Self-assembled templates and field-flow fractionation for controlling and organizing the sizes of nanomaterials.

摘要

Nanostructured materials have received a tremendous amount of attention because of their novel properties, which are remarkably different from those of the bulk. These materials include nanoparticles, atomic clusters, nanocomposites, and multilayer films, all of which can demonstrate quantum confinement effects and size tunable properties. While great strides have been made in the synthesis of novel nanomaterials, the goal of obtaining monodisperse products remains a challenge. This thesis seeks to demonstrate two diverse methods for controlling and organizing the sizes of nanostructured materials.;The first project is the development of aqueous and organic solvent compatible asymmetric flow field-flow fractionation (AsFlFFF) for the separation and purification of passivated silicon quantum dots (Si QDs). Polydisperse as-synthesized allylamine-, dodecene-, and pyrenebutanol-terminated Si QDs were separated into narrower size distribution subpopulations. Fractions of dodecene-Si QDs, which were collected at the channel outlet, were further characterized using transmission electron microscopy (TEM) and were confirmed to have significantly reduced dispersities, e.g., 3.8 +/- 0.67 nm, 5.3 +/- 0.78 nm, and 6.3 +/- 0.82 nm, compared to the 3-16 nm present in the unfractionated sample. An additional complication presents itself when using these fractions in subsequent studies because of the presence of excess reactants used in the passive reaction. This was the case for pyrenebutanol whose photoluminescence spectrum is similar whether it is covalently bonded to the Si QDs or present as unattached ligands in solution. AsFlFFF's ability for on-line removal of excess pyrenebutanol as part of the separation process was demonstrated. This work represents the first use of AsFlFFF for separating Si QDs and the first application of an organic solvent compatible system for isolating nanoparticles.;The results of separating allylamine-terminated Si QDs using aqueous AsFlFFF was also demonstrated with subpopulations that had average diameters of 4.3 nm, 7.2 nm, 9.8 nm, and 21 nm. In this case, the as-synthesized QDs had an initial size range of 3.8 - 300 nm with large aggregates present. Here, the advantage of an externally applied field that can be turned off was demonstrated by turning of the cross-flow to allow undesired large Si NPs to rapidly elute from the channel and thus shorten the overall separation time. The ability to readily collect separated subpopulations, the possibility to monitor the onset of aggregation, the high sample recovery, and the on-line removal of excess reagents and other small molecules make AsFlFFF highly attractive for separating and purifying passivated Si QDs and other nanomaterials in aqueous and organic solvents.;The second project addresses nanostructured thin films. Self-assembly is a bridge for connecting micro and macro worlds by making ordered arrays of nanomaterials. Electrochemically assisted self-assembly (EASA) was used to synthesize a nanoporous silica thin film on nontraditional conductive substrates. Grazing-incidence small angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM) provide evidence that these nanopores were highly ordered, ~ 3nm, and aligned perpendicular to the substrate surface. The use of a sacrificial conductive polymer, PEDOT:PSS provided a novel approach to synthesizing these films on nonconductive surfaces and to obtaining free standing nanoporous films that can be transferred to different substrates. The tunable pore size and controllable thickness of this silica thin film make it a good candidate for use as a template for synthesizing ordered nanowire/nanorod arrays and applications such as membranes where high pore densities are required.