Routes to different-sized Si nanowires with erbium-rich shell structures have been developed. Tunable nanowire widths with average values of 50, 75, 90, and 160 nm can be controlled by the nature of the gold catalyst employed in the vapor-liquid-solid reaction used to obtain the Si core structure. Lattice imaging of selected nanowires permit structural characterization as a single-crystal Si core surrounded by a polycrystalline erbium-rich surface layer. Furthermore, it appears that in relatively small wires (< 100 nm in diameter), negligible near-infrared photoluminescence is observed, suggesting that energy transfer quenching by Er3+ clustering or surface hydroxyl groups on a relatively smaller surface area are likely origins for deactivation of the 1540 nm emission.