Synergistically designed antireflective cover for improving wide-angle photovoltaic efficiencies.

We demonstrated that a well-designed nanopatterned cover improves photovoltaic efficiency across a wide range of incident angles (θ). A nanopatterned cover was created using an integrated ray-wave optics simulation to maximize the light absorption of the surface-textured Si photovoltaic device. A hexagonally arranged nanocone array with a 300 nm pitch was formed into a polymer using nanoimprinting, and the nanostructured polymer was then attached to a glass cover with an index-matching adhesive. Angle-resolved current density-voltage measurements on Si photovoltaic devices showed that the nanopatterned glass cover yielded a 2-13% enhancement in power conversion efficiency at θ = 0-60°, which accounted for its broadband antireflective feature. We performed all-season-perspective simulations based on the results of the integrated ray-wave optics simulations and solar altitude database of South Korea, which validated the sustainability of the developed nanopatterned cover during significant seasonal fluctuations.

Enhancement of refractive index sensing for an infrared plasmonic metamaterial absorber with a nanogap.

An infrared plasmonic metamaterial absorber with a nanogap was numerically and experimentally investigated as a refractive index sensor. We experimentally demonstrated large enhancements of both sensitivity (approximately 1091 nm/refractive index unit) and figure of merit (FOM*; approximately 273) owing to the nanogap formation in the metamaterial absorber to achieve perfect absorption (99%). The refractive index sensing platform was fabricated by producible nanoimprint lithography and isotropic dry etching processes to have a large area and low cost while providing a practical solution for high-performance plasmonic biosensors.

Optical Tunneling Mediated Sub-Skin-Depth High Emissivity Tungsten Radiators.

Tailoring the spectrum of thermal radiation at high temperatures is a central issue in the study of thermal radiation harnessed energy resources. Although bulk metals with periodic cavities incorporated into their surfaces provide high emissivity, they require a complicated micron metal etch, thereby precluding reliable, continuous operation. Here, we report thermally stable, highly emissive, ultrathin (<20 nm) tungsten (W) radiators that were prepared in a scalable and cost-effective route. Alumina/W/alumina multiwalled, submicron cavity arrays were fabricated sequentially using nanoimprinting lithography, thin film deposition, and calcination processes. To highlight the practical importance of high-temperature radiators, we developed a thermophotovoltaic (TPV) system equipped with fabricated W radiators and low-bandgap GaSb photovoltaic cells. The TPV system produced electric power reliably during repeated temperature cycling between 500 and 1200 K; the power density at 1200 K was fixed to be approximately 1.0 W/cm2. The temperature-dependent electric power was quantitatively reproduced using a one-dimensional energy conversion model. The symmetric configuration of alumina/W/alumina multiwall together with the presence of a void inside each cavity alleviated thermal stress, which was responsible for the stable TPV performance. The short-current-density (JSC) of developed TPV system was augmented significantly by decreasing the W thickness below its skin depth. A 17 nm thick W radiator yielded a 32% enhancement in JSC compared to a 123 nm thick W radiator. Electromagnetic analysis indicated that subskin-depth W cavity arrays led to suppressed surface reflection due to the mitigated screening effect of free electrons, thereby enhancing the absorption of light within each W wall. Such optical tunneling-mediated absorption or radiation was valid for any metal material and morphology (e.g., planar or patterned).

Dual nanotransfer printing for complementary plasmonic biosensors.

One of the main challenges in the widespread utilization of localized plasmon resonance-based biosensors is the fabrication of large-area and low-cost plasmonic nanostructures. In this work, we fabricated large-area and low-cost complementary plasmonic biosensors such as nanohole and nanodisk arrays using dual nanotransfer printing (NTP) with a single metal deposition and a single reusable mold. The suspended nanohole arrays and the suspended nanodisk arrays were fabricated using the subsequent dry etching process. We confirmed a maximum enhancement in bulk sensitivity in experiments and simulations by controlling the vertical and lateral etching depths of the dielectric layer underneath the gold (Au) nanohole and nanodisk arrays. Furthermore, we show that the surface sensitivity evaluated by atomic layer deposition of aluminum oxide increased because appropriate vertical and lateral etching depths allow the target analyte to access the additional near-field formed at the bottom of the Au nanostructure. The dual NTP method provides a practical solution for the realization of large-area and low-cost label-free plasmonic biosensing systems, with a reduction in complexity and cost of the fabrication process of complementary plasmonic structures and metasurfaces.

