Here we report a simple self-masking technique for fabricating bioinspired broadband antireflection coatings on both single-crystalline and multicrystalline silicon wafers with the assistance of a polyimide tape. Subwavelength-structured moth-eye nanopillars, which exhibit superior antireflection performance over a broad range of visible and near-IR wavelengths, can be patterned uniformly on the wafer surface by applying a chlorine-based reactive ion etching (RIE) process. The resulting random nanopillars show improved antireflection properties compared with ordered nanopillars templated by colloidal lithography under the same RIE conditions. X-ray photoelectron spectroscopy analysis suggests that energetic bombardment by reactive ions and radicals during RIE sputters the polyimide tape and spontaneously forms nanomasks over the wafer.

The diffusion of uncured polydimethylsiloxane (PDMS) oligomers out of bulk PDMS elastomers is usually detrimental to many biomedical and microfluidic applications due to the inevitable contamination of the contacting fluids and substrates. Here, we transform this detrimental process into an enabling technology for achieving novel reconfigurable antireflection (AR) coatings, which are of great technological importance in the development of new nano-optical and optoelectronic applications. Self-assembled monolayer silica colloidal crystals are first used as sacrificial templates in fabricating nanoporous polymer AR coatings. When air in the templated nanopores is replaced with infused PDMS oligomers simply by pressing a PDMS stamp on a nanoporous AR film, the original antireflection conditions are lost, and the coating transforms from a low-reflection configuration to a high-reflection state. The original antireflection performance can be fully recovered by dissolving the infused oligomers in the appropriate solvents (e.g., hexane). This novel tuning mechanism for achieving reconfigurable AR properties has been confirmed by systematic investigations using various microscopes, optical spectroscopy, nanoindentation, thermomechanical tests, and X-ray photoelectron spectroscopy. Complex micropatterns with micrometer-scale spatial resolution and drastically different AR performances can be easily printed on nanoporous AR films by using a soft lithography-based microcontact printing process. Numerical finite-difference time-domain simulations match well with experimental antireflection measurements and reveal a linear relationship between the optical transmission and the amount of infused PDMS oligomers in nanopores.

In situ monitoring of small molecule diffusion at solid-solid interfaces is challenging, even with sophisticated equipment. Here, novel chromogenic photonic crystal detectors enabled by integrating bioinspired structural color with stimuli-responsive shape memory polymer (SMP) for detecting trace amounts of small molecule interfacial diffusion are reported. Colorless macroporous SMP membranes with deformed macropores can recover back to the "memorized" photonic crystal microstructures and the corresponding iridescent structural colors when triggered by diffused small molecules. Systematic experimental and theoretical investigations using various microscopes, optical spectroscopy and modeling, spatio-resolved energy-dispersive X-ray spectroscopy, and theoretical diffusion calculations confirm the diffusion-induced shape memory and chromogenic mechanisms. Importantly, proof-of-concept sensing of temporospatial-resolved diffusion of bioactive ingredients used in drug delivery, including anti-inflammatory methyl salicylate in pain relieving patches and vitamin E barriers loaded in contact lens, and phthalates plasticizers in commercial PVC products has been demonstrated. These innovative detectors are inexpensive, reusable, and easy to operate and deploy for both qualitative and quantitative analyses, promising for opening new avenues in biomedical research, threat detection, and monitoring of plastics, food, and environmental safety. Moreover, reconfigurable photonic crystals with micrometer-scale resolution, which are of great importance in tunable and integrated nanooptics, can be fabricated by diffusion-enabled microcontact printing.

