Dimer reconstruction and electronic surface states on clean and hydrogenated diamond (100) surfaces

We present ab initio investigations of the structural and electronic properties of clean and hydrogen-covered diamond (100) surfaces within local-density-functional theory. Our calculations are based on a variational solution of the Kohn-Sham equations using a preconditioned conjugate-gradient approach and on the optimization of the atomic structure via a quasi-Newton quench based on the exact Hellmann-Feynman forces. The computations are performed in a plane-wave basis, the electron-ion interaction is described by optimized ultrasoft pseudopotentials. We find that the clean and the monolayer-covered surfaces reconstruct in a (2 X 1) cell via the formation of rows of symmetric pi-bonded dimers. Further hydrogenation to a coverage of 1.5 ML stabilizes a surface with a (1X1) periodicity in the C layers, albeit with a low H-desorption energy for the formation of the reconstructed monohydride surface. The two-step desorption process is in good agreement with experimental observations. Electronic surface states within the bulk gap are predicted for the clean surface, but not for the monohydride case. The detailed analysis of the layer-resolved local densities of states and of the dispersion of the surface states demonstrates that the results are in good agreement with recent photoemission experiments. A negative electron affinity is predicted for the monohydride surface, but not for the clean surface.