Antireflective Nanostructures Research Articles

A highly efficient, eco‐friendly method for antireflection nanostructures on poly (ethylene terephthalate)

AbstractAntireflection surfaces are widely used in optical devices (such as vehicle display, solar cells, and architectural glass) to reduce the reflection and increase transmittance. Plasma etching shows great potential in making subwavelength antireflection structures for its advantage of scalable, low‐cost, and applicable in flexible substrates. Here, we demonstrate an antireflection nanostructure by O2 plasma etching on poly (ethylene terephthalate) (PET) substrate without additional corrosive gas species or antireflective coatings. Nanopores on the surface were formed due to the different etching rates of the organic region and silica region of the surface. The solar weighted average transmittance was improved from 91.6% to 94.8% for single‐side treated PET with silica antiblocking layer. The transmission increment was attributed to the gradient refractive index of the nanostructured surface due to the elimination of step discontinuity in refractive index. The result shows a highly efficient, eco‐friendly, solvent‐free, economical, and sputtering target‐free method for reducing the reflection and increasing the transmittance of the substrates.

Biomimetic Multifunctional Graphene-Based Coating for Thermal Management, Solar De-Icing, and Fire Safety: Inspired from the Antireflection Nanostructure of Compound Eyes.

Due to the ubiquitous and inexhaustible solar source, photothermal materials have gained considerable attention for their potential in heating and de-icing. Nevertheless, traditional photothermal materials, exemplified by graphene, frequently encounter challenges emanating from their elevated reflectance. Inspired by ocular structures, this study uses the Fresnel equation to enhance the photo-thermal conversion efficiency of graphene by introducing a polydimethylsiloxane (PDMS)/silicon dioxide (SiO2) coating, which reduces the light reflectance (≈20%) through destructive interference. The designed coating achieves an equilibrium temperature of ≈77°C at one sun and a quick de-icing in ≈65 s, all with a thickness of 5µm. Simulations demonstrate that applying this coating to high-rise buildings results in energy savings of ≈31% in winter heating. Furthermore, the combination of PDMS/SiO2 and graphene confers a notable enhancement in thermal stability through a synergistic flame-retardant mechanism, effectively safeguarding polyurethane against high temperatures and conflagrations, leading to marked reduction of 58% and 28% in heat release rate and total heat release. This innovative design enhances the photo-thermal conversion, de-icing function, and flame retardancy of graphene, thereby advancing its applications in outdoor equipment, high-rise buildings, and aerospace vessels.

Analysis of nanostructured anti reflection coating for various perovskite layer thicknesses in perovskite silicon tandem solar cells

The anti-reflection coating (ARC) plays an important role in the design of every kind of solar cell. The suitable optimization of the ARC layer can make a lot of difference in the final output of the cell, by reducing the reflections at the surface. In this regard, the present paper highlights and analyses numerically the effect of nanostructured ARC for different top perovskite layer thicknesses in perovskite-silicon tandem solar cells. In the present case, the nanostructures for ARC are considered to be made up of SiO2 nanoparticles (NP) embedded in ITO. To evaluate the effect of nanostructure for this proposed cell, the nanostructured tandem cell is compared with its planar ARC-based reference cell. The top perovskite active thickness is varied from 100 nm to 800 nm. It has been found that the effect of nanostructured ARC is more pronounced for thinner perovskite layer-based cells than for thicker layers. To reduce reflections at the front surface, the SiO2 NP diameter and inter-particle spacing are optimized. With the nanostructured ARC at the top, the cell achieved the current density rise of 11.3% as compared to the reference cell for a 100 nm thick perovskite-based tandem cell design. As both the sub-cells are in series in tandem design, the top cell current is matched to the bottom silicon layer current by optimizing the bottom cell too. The proposed ARC design has the added advantage that it can simply be done with sol–gel processes.

Discretely-supported transfer nanoimprint anti-reflection nanostructures on complex uneven surface of Fresnel lenses

Nanopatterning complex uneven surface of numerous functional devices to improve their performance is significantly appealing; however, it is extremely challenging. This study proposes a discretely-supported transfer nanoimprint technique to fabricate nanostructures on complex device surfaces containing multi-spatial frequencies. First, a discretely-supported nanoimprint template was designed based on the built energy criterion. A contact fidelity of over 99% was achieved between the designed template and the targeted complex uneven substrate surface. Next, the prefilled nanostructures on the template were transferred to the target surface after contact. By precisely controlling the amount of micro-droplet jetting on the template on-demand, the accumulation of the polymer in the micro-valley sites on the complex substrate was avoided, thus maintaining the morphology and generating function of the devices. Finally, high-quality Fresnel lenses with broadband wide-directional antireflection and excellent imaging performance were developed by imprinting subwavelength-tapered nanostructures on the relief surface.

SWIR anti-reflective nanostructures on nonlinear crystals by direct UV femtosecond laser printing

Nonlinear infrared (IR) crystals for radiation conversion are of paramount importance for realization of advanced laser spectrometers for medical diagnostics, environmental monitoring, and advanced sensing. However, performance of such crystals suffers from substantial surface reflectivity coming from rather high (over 2.5) refractive index of the key nonlinear materials used. Here, based on the example of promising BaGa4Se7 nonlinear crystal, we attested direct surface patterning with ultraviolet (257 nm) femtosecond laser pulses used to engrave anti-reflective microstructures (ARMs) directly on both output sides of the crystal. Imprinted surface nanotrenches arranged into a fish-net morphology with a periodicity down to 500 nm was found to increase transmittance of the crystals from 65% to 84% within a practically relevant shortwave IR spectral range. Formation of the ARMs with an optimized geometry is expected to weakly reduce the laser damage threshold of a pristine crystal material as it was also evidenced from supporting full-wave simulations and tests.

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.

Enhanced Performance of Transfer‐Free Graphene Transparent Conductive Films on Insulating Substrates by Introducing Array Nanostructure

AbstractGraphene is one of the most prevailing materials to replace traditional transparent conductive films (TCFs). In order to improve the performance of graphene TCFs, it is proposed to directly deposit graphene on insulating substrates via assisted catalysis of Cu foil to avoid intricate transfer process, and further effectively enhance the optical performance by introducing the close‐packed antireflection array nanostructure. The graphene TCFs grown on bare quartz substrates have sheet resistance of 0.766 kΩ sq−1 and transmittance of 86.83 %. Graphene composite TCFs with transmittance of 95.79 % were obtained by introducing the close‐packed solid/hollow SiO2 nanospheres antireflection array nanostructure. Compared with graphene TCFs, transmittance of graphene composite TCFs increased significantly, while the sheet resistance remained at the same level. This method combining direct synthesis technique with novel antireflection array nanostructure provides a significant design idea to grow transfer‐free graphene with controllable optoelectronic properties and promotes the application and development of the future electronic devices.

Antireflective GaN Nanoridge Texturing by Metal-Assisted Chemical Etching via a Thermally Dewetted Pt Catalyst Network for Highly Responsive Ultraviolet Photodiodes.

Antireflective (AR) surface texturing is a feasible way to boost the light absorption of photosensitive materials and devices. As a plasma-free etching method, metal-assisted chemical etching (MacEtch) has been employed for fabricating GaN AR surface texturing. However, the poor etching efficiency of typical MacEtch hinders the demonstration of highly responsive photodetectors on an undoped GaN wafer. In addition, GaN MacEtch requires metal mask patterning by lithography, which leads to a huge processing complexity when the dimension of GaN AR nanostructure scales down to the submicron range. In this work, we have developed a facile texturing method of forming a GaN nanoridge surface on an undoped GaN thin film by a lithography-free submicron mask-patterning process via thermal dewetting of platinum. The nanoridge surface texturing effectively reduces the surface reflection in the ultraviolet (UV) regime, which can be translated to a 6-fold enhancement in responsivity (i.e., 115 A/W) of the photodiode at 365 nm. The results demonstrated in this work show that MacEtch can offer a viable route for enhanced UV light-matter interaction and surface engineering in GaN UV optoelectronic devices.

Combined antifogging and antireflective double nanostructured coatings for LiDAR applications.

To increase the performance of optical systems, a good antireflective coating is required to ensure low reflectance and high transmittance of optical surfaces. Further problems, such as fogging that causes light scattering, negatively affect the image quality. This implies that other functional properties are also required. Presented here is a highly promising combination of an antireflective double nanostructure on top of an antifog coating with long-term stable properties, generated in a commercial plasma-ion-assisted coating chamber. It is demonstrated that the nanostructures do not affect the antifog properties and can be successfully used for many applications.

High-performance antireflection nanostructure arrays on aluminum-doped zinc oxide film fabricated with femtosecond laser near-field processing

Transmittance is critical to the performance of optoelectronic devices, and the fabrication of nanostructure arrays with sizes and periods similar to wavelengths on optical surfaces can effectively reduce reflectance and increase transmittance. A method combining a femtosecond laser and the near-field effect of polystyrene (PS) microspheres to prepare integrated high-performance anti-reflective nanostructure arrays on aluminum-doped zinc oxide (AZO) film was proposed in this study. The near-field effect of microspheres with different diameters on a laser light field for several laser wavelengths was simulated with the finite-difference time-domain (FDTD) method. The simulation results showed that a 10-fold improvement of the intensity of a focused laser beam could be obtained with the combination of an 800-nm laser and 1-μm PS microspheres. Nano-craters on the surface of the AZO film were obtained with femtosecond laser irradiation. The depth and the area of these nano craters both increased with the increase of the laser fluences and tended toward saturation when the fluence went from 0.67 to 0.83 J/cm2. Facilitated by the gradient refractive index formed by the nano-craters, the transmittance of the sample treated with 0.83 J/cm2 increased by 8% in the 400–1100 nm band and even by 15% in the 400–600 nm band. The ablation mechanism was revealed by the time-resolved pump-probe technique, which proved that the nano-craters were formed by the phase explosion, and the smooth bottom with fine cracks was induced by the inhomogeneous resolidification of the superheated liquid phase. Our method could endow a photoelectric device with high-performance integrated anti-reflection capability, eliminating the need for additional anti-reflection coating.

Bioinspired antireflective subwavelength nanostructures induced by femtosecond laser for high transparency glass

In nature, the Cicada wing exhibiting a high transparency at wide viewing angles can be bioinspired for the formation of broadband antireflective subwavelength nanostructures (ASS), which can significantly improve the transmittance of optical devices. In this study, a femtosecond laser micro-machining is developed to fabricate a biomimic nanostructure array on optical glass for antireflection. The underlying mechanism for the transmittance enhancement and geometry design of the artificial structure is thoroughly explored by applying a rigorous coupled-wave analysis method both in the visible and the near-infrared band. In addition, the height, diameter, and period of the ASS experimentally induced by the femtosecond laser are adjusted by controlling the laser-processing parameters of pulse energy and scanning velocity. Accordingly, the proposed bioinspired nanostructures featuring a wide-angle omnidirectional antireflection in the visible band, as well as hydrophobicity properties, are anticipated to be widely used in glass displays applications.

Fresnel reflection suppression from deterministic illumination diffusers using antireflection random nanostructures

Deterministic illumination diffractive-diffusers have nonperiodic short and medium-scale topography. Because of the deterministic locations of vertical sidewalls at the phase transition boundaries, over-coating diffractive diffusers with thin-film antireflection layers perturbs their function, resulting in performance deviations and nonuniformities. To mitigate these effects, we added antireflection random nanostructures on the surface of three different classes of fused-silica multiphase diffractive diffusers, using reactive-ion plasma etching. The diffusers were measured before and after the random nanostructures addition, using a polarized-laser scatterometer with a dynamic range of nine orders of magnitude. The bidirectional scatter distribution function was measured over the entire equatorial plane of incidence, to analyze the directionality of scattered light and the impact of the antireflective nanostructure presence on the optical performance of the diffusers. The overall reflectivity suppression was measured across the illumination patterns directions, as well as, across the entire 180-deg angle-sweep. The designed deterministic illumination patterns and their contrast were unaffected by the presence of the random antireflective structures, whereas Fresnel reflectivity was reduced by an order of magnitude on average.

Single-Step Fabrication of Longtail Glasswing Butterfly-Inspired Omnidirectional Antireflective Structures.

Most bio-inspired antireflective nanostructures are extremely vulnerable and suffer from complicated lithography-based fabrication procedures. To address the issues, we report a scalable and simple non-lithography-based approach to engineer robust antireflective structures, inspired by the longtail glasswing butterfly, in a single step. The resulting two-dimensional randomly arranged 80/130/180 nm silica colloids, partially embedded in a polymeric matrix, generate a gradual refractive index transition at the air/substrate interface to suppress light reflection. Importantly, the randomly arranged subwavelength silica colloids display even better antireflection performance for large incident angles than that of two-dimensional non-close-packed silica colloidal crystals. The biomimetic coating is of considerable technological importance in numerous practical applications.

Anti-reflective nanostructures for Efficiency Improvement of GaAs based Solar Cells

In this study, Gallium Arsenide (GaAs) and Silicon (Si) nanostructure based solar cells are investigated for efficiency enhancement. The optical absorption capability of triangular, hemispherical, and truncated nanocone geometries used as anti-reflection coatings are analyzed using COMSOL Multiphysics based on Finite Element Method calculations. The optical effects of different nanostructures with constant period of 400 nm on the top of the substrate are studied. Truncated nanocone structures can achieve optical absorption improvements as compared to the other two nanostructures. Furthermore, (GaAs)-based solar cell nanostructures exhibit higher optical absorption as compared to Si-based nanostructures.

Fabrication of Non‐Uniform Nanolattices with Spatially Varying Geometry and Material Composition

AbstractThe fabrication of periodic 3D nanostructures with uniform material properties has been widely investigated and is important for applications in photonics, mechanics, and energy storage. However, creating nanostructures with spatially varying lattice geometry and material composition is still largely an unexplored challenge in nanofabrication. This work presents the fabrication of non‐uniform nanolattices by patterning multiple layers of 3D nanostructures using phase shift lithography and atomic layer deposition. By controlling the processing parameters, the lattice geometry and material composition of each individual nanolattice layer can be tailored to create arbitrary material property profiles. Using the proposed method, a five‐layer nanolattice with spatially varying porosity and oxide materials has been demonstrated. This process can be used to create gradient‐index antireflection nanostructures, and a fabricated four‐layer nanolattice structure consisting of TiO2 and Al2O3 with gradually varying porosity reduces more than 90% of the specular reflectance from a silicon substrate. By enabling nanolattices with arbitrary profiles in physical properties, the demonstrated technique can find broad applications in nanophotonics, graded filters, energy storage systems, and nanoarchitected films.

Anti-reflective biomimetic nanostructures formed by 2D arrays of silica colloidal particles via self-assembly using sublimation, polymer solidification, and thermal fusion

Nano-size protrusions have attracted significant interest in biomimetic applications, such as mechanical engineering, optics, and biosciences. One of the most effective methods for fabricating such nanostructures is the use of two-dimensional (2D) self-assembly of colloidal particles. Herein, three types of novel self-assembly approaches for synthesizing homogenous 2D colloid arrays using sublimation, polymer solidification, and thermal fusion are presented. In the first approach, the sublimation method was applied by coating silica colloids and sublimable compounds on a substrate, followed by exposing them via sublimation. In the second one, polymer solidification method was developed by using the colloids, polymers, and fluorinated solvents with high boiling points, followed by evaporation of the solvents. Final approach involved forming 2D arrays of silica-polymer core-shell nanoparticles using layer-by-layer techniques and the subsequent thermal fusion of polymer shells on the particles. Homogenous nano-protrusions were obtained using these three methods. In addition to these uniform arrangement methods, application of colloid monolayers in moth-eye-type anti-reflection films were confirmed. These bottom-up technologies are expected to contribute to multifaceted applications and high-throughput techniques for fabricating nanostructures.

Fabrication of Moth-eye Antireflective Nanostructures via Oxygen Ion-beam Etching on a UV-curable Polymer

Fabrication of Moth-eye Antireflective Nanostructures via Oxygen Ion-beam Etching on a UV-curable Polymer

A whispering gallery mode-based surface enhanced electrochemiluminescence biosensor using biomimetic antireflective nanostructure

In this work, we developed a whispering gallery mode-based surface enhanced electrochemiluminescence (SE-ECL) sensing system with strong electromagnetic field strength. Ni doped MoS2 QDs were synthesized as a novel ECL emitter which can effectively catalyze coreactant and generate strong ECL signal. Au NPs were electrodeposited on the surface of MoS2 nanosheets to construct biomimetic antireflective nanostructure. And SiO2 dielectric microspheres with the whispering gallery mode characteristic were placed on the top of Au NPs/MoS2 nanosheets. The optoplasmonic hybrid structures can efficiently enhance the localization electric field and couple hot spot area. Therefore, the ECL intensity has been amplified as 10 times stronger compared to the original Ni doped MoS2 QDs. This sandwich type SE-ECL sensor can quantify K-RAS gene in 1 fM to 1 nM concentration range with the detection limit as 0.3 fM.

Developmental, cellular and biochemical basis of transparency in clearwing butterflies.

ABSTRACTThe wings of butterflies and moths (Lepidoptera) are typically covered with thousands of flat, overlapping scales that endow the wings with colorful patterns. Yet, numerous species of Lepidoptera have evolved highly transparent wings, which often possess scales of altered morphology and reduced size, and the presence of membrane surface nanostructures that dramatically reduce reflection. Optical properties and anti-reflective nanostructures have been characterized for several ‘clearwing’ Lepidoptera, but the developmental processes underlying wing transparency are unknown. Here, we applied confocal and electron microscopy to create a developmental time series in the glasswing butterfly, Greta oto, comparing transparent and non-transparent wing regions. We found that during early wing development, scale precursor cell density was reduced in transparent regions, and cytoskeletal organization during scale growth differed between thin, bristle-like scale morphologies within transparent regions and flat, round scale morphologies within opaque regions. We also show that nanostructures on the wing membrane surface are composed of two layers: a lower layer of regularly arranged nipple-like nanostructures, and an upper layer of irregularly arranged wax-based nanopillars composed predominantly of long-chain n-alkanes. By chemically removing wax-based nanopillars, along with optical spectroscopy and analytical simulations, we demonstrate their role in generating anti-reflective properties. These findings provide insight into morphogenesis and composition of naturally organized microstructures and nanostructures, and may provide bioinspiration for new anti-reflective materials.

Large-Scale Bio-Inspired Flexible Antireflective Film with Scale-Insensitivity Arrays

Natural creatures can always provide perfect strategies for excellent antireflection (AR), which is valuable for photovoltaic industry, optical devices, and flexible displays. However, limited by precision, it is still difficult to guarantee the consistency between the artificial structures and the original biological structures. Here, a novel large-scale flexible AR film is inspired by the cicada wings and successfully fabricated with a recycled template. On the one hand, the adjustable structures on porous templates make it possible to optimize the design of AR structure parameters toward the practical demand. On the other hand, it breaks the limitation of the biological organism size, accomplishing the replication of AR nanostructure units in a large scale. Interestingly, even if the film is covered by enlarged dome cone arrays, it still maintains almost perfect AR property, achieving excellent scale-insensitivity AR performance. This work numerically and experimentally investigates its scale-insensitivity AR performance in detail. Compared with subwavelength nanocones, enlarged cones change the original optical behaviors, and the proportion of transmitted light is reduced while scattering and absorption increase. Based on this, these bio-inspired scale-insensitivity AR arrays could be used in flexible displays, photothermic conversion, solar cells, and so on.