Stimuli‐responsive photonic crystals with patterned microstructures are of great interest in developing reconfigurable nano‐optical devices. Leveraging unconventional all‐room‐temperature shape memory efforts and spatially resolved photopolymerization, herein, a facile method for micropatterning stimuli‐responsive photonic crystals is reported. Macroporous shape memory polymer (SMP) photonic crystals fabricated by colloidal templating can be deformed by cold programming, triggering the disappearance of their original structural colors. Exposure of the deformed samples to UV light through a photomask selectively disables the shape memory capabilities in the UV‐exposed regions. Hidden micropatterns defined by the photomask can be revealed by exposing the colorless SMP films to ethanol vapor, which triggers the shape memory recovery of the “memorized” ordered microstructures and the corresponding structural colors. Extensive nanoindentation experiments indicate that the exposure to UV light increases the crosslinking density and enhances the elastic modulus and toughness of the exposed regions by a factor of ≈2.0 and ≈5.2, respectively. Due to the formation of these extra crosslinks in the deformed configuration, they prevent normal shape memory behavior where the strained polymer chains rearrange from the temporary to permanent configuration when triggered by an external stimulus. This simple micropatterning technology can enable multistimuli‐responsive reconfigurable nanophotonic devices and chromogenic anticounterfeiting labels.

Antireflection (AR) coatings are widely used in reducing unwanted light reflection loss from optical surfaces. They are critical components for many important technological applications, such as improving conversion efficiencies of photovoltaic cells, enhancing light extraction efficiencies of light emitting diodes, and reducing glare from lenses. Traditional dielectric AR coatings rely on expensive top-down vacuum deposition technologies, which greatly impede the manufacturing cost and production throughput. Bottom-up colloidal assembly provides a much simpler, faster, and cheaper alternative to top-down approaches in fabricating high-quality AR coatings on a large variety of optical materials. In this book chapter, we will first review on the state-of-the-art of various AR coating manufacturing technologies, and then focus on three colloidal assembly platforms developed in our lab for scalably fabricating AR coatings on different optical substrates ranging from glass and plastics to crystalline silicon and GaAs. Langmuir-Blodgett assembly and electrostatics-assisted self-assembly of silica nanoparticles render simple and parallel approaches for making narrowband quarter-wavelength AR coatings on both planar and nonplanar optical surfaces. Spin coating of silica nanoparticles dispersed in a nonvolatile monomer enables wafer-scale production of colloidal crystals with unusual non-close-packed crystalline structures. The self-assembled nanoparticles can be used as structural templates in making bioinspired broadband moth-eye AR coatings, which can effectively suppress light reflection over a brand range of wavelengths and incident angles.

It is essential to secure the optical performance (sensitivity, FOM, plasmonic absorption strength, etc.), large-area fabrication, and physical durability to improve the usability of nanostructured SPR sensors. In this study, to ensure the physical durability of an Au-covered silica sphere monolayer (Au film over nanosphere (AuFON)) manufactured for use as an SPR sensor platform, the sensing performance and physical durability after TMOS treatment to bind spherical silica particles were investigated. The peel-off test showed that Au and silica constituting the AuFON did not fall off, and there was no significant difference in the sensing sensitivity and plasmonic absorption intensity before and after TMOS treatment. In addition, when colloidal Au nanoparticles (diameter of 5 nm) were applied to as-prepared AuFON and heat-treated, it was confirmed that the Au shells of the plasmonic particles were interconnected, and the Fano intensity increased from 3–5% to 22%.

The figure of merit (FOM, 1/RIU) 112.34 is achieved in the Au-covered epoxy grating (Au/Epoxy) with the inverse structure of the original track pitch DVD-R. The feasible exfoliation of the cured epoxy resin from the Au-covered polycarbonate (PC) gratings (Au/PC) of DVD-R grating creates a narrow deep channel structure. As well as Au/PC, newly created Au/Epoxy gratings glow beautifully with rainbow-colored stripes on the surface due to the constructive interference of visible light. The SEM and 3D-AFM images show that the surface distortion of the Au/Epoxy grating is -0.58 (negative) and that of the Au/PC grating is 0.14 (positive). The Au/Epoxy grating exhibits 872.01 nm/RIU with slightly lower sensitivity than the Au/PC grating (887.59 nm/RIU). However, the Au/Epoxy grating shows a total of 7.76 +/- 0.53 nm with up to full width half maximum (FWHM) for the SPR dip during the detection test, so a high FOM of 112.34 can be achieved. In addition, the FDTD simulation shows the experimental results as well as the relevant results of the SPR deep wavelength and FWHM. Finally, the Au/Epoxy grating shows a more pronounced resonance for methanol concentration measurements than the Au/PC grating due to the high FOM and strong adhesion between the Au layer and the epoxy substrate. (c) 2022 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

