  Atomic structures of quasi-one-dimensional (1D) character can be grown on
semiconductor substrates by metal adsorption. Significant progress concerning
study of their 1D character has been achieved recently by condensing noble
metal atoms on the Ge(001) surface. In particular, Pt and Au yield high quality
reconstructions with low defect densities. We reported on the self-organized
growth and the long-range order achieved, and present data from scanning
tunneling microscopy (STM) on the structural components. For Pt/Ge(001), we
find hot substrate growth is the preferred method for self-organization.
Despite various dimerized bonds, these atomic wires exhibit metallic conduction
at room temperature, as documented by low-bias STM. For the recently discovered
Au/Ge(001) nanowires, we have developed a deposition technique that allows
complete substrate coverage. The Au nanowires are extremely well separated
spatially, exhibit a continuous 1D charge density, and are of solid metallic
conductance. In this review we present structural details for both types of
nanowires, and discuss similarities and differences. A perspective is given for
their potential to host a one-dimensional electron system. The ability to
condense different noble metal nanowires demonstrates how atomic control of the
structure affects the electronic properties.
