(a) Scanning electron microscope image of GaAsP nanowires, each having a Ga droplet on top. The nanowires are seen from above at an angle of 25°. Scale bar, 1 μm. (b) Transmission electron microscope image showing the upper part of a GaAsP nanowire with only a few twinnings visible just below the Ga droplet and further down the nanowire. Scale bar, 200 nm. Inset: diffraction image showing the nanowires zinc-blende crystal structure, expanded in Supplementary Fig. S3. (c) Energy-dispersive X-ray spectroscopy characterization of P content (x-value) in two nanowires from the same growth. The variation in P during the first 1 μm of the growth is due to a deliberate flux change, whereas the downward slope is under a constant flux.