The colloidal Langmuir-Blodgett coating process is used to fabricate Au-covered silica sphere monolayer (Au film over silica nanosphere (AuFON)) and study the effects of silica diameter on surface plasmon resonance (SPR) sensing sensitivity. The resulting hexagonal close-packed (HCP) monolayers are prepared with silica sphere diameters of 200, 400, 700, and 1000 nm. In SPR sensing applications, the optical properties of Au-covered silica sphere monolayer are evaluated by measuring normal-incidence reflection spectra and sensing tests. The high sensitivity (nm/RIU) is observed in silica sphere diameter (1000 > 700 > 400 > 200 nm) and plasmon mode (dipole > Fano resonance (FR) > and high order) while the highest sensitivity is 968 nm/RIU (dipole mode, 1000 nm of silica sphere diameter). 3-D Finite Difference Time Domain (FDTD) simulation shows a sensitivity trend similar to the experimental results. (c) 2021 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

In order to apply the surface plasmon resonance (SPR) sensing to the study of heterogeneous catalytic reactions, in addition to the metals and dielectrics that the SPR platform already has, a layer or particle that can additionally act as a catalyst is required. The Au layer (50 nm thick) was formed by sputter deposition on the polycarbonate (PC) grating surface of the original DVD-R structure, and the Pt layer was additionally formed by electrochemical deposition. As-prepared Au-covered PC grating exhibits the sensing sensitivity of 973.95 nm/RIU and a figure of merit (FOM) of 69.17. Even after electrochemical Pt deposition, it still showed high sensing sensitivity of 956.57 nm/RIU and FOM of 55.96. In particular, the methanol oxidation and CO coverage on the Pt surface, which are one of the major reactions of the polymer electrolyte membrane fuel cells (PEMFCs), were monitored using Pt deposited SPR sensor. As a result, as the applied potential for methanol oxidation increased and the CO coverage increased, the SPR sensor showed a blue-shift trend. This is probably due to the process of dehydrogenation of methanol (CH3OH) molecules to CO and CO2, and the replacing process the water on the Pt surface with CO.

The progressively improved sensing sensitivity (Delta lambda(SPR)/Delta n, nm/RIU) to detect the refractive index is observed on the SPR platform of an Au-covered epoxy gratings in an increase in potential cycling in a typical three-electrode cell. Here, a DVD-R optical disc was used as a structure template to prepare an Au-covered epoxy gratings, and the newly formed reverse track pitch structure on the epoxy substrate was used as a working electrode directly in aqueous sulfuric acid solution. It is expected that Au reconstruction by potential cycling in sulfuric acid electrolyte increases the packing density of Au atoms in the grain boundary and improves the propagation of electromagnetic waves.

Here, a facile bottom‐up nanofabrication technology for making smart superhydrophobic coatings with switchable wettability and tunable optical properties by integrating shape memory polymers (SMPs) with templated macroporous photonic crystals is reported. Thermoresponsive shape memory efforts exhibited by a polyurethane‐based shape memory copolymer enable reversible microstructural transitions between a “memorized” permanent configuration composing of highly ordered arrays of macropores and a temporary deformed structure with collapsed macropores. The unique macroporous SMP photonic crystal arrays with large surface roughness can entrap a large portion of air in the interconnecting macropores, resulting in superior superhydrophobic properties including high apparent water contact angle (CA > 160°) and low CA hysteresis. Importantly, the shape memory‐enabled microstructural transitions lead to drastic wettability switching from superhydrophobic (CA > 160°) to hydrophobic (CA ≈ 110°) or from oleophobic (CA ≈139° for hexadecane) to oleophilic (CA ≈ 80°). The large CA tunability (>50°) coupled with the simultaneous color change from shining iridescence to colorless associated with the same microstructural transition can not only provide a noninvasive means for visually indicating the surface wetting states, this favorable coupling between the wetting and optical behaviors of macroporous SMP photonic crystal membranes can also pave the way for developing new tunable nanooptical devices.

