The electronic properties of insulators such as diamond are of interest not only for their passive dielectric capabilities for use in electronic devices, but also for their strong electron confinement on atomic scales. However, the inherent lack of electrical conductivity in insulators usually prevents the investigation of their surfaces by atomic-scale characterization techniques such as scanning tunnelling microscopy (STM). And although atomic force microscopy could in principle be used, imaging diamond surfaces has not yet been possible. Here, we demonstrate that STM can be used in an unconventional resonant electron injection mode to image insulating diamond surfaces and to probe their electronic properties at the atomic scale. Our results reveal striking electronic features in high-purity diamond single crystals, such as the existence of one-dimensional fully delocalized electronic states and a very long diffusion length for conduction-band electrons. We expect that our method can be applied to investigate the electronic properties of other insulating materials and so help in the design of atomic-scale electronic devices.

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Surface electronic states of the partially hydrogenated diamond C(100)-(2ϫ1):H surface were studied by near-edge x-ray absorption fine structure and C 1s core level photoemission. Partially hydrogenated surfaces were prepared by synchrotron irradiation of the monohydride-terminated surface or by hydrogen adsorption on the clean surface. A new surface core-exciton state produced at a photon energy of 282.5 eV has been assigned to single dangling bonds of the partially hydrogenated surface. Monitoring this new feature has been found to be a powerful method to study hydrogen kinetics during ͑i͒ photon irradiation of a fully hydrogenated diamond surface, ͑ii͒ adsorption of atomic hydrogen on a clean diamond surface, and ͑iii͒ photon irradiation of a fully hydrogenated surface followed by thermal annealing. From the analysis of dangling-bond distribution, it follows that no preferential pairing of hydrogen on the C-C dimers occurs during hydrogen adsorption at room temperature. In contrast, thermal annealing induces pairing of the single dangling bonds into the -bonded configuration, the pairing process being accompanied by hydrogen desorption. This observation suggests that the activation barrier of hydrogen thermal diffusion is only slightly lower than that of thermal desorption.