Magnetic order emerging in otherwise non-magnetic materials as carbon is a paradigmatic example of a novel type of s-p electron magnetism predicted to be of exceptional hightemperature stability 1 . It has been demonstrated that atomic scale structural defects of graphene can host unpaired spins 2,3 . However, it is still unclear under which conditions longrange magnetic order can emerge from such defect-bound magnetic moments. Here we propose that in contrast to random defect distributions, atomic scale engineering of graphene edges with specific crystallographic orientation -comprising edge atoms only from one sublattice of the bipartite graphene lattice -can give rise to a robust magnetic order. We employ a nanofabrication technique 4 based on Scanning Tunneling Microscopy to define graphene nanoribbons with nanometer precision and well-defined crystallographic edge orientations.While armchair ribbons display quantum confinement gap, zigzag ribbons narrower than 7 nm reveal a bandgap of about 0.2 -0.3 eV, which can be identified as a signature of 2 interaction induced spin ordering along their edges. Moreover, a semiconductor to metal transition is revealed upon increasing the ribbon width, indicating the switching of the magnetic coupling between opposite ribbon edges from antiferromagnetic to ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hope for graphene-based spintronic devices operating under ambient conditions.The intrinsic magnetism of graphite has a long and controversial history 1 . The origin of the measured magnetic signal is generally attributed to atomic scale structural defects locally breaking the sub-lattice balance of the bipartite hexagonal lattice 5,6 . However, the unambiguous identification of the structural sources of the measured magnetic signal has proven challenging as they are buried inside the bulk of the material. The isolation of single graphene layers 7 opens new prospects in this direction 8,9 as their atomic structure is fully accessible for imaging and controlled modification. In particular, graphene edges of specific (zigzag) crystallographic orientation comprising carbon atoms from only one sub-lattice of the bipartite hexagonal lattice are predicted to host magnetic order 10 , in striking contrast to armchair edges incorporating an equal number of carbon atoms from both sublattices.The strong influence of edge orientation on the electronic structure of graphene nanoribbons had However, the random orientation of the edges and the influence of a possible strong edge-substrate hybridization 21 did not allow full access to the nature of edge-magnetism in graphene. Though the 4 magnetic order is expected to persist to some extent on zigzag segments of randomly oriented graphene edges, the mixing of different edge types are expected to substantially weaken the effect 19,22 . Therefore, the lack of experimental control over the edge orientation seems one of the main ...

The chemical inertness of the defect-free basal plane confers environmental stability to MoS single layers, but it also limits their chemical versatility and catalytic activity. The stability of pristine MoS basal plane against oxidation under ambient conditions is a widely accepted assumption however, here we report single-atom-level structural investigations that reveal that oxygen atoms spontaneously incorporate into the basal plane of MoS single layers during ambient exposure. The use of scanning tunnelling microscopy reveals a slow oxygen-substitution reaction, during which individual sulfur atoms are replaced one by one by oxygen, giving rise to solid-solution-type 2D MoSO crystals. Oxygen substitution sites present all over the basal plane act as single-atom reaction centres, substantially increasing the catalytic activity of the entire MoS basal plane for the electrochemical H evolution reaction.

MoS2 single layers have recently emerged as strong competitors of graphene in electronic and optoelectronic device applications due to their intrinsic direct bandgap. However, transport measurements reveal the crucial role of defect-induced electronic states, pointing out the fundamental importance of characterizing their intrinsic defect structure. Transmission Electron Microscopy (TEM) is able to image atomic scale defects in MoS2 single layers, but the imaged defect structure is far from the one probed in the electronic devices, as the defect density and distribution are substantially altered during the TEM imaging. Here, we report that under special imaging conditions, STM measurements can fully resolve the native atomic scale defect structure of MoS2 single layers. Our STM investigations clearly resolve a high intrinsic concentration of individual sulfur atom vacancies, and experimentally identify the nature of the defect induced electronic mid-gap states, by combining topographic STM images with ab intio calculations. Experimental data on the intrinsic defect structure and the associated defect-bound electronic states that can be directly used for the interpretation of transport measurements are essential to fully understand the operation, reliability and performance limitations of realistic electronic devices based on MoS2 single layers.