An enhanced finite-difference time-domain (FDTD) algorithm is built to solve the transverse electric two-dimensional Maxwell's equations with inhomogeneous dielectric media where the electric fields are discontinuous across the dielectric interface. The new algorithm is derived based upon the integral version of the Maxwell's equations as well as the relationship between the electric fields across the interface. To resolve the instability issue of Yee's scheme (staircasing) caused by discontinuous permittivity across the interface, our algorithm revises the permittivities and makes some corrections to the scheme for the cells around the interface. It is also an improvement over the contour-path effective permittivity algorithm by including some extra terms in the formulas. The scheme is validated in solving the scattering of a dielectric cylinder with exact solution from Mie theory and is then compared with the above contour-path method, the usual staircasing and the volume-average method. The numerical results demonstrate that the new algorithm has achieved significant improvement in accuracy over other methods. Furthermore, the algorithm has a simple structure and can be merged into current FDTD software packages easily. The C++ source code for this paper is provided as supporting information for public access.

The perpetual health and safety concerns caused by bacterial adhesion on surfaces demand the development of next-generation antibacterial materials. Inspired by bactericidal surfaces on cicada wings with protrusive nanostructures, which induce mechanical rupture of adhered bacterial membranes without antibacterial, chemical treatments, analogous structures have been fabricated on various synthetic materials to achieve such mechanical bactericidal efficacy. Herein, we developed a series of protrusive nanopillars on hard silicon (Si) substrates and soft poly(ethylene glycol) diacrylate (PEGDA) thin films by colloidal lithography. We first investigated the correlation of interpillar distance and bactericidal efficacy against a model Gram-negative bacterium, Escherichia coli, using Si surfaces with different nanopillar number densities. We demonstrated that the bactericidal efficacy increased with decreasing nanopillar number density, which occurred when the average interpillar distance was smaller than the cell size. The bactericidal efficacy decreases when the average interpillar distance becomes larger than the comparable size of bacteria. We then designed PEGDA thin films with optimized bactericidal nanopillar density to improve their antibacterial and antireflective performance. The results indicate that the surface nanostructure plays a critical role in dictating antibacterial performance, regardless of the material type. This work provides insight into the understanding of physical interactions between nanostructured surfaces and bacterial cells as well as practical solutions for the development of antibacterial polymer surfaces for the application of optical lenses or screen displays to prevent public pathogenic infections.

Intelligent control of friction and adhesion has attracted much attention for use in soft robotics, human-sensor interfaces, and bionics. Here we introduce a shape memory photonic crystal (SMPC) polymer that can be programmed and recovered by solvent to realize switchable surface friction. Micro sliding test show that the friction coefficient on this SMPC in the programmed and recovered state can vary by three times. We also show that the mechanism behind this switchable friction coefficient is the surface roughness related adhesion.

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.

Here, we report a new type of reconfigurable anticounterfeiting coating enabled by integrating the scientific principles of photonic crystal and shape memory polymer (SMP). The autonomous infusion of uncured oligomers in a polydimethylsiloxane (PDMS) stamp into a templated macroporous SMP photonic crystal coating, which was confirmed by quantitative X-ray photoelectron spectroscopy analysis, can program an iridescent pattern on the transparent SMP membrane with deformed macropores. By manipulation of the unconventional all-room-temperature shape memory effects exhibited by the shape memory copolymer comprising polyethylene glycol diacrylate and ethoxylated trimethylolpropane triacrylate, the iridescent pattern can be easily and instantaneously concealed and revealed by immersing in common household liquids (e.g., ethanol and water). Systematic experiments and theoretical simulations using scanning electron microscopy, atomic force microscopy, optical spectroscopy, scalar wave approximation modeling, and contact angle measurements reveal the major mechanism underlying the anticounterfeiting concealing and revealing processes: the compressive deformation of periodic macropores induced by capillary pressure created by solvent evaporation. Apparent water contact angle measurements show that the infusion of hydrophobic PDMS oligomers into hydrophilic macroporous SMP coatings leads to a large increase in water contact angle from similar to 35 degrees to, similar to 70 degrees, which significantly changes the capillary pressure and the final configuration of the SMP photonic crystals. In addition to rendering a facile anticounterfeiting mechanism, the novel oligomer-infusion-induced chromogenic effects and modification of surface wettability might lead to important applications in developing new chromogenic sensors for noninvasively monitoring molecular diffusion at solid-solid interfaces and durable superhydrophobic and/or superomniphobic coatings.

