Optimal moth eye nanostructure array on transparent glass towards broadband antireflection

Mechanically-Durable Antireflective Subwavelength Nanoholes on Glass Surfaces Using Lithography-Free Fabrication

Traditional multilayer antireflection (AR) surfaces are of significant importance for numerous applications, such as laser optics, camera lenses, and eyeglasses. Recently, technological advances in the fabrication of biomimetic AR surfaces capable of delivering broadband omnidirectional high transparency combined with self-cleaning properties have opened an alternative route toward realization of multifunctional surfaces which would be beneficial for touchscreen displays or solar harvesting devices. However, achieving the desired surface properties often requires sophisticated lithography fabrication methods consisting of multiple steps. In the present work, we show the design and implementation of mechanically robust AR surfaces fabricated by a lithography-free process using thermally dewetted silver as an etching mask. Both-sided nanohole (NH) surfaces exhibit transmittance above 99% in the visible or the near-infrared ranges combined with improved angular response at an angle of incidence of up to θi = 60°. Additionally, the NHs demonstrate excellent mechanical resilience against repeated abrasion with cheesecloth due to favorable redistribution of the shearing mechanical forces, making them a viable option for touchscreen display applications.

A Novel Approach for Colloidal Lithography: From Dry Particle Assembly to High-Throughput Nanofabrication

We introduce a novel approach for colloidal lithography based on the dry particle assembly into a dense monolayer on an elastomer, followed by mechanical transfer to a substrate of any material and curvature. This method can be implemented either manually or automatically and it produces large area patterns with the quality obtained by the state-of-the-art colloidal lithography at a very high throughput. We first demonstrated the fabrication of nanopatterns with a periodicity ranging between 200 nm and 2 μm. We then demonstrated two nanotechnological applications of this approach. The first one is antireflective structures, fabricated on silicon and sapphire, with different geometries including arrays of bumps and holes and adjusted for different spectral ranges. The second one is smart 3D nanostructures for mechanostimulation of T cells that are used for their effective proliferation, with potential application in cancer immunotherapy. This new approach unleashes the potential of bottom-up nanofabrication and paves the way for nanoscale devices and systems in numerous applications.

Asymmetric Plasmonic Moth-Eye Nanoarrays with Side Opening for Broadband Incident-Angle-Insensitive Antireflection and Absorption

Plasmonic absorbers with broadband angle-insensitive antireflection have attracted intense interests because of its wide applications in optical devices. Hybrid surfaces with multiple different sub-wavelength array units can provide broadened antireflection, while many of these antireflective surfaces only work for specific angles and require high complexity of nanofabrication. Here, a plasmonic asymmetric nanostructure composed of the moth-eye dielectric nanoarray partially modified with the top Ag nanoshell providing a side opening for broadband incident-angle-insensitive antireflection and absorption, is rationally designed by nanoimprinting lithography and oblique angle deposition. This study illustrates that the plasmonic asymmetric nanostructure not only excites strong plasmonic resonance, but also induces more light entry into the dielectric nanocavity and then enhances the internal scattering, leading to optimized light localization. Hence, the asymmetric nanostructure can effectively enhance light confinement at different incident angles and exhibit better antireflection and the corresponding absorption performance than that of symmetric nanostructure over the visible wavelengths, especially suppressing at least 16.4% lower reflectance in the range of 645-800 nm at normal incidence.Moreover, the reflectance variance of asymmetric nanostructure with the incident angle changing from 5° to 60° is much smaller than that of symmetric nanostructure, making our approach relevant for various applications in photocatalysis, photothermal conversion, and so on.

Optical gratings fabricated using the capillary-assisted self-assembly of nanoparticles on a flexible substrate

We demonstrate a cost-effective and high-throughput fabrication technique to deposit colloidal nanoparticles on a patterned polymer substrate using a capillary-assisted self-assembly method over a large area. In particular, we fabricate optical gratings using gold nanoparticles and a polymer substrate. We show the versatility of the technique over different nanoparticle diameters and grating periodicities. Through both experiments and simulations, we show enhanced transmission in the first-order diffraction of the gold-polymer grating as compared to the air-polymer grating. Our fabrication technique also enables the transfer of the nanoparticle pattern from the polymer substrate to any desired surface. Here we demonstrate the transfer of the nanoparticle grating structure to the tip of optical fibers.

Antireflection Structures for VIS and NIR on Arbitrarily Shaped Fused Silica Substrates with Colloidal Polystyrene Nanosphere Lithography

Antireflective (AR) nanostructures offer an effective, broadband alternative to conventional AR coatings that could be used even under extreme conditions. In this publication, a possible fabrication process based on colloidal polystyrene (PS) nanosphere lithography for the fabrication of such AR structures on arbitrarily shaped fused silica substrates is presented and evaluated. Special emphasis is placed on the involved manufacturing steps in order to be able to produce tailored and effective structures. An improved Langmuir-Blodgett self-assembly lithography technique enabled the deposition of 200 nm PS spheres on curved surfaces, independent of shape or material-specific characteristics such as hydrophobicity. The AR structures were fabricated on planar fused silica wafers and aspherical planoconvex lenses. Broadband AR structures with losses (reflection + transmissive scattering) of <1% per surface in the spectral range of 750-2000 nm were produced. At the best performance level, losses were less than 0.5%, which corresponds to an improvement factor of 6.7 compared to unstructured reference substrates.

Biomimicry in nanotechnology: a comprehensive review

Biomimicry has been utilized in many branches of science and engineering to develop devices for enhanced and better performance. The application of nanotechnology has made life easier in modern times. It has offered a way to manipulate matter and systems at the atomic level. As a result, the miniaturization of numerous devices has been possible. Of late, the integration of biomimicry with nanotechnology has shown promising results in the fields of medicine, robotics, sensors, photonics, etc. Biomimicry in nanotechnology has provided eco-friendly and green solutions to the energy problem and in textiles. This is a new research area that needs to be explored more thoroughly. This review illustrates the progress and innovations made in the field of nanotechnology with the integration of biomimicry.

Photonic nanostructures mimicking floral epidermis for perovskite solar cells

Here, we report photonic nanostructures replicated from the adaxial epidermis of flower petals onto light-polymerized coatings using low-cost nanoimprint lithography at ambient temperature. These multifunctional nanocoatings are applied to confer enhanced light trapping, water repellence, and UV light and environmental moisture protection features in perovskite solar cells. The former feature helps attain a maximum power conversion efficiency of 24.61% (21.01% for the reference cell) without any additional device optimization. Added to these merits, the nanocoatings also enable stable operation under AM 1.5G and UV light continuous illumination or in real-world conditions. Our engineering approach provides a simple way to produce multifunctional nanocoatings optimized by nature's wisdom.

