Quantum dot superlattices, typically thought of as arrays of quantum dot nanocrystals, can also be thought of as arrays of nanopores, representing a new class of ultra-thin nanoporous membranes. Epitaxially connected quantum dot membranes have pores periodically spaced 6 nm apart with diameters between 2 and 3 nm. The pore diameters are ideal for designing membranes with high ion selectivity in 2D nanofluidics due to overlapping electric double layers between nanopore walls. The near-atomic thinness of the membranes, down to 5 nm, also enables fast transport while maintaining high selectivity. Tunable surface chemistries enable programmable ion selectivity. Furthermore, flux through the membranes can be dynamically tuned via electrical or light-based biasing because of the semiconducting properties of the quantum dots. Demonstration of quantum dot superlattices for ionic separations allows for understanding of 2D nanofluidic transport, which I explored using a combination of experimental reverse electrodialysis and molecular dynamic simulations. I also showed that the membranes can be tuned to be selective to either anions or cations via modification to the surface chemistry of the pores and explain differences in transport behavior using molecular dynamics simulations. Fabrication of the quantum dot superlattices can be challenging. The superlattices are formed via deposition of colloidal particles onto an immiscible ethylene glycol subphase. While the process is ostensibly simple, there are several interrelated dynamics at work, including spreading and drying of the solvent, assembly of the particles, and the final step of ligand removal resulting in attachment of the particles. After evaporation of the top solvent, usually hexane or decane, the particles are left in a dry, unconnected state. Oriented attachment between particles is then induced via either chemical or thermal initiation. Uncoupling the complex aspects of assembling the particles into ordered films for application as membranes has been a concurrent direction of my work. To that end, I have studied the assembly process from formation of a droplet containing quantum dots to the final step of oriented attachment between quantum dots. Elucidation of these phenomena was achieved primarily using a combination of in situ X-ray scattering techniques and ex situ electron microscopy. To understand these processes, I have focused on different superlattice polymorphs in which the superlattices can have three distinct crystallographic orientations relative to the liquid subphase and can form different structures ranging from 1D wires to 3D multilayers. In this vein, this dissertation contributes to both 2D nanofluidics and fundamental crystallography of superlattices formed on fluid interfaces.