Here we report a simple and scalable electrostatics-assisted colloidal self-assembly technology for fabricating monolayer nanoparticle antireflection coatings on geometrically complex optical surfaces. By using a surface-modified glass volumetric flask with a long neck as a proof-of-concept demonstration, negatively charged silica nanopartides with 110 nm diameter are electrostatically adsorbed on both the interior and exterior surfaces of the flask possessing positive surface charges. The self-assembled monolayer nanoparticle antireflection coatings can significantly improve light transmission through different regions of the flask with varied curvatures, as revealed by optical transmission measurements and numerical simulations using a simplified thin-film multilayer model. (C) 2018 Optical Society of America

Here novel chromogenic photonic crystal sensors based on smart shape memory polymers (SMPs) comprising polyester/polyether-based urethane acrylates blended with tripropylene glycol diacrylate are reported, which exhibit nontraditional all-room-temperature shape memory (SM) effects. Stepwise recovery of the collapsed macropores with 350 nm diameter created by a "cold" programming process leads to easily perceived color changes that can be correlated with the concentrations of swelling analytes in complex, multicomponent nonswelling mixtures. High sensitivity (as low as 10 ppm) and unprecedented measurement range (from 10 ppm to 30 vol%) for analyzing ethanol in octane and gasoline have been demonstrated by leveraging colorimetric sensing in both liquid and gas phases. Proof-of-concept tests for specifically detecting ethanol in consumer medical and healthcare products have also been demonstrated. These sensors are inexpensive, reusable, durable, and readily deployable with mobile platforms for quantitative analysis. Additionally, theoretical modeling of solvent diffusion in macroporous SMPs provides fundamental insights into the mechanisms of nanoscopic SM recovery, which is a topic that has received little examination. These novel sensors are of great technological importance in a wide spectrum of applications ranging from environmental monitoring and workplace hazard identification to threat detection and process/product control in chemical, petroleum, and pharmaceutical industries.

Shape memory photonic crystals may hold the key to the continuing development of smart optical coatings and next-generation all optical integrated circuits. The reconfigurability of these materials in response to various external stimuli is not only aesthetically appealing but also fundamentally important in guiding the design of emerging reconfigurable nanophotonic devices. Here we report a new type of polymer shape memory photonic crystal (PSMPC) that shows autonomous Laplace pressure-driven, elastic modulus-dependent microstructural programming and solvent-swelling-triggered shape memory recovery, all occurring at room temperature. By varying the compositions of their constituent polymers, the elastic moduli of the PSMPCs can be systematically modulated, leading to different photonic bandgaps (i.e., diffractive colors) in response to different solvents, such as water, ethanol, and acetonitrile. The different diffractive colors represent varied strains stored in the semideformed nanoporous PSMPCs. A new physical competing relationship, denoted by a dimensionless parameter, mu between the elastic modulus of polymer and the surface tension of solvent, was established to characterize the range within which the nanoporous photonic crystal structure can stay in the semideformed stable state upon the application of solvents with different surface tensions. Good overlapping in the range of mu was observed from PSMPCs deformed by water, ethanol, and acetonitrile, which verified the applicability of this competing relationship in predicting the "cold" programming behaviors of the nanoporous PSMPCs. This fundamental study will pave the way for the rational design of nanoporous PSMPCs with optimized mechanochromic properties that can be used in a broad spectrum of tunable nano-optical applications.