Connecting the dots: shedding light on the self-assembly of semiconductor nanocrystals with synchrotron X-ray scattering techniques

Abstract

We studied the formation of two-dimensional crystals from nanocrystals using X-ray scattering techniques. Inside these nanocrystals, with sizes between 5-10 nm, the atoms are ordered in an atomic lattice. We use the nanocrystals as building blocks to create larger lattices in two dimensions. By adsorbing them at a liquid-air interface we are able to grow the crystal in only two dimensions, instead of the usual three dimensions. The particles and the resulting superlattices are still too small to visualize them through conventional microscopy. To study their self organization in real time, we use synchrotron based X-ray scattering. Any periodic structure will reflect the X-ray photons in directions governed by the crystal lattice. By looking at the position of these reflections at each point in time, we can calculate back what the particles look like at the liquid-air interface. For example, we can follow the distance between the particles, how they rotate and how they fuse together all in real time. We also studied the organization of novel perovskite nanocrystals into larger three-dimensional ordered structures, and show that we can alter their optical properties by exchanging the cations in these lattices.