Moth Eye Research Articles

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.

Investigate on structure for transparent anti-icing surfaces

The ice-phobic and transparent surface based on the distinctive structure of a coating material has been investigated. Moth eye structure fabricated on the quartz substrate was covered with a flat paraffin layer to isolate it in a cold and humid environment. Paraffin wax was chosen as the coating material due to low thermal conductivity, easy coating, and original water repellency. The paraffin layer only stayed on the top of the nanostructure, separated it from the outside environment to obstruct heat energy being transferred to the cold substrate, and prevented the wetting transition, which was observed regularly on the rough surface. The uncountable number of air blocks trapped inside the nanostructure also contributed to delayed heat transfer, leading to an increase in the freezing time of the attached water droplet. The anti-icing performance was evaluated in terms of adhesion strength, freezing time, and freezing rain sustainability. The nanostructure coated sample was compared with barely coated and superhydrophobic nanostructure surface and demonstrated a preeminent performance.

Hierarchical Nano/Micro Moth Eyelike Polymer Film Using Solid/Liquid Interfacial Reaction at Room Temperature.

A simple pathway for the fabrication of real moth eyelike patterned (MEP) polymer film with a double-layered nano/microhierarchical structure is demonstrated through a solid/liquid interfacial reaction at atmospheric conditions. A convex-structured polyvinyl alcohol (PVA) film containing CdCl2 was first fabricated using a self-organized honeycomb-patterned porous film as a template. The CdCl2/PVA convex film was immersed into Na2S/ethanol solution to facilitate the reaction between CdCl2 and Na2S at the solid/liquid interface, which led to the functionalization of CdS nanoparticles in the convex-structured PVA film. The tunable introduction of interfacial reaction resulted in the formation of a CdS moth eyelike nanoarray on the top surface of the PVA convex microarray, which mimicked the real moth eye (PVA-CdS MEP). PVA-CdS MEP film with a double moth eyelike structure showed improved antireflective property in comparison with flat and convex-structured PVA films. The PVA-CdS MEP film showed photoresponse under simulated solar light radiation and flexible duration after 500 cycles of folding.

Evolutionary constraints on flicker fusion frequency in Lepidoptera.

Flying insects occupy both diurnal and nocturnal niches, and their visual systems encounter distinct challenges in both conditions. Visual adaptations, such as superposition eyes of moths, enhance sensitivity to low light levels but trade off with spatial and temporal resolution. Conversely, apposition eyes of butterflies enable high spatial resolution but are poorly sensitive in dim light. Although diel activity patterns of insects influence visual processing, their role in evolution of visual systems is relatively unexplored. Lepidopteran insects present an excellent system to study how diel activity patterns and phylogenetic position influence the visual transduction system. We addressed this question by comparing electroretinography measurements of temporal response profiles of diverse Lepidoptera to light stimuli that were flickering at different frequencies. Our data show that the eyes of diurnal butterflies are sensitive to visual stimuli of higher temporal frequencies than nocturnal moths. Hesperiid skippers, which are typically diurnal or crepuscular, exhibit intermediate phenotypes with peak sensitivity across broader frequency range. Across all groups, species within families exhibited similar phenotypes irrespective of diel activity. Thus, Lepidopteran photoreceptors may have diversified under phylogenetic constraints, and shifts in their sensitivity to higher temporal frequencies occurred concomitantly with the evolution of diurnal lifestyles.

Improvement of sensitivity of Si based infrared detector using moth eye structure

Improvement of sensitivity of Si based infrared detector using moth eye structure

Mechanically robust antireflective moth-eye structures with a tailored coating of dielectric materials

Bioinspired moth-eye surface provides broadband antireflection features, which significantly enhance performances in optical components/devices. However, their practical uses are strictly limited due to poor mechanical stability of nano-patterns. In this study, we artificially engineered moth-eye structures on polycarbonate substrate through a thin-film coating of mechanically stable dielectric materials (i.e., Al2O3, Cr2O3, ZrO2, and TiO2 etc). The geometry of Al2O3-coated moth-eye surface is designed by considering the effective medium theory and confirmed by calculation of diffraction efficiency based on a rigorous coupled-wave analysis method. The tailored Al2O3-coating on moth eye surface exhibit the improved hardness while maintaining high optical transmittance.

Optical Modelling and Phylogenetic Analysis Provide Clues to the Likely Function of Corneal Nipple Arrays in Butterflies and Moths.