Temporally Arrested Breath Figure

Since its original conception as a tool for manufacturing porous materials, the breath figure method (BF) and its variations have been frequently used for the fabrication of numerous micro- and nanopatterned functional surfaces. In classical BF, reliable design of the final pattern has been hindered by the dual role of solvent evaporation to initiate/control the dropwise condensation and induce polymerization, alongside the complex effects of local humidity and temperature influence. Herein, we provide a deterministic method for reliable control of BF pore diameters over a wide range of length scales and environmental conditions. To this end, we employ an adapted methodology that decouples cooling from polymerization by using a combination of initiative cooling and quasi-instantaneous UV curing to deliberately arrest the desired BF patterns in time. Through in situ real-time optical microscopy analysis of the condensation kinetics, we demonstrate that an analytically predictable self-similar regime is the predominant arrangement from early to late times O(10-100 s), when high-density condensation nucleation is initially achieved on the polymer films. In this regime, the temporal growth of condensation droplets follows a unified power law of D ∝ t. Identification and quantitative characterization of the scale-invariant self-similar BF regime allow fabrication of programmed pore size, ranging from hundreds of nanometers to tens of micrometers, at high surface coverage of around 40%. Finally, we show that temporal arresting of BF patterns can be further extended for selective surface patterning and/or pore size modulation by spatially masking the UV curing illumination source. Our findings bridge the gap between fundamental knowledge of dropwise condensation and applied breath figure patterning techniques, thus enabling mechanistic design and fabrication of porous materials and interfaces.

Thin film block copolymer self-assembly for nanophotonics

The nanophotonic engineering of light-matter interactions has profoundly changed research behind the design and fabrication of optical materials and devices. Metasurfaces-arrays of subwavelength nanostructures that interact resonantly with electromagnetic radiation-have emerged as an integral nanophotonic platform for a new generation of ultrathin lenses, displays, polarizers and other devices. Their success hinges on advances in lithography and nanofabrication in recent decades. While existing nanolithography techniques are suitable for basic research and prototyping, issues of cost, throughput, scalability, and substrate compatibility may preclude their use for many metasurface applications. Patterning via spontaneous self-assembly of block copolymer thin films offers an enticing alternative for nanophotonic manufacturing that is rapid, inexpensive, and applicable to large areas and diverse substrates. This review discusses the advantages and disadvantages of block copolymer-based nanopatterning and highlights recent progress in their use for broadband antireflection, surface enhanced Raman spectroscopy, and other nanophotonic applications. Recent advances in diversification of self-assembled block copolymer nanopatterns and improved processes for enhanced scalability of self-assembled nanopatterning using block copolymers are also discussed, with a spotlight on directions for future research that would enable a wider array of nanophotonic applications.

Broadband Enhancement of Anti-reflectivity for a High Angle of Incidence Using Nanocone Geometry

The broadband antireflective (AR) effect for wide incident angles has a significant effect on the photoconversion efficiency of photovoltaics and visibility of large-format display panels. The fabrication of surface nanostructures has continued to attract research interest as an effective way to provide such optical performance. However, the effects of different nanostructure geometries are not fully understood, especially for wide-angle AR effects. In this work, we conduct a systematic analysis of the effect of periodic nanostructures such as nanocones (NCs) and inverted nanocones (INCs) on anti-reflectivity at high angles of incidence (AOIs) in terms of light scattering, guided-mode resonance (GMR), and internal reflections. NCs provide good coupling of light scattering and GMR because of their protruding geometry; hence, reduced reflectance can be obtained in the short wavelength region. Further, NCs exhibit relatively weaker GMR intensities and internal reflections, resulting in low reflectance in the long wavelength region. Therefore, NCs offer a superior broadband AR effect for high AOIs compared with INCs. Based on this analysis, we demonstrate an extremely low average reflectance (5.4%) compared to that of the bare substrate (34.7%) for the entire visible range at an AOI of 75° by fabricating NCs on both sides of the substrate.

Surface nanostructuring of alkali-aluminosilicate Gorilla display glass substrates using a maskless process

This paper reports on the formation of moth-eye nanopillar structures on surfaces of alkali-aluminosilicate Gorilla glass substrates using a self-masking plasma etching method. Surface and cross-section chemical compositions studies were carried out to study the formation of the nanostructures. CF_xinduced polymers were shown to be the self-masking material during plasma etching. The nanostructures enhance transmission at wavelengths over 525 nm may be utilized for fluid-induced switchable haze. Additional functionalities associated with nanostructures may be realized such as self-cleaning, anti-fogging, and stain-resistance.

Analysis of Surface Patterns and Electric Field Simulation of Antireflective Green Lacewing Wings

The structural coloration and decoloration are problems of scientific interest for a long time. Hence, the fundamental investigations on structures and the optical properties of insect wings have been performed. As a part of such studies, we elucidate the optical properties of green lacewing wings via observation and simulation. First, we elucidate the surface pattern of green lacewing wings using a two-dimensional fast Fourier transform. A cross-shaped pattern of a Fourier spectrum is obtained, and the concise wing model with the surface protrusions arranged in a square grid on a base substrate is constructed in reference to the obtained Fourier spectrum. Next, we perform a finite-difference time-domain (FDTD) simulation to elucidate a light path through wings with and without surface protrusions. The FDTD simulation results indicate that the surface protrusions of a wing increase and decrease the intensity of the transmitted and reflected light, respectively, which is an antireflection behavior. This phenomenon was also observed in the case of 45° incident light. The intensity of transmitted light coupled to wings is induced by surface protrusions with a stepwise refractive index between air and a substrate, which induces antireflection. In particular, transmitted light is increased by the surface protrusions of wings in the range of 500-800 nm wavelength. The intensities of transmitted and reflected light are affected by the direction of incident electric field (polarization) in the case of wings with protrusions arranged in the same direction (parallel). Hence, the surface protrusions are arranged in a square grid to reduce the influence of the polarization direction.

Bioinspired nanoscale hierarchical pillars for extreme superhydrophobicity and wide angular transmittance

Hierarchical structures in nature provide unique functions for living organisms that can inspire technology. Nanoscale hierarchical structured surfaces are essential to realize the dual functions of non-wetting and transparency for applications such as cover glasses and windows; however, these structures are challenging to fabricate. In this study, nano-hierarchical structured glass surfaces were fabricated using multi-step colloidal lithography and etching to obtain tunable morphology. Nanostructured surfaces of mono-pillar structures of diameter 120 and 350 nm and hierarchical-pillar structures of their combinations exhibited superhydrophobicity after perfluoropolyether coating. In particular, the hierarchical nanosurfaces showed excellent non-wetting properties with the apparent, advancing, and receding water contact angles exceeding 177° and contact angle hysteresis below 1°. Water bouncing behaviors - contact time, spreading diameter, and shape of the bouncing motion were also evaluated according to the Weber number to examine the robustness of superhydrophobicity. Hierarchical nanosurfaces showed larger spreading diameters than mono-nanosurfaces with 14 bounces, indicating minimal energy loss from friction, as can be explained by the effective slip length. Furthermore, the nano-hierarchical structures exhibited better transmittance for wide angles of incidence up to 70° than mono-nanostructures owing to their reduced scattering area and multi-periodicity.

