Colloidal cadmium selenide quantum nanowires: Synthesis, photo-induced photoluminescence enhancement, and fabrication of core/shell, water-dispersible structures

The goal of this project is to apply the solution-liquid-solid (SLS) mechanism developed in our group to optimize the growth of colloidal CdSe semiconductor nanowires (NWs) with rationally controlled composition, diameter, structure, and morpology. Bi nanoparticles are used to catalyze the CdSe nanowire growth in the coordinating solvent tri-n-octylphosphine oxide (TOPO). The availability of high quality CdSe quantum wires with both good morphology control and emission efficiency allow us to investigate the quantum-confinement effect in one dimensional (1D) semiconductor quantum structures.;High quality wurtzite CdSe quantum nanowires have been successfully synthesized with controlled diameters in the range of 5-16 nm. The best synthetic conditions are determined by varying precursors, amounts, ratios, and beneficial additives. The diameter control of the CdSe NWs is achieved by choosing different Bi nanoparticle sizes and reaction temperatures. The size dependence of the effective bandgaps in the CdSe quantum wires are determined from the absorption spectra.;The impurities in different batches of commercial TOPO significantly influence the rates of nanocrystal formation, the nanocrystal morphologies, and the reproducibilities of CdSe nanowire syntheses. By studying the effects of the different impurities on the syntheses of the CdSe NWs, the important beneficial impurity dioctylphosphinic acid (DOPA) is proven to be critical for high quality nanowire formation. Now the use of purified TOPO with controlled amounts of specific additives will allow rational and reproducible nanocrystal syntheses.;The as-prepared CdSe quantum wires dispersed in organic solvents undergo a photo-induced photoluminescence enhancement when illuminated under fluorescent lighting. This process is investigated with different light intensities, various additives, and various solvents. The partial reversibility of the photo-enhancement effect is observed. The quantum yields of CdSe NWs are carefully calculated. The PL quantum yield of photo-enhanced NWs is improved to ∼ 10% from the original 0.2%.;Type I CdSe/CdS and CdSe/ZnS core/shell nanowires are successfully fabricated via epitaxial growth. The optical properties of the core/shell structures are investigated. The PL quantum yield of the core/shell structures is enhanced to ∼ 10%. Water-dispersible CdSe quantum wires, such as PEI-coated, PEG-phosphine-oxide coated and CdSe/SiO2 core/shell structures, are also successfully synthesized. These structures will allow the applications of semiconductor quantum wires in biological systems.