Emma Vargo1,Ting Xu1
UC Berkeley1
Blends of nanoparticles, polymers, and small molecules can self-assemble into optical, magnetic, and electronic devices with structure-dependent properties. Across this range of applications, a nanocomposite blend’s performance is a function of both its microscopically-averaged structure and its individual nanoscopic defects. Previously, top-down approaches have been employed to control nanocomposite structures and defect densities, including lithographically-patterned templates, external fields, and/or lengthy annealing processes. The strength and beauty of self-assembly, however, comes from its bottom-up nature. We hypothesized that nanocomposite systems could achieve precise structures and extremely low defect densities through self-regulation alone. Guided by this principle, we successfully fabricated nanocomposite multilayer films with large periodicities (172 nm) and the lowest defect densities reported to date (< 0.06 stacking defects / µm<sup>2</sup>). To produce each film, a solution of diblock copolymers, organic small molecules, and nanoparticles was drop-casted and left to dry. Successful self-assembly was achieved across a range of substrates (silicon, glass, polyester), thicknesses (1 µm films to bulk solids), and drying times (30 minutes to multiple days). The films’ excellent order does not come from optimized processing conditions, but from self-regulation across increasing distances during the assembly process. Systematic neutron and x-ray scattering experiments suggest that self-assembly occurs in two stages. First, chain entanglement drives the growth of high-aspect-ratio sheets from the polymer, nanoparticle, and small molecule building blocks. After the sheets form, they proceed to self-assemble as colloidal platelets, neatly stacking to maximize entropy. The self-regulation demonstrated by the nanocomposite spans six orders of magnitude in size, from the molecular building blocks to the millimeter-sized grains of nearly-perfect layers. The combination of low defect densities and large microdomains make the nanocomposite an efficient dielectric coating material. The nanocomposite blend can be redissolved and recast without loss of structure, providing a readily-recycled alternative to existing laminated composites. Overall, this work demonstrates how microscopic and macroscopic features contribute to the performance of a technologically-useful nanocomposite, and proposes self-regulation as a means of achieving order at—and beyond—the nanoscale.