Performance Enhancement of All-Inorganic Carbon-Based CsPbIBr2 Perovskite Solar Cells Using a Moth-Eye Anti-Reflector

All-inorganic carbon-based CsPbIBr₂ perovskite solar cells (PSCs) have attracted increasing interest due to the low cost and the balance between bandgap and stability. However, the relatively narrow light absorption range (300 to 600 nm) limited the further improvement of short-circuit current density (J_SC) and power conversion efficiency (PCE) of PSCs. Considering the inevitable reflectance loss (~10%) at air/glass interface, we prepared the moth-eye anti-reflector by ultraviolet nanoimprint technology and achieved an average reflectance as low as 5.15%. By attaching the anti-reflector on the glass side of PSCs, the J_SC was promoted by 9.4% from 10.89 mA/cm² to 11.91 mA/cm², which is the highest among PSCs with a structure of glass/FTO/c-TiO₂/CsPbIBr₂/Carbon, and the PCE was enhanced by 9.9% from 9.17% to 10.08%. The results demonstrated that the larger J_SC induced by the optical reflectance modulation of moth-eye anti-reflector was responsible for the improved PCE. Simultaneously, this moth-eye anti-reflector can withstand a high temperature up to 200 °C, and perform efficiently at a wide range of incident angles from 40° to 90° and under various light intensities. This work is helpful to further improve the performance of CsPbIBr₂ PSCs by optical modulation and boost the possible application of wide-range-wavelength anti-reflector in single and multi-junction solar cells.

Light absorption enhancement and radiation hardening for triple junction solar cell through bioinspired nanostructures

Multi-junction solar cells constitute the main source of power for space applications. However, exposure of solar cells to the space radiation environment significantly degrades their performance across the mission lifetime. Here, we seek to improve the radiation hardness of the triple junction solar cell, GaInP/Ga(In)As/Ge, by decreasing the thickness of the more sensitive middle junction. Thin junctions facilitate the collection of minority carriers and show slower degradation due to defects. However, thinning the junction decreases the absorption, and consequently, the expected photocurrent. To compensate for this loss, we examined two bioinspired surface patterns that exhibit anti-reflective and light-trapping properties: (a) the moth-eye structure which enables vision in poorly illuminated environments and (b) the patterns of the hard cell of a unicellular photosynthetic micro-alga, the diatoms. We parametrize and optimize the biomimetic structures, aiming to maximize the absorbed light by the solar cell while achieving significant reduction in the middle junction thickness. The density of the radiation-induced defects is independent of the junction thickness, as we demonstrate using Monte Carlo simulations, allowing the direct comparison of different combinations of middle junction thicknesses and light trapping structures. We incorporate the radiation effects into the solar cell model as a decrease in minority carrier lifetime and an increase in surface recombination velocity, and we quantify the gain in efficiency for different combinations of junction thickness and the light-trapping structure at equal radiation damage. Solar cells with thin junctions compensated by the light-trapping structures offer a promising approach to improve solar cell radiation hardness and robustness, with up to 2% higher end-of-life efficiency than the commonly used configuration at high radiation exposure.

Broadband Anti-Reflection Coatings Fabricated by Precise Time-Controlled and Oblique-Angle Deposition Methods

Broadband anti-reflection (AR) coatings are essential elements for improving the photocurrent generation of photovoltaic modules and enhancing visibility in optical devices. In this paper, we report a hybrid-structured, anti-reflection coating that combines multi-layer thin films with a single top-oblique deposited layer. By simply introducing this low-refractive index layer, the broadband anti-reflection properties of optical thin films can be improved while simplifying the preparation. Precise time-controlled and oblique-angle deposition (OAD) methods were used to fabricate the broadband AR coating. By accurately measuring and adjusting the design errors for the thin and thick film layers, 22-layer and 36-layer AR coatings on a sapphire substrate with a 400–2000 nm wideband were obtained. This bottom-up preparation process and AR coating design have the potential to significantly enhance the broadband antireflective properties for many optical systems and reduce the manufacturing cost of broadband AR coatings.

Glass Flow Evolution and the Mechanism of Antireflective Nanoprotrusion Arrays in Nanoholes by Direct Thermal Imprinting

Moth-eye-mimicking nanoprotrusion arrays are typical bioinspired broadband antireflection patterns that improve the transmittance and visibility of optical devices by adjusting different geometrical parameters of nanostructures, such as diameter, height, shape, and periodic arrangement, and widely used in solar cells, electronic displays, and so on. Rapid, net-shape, less complicated, and low-cost fabrication of the glass-based moth-eye nanostructure array is a huge challenge. This work adopted the nanohole array template to transform the moth-eye nanostructures on the optical glass by hot embossing combined with ultrasonic-assisted demolding. To investigate the mode transition and filling behavior of the glass nanostructures when compressed into the nanoholes, we conducted a series of hot embossing tests with various processing parameters and characterized the geometrical morphology of the glass-based nanostructure array, such as height and shape. In these tests, surface defects such as nanocracks will occur when inappropriate processing parameters were applied and we evaluated the transmittance performance of defective and fine glass nanostructures and surface with no nanostructures to reveal the effect of nanostructures with different levels of quality on antireflection. This work provides an effective and environmental-friendly method for the fabrication of moth-eye nanostructure arrays with an improved antireflection performance.

Fabrication of Oblique Submicron-Scale Structures Using Synchrotron Hard X-ray Lithography

Oblique submicron-scale structures are used in various aspects of research, such as the directional characteristics of dry adhesives and wettability. Although deposition, etching, and lithography techniques are applied to fabricate oblique submicron-scale structures, these approaches have the problem of the controllability or throughput of the structures. Here, we propose a simple X-ray-lithography method, which can control the oblique angle of submicron-scale structures with areas on the centimeter scale. An X-ray mask was fabricated by gold film deposition on slanted structures. Using this mask, oblique ZEP520A photoresist structures with slopes of 20° and 10° and widths of 510 nm and 345 nm were fabricated by oblique X-ray exposure, and the possibility of polydimethylsiloxane (PDMS) molding was also confirmed. In addition, through double exposure with submicron- and micron-scale X-ray masks, dotted-line patterns were produced as an example of multiscale patterning.

Broadband and Tunable Light Harvesting in Nanorippled MoS2 Ultrathin Films

Nanofabrication of flat optic silica gratings conformally layered with two-dimensional (2D) MoS₂ is demonstrated over large area (cm²), achieving a strong amplification of the photon absorption in the active 2D layer. The anisotropic subwavelength silica gratings induce a highly ordered periodic modulation of the MoS₂ layer, promoting the excitation of Guided Mode Anomalies (GMA) at the interfaces of the 2D layer. We show the capability to achieve a broadband tuning of these lattice modes from the visible (VIS) to the near-infrared (NIR) by simply tailoring the illumination conditions and/or the period of the lattice. Remarkably, we demonstrate the possibility to strongly confine resonant and nonresonant light into the 2D MoS₂ layers via GMA excitation, leading to a strong absorption enhancement as high as 240% relative to a flat continuous MoS₂ film. Due to their broadband and tunable photon harvesting capabilities, these large area 2D MoS₂ metastructures represent an ideal scalable platform for new generation devices in nanophotonics, photo- detection and -conversion, and quantum technologies.

