We report how the structural arrangement of germanium nanowires optimizes surface stability and enables the formation of very thin free-standing crystalline wires. Ge nanowires consisting of a crystalline Ge core and an amorphous ${\mathrm{GeO}}_2$ sheath have been formed by laser ablation of a mixture of Ge and ${\mathrm{GeO}}_2.$ The crystalline Ge core lies in the axial [211] direction and is terminated by the {111} facets on the surface. The ${\mathrm{GeO}}_2$ sheath saturates the surface bonds of the core, adapts to the core surface roughness, and prohibits the growth of the nanowire in the lateral direction. With such a core and sheath, the surface energy of the nanowire is reduced and the formation of very thin nanowires is thereby permitted. Phonon confinement in the Ge nanowires has been observed by Raman scattering.

• Received 24 March 1999

DOI:https://doi.org/10.1103/PhysRevB.61.4518