Electronic structure of 2D van der Waals heterostructures

The UK leads in the fundamental science of 2D materials, with world class expertise in both experimental and theoretical research. In the decade since the 'discovery' of graphene, the field has expanded in many directions: there is an ever-growing library of materials which can be made atomically-thin, including graphene, hexagonal boron nitride, transition metal dichalcogenides, black phosphorus, and many more; and the diversity of electronic applications of major technological importance has grown to encompass areas such as sensors, low-power electronics, high speed electronics and optoelectronics, all harnessing different aspects of their many unique properties.

Real devices harnessing these unique properties inevitably involve interfaces between different layered materials: indeed in many cases it is the interface which is itself the device. Layered materials can be combined in innumerable ways, much like stacking and shuffling playing cards. The same structure that makes it possible to obtain a monolayer form means that one layer only interacts with another via the rather weak van der Waals interaction. Different layers, particularly of lattice-mismatched materials, are therefore not constrained to specific alignments and a large and complex phase space of possible combinations is readily imaginable.

The electronic structure of the individual layers is important, but it is also imperative to understand how these change due to the (weak but significant) interlayer interactions and due to the electric fields that are applied in electronic and optoelectronic devices. Device engineering requires fundamental and quantitative understanding of these effects which are drastically different in 2D material devices compared to their 3D counterparts. This project will synergistically combine two state of the art tools that promise to revolutionise our ability to measure, model and ultimately design 2D material interfaces tailored to applications.

On the experimental side, Neil Wilson's expertise in angle resolved photoemission spectroscopy with (sub)micrometre spatial resolution (micro-ARPES) will provide a direct measurement tool for electronic structure, combining high spatial, angular and energetic resolution. Its results can, however, be challenging to interpret, and theoretical modelling is therefore vital. Density Functional Theory (DFT) is a well-established tool which balances accuracy and computational cost for materials modelling. While the computational effort of traditional approaches to DFT scales cubically with system size, precluding application to the very large atomistic models required to simulate a low-strain 2D material interface, in recent years the UK has pioneered the development of linear-scaling approaches to DFT. Here we will use the linear-scaling code ONETEP, which has been co-developed by Nick Hine.

We thus combine world-leading expertise in micro-ARPES, harnessing timely availability of novel synchrotron capabilities, with the state-of-the-art simulation tools which capitalise on EPSRCs investment in infrastructure for high-performance computing. We will apply these tools to study the electronic structure of 2D materials and how these change in heterostructures and operating electronic devices, and use the electronic structure to predict device performance. Having previously collaborated on proof-of-principle projects in this field, we have an excellent opportunity to make a significant impact on this expanding field. The devices that can be made from these materials and their interfaces promise, in the long term, to revolutionise many areas of technology. First, though, we must develop the ability to accurately measure and simulate their unique electronic structure. Combining our expertise, this proposal represents a timely and unique opportunity to do just that, and continue to advance the UK's leading role in this field on the international stage.

There is great hope that the impact of 2D materials will rapidly move from fundamental science to applications and hence from advances in our knowledge and understanding to real contributions to our economy and society. This work aims to underpin the future application of 2D materials for electronic and optoelectronic applications, providing key material parameters, new methodology, and insight. As such, although we hope that the long term impact will be to help 2D materials reach their full potential and hence make a significant economic and societal impact, we expect the key short term impacts to be associated with the knowledge, people and outreach that will be delivered. These short term impacts are of relevance to academics working in the areas of two dimensional materials, devices and electronic structure measurement and theory, the people associated with the proposal, and members of the public (particularly local schools) who will be engaged by our outreach activities.

The knowledge gained will be of great practical use for researchers looking to design devices with 2D materials. Such device design requires understanding of the electronic properties of the constituent materials and how they interact in heterostructures: our electronic structure parameters will enable the accurate and quantitative models to be made that are essential to device engineering. These same parameters will also be used by other condensed matter theoreticians to parameterise quantum transport models, or to compare to ab initio simulations. The technical advances in electronic structure measurement (by micro-ARPES) and electronic structure prediction (by linear scaling DFT) will be relevant to the wider communities working in these fields. Through engagement with CCP9 (Computational Electronic Structure of Condensed Matter) and IO5 at the Diamond Light Source, we will reach out to these researchers.

As a growing area, there is a need for people experienced in 2D materials research. A quick search on jobs.ac.uk shows PhD studentships, postdoctoral positions and permanent academic posts specifically advertised in this area. The people pipeline here is thus important. The project is designed to involve undergraduates through summer research studentships (we expect 4), PhD students (3), and a postdoctoral researcher as well as the two investigators. Each will benefit from interactions with the others, as well as with the project partners who add further international excellence. The investigators will ensure that all members of the team receive appropriate training and career advice to help the delivery of skilled and motivated researchers. Neil Wilson (as PI) will take overall responsibility for the management of the team.

The Physics Department at Warwick has an extremely active outreach programme, organised by the full-time school teacher-fellow (Ally Caldecote). This includes visits to and from local schools of all ages and backgrounds, as well as larger projects like the Warwick Christmas Lecture series (which last year had a combined live audience of >2800) and exhibits at the Cheltenham Science Festival and the Big Bang Fair. Neil Wilson (PI) organised the department's outreach for more than 5 years, is on the Royal Microscopical Society's Outreach committee, and has given talks and demonstrations to 1000s of pupils and members of the public over the last 10 years. The subject matter here is particularly apt to engage with school children and the general public: it is highly topical, visual, has clear applications and relevance, and is frequently covered in the popular press. We will capitalise on this by producing a hands-on outreach activity, 2D material playing cards, and working with Ally Caldecote to deliver them effectively to a wide audience.