Toward Improving the Overall Uniformity in Langmuir Films of Nanoparticles by Controlling the Initial Deposition Parameters

This work defines how the deposition method, deposition volume, dispersion density, and film expansion mode influence and therefore may be controlled to improve uniformity in Langmuir films of nanoparticles. We used fluorescence imaging of quantum dots with oleic acid ligands. We first establish why future work with Langmuir films of nanoparticles should avoid using a drop method of deposition, whereby the dispersion falls onto the subphase. The preferred method should be a touch method, whereby a meniscus of the dispersion touches the surface of the subphase, and either is allowed to be captured by or is pushed onto the subphase. For free expansion of films, deposition volume defines the final spread area of films, whereas dispersion mass loading (volume times dispersion density) establishes the average overall area number density of nanoparticles in the final film. We demonstrate that films spreading in free expansion retain a coffee-ring pattern even after they collapse freely to a final, steady-state size. The final films contain various microstructures, including layers in the outer rings as well as clusters, chains, or networks in the inner open region, depending on initial dispersion mass loading. We propose that allowing free expansion of Langmuir films of nanoparticles should not be preferred to achieve consistently uniform films over the entire area, even when the final films might be post-processed by compression + expansion isotherm steps. Finally, we explore the effects of setting a physical boundary on the expansion. Constrained expansion is defined as a ratio of constraint area to maximum spread area below unity. We establish that the deposition of Langmuir films of nanoparticles using constrained expansion offers a reproducible and robustly viable method to create spontaneously self-assembled, uniform (monolayer) Langmuir films of nanoparticles, with the uniformity extending over the entire film.