Seed-free growth of strongly UV emitting self-supported hybrid ZnO nanowire/graphite membranes

Self-supported membranes consisting of entangled 2 to 6 mu m long ZnO nanowires (NWs) on graphite with excellent UV emission were fabricated by a vapor transport technique at high temperature in a tubular furnace. The NWs grew on catalyst-free graphite flakes that had been mechanically compacted within an alumina boat crucible (the substrate crucible). The self-supported ZnO NW/graphite membrane detached spontaneously from the graphite substrate during the cooling stage after NW growth. The inversion of the substrate crucible orientation to an upside down position (usually used by our group for rigid substrates) resulted in strong reductions of the UV/visible emission ratio; by a factor of similar to 20 for NWs grown on compacted graphite and of similar to 1000 for NWs grown on Au-catalyzed Si wafers. Simulations of the transport fluid dynamics within the synthesis tube demonstrate that the substrate holder shape and substrate position within the holder play crucial roles in the reduction of precursor velocities, which is likely a key factor for achieving the long entangled NWs with enhanced UV/visible emission ratios that conform the self-supported ZnO NW/graphite membranes. (C) 2020 Elsevier B.V. All rights reserved.