Local and long-range order of carbon impurities on Fe(100): Analysis of self-organization at a nanometer scale

Bulk carbon impurities segregate at the Fe(100) surface and, upon thermal annealing, can form metastable surface phases, with local and long-range order, that show peculiar electronic properties. We present a surface science study of C-segregated Fe(100) with scanning tunneling microscopy/spectroscopy (STM/STS), core level spectroscopy, and ab initio calculations of the surface structure. In particular, we investigate a c (32×2) structure, observed for 0.67±0.05 atomic layers of C segregated at the iron surface. This structure is found to be due to self-organized carbon stripes, which form a regular pattern on a nanometer lateral scale and are made of zig-zag chains. The C atoms in the chains lie slightly off center in the fourfold hollow site and are bonded to 5 Fe neighbors. Striking features of this structure are the self-avoiding chains, the passivation effect of the iron surface, and the presence of one-dimensional-like Fe surface states close to the Fermi energy.