All-Nanoparticle Monolayer Broadband Antireflective and Self-Cleaning Transparent Glass Coatings

The vast majority of light-emitting diode and liquid-crystal displays, solar panels, and windows in residential and industrial buildings use glass panels owing to their high mechanical stability, chemical resistance, and optical properties. Glass surfaces reflect about 4-5% of incident light if no antireflective coating is applied. In addition to energy losses in displays, surface reflections diminish picture quality. Engineering of antireflective coatings can be beneficial for all types of glass screens, specifically for large screens and touch-screen devices when scratch-resistance and self-cleaning properties of the glass surface are also desired. A scalable and robust approach to produce antireflective coatings for glass surfaces with desired optical and mechanical properties is introduced in this work. The developed coating mimics the structure of a moth-eye cornea. The coating is a subwavelength-microstructured thin layer on the glass surface made of a monolayer of hemispherical silica nanoparticles obtained by hydrothermal fusion of spherical particles to the glass substrate. The sequence of the particle deposition in the layer-by-layer process is adjusted to balance attractive-repulsive interactions among nanoparticles and between the nanoparticles and the glass surface to generate coatings with a high surface coverage of up to 70%, which exceeds the 54.7% limit of the random sequential addition model. This level of surface coverage allows for a combination of properties beneficial for the described applications: (i) an average reflectance of 0.5 ± 0.2% for a visible and near-infrared optical spectrum, (ii) an improved mechanical stability and scratch resistance, and (iii) non-wetting behavior.

Bioinspired antireflective flexible films with optimized mechanical resistance fabricated by roll to roll thermal nanoimprint

This work describes the fabrication process of moth eye antireflective poly (methyl methacrylate) transparent films via roll to roll thermal nanoimprint lithography. The process parameters are investigated and adjusted in order to obtain from a single moth-eye structured mold, a range of antireflective topographies that gradually vary their geometry from protruding to intruding nanocones. A correlation between the process parameters with the optical and mechanical properties of the films is established to illustrate the influence of the processing parameters and serve as guideline to produce antireflective flexible films with balanced properties and optimized performance adequate to the application environment. A finite element model is described predicting the mechanical behavior of the moth-eye PMMA imprinted nanostructures.

Bilayer PMMA antireflective coatings via microphase separation and MAPLE

A poly(methyl methacrylate) (PMMA) bilayer antireflective coating (ARC) is designed based on polymeric microphase separation and matrix-assisted pulsed laser evaporation (MAPLE). The spin-coated layer shows subwavelength porous network structures, after phase separation via annealing and removal of the polystyrene (PS) phase, while the MAPLE deposited surface layer exhibits a biomimic moth-eye structure on glass to trap the incident light. The elaborate spin coated structure can be controlled flexibly by changing the ratio of mixture, annealing time and temperature, and the moth-eye structure can also be tuned by deposition parameters. The transmittance of the ARC presents a maximum of 95.64% and an average of 94.81% in visible range. The moth-eye structure on glass substrate formed by nanoglobules makes positive contributions to the improvement of transmittance according to UV–Vis result and simulation. The wetting motion of PMMA globules is observed as well by the comparison of AFM surface morphologies and cross-sectional profiles of globules on glass and polymer thin film. This work is a novel attempt to fabricate bilayer ARC with two different structures by a single polymeric material and will provide new route for fabrication of multilayer ARCs.

Challenges and Prospects of Bio-Inspired and Multifunctional Transparent Substrates and Barrier Layers for Optoelectronics

Bio-inspiration and advances in micro/nanomanufacturing processes have enabled the design and fabrication of micro/nanostructures on optoelectronic substrates and barrier layers to create a variety of functionalities. In this review article, we summarize research progress in multifunctional transparent substrates and barrier layers while discussing future challenges and prospects. We discuss different optoelectronic device configurations, sources of bio-inspiration, photon management properties, wetting properties, multifunctionality, functionality durability, and device durability, as well as choice of materials for optoelectronic substrates and barrier layers. These engineered surfaces may be used for various optoelectronic devices such as touch panels, solar modules, displays, and mobile devices in traditional rigid forms as well as emerging flexible versions.

A Comparative Study of Crystallography and Defect Structure of Corneal Nipple Array in Daphnis nerii Moth and Papilio polytes Butterfly Eye

Moth and butterfly ommatidial nanostructures have been extensively studied for their anti-reflective properties. Especially, from the point of view of sub-wavelength anti-reflection phenomena, the moth eye structures are the archetype example. Here, a comparative analysis of corneal nipples in moth eye (both Male and Female) and butterfly eye (both Male and Female) is given. The surface of moth(Male and Female) and butterfly(Male and Female) eye is defined with regularly arranged hexagonal facets filled with corneal nipples. A detailed analysis using high-resolution scanning electron microscopy images show the intricate hexagonal arrangement of corneal nipples within the individual hexagonal facet. Individual nipples in moth are circular with an average diameter of about 140/165 nm (Male/Female) and average internipple separation of 165 nm. The moth eye show the ordered arrangement of the corneal nipples and the butterfly eye (Male/Female) show an even more complex arrangement of the nipples. Structurally, the corneal nipples in both male and female butterflies are not circular but are polygons with 5, 6, and 7 sides. The average center-to-center separation in the butterfly(Male/Female) is about 260 nm/204 nm, respectively. We find that these corneal nipples are organized into much more dense hexagonal packing with the internipple (edge-to-edge) separation ranging from 20 to 25 nm. Each hexagonal facet is divided into multiple grains separated by boundaries spanning one or two crystallographic defects. These defects are seen in both moth and butterfly. These are typical 5-coordinated and 7-coordinated defect sites typical for a solid-state material with the hexagonal atomic arrangement. Even though the isolated defects are a rarity, interwoven (7-5) defects form a grain boundary between perfectly ordered grains. These defects introduce a low-angle dislocation, and a detailed analysis of the defects is done. The butterfly eye (Male/Female) is defined with extremely high-density corneal nipple with no apparent grains. Each corneal nipple is a polygon with "n" sides (n = 5, 6, and 7). While the 5- and 7-coordinated defects exist, they do not initiate a grain rotation as seen in the moth eyes. To find out the similarity and the difference in the reflectivity of these nanostructured surfaces, we used the effective medium theory and calculated the reflectivity in moth and butterfly eyes. From this simple analysis, we find that females have better anti-reflective properties compared to the males in both moth and butterfly.