The lenses in compound eyes of butterflies and moths contain an array of nipple-shaped protuberances, or corneal nipples. Previous work has suggested that these nipples increase light transmittance and reduce the eye glare of moths that are inactive during the day. This work builds on but goes further than earlier analyses suggesting a functional role for these structures including, for the first time, an explanation of why moths are attracted to UV light. Using a phylogenetic approach and 3D optical modelling, we show empirically that these arrays have been independently lost from different groups of moths and butterflies and vary within families. We find differences in the shape of nipples between nocturnal and diurnal species, and that anti-glow reflectance levels are different at different wave-lengths, a result thereby contradicting the currently accepted theory of eye glow for predator avoidance. We find that there is reduced reflectance, and hence greater photon absorption, at UV light, which is probably a reason why moths are attracted to UV. We note that the effective refractive index at the end of the nipples is very close to the refractive index of water, allowing almost all the species with nipples to see without distortion when the eye is partially or completely wet and providing the potential to keep eyes dry. These observations provide a functional explanation for these arrays. Of special interest is the finding that their repeated and independent loss across lepidopteran phylogeny is inconsistent with the explanation that they are being lost in the ‘higher’, more active butterflies.

Bioinspired Metamaterials: Multibands Electromagnetic Wave Adaptability and Hydrophobic Characteristics.

Although various photonic devices inspired by natural materials have been developed, there is no research focusing on multibands adaptability, which is not conducive to the advancement of materials science. Herein, inspired by the moth eye surface model, state-of-the-art hierarchical metamaterials (MMs) used as tunable devices in multispectral electromagnetic-waves (EMWs) frequency range, from microwave to ultraviolet (UV), are designed and prepared. Experimentally, the robust broad bandwidth of microwave absorption greater than 90% (reflection loss (RL) < -10 dB) covering almost entire X and Ku bands (8.04-17.88 GHz) under a deep sub-wavelength thickness (1 mm) is demonstrated. The infrared emissivity is reduced and does not affect the microwave absorption simultaneously, further realizing anti-reflection and camouflage via the strong visible light scattering by the microstructure, and can prevent degradation by reducing the transmittance to less than 10% over the whole near UV band, as well as having hydrophobic abilities. The mechanism explored via simulation model is that topological effects are found in the bio-structure. This discovery points to a pathway for using natural models to overcome physical limits of MMs and has promising prospect in novel photonic materials.

Fabrication of moth eye-like patterned polystyrene films and their functionalization with polyaniline via interfacial reaction

Moth eye-like patterned (MEP) polystyrene (PS) films with a hexagonal array of microdomes mimicking natural moth eye were fabricated by pouring a PS solution on a honeycomb-patterned porous polyvinyl alcohol film using as a template. The surface of microdomes on the PS MEP film containing benzoyl peroxide (BPO) was functionalized with polyaniline (PANI) through interfacial reaction by using aniline hydrochloride to enhance their functionality of the MEP patterned film. The PANI functionalization on the MEP film was confirmed by UV–visible spectroscopy, scanning electron microscopy and EDX analysis.

(Invited) Biomimetic Approaches for the Practical Utilization of Discrete Liquid-Solid Contact Electrification

The concept of “discrete liquid-solid contact electrification”, which is result of sequential process of the liquid contact on the solid surface and successive detachment of contact liquid from the solid surface, was reported. After the first report on discrete liquid-solid contact electrification inside the conventional pipet tip surface, the phenomenon is deeply investigated and explored with the help of utilization on a hydrophobic surfaces, which enables easy detachment of liquid from its surface. As a result, the phenomenon can be intentionally enhanced. Recently, discrete liquid-solid contact electrification has been utilized as a fundamental operation mechanism of the energy harvester, called triboelectric nanogenerator (TENG), which utilizes the contact electrification phenomenon with electrical induction to efficiently convert ambient mechanical energy to electrical energy. In this presentation, the history from the discovery of discrete liquid-solid contact electrification to its application as a TENG is introduced. Especially, the biomimetic approaches (mimicking natural lotus leaf and moth's eye) in the fabrication the TENG are introduced not only to enhance the electrical output as an energy harvester, but also to give an additional function for practical utilization. The present study would broaden the applications of discrete liquid-solid contact electrification, which is ubiquitous but previously inconspicuous in our daily life.

