Evaporation induced self-assembly of ordered structures from a capillary-held solution
The use of spontaneous self-assembly as a lithography and external fields-free means to construct well-ordered, often intriguing structures has received much attention as a result of the ease of producing complex structures with small feature sizes. Self-assembly via irreversible solvent evaporation of a droplet containing nonvolatile solutes (polymers, nanoparticles, and colloids) represents one such case. However, the flow instabilities within the evaporating droplet often result in irregular dissipative structures (e.g., convection patterns and fingering instabilities). Therefore, fully utilizing evaporation as a simple tool for creating well-ordered structures that have numerous technological applications requires delicate control over several factors, including the evaporative flux, solution concentration, interfacial interaction between the solute and the substrate, etc.;In this study, we developed a simple route to produce highly regular polymeric structures in an easily controllable, cost-effective, and reproducible manner simply by allowing a drop to evaporate in a confined geometry consisting of a sphere on a Si surface (i.e., a sphere-on-Si geometry). The confined geometry provides unique environment for controlling the flow within the evaporating droplet, which, in turn, regulates the structure formation. A variety of polymers, including poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly(ferrocenyldimethylsilane) (PFS), polystyrene (PS), poly(methyl methacrylate) (PMMA), and polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), are selected as nonvolatile solutes. A number of parameters are found to effectively mediate the structure formation, including the solution concentration, the interfacial interaction between the solute and the substrate, curvature and molecular effect. This simple, lithography-free route allows subsequent preparation of various metal, metal oxide, and carbon nanotube patterns with controlled spacing, size, and thickness.