High-purity, type IIa diamond is investigated by noncontact atomic force microscopy (NC-AFM). We present atomic-resolution images of both the electrically conducting hydrogen-terminated C(100)-$\left(2\times 1\right):\text{H}$ surface and the insulating C(100)-$\left(2\times 1\right)$ surface. For the hydrogen-terminated surface, a nearly square unit cell is imaged. In contrast to previous scanning tunneling microscopy experiments, NC-AFM imaging allows both hydrogen atoms within the unit cell to be resolved individually, indicating a symmetric dimer alignment. Upon removing the surface hydrogen, the diamond sample becomes insulating. We present atomic-resolution images, revealing individual C-C dimers. Our results provide real-space experimental evidence for a $\left(2\times 1\right)$ dimer reconstruction of the truly insulating C(100) surface.

• Received 22 April 2010

DOI:https://doi.org/10.1103/PhysRevB.81.201403