Enhanced synthetic control of the morphology, crystal structure, and composition of nanostructures can drive advances in nanoscale devices. Axial and radial semiconductor nanowires are examples of nanostructures with one and two structural degrees of freedom, respectively, and their synthetically tuned and modulated properties have led to advances in nanotransistor, nanophotonic, and thermoelectric devices. Similarly, developing methods that allow for synthetic control of greater than two degrees of freedom could enable new opportunities for functional nanostructures. Here we demonstrate the first regioselective nanowire shell synthesis in studies of Ge and Si growth on faceted Si nanowire surfaces. The selectively deposited Ge is crystalline, and its facet position can be synthetically controlled in situ. We use this synthesis to prepare electrically addressable nanocavities into which solution soluble species such as Au nanoparticles can be incorporated. The method furnishes multicomponent nanostructures with unique photonic properties and presents a more sophisticated nanodevice platform for future applications in catalysis and photodetection.