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 nonclose-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 nonclose-packed structure of the original colloidal template and the close-to-ideal mechanical property enables 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.

Templated fabrication of metal half-shells for surface-enhanced Raman scattering.

Here we report a scalable colloidal templating approach for producing metal half-shells as efficient surface-enhanced Raman scattering (SERS) substrates. Nonclose-packed monolayer colloidal crystals created by a spin-coating technology are used as structural template to fabricate both water-dispersed half-shells and disordered arrays of half-shells with preferential upright orientation. The sharp edges of the half-shells and the small gaps between adjacent shells can significantly enhance the local electromagnetic field, resulting in high SERS enhancement factor (up to 10(10)) which is nearly 2 orders of magnitude higher than those of periodic substrates produced by other colloidal templating approaches. We have also demonstrated that the thickness of the half-shells determines the surface plasmon resonance and the resulting SERS enhancement. Counterintuitively, the disordered arrays of oriented half-shells show reproducible enhancement with a standard deviation of less than 20%. This new bottom-up approach enables the large-scale production of SERS substrates that are at least 2 orders of magnitude larger in area than those made by other colloidal lithography technologies. The resulting substrates with high and reproducible SERS enhancement are promising for ultrasensitive chemical and biological sensing.

Surface-enhanced Raman scattering on periodic metal nanotips with tunable sharpness.

This paper reports on a scalable bottom-up technology for producing periodic gold nanotips with tunable sharpness as surface-enhanced Raman scattering (SERS) substrates. Inverted silicon pyramidal pits, which are templated from non-close-packed colloidal crystals prepared by a spin-coating technology, are used as structural templates to replicate arrays of polymer nanopyramids with nanoscale sharp tips. The deposition of a thin layer of gold on the polymer nanopyramids leads to the formation of SERS-active substrates with a high enhancement factor (up to 10(8)). The thickness of the deposited metal determines the sharpness of the nanotips and the resulting Raman enhancement factor. Finite-element electromagnetic modeling shows that the nanotips can significantly enhance the local electromagnetic field and the sharpness of nanotips greatly affects the SERS enhancement.

Biomimetic subwavelength antireflective gratings on GaAs.

We have developed a simple and scalable bottom-up approach for fabricating moth-eye antireflective coatings on GaAs substrates. Monolayer, non-close-packed silica colloidal crystals are created on crystalline GaAs wafers by a spin-coating-based single-layer reduction technique. These colloidal monolayers can be used as etching masks during a BCl(3) dry-etch process to generate subwavelength-structured antireflective gratings directly on GaAs substrates. The gratings exhibit excellent broadband antireflective properties, and the specular reflection matches with the theoretical prediction using a rigorous coupled-wave analysis model. These bioinspired antireflection coatings have important technological applications ranging from efficient solar cells to IR detectors.

Templated fabrication of sub-100 nm periodic nanostructures.

Periodic polymer nanoposts and metal nanohole arrays with tunable size have been fabricated by templating from spin-coated two-dimensional non close-packed colloidal crystal-polymer nanocomposites.