In this paper the fundamental properties of heterostructures based on semiconductor nanowires synthesized with molecular beam epitaxy are reviewed. Special focus is given on surface passivation mechanisms with radial epitaxial passivation shells. The growth of radial p-i-n junctions in GaAs nanowires is discussed. Characterization of such nanowires on a single nanowire level is presented. The fundamental limits of single nanowire optical device performance are obtained by numerical simulation and discussed.