Probing local electronic structure in 2D heterostructures

Starting with the semi-metallic graphene, the library of two-dimensional materials (2DMs) has expanded rapidly to include metals, semiconductors, insulators and more. Strongly correlated 2D materials are emerging as a fascinating area of research, with reports of 2D superconductors, magnets and even 2D topological insulators (2D TIs). Associated with this, phase transitions in 2D are fundamentally different from in three dimensions, leading to new Physics to explore. With the award of the Nobel Prize in Physics 2017 for the study of topological phase transitions and topological phases of matter, and the continued rise of 2DMs, this highlights exciting new avenues of research. The ability to simply stack different 2DMs to create complex heterostructures with new functional properties adds not only important degrees of freedom in designing devices, but also great hope for achieving real technological advances.

Probing the local electronic structure and properties of 2D materials is thus an outstanding challenge and opportunity. The 2D stacks are typically only a few micrometres across, necessitating local probes. We have shown that angle resolved photoemission spectroscopy with submicrometre spatial resolution (nanoARPES) is capable of directly measuring the layer dependent electronic structure in these stacks. This opens new avenues of research, allowing us to directly interrogate interaction phenomena such as moire effects and renormalisation. To complement the nanoARPES experiments, scanning probe microscopy (SPM) can resolve topographic, electronic, ferroic and magnetic structure with nanometre precision and under a range of environmental conditions. For 2DM, this enables studying how local electronic and ferroic properties change with the number of layers, distinguishing edge and 'bulk' effects, and probing spatial inhomogeneities. For example, with a 2D TI, SPM will be able to probe the bulk insulating and edge conducting states and examine spatial correlations at the phase transition.

This project is thus an exciting opportunity to study new Physics in new materials using new technology. The primary focus will be to apply high-resolution photoemission and SPM techniques to study electronic and functional properties of 2D materials at the nanoscale. This will capitalise on the unique SPM facility at Warwick for low temperature (base temperature < 2 K) atomic force microscopy with vectoral magnetic field (9x1x1 T) and optical access for time-resolved photodependent measurements, complemented by a new state-of-the-art ambient AFM. Although some of the 2D heterostructure samples will be fabricated in house, others will be sourced from our international collaborators.