Using colloidal lithography to fabricate and optimize sub-wavelength pyramidal and honeycomb structures in solar cells

A laser-processed micro/nanostructures surface and its photothermal de-icing and self-cleaning performance

Micro/nanostructures have garnered significant attention and widespread applications in areas such as photocatalysis, coated fabrics, microchips, and sensors. However, high-resolution and multifunctional micro/nanostructures fabrication remains a great challenge. In this work, a novel self-assembly-femtosecond laser processing for the regular micro squares and nano bumps surface on steel substrates is proposed, and a great potential in the field of anti-icing/de-icing and self-cleaning is demonstrated. The surface tension gradient-driven liquid-air self-assembly provides a silica microsphere monolayer, while the post-femtosecond laser process can give precise micro/nano decoration. We systematically explore the impact of laser repetition frequency, scanning speed, and laser incident power on the size and shape of micro/nano decorations that have been studied. The different performances of self-cleaning effects, ice adhesion, and the photothermal de-icing capability due to the change in surface wettability have been demonstrated. This research shows a new pathway for the creation of smart micro/nanostructures surface which possess stable super hydrophilic and highly adhesive superhydrophobic properties, as well as high abrasion resistance. The discovery achieves a suitable blend of multiple functions on the surface of a single material, which can be applied to various surface engineering fields.

Wafer-scale synthesis of a morphologically controllable silicon ordered array as a platform and its SERS performance

In this study, we fabricated four different structures using single crystal silicon wafers for surface-enhanced Raman spectroscopy (SERS) applications. For single crystal silicon, different crystal orientations exhibit different physical and chemical properties. In chemical etching, the etching speed of different crystal planes also exhibits significant differences. We first used reactive ion etching (RIE) to process the surface of the substrate, and subsequently used KOH anisotropic wet etching technology to modify the surface of silicon wafers with different crystal orientations and produced four different results. In the RIE stage in an O2 atmosphere, the (110) silicon wafer formed a hexagonal hole structure, and the (100) silicon wafer formed an inverted pyramid hole structure; however, in the RIE-treated substrates in O2 and SF6 atmosphere, the (110) silicon wafer formed a pyramid with a diamond-shaped base, and the (100) silicon wafer showed a columnar structure with a "straw hat" at the top. The formation mechanisms of these four structures were elucidated. We also performed structure-related SERS characterizations of the four different structures and compared their performance differences.

Massively parallel direct writing of nanoapertures using multi-optical probes and super-resolution near-fields

Laser direct-writing enables micro and nanoscale patterning, and is thus widely used for cutting-edge research and industrial applications. Various nanolithography methods, such as near-field, plasmonic, and scanning-probe lithography, are gaining increasing attention because they enable fabrication of high-resolution nanopatterns that are much smaller than the wavelength of light. However, conventional methods are limited by low throughput and scalability, and tend to use electron beams or focused-ion beams to create nanostructures. In this study, we developed a procedure for massively parallel direct writing of nanoapertures using a multi-optical probe system and super-resolution near-fields. A glass micro-Fresnel zone plate array, which is an ultra-precision far-field optical system, was designed and fabricated as the multi-optical probe system. As a chalcogenide phase-change material (PCM), multiple layers of Sb₆₅Se₃₅ were used to generate the super-resolution near-field effect. A nanoaperture was fabricated through direct laser writing on a large-area (200 × 200 mm²) multi-layered PCM. A photoresist nanopattern was fabricated on an 8-inch wafer via near-field nanolithography using the developed nanoaperture and an i-line commercial exposure system. Unlike other methods, this technique allows high-throughput large-area nanolithography and overcomes the gap-control issue between the probe array and the patterning surface.

The Optical Properties of Metal-Free Polymer Films with Self-Assembled Nanoparticles

In this paper, it is reported that a metal-free and non-conjugated polymer, MA-PEG 8000-BADGE (MP8B), exhibits an antireflective property and substrate-dependent photoluminescence (SDP). MP8B was constructed from maleic anhydride, poly(ethylene glycol) and bisphenol-A diglycidyl ether. Self-assembled nanoparticles are found in MP8B and can prospectively act as scattering centers to improve light trapping and extraction. MP8B films prepared from MP8B solutions have been characterized by photoluminescence (PL), atomic force microscopy (AFM), tunnelling electron microscope (TEM), reflectance, transmittance, and UV-Vis absorption spectrum. MP8B films can suppress light reflection and enhance light transmission. The PL spectrum of MP8B film on ITO peaks at approximately 538 nm, spanning from 450 to 660 nm at a concentration of 25 mM. Meanwhile, the effects of concentration and substrate on the PL of MP8B films are also investigated in this study. Surface roughness becomes larger with concentration. A red shift of the PL spectrum is observed as solution concentration increases. Meanwhile, aggregation-caused quenching (ACQ) is insignificant. Moreover, the PL spectra of MP8B films show a substrate-dependent phenomenon due to dielectric screening. The optical band-gap energy of MP8B is approximately 4.05 eV. It is concluded that MP8B is a promising candidate for a host material, and its film can be utilized as a multifunctional layer (i.e., antireflective and light-scattering functions) for optoelectronic applications.

