Antireflective Nanostructures Research Articles

Enhanced performance of graphene transparent conductive films by introducing SiO2 bilayer antireflection nanostructure

The performance of graphene transparent conductive films (TCFs) can be greatly enhanced by introducing silica nanospheres/acid-catalyzed silica bilayer antireflection (AR) nanostructure.

Fabrication of antireflective hierarchical TiO2 nanostructures by moth-eye patterning of anodic anodized nanotubes.

Herein, we report the fabrication and characterization of a hierarchical TiO2 structure that exhibits reduced surface reflection. The hierarchical structure, which is a moth-eye-shaped array containing nanotubes, was fabricated by dry-etching a TiO2 nanotube layer, by using colloidal lithography. The fabricated structure shows a reduced reflectance, compared with that of non-patterned TiO2 nanotubes. This is because of the graded refractive index of the moth-eye pattern. Furthermore, we investigated the optical properties of gold-decorated moth-eye TiO2 nanotubes and found that the absorption, which was caused by the plasmonic resonance of gold nanostructures, was further enhanced by coupling with the light-trapping effect.

Moth-eye antireflection nanostructure on glass for CubeSats

A CubeSat is a type of miniaturized and modular satellite designed for space research or technology demonstration. By filling the unused capacity of major launch vehicles, CubeSats significantly lower the cost of entry to the space. To reduce the energy loss at the solar panel coverglass on CubeSats, the authors investigated the potential of the biomimetic moth-eye antireflection (MEAR) technique which features a gradient refractive index. A novel fabrication process combining nanosphere lithography, reactive-ion etching, and dry oxidation for patterning glass with moth-eye structures was reported that allows for the production of well-defined nanopillars with tunable sidewall profiles. The thermal annealing treatment of colloidal particles was introduced for making MEAR structures for which partial deformation of nanospheres was achieved. The broadband and omnidirectional antireflective performance of fabricated glasses was experimentally demonstrated by using a spectrophotometer and ellipsometer. A significant 10% decrease of reflectance throughout the measured spectral range was observed at a high incident angle of 75°.

Fabrication of Highly Transparent and Cost‐Effective Soda Lime Glass with Antireflective Nanostructures Using Silver Ink

Highly transparent single‐side subwavelength structures (SWSs)‐integrated glass for low‐cost antireflective glass are fabricated. Using the silver ink and a RIE process, various SWSs on soda lime glass are fabricated simply and cost‐effectively. The fabricated SWSs have different distributions and shapes as the silver ink ratio. The average transmittance (Tavg), solar‐weighted transmittance (SWT), and angle‐dependent transmittance values of bare and SWSs‐integrated glasses for investigation of optical properties are measured. The SWSs sample with a silver ink ratio of 33% exhibited good Tavg and SWT compared to the samples with silver ink ratios of 25 and 50%. This shows that the Tavg and SWT values can be attained with the soda lime glass are higher compared to those attained using bare glass, over a broad spectral range and a wide incident angle range. As results, optical properties of glass and the performance of photovoltaic (PV) modules can be enhanced by integrating SWSs. The fabricated SWSs‐integrated glass is mounted on the top of PV modules for economical coverglass. The PV module with the SWSs‐integrated coverglass shows efficiency of 36.6% and the enhanced efficiency at all angle of incidence compared to that without coverglass. It indicates that the highly transparent glasses with SWSs can be used for low‐cost antireflective glasses such as large‐scale framed glasses for pictures, optical lens, displays, LEDs, and other optoelectronic devices as well as PV modules.

Enhanced Performance of Ge Photodiodes via Monolithic Antireflection Texturing and α-Ge Self-Passivation by Inverse Metal-Assisted Chemical Etching.

Surface antireflection micro and nanostructures, normally formed by conventional reactive ion etching, offer advantages in photovoltaic and optoelectronic applications, including wider spectral wavelength ranges and acceptance angles. One challenge in incorporating these structures into devices is that optimal optical properties do not always translate into electrical performance due to surface damage, which significantly increases surface recombination. Here, we present a simple approach for fabricating antireflection structures, with self-passivated amorphous Ge (α-Ge) surfaces, on single crystalline Ge (c-Ge) surface using the inverse metal-assisted chemical etching technology (I-MacEtch). Vertical Schottky Ge photodiodes fabricated with surface structures involving arrays of pyramids or periodic nano-indentations show clear improvements not only in responsivity, due to enhanced optical absorption, but also in dark current. The dark current reduction is attributed to the Schottky barrier height increase and self-passivation effect of the i-MacEtch induced α-Ge layer formed on top of the c-Ge surface. The results demonstrated in this work show that MacEtch can be a viable technology for advanced light trapping and surface engineering in Ge and other semiconductor based optoelectronic devices.