Shape-Controlled 3D Periodic Metal Nanostructures Fabricated via Nanowelding.

A novel method for fabricating 3D metallic nanostructures to be used in polarized color filters based on nanoimprint lithography, electron-beam evaporation, and nanowelding is proposed. The shape of the nanostructures can be controlled by adjusting the temperature for the nanowelding process. Ag nanowires deposited on polymer patterns are accumulated by the nanowelding process to build up diverse 3D nanostructures. The morphologies of the fabricated 3D nanostructures are analyzed using scanning electron microscopy, atomic force microscopy, and focused ion beam; the heating temperature is varied from 90 to 130 °C in steps of 10 °C. In order to analyze the recrystallization phenomenon after welding, transmission electron microscopy is utilized. The 3D metallic nanostructure has different morphologies and optical properties corresponding to welding temperature conditions and accumulated layer thicknesses. Based on preliminary experimental results, the process parameters are optimized and a polarized color filter is fabricated. Optical characteristics of the filter are evaluated using polarizer and spectrometer. Through this work, it is shown that the proposed method is an effective way to realize various 3D metallic nanostructures for special optical properties, therefore the method based on nanowelding can be utilized in fabrication of functional metamaterials, optical filters, biosensors, and others.

Rapid Low-Temperature 3D Integration of Silicon Nanowires on Flexible Substrates.

The vertical integration of 1D nanostructures onto the 2D substrates has the potential to offer significant performance gains to flexible electronic devices due to high integration density, large surface area, and improved light absorption and trapping. A simple, rapid, and low temperature transfer bonding method has been developed for this purpose. Ultrasonic vibration is used to achieve a low temperature bonding within a few seconds, resulting in a polymer-matrix-free, electrically conducting vertical assembly of silicon nanowires (SiNWs) with a graphene/PET substrate. The microscopic structure, and mechanical and electrical characteristics of the interface between the transferred SiNW array and graphene layer are subsequently investigated, revealing that this creates a mechanically robust and electrically Ohmic contact. This newly developed ultrasonic transfer bonding technique is also found to be readily adaptable for diverse substrates of both metal and polymer. It is therefore considered as a valuable technique for integrating 1D vertical nanostructures onto the 2D flexible substrates for flexible photovoltaics, energy storage, and water splitting systems.

A facile patterning of silver nanowires using a magnetic printing method.

Patterning of metal nanowires (NWs) is vital for the fabrication of NW-based, high-performance devices such as sensors, transparent conducting electrodes, and optoelectronics. However, the majority of existing patterning methods require complex and expensive technologies. For this reason, we report for the first time a facile and quick patterning method of silver (Ag) NWs using a magnetic printing method. We successfully demonstrated a patterned AgNW grid structure ona flexible substrate as transparent electrodes. The flexible AgNW grid electrode exhibited optical and electrical properties comparable to those of commercial transparent conducting electrodes.We believe our work will be broadly applicable to other NW-based devices such as sensors,energy storage devices, meta devices, nanoscale electronics, and optoelectronics.

High-durable AgNi nanomesh film for a transparent conducting electrode.

Uniform metal nanomesh structures are promising candidates that may replace of indium-tin oxide (ITO) in transparent conducting electrodes (TCEs). However, the durability of the uniform metal mesh has not yet been studied. For this reason, a comparative analysis of the durability of TCEs based on pure Ag and AgNi nanomesh, which are fabricated by using simple transfer printing, is performed. The AgNi nanomesh shows high long-term stability to oxidation, heat, and chemicals compared with that of pure Ag nanomesh. This is because of nickel in the AgNi nanomesh. Furthermore, the AgNi nanomesh shows strong adhesion to a transparent substrate and good stability after repeated bending.