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.

Ultra-Broadband Directional Scattering by Colloidally Lithographed High-Index Mie Resonant Oligomers and Their Energy-Harvesting Applications

Emerging high-index all-dielectric nanostructures, capable of manipulating light on the subwavelength scale, empower designing and implementing novel antireflection and light-trapping layers in many photonic and optoelectronic devices. However, their performance and practicality are compromised by relatively narrow bandwidths and highly sophisticated fabrications. In this paper, we demonstrate an ultra-broadband (300-1200 nm) directional light scattering strategy using high-index surface silicon oligomer resonators fabricated by a facile, scalable, and low-cost colloidal lithography technique. The exceptional broadband forward scattering stems from a combined effect of strongly intercoupled Mie resonances within the oligomers composed of randomly positioned nanodisks in the visible region and a strong electric mode coupling between the oligomers and the high-index substrate in the red-to-near-infrared region. By implementing this efficient approach in silicon solar cells, the integrated optical reflection loss across the wavelength range 300-1200 nm can be as low as 7%. Consequently, the short-circuit current density determined from the external quantum efficiency of solar cells can be increased to 35.1 from 25.1 mA/cm², representing an enhancement of 40%, with a demonstrated energy conversion efficiency exceeding 15.0%. The insights in this paper hold great potentials for new classes of light management and steering photonic devices with drastically improved practicality.

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.

Flexible Semiconductor Technologies with Nanoholes-Provided High Areal Coverages and Their Application in Plasmonic-Enhanced Thin Film Photovoltaics

Mechanical flexibility and advanced light management have gained great attentions in designing high performance, flexible thin film photovoltaics for the realization of building-integrated optoelectronic devices and portable energy sources. This study develops a soft thermal nanoimprint process for fabricating nanostructure decorated substrates integrated with amorphous silicon solar cells. Amorphous silicon (a-Si:H) solar cells have been constructed on nanoholes array textured polyimide (PI) substrates. It has been demonstrated that the nanostructures not only are beneficial to the mechanical flexibility improvement but also contribute to sunlight harvesting enhancement. The a-Si:H solar cells constructed on such nanopatterned substrates possess broadband-enhanced light absorption, high quantum efficiency and desirable power conversion efficiency (PCE) and still experience minimal PCE loss even bending around 180°. The PCE performance without antireflection coatings increases to 7.70% and it improves 40% compared with the planar devices. Although the advantages and feasibility of the schemes are demonstrated only in the application of a-Si:H solar cells, the ideas are able to extend to applications of other thin film photovoltaics and semiconductor devices.

Nanostructured GaAs solar cells via metal-assisted chemical etching of emitter layers

GaAs solar cells with nanostructured emitter layers were fabricated via metal-assisted chemical etching. Au nanoparticles produced via thermal treatment of Au thin films were used as etch catalysts to texture an emitter surface with nanohole structures. Epi-wafers with emitter layers 0.5, 1.0, and 1.5 um in thickness were directly textured and a window layer removal process was performed before metal catalyst deposition. A nanohole-textured emitter layer provides effective light trapping capabilities, reducing the surface reflection of a textured solar cell by 11.0%. However, because the nanostructures have high surface area to volume ratios and large numbers of defects, various photovoltaic properties were diminished by high recombination losses. Thus, we have studied the application of nanohole structures to GaAs emitter solar cells and investigated the cells' antireflection and photovoltaic properties as a function of the nanohole structure and emitter thickness. Due to decreased surface reflection and improved shunt resistance, the solar cell efficiency increased from 4.25% for non-textured solar cells to 7.15% for solar cells textured for 5 min.

Bioinspired Omnidirectional Self-Stable Reflectors with Multiscale Hierarchical Structures

Structured surfaces, demonstrating various wondrous physicochemical performances, are ubiquitous phenomena in nature. Butterfly wings with impressive structural colors are an interesting example for multiscale hierarchical structures (MHSs). However, most natural structural colors are relatively unstable and highly sensitive to incident angles, which limit their potential practical applications to a certain extent. Here, we reported a bioinspired color reflector with omnidirectional reflective self-stable (ORS) properties, which is inspired by the wing scales of Papilio palinurus butterfly. Through experimental exploration and theoretical analysis, it was found that the vivid colors of such butterfly wings are structure-based and possess novel ORS properties, which attributes to the multiple optical actions between light and the complex structures coupling the inverse opal-like structures (IOSs) and stacked lamellar ridges (SLRs). On the basis of this, we designed and successfully fabricated the SiO₂-based bioinspired color reflectors (BCRs) through a facile and effective biotemplate method. It was confirmed that the MHSs in biotemplate are inherited by the obtained SiO₂-based BCRs. More importantly, the SiO₂-based BCRs also demonstrated the similar ORS properties in a wide wavelength range. We forcefully anticipate that the reported MHS-based ORS performance discovered in butterfly wing scales here could offer new thoughts for scientists to solve unstable reflection issues in particular optical field. The involved biotemplate fabrication method offers a facile and effective strategy for fabricating functional nanomaterials or bioinspired nanodevices with 3D complex nanostructures, such as structured optical devices, displays, and optoelectronic equipment.

