Powered by the mutual developments in instrumentation, materials and theoretical descriptions, sensing and imaging capabilities of quantum emitters in solids have significantly increased in the past two decades. Quantum emitters in solids, whose properties resemble those of atoms and ions, provide alternative ways to probing natural and artificial nanoscopic systems with minimum disturbance and ultimate spatial resolution. Among those emerging quantum emitters, the nitrogenvacancy (NV) color center in diamond is an outstanding example due to its intrinsic properties at room temperature (highly-luminescent, photo-stable, biocompatible, highly-coherent spin states). This review article summarizes recent advances and achievements in using NV centers within nano-and single crystal diamonds in sensing and imaging. We also highlight prevalent challenges and material aspects for different types of diamond and outline the main parameters to consider when using color centers as sensors. As a novel sensing resource, we highlight the properties of NV centers as light emitting electrical dipoles and their coupling to other nanoscale dipoles e.g. graphene. ‡ Present address: Istituto Nazionale di Ricerca Metrologica, Strada delle cacce 91, 10137 Torino (To), Italy arXiv:1909.03719v1 [physics.app-ph] 9 Sep 2019 Nanoscale sensing based on nitrogen vacancy centers 2

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We investigate native nitrogen (NV) and silicon vacancy (SiV) color centers in commercially available, heteroepitaxial, wafer-sized, mm thick, single-crystal diamond. We observe single, native NV centers with a density of roughly 1 NV per µm 3 and moderate coherence time (T 2 = 5 µs) embedded in an ensemble of SiV centers. Low-temperature spectroscopy of the SiV zero phonon line fine structure witnesses high crystalline quality of the diamond especially close to the growth surface, consistent with a reduced dislocation density. Using ion implantation and plasma etching, we verify the possibility to fabricate nanostructures with shallow color centers rendering our diamond material promising for fabrication of nanoscale sensing devices. As this diamond is available in wafer-sizes up to 100 mm it offers the opportunity to up-scale diamond-based device fabrication. PACS numbers: Valid PACS appear hereDiamond nanostructures are of significant importance for various applications in science and industry including nanomechanical devices, 1 photonics 2 and sensing. 3 A major challenge for most of these applications is the scalability of the fabrication process predominantly due to a lack of large area single-crystal diamonds with good crystalline quality and high purity. Manufacturing synthetic, single-crystal diamond on wafer-scale has been an active field of research 4,5 leading to the commercial availability of single-crystal diamonds with a diameter of ≈ 100 mm recently. This progress opens the road towards up-scaling the fabrication of single-crystal nanostructures especially for diamond related sensing applications. 6 Color centers in diamond, in particular the negatively-charged nitrogen vacancy (NV) center in nanostructures, have been extensively used to sensitively measure e.g. magnetic fields in the last decade. Recently, silicon vacancy (SiV) centers emerged as alternative enabling all optical sensing of temperatures using their narrow electronic transitions. 7 Single color centers allow for sensing with high spatial resolution and offer bright, photostable photoluminescence (PL). In addition, NV centers provide highly-coherent, controllable spin states 3 and show optically-detected magnetic resonance (ODMR) enabling to read out their spin states via PL detection. As a consequence, even single NV centers can serve as quantum-enhanced sensors. Magnetic field imaging using NV centers has various applications ranging from material characterization in superconductors 8 or magnetic materials for spintronics 9 to life science applications where nuclear magnetic resonance detection of single proteins is of interest. 10 We here demonstrate the basic applicability of commercial, single-crystal, wafer-sized diamonds for quantum a) Electronic mail: elkeneu@physik.uni-saarland.de technology applications. To this end, we demonstrate coherent manipulation of single native NV center spins in the material, while low-temperature spectroscopy of SiV center PL indicates high crystalline quality of the material. In addition, we im...

In 2 O 3 has emerged as a promising catalyst for CO 2 activation, but a fundamental understanding of its mode of operation in CO 2 hydrogenation is still missing, as the application of operando vibrational spectroscopy is challenging due to absorption effects. In this mechanistic study, we systematically address the redox processes related to the reverse water-gas shift reaction (rWGSR) over In 2 O 3 nanoparticles, both at the surface and in the bulk. Based on temperature-dependent operando UV/Vis spectra and a novel operando impedance approach for thermal powder catalysts, we propose oxidation by CO 2 as the rate-determining step for the rWGSR. The results are consistent with redox processes, whereby hydrogen-containing surface species are shown to exhibit a promoting effect. Our findings demonstrate that oxygen/hydrogen dynamics, in addition to surface processes, are important for the activity, which is expected to be of relevance not only for In 2 O 3 but also for other reducible oxide catalysts.

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