Improved antireflection based on biomimetic nanostructures at material interface

Reducing light reflections on the surface of materials has important applications in many fields, such as solar cells, photodetectors, and optical sensors, etc. An effective method of decreasing reflection is using the anti-reflective coating with a gradient refractive index. In this study, we designed a nanostructure composed of optimized cone arrays on the flat thin film surface. The tapered nanostructure forms an anti-reflection layer. The effective refractive index of the anti-reflection layer changes smoothly with the depth so that the surface can efficiently reduce the reflection in a wide visible light range. Moreover, the reflection can also be modulated by adjusting the height and the period of the nanocones. Furthermore, there is an optimal wavelength at which the highest anti-reflection efficiency is achieved. The results here provide a theoretical guidance for the practical design of broadband anti-reflection nanostructures at the device surface.

A broadband antireflective nanostructure with Ag nanoparticles on SiO2 nanocolumns

A broadband antireflective nanostructure with Ag nanoparticles on SiO2 nanocolumns has been presented. Ag nanoparticles are located at the top of SiO2 nanocolumns which are deposited on Si substrates. SiO2 nanocolumns are fabricated by oblique angle deposition through electron beam evaporation, and Ag nanoparticles are fabricated by thermal evaporation. Experimental results show that the average reflection can reach 3.84% in the range of 400–700 nm and 5.75% in the range of 400–1100 nm, much lower than that of Ag islands on SiO2 thin films. The simulation shows that the broadband low reflection can be attributed to the localized surface plasmon resonance of Ag nanoparticles whose resonance wavelengths depends on the size of Ag nanoparticles. The different diameters of SiO2 nanocolumns determine the size distribution of Ag nanoparticles which resonate in a wide wavelength range and lead to a broadband low reflection. This provides a possible way to fabricate a broadband antireflection structure.

Emerging technologies for high performance infrared detectors

Abstract Infrared photodetectors (IRPDs) have become important devices in various applications such as night vision, military missile tracking, medical imaging, industry defect imaging, environmental sensing, and exoplanet exploration. Mature semiconductor technologies such as mercury cadmium telluride and III–V material-based photodetectors have been dominating the industry. However, in the last few decades, significant funding and research has been focused to improve the performance of IRPDs such as lowering the fabrication cost, simplifying the fabrication processes, increasing the production yield, and increasing the operating temperature by making use of advances in nanofabrication and nanotechnology. We will first review the nanomaterial with suitable electronic and mechanical properties, such as two-dimensional material, graphene, transition metal dichalcogenides, and metal oxides. We compare these with more traditional low-dimensional material such as quantum well, quantum dot, quantum dot in well, semiconductor superlattice, nanowires, nanotube, and colloid quantum dot. We will also review the nanostructures used for enhanced light-matter interaction to boost the IRPD sensitivity. These include nanostructured antireflection coatings, optical antennas, plasmonic, and metamaterials.

Al2O3 Encapsulated Teflon Nanostructures with High Thermal Stability and Efficient Antireflective Performance.

Scientific advancement is highly inspired and imitative of natural phenomenon's, which exhibits extremely developed and well-organized nanostructures to cope with challenges under different environmental circumstances, such as moth eyes protuberances for efficient antireflective (AR) performance. Innovative researches have been performed in the past to exterminate the undesirable reflectance in common optical components and optoelectronic industrial applications by biomimetic and replicating moth eye nanostructures creating gradient effect using metal oxides, composites, or polymers in multilayer AR coatings. However, in few multilayer AR designs, the properties mismatch at interfaces, high cost, low mechanical durability, wetting issues, or thermal stability bounds their practical applicability. Herein, we develop an approach for fabricating efficient, high-performance Teflon (polytetrafluoroethylene [PTFE]) AR nanostructures for glass-based supporting materials. Nanotailoring, the morphology and structure of PTFE, have been efficaciously carried out for fabricating high-performance AR coatings according to predicted optical simulation. The total reflectance from polymer AR coating lessens to <0.05% in a visible wavelength range which according to our best knowledge seems to be the superior AR performance by a polymer coating ever reported. Furthermore, the fabricated polymer AR coatings are omnidirectional, mechanically durable, and thermally stable up to 200 °C. Moreover, we modify and tune the refractive index of PTFE from 1.34 to 1.156 by inducing porosity and changing deposition angle.

Inverse Moth Eye Nanostructures with Enhanced Antireflection and Contamination Resistance.