Three dimensional printing of metamaterial embedded geometrical optics (MEGO)

Three-dimensional printers have revolutionized many scientific fields with its low-cost, accessibility and ease of printing. In this paper, we show how stereolithography (SLA) based 3D printers can enable realization of innovative 3D optical devices formed through the fusion of metamaterials with geometrical optics or MEGO. It utilizes a combination of desktop SLA 3D printer and metal deposition/coating systems. Using this approach, we present innovative metamaterial embedded optical components such as mushroom-type metamaterials, curved wide-angle metamaterial absorbers/reflectors and a frequency selective moth eye hemispherical absorber. Finally a unique MEGO device formed through the fusion of a frequency selective metamaterial with an optical parabolic reflector has been demonstrated that combines their individual properties in a single device. The fabricated MEGO devices operate in the millimeter wave frequency range. Simulation and measurement results using terahertz continuous-wave spectrometer validate their functionality and performance. With improving resolution in 3D printing, MEGO devices will be able to reach Terahertz and optical frequencies in the near future.

Metallic triangular pillar grating arrays for high transmission polarizers for air:glass interfaces

A metallic one-dimensional moth eye structure that can be applied at the interface between glass and air to act as an optical polarizer with high transmission and high polarization extinction ratio (PER) is introduced. The metallic triangular grating array (MTGA) is designed by effective medium approach, with the expected performance confirmed by numerical simulations. A transmission of 99.12% and PER of 56.02 dB are predicted. Furthermore, the structure provides >99% transmission over a 450 nm spectral bandwidth for a 4 degree angled surface. In addition, the MTGA has been applied to a Fresnel zone plate (FZP), which enables both polarizing and focusing with 9.27% higher field intensity than the conventional FZP. The proposed metallic triangular array structure proposed here will provide highly-efficient polarization state selection.

Silica single-layer inverse opal films: large-area crack-free fabrication and the regulation of transmittance in the visible region

Crack-free large-area single-layer SiO2 inverse opal (IO) films similar to an inverse “moth's eye” structure, are fabricated. The effects of their porous structure on transmittance in the visible region are investigated.

Formation of Hierarchical Zinc Oxide Nanostructures for Solar Energy Converters and Photovoltaics

Zinc oxide is a promising material for the fabrication of thin film layers for a new generation of photovoltaic devices and solar thermal collectors, due to the unique optical and electrical properties as well as the propensity to form fil­amentous one-dimensional submicron structures, namely nanowires, nanorods, nanotubes, nanobelts, and hierarchical nanostructures with developed surface and projected superhydrophobicity. We identified the possibility for a develop­ment of planar single-layer antireflection coatings and/or arrays of nanorods of this material as having the shape of hexagonal prisms, and demonstrating the moth eye effect on the substrates of transparent conductive tin dioxide and on silicon wafers with embedded homojunctions. The optimization of the pulsed electrodeposition of zinc oxide arrays adjust the size of parabolic nanonipples for the implementation of antireflection coatings with the moth eye effect on var­ious substrates, including flexible. The antireflection coatings will be developed for thin-film photovoltaic devices of substrate configuration based on kesterite, tin sulphide, and fullerene layers, for the cadmium telluride based photovoltaic cells with bilateral sensitivity of superstrate configuration on the flexible sub­strates and for optoelectronic devices based on zinc selenide for the ultraviolet spectra.

Biomimetic anti-reflective triboelectric nanogenerator for concurrent harvesting of solar and raindrop energies

As a combination of solar cell and water-based triboelectric nanogenerator (TENG), new concept of a solar-cell-based hybrid energy harvester has been proposed. However, previous studies have not fully considered the degradation of optical characteristics due to water-based TENG and energy loss due to an ineffective electrical connection between solar cell and water-based TENG. The 1% degradation in light transmittance by applying the water-based TENG on its outermost part results in more than 1 mW/cm2 output power loss in a solar cell, which cannot be recovered by the instantaneous electrical energy output of water-based TENG. Herein, we report a moth's eye mimicking TENG (MM-TENG), which can play a role of complementary energy harvester to a conventional solar cell due to its superior specular transmittance (maximum of 91% for visible light). For the first time, we deeply analyze the optical effect of the MM-TENG on a solar cell by investigating solar-weighted transmittance (SWT). The 0.01% improved SWT in the MM-TENG increases the fill factor and power conversion efficiency of solar cell by 0.5% and 0.17%, respectively, compared with a conventional protective glass plate which is always applied in a solar panel. In addition to such prominent high transmittance, the self-cleaning property of the MM-TENG enables the long-term performance of the solar panel. And particularly, this paper reports a novel electric circuit for effective management in a hybrid energy harvester by intermittently transferring the stabilized electrical energy output of the MM-TENG. This work, which addresses issues for the practical utilization of the water-based TENG as a complementary energy harvester to solar cell, would move the water-based TENG one step closer to its practical utilization by resolving critical concerns.

