From colloidal nanoparticles in non-polar solvents to 3D microstructures: a new paradigm in Convective Self-Assembly

Colloidal nanoparticles (NPs) exhibit unique and tunable physical properties, making them highly attractive for advanced applications in sensing, photonics, flexible electronics, and beyond. Harnessing these properties in functional devices often requires assembling NPs into three-dimensional (3D) microstructures that combine substantial thickness to enhance their functional response, micrometer-scale resolution for precise pattern definition, and high selectivity to minimize material loss. Directed assembly techniques aim to meet these stringent requirements but face significant challenges when NPs are dispersed in non-polar solvents, commonly used for their role in synthesis and colloidal stabilization. Indeed, such solvents hinder NP assembly because of their high wettability and volatility. These limitations are particularly pronounced in Convective Self-Assembly (CSA), which performs efficiently with aqueous dispersions but struggles with non-polar media. In this study, we investigate how the properties of non-polar solvents influence CSA on topographically patterned substrates. To overcome these issues, we propose an upgraded CSA method that combines solvent selection, surface functionalization, and controlled evaporation. This approach enables reproducible and precise assembly of NPs into micrometer-scale 3D structures. The method is versatile, compatible with various nanoparticle types and patterns, and allows sequential co-assembly on a single substrate, offering a scalable route for high-resolution nanomanufacturing of complex multifunctional architectures.