Highly sensitive and label-free protein immunoassay-based biosensor comprising infrared metamaterial absorber inducing strong coupling.

A mid-infrared label-free immunoassay-based biosensor is an effective device to help identify and quantify biomolecules. This biosensor employs a surface-enhanced infrared absorption spectroscopy, which is a highly potent sensing technique for detecting minute quantities of analytes. In this study, a biosensor was constructed using a metamaterial absorber, which facilitated strong coupling effects. For maximum coupling effect, it is necessary to enhance the near-field intensity and the spatial and spectral overlap between the optical cavity resonance and the vibrational mode of the analyte. Due to significant peak splitting, conventional baseline correction methods fail to adequately analyze such a coupling system. Therefore, we employed a coupled harmonic oscillation model to analyze the spectral distortion resulting from the peak splitting induced by the strong coupling effect. The proposed biosensor with a thrombin-binding aptamer-based immunoassay could achieve a limit of detection of 267.4 pM, paving the way for more efficient protein detection in clinical practice.

Superamphiphobic surface by nanotransfer molding and isotropic etching.

We present a novel method of fabricating superhydrophobic and superoleophobic surfaces with nanoscale reentrant curvature by nanotransfer molding and controlled wet etching of the facile undercut. This method produces completely ordered re-entrant nanostructures and prevents capillary-induced bundling effects. The mushroom-like, re-entrant, overhanging structure demonstrates superhydrophobic and superoleophobic characteristics, as tested by water droplet bouncing and contact angle measurements, and has high transparency on a flexible substrate. Widespread use as self-cleaning surfaces is expected in the near future.

Layer-by-layer all-transfer-based organic solar cells.

For the first time, we describe a novel cost- and time-effective vacuum-free process to fabricate bulk-heterojunction (BHJ) organic photovoltaics (OPVs) via layer-by-layer selective stamping transfer of all layers. By controlling the surface properties of polyurethane acrylate (PUA) stamping molds with ultraviolet (UV)-ozone (UVO) exposure, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), BHJ layer, and metal cathode were uniformly transferred layer by layer onto each of the bottom layers. Among several interfaces between each layer, we found that the interface between the active layer and metal cathode is a critical factor in obtaining conventional device-like efficiency. To enhance the interfacial connectivity between the BHJ layer and metal cathode and increase electron extraction from the BHJ layer, a titanium oxide (TiOx) interlayer was introduced. Cell performance was optimized by controlling the concentration of TiOx solution. The poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PC60BM) BHJ device fabricated by transferring PEDOT/PSS, TiOx/active layer, and Al cathode showed 2.01% power conversion efficiency. This efficiency is not comparable to those of conventional OPVs, but our approach shows the possibility of fabricating OPVs via the layer-by-layer transfer method for the first time.

Graphoepitaxy of block-copolymer self-assembly integrated with single-step ZnO nanoimprinting.

A highly efficient, ultralarge-area nanolithography that integrates block-copolymer lithography with single-step ZnO nanoimprinting is introduced. The UV-assisted imprinting of a photosensitive sol-gel precursor creates large-area ZnO topographic patterns with various pattern shapes in a single-step process. This straightforward approach provides a smooth line edge and high thermal stability of the imprinted ZnO pattern; these properties are greatly advantageous for further graphoepitaxial block-copolymer assembly. According to the ZnO pattern shape and depth, the orientation and lateral ordering of self-assembled cylindrical nanodomains in block-copolymer thin films could be directed in a variety of ways. Significantly, the subtle tunability of ZnO trench depth enabled by nanoimprinting, generated complex hierarchical nanopatterns, where surface-parallel and surface-perpendicular nanocylinder arrays are alternately arranged. The stability of this complex morphology is confirmed by self-consistent field theory (SCFT) calculations. The highly ordered graphoepitaxial nanoscale assembly achieved on transparent semiconducting ZnO substrates offers enormous potential for photonics and optoelectronics.

Conformally direct imprinted inorganic surface corrugation for light extraction enhancement of light emitting diodes.

