PLANAR scanning probe microscopy FOCUS have been demonstrated with an extended planar bulk diamond. The technique here solves an outstanding challenge: NV centers have ideal properties when they are under a well-defined planar surface, but these properties easily degrade if nanofabrication is performed in their vicinity, e. g. plasma etching required to shape a nanoscale tip. Also, fabrication of a planar probe is considerably simpler than nanofabrication of nanoscale tips.
THE FUTURE – DIFFERENT SENSORS AND MASSIVE PARALLELISM The most interesting question, however, is: what will we be able to do with planar scanning probes that could not be done with a scanning tip? One answer could be the wide variety of sensors that can potentially be used. Many emerging sensors are extended circuits, for example microfabricated superconducting magnetic resonance cavities which have recently become sufficiently sensitive to detect single electron spins. Even for pointlike sensors the ability to augment them by extended circuits is attractive. For instance, nitrogen-vacancy centers could be augmented by switchable nanomagnets to perform gradient-based magnetic resonance imaging of samples.
Another answer could be the ability to scan at a controlled nanoscale fly height without ever touching the sample. This could pave the way to using fragile nanoparticles as scanning probes, e. g. solution-synthesized epitaxial nano-cubes which have excellent plasmonic properties and promise much higher plasmonic enhancement than top-down-fabricated structures. Notably, arrangements of these nanoparticles, e. g. in the form of particle-on-mirror nanogap cavities, have achieved spectacular performance in stationary nano-fabricated assemblies, amplifying luminescence by up to four orders of magnitude and confining light to atomic scales [ 7 ]. Turning these plasmonic arrangements into a scanning probe scheme would open the door to optical microscopy with atomic resolution. Finally, massive parallelism is an attractive perspective. More than one sensor can be embedded into a single planar probe and can be operated in parallel. This is especially promising for sensors like NV centers, which suffer from a slow readout. Using massively parallel arrays of centers could enable scanning at high resolution( e. g. 4K pixels) and / or large( 10 µ m-mm) field of view, while fully preserving the nanometer-scale resolution of scanning defect centers. At a high density of NV centers, scanning could even be replaced by superresolution microscopy. Simultaneous scanning with multiple centers could also enable imaging of spatial correlations of signals by correlating the signal of different centers or even by creating quantum entanglement between them, which would open a new window into transport phenomena and spatial correlations in solid-state physics.
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