We report on the growth of ZnSe nanowires by molecular beam epitaxy using solid Au nanoparticles as growth seeds. We show that nanowires with diameter below 15 nm are reproducibly straight, while thicker nanowires display a 'worm-like' morphology characterized by multiple kinks. This size-dependent morphology does not depend on growth parameters such as substrate temperature, Zn/Se pressure ratio or substrate chemistry, but we believe it is related to the solid state of the metal seed driving the nanowire growth. We finally propose three alternative methods to selectively obtain thin, straight and oriented ZnSe nanowires on GaAs(1 1 1)B.
