Self-Assembly of Tunable Nanocrystal Superlattices Using Poly-(NIPAM) Spacers

Understanding and controlling 3D nanocrystal self-assembly is a fundamental challenge in materials science. Assembly enables the unique optical and electronic properties of nanocrystals to be exploited in macroscopic materials, and also opens up the possibility to couple the optical response of nanocrystals to the optical modes of the superlattice. To date, assembly of such nanocrystal superlattices (NCSL) has focussed on fixed, close packed structures with particle separations of just 13 nm. To achieve highly crystalline structures with tunable optical response, the nanocrystal interparticle separation needs to be precise and easily variable but >50 nm. Here, we show the preparation of nanocrystal superlattices with spacings of 50500 nm assembled from gold-poly-N-isopropylacrylamide core-shell particles and the characterization of their fascinating diffraction behavior by means of UV-vis spectroscopy. These nanocrystal superlattices exhibit pronounced diffraction in the visible (440-560 nm) with peak half-widths of the order of 10 nm. The position of the Bragg peak is simply tuned by adjusting the particle volume fraction. Due to the thermoresponsive nature of the polymer shell, temperature is used to initiate crystallization or melting of the superlattice. Heating and cooling cycles cause highly reversible melting/recrystallization in less than a minute.