Model for large-area monolayer coverage of polystyrene nanospheres by spin coating

Nanosphere lithography, an inexpensive and high throughput technique capable of producing nanostructure (below 100 nm feature size) arrays, relies on the formation of a monolayer of self-assembled nanospheres, followed by custom-etching to produce nanometre size features on large-area substrates. A theoretical model underpinning the self-ordering process by centrifugation is proposed to describe the interplay between the spin speed and solution concentration. The model describes the deposition of a dense and uniform monolayer by the implicit contribution of gravity, centrifugal force and surface tension, which can be accounted for using only the spin speed and the solid/liquid volume ratio. We demonstrate that the spin recipe for the monolayer formation can be represented as a pathway on a 2D phase plane. The model accounts for the ratio of polystyrene nanospheres (300 nm), water, methanol and surfactant in the solution, crucial for large area uniform and periodic monolayer deposition. The monolayer is exploited to create arrays of nanoscale features using ‘short’ or ‘extended’ reactive ion etching to produce 30–60 nm (diameter) nanodots or 100–200 nm (diameter) nanoholes over the entire substrate, respectively. The nanostructures were subsequently utilized to create master stamps for nanoimprint lithography.

Nano/Micro-Manufacturing of Bioinspired Materials: a Review of Methods to Mimic Natural Structures

Through billions of years of evolution and natural selection, biological systems have developed strategies to achieve advantageous unification between structure and bulk properties. The discovery of these fascinating properties and phenomena has triggered increasing interest in identifying characteristics of biological materials, through modern characterization and modeling techniques. In an effort to produce better engineered materials, scientists and engineers have developed new methods and approaches to construct artificial advanced materials that resemble natural architecture and function. A brief review of typical naturally occurring materials is presented here, with a focus on chemical composition, nano-structure, and architecture. The critical mechanisms underlying their properties are summarized, with a particular emphasis on the role of material architecture. A review of recent progress on the nano/micro-manufacturing of bio-inspired hybrid materials is then presented in detail. In this case, the focus is on nacre and bone-inspired structural materials, petals and gecko foot-inspired adhesive films, lotus and mosquito eye inspired superhydrophobic materials, brittlestar and Morpho butterfly-inspired photonic structured coatings. Finally, some applications, current challenges and future directions with regard to manufacturing bio-inspired hybrid materials are provided.

Geometrical shape design of nanophotonic surfaces for thin film solar cells

We present the effect of geometrical parameters, particularly shape, on optical absorption enhancement for thin film solar cells based on crystalline silicon (c-Si) and gallium arsenide (GaAs) using a rigorous coupled wave analysis (RCWA) method. It is discovered that the "sweet spot" that maximizes efficiency of solar cells exists for the design of nanophotonic surfaces. For the case of ultrathin, rod array is practical due to the effective optical resonances resulted from the optimum geometry whereas parabola array is viable for relatively thicker cells owing to the effective graded index profile. A specific value of thickness, which is the median value of other two devices tailored by rod and paraboloid, is optimized by truncated shape structure. It is therefore worth scanning the optimum shape of nanostructures in a given thickness in order to achieve high performance.

Antireflective Coatings: Conventional Stacking Layers and Ultrathin Plasmonic Metasurfaces, A Mini-Review

Reduction of unwanted light reflection from a surface of a substance is very essential for improvement of the performance of optical and photonic devices. Antireflective coatings (ARCs) made of single or stacking layers of dielectrics, nano/microstructures or a mixture of both are the conventional design geometry for suppression of reflection. Recent progress in theoretical nanophotonics and nanofabrication has enabled more flexibility in design and fabrication of miniaturized coatings which has in turn advanced the field of ARCs considerably. In particular, the emergence of plasmonic and metasurfaces allows for the realization of broadband and angular-insensitive ARC coatings at an order of magnitude thinner than the operational wavelengths. In this review, a short overview of the development of ARCs, with particular attention paid to the state-of-the-art plasmonic- and metasurface-based antireflective surfaces, is presented.

Optimized antireflective silicon nanostructure arrays using nanosphere lithography

Broadband optical antireflective arrays of sub-wavelength structures were fabricated on silicon substrates using colloidal nanosphere lithography in conjunction with reactive ion etching. The morphology of the nanostructures, including the shape, base diameter and height, was precisely controlled by modifying the conventional process of nanosphere lithography. We investigated their effects on the optical characteristics based on experimentally measured reflectance performance. The Si nanostructure arrays demonstrated optical antireflection performance with an average reflectance of about 1% across the spectral range from 300 to 800 nm, i.e. near-ultraviolet to visible wavelengths. This fabrication method can be used to create a large surface area and offers a promising approach for antireflective applications.

