Antireflection behavior of multidimensional nanostructures patterned using a conformable elastomeric phase mask in a single exposure step

Through-hole composite membrane with an ultrathin oxide shell for highly robust and transparent air filters

Exploring pore structures that are optically transparent and have high filtration efficiency for ultrafine dust is very important for realizing passive window filters for indoor air purification. Herein, a polyester track-etched (PETE) membrane with vertically perforated micropores is investigated as a cost-effective candidate for transparent window filters. The pore size, which governs transparency and filtration efficiency, can be precisely tuned by conformally depositing an ultrathin oxide layer on the PETE membrane via atomic layer deposition. The maximum visible light transmittance (∼81.2 %) was achieved with an alumina layer of approximately 55 nm, and the resulting composite membrane exhibited competitive filtration efficiency compared to commercial products. The chemically inert alumina layer also increased resistance to various external stimuli and enabled simple cleaning of the contaminated membrane surface with a solvent. The membrane installed on an insect screen effectively maintained its filtration performance (∼85 % for PM_2.5) even after 10 reuse cycles under extremely harsh conditions (PM_2.5 concentration: ∼5000 μg cm^-3). The proposed through-hole composite membrane can expand the choice of aesthetic window filters to situations that require high outside visibility and daylighting.

Optically Activated 3D Thin-Shell TiO2 for Super-Sensitive Chemoresistive Responses: Toward Visible Light Activation

One of the well-known strategies for achieving high-performance light-activated gas sensors is to design a nanostructure for effective surface responses with its geometric advances. However, no study has gone beyond the benefits of the large surface area and provided fundamental strategies to offer a rational structure for increasing their optical and chemical performances. Here, a new class of UV-activated sensing nanoarchitecture made of highly periodic 3D TiO2, which facilitates 55 times enhanced light absorption by confining the incident light in the nanostructure, is prepared as an active gas channel. The key parameters, such as the total 3D TiO2 film and thin-shell thicknesses, are precisely optimized by finite element analysis. Collectively, this fundamental design leads to ultrahigh chemoresistive response to NO2 with a theoretical detection limit of ≈200 ppt. The demonstration of high responses with visible light illumination proposes a future perspective for light-activated gas sensors based on semiconducting oxides.

Electroplated Functional Materials with 3D Nanostructures Defined by Advanced Optical Lithography and Their Emerging Applications

Electroplating has been favored to date as a surface treatment technology in various industries in the development of semiconductors, automobiles, ships, and steel due to its advantages of being a simple, solution-based process, with low cost and high throughput. Recently, classical electroplating has been reborn as an advanced manufacturing process for functional materials by combining it with unconventional optical three-dimensional (3D) nanofabrication techniques capable of generating polymer templates with high-resolution 3D periodic nanostructures. The bottom-up filling behavior of electroplating rising from a seed layer makes it possible to densely fill the nanoporous network of the template with heterogeneous inorganic materials. At this time, understanding and optimizing the process parameters (e.g., additive, current density, type of current waveform, etc.) of electroplating is critical for defect control. In addition, since electroplating is generally performed near room temperature, unlike other thin film deposition techniques, structural damage to the polymer template by heat during electroplating is almost negligible. Based on the excellent compatibility of electroplating and optical 3D nanofabrication, innovative functional materials with 3D periodic nanostructures targeting electrochemical or energy-related applications have been created. In this mini review, a strategy for producing functional materials with 3D periodic nanostructures through a templating process will be covered, and the recent cases of successful applications to electrodes for energy storage devices, electrocatalysts, and thermoelectric materials will be summarized. We will also discuss technical issues that need to be considered in the process to improve the quality of the resulting functional materials with 3D nanoarchitectures.

Focused Electric-Field Polymer Writing: Toward Ultralarge, Multistimuli-Responsive Membranes

The cost-effective direct writing of polymer nanofibers (NFs) has garnered considerable research attention as a compelling one-pot strategy for obtaining key building blocks of electrochemical and optical devices. Among the promising applications, the changes in optical response from external stimuli such as mechanical deformation and changes in the thermal environment are of great significance for emerging applications in smart windows, privacy protection, aesthetics, artificial skin, and camouflage. Herein, we propose a rational design for the mass production of customized NFs through the development of focused electric-field polymer writing (FEPW) coupled with the roll-to-roll technique. As a proof of key applications, we demonstrate multistimuli-responsive (mechano- and thermochromism) membranes with an exceptional production scale (over 300 cm²). Specifically, the membranes consist of periodically aligned ultrathin (∼60 nm) alumina nanotubes inserted in the elastomers. We performed a two-phase finite element analysis of the unit cells to verify the underlying physics of light scattering at heterogeneous interfaces of the strain-induced air gaps. By adding thermochromic dye during the FEPW, the optical modulation of transmittance change (∼83% to 37% at visible wavelength) was successfully extended to high-contrast thermal-dependent coloration.

