ZnO Nanowires Grown by Chemical Bath Deposition in a Continuous Flow Microreactor

We report on a continuous flow microreactor for chemical bath deposition that enables rapid process characterization. The chemical bath flows through a submillimeter channel and material is deposited on a heated glass/silicon substrate that serves as one reactor wall. The microreactor operates in plug flow; bath composition changes as a function of distance down the reaction channel but the concentration profile is time-invariant. Spatially resolved characterization of the substrate enables rapid and direct correlation of material properties to growth conditions, which is not possible with a batch reactor where bath composition changes with time. We have used this microreactor to grow dense arrays of well-aligned, single-crystal ZnO nanowires. Slow flow rates result in nanowires whose lengths, growth mechanisms, and optical properties vary along the length of the substrate; fast flow rates produce nanowires that are more spatially uniform. Spatially resolved characterization of a single substrate reveals that, along the direction of flow, nanowire lengths decreased, morphology changed from pyramidal tops to flat tops, growth mechanism transitioned from two-dimensional nuclei to spiral growth, and band gap blue-shifted because of compressive strain. The continuous flow microreactor, demonstrated here for ZnO, can also be used to deposit other oxide and chalcogenide nanowires and thin films.