Graphene nanosensors for scanning Hall microscopy and susceptometry

Many of the most important advances in science and technology have only been made possible by parallel developments in instrumentation. For example, the development of microchips, which are now so common in our everyday lives, could not have taken place without the availability of electron microscopy to image cross-sections through prototype devices. Scanning Hall microscopy is a so-called "scanning probe" imaging technique where a tiny sensor is rastered across the surface of a sample to create a map of the magnetic fields. In this case the sensors rely on the Hall effect which arises when the electron flow in a conducting sample is bent by a magnetic field creating a Hall voltage at right angles to the main current direction. At present scanning Hall microscopy is a relatively niche technique that is mainly confined to making measurements of magnetic materials at low temperatures (typically less than -170C). This is due to the fact that although existing Hall effect sensors have high sensitivity at low temperatures, this becomes very much worse at room temperature when other scanning probe imaging methods, for example magnetic force microscopy, are preferred. Recent developments in graphene technology mean that this situation is about to change. Graphene is a single atomic layer of carbon that was first isolated by scientists in Manchester in 2004, leading to the award of the physics Nobel Prize in 2010. It is remarkable for its very high conductivity and mechanical strength, and the electrical carriers in graphene are able to move very much more freely than electrons in copper. Recently scientists have shown that still higher conductivities can be obtained if the graphene is sandwiched between thin layers of an insulator called boron nitride. In this way an improvement in Hall sensor performance of more than a hundred times is possible at room temperature, rivalling the other available magnetic imaging techniques. We also plan to develop new "susceptibility" imaging modes when the Hall probe measures the response of a sample to a small oscillating magnetic field generated by a tiny coil integrated into the sensor. This will allow new types of samples to be studied, and different types of problems can be addressed. Our new sensors target applications in three important technological areas. We will use Hall microscopy to map the nanoscale current distribution in second generation high temperature superconducting tapes that have enormous potential for applications in lossless power transmission and energy storage. Hall susceptometry will be used for the non-invasive detection of defects in "3D printed" materials (for example steel) which are known to play a critical role in structural failure. Finally we will explore how Hall susceptometry can be used for routine process control of the uniformity of the magnetic properties of thin film ferromagnetic materials for applications in data storage.

The requested PDRA and the ICON-funded PhD student working on the project will benefit enormously from the rich and varied training environment it offers. All researchers involved will develop expertise in a wide range of complementary cutting-edge experimental and theoretical techniques including nanoscale graphene device design and modelling, cutting-edge device fabrication and novel modes of scanning probe microscopy. These include many skills that have direct relevance to industry and commerce e.g. nanolithography and thin film deposition; numerical simulation and modelling; novel electronics and instrument design. The project offers the opportunity to learn about the latest exciting developments in the fields of Graphene & 2D materials, nanoscale sensor development and scanning probe microscopy. We will encourage our researchers to acquire advanced theoretical and practical knowledge by participation in appropriate Graduate School programmes, e.g., via training offered within the Bristol-Bath EPSRC Centre for Doctoral Training in Condensed Matter Physics. Comprehensive training and experience in these important technological areas will leave the PDRA and PhD student highly prized for subsequent positions in industry or academe.
Significant economic impact could arise from the project which has strong potential for the generation of important intellectual property. First and foremost we target the development of sensors that are directly compatible with commercial atomic force microscope systems. Subject to IPR protection with the support of Bath Ventures, the impact could be enabled through licensing arrangements with the partner company NanoMagnetic Instruments (letter of support attached) for the developed sensor technologies. However, the applications for the new sensors will have major impact in far more wide ranging areas including mapping the current distribution in 2G-HTS superconducting tapes, defect characterisation in additive manufactured materials and process control of thin ferromagnetic films for data storage. Strong interest in our project is reflected by the attached letters of support from superconducting tape manufacturers AMSC and Shanghai Superconductor, as well as Renishaw who are very active in the area of additive manufacturing. However, this only represents a small sub-set of potential stakeholders and many areas in the broad field of Non Destructive Testing (NDT) would, in particular, benefit from the implementation of eddy current testing technologies to be developed.
The project will also generate impact through public engagement and outreach. We routinely showcase our scientific research during visits to schools, open days and science fairs (e.g., Bath Taps into Science) and a focussed set of demonstrations will be developed to highlight the research achievements made within the project.