High-Contrast Optical Modulation from Strain-Induced Nanogaps at 3D Heterogeneous Interfaces

The realization of high-contrast modulation in optically transparent media is of great significance for emerging mechano-responsive smart windows. However, no study has provided fundamental strategies for maximizing light scattering during mechanical deformations. Here, a new type of 3D nanocomposite film consisting of an ultrathin (≈60 nm) Al2O3 nanoshell inserted between the elastomers in a periodic 3D nanonetwork is proposed. Regardless of the stretching direction, numerous light-scattering nanogaps (corresponding to the porosity of up to ≈37.4 vol%) form at the interfaces of Al2O3 and the elastomers under stretching. This results in the gradual modulation of transmission from ≈90% to 16% at visible wavelengths and does not degrade with repeated stretching/releasing over more than 10 000 cycles. The underlying physics is precisely predicted by finite element analysis of the unit cells. As a proof of concept, a mobile-app-enabled smart window device for Internet of Things applications is realized using the proposed 3D nanocomposite with successful expansion to the 3 × 3 in. scale.

Suspended-Template Electric-Assisted Nanoimprinting for Hierarchical Micro-Nanostructures on a Fragile Substrate

Coating hierarchical micro-nanostructures on the surface of optoelectronic devices has been demonstrated to improve the overall performance. However, fabricating desired structures on a fragile optoelectronic device substrate is still challenging. A suspended-template electric-assisted nanoimprinting technique is proposed herein to controllably fabricate hierarchical micro-nanostructures on a fragile substrate. The suspension design of the template ensures that it conveniently deforms to fully fit the surface fluctuation of the substrate. The deformation of template and the filling of liquid polymer in the micro/nanocavities of the template are both driven by the powerful surface/interface force generated by an electric field applied between the template and substrate surface, thus protecting the fragile substrate from squeezing damage. Different morphologies of hierarchical micro-nanostructures are fabricated by changing the electric field. Based on suspended-template electric-assisted nanoimprinting, the environmentally adaptable fully covering hierarchical micro-nanostructures are encapsulated on the surface of flip-film light-emitting diode chips, thus significantly enhancing their light management in complex environments.

Three-Component Regio- and Stereoselective Polymerizations toward Functional Chalcogen-Rich Polymers with AIE-Activities

Polymers containing rich chalcogen elements are rarely reported due to the lack of facile synthesis methods. Herein, a novel multicomponent polymerization route toward chalcogen-rich polymers was introduced. A series of poly(vinyl sulfones) (PVSs) were synthesized at room temperature using readily prepared monomers. PVSs were generated with high regio- and stereo-selectivity in high yields (up to 92.3%). Rich chalcogen elements endowed PVSs with distingctive multifunctionalities. The PVSs possessed good solubility and film-forming ability. Their thin films exhibited outstanding refractive indices up to 1.8062 at 550.0 nm together with good optical transparency in the visible region. Thin films of some polymers can also be fabricated into well-resolved fluorescent photopatterns by photolithography. Thanks to the unique redox properties of selenium, postmodification by oxidation reaction of P1a/2/3a successfully eliminates the caused heavy atom effect and endow resulting polymers with novel functionality as fluorescent bioprobes for cellular imaging.

Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks

Atomic layer deposition (ALD) is a unique tool for conformally depositing inorganic thin films with precisely controlled thickness at nanoscale. Recently, ALD has been used in the manufacture of inorganic thin films using a three-dimensional (3D) nanonetwork structure made of polymer as a template, which is pre-formed by advanced 3D nanofabrication techniques such as electrospinning, block-copolymer (BCP) lithography, direct laser writing (DLW), multibeam interference lithography (MBIL), and phase-mask interference lithography (PMIL). The key technical requirement of this polymer template-assisted ALD is to perform the deposition process at a lower temperature, preserving the nanostructure of the polymer template during the deposition process. This review focuses on the successful cases of conformal deposition of inorganic thin films on 3D polymer nanonetworks using thermal ALD or plasma-enhanced ALD at temperatures below 200 °C. Recent applications and prospects of nanostructured polymer–inorganic composites or hollow inorganic materials are also discussed.

Emergence of New Density-Strength Scaling Law in 3D Hollow Ceramic Nanoarchitectures

Density-strength tradeoff appears to be an inherent limitation for most materials and therefore design of cell topology that mitigates strength decrease with density reduction has been a long-lasting engineering pursue for porous materials. Continuum-mechanics-based analyses of mechanical responses of conventional porous materials with bending-dominated structures often give the density-strength scaling law following the power-law relationship with an exponent of 1.5 or higher, which consequentially determines the upper bound of the specific strength for a material to reach. In this work, a new design criterion capable of significantly abating strength degradation in lightweight materials is presented, by successfully combining the size-induced strengthening effect in nanomaterials with the architectural design of cellular porous materials. Hollow-tube-based 3D ceramic nanoarchitectures satisfying such criterion are fabricated in large area using proximity field nano-patterning and atomic layer deposition. Experimental data from micropillar compression confirm that the strengths of these nanoarchitectural materials scale with relative densities with a power-law exponent of 0.93, a hardly observable value in conventional bending-dominated porous materials. This discovery of a new density-strength scaling law in nanoarchitectured materials will contribute to creating new lightweight structural materials attaining unprecedented specific strengths overcoming the conventional limit.

