Discovered almost two decades ago, the solution-liquid-solid (SLS) method for semiconductor nanowire (SC-NW) synthesis has since proven to be an important route to high-quality, single-crystalline anisotropic nanomaterials. Mechanistically, SLS is a solution-phase analogue to vapor-liquid-solid (VLS) SC-NW growth. Both SLS and VLS employ molten metal nanoparticles to catalyze nucleation and elongation of SC-NWs. With respect to execution, however, SLS has more in common with colloidal quantum dot synthesis in that it entails injection of chemical precursors into a hot surfactant solution. In contrast, in VLS growth precursors are introduced to catalytic liquid droplets through the vapor phase, carried by a flow of gas through a reaction chamber that likewise removes unreacted precursor and by-products. The inherent ability of the VLS approach to dynamically control the introduction of reactants and removal of by-products has enabled a degree of synthetic control not possible with SLS growth.We show here that SLS can be transformed into a flow-enabled technique with the resulting synthetic control affording unprecedented mechanistic insight and a novel platform for designed synthesis of technologically significant axially heterostructured SC-NWs. For two model systems � CdSe and ZnSe � we use observed trends in the temporal evolution of SC-NW lengths and diameters to discern information pertaining to growth modes active during flow-SLS synthesis. We find that under the conditions employed, catalyzed growth is unimpeded by contributions from sidewall addition and that growth can be modeled according to a combined Gibbs-Thomson/diffusion-limited growth model. Due to the controlled nature of flow-SLS growth, we are able to reveal that even small changes in temperature can effect significant enhancements in length/diameter. Furthermore, we demonstrate the possibility to tune catalyst-droplet supersaturation by, for example, changing reactant flow rates and, thereby, the Gibbs-Thomson cutoff diameter. Taken together, the new method also allows us to fabricate the first extensively-segmented superlattice SLS-SC-NWs.

Flow solution-liquid-solid nanowire synthesis: New platform for understanding growth mechanisms and advancing axial heterostructuring

246th National Meeting (2013)

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