Evaporation-Induced Hierarchical Assembly of Rigid Silicon Nanopillars Fabricated by a Scalable Two-Level Colloidal Lithography Approach.

Periodic arrays of silicon nanowires/nanopillars are of great technological importance in developing novel electrical, optical, biosensing, and electromechanical devices. Here we report a novel two-level colloidal lithography technology for making periodic arrays of single-crystalline silicon nanopillars (or nanocolumns) over large areas. Spin-coated monolayer silica colloidal crystals with unusual non-close-packed structures are utilized as first-level etching masks in generating ordered polymer posts whose sizes can be much smaller than the templating silica microspheres. These polymer posts can then be used as second-level structural templates in fabricating highly ordered silicon nanopillars with broadly tunable geometries by employing metal-assisted chemical etching. As the silicon nanopillars are produced by direct wet-etching on the surface of a single-crystalline silicon wafer, they are relatively free of volume defects and thus their bending strength approaches the predicted theoretical maximum. Most importantly, the unique non-close-packed structure of the original colloidal template and the close-to-ideal mechanical property enable the formation of unusual open-structured hierarchical assemblies of rigid silicon nanopillars during water evaporation. Both experiments and numerical finite-difference time-domain modeling confirm the importance of high aspect ratios of the templated silicon nanopillars in achieving superior broadband antireflection properties. The large fraction of entrapped air in the hierarchically assembled silicon nanopillars further facilitates to accomplish superhydrophobic surface states, promising for developing self-cleaning antireflection coatings for many important optoelectronic applications.