(169a) Solution Growth of ZnO Nanowires in a Continuous Flow Microreactor

Crystal growth from solution is used in the optoelectronic and photovoltaic industries, as well as in the laboratory, for the production of oxide and chalcogenide nanowires and thin films. Chemical bath deposition (CBD) offers advantages over high-temperature vapor-phase growth in terms of both cost and compatibility with flexible substrates. However, its widespread use is limited by low process yield and excessive waste solvent, which result because precipitation in solution competes with deposition on the substrate, and also by lack of detailed understanding of how growth conditions impact material properties and morphology.

We report on the implementation of a continuous flow microreactor, where the substrate serves as one reactor wall and the chemical bath is contained within a sub-millimeter channel. We have used the microreactor to grow dense arrays of well-aligned single-crystal ZnO nanowires. The smaller transport lengths offered by the microreactor design reduce mass transport limitations and mitigate homogenous precipitation, resulting in microreactor deposition yields that can be up to an order of magnitude higher than yields using CBD with conventional reactor geometries. Furthermore, the continuous flow microreactor operates in plug flow, where bath composition changes as a function of position but is time-invariant. Slow flow rates result in nanowires whose lengths, growth mechanisms, and optical properties vary along the length of a single substrate, while fast flow rates produce nanowires that are more uniform across the substrate. Spatially-resolved characterization of the substrate enables rapid and direct correlation of material properties to growth conditions, which is not possible for batch growth where bath composition evolves with time. Here we explore growth at low flow rates to create combinatorial libraries of materials; and we describe the relationship between growth mechanism, strain, and photoluminescence of solution-deposited ZnO nanowires in unprecedented detail.