A High-Transmission, Multiple Antireflective Surface Inspired from Bilayer 3D Ultrafine Hierarchical Structures in Butterfly Wing Scales

A high-transmission, multiple antireflective surface inspired by bilayer 3D ultrafine hierarchical structures in butterfly wing scales is fabricated on a glass substrate using wet chemical biomimetic fabrication. Interestingly, the biomimetic antireflective surface exhibits excellent antireflective properties and high transmission, which provides better characteristics than the butterfly wings and can significantly reduce reflection without losing transparency. These findings offer a new path for generating nanostructured antireflectors with high transmission properties.

Highly-ordered silicon inverted nanocone arrays with broadband light antireflectance

In this work, highly-ordered silicon inverted nanocone arrays are fabricated by integration of nanosphere lithography with reactive ion etching (RIE) method. The optical characteristics of as-prepared Si inverted nanocone arrays are investigated both by experiments and simulations. It is found that the Si nanocone arrays present excellent broadband light antireflectance properties, which are attributed to the gradient in the effective refractive index of nanocones and enhanced light trapping owing to optical diffraction. The inverted Si nanocone arrays might find a variety of applications in solar cells and photodetectors.

Inverted nanocone-based thin film photovoltaics with omnidirectionally enhanced performance

Thin film photovoltaic (PV) technologies are highly attractive for low-cost solar energy conversion and possess a wide range of potential applications from building-integrated PV generation to portable power sources. Inverted nanocones (i-cones) have been demonstrated as a promising structure for practical thin film PV devices/modules, owning to their antireflection effect, self-cleaning function, superior mechanical robustness, and so forth. In this work, we have demonstrated a low-cost and scalable approach to achieve perfectly ordered i-cone arrays. Thereafter, thin film amorphous silicon (a-Si:H) solar cells have been fabricated based on various i-cone substrates with different aspect ratios and pitches to investigate the impact of geometry of i-cone nanostructures on the performance of the as-obtained PV devices. Intriguingly, the optical property investigations and device performance characterizations demonstrated that the 0.5-aspect-ratio i-cone-based device performed the best on both light absorption capability and energy conversion efficiency, which is 34% higher than that of the flat counterpart. Moreover, the i-cone-based device enhanced the light absorption and device performance over the flat reference device omnidirectionally. These results demonstrate a viable and convenient route toward scalable fabrication of nanostructures for high-performance thin film PV devices based on a broad range of materials.

Fabrication of large-sized two-dimensional ordered surface array with well-controlled structure via colloidal particle lithography

Epoxy resin coated glass slides were used for colloidal particle lithography, in order to prepare well-defined 2D surface arrays. Upon the assistance of a large-sized 2D colloidal single crystal as template, centimeter-sized ordered surface arrays of bowl-like units were obtained. Systematic studies revealed that the parameters of obtained surface arrays could be readily controlled by some operational factors, such as temperature, epoxy resin layer thickness, and template particle size. With epoxy resin substituting for normal linear polymer, the height/diameter ratio of bowls in the formed surface arrays can be largely increased. With further reactive plasma etching, the parameters of ordered surface arrays could be finely tuned through controlling etching time. This study provides a facile way to prepare large-sized 2D surface arrays with tunable parameters.

Microstructured gradient-index antireflective coating fabricated on a fiber tip with direct laser writing

We present a simple broadband gradient-index antireflective coating, fabricated directly on a single mode telecom fiber tip. A regular array of hemi-ellipsoidal protrusions significantly reduce the Fresnel reflection from the glass-air interface. The parameters of the structure were optimized with numerical simulation for the best performance at and around 1550 nm and the coating was fabricated with Direct Laser Writing. The measured reflectance decreased by a factor of 30 at 1550 nm and was below 0.28% for the 100 nm spectral band around the central wavelength. Compared to quarter wavelength antireflective coatings the demonstrated approach offers significantly reduced technological challenges, in particular processing of a single optical material with low sensitivity to imperfections in the fabrication process.

Numerical Modeling of Sub-Wavelength Anti-Reflective Structures for Solar Module Applications

This paper reviews the current progress in mathematical modeling of anti-reflective subwavelength structures. Methods covered include effective medium theory (EMT), finite-difference time-domain (FDTD), transfer matrix method (TMM), the Fourier modal method (FMM)/rigorous coupled-wave analysis (RCWA) and the finite element method (FEM). Time-based solutions to Maxwell's equations, such as FDTD, have the benefits of calculating reflectance for multiple wavelengths of light per simulation, but are computationally intensive. Space-discretized methods such as FDTD and FEM output field strength results over the whole geometry and are capable of modeling arbitrary shapes. Frequency-based solutions such as RCWA/FMM and FEM model one wavelength per simulation and are thus able to handle dispersion for regular geometries. Analytical approaches such as TMM are appropriate for very simple thin films. Initial disadvantages such as neglect of dispersion (FDTD), inaccuracy in TM polarization (RCWA), inability to model aperiodic gratings (RCWA), and inaccuracy with metallic materials (FDTD) have been overcome by most modern software. All rigorous numerical methods have accurately predicted the broadband reflection of ideal, graded-index anti-reflective subwavelength structures; ideal structures are tapered nanostructures with periods smaller than the wavelengths of light of interest and lengths that are at least a large portion of the wavelengths considered.

