Graphene nanosensors for scanning Hall microscopy and susceptometry

Lead Research Organisation: University of Bath
Department Name: Physics


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.

Planned Impact

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.
Description We have developed deep sub-micron CVD graphene Hall sensors based on the intersection of wires with widths down to 50nm, which have superior minimum detectable fields to all other existing sensors of comparable dimensions at room temperature. Work is now in motion to improve the figures of merit still further by encapulation the graphene in layers of hBN. We are also developing very sensitive susceptometers by incorporating a field coil around the graphene Hall sensors. In addition new types of "edge-free" graphene Hall sensors have been developed and are soon to be tested for imaging applications.
Exploitation Route Advanced characterisation of magnetic storage media and NDE of novel materials, e.g., additive manufactured metal artefacts.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy,Manufacturing, including Industrial Biotechology

Title Dry transfer of exfoliated 2D flakes for van der Waals heterostructures 
Description A dry transfer rig has been developed in a glove box for the fabrication of high quality van der Waals heterostructures from exfoliated flakes from 2D crystals. 
Type Of Material Improvements to research infrastructure 
Year Produced 2019 
Provided To Others? No  
Impact Has enabled a new line f research in high quality "twisted" van der Waals heterostrutcures, using proximity effects at interfaces to engineer the electronic an optical properties of structures. 
Title Dataset for "High quality hydrogen silsesquioxane encapsulated graphene devices with edge contacts" 
Description Datasets underpinning the four figures for "High quality hydrogen silsesquioxane encapsulated graphene devices with edge contacts" in Materials Letters. The primary data files are transmission line measurements for edge contacts and Hall mobilities of devices with and without hydrogen silsesquioxane (HSQ) encapsulation. Also included are a device processing chart and atomic force microscopy (AFM) profiles. Raman data presented in the Supporting Information are also included. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Title Dataset for: "Nanoscale graphene Hall sensors for high-resolution magnetic imaging" 
Description This dataset contains data from the characterisations of chemical vapour deposition (CVD) graphene Hall sensors with wire widths between 50nm and 1500nm. Characterisations include noise amplitude at various drive currents and back gate voltages, Hall voltage measurements at various magnetic fields and back gate voltages, 2-wire voltage measurements with back gate voltage, and an SEM image used in publications/dissemination. The data files are named according to a convention that indicates to which wire width each file corresponds. Data in figures 7 and 8b of the associated paper are based on all of the complete sets of data for each of the wire widths. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Description Graphene Hall sensor development partner, Nanomagnetic Instruments Ltd 
Organisation NanoMagnetics Instruments Ltd
Country United Kingdom 
Sector Private 
PI Contribution Developing CVD graphene and encapsulated graphene nanoscale Hall sensors for scanning Hall probe microscopy.
Collaborator Contribution Provision of CVD graphene and access to Nanomagnetics low temperature scanning probe hardware to test sensors developed in Bath.
Impact Nanoscale graphene Hall sensors have been developed. Collaborative measurements at Nanomagnetic Instruments factory in Ankara are planned.
Start Year 2018