Quantum sensing with shallow nitrogen-vacancy (NV) centers in diamond offer promise for chemical analysis. Preserving favorable NV spin and charge properties while enabling molecular surface functionalization remains a critical challenge.

Quantum sensors based on spin defects in diamond have recently enabled detailed imaging of nanoscale magnetic patterns, such as chiral spin textures, two-dimensional ferromagnets, or superconducting vortices, based on a measurement of the static magnetic stray field. Here, we demonstrate a gradiometry technique that significantly enhances the measurement sensitivity of such static fields, leading to new opportunities in the imaging of weakly magnetic systems. Our method relies on the mechanical oscillation of a single nitrogen-vacancy center at the tip of a scanning diamond probe, which up-converts the local spatial gradients into ac magnetic fields enabling the use of sensitive ac quantum protocols. We show that gradiometry provides important advantages over static field imaging: (i) an order-of-magnitude better sensitivity, (ii) a more localized and sharper image, and (iii) a strong suppression of field drifts. We demonstrate the capabilities of gradiometry by imaging the nanotesla fields appearing above topographic defects and atomic steps in an antiferromagnet, direct currents in a graphene device, and para- and diamagnetic metals.

We describe the atomic structure of the silver film grown on Si(001) at room temperature, as studied by low-temperature scanning tunneling microscopy and density functional theory. Experiment and theory agree on a film structure in which Ag tetramers are identified for the first time. Ag tetramers are found to be adsorbed exclusively at the trough between two Si rows, interacting with four adjacent Si dimers via covalent bonding. Consequently, the π bonds of the Si dimers underneath the silver film are eliminated.

We report on nanometer magnetic imaging of two-dimensional current flow in bilayer graphene devices at room temperature. By combining dynamical modulation of the source-drain current with ac quantum sensing of a nitrogen-vacancy center in the diamond probe tip, we acquire magnetic field and current density maps with excellent sensitivities of 4.6 nT and 20 nA/μm, respectively. The spatial resolution is 50-100 nm. We introduce a set of methods for increasing the technique's dynamic range and for mitigating undesired back-action of magnetometry operation (scanning tip, laser and microwave pulses) on the electronic transport. Finally, we show that our imaging technique is able to resolve small variations in the current flow pattern in response to changes in the background potential. Our experiments demonstrate the feasibility for detecting and imaging subtle spatial features of nanoscale transport in two-dimensional materials and conductors.

When a silver film is grown on the Si(001) surface at room temperature, it forms a monolayer density film with a (2 × 2) periodicity. Previous models of this (2 × 2) surface, simulated by density functional theory (DFT), are found to be inconsistent with experimental observations. The DFT simulations provide evidence that a new model, the square tetramer model, describes the structure of the observed (2 × 2)-Ag phase and is fully consistent with scanning tunneling microscopy data. Theoretical evidence of a covalent bond shared between the Ag and Si is found that matches previous experiments. Interestingly, the simulations also show that the stoichiometry between Si and Ag changes with metal coverage as adsorbate−adsorbate and adsorbate−substrate interactions balance out. At low coverages, when individual Ag adatoms interact solely with Si, a two-to-one Si−Ag−Si interaction scheme is energetically preferred. At 1 monolayer, when Ag−Ag interactions must be considered, a one-to-one Si− Ag interaction scheme is preferred, as it maximizes Ag−Ag interactions.