Ordering of ZnS Nanorods at the Air–Liquid Interface: An In Situ X-ray Scattering Study

Self-assembly of anisotropic nanomaterials into ordered multidimensional (2D and 3D) arrays facilitate versatile collective properties. Monitoring of this self-assembly processes is essential to control the emerging properties of these nanostructured materials. We report here results of in situ X-ray reflectivity (XRR) and grazing incidence small-angle X-ray scattering (GISAXS) studies carried out at the air–liquid interface to understand the self-assembly process of ZnS nanorods (NRs) in real-time. Our study shows that at high surface pressure, ZnS NRs self-assemble into a bilayer 2D square superlattice, coupled by their organic surface ligands along the out-of-plane direction with NRs lying side by side flat in the in-plane direction at the liquid surface. Upon increasing subphase temperature, we observed a transformation of the 2D superlattice into a 3D multilayer structure of the nanorods. On the basis of the extracted parameters (film thickness, electron density of each layer, and in-plane lattice parameters), we propose a packing model of the hierarchical self-assembly of anisotropic NRs on the water surface.