We investigate a new method for forming large-area (> cm2) ordered monolayers of colloidal nanocrystal quantum dots (QDs). The QD thin films are formed in a single step by spin-casting a mixed solution of aromatic organic materials and aliphatically capped QDs. The two different materials phase separate during solvent drying, and for a predefined set of conditions the QDs can assemble into hexagonally close-packed crystalline domains. We demonstrate the robustness and flexibility of this phase-separation process, as well as how the properties of the resulting films can be controlled in a precise and repeatable manner. Solution concentration, solvent ratio, QD size distribution, and QD aspect ratio affect the morphology of the cast thin-film structure. Controlling all of these factors allows the creation of colloidal-crystal domains that are square micrometers in size, containing tens of thousands of individual nanocrystals per grain. Such fabrication of large-area, engineered layers of nanoscale materials brings the beneficial properties of inorganic QDs into the realm of nanotechnology. For example, this technique has already enabled significant improvements in the performance of QD light-emitting devices.