An overview of solution-based semiconductor nanowires: synthesis and optical studies.

This Perspective describes recent progress in the synthesis and optical characterization of high quality solution-based semiconductor nanowires (NWs). The described solution-phase NW syntheses are analogous to conventional vapor-liquid-solid (VLS) growth schemes for 1D materials. However, a primary difference is that low melting catalyst particles are used to induce the asymmetric crystallization of wires at temperatures sustainable by solution chemistry (T < 400 degrees C). Reactions are conducted in the presence of mild coordinating solvents such as trioctylphosphine oxide, which modulate NW growth kinetics and passivate their surfaces. This approach borrows from concurrent advances in colloidal quantum dot (QD) syntheses. In particular, the appropriate choice of solvent, coordinating ligands, growth conditions and precursors all originate from existing preparations for high quality semiconductor QDs. Subsequent structural characterization of the nanowires reveals their high degree of crystallinity, low ensemble size distributions and intrawire uniformity. Variations of these syntheses yield branched CdSe, CdTe and PbSe NWs with characteristic tripod, v-shape, y-shape, t-shape and "higher order" morphologies. A "geminate" NW nucleation mechanism is used to explain this phenomenon. The proposed branching model is also predictive and can be tested by additional nanowire syntheses that we or others conduct. Corresponding optical properties of both straight and branched wires are described, including measurements of their frequency-dependent absorption cross sections. Preliminary ensemble experiments focus on transient differential absorption measurements to study relevant carrier relaxation pathways and timescales over which these processes occur. Additional studies reveal intrawire optical heterogeneity and fluorescence intermittency at the single NW level.