We describe the fabrication of corrugated inorganic oxide surface via direct single step conformal nanoimprinting to achieve enhanced light extraction in light emitting diodes (LEDs). Nanoscale zinc oxide (ZnO) and indium tin oxide (ITO) corrugated layer were created on a nonplanar GaN LED surface including metal electrode using ultraviolet (UV) assisted conformal nanoimprinting and subsequent inductively coupled plasma reactive ion etching (ICP-RIE) treatment. The total output powers of the surface corrugated LEDs increased by 45.6% for the patterned sapphire substrate LED and 41.9% for the flat c-plane substrate LED without any degradation of the electrical characteristics. The role of the nanoscale corrugations on the light extraction efficiency enhancement was examined using 3-dimensional finite-difference time-domain (FDTD) analysis. It was found that light scattering by subwavelength scale surface corrugation plays important role to redirect the trapped light into radiative modes. This straightforward inorganic oxide imprint method with inherent flexibility provides an efficient way to generate nanoscale surface textures for the production of high power LEDs and optoelectronic devices.

Surface plasmon-waveguide hybrid polymer light-emitting devices using hexagonal Ag dots.

We investigated surface plasmon-waveguide hybrid resonances for enhancement of light emission in polymer light-emitting diodes (PLEDs). Hybrid waveguide-plasmon resonances in the visible range for waveguide mode and near IR range for surface plasmons were observed by incorporation of hexagonal Ag dot arrays. Considerable overlap between the emission wavelength of the PLEDs and the waveguide mode by an Ag dot array with a lattice constant of 500 nm was observed. Because of enhanced light extraction by Bragg scattering of waveguide modes, photoluminescence (PL) and electroluminescence (EL) were increased by 70% and 50%, respectively.

Effect of surface tension and coefficient of thermal expansion in 30 nm scale nanoimprinting with two flexible polymer molds.

We report on nanoimprinting of polymer thin films at 30 nm scale resolution using two types of ultraviolet (UV)-curable, flexible polymer molds: perfluoropolyether (PFPE) and polyurethane acrylate (PUA). It was found that the quality of nanopatterning at the 30 nm scale is largely determined by the combined effects of surface tension and the coefficient of thermal expansion of the polymer mold. In particular, the polar component of surface tension may play a critical role in clean release of the mold, as evidenced by much reduced delamination or broken structures for the less polarized PFPE mold when patterning a relatively hydrophilic PMMA film. In contrast, such problems were not notably observed with a relatively hydrophobic PS film for both polymer molds. In addition, the demolding characteristic was also influenced by the coefficient of thermal expansion so that no delamination or uniformity problems were observed when patterning a UV-curable polymer film at room temperature. These results suggest that a proper polymeric mold material needs to be chosen for patterning polymer films under different surface properties and processing conditions, providing insights into how a clean demolding characteristic can be obtained at 30 nm scale nanopatterning.

A micro-system based on glass-nanoporous silicon for optical sensing of organic solvent vapor.

We present a recent experimental study on the application of nanoporous silicon (np-Si) to an optical vapor sensor. We fabricated the micro-system based on a glass-nanoporous silicon layer on a p(+)-type silicon wafer. To check the selectivity and sensitivity of the np-Si layer to organic vapors, we prepared three types of np-Si layer samples--a single layer, distributed Bragg reflector (DBR) layer, and microcavity layer--and investigated its reflectance spectra upon exposure to different concentrations of various organic vapors. When the np-Si layer samples were exposed to the organic vapors, a red-shift occurred in the reflectance spectrum, and we determined that this red-shift can be attributed to the changes in the refractive index induced by the capillary condensation of the organic vapor within the pores of the np-Si layer. The np-Si layer samples showed excellent sensing ability to different types and concentrations of organic vapors. After removing the organic vapors, the reflectance spectrum immediately returned to its original state.

Optimized film processing of nanosilver colloids for photoluminescence enhancement.

