Single-Molecule Plasmoelectronics

Continuing miniaturization of electronic components in computer chips will eventually lead to component sizes on the molecular scale. Conventional semiconductor nanostructures at these length scales will suffer from increased leakage currents due to tunnelling as well as increased thermal effects due to higher power densities. The need for developing alternative approaches has created over the last two decades the field of molecular electronics, in which electronic components are realized using single molecules. Numerous examples of prototypical devices such as diodes, memory elements and transistors employing individual molecules have been demonstrated.

One of the most important functions is the control of the current through a device with an external stimulus, i.e. gating. Stimuli which have been employed include electrostatic and electrochemical potentials, temperature, and light. Light is one of the most attractive options since it potentially allows coupling single-molecular devices with future optoelectronic circuitry, holding the promise of ultimate speed and miniaturization.

Efficient coupling of light with nanoscale objects can be achieved using plasmonic nanostructures that concentrate and focus light beyond the diffraction limit. In combination with electronic devices one speaks of plasmoelectronics. Such efficient and spatially confined coupling is a pre-requisite for the tight integration of optically gate-able molecular devices on the sub-100 nm scale. The proposed research aims at realizing single-molecular plasmoelectronic devices in which the current through a single molecule coupled to a plasmonic nanostructure is gated by external illumination. The envisaged device structures will take advantage of the plasmonic properties of noble metal nanoparticles that serve as the electrodes of the single-molecule junction. This research will open new opportunities for miniaturization, integration, and control of optoelectronic devices to the single-molecule level.

The research is interdisciplinary spanning physics, chemistry, molecular electronics and plasmonics. This is reflected in the research team which brings together expertise in organic synthesis of single-molecular conductors (Beeby, Durham), single-molecule conduction measurements (Nichols, Higgins, Liverpool), and nanoplasmonics (Jaeckel, Liverpool). This broad expertise will allow for a systematic approach varying the chemical nature of the molecular conductor and matching it with the plasmonic properties of the single-molecule junction. This will allow detailed characterization of parameters such as spectral overlap and electronic coupling in the junction and their relation to the optical gating effect in the device. The single-molecule approach will eliminate both ensemble averaging effects which can mask important effects in macroscopic measurements and sample heterogeneity which makes interpretation of results more complex. The project will deliver a fundamental understanding of plasmoelectronic single-molecule junctions and formulate design rules for future devices. The results will also open new opportunities in related research areas such photovoltaics, organic electronics, and catalysis.

This proposal represents early stage work in the development of "plasmoelectronics" in which plasmon mediated optical excitation of molecular electrical junctions is used to control their electrical properties. We aim to do achieve this down to the single-molecule and single-nanoparticle level, hence realizing plasmonically gate-able single-molecular junctions. This will be an interdisciplinary effort with the team having the expertise to design and synthesise the molecular and plasmonic nanostructures, to characterise and understand their photo-physical properties and to perform the optically gated scanning probe microscopy studies necessary for device characterisation under optical illumination. Through defined experiments we aim to both fundamentally understand the plasmonic gating effect and to establish design rules for efficient plasmonic gating of future molecular devices.
We expect impact of our research in the following areas:
i) Economy

By establishing design rules for future plasmonically gate-able devices and securing connected intellectual property we will lay foundations for future applications of plasmoelectronic devices. Although any future technological implementation of plasmoelectronics is unlikely to be initially at the single-molecule level, the measurements we propose at the single molecule level offer us a defined way for establishing understanding and design rules through measurements in which the molecular bridge and plasmonic nanostructure are varied. The technological impact of plasmoelectronics has recently been highlighted in the literature by Nitzan and Galperin. These will include new molecular scale devices or sensors or also spin-off applications in organic photovoltaics or organic electronics. According to the International Technology Roadmap for Semiconductors (ITRS) the anticipated timeline for developing molecular electronics is currently in a fundamental research phase (until 2018), then transitions into development (2019-2021) and then onwards to production (from 2022). Thus we expect technologically relevant impact in terms of technologies applicable in electronic devices on the 5-10 year time scale. In the long term this will strengthen the UK's position in this future technology sector.

ii) People
The project will train young researchers and provide them with necessary skills to shape and drive the future development and production phase outlined in the ITRS starting in 2019. These include specific knowledge of the realisation and characterisation of molecular electronic devices but also more generic skill such as project management and effective communication.
iii) Society

In the short term dissemination and description of our research in accessible formats will contribute to the education of the public about the opportunities and risks associated with nanotechnology. In the long term, and intimately connected to the outlined technological impact, quality of life will be improved by the introduction of new products utilizing plasmoelectronics. These may include sectors of electronics (new device structures), health care (novel sensors), or energy (more efficient photovoltaics).

iv) Knowledge
We also expect major academic impact which is outlined in more detail in the academic beneficiary section.