Molecular assembly of spintronic circuits with DNA

The smallest scale on which it is possible to design functional devices, including electronics, is the molecule scale (about 100,000 times smaller than the width of a human hair). This is the ultimate limit for miniaturisation and motivates research to manipulate and study the properties of individual molecules for applications in, e.g., information technologies and sensors. It is also the scale at which quantum phenomena dominate properties, so single-molecule structures offer a domain for investigations ranging from fundamental tests of quantum theory to developing components for future quantum technologies.

To realise such experiments and technologies, it is necessary to incorporate individual molecules into electrical circuits. This is challenging because the typical size of a useful functional molecule is much smaller than the smallest wires that it is possible to fabricate, even with the most sophisticated lithography systems available today. Most researchers use one of two approaches.

The first uses an electrical current or mechanical strain to make a tiny gap, a few nanometres across, in a thin wire, and then deposit the molecules of interest randomly, hoping that one and only one bridges the gap. This method relies on chance, and so it very rarely yields a working device: typically, only a very small proportion of devices fabricated show behaviour consistent with a single molecule in the gap and, because the shape of the gap and the orientation of the molecule are uncontrolled, it is rare for even such "working" devices to exhibit reproducible properties.

The second method uses a scanning tunnelling microscope to locate and investigate molecules that are deposited on a conducting surface. This process is much more reliable and reproducible than the break junction method but it involves bulky experimental apparatus and it tightly limits the experimental geometry, ruling out the development of more complicated experiments or practical devices.

These limitations in the existing methods have hamstrung the development of molecule-scale devices and technologies. Further progress in this field now requires the development of controlled and reliable methods that can be scaled to high volume production. This project will provide this methodology and demonstrate a range of prototype molecular devices.

Our approach is based on DNA nanotechnology, which has, over the last decade, proved itself to be a powerful tool for controlled self-assembly of structures at the molecular scale. We will use these methods to direct the assembly of "packages" about 100 nanometres across. Constructed mainly from DNA with a precisely programmed structure, these packages will position gold nanoparticle contacts and the "target" molecular components, whose electrical transport properties we would like to exploit, with sub-nanometre accuracy.

Our method produces trillions of packages at a time in a test-tube and ensures that each one has exactly the correct molecules incorporated in the correct positions and orientations between contacts. These gold nanoparticle contacts are large enough that we can connect them to laboratory equipment using standard nanolithography techniques. The technology has the potential for future development to connect multiple molecules in three-dimensional device architectures, and for the assembly of large-scale integrated molecular circuits.

We propose to create several families of devices, designed to develop and prove this radically new molecular device fabrication methodology. These devices will give us an unprecedented experimental tool for probing electrical and magnetic properties of molecules, but they will also establish the potential for the industrial deployment of our technology. Central to the project are close interactions with industrial partners and knowledge transfer activities designed to accelerate commercial applications.

By introducing a scaleable, high-yield methodology for assembling devices that exploit single molecules as functional units, this project will impact both commercial and academic beneficiaries.

In the long term, our results will introduce an industrially-deployable platform by which the properties of individual molecules may be exploited in devices. This will enable a new era of miniaturisation, beyond the hard limits of conventional semiconductor processing that will soon be encountered, down to the fundamental minimum scale on which functional materials can be engineered. This is also the scale at which quantum mechanics dominates the properties of materials: we will facilitate a new route for the commercialisation of quantum technologies.

By sidestepping long-standing roadblocks impeding scientific investigation of the electrical properties of single molecules, our results will have a major and immediate academic impact. There is worldwide interest in academic research in this subject, with motivations ranging from the development of sensors and information processing technologies all the way to exploration of fundamental questions in quantum mechanics.

This project represents a ground-breaking application to "hard" condensed matter physics and materials science of a technology, DNA self-assembly, that is conventionally thought of as lying within the domains of biology or "soft" matter physics. As DNA self assembly matures, we envisage a much wider range of examples of this transition (for example, DNA nanostructures might offer a route to spatially-periodic perturbation of interesting two-dimensional systems such as graphene or surface states in topological insulators). Our success will place the UK in a leading position in this broader field of academic research.

The project will expose a cohort of postdocs and graduate students to highly innovative research with the potential for substantial academic and industrial impacts. This will provide excellent career development opportunities and outstanding interdisciplinary training. These researchers will further benefit from the extensive career and professional skills development opportunities offered by their host institutions.