Thermal Nanoimprint Process Research Articles

Triple and Quadruple Surface Pattern Memories in Nanoimprinted Polymer Blends

Trigger-responsive surfaces with multiple surface properties have wide-ranging application potential from surfaces with trigger-responsive fluid flow to cell culture to optical effects; such surfaces can be achieved through surface morphological changes. Although multiple shape-memory effects are successful in bulk polymers, there is limited programing and recovery of multiple surface memories due to the challenges in fabricating multiple surface topographies with good controllability. Here, we report the synergy between the polymer blend formulation and the thermal nanoimprinting process to achieve multiple microtopography memories. A series of immiscible blends consisting of poly(caprolactone) (PCL) and polyethylene (PE) with distinct thermal transitions governed by distinct crystallization events were augmented with improved elasticity through preferential cross-linking in the polymer blend. The effect of preferential cross-linking by dicumyl peroxide on the elastic property of the PCL/PE has been found to be nonlinearly dependent on the blend composition. This approach enabled triple and quadruple surface pattern fixity and recovery in nanoimprinted PCL/PE blends. Specifically, we demonstrated the recovery of a micropillar structure (diameter: 20 μm and height: 10 μm) from a hierarchical micrograting topography (width: 2 μm and height: 2 μm) when exposed to a thermal stimulus at 60 °C for 180 s. Furthermore, we also demonstrated the recovery of a deformed micrograting followed by a secondary recovery of the micropillar structure.

Guided-mode resonance pressure sensor based on a stretchable low-density polyethylene film

In this study, we demonstrate a method to fabricate a guided-mode resonance (GMR) device on a flexible and transparent low-density polyethylene (LDPE) film and present the measurement results of this device as a pressure sensor. A simple thermal-nanoimprinting process was used to fabricate a grating structure on the LDPE film substrate. This very flexible film was attached to a glass plate using an adhesive and sacrificial layer for coating high-refractive-index titanic oxide on the grating surface to form the GMR device. The LDPE-GMR device was equipped with a gas chamber to act as a pressure sensor. When the pressure inside the chamber was increased, the grating period of the GMR sensor also increased, resulting in a shift in the resonance angle of the GMR device. Owing to the higher flexibility of the LDPE film, a better pressure detection sensitivity and resolution can be obtained. Using the transmitted-intensity detection approach, we show that the transmitted laser power changes proportionally with the pressure increase. The experimental results showed that the LDPE-GMR pressure sensor could achieve a sensitivity of 8.27 µW/mbar and a limit of detection of 0.012 mbar at a power meter noise of 0.1 µW.

Distribution of Relaxation Time Analysis of Cathode Micro-Patterned PEFC

1. Introduction Fuel cells have been attracting worldwide attention because they are a key technology for producing electric energy for a future hydrogen-based society. In particular, polymer electrolyte fuel cells (PEFCs) are important among other fuel cells. This is because they can be operated at relatively low temperatures, which means they have many practical applications, e.g., fuel cell vehicles. However, PEFCs still have some problems. One is their performance because they are not as efficient as other forms of fuel cells. There are various approaches for achieving this, but it is common theory that the cathode chemical process limits the performance of PEFCs. Heinzmann et al. applied electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analysis to break down polarization resistance in detail [1]. They clearly showed that the dominant process is the cathode gas diffusion and oxygen reduction reaction (ORR). One way to improve the ORR is to increase the chemical reaction surface. Jeon et al. fabricated a micro-patterned membrane and applied it to membrane electrode assembly (MEA) [2]. As a result, they confirmed that the micro-patterned membrane can improve the performance of the entire PEFC. However, the physical phenomenon of its improvement has yet to be clarified. In detail, these patterns can certainly increase the surface area but may decrease mass transportation, especially gas diffusion. In this study, we performed DRT analysis with micro-patterned MEA to investigate the mechanism of performance improvement from using micro-patterned MEA (Fig. 1).2.Experiment A cathode micro-patterned MEA was fabricated, and then EIS and DRT analysis were performed. The patterned membrane was produced by a thermal nanoimprinting process. A Ni mold with a 5.0-mm line pattern was prepared, and the membrane was pressed onto the mold. Through this process, the pattern of the Ni mold was replicated to a Nafion membrane (NRE-212, Sigma-Aldrich Co. LLC) with a thickness of 50 mm. Then, the catalyst ink was prepared with Pt/C (TEC10V50E, Tanaka Kikinzoku Kogyo K.K.), isopropyl alcohol, water, and a 10% ionomer solution. We sprayed this ink onto the patterned membrane to fabricate the MEA. The catalyst loading was approximately 0.4 mg/cm2. Using a commercial gas diffusion layer (Sigracet®︎ 29BC, SGL carbon GmbH), the cell was assembled, and EIS was operated at 0.1 Hz to 1 MHz and at 0.7, 0.65, and 0.60 V. The acquired data were then applied to DRT analysis using the DRT tools by Wan et al. [3]. Noisy data points were excluded before application because the DRT analysis is very sensitive to noise.3.Results and discussion Figure 2 shows a cross-sectional SEM image of the micro-patterned MEA. Clearly, the membrane is patterned via the designed geometry, and the catalyst layer is fully integrated into the micro-pattern. Figure 3 shows the EIS results. The polarization resistance decreased as the potential was decreased, similar to the general behavior of conventional fuel cells. Figure 4 shows the DRT analysis. Two clear peaks are observable at approximately 10 Hz and 100 Hz, which we call P1 and P2, respectively. P1 remained the same or became slightly larger with decreasing potential. However, P2 decreased as the potential decreased. In addition, P2 of 0.60 V indicated that this peak split into two peaks as the potential decreased. P2 appeared as a combined peak of gas diffusion and the oxygen reduction reaction (ORR) upon comparing it to previous DRT analysis research of commercial cells [1]. We could determine which peak represents what physical phenomena if we perform additional DRT analysis while changing the operating environment, as in previous studies. By comparing the analysis of micro-patterned samples with conventional sample, it will be possible to evaluate the effect of the micro-pattern and its physical phenomena.4.Conclusion We performed DRT analysis with a cathode micro-patterned PEFC. DRT analysis has the ability to clarify the physical phenomena of how a patterned MEA increases cell performance with patterned membranes. Our results indicated that there is a peak at approximately 100 Hz, and it splits into two peaks representing gas diffusion and ORR with a lower potential, such as 0.60 V. As mentioned earlier, further work is required to assign physical phenomena to each peak. A comparison of DRT data using a conventional sample can possibly reveal the mechanism of performance increase owing to membrane patterning.