Enhancing photovoltaics with broadband high-transparency glass using porosity-tuned multilayer silica nanoparticle anti-reflective coatings

Multilayer stacks of nanoparticle films with varying degrees of porosity were fabricated by sequentially spin coating and sintering silica/polystyrene mixtures. This increased the average transmittance of glass, giving the highest reported values for facile synthesized multilayer structures.

Fabrication and functionalization of periodically aligned metallic nanocup arrays using colloidal lithography with a sinusoidally wrinkled substrate

We propose a general strategy for fabricating ultrasmall attoliter-sized (10(-18) L) one-dimensional (1D) aligned nanocup arrays embedded in poly(dimethylsiloxane) (PDMS) films based on a combination of colloidal soft-lithography and wrinkle processing. The nanocup consists of a metallic shell (silver-single or double-layer silver/gold type) with a thickness of several tens of nanometers and whose diameter was ca. 500 nm and cavity depth was ca. 250 nm. First, monodisperse polystyrene (PS) colloids (d = 500 nm) were arranged onto a sinusoidally wrinkled PDMS substrate. Then, the colloid particle arrays were transferred onto another flat PDMS substrate, and a metal film was vacuum deposited over the array to form a nanostructured surface consisting of half-shell metal-coated colloid particle arrays. After the metal-coated PS array was gently transferred onto another soft PDMS substrate prepared by nonthermal curing, the attached films were thermally cured. After that, both films were carefully separated to selectively transfer the metal-coated PS particle arrays, since the metallic shell on the PS surface can adhere to the soft PDMS. Finally, the PS colloids were removed by plasma etching, leaving behind the 1D hemispherical metallic shells, called here the "metallic nanocup array structure". This structure was evaluated by performing atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy measurements. We further demonstrate chemical modification of the inner nanocup surface through construction of a self-assembled monolayer, and we also fill them with nanomaterials (silica nanoparticles) to demonstrate their application to size-selecting devices. The obtained metallic nanocup arrays could be components in a new class of chemical and/or biological nanoreactors with small reaction vessels, surface-enhanced Raman scattering (SERS)-based sensors, and size separators for nanoparticles.

Optimal moth eye nanostructure array on transparent glass towards broadband antireflection

Broadband antireflection (AR) is essential for improving the photocurrent generation of photovoltaic modules or the enhancement of visibility in optical devices. Beyond conventional AR coating methods, moth eye mimicking nanostructures give new directions to enhance broadband antireflection through the selection of geometrical parameters, such as height, periodic distance, shape, and arrangement. This study numerically and experimentally investigates the behavior of light on complex nanostructures designed to mimic the surface of the moth eye with mixed shapes and various arrangements. To obtain broadband AR, we rigorously study the design parameters, such as height, periodic distance, shape, and arrangement, on a transparent quartz substrate. Several kinds of nanopillar arrays are elaborately fabricated including mixed nanostructures comprising pointy and round shapes in ordered and random arrangements via colloidal lithography. The optimal morphology of moth eye nanostructure arrays for broadband antireflection is suggested in view of reflectance and average weight transmittance.

Antireflection effects at nanostructured material interfaces and the suppression of thin-film interference

Thin-film interference is a well-known effect, and it is commonly observed in the colored appearance of many natural phenomena. Caused by the interference of light reflected from the interfaces of thin material layers, such interference effects can lead to wavelength and angle-selective behavior in thin-film devices. In this work, we describe the use of interfacial nanostructures to eliminate interference effects in thin films. Using the same principle inspired by moth-eye structures, this approach creates an effective medium where the index is gradually varying between the neighboring materials. We present the fabrication process for such nanostructures at a polymer-silicon interface, and experimentally demonstrate its effectiveness in suppressing thin-film interference. The principle demonstrated in this work can lead to enhanced efficiency and reduce wavelength/angle sensitivity in multilayer optoelectronic devices.