Design and Fabrication of Moth-Eye Subwavelength Structure with a Waist on Silicon for Broadband and Wide-Angle Anti-Reflection Property

Reflection loss on the optical component surface is detrimental to performance. Several researchers have discovered that the eyes of moths are covered with micro- and nanostructured films that reduce broadband and wide-angle light reflection. This research proposes a new type of moth-eye subwavelength structure with a waist, which is equivalent to a gradient refractive index film layer with high–low–high hyperbolic-type fill factor distribution. The diffraction order characteristics of a moth-eye subwavelength structure are first analyzed using a rigorous coupled wave analysis. The moth-eye structural parameters are optimized within the spectral range of 2–5 μm using the finite-difference time-domain method. The experimental fabrication of the moth-eye structure with a waist array upon a silicon substrate is demonstrated by using three-beam laser interferometric lithography and an inductively coupled plasma process. The experimental and simulation results show good agreement. The experimental results show that the reflectivity of the moth-eye structure with a waist is less than 1.3% when the incidence angle is less than 30°, and less than 4% when the incidence angle is less than 60°. This research can guide the development of AR broadband optical components and wide-angle applications.

Approaching Perfect Light Incoupling in Perovskite and Silicon Thin Film Solar Cells by Moth Eye Surface Textures

AbstractSolar cells with increased short‐circuit current density and energy conversion efficiency can be realized by integrating moth eye textures in the design of perovskite and amorphous silicon thin film solar cells. Broadband light incoupling in solar cells can be achieved by using hexagonally arranged arrays of nipples or domes with parabolically shaped surface profiles. The moth eye surface texture represents a refractive index grating that allows for an efficient incoupling of light in the solar cell while minimizing reflection losses. The light incoupling is studied for perovskite and amorphous silicon solar cells. Perovskite has a rather low refractive index of ≈2.5, while amorphous silicon exhibits a refractive index of ≈4.5 comparable to that of crystalline silicon. Due to largely different refractive indices, different device designs must be selected to allow for an efficient light incoupling in the solar cell. 3D finite‐difference time‐domain simulations are used for the optical modeling. Design guidelines are provided on how to realize perovskite and silicon thin film solar cells with high quantum efficiency and short‐circuit current by using moth eye textures.

Antireflective glass nanoholes on optical lenses.

Antireflective structures, inspired from moth eyes, are still reserved for practical use due to their large-area nanofabrication and mechanical stability. Here we report an antireflective optical lens with large-area glass nanoholes. The nanoholes increase light transmission due to the antireflective effect, depending on geometric parameters such as fill factor and height. The glass nanoholes of low effective refractive index are achieved by using solid-state dewetting of ultrathin silver film, reactive ion etching, and wet etching. An ultrathin silver film is transformed into nanoholes for an etch mask in reactive ion etching after thermal annealing at a low temperature. Unlike conventional nanopillars, nanoholes exhibit high light transmittance with enhancement of ~4% over the full visible range as well as high mechanical hardness. Also, an antireflective glass lens is achieved by directly employing nanoholes on the lens surface. Glass nanoholes of highly enhanced optical and mechanical performance can be directly utilized for commercial glass lenses in various imaging and lighting applications.

Transparent Surfaces Inspired by Nature

AbstractNature has long inspired scientists and engineers. As one ubiquitous example of this, nature has provided all with several clever methods to absorb, repel, and/or allow both sunlight and water to pass through surfaces. Moth's eyes (highly antireflective) and lotus leaves (highly hydrophobic and self‐cleaning) represent durable natural surfaces which exhibit nearly ideal physical and optical properties. Man‐made transparent surfaces must also be able to cope with water and dust while reaching the maximum possible light transmission for solar collectors, displays, and other optical devices. To explore the link between these – particularly for transparent surfaces – this review puts the physics, progress, and limitations of synthetic materials in context with natural materials. This perspective reveals that there is still much more to learn (and implement) if it is hoped to match the multifunctionality and resilience of natural materials.

It’s About Making Surfaces Invisible

Antireflection is an essential requirement for certain optoelectronic devices. Antireflective surfaces can be generated by different methods. One method mimics the nanofeatures on the moth eye surface. This can be done by ‘soft lithography’ (SL), where nanofeatures are created on an elastomeric stamp. This stamp is then used for imprinting the pattern on other surfaces.