Moth-Eye Mimicking Solid Slippery Glass Surface with Icephobicity, Transparency, and Self-Healing

Slippery liquid-infused porous surfaces (SLIPSs) have been actively studied to improve the limitations of superhydrophobic (SHP) surfaces, especially the defects of the nonwetting chemical coating layer and the weak mechanical robustness of surface micro/nanostructures. However, the SLIPSs also have several drawbacks including volatilization and leakage of lubricant caused by long-term usage. In this study, we suggest the use of icephobic, highly transparent, and self-healing solid slippery surface to overcome the limitations of both surfaces (SLIPS and SHP) by combining specific biomimetic morphology and intrinsic properties of paraffin wax. A moth-eye mimicking nanopillar structure was prepared instead of a porous structure and was coated with solid paraffin wax for water repellence. Moth-eye structures enable high surface transparency based on antireflective effect, and the paraffin layer can recover from damage due to sunlight exposure. Furthermore, the paraffin coating on the nanopillars provides an air trap, resulting in a low heat transfer rate, increasing freezing time and reducing adhesion strength between the ice droplet and the surface. The heat transfer model was also calculated to elucidate the effects of the nanopillar height and paraffin layer thickness. The antireflection and freezing time of the surfaces are enhanced with increase in nanopillar height. The paraffin layer slightly deteriorates the transmittance but enhances the icephobicity. The solar cell efficiency using a biomimetic solid slippery surface is higher than that of bare glass due to the antireflective effect. This integrated biomimetic solid slippery surface is multifunctional due to its self-cleaning, anti-icing, antireflection, and self-healing properties and may replace SLIPS and SHP surfaces.

Deformation Behavior of Glass Nanostructures in Hot Embossing

In the nanoscale glass formation, the flow and deformation behavior of glass materials are quite different from those in the macroscale because the mold cavity influences the viscous flow behaviors of glass because of the size effect. The knowledge of macroglass molding process no longer applies to the fabrication of glass microparts by hot embossing. To investigate the size effect of the mold cavity on glass flow behavior during squeeze flow, patterned molds with different length scales and shapes were used for glass embossing. The experimental results demonstrated that glass structures with ultrafine and atomic scale surface could be fabricated by using precision embossing. The nanostructures of embossed glass at 100 and 500 nm wide cavity were found to exhibit nanoscale effect during squeeze flow. Molecular confinement accelerates the tectonic deformation of embossed glass at smaller length scales. At the microscale filling, the tectonic deformation of embossed glass is mainly dominated by elastic recovery, surface tension, hydrostatic pressure, and viscous flow. As the length scale reduces to submicron, the dual-peak filling mode gradually transfers to the single-peak filling mode. Additionally, deformation modes have little influence on the shapes of the mold cavity. This work sheds light on the fabrication of glass nano/microstructures.

Broadband transparency with all-dielectric metasurfaces engraved on silicon waveguide facets: effect of inverted and extruded features based on Babinet's principle

Building blocks of photonic integrated circuitry (PIC), optical waveguides, have long been considered transparent. However, the inevitable Fresnel reflection from waveguide facets limits their transparency. This limitation becomes more severe in high-index waveguides in which the transparency may drop to 65%. We overcome this inherent optical property of high-index waveguides by engineering an appropriate facet landscape made of sub-wavelength artificial features unit cells. For this, we develop a semi-analytical formalism for predicting the metasurface parameters made of high-index dielectric materials, to be engraved on the facets of optical waveguides, based on Babinet's principle: either extruded from the waveguide facet or etched into it. Our semi-analytical model predicts the shape of anti-reflective metasurface unit cells to achieve transmission as high as 98.5% in near-infrared from 1 μm to 2 μm. This new class of metasurfaces may be used for the improvement of PIC devices for communication and sensing, where device transparency is crucial for high signal-to-noise ratios.

Anti-Reflective Coating Materials: A Holistic Review from PV Perspective

The solar photovoltaic (PV) cell is a prominent energy harvesting device that reduces the strain in the conventional energy generation approach and endorses the prospectiveness of renewable energy. Thus, the exploration in this ever-green field is worth the effort. From the power conversion efficiency standpoint of view, PVs are consistently improving, and when analyzing the potential areas that can be advanced, more and more exciting challenges are encountered. One such crucial challenge is to increase the photon availability for PV conversion. This challenge is solved using two ways. First, by suppressing the reflection at the interface of the solar cell, and the other way is to enhance the optical pathlength inside the cell for adequate absorption of the photons. Our review addresses this challenge by emphasizing the various strategies that aid in trapping the light in the solar cells. These strategies include the usage of antireflection coatings (ARCs) and light-trapping structures. The primary focus of this study is to review the ARCs from a PV application perspective based on various materials, and it highlights the development of ARCs from more than the past three decades covering the structure, fabrication techniques, optical performance, features, and research potential of ARCs reported. More importantly, various ARCs researched with different classes of PV cells, and their impact on its efficiency is given a special attention. To enhance the optical pathlength, and thus the absorption in solar PV devices, an insight about the advanced light-trapping techniques that deals with the concept of plasmonics, spectral modification, and other prevailing innovative light-trapping structures approaching the Yablonovitch limit is discussed. An extensive collection of information is presented as tables under each core review section. Further, we take a step forward to brief the effects of ageing on ARCs and their influence on the device performance. Finally, we summarize the review of ARCs on the basis of structures, materials, optical performance, multifunctionality, stability, and cost-effectiveness along with a master table comparing the selected high-performance ARCs with perfect AR coatings. Also, from the discussed significant challenges faced by ARCs and future outlook; this work directs the researchers to identify the area of expertise where further research analysis is needed in near future.

Plasma-Polymer-Fluorocarbon Thin Film Coated Nanostructured-Polyethylene Terephthalate Surface with Highly Durable Superhydrophobic and Antireflective Properties

Herein, an antireflection and superhydrophobic film was obtained by uniformly forming nanostructures on the surface of polyethylene terephthalate (PET) substrate using oxygen plasma without a pattern mask and coating plasma-polymer-fluorocarbon (PPFC) on the nanostructured surface by mid-range frequency sputtering. PPFC/nanostructured-PET showed a reflectance of 4.2%, which is 56% lower than that of the PET film. Haze was also improved. Nanostructured-PET exhibited a superhydrophilic surface due to plasma deformation and a superhydrophobic surface could be realized by coating PPFC on the nanostructured surface. The PPFC coating prevented the aging of polymer film nanostructures and showed excellent durability in a high-temperature and high-humidity environment. It exhibited excellent flexibility to maintain the superhydrophobic surface, even at a mechanical bending radius of 1 mm, and could retain its properties even after repeated bending for 10,000 times.