3D nanostructured N-doped TiO2 photocatalysts with enhanced visible absorption

Considering the environmental issues, it is essential to develop highly efficient and recyclable photocatalysts in purification systems. Conventional TiO2 nanoparticles have strong intrinsic oxidizing power and high surface area, but are difficult to collect after use and rarely absorb visible light, resulting in low photocatalytic efficiency under sunlight. Here we develop a new type of highly efficient and recyclable photocatalyst made of a three-dimensional (3D) nanostructured N-doped TiO2 monolith with enhanced visible light absorption. To prepare the sample, an ultrathin TiN layer (∼10 nm) was conformally coated using atomic layer deposition (ALD) on 3D nanostructured TiO2. Subsequent thermal annealing at low temperature (550 °C) converted TiN to anatase phase N-doped TiO2. The resulting 3D N-doped TiO2 showed ∼33% enhanced photocatalytic performance compared to pure 3D TiO2 of equivalent thickness under sunlight due to the reduced bandgap, from 3.2 eV to 2.75 eV through N-doping. The 3D N-doped TiO2 monolith could be easily collected and reused at least 5 times without any degradation in photocatalytic performance.

Three-Dimensional Continuous Conductive Nanostructure for Highly Sensitive and Stretchable Strain Sensor

The demand for wearable strain gauges that can detect dynamic human motions is growing in the area of healthcare technology. However, the realization of efficient sensing materials for effective detection of human motions in daily life is technically challenging due to the absence of the optimally designed electrode. Here, we propose a novel concept for overcoming the intrinsic limits of conventional strain sensors based on planar electrodes by developing highly periodic and three-dimensional (3D) bicontinuous nanoporous electrodes. We create a 3D bicontinuous nanoporous electrode by constructing conductive percolation networks along the surface of porous 3D nanostructured poly(dimethylsiloxane) with single-walled carbon nanotubes. The 3D structural platform allows fabrication of a strain sensor with robust properties such as a gauge factor of up to 134 at a tensile strain of 40%, a widened detection range of up to 160%, and a cyclic property of over 1000 cycles. Collectively, this study provides new design opportunities for a highly efficient sensing system that finely captures human motions, including phonations and joint movements.

Flexible Near-Field Nanopatterning with Ultrathin, Conformal Phase Masks on Nonplanar Substrates for Biomimetic Hierarchical Photonic Structures

Multilevel hierarchical platforms that combine nano- and microstructures have been intensively explored to mimic superior properties found in nature. However, unless directly replicated from biological samples, desirable multiscale structures have been challenging to efficiently produce to date. Departing from conventional wafer-based technology, new and efficient techniques suitable for fabricating bioinspired structures are highly desired to produce three-dimensional architectures even on nonplanar substrates. Here, we report a facile approach to realize functional nanostructures on uneven microstructured platforms via scalable optical fabrication techniques. The ultrathin form (∼3 μm) of a phase grating composed of poly(vinyl alcohol) makes the material physically flexible and enables full-conformal contact with rough surfaces. The near-field optical effect can be identically generated on highly curved surfaces as a result of superior conformality. Densely packed nanodots with submicron periodicity are uniformly formed on microlens arrays with a radius of curvature that is as low as ∼28 μm. Increasing the size of the gratings causes the production area to be successfully expanded by up to 16 in(2). The "nano-on-micro" structures mimicking real compound eyes are transferred to flexible and stretchable substrates by sequential imprinting, facilitating multifunctional optical films applicable to antireflective diffusers for large-area sheet-illumination displays.

Recent advancement on micro-/nano-spherical lens photolithography based on monolayer colloidal crystals

Highly ordered nanostructures have gained substantial interest in the research community due to their fascinating properties and wide applications.Micro-/nano-spherical lens photolithography (SLPL) has been recognized as an inexpensive, inherently parallel, and high-throughput approach to the creation of highly ordered nanostructures. SLPL based on monolayer colloidal crystals (MCCs) of self-assembled colloidal micro-/nano-spheres have recently made remarkable progress in overcoming the constraints of conventional photolithography in terms of cost, feature size, tunability, and pattern complexity. In this review, we highlight the current state-of-the-art in this field with an emphasis on the fabrication of a variety of highly ordered nanostructures based on this technique and their demonstrated applications in light emitting diodes, nano-patterning semiconductors, and localized surface plasmon resonance devices. Finally, we present a perspective on the future development of MCC-based SLPL technique, including a discussion on the improvement of the quality of MCCs and the compatibility of this technique with other semiconductor micromachining process for nanofabrication.

