Stepwise colloidal lithography toward scalable and various planar chiral metamaterials

Chiral metamaterials(CMs)composed by artificial chiral resonators have attracted great attentions in the recent decades due to their strong chiroptical resonance and identifiable interaction with chiral materials,facilitating practical applications in chiral biosensing,chiral emission,and display technology.However,the complex geometry of CMs improves the fabrication difficulty and hinders their scalable fabrication for practical applications,especially in the visible and ultraviolet wavelengths.One potential strategy is the colloidal lithography that enables parallel fabrication for scalable and various planar structures.Here,we demonstrate a stepwise colloidal lithography technique that uses sequential deposition from multiple CMs and expand their variety and complexity.The geometry and optical chirality of building blocks from single deposition are systematically investigated,and their combination enables a significant extension of the range of chiral patterns by multiple-step depositions.This approach resulted in a myriad of complex designs with different characteristic sizes,compositions,and shapes,which are particularly beneficial for the development of nanophotonic materials.In addition,we designed a flexible chiral device based on PDMS,which exhibits a good CD value and excellent stability even after multiple inward and outward bendings.The excellent compatibility to various substrates makes the planar CMs more flexible in practical applications in microfluidic biosensing.

Tuning a nano-pillar array for enhancing the photoluminescence extraction efficiency of GaN-based light-emitting diodes

We demonstrate the fabrication of hexagonal nano-pillar arrays at the surface of GaN-based light-emitting diodes （LEDs） by nanosphere lithography. By varying the oxygen plasma etching time, we could tune the size and shape of the pillar. The nano-pillar has a truncated cone shape. The nano-pillar array serves as a gradual effective refractive index matcher, which reduces the reflection and increases light cone. It is found that the patterned surface absorbs more pumping light. To compare extraction efficiencies of LEDs, it is necessary to normalize the photoluminescence power spectrum with total absorption rate under fixed pumping power, then we could obtain the correct enhancement factor of the photoluminescence extraction efficiency and optimized structure. The highest enhancement factor of the extraction efficiency is 10.6.

Polymer-assisted fabrication of gold nanoring arrays

In this paper, we report a new strategy for the fabrication of gold nanoring arrays via colloidal lithography and polymer-assisted self-assembly of gold nanoparticles （Au NPs）. First, multi-segmented polymer nanorod arrays were fabricated via colloidal lithography. They were then used as templates for Au NP adsorption, which resulted in nanoparticles on the poly（4-vinyl pyridine） （P4VP） segments. Continuous gold nanorings were formed after electroless deposition of gold. The diameter, quantity, and spacing of the gold nanorings could be tuned. Three dimensional coaxial gold nanorings with varying diameters could be fabricated on a polymer nanorod by modifying the etch parameters. The nanorings exhibited optical plasmonic resonances at theoretically predicted wavelengths. In addition, the polymer-assisted gold nanorings were released from the substrate to generate a high yield of flee-standing nanorings. This simple, versatile method was also used to prepare nanorings from other metals such as palladium.

Au nanoring arrays with tunable morphological features and plasmonic resonances

Gold nanoring arrays are widely applied in various fields benefitting from their localized surface plasmon resonance(LSPR)properties.A key advantage of gold nanoring arrays is that the dipole resonance peak can be systematically tuned by changing the dimensions of gold nanoring arrays.However,most of the currently reported methods for preparing gold nanoring arrays cannot conveniently control the heights of the nanorings at a low cost.Here we introduce a facile method for preparing gold nanoring arrays with tunable plasmonic resonances using colloidal lithography.The dimensions of the nanorings including diameters,lattice constants,even the heights of the nanorings can be conveniently varied.Fourier transform near-infrared(FT-NIR)absorption spectroscopy was used to obtain the plasmonic resonance spectra of the nanoring arrays.All the prepared gold nanoring arrays exhibited a strong NIR or infrared(IR)plasmonic resonance which can be tuned by varying the nanoring dimensions.This versatile method can also be used to fabricate other types of plasmonic nanostructures,such as gold nanocone arrays.The obtained gold nanoring arrays as well as nanocone arrays may have potential applications in surface-enhanced spectroscopy or plasmonic sensing.

Self-generating nanogaps for highly effective surface-enhanced Raman spectroscopy

The fabrication of surface enhanced Raman spectroscopy(SERS)substrates with controlled high density hot spots still remains challenging.Herein,we report highly effective SERS substrates containing the self-generating(SG)nanogaps from polystyrene nanosphere monolayer through isotropic plasma etching.The emergence of multimode hot spots,i.e.,metal film over nanosphere(MFON)-like hot spots(closed gaps,0 nm),individual self-aligned hot spots(discrete gaps,>20 nm)and threedimensional(3D)hot spots(nanogaps,1-10 nm),makes the SG SERS substrates superior as compared to the traditional MFON or the well-ordered self-aligned SERS substrates in terms of enhancement,uniformity,and reproducibility.The SG SERS substrates can function as the excellent SERS platforms for trace molecule detection in the practical application fields.

Highly sensitive deep-silver-nanowell arrays （d-AgNWAs） for refractometric sensing

Large-area deep-silver-nanowell arrays （d-AgNWAs） for plasmonic sensing were manufactured by combining colloidal lithography with metal deposition. In contrast to most previous studies, we shed light on the outstanding sensitivity afforded by deep metallic nanowells （up to 400 nm in depth）. Using gold nanohole arrays as a mask, a silicon substrate was etched into deep silicon nanowells, which acted as a template for subsequent Ag deposition, resulting in the formation of d-AgNWAs. Various geometric parameters were separately tailored to study the changes in the optical performance and further optimize the sensing ability of the structure. After several rounds of selection, the best sensing d-AgNWA, which had a Ag thickness of 400 nm, template depth of 400 nm, hole diameter of 504 nm, and period of 1 ~m, was selected. It had a sensitivity of 933 nm.RIU-1, which is substantially higher than those of most common thin metallic nanohole arrays. As a proof of concept, the as-prepared structure was employed as a substrate for an antigen-antibody recognition immunoassay, which indicates its great potential for label-free real-time biosensing.