Broadband absorption of nanostructured stainless steel surface fabricated by nanosecond laser irradiation

Highly efficient broadband absorbing surfaces covering the UV, visible and near-IR regions are of great importance for low-light imaging devices, optical devices and optoelectronic devices. In this work, we demonstrate the fabrication of remarkably efficient absorbing surfaces due to the formation of nanoflower-like cavity structures on a stainless steel (SS304) surface, along with micropatterning in a hierarchical fashion. The fabrication process is carried out using noncontact, programmable, single-step laser irradiation by an inexpensive and robust 532 nm nanosecond laser. The measured specular antireflection properties over a wide spectral region (250-1800 nm) are extremely low, less than 0.5%, over a large range of incident angles and for both orthogonal polarizations. These special hierarchical structures with nanorods, nanoparticles, and nanocavities, completely trap the photon incident on these surfaces due to multiple reflections. These surface structures evolve with time to give better nanostructured features with higher oxygen content on the surfaces, revealed by FESEM elemental analysis, which increases the ability to trap photons. We believe these antireflection surfaces, with high efficiencies and long-term stability, will play a vital role in many modern technological applications.

Antireflection in Green Lacewing Wings with Random Height Surface Protrusions

Wings of insects exhibit many functions apart from flying. In particular, their antireflection function is important for insects to avoid detection by their enemies. This function can be applied to antireflection biomimetic films in engineering fields. For such applications, confirming the antireflection mechanisms of insect wings is important. Herein, we used electron microscopy to compare the surfaces of green lacewing wings with and without a surface wax structure and recorded the transmittance spectra to clarify the surface structural and optical properties of insect wings. The spectral transmittance was higher for wings with a surface wax structure than for wings without a wax layer in the light wavelength regime from 500 to 750 nm. We constructed a concise model of the green lacewing wing with flake-like surface structure with a graded effective refractive index corresponding to the wing samples with a surface wax layer; we also constructed a simple thin-film model corresponding to the wing samples without a wax layer. The graded refractive indices were calculated using the effective medium theory, and the transmittance spectra of such models were then calculated using the transfer-matrix method. It was observed that the calculated spectra are in good agreement with the experimental results. In addition, wing samples without a surface structure induce thin-film interference. These results suggest that a wax structure can reduce the reflectance and increase the transmittance enabling the green lacewings to avoid detection by their enemies. These findings may lead to further advances in both the biomimetic field and fundamental research fields.

Nanostructured front electrodes for perovskite/c-Si tandem photovoltaics

The rise in the power conversion efficiency (PCE) of perovskite solar cells has triggered enormous interest in perovskite-based tandem photovoltaics. One key challenge is to achieve high transmission of low energy photons into the bottom cell. Here, nanostructured front electrodes for 4-terminal perovskite/crystalline-silicon (perovskite/c-Si) tandem solar cells are developed by conformal deposition of indium tin oxide (ITO) on self-assembled polystyrene nanopillars. The nanostructured ITO is optimized for reduced reflection and increased transmission with a tradeoff in increased sheet resistance. In the optimum case, the nanostructured ITO electrodes enhance the transmittance by ∼7% (relative) compared to planar references. Perovskite/c-Si tandem devices with nanostructured ITO exhibit enhanced short-circuit current density (2.9 mA/cm² absolute) and PCE (1.7% absolute) in the bottom c-Si solar cell compared to the reference. The improved light in-coupling is more pronounced for elevated angle of incidence. Energy yield enhancement up to ∼10% (relative) is achieved for perovskite/c-Si tandem architecture with the nanostructured ITO electrodes. It is also shown that these nanostructured ITO electrodes are also compatible with various other perovskite-based tandem architectures and bear the potential to improve the PCE up to 27.0%.

Newly Developed Broadband Antireflective Nanostructures by Coating a Low-Index MgF2 Film onto a SiO2 Moth-Eye Nanopattern

A newly developed nanopatterned broadband antireflective (AR) coating was fabricated on the front side of a glass/indium tin oxide/perovskite solar cell (PSC) by depositing a single interference layer onto a two-dimensional (2D)-patterned moth-eye-like nanostructure. The optimized developed AR nanostructure was simulated in a finite-difference time domain analysis. To realize the simulated developed AR nanostructure, we controlled the SiO₂ moth-eye structure with various diameters and heights and a MgF₂ single layer with varying thicknesses by sequentially performing nanosphere lithography, reactive ion etching, and electron-beam evaporation. Optimization of the developed AR nanostructure, which has a 100 nm-thick MgF₂ film coated onto the SiO₂ moth-eye-like nanostructure (diameter 165 nm and height 400 nm), minimizes the reflection loss throughout the visible range. As a result, the short-circuit current density (J_SC) of the newly AR-coated PSC increases by 11.80%, while the open-circuit voltage (V_OC) remains nearly constant. Therefore, the power conversion efficiency of the newly developed AR-decorated PSC increases by 12.50%, from 18.21% for a control sample to 20.48% for the optimum AR-coated sample. These results indicate that the newly developed MgF₂/SiO₂ AR nanostructure can provide an advanced platform technology that reduces the Fresnel loss and therefore increases the possibility of the commercialization of glass-based PSCs.

Bio-inspired broadband absorbers induced by copper nanostructures on natural leaves

In this work, two copper-based biometamaterials were engineered using leaves of water cabbage (Pistia stratiotes) and purple bauhinia (Phanera purpurea) as templates. The copper sputtering was implemented to produce a thin copper film on the surface of leaves. The scanning electron microscopy (SEM) images exhibited the root hair-like nanostructure of water cabbage leaf and single comb-like nanostructure of purple bauhinia leaf. In spite of copper coating, the leaf surfaces of water cabbage and purple bauhinia were black and exhibited excellent light absorption at visible and near infrarrred wavelengths. It was estimated that these two types of leaves could absorb roughly 90% of light. Finite-difference time-domain (FDTD) calculations predicted the low reflectance stemming from the leaf nanostructures and copper coating layer. Because of the low cost of copper as a coating metal and simple procedure, this can be a promising method for quick fabrication of a thin copper film on the leaf nanostructure for application in blackbody or as the light absorbers.

Subwavelength Hollow-Nanopillared Glass with Gradient Refractive Index for Ultralow Diffuse Reflectance and Antifogging

Nanostructured glass with subwavelength hollow nanopillars of diameters of sub-65 nm was fabricated, showing high optical transmittance and ultralow diffuse reflectance. A simple process involving single-step plasma etching was used on a glass slide coated with a SiO₂ sacrificial film. First, SiO₂ nanodot structures were formed using plasma-induced anisotropic etching with CF₄ plasma. The SiO₂ nanodot array then became a secondary etching mask to form hollow nanopillars on the glass. The hollow structures formed at the upper part reaching up to the apex of the nanopillar had a lower solid fraction, while the lower part had a higher fraction. The refractive index (RI) gradually increased from 1.09 (near the value for air) to 1.42 (near the value for glass). Geometry-induced RI gradient enhanced light transmi, while it significantly reduced diffuse reflectance, particularly in the shorter wavelengths, thus suppressing the haziness or milky appearance of the nanostructured glass. Superhydrophilic and antifogging properties of nanostructured glasses and dental mirrored glasses were also demonstrated with water spraying and exhaled breath tests. Results showed that the wettability was enhanced in hydrophilicity and antifogging property by both the hydrophilic nature of the glass and the newly formed nanostructures. The nanostructured, superhydrophilic glass was also found to have easy cleaning nature against fine sand dust adhesion by simply blowing air or spraying water. Results of this study showed that such a hollow-pillared glass surface with gradient RI and special wettability could be applied in a variety of optical and optoelectronic applications requiring superwetting, such as optical windows for solar cell panels, display panels, light-emitting diodes, and medical devices even with curved surfaces.