Designing unit cell in three-dimensional periodic nanostructures using colloidal lithography

Colloidal phase-shift lithography, the illumination of a two-dimensional (2D) ordered array of self-assembled colloidal nanospheres, is an effective method for the fabrication of periodic three-dimensional (3D) nanostructures. In this work, we investigate the design and control of the unit-cell geometry by examining the relative ratio of the illumination wavelength and colloidal nanosphere diameter. Using analytical and finite-difference time-domain (FDTD) modeling, we examine the effect of the wavelength-diameter ratio on intensity pattern, lattice constants, and unit-cell geometry. These models were validated by experimental fabrication for various combination of wavelength and colloid diameter. The developed models and fabrication tools can facilitate the design and engineering of 3D periodic nanostructure for photonic crystals, volumetric electrodes, and porous materials.

3D hierarchical architectures prepared by single exposure through a highly durable colloidal phase mask

Three-dimensional hierarchical architectures are fabricated using a simple, cost-effective, durable colloidal phase mask containing a colloidal monolayer embedded in a flexible polydimethylsiloxane (PDMS) membrane. These structures give rise to a photonic bandgap that can be tuned over a wide spectral range from the visible to the near-infrared regions.

Monolithic 3D titania with ultrathin nanoshell structures for enhanced photocatalytic activity and recyclability

Titania has attracted considerable interest for use in water purification applications due to its excellent photocatalytic activity. To further improve the efficiency of photocatalysis, numerous nanostructures (i.e. nanoparticles, nanotubes, and nanowires) have been proposed to increase the surface area of titania. Despite the high photocatalytic performance of the nanostructured titania, subsequent difficulties encountered in recollection and reuse of titania inhibit the practical application for water purification systems. Here we successfully fabricate monolithic, three dimensional (3D) nanoshell titania with high uniformity over large areas (~1 × 1 inch(2)) through proximity field nanopatterning (PnP) and low-temperature atomic layer deposition (ALD) techniques. The higher surface area of 3D nanoshell titania increases the photocatalytic performance more than three-fold relative to that of a thin film of equivalent sample size. Also, the monolithic form of titania enables it to be reused without any degradation of photocatalytic activity. The newly developed nanomaterials in this study can serve as an efficient and reusable photocatalyst for water purification systems.

Size-dependent optical behavior of disordered nanostructures on glass substrates

We demonstrate the distinctive optical properties of disordered nanostructures on glass substrates in accordance with changes in the average size of the nanostructures. Dissimilar sizes of nanostructures were fabricated by using different thicknesses of thermally dewetted Ag nanoparticles as etch masks. Unlike a flat glass substrate, the nanostructured glasses (NSGs) show a changed optical characteristic. By increasing the size of the nanostructures, the wavelength of the peak transmittance of about 99% gradually moved from 730 to 2000 nm. To clearly discern the effect of the different sizes of nanostructures, the normalized angle-dependent transmittance spectra of the NSGs were analyzed. Only if the size becomes relatively larger than the wavelength of the incident light are the transmittance spectra more strongly affected by the incident angle as well as by the relative size, rather than by the Fresnel reflection.

Maskless laser tailoring of conical pillar arrays for antireflective biomimetic surfaces

Herein, we report a facile approach for rapid and maskless production of subwavelength structured antireflective surfaces with high and broadband transmittance-direct laser interference ablation. The interfered laser beams were introduced into the surface of a bare optical substrate, where structured surfaces consisting of a micropillar array were produced by two-step laser irradiation in the time frame of seconds. A multiple exposure of the two-beam interference approach was proposed instead of multiple-beam interference to simply realize planar patterns of a high aspect ratio. Tall sinusoidal pillars were created and shaped by pulse shot number control. As an example of the application, zinc sulfide substrates were processed with the technology, from which high transmission at an infrared wavelength, over 92%, at normal incidence was experimentally achieved.

Enhancement of optical transmission with random nanohole structures

We demonstrate an enhancement of optical transmission by creating randomly distributed nanoholes in a glass surface using a simple bottom-up fabrication process. V-shaped holes with sub-100 nm diameter are created by anodized aluminum oxide template and dry etching on glass substrates. The broadband and omnidirectional antireflective effect of the proposed nanostructures is confirmed by measuring the transmittance of the patterned glasses, leading to 3% better transmission. Subsequently, the short-circuit current and the open-circuit voltage of a solar cell with nanostructures are enhanced by 3-4%, improving the solar cell efficiency from 10.47% to 11.20% after two weeks of outdoor testing.