Nanoparticle-directed synthesis of organic nanorods

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Guangzhao Mao from Wayne State University is supported by an award from the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry to investigate organic nanorod growth on an inorganic nanoparticle, the "seed" which is several nanometers (billionths of a meter) across. The long-range goal is to learn to build tiny devices that combine the functions of organic and inorganic nanomaterials. The specific objectives are: 1) to investigate growth of these organic structures using gold nanoparticles seeds, 2) to show what effects on the nanorods depend on the seed size and shape, and 3) to make conductive organic nanorods by crystallizing charged organic molecules using electric current at the nanoparticles. The research contributes knowledge of nucleation and crystallization at the nanoscale, which is critical in nanomaterials synthesis and applications. The proposed real-time measurements are challenging because of the time and length scales of nucleation and growth processes and their inherent transient nature. To date, very few reliable measurements of the microscopic mechanisms and kinetics of molecular nucleation have been made. The synthesis of nanorods and nanowires from small organic molecule electrochemistry is transformative because most nanorods synthesized before have been of inorganic materials. The seed-mediated process is applicable to the manufacturing of inexpensive and field-ready electrochemical sensors. The educational plan focuses on three areas: 1) integration of research and teaching infrastructure by utilizing NSF MRI resources, 2) mentoring underrepresented students by working with the Wayne State University GO-GIRLS program, and 3) providing research experiences for undergraduates.Nanoparticles as nucleation seeds inhibit nucleation below a critical seed size and promote nucleation of crystals with different morphology than bulk in a size range above the critical size. The primary hypothesis is that the small size of nanoparticles imposes an unsustainable strain in the nucleated crystal and leads to the nanorod shape. An alternative hypothesis is the crystallographic confinement imposed by crystalline facets, edges, or corners of the nanoparticle seed. Two different systems, carboxylic acid crystals in evaporative crystallization and tetrathiafulvalene charge-transfer salt crystals in electrocrystallization allow a rigorous test of the primary and alternative hypotheses. Electrocrystallization affords a precise control of the nucleation and crystallization process, which is critical for understanding the seed-mediated process. Unlike work by others using seeds of similar building blocks as the nuclei, the work investigates the formation of the nanoparticle/nanorod architecture using heterogeneous and non-epitaxial seed-mediated nucleation. The hypotheses are examined by real-time electrochemical AFM experiments of the charge-transfer salt crystallization on gold nanoparticle-decorated electrodes. The proposed study explores an alternative strategy to integrate molecular and supramolecular components into nanodevices. In order for organic species to be used in nanodevices, the effect of area or shape confinement on molecular self-assembly must be addressed. The proposed research is significant because 1) the method incorporates organic species to make truly hybrid nanostructures; 2) the modular approach facilitates divergent combinatorial electrochemistry; and 3) the solution-based room-temperature process is potentially scalable for the manufacturing of molecular conductors, connectors, switches, and networks.