Optimization of Shapes and Sizes of Moth-Eye-Inspired Structures for the Enhancement of Their Antireflective Properties

Novel antireflective (AR) structures have attracted tremendous attention and been used in various applications such as solar cells, displays, wearable devices, and others. They have also stimulated the development of several other methods, including moth-eye-inspired technologies. However, the analyses of the shapes and sizes of nanostructures remain a critical issue and need to be considered in the design of effective AR surfaces. Herein, moth-eye and inverse-moth-eye patterned polyurethane-acrylate (PUA) structures (MPS and IMPS) with three different sizes are analyzed and compared to optimize the designed nanostructures to achieve the best optical properties pertaining to maximum transmittance and minimum reflectance. We fabricated moth-eye-inspired conical structures with three different sizes using a simple and robust fabrication method. Furthermore, the fabricated surfaces of the MPS and IMPS structures were analyzed based on the experimental and theoretical variation influences of their optical properties according to their sizes and shapes. As a result of these analyses, we herein propose a standard methodology based on the optimal structure of IMPS structure with a 300 nm diameter.

Nanostructured Hybrid-Material Transparent Surface with Antireflection Properties and a Facile Fabrication Process

Highly transparent optical surfaces with antireflection (AR) properties have the potential to increase the performance of a wide range of applications, such as windows for photovoltaic cells, photodetectors, and display screens among others. Biomimetic structures inspired by the moth-eye have attracted much attention as they can offer superior AR properties, which can generate broadband, omnidirectional optical transmission, and water-repellent self-cleaning behavior. However, many biomimetic surfaces suffer from time-consuming and complex processing, for example, electron beam and nanoimprint lithography, and/or sub-optimal mechanical reliability. In this paper, we introduce a hybrid material approach-nanostructured polyimide on a substrate-for demonstrating a surface with significant AR and hydrophobic properties together with low scattering (haze) and high mechanical resistance. As an example of applications, we demonstrate an indium tin oxide transparent conductive substrate with a large AR effect and optical transmission associated to the nanostructured polyimide coating. The proposed design and method based on conventional spin-coating and lithography-free metal dewetting have the potential to be a low-cost processing path of nanostructured AR transparent substrates.

Synthesis of Surface-Reinforced Biodegradable Chitosan Nanoparticles and Their Application in Nanostructured Antireflective and Self-Cleaning Surfaces

Surface-reinforced chitosan nanoparticles were used instead of polystyrene nanoparticles in the nanostructuring of antireflective, self-cleaning surfaces. Nanosphere lithography is a fascinating method to fabricate functional surfaces, but a large amount of nanoparticles are used and drained. Because synthetic polymer nanoparticles cause serious ecological and biological problems, the preparation of spherical nanoparticles was attempted with biodegradable, natural polymers, including chitosan and cellulose for application in nanosphere lithography. Chitosan nanospheres can be formed with a controlled size and surface charge, whereas cellulose spherical nanoparticles are hard to make. Therefore, chitosan nanoparticles were chosen and enclosed with trichloro(phenyl)silane to enhance their stability under plasma etching. A monolayer of the surface-reinforced chitosan nanoparticles was coated on a glass surface via a floating method for nanosphere lithography to act as a mask under reactive ion etching. After etching, the nanostructured glass showed a 2% increased transmittance compared with bare glass at 550 nm due to an antireflective effect. Moreover, the nanostructured glass with perfluoropolyether coating had a water contact angle of 152° and exhibited superhydrophobicity and a self-cleaning effect. This work addresses the issues of ecofriendly nanostructuring based on biodegradable, natural polymer nanoparticles for energy- and water-saving applications of nanostructured surfaces, by demonstrating the practical utilization of chitosan nanoparticles in nanosphere lithography.

Biomimetic Omnidirectional Antireflective Glass via Direct Ultrafast Laser Nanostructuring

Here, a single-step, biomimetic approach for the realization of omnidirectional transparent antireflective glass is reported. In particular, it is shown that circularly polarized ultrashort laser pulses produce self-organized nanopillar structures on fused silica (SiO₂ ). The laser-induced nanostructures are selectively textured on the glass surface in order to mimic the spatial randomness, pillar-like morphology, as well as the remarkable antireflection properties found on the wings of the glasswing butterfly, Greta oto, and various Cicada species. The artificial structures exhibit impressive antireflective properties, both in the visible and infrared frequency ranges, which are remarkably stable over time. Accordingly, the laser-processed glass surfaces show reflectivity smaller than 1% for various angles of incidence in the visible spectrum for s-p linearly polarized configurations. However, in the near-infrared spectrum, the laser-textured glass shows higher transmittance compared to the pristine. It is envisaged that the current results will revolutionize the technology of antireflective transparent surfaces and impact numerous applications from glass displays to optoelectronic devices.

Hybrid Nanostructured Antireflection Coating by Self-Assembled Nanosphere Lithography

Broadband antireflection (AR) coatings are essential elements for improving the photocurrent generation of photovoltaic modules or the enhancement of visibility in optical devices. In this paper, we report a hybrid nanostructured antireflection coating combination that is a clean and efficient method for fabricating a nanostructured antireflection coating (ARC). A multilayer thin-film was introduced between the ARC and substrate to solve the significant problem of preparing nanostructured ARCs on different substrates. In this way, we rebuilt a gradient refractive index structure and optimize the antireflective property by simply adjusting the moth-eye structure and multilayers. Subwavelength-structured cone arrays were directly patterned using a self-assembled single-layer polystyrene (PS) nanosphere array as an etching mask. Nanostructure coatings exhibited excellent broadband and wide-angle antireflective properties. The bottom-up preparation process and hybrid structural combination have the potential to significantly enhance the broadband and wide-angle antireflective properties for a number of optical systems that require high transparency, which is promising for reducing the manufacturing cost of nanostructured AR coatings.

