Polymer Self-Assembly and High Resolution Structural Control in Supercritical Fluids

A myriad of technologies rely on the formation of structures at the smallest length scales. Traditionally, this is accomplished by photolithography, whereby structures are formed by controlling the removal of material from a monolithic substrate or by controlling the location of material deposition. This process is robust, scalable, but can only be used for relatively simple inorganic materials, for example elemental silicon, gold, or compounds like GaAs and AlGaAs. Contrast this with solution-phase self-assembly which relies on intermolecular interactions and entropy to create structures of arbitrary complexity, as exemplified by living organisms. Solution-phase self-assembly has had the benefit of billions of years of evolution to overcome the challenges imposed by entropy and the relative weakness of intermolecular forces. My group has been developing self-assembly in supercritical fluids as an additive manufacturing technique that can leverage the benefits of solution phase self assembly and photolithography to produce additively manufactured structures at virtually any length-scale. Supercritical fluids can act as solvents whose properties can be tuned between liquid-like and gas-like by varying the temperature and pressure of the system in the vicinity of the critical point. When a polymer solution is heated, its saturation solubility typically increases. However, if the solvent is pressurized above its critical point, the saturation solubility will reach a maximum and decrease with further increases in temperature. We have demonstrated that this maximum in the isobaric saturation solubility can be exploited to deposit thin films of polymer by heating the substrate beyond the solubility maximum. Vitally, the location of material deposition can be controlled by controlling the local temperature of the substrate, allowing us to create patterns with linewidths less than 2 microns. This is accomplished by patterning a substrate via photolithography, allowing us to couple solution-phase self-assembly with photolithography in a manner that allows us to obtain the benefits of both techniques. We also demonstrate that deposition can be carried out on surfaces of nearly arbitrary curvature, a key advantage of additive manufacturing. The technique allows control over the nanomorphology of the deposited material demonstrating the potential of the technique to control materials at all length scales.