Efficient light absorption with integrated nanopillar/nanowell arrays for three-dimensional thin-film photovoltaic applications

Efficient light absorption in thin-film photovoltaic (PV) devices is crucial for improving their efficiency and reducing cost. Here we have not only developed a low-cost and scalable method to fabricate a unique type of integrated-nanopillar-nanowell (i-NPW) structure by integrating nanopillar and nanowell arrays together vertically, but also demonstrated the attractive optical property of the i-NPW arrays by leveraging the advantages of "positive" and "negative" nanostructures for photon harvesting. Impressively, the 2 μm thick i-NPW arrays with only 40 nm a-Si coating obtained a day-integrated absorption of 89.27%, as opposed to only 33.45% for the planar control sample. These results suggest the feasibility and clear advantage of vertical integration of three-dimensional (3-D) nanophotonic structures, and meanwhile also pave a viable and convenient way toward a 3-D ultrathin film PV module with potency for high energy conversion efficiency.

Fabrication of metal nanoparticle arrays by controlled decomposition of polymer particles

We report a novel fabrication method for ordered arrays of metal nanoparticles that exploits the uniform arrangement of polymer beads deposited as close-packed monolayers. In contrast to colloidal lithography that applies particles as masks, we used thermal decomposition of the metal-covered particles to precisely define metal structures. Large arrays of noble metal (Au, Ag, Pt) nanoparticles were produced in a three-step process on silicon, fused silica and sapphire substrates, demonstrating the generality of this approach. Polystyrene spheres with diameters ranging between 110 nm and 1 μm were convectively assembled into crystalline monolayers, coated with metal and annealed in a resistive furnace or using an ethanol flame. The thermal decomposition of the polymer microspheres converted the metal layer into particles arranged in hexagonal arrays that preserved the order of the original monolayer. Both the particle size and the interparticle distance were adjusted via the thickness of the metal coating and the sphere diameter, respectively.

Optimal broadband antireflective taper

We have solved the optimal taper design for minimizing reflection over a broad frequency band for a given tapered length. We believe this work will lead to significant improvement in developing broadband antireflective devices in many areas.

Experimental and simulation studies of anti-reflection sub-micron conical structures on a GaAs substrate

In order to reduce surface reflection, anti-reflective (AR) coatings are widely used on the surfaces of solar cells to improve the efficiency of photoelectric conversion. This study employed colloidal lithography with a dry etching process to fabricate sub-micron anti-reflection structures on a GaAs substrate. Etching parameters, such as RF power and etching gas were investigated to determine their influence on surface morphology. We fabricated an array of conical structures 550 nm in diameter and 450 nm in height. The average reflectance of a bare GaAs wafer was reduced from 35.0% to 2.3% across a spectral range of 300 nm - 1200 nm. The anti-reflective performance of SWSs was also calculated using Rigorous Coupled Wave Analysis (RCWA) method. Both simulation and experiment results demonstrate a high degree of similarity.

Improved antireflection properties of moth eye mimicking nanopillars on transparent glass: flat antireflection and color tuning

The sub-wavelength structures in moth eyes exhibit fascinating antireflective properties over the broadband wavelength region and at large incident angle by generating an air-mixed heterogeneous optical interface. In this work, antireflective behavior of transparent glass is observed with the elaborate controls of the nanopillar morphology. The reflectance spectrum shows a red shift and a notable light scattering with increase of the height of the nanopillars. The nanopillar arrays with a pointed cone shape have better optical performance in visible range than the rounded cone shape which is typical antireflective nanostructures in nature. Based on the observed antireflective behaviors, the flat and low value reflectance spectrum in the visible wavelength range is demonstrated by moth eye mimicking nanostructures on both sides of a glass surface. It is a unique strategy to realize a flat and broadband spectrum in the visible range showing 99% transparency via the appropriate matching of nanopillar height on the front and back sides of glass. The controlled reflection based color tuning on the antireflective and transparent glass is also obtained by adjusting the height of the nanopillar arrays on both sides. The visibility and self-cleaning ability of moth eye mimicking glass are examined for practical applications such as antireflection and self-cleaning.

Durability of stochastic antireflective structures--analyses on damage thresholds and adsorbate elimination

We fabricated stochastic antireflective structures (ARS) and analyzed their stability against high power laser irradiation and high temperature annealing. For 8 ps pulse duration and 1030 nm wavelength we experimentally determined their laser induced damage threshold to 4.9 (±0.3) J/cm(2), which is nearly as high as bulk fused silica with 5.6 (±0.3) J/cm(2). A commercial layer stack reached 2.0 (±0.2) J/cm(2). An annealing process removed adsorbed organics, as shown by XPS measurements, and significantly increased the transmission of the ARS. Because of their monolithic build the ARS endure such high temperature treatments. For more sensitive samples an UV irradiation proved to be capable. It decreased the absorbed light and reinforced the transmission.

Efficient photon capturing with ordered three-dimensional nanowell arrays

Unique light-matter interaction at nanophotonic regime can be harnessed for designing efficient photonic and optoelectronic devices such as solar cells, lasers, and photodetectors. In this work, periodic photon nanowells are fabricated with a low-cost and scalable approach, followed by systematic investigations of their photon capturing properties combining experiments and simulations. Intriguingly, it is found that a proper periodicity greatly facilitates photon capturing process in the nanowells, primarily owing to optical diffraction. Meanwhile, the nanoengineered morphology renders the nanostructures with a broad-band efficient light absorption. The findings in this work can be utilized to implement a new type of nanostructure-based solar cells. Also, the methodology applied in this work can be generalized to rational design of other types of efficient photon-harvesting devices.