Superhydrophobic surfaces with embedded protective sawtooth ring microstructures

Despite the achievements in superhydrophobic surface obtained via geometrically uniform micro-nano scale surface texture, the mechanical robustness of superhydrophobic surfaces remains a challenge where service conditions involve solid-body contact. We introduce a unique sawtooth-ring protective microstructure design as an integrated shield for underlying superhydrophobic micro-pillar arrays. The sawtooth-ring design minimizes the areal occupancy of the surface and merges directly with the anti-wetting micro-pillar array, giving rise to an integrated micro-structure. The integrated surface micro-structure has been fabricated in an intermediate polymer mold via an innovative two-step thermal nanoimprinting process. The integrated sawtooth ring and micro-pillar array have been replicated on UV polymerized resin, thermally cured polydimethylsiloxane (PDMS) and solvent cast polycarbonate (PC) using the intermediate polymer mold. The post-compression water contact angle results demonstrate the reliance of the sawtooth ring design on the underlying material’s resistance to plastic yield. This reliance can be met either in the form of relatively high yield stress, as is the case with PC or in the form of a large elastic strain tolerance, as is the case with PDMS. In both cases, compressive testing of sawtooth ring protected anti-wetting surfaces resulted in the retention of the Cassie-Baxter 2-component wetting state and negligible degradation in their respective contact angles.

Micro/Nano Dual‐Scale Crossed Sinusoidal Wavy Patterns for Synergistic Promotion of Proliferation and Endothelial Differentiation of Human Adipose‐Derived Stem Cells

AbstractAlthough human adipose‐derived stem cells (hASCs) have great potential as a cell source for stem cell therapy due to their capacity of self‐renewal, long‐term growth, and multipotency with high obtainability through lipoaspiration, little is known about the effects of micro/nano dual‐scale topography on the proliferation and differentiation of the hASCs. In this study, the effects of micro/nano dual‐scale crossed sinusoidal wavy patterns (SIMNs) on the hASC behaviors including cellular alignment, proliferation, and endothelial differentiation are investigated. Various types of microscale, nanoscale, and SIMNs are fabricated on a single polystyrene surface in a step‐gradient manner based on the combined fabrication processes of deep X‐ray lithography, polydimethylsiloxane surface wrinkling, and thermal nanoimprinting process. The cellular alignment, proliferation, and endothelial differentiation of the hASCs are promoted on the SIMNs compared to the microscale or nanoscale sinusoidal wavy patterns. A micro/nano dual‐scale pattern of SIMN160_500, which has a micro‐scale sinusoidal wavy pattern with a wavelength of 160 µm crossed with a nanoscale sinusoidal wavy pattern with a wavelength of 500 nm, is found to be a most effective pattern for synergistically promoting both proliferation and endothelial differentiation of the