Nanocrystal size dependent adsorption geometry in self-assembled 2-D superlattices

Abstract

The self-assembly of colloidal semiconductor nanocrystals at the ethylene glycol-air interfaces results in the formation of two-dimensional nanocrystal superlattices. These superlattices exhibit atomic ordering and a superimposed nanoscale geometry. The adsorption geometry of the nanocrystals, before attachment, is ex- pected to have to have a great impact on the atomically connected two-dimensional superlattices. Here, the adsorption geometry of PbSe nanocrystal monolayers are studied by combining grazing incidence small and wide angle X-ray scattering (GISAXS/GIWAXS) with specular X-ray reflectivity (XRR) measurements. The GISAXS and GIWAXS measurements allowed information on the order of the nanocrystal monolayer in the nanocrystal and atomic length scales to be obtained, respectively. With these two techniques we observed that the nanocrystals with a diameter ≥ 5.2 nm align crystallographically with a [100] axis perpendicular to the interface while nanocrystals with a diameter < 5.2 nm exhibit more random orientations. The XRR measurements enabled information on the PbSe nanocrystals adsorbed on the ethylene glycol- air interface in the z-direction to be attained. With these measurements we observed that as the nanocrystals increase in diameter, they stick out more of the ethylene glycol substrate. The combination of all three techniques enabled a three-dimensional picture of how the PbSe nanocrystals monolayers float on top of the ethylene glycol- air interface to be fabricated.