Chemistry Weekly Seminar - Dr. David Vaknin, Iowa State University

Title

Novel robus routes to crystallizing nanoparticles in two- and three-dimensions

Abstract

I will show how by grafting nanoparticles (NPs) with various polymers, and by tweaking the aqueous NPs-suspension, crystallization of the NPs is achieved. Surface sensitive X-ray scattering methods, small-angle X-ray scattering (SAXS) and X-ray spectroscopy techniques are primarily used to determine the conditions under which two- and three-dimensional (2D and 3D) NP crystallization occurs. Here, we use mainly AuNPs as model systems for self-assembly with a long-term goal to use these methods to assemble more exotic nano-structures for applications in photonics. The AuNPs are grafted with polyethylene glycol (PEG), Poly(N-isopropylacrylamide) (PNIPAM), DNA, or polyelectrolytes and are dispersed in aqueous solutions to form homogeneous suspensions.  Upon addition of salt to the suspensions, the capped-AuNPs migrate to the aqueous/vapor interface and at a threshold salt concentration the PEG-AuNPs order in 2D hexagonal structure. At high enough salt concentrations, PEG-AuNPs form 3D superstructures compatible with FCC symmetry (short-range-order at that). 3D assembly is also achieved by adding polyelectrolytes to the PEG-AuNPs suggestive of interpolymer complexation, in our case -- PEG-PAA complexation.  We also find that raising the temperature of the suspensions can induce 3D superstructures at lower salt concentrations and also improve crystallinity.  I will end by presenting recent results where superstructures are formed by mixing two types of PEG-AuNPs that differ in their core-size and their polymer-length.

Date:    Friday, September 24

Time:    1:30 pm

Via Zoom video conference (link available by request to chem.dept@usask.ca)