Moth-eye-inspired nanostructures are highly useful for antireflection applications. However, block copolymer micelle lithography, an effective method to prepare moth eye nanopillars, can only be used on a limited choice of substrates. Another drawback of nanopillar substrates is that contamination is easily absorbed, thereby reducing transmittance. The production of antireflective surfaces that are contamination-resistant or that can be cleaned easily without the loss of optical properties remains challenging. Here, we describe an approach for creating inverse moth eye nanostructures on other optical substrates than the most commonly used fused silica. We demonstrate its feasibility by fabricating a borosilicate substrate with inverse nanostructures on both sides. The etching of nanoholes on both sides of the substrate improves its transmittance by 8%, thereby surpassing the highest increase of transmittance yet to be obtained with nanopillars on fused silica. More importantly, the substrate with inverse moth eye nanostructures is more robust against contaminations than the substrates with nanopillars. No significant decrease in performance is observed after five cycles of repeated contamination and cleaning. Our approach is transferable to a variety of optical materials, rendering our antireflection nanostructures ideal for applications in touch devices such as touch screens and display panels.

Graphene-based transparent conductive films with enhanced transmittance and conductivity by introducing antireflection nanostructure

Graphene-based films have intrinsic good performances among various transparent conductive films (TCFs). Here we described a promising way to further improve the transmittance of graphene-based TCFs under the premise of good conductivity by introducing antireflection nanostructure. A SiO2 nanoporous structure dip-coated on quartz substrate acts as an antireflection layer, followed with direct growth of graphene using remote catalyzation of Cu nanoparticles by ambient pressure chemical vapor deposition to obtain the composite TCFs. Further investigations show that the composite TCFs achieve a transmittance of 95.9%, increased by 6.2% to graphene/quartz film and a sheet resistance of 0.6kΩ·sq−1, reduced by 0.65kΩ·sq−1. This method avoids transferring graphene and enhances the transmittance and conductivity of TCFs simultaneously, which has the advantages of low cost, easy operating and indicates the potential applications of composite TCFs in many photoelectric devices.

Enhanced Optical Transmittance by Reduced Reflectance of Curved Polymer Surfaces

Subwavelength nanostructure arrays on surfaces improve their optical transmittance by reducing the reflection of light over a wide range of wavelengths and angles of incidence. A method to imprint a sub‐100 nm nanostructure array on a large surface (Ø 20 mm) made from thermoplastic materials is reported. Transmittance through the flat polymer is improved by ≈6.5%, reaching values of up to 97.5%, after imprinting. The optical properties of the nanostructured samples are highly reproducible. After eight repeated imprinting operations with the same stamp, the transmittance of the nanostructured surface is decreased by less than 0.2%. Moreover, the nanostructures can also be imprinted on curved polymethylmethacrylate surfaces, achieving a maximum transmittance of 97%. This method to prepare large‐scale antireflective nanostructures on flat and flexible curved polymer surfaces is of interest for the production of antireflective screens, optical devices, and biomedical devices such as contact lenses and intraocular lenses. image

Sustainable antireflection using recoverable nanopattern arrays

This research proposes a unique antireflection (AR) strategy which is to infuse shape recovery ability into nanopattern arrays for obtaining high sustainability of the AR nanostructures.

Performance enhancement of InAs quantum dots solar cells by using nanostructured antireflection coating with hydrophobic properties

Enhancement of the performance of an InAs quantum dots (QDs) solar cell was investigated by using a nanostructured antireflection coating with hydrophobic properties. The surface modification was performed by growing ZnO nanoneedles on top of an InAs QDs solar cell’s surface, then the nanoneedles’ hydrophobicity treatment was achieved by using stearic acid. The QDs solar cell’s performance remarkably improved by 50% as noticed in the power conversion efficiency, external quantum efficiency, and spectral response after the surface modification. Additionally, the contact angle of the hydrophobic surface was found to be 153 deg.

Broadband and wide angle anti-reflective nanoporous surface on poly (ethylene terephthalate) substrate using a single step plasma etching for applications in flexible electronics

Poly (ethylene terephthalate) (PET) substrate is a widely used polymer in the optoelectronics industry, its optical properties, such as light transmittance, haze, etc., are of significant importance in organic light emitting diodes and photovoltaics. In this paper, we present a broadband anti-reflective nanostructure on PET substrate prepared by means of plasma etching in a mixture of argon and oxygen gas. Etching time was varied for optimizing the optical transmittance. Field emission scanning electron microscopy images revealed formation of nanostructures of sub-wavelength dimensions resembling pores at the surface. Atomic force microscopy confirmed that the graded porosity was generated from the randomly distributed pores of different sizes and depths. Due to the formation of a gradient refractive index, nanostructured surface exhibited broadband and quasi-omnidirectional anti-reflection properties. A maximum total transmittance of ~99% at 835nm was achieved for both sides treated PET without significant increase in light scattering or haze. Moreover, both sides treated PET showed polarization insensitivity upto 48°, which was not exhibited by untreated PET. X-ray photoelectron spectroscopy revealed generation of degradation products, which consisted of carbon and oxygen containing functional groups. Durability of the anti-reflection layer was tested and proved to be satisfactory.