Enhancement in broadband and quasi-omnidirectional antireflection of nanopillar arrays by ion milling

A new technique is developed to fabricate biomimetic antireflection coatings (ARCs). This technique combines a bottom-up fabrication approach (glancing angle deposition, or GLAD) with a top-down engineering process (ion milling). The GLAD technique is first utilized to produce nanopillar arrays (NPAs) with broadened structures, which are subsequently transformed into biomimetic tapered geometries by means of post-deposition ion milling. This structure transformation, due to milling-induced mass redistribution, remarkably decreases reflection over a wide wavelength range (300-1700 nm) and field of view (angle of incidence < 60° with respect to the substrate normal). The milling-induced antireflection enhancement has been demonstrated in the NPAs made of Si, SiO(x) and TiO(2), illustrating that this integrated technique is readily adapted to a wide variety of materials. Good agreement between simulation and experiment indicates that the enhanced antireflection performance is ascribed to a smoother refractive index transition from the substrate to the air, which improves the impedance match and reduces reflection losses. Additionally, ion bombardment tends to alter the stoichiometry and diminish the crystallographic structure of the NPA materials. The broadband and quasi-omnidirectional antireflection observed establishes the strong competitiveness of this technique with the methods previously reported.

Experimental design applied to spin coating of 2D colloidal crystal masks: a relevant method?

Monolayers of colloidal spheres are used as masks in nanosphere lithography (NSL) for the selective deposition of nanostructured layers. Several methods exist for the formation of self-organized particle monolayers, among which spin coating appears to be very promising. However, a spin coating process is defined by several parameters like several ramps, rotation speeds, and durations. All parameters influence the spreading and drying of the droplet containing the particles. Moreover, scientists are confronted with the formation of numerous defects in spin coated layers, limiting well-ordered areas to a few micrometers squared. So far, empiricism has mainly ruled the world of nanoparticle self-organization by spin coating, and much of the literature is experimentally based. Therefore, the development of experimental protocols to control the ordering of particles is a major goal for further progress in NSL. We applied experimental design to spin coating, to evaluate the efficiency of this method to extract and model the relationships between the experimental parameters and the degree of ordering in the particles monolayers. A set of experiments was generated by the MODDE software and applied to the spin coating of latex suspension (diameter 490 nm). We calculated the ordering by a homemade image analysis tool. The results of partial least squares (PLS) modeling show that the proposed mathematical model only fits data from strictly monolayers but is not predictive for new sets of parameters. We submitted the data to principal component analysis (PCA) that was able to explain 91% of the results when based on strictly monolayered samples. PCA shows that the ordering was positively correlated to the ramp time and negatively correlated to the first rotation speed. We obtain large defect-free domains with the best set of parameters tested in this study. This protocol leads to areas of 200 μm(2), which has never been reported so far.

Transmission enhanced optical lenses with self-organized antireflective subwavelength structures for the UV range

We present transmission increased fused silica lenses produced by using self-organized antireflective structures for which we developed an efficient manufacturing process. The spectral transmission measured over the whole lens aperture shows a significant transmission enhancement of up to 3.5% in the UV range. Local measurements on the lens's surface reveal a strongly reduced reflection of below 0.1% for 300 nm wavelength, which is homogeneous over the whole lens. Further, the lenses show a broadband spectral antireflection behavior. For 600 nm wavelength the reflection was measured at about 1%.

Biomimetic nanostructured antireflection coating and its application on crystalline silicon solar cells

In this report, we demonstrate the implementation of biomimetic nanostructured antireflection coatings with polymethyl methacrylate (PMMA) layer on the micro-textured surface of silicon crystalline solar cells. To reduce cost, the process combines colloidal lithography, cast molding method, and reversal nanoimprint lithography. The technique is simple, low cost, and does not cause damage to the thin and brittle conventional crystalline solar cells. The antireflection properties of this biomimetic nanostructure coating are considered as effective as those of a conventional single-layer SiNx thin film. The resultant structures alone could reduce the average reflectance of solar cell from 13.2% to 7.8% and enhance power conversion efficiency from 12.85% to 14.2%.

Nanostructured gradient-index antireflection diffractive optics

We describe the fabrication and characterization of a nanostructured diffractive element with near-zero reflection losses. In this element, subwavelength nanostructures emulating adiabatic index matching are integrated on the surface of a diffractive microstructure to suppress reflected diffraction orders. The fabricated silicon grating exhibits reflected efficiencies that are suppressed by 2 orders of magnitude over broad wavelength bands and wide incident angles. Theoretical models of the fabricated structure based on rigorous coupled-wave analysis and effective medium theory are in agreement with the experimental data. The proposed principles can be applied to improve the performance of any diffractive structures, potentially leading to more efficient Fresnel lenses, holographic elements, and integrated optical systems.