Recently, various research strategies have been employed to improve light extraction efficiency in organic LEDs, including the recent development of localized surface plasmon resonance (LSPR), as well as the more widely-known application of a photonic crystal layer. Here, we report on the development of a process method for forming a two-dimensional nanosilver patterned array to achieve LSPR-coupled light-emission efficiency enhancement. The process scheme involves the spin-coating of nanosilver colloidal ink onto a glass substrate, followed by optimized thermal annealing to create an array of isolated nanosilver islands. The resulting Ag islands are in the size range 50 approximately 80 nm, which is larger than the diameter of the Ag nanoparticles in the colloidal suspension. Then, silicon oxide is thermally sputtered to provide a spacer layer to prevent luminescence quenching of the red-emitting nanocrystal quantum dot (NQD) layer, which is deposited in a subsequent spin-coating process. When the NQD layer is excited, the energy of the photoelectron is confined to the surfaces of the nanosilver islands in the near-field. In this study, the localized surface plasmon resonance peaks were at a wavelength of 625 nm, and out-coupling efficiency was enhanced more than sixfold.

Photovoltaic devices with an active layer from a stamping transfer technique: single layer versus double layer.

In this study, organic photovoltaic devices with single or double-layered active film were prepared from a stamping transfer technique. A P3HT/PCBM single-layered active layer and a ratio-controlled P3HT/PCBM double-layered active can be successfully fabricated with the help of ultraviolet curable polycarbonate films via a stamping transfer technique. The maximum conversion efficiency values 2.85 for a single active layer transferred device and 3.24% for an optimized double active layer transferred device. Even though transferred double layers should have a sharp interface boundary, an intermixed zone with a concentration gradient was generated by the interpenetration of a donor-rich layer and an acceptor-rich layer in a thermal annealing process. The generation of the intermixed zone is confirmed by Auger electron spectroscopy. The enhanced conversion efficiency levels are attributed to the increased efficiency of the carrier transporting process, which is due to the fact that the concentration gradient is combined with the efficient charge generation from the bulk heterojunction layers.

A simple soft lithographic nanopatterning of gold on gallium arsenide via galvanic displacement.

Nanoscale patterning of gold layers on GaAs substrate is demonstrated using a combination of soft lithographic molding and galvanic displacement deposition. First, an electroless deposition method has been developed to plate gold on GaAs with ease and cost-effectiveness. The electroless metallization process is performed by dipping the GaAs substrates into a gold salt solution without any reducing agents or additives. The deposition proceeds via galvanic displacement in which gold ions in the aqueous solution are reduced by electrons arising from the GaAs substrate itself. The deposition rate, surface morphology and adhesion property can be modulated by the plating parameters such as the choice of acids and the immersion time. Second, soft lithographic patterning of nanodots, nanorings, and nanolines are demonstrated on GaAs substrates with hard-polydimethylsiloxane (h-PDMS) mold and plasma etching. This method can be easily applied to the metallization and nanopatterning of gold on GaAs surfaces.

Pattern-definable and low cost fabrication of nanopatterned conducting polymer film on flexible substrates.

This study reports the pattern definable and low cost fabrication of nanopatterned conducting polymer film on flexible substrates. Noble nanopatterned polymer hard template was fabricated by using nanoimprint lithography (NIL) and used for electropolymerization of conducting polymer. Conducting polymer was electrochemically deposited on the template and transferred over to flexible substrates. Eventually conducting polymer films with various nanopatterns were fabricated on flexible substrates. High pattern definability was achieved by nanoimprinted polymer template, which was molded from lithographically fabricated stamp. Low cost fabrication was accomplished due to low cost NIL, reusable polymer templates, and low material consumption of electrodeposition. The electrodeposited films were transferred using double sided tape. Because the templates are made of flexible polymer, the transfer bonding method applied in this study is adaptable to both wafers and flexible polymer substrates. The fabricated nanopatterned conducting polymer film can be applied to gas sensors, super capacitors, super wetting films, and neuron interfaces due to its characteristic of high surface to volume. For an illustrative application, the gas sensing properties of films were tested. The result showed enhanced sensing characteristic with nanopatterned film, which are attributed to the high surface to volume ratio of nanopatterned films.