Wide-Angle Broadband Antireflection Coatings Prepared by Atomic Layer Deposition

A novel broadband antireflective coating with ultra-low residual reflectance for light incidence angles from 0° up to 60° is presented. The system consists of an interference multilayer coating made by atomic layer deposition (ALD) combined with a low- n nanoporous SiO₂ top-layer obtained by wet-chemical etching of an atomically mixed SiO₂/Al₂O₃ ALD composite. The average residual reflectance measured at normal incidence for double-sided coated B270 glass substrates is only 0.5% in a broad spectral range from 400 to 1100 nm. The average reflectance of the substrate considering both front and rear sides decreased in the visible spectral range of 420-680 nm from 9.9 and 15.8 to 0.4 and 1.8% at an oblique angle of incidence (AOI) of 45° and 60°, respectively, by applying the hybrid ALD antireflection coatings. The measured average transmittance reaches 99.5% at AOI 6° in the 400-950 nm spectral range. Measurements three weeks after preparation show only a small reduction of the average transmittance to 99.3% in this spectral range spanning 550 nm. Ten months later, the average transmittance is still 99.0%, whereby the sample handling might have also affected the performance. The hybrid ALD system shows excellent conformal AR performance on a strongly curved B270 aspheric lens with a diameter of 50 mm and a height of 25 mm. The presented process is a promising route toward omnidirectional AR coatings on complex 3D optics, which are increasingly important for consumer and high-performance optical systems.

Design and Fabrication of Wafer-Level Microlens Array with Moth-Eye Antireflective Nanostructures

Wafer-level packaging (WLP) based camera module production has attracted widespread industrial interest because it offers high production efficiency and compact modules. However, suppressing the surface Fresnel reflection losses is challenging for wafer-level microlens arrays. Traditional dielectric antireflection (AR) coatings can cause wafer warpage and coating fractures during wafer lens coating and reflow. In this paper, we present the fabrication of a multiscale functional structure-based wafer-level lens array incorporating moth-eye nanostructures for AR effects, hundred-micrometer-level aspherical lenses for camera imaging, and a wafer-level substrate for wafer assembly. The proposed fabrication process includes manufacturing a wafer lens array metal mold using ultraprecise machining, chemically generating a nanopore array layer, and replicating the multiscale wafer lens array using ultraviolet nanoimprint lithography. A 50-mm-diameter wafer lens array is fabricated containing 437 accurate aspherical microlenses with diameters of 1.0 mm; each lens surface possesses nanostructures with an average period of ~120 nm. The microlens quality is sufficient for imaging in terms of profile accuracy and roughness. Compared to lenses without AR nanostructures, the transmittance of the fabricated multiscale lens is increased by ~3% under wavelengths of 400-750 nm. This research provides a foundation for the high-throughput and low-cost industrial application of wafer-level arrays with AR nanostructures.

Fabrication of broadband anti-reflective layers by mask-free etching TiO2 films

We present a simple way to make TiO₂ anti-reflective layers on top of silicon substrates. Surfaces of TiO₂ films have been modified by radio frequency plasma with CF₄ as an etchant. Mask-free etching process on the polycrystalline films leads to the formation of random sub-wavelength textures. The reflection of the etched samples are significantly suppressed in the wavelength range of 400~800 nm (2.9~4.6%, 3% compared with 34% on bare silicon at the wavelength of 600 nm). We have numerically simulated the optical properties of TiO₂ layers using the finite-difference time-domain method. The anti-reflective effects are attributed to random roughness on TiO₂ surfaces. The etching porcess increases the surface roughness, therefore, the gradient of refractive index between air and silicon substrate is reduced. As a result, the Fresnel reflection is supressed. Our results demonstrate an efficient way of anti-reflective coating for solar cells.

Solution processes for ultrabroadband and omnidirectional graded-index glass lenses with near-zero reflectivity in high concentration photovoltaics

Concentrator photovoltaic (CPV) systems, where incident direct solar radiation is tightly concentrated onto high-efficiency multi-junction solar cells by geometric optical elements, exhibit the highest efficiencies in converting the sun’s energy into electric power. Their energy conversion efficiencies are greatly limited, however, due to Fresnel reflection losses occurring at three air/optics interfaces in the most sophisticated dual-stage CPV platforms. This paper describes a facile one-step wet-etching process to create a nanoporous surface with a graded-index profile on both flat and curved glasses, with capabilities of achieving ~99% average transmission efficiency in a wide wavelength range from 380 nm to 1.3 µm and for a wide range of incident angles up to ±40° regardless of the polarization state of incident sunlight. The simplicity of the etching process remarkably increases their versatility in various optical elements that require unconventional form factors such as Fresnel lenses and microlens arrays, and/or demanding curvatures along with much reduced dimensions such as ball lenses. Etched glass surfaces on two-stage optical concentrating systems yield enhancements in total optical transmission efficiencies by 13.8% and in the photocurrent by 14.3%, as experimentally determined by measurements on microscale triple-junction solar cells. The presented strategy can be widely adapted in a variety of applications such as image sensors, display systems, and other optoelectronic devices.

Carbon Nanotube Embedded Nanostructure for Biometrics

Low electric energy loss is a very important problem to minimize the decay of transferred energy intensity due to impedance mismatch. This issue has been dealt with by adding an impedance matching layer at the interface between two media. A strategy was proposed to improve the charge transfer from the human body to a biometric device by using an impedance matching nanostructure. Nanocomposite pattern arrays were fabricated with shape memory polymer and carbon nanotubes. The shape recovery ability of the nanopatterns enhanced durability and sustainability of the structure. It was found that the composite nanopatterns improved the current transfer by two times compared with the nonpatterned composite sample. The underlying mechanism of the enhanced charge transport was understood by carrying out a numerical simulation. We anticipate that this study can provide a new pathway for developing advanced biometric devices with high sensitivity to biological information.

Multifunctional Moth-Eye TiO2/PDMS Pads with High Transmittance and UV Filtering

This work reports a facile fabrication method for constructing multifunctional moth-eye TiO₂/polydimethylsiloxane (PDMS) pads using soft nano-imprinting lithography and a gas-phase-deposited thin sacrificial layer. Mesoporous TiO₂ nanoparticles act as an effective UV filter, completely blocking high-energy UVB light and partially blocking UVA light and forming a robust TiO₂/PDMS composite pad by allowing the PDMS solution to easily fill the porous TiO₂ network. The paraboloid-shaped moth-eye nanostructures provided high transparency in the visible spectrum and also have self-cleaning effects because of nanoroughness on the surface. Furthermore, we successfully achieved a desired multiscale-patterned surface by partially curing select regions using TiO₂/PDMS pads with partial UVA ray blockers. The ability to fabricate multifunctional polymeric pads is advantageous for satisfying increasing demands for flexible and wearable electronics, displays, and solar cells.

Structural and optical investigation on the wings of Idea malabarica (Moore, 1877)

The nanostructures on the wings of Idea malabarica (Moore, 1877) were analysed using scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, Fourier transform-infrared spectroscopy, and reflectance measurements. The chemical and morphological analyses revealed the chitin-based intricate nanostructures. The influence of the nanostructures on the wetting characteristics of the wing was investigated using optical imaging. Applying the Maxwell-Garnet approximation to the porosities within the nanostructures, the refractive indices, which relate the reflectance response, were estimated. It was concluded that the colour seen on the wings of the Idea malabarica originate from the nanostructural configurations of the chitin-based structures and the embedded pigment.