transfer in redox systems (ligand and multi-metallic complex syntheses, spectroelectrochemistry) positions me particularly well to tackle this highly interdisciplinary area. The Fellowship will significantly enhance my international profile through publications in high impact journals and multinational networking activities. The new synthetic methods, scanning probe/EGaIn techniques and fundamental concepts explored here will broaden my capabilities in nanoscience, inspiring further novel ideas which will be exploited subsequently in pursuit of my independent academic career.
To exploit and improve our understanding of hopping processes on the nanoscale, the specific objectives of this project are to: (i) identify different classes of redox-active complexes that are stable under the experimental conditions used to probe their charge transport properties; (ii) develop methods to covalently link these individual complexes together into multi-site molecular systems; (iii) synthesize and demonstrate the first single-molecule current oscillator by exploiting intramolecular charge hopping in a junction (Fig 1a); (iv) prepare linear multi-site systems comprising a redox-potential gradient as hopping-based current rectifiers (Fig 1b); (v) compare charge transport through the same molecules at the single-molecule scale using STM-BJ and in large-area measurements using EGaIn. My previous experience studying electron transfer in redox systems (ligand and multi-metallic complex syntheses, spectroelectrochemistry) positions me particularly well to tackle this highly interdisciplinary area. The Fellowship will significantly enhance my international profile through publications in high impact journals and multinational networking activities. The new synthetic methods, scanning probe/EGaIn techniques and fundamental concepts explored here will broaden my capabilities in nanoscience, inspiring further novel ideas which will be exploited subsequently in pursuit of my independent academic career.
Contrary to the metal terpy systems outlined in the proposal to this project, we could identify triarylamines (TAAs) as highly-performant building blocks in molecular junctions which display a large on/off ratio between their reduced and oxidized state. This could be traced back to the delocalized nature of the half-filled state which importantly stretches across the conducting channel between the two metal electrodes. Maintaining this open-shell character turned out to be inherently difficult due to the tendency of these systems towards a closed-shell configuration. This was expressed through the large influence of environmental factors (i.e. bias, solvent or counterion) on the observed conductance. We there put a lot of emphasis on controlling these parameters in order before proceeding to the next step. In order to lower the electron transfer rate, a prerequisite to observe any supposed current oscillations, oligomers built from triarylamines were synthesized and characterized. These display surprisingly high conductances at high bias or in their oxidized form. Electronic coupling between the individual entities was found to be still too strong, however, with (kET > 10-5 s) severely complicating the creation of a system with an observable current oscillation.
Additional work was conducted on the successful conductance studies on building blocks for covalent-organic frameworks (COFs) which let to the co-authorship in a Nano Letter publication (Nano Lett. 2022, 22, 20, 8331–8338, https://doi.org/10.1021/acs.nanolett.2c03288(si apre in una nuova finestra))
In the second part of the project attempts were made to translate the findings made for these triarylamine-based systems in a single-molecule setting to large area junctions. To this end, self-assembled bilayers were used. These are comprised of a self-organized monolayer of C60 fullerenes on gold, which through functionalization with ethylene glycol (EG) chains can interdigitate with a second top, which has also been functionalized with EG chains. Here a strongly electron-accepting perchloro triphenylmethane (PTM) unit was placed in between the C60 fullerene and the EG chain forming the bottom layer whilst EG-modified TAAs comprise the top layer. It was anticipated that analogously to covalently bound dyads of PTM and TAA, intramolecular electron transfer could be achieved. This would yield a positively charged TAA along with the PTM anion resulting in a system with a strong dipole moment and electron density gradient, as a function of the applied bias voltage. Whilst for the mixed bilayer of C60-PTM-TEG (TEG=triethylene glycol) at the bottom and a TEG-functionalized TAA as the top layer, does display current rectification the effect is more subtle than expected (6x at 1V). Furthermore, analysis of the recorded current-voltage curves did not yield direct evidence for a redox-event, such as hysterises.
Each project was presented at at least one scientific conference and was able to gain further support through collaborations with theoretical chemists. To this end, manuscripts are in preparation in order to disseminate the findings made during these studies in the form of publications.
Additional work was conducted on the successful conductance studies on building blocks for covalent-organic frameworks (COFs) which let to the co-authorship in a Nano Letter publication (Nano Lett. 2022, 22, 20, 8331–8338, https://doi.org/10.1021/acs.nanolett.2c03288(si apre in una nuova finestra))
In the second part of the project attempts were made to translate the findings made for these triarylamine-based systems in a single-molecule setting to large area junctions. To this end, self-assembled bilayers were used. These are comprised of a self-organized monolayer of C60 fullerenes on gold, which through functionalization with ethylene glycol (EG) chains can interdigitate with a second top, which has also been functionalized with EG chains. Here a strongly electron-accepting perchloro triphenylmethane (PTM) unit was placed in between the C60 fullerene and the EG chain forming the bottom layer whilst EG-modified TAAs comprise the top layer. It was anticipated that analogously to covalently bound dyads of PTM and TAA, intramolecular electron transfer could be achieved. This would yield a positively charged TAA along with the PTM anion resulting in a system with a strong dipole moment and electron density gradient, as a function of the applied bias voltage. Whilst for the mixed bilayer of C60-PTM-TEG (TEG=triethylene glycol) at the bottom and a TEG-functionalized TAA as the top layer, does display current rectification the effect is more subtle than expected (6x at 1V). Furthermore, analysis of the recorded current-voltage curves did not yield direct evidence for a redox-event, such as hysterises.
Each project was presented at at least one scientific conference and was able to gain further support through collaborations with theoretical chemists. To this end, manuscripts are in preparation in order to disseminate the findings made during these studies in the form of publications.
Progress in all outlined projects was severely hampered by the COVID 19 pandemic along with unanticipated scientific challenges throughout the outgoing as well as the return phase.
This unfortunately negatively impacted the output of this project.
The observed impact different counterions can have on the conductance of charged species can be considered a significant contribution to the contested discussion on how pronounced such effects should be. These findings, therefore, act as a guide for further studies on open-shell/charged species in single-molecule junctions to not disregard but carefully evaluate even subtle environmental changes.
The extension of these organic radicals to systems several nanometers in length, whilst maintaining excellent observed conductance and high on/off ratios as a function of bias voltage or redox state, opens up further investigations into the fabrication of actual molecule-scale electronic devices.
Unfortunately the set goal of creating a molecular oscillator could not be achieved within the timeframe of this project, however, the studied systems can be considered an ideal launching pad for its realization.
The insight gathered during this project is however of direct interest for any further research into the creation of molecular-scale devices, in order to push the boundaries of future electronic components.
This unfortunately negatively impacted the output of this project.
The observed impact different counterions can have on the conductance of charged species can be considered a significant contribution to the contested discussion on how pronounced such effects should be. These findings, therefore, act as a guide for further studies on open-shell/charged species in single-molecule junctions to not disregard but carefully evaluate even subtle environmental changes.
The extension of these organic radicals to systems several nanometers in length, whilst maintaining excellent observed conductance and high on/off ratios as a function of bias voltage or redox state, opens up further investigations into the fabrication of actual molecule-scale electronic devices.
Unfortunately the set goal of creating a molecular oscillator could not be achieved within the timeframe of this project, however, the studied systems can be considered an ideal launching pad for its realization.
The insight gathered during this project is however of direct interest for any further research into the creation of molecular-scale devices, in order to push the boundaries of future electronic components.