Quantum imaging of current flow in graphene

Abstract: Since its first isolation in 2004, graphene has been found to host a plethora of unusual electronic transport phenomena, making it a fascinating system for fundamental studies in condensed-matter physics as well as offering tremendous opportunities for future electronic and sensing devices. However, to fully realise these goals a major challenge is the ability to non-invasively image charge currents in monolayer graphene structures and devices. Typically, electronic transport in graphene has been investigated via resistivity measurements, however, such measurements are generally blind to spatial information critical to observing and studying landmark transport phenomena such as electron guiding and focusing, topological currents and viscous electron backflow in real space, and in realistic imperfect devices. Here we bring quantum imaging to bear on the problem and demonstrate high-resolution imaging of current flow in graphene structures. Our method utilises an engineered array of near-surface, atomic-sized quantum sensors in diamond, to map the vector magnetic field and reconstruct the vector current density over graphene geometries of varying complexity, from mono-ribbons to junctions, with spatial resolution at the diffraction limit and a projected sensitivity to currents as small as 1 {\mu}A. The measured current maps reveal strong spatial variations corresponding to physical defects at the sub-{\mu}m scale. The demonstrated method opens up an important new avenue to investigate fundamental electronic and spin transport in graphene structures and devices, and more generally in emerging two-dimensional materials and thin film systems.

• The paper presents a novel quantum imaging method using NV centers in diamond to non-invasively map current flow in monolayer graphene with diffraction-limited resolution.
• It employs advanced fabrication techniques and detects currents as low as 1 μA, enabling the identification of sub-micrometer defects in the graphene structure.
• The results underscore the technique’s potential for probing non-classical electronic transport phenomena and device imperfections in graphene-based technologies.

Quantum Imaging of Current Flow in Graphene

The paper presents a novel method for high-resolution imaging of current flow in graphene structures, utilizing quantum sensors embedded within a diamond chip. This approach addresses the critical challenge in condensed-matter physics and electronics of non-invasively imaging charge currents in monolayer graphene. Traditional methods, primarily resistivity measurements, lack spatial resolution, limiting insights into electronic phenomena. The presented quantum imaging technique offers a spatial resolution at the diffraction limit, providing detailed mappings of vector magnetic fields and current densities over graphene devices.

The methodology involves fabricating graphene devices directly onto a diamond chip that hosts an array of nitrogen-vacancy (NV) centers near its surface. These NV centers function as atomic-sized magnetic sensors. The graphene is transferred onto the diamond substrate through a carefully controlled process involving chemical vapor deposition, electron-beam lithography, and plasma etching. The resulting devices are then imaged using photoluminescence (PL) quenching, where the NV centers' response to laser and microwave excitation enables the mapping of magnetic fields generated by the current flow in graphene.

A significant aspect of the study is the ability to resolve current flow features with the sensitivity to detect currents as small as 1 μA under the acquisition parameters used. Additionally, the study reveals the practicality of this approach by demonstrating its applicability through measurements that highlight sub-micrometer scale defects, which manifest as spatial irregularities in the current density maps. These defects, including cracks and tears in the graphene, are corroborated by PL and scanning electron microscopy (SEM) images, emphasizing the method's capability to identify and analyze imperfections impacting electronic transport at microscale.

The results extend beyond simple linear graphene ribbons, with the quantum imaging platform used to analyze current flow near junctions with metal contacts. This is crucial for understanding how geometry and material interfaces influence current distribution, which extends the application potential of this technique to complex device architectures that are increasingly common in graphene-based technologies.

From a theoretical standpoint, this integrated imaging approach allows for the study of non-classical electronic transport phenomena in two-dimensional materials, such as topological currents and electron backflow. Practically, it provides a tool for real-time investigation of conducting properties in devices that suffer from spatial inhomogeneities due to fabrication imperfections or operational conditions.

Future implications of this work are substantial, particularly in the realms of graphene spintronics and the study of orbital magnetism. The NV centers' capability to sense both static and dynamic magnetic fields, including fluctuating fields, positions this technique as potentially influential in probing advanced electronic properties such as spin injection and the spin Hall effect.

Overall, the paper successfully demonstrates a sophisticated quantum imaging technique that advances the capabilities for analyzing electronic and spin transport in graphene and other emerging two-dimensional materials, thereby providing a powerful tool for researchers in the field of condensed-matter physics and quantum technology.