Formation and Order Enhancement of Submicrometer and Nanoscale Features in Thin Films

Abstract

The wide applications of two-dimensionally ordered submicrometer features have stimulated the development of cheap and fast fabrication techniques throughout the years. We showed that electrohydrodynamic patterning, which is known to produce ordered pillar arrays on polymer thin film at micrometer scale, could be extended to create submicrometer features as small as 351 ± 78 nm quickly and economically. However, the dielectric breakdown of polymer imposed a fundamental limit on further reducing the feature size to nanoscale. We overcame the limited resolution by bottom-up colloidal assemblies which used nanoparticles between 49.8 ± 8.7 nm and 117.6 ± 6.7 nm as building blocks. By employing flow-coating deposition, we were able to control the deposited amount and achieve large area of uniform colloidal film easily. However, the single crystalline domain of a close-packed monolayer remained limited due to the restricted time for ordering through lateral capillary pressure before the particles were in contact. That motivated the deposition of initially ordered colloidal dispersions, attained through deionizing the solutions to extend electrostatic double layers for long-range repulsion. Although the deposited amount agreed reasonably well with our model adapted for a power-law fluid, the shear-thinning viscosity and final structure both revealed that the initial order was destroyed at high shear. On the other hand, the particle order was partially sustained during low shear depositions, yet the domain size was not particularly extensive. We attributed the limited order to the high compressibility of double layer during the evaporation phase. To improve the order of a colloidal monolayer, we investigated the effect of excess free ligands on 8.0 ± 0.3 nm gold nanoparticles deposited by the Langmuir-Blodgett technique. We showed that the excess ligands, oleylamine, not only improved the order of particles that were deposited together with them, but could also anneal a poorly ordered two-dimensional array to form an ordered monolayer containing around 20000 particles.