From two-dimensional colloidal self-assembly to three-dimensional nanolithography

A number of "top-down" lithographic and "bottom-up" self-assembly methods have been developed to fabricate three-dimensional (3D) nanostructures to support the recent advances in nanotechnology. But they are limited by a number of factors such as fabrication cost, pattern resolution, and/or flexibility of geometry. Here we present a 3D nanolithography process that utilizes self-assembled nanospheres to create a periodic array of focal spots, which are then replicated across multiple depth in a transparent medium according to the Talbot effect. The Talbot field then exposes a pattern onto the underlying photoresist, recording the 3D intensity distribution. We have demonstrated designable complex 3D periodic structures with 80 nm minimum feature size, roughly one-fourth of the operating wavelength. This approach combines 2D colloidal self-assembly and 3D phase lithography, is robust, cost-effective, and widely applicable to nanoscale research and manufacturing.

Fabrication of GaAs subwavelength structure (SWS) for solar cell applications

We developed a novel GaAs subwavelength structure (SWS) as an antireflective layer for solar cell applications. The GaAs SWS patterns were fabricated by a combination of nanosphere lithography (NSL) and reactive ion etching (RIE). The shape and height of the GaAs SWS were controlled by the diameter of the SiO2 nanospheres and the etching time. Various GaAs SWS were characterized by the reflectance spectra. The average reflectance of the polished GaAs substrate from 200nm to 800nm was 35.1%. However, the average reflectance of the tapered GaAs SWS was reduced to 0.6% due to scattering and moth-eye effects.

Minimizing light reflection from dielectric textured surfaces

In this paper, we consider antireflective properties of textured surfaces for all texture size-to-wavelength ratios. Existence and location of the global reflection minimum with respect to geometrical parameters of the texture is a subject of our study. We also investigate asymptotic behavior of the reflection with the change of the texture geometry for the long and short wavelength limits. As a particular example, we consider silicon-textured surfaces used in solar cells technology. Most of our results are obtained with the help of the finite-difference time-domain (FDTD) method. We also use effective medium theory and geometric optics approximation for the limiting cases. The FDTD results for these limits are in agreement with the corresponding approximations.

Biomimetic antireflective hierarchical arrays

We report a simple method for the fabrication of biomimetic antireflective hierarchical arrays based on the combination of self-assembled polymer spheres and nanoimprint lithography (NIL). The hierarchical structures are fabricated by creating nanopillars on the microscale round protrusion arrays, which are similar to natural mosquito eyes consisting of combined micro- and nanostructures. The hierarchical arrays dramatically suppress the surface reflection from visible to near-infrared regions with an angle of incidence of up to 70°.

Nonlinear dependence between the surface reflectance and the duty-cycle of semiconductor nanorod array

The nonlinear dependence between the duty-cycle of semiconductor nanorod array and its surface reflectance minimization is demonstrated. The duty-cycle control on thin-SiO2 covered Si nanorod array is performed by O(2-) plasma pre-etching the self-assembled polystyrene nanosphere array mask with area density of 4 × 10(8) rod/cm(-2). The 120-nm high SiO2 covered Si nanorod array is obtained after subsequent CF4/O2 plasma etching for 160 sec. This results in a tunable nanorod diameter from 445 to 285 nm after etching from 30 to 80 sec, corresponding to a varying nanorod duty-cycle from 89% to 57%. The TM-mode reflection analysis shows a diminishing Brewster angle shifted from 71° to 54° with increasing nanorod duty-cycle from 57% to 89% at 532 nm. The greatly reduced small-angle reflectance reveals a nonlinear trend with enlarging duty-cycle, leading to a minimum surface reflectance at nanorod duty-cycle of 85%. Both the simulation and experiment indicate that such a surface reflectance minimum is even lower than that of a uniformly SiO2 covered Si substrate on account of its periodical nanorod array architecture with tuned duty-cycle.

Colloidal self-assembly meets nanofabrication: from two-dimensional colloidal crystals to nanostructure arrays

Self-assembly of colloidal microspheres or nanospheres is an effective strategy for fabrication of ordered nanostructures. By combination of colloidal self-assembly with nanofabrication techniques, two-dimensional (2D) colloidal crystals have been employed as masks or templates for evaporation, deposition, etching, and imprinting, etc. These methods are defined as "colloidal lithography", which is now recognized as a facile, inexpensive, and repeatable nanofabrication technique. This paper presents an overview of 2D colloidal crystals and nanostructure arrays fabricated by colloidal lithography. First, different methods for fabricating self-assembled 2D colloidal crystals and complex 2D colloidal crystal structures are summarized. After that, according to the nanofabrication strategy employed in colloidal lithography, related works are reviewed as colloidal-crystal-assisted evaporation, deposition, etching, imprinting, and dewetting, respectively.