Optimized moth-eye anti-reflective structures for As2S3 chalcogenide optical fibers.

We computationally investigate moth-eye anti-reflective nanostructures imprinted on the endfaces of As2S3 chalcogenide optical fibers. With a goal of maximizing the transmission through the endfaces, we investigate the effect of changing the parameters of the structure, including the height, width, period, shape, and angle-of-incidence. Using these results, we design two different moth-eye structures that can theoretically achieve almost 99.9% average transmisison through an As2S3 surface.

Sub-wavelength printing in the deep ultra-violet region using Displacement Talbot Lithography

Printing of sub-100nm half-pitch periodic structures is demonstrated using Displacement Talbot Lithography (DTL) and a deep ultra-violet light source. DTL is a recently developed mask-based photolithography for forming high-resolution periodic structures over large areas using a relatively simple and low-cost system. A phase shift mask consisting of a 40×40mm2 transmission grating with a half-pitch of 150nm is fabricated by electron-beam lithography followed by reactive ion etching. The geometry of the phase grating is optimized by numerical simulation. Using this mask, dense line/space patterns and two-dimensional arrays of pillars, both with a half-pitch of 75nm, are printed onto silicon wafers. This new technique is suitable for uniform and high-throughput patterning of large-areas for such applications as moths-eye anti-reflective nanostructures, distributed feedback lasers, zeroth-order gratings, wire-grid polarizers and engineered substrates for LEDs.

Micro/nano-hybrid lens for enhancing light extraction using micro-milling and anodic aluminium oxide (AAO)

In the recent past there has been much research towards increasing the transmission of light in optical systems by reducing the Fresnel reflection of radiation, as the reflection of light from surfaces seriously decreases the performance of an optical device. These drawbacks have been overcome by mainly two methods, which are anti-reflective coating and anti-reflective nanostructure formation.In this study, we developed a simple fabrication process for Al micro/nano hybrid lens (MNHL) moulds for efficient light extraction using micro-milling and anodic aluminum oxide (AAO). From these moulds, two different types of polymer MNHL were fabricated using hot-embossing; one was a polymer MNHL that was covered with nanostructures over the entire surface, and the other was one for which only the microlens surface was covered with nanostructures. Two different types of polymer MNHLs were evaluated and compared with each other concerning the light extraction performance. The MNHL with nanostructures only on the microlens surface exhibited a higher light extraction performance than the other by 20.7%. It is expected that the fabricated MNHL can be used for the amplification of small signals when observing the presence of bio-molecules dyed with a fluorescent material.

Absorption Enhancement in Ultrathin Structures Based on Crystalline-Si/Ag Parabola Nanocones Periodic Arrays with Broadband Antireflection Property

In this study, we examine the optical properties and unique features of a novel design of a parabola nanocone consisting of a homogenous shell-like cover layer of crystalline silicon (c-Si) and an Ag core which provides an enhanced absorption efficiency and significant photocurrent conversion during exposure to an incident light. Determining the geometrical sizes of the c-Si/Ag parabola nanocone, we designed an antireflection nanostructure based on certain arrays of investigated cone arrays on a GaAs substrate. We proved that the examined nanostructure shows a low percentage of reflectance of 6.24 % and a significant short current density of ~37.2 mA/m 2 as well as broadband antireflection facility. This understanding paves the way for novel methods toward the use of a simple and two layer nanoparticle in designing efficient and high performance antireflection layers of photovoltaics and solar cells that are able to function over a wide range of spectrum.

Highly Efficient Flexible Perovskite Solar Cells with Antireflection and Self-Cleaning Nanostructures.

Flexible thin film solar cells have attracted a great deal of attention as mobile power sources and key components for building-integrated photovoltaics, due to their light weight and flexible features in addition to compatibility with low-cost roll-to-roll fabrication processes. Among many thin film materials, organometallic perovskite materials are emerging as highly promising candidates for high efficiency thin film photovoltaics; however, the performance, scalability, and reliability of the flexible perovskite solar cells still have large room to improve. Herein, we report highly efficient, flexible perovskite solar cells fabricated on ultrathin flexible glasses. In such a device structure, the flexible glass substrate is highly transparent and robust, with low thermal expansion coefficient, and perovskite thin film was deposited with a thermal evaporation method that showed large-scale uniformity. In addition, a nanocone array antireflection film was attached to the front side of the glass substrate in order to improve the optical transmittance and to achieve a water-repelling effect at the same time. It was found that the fabricated solar cells have reasonable bendability, with 96% of the initial value remaining after 200 bending cycles, and the power conversion efficiency was improved from 12.06 to 13.14% by using the antireflection film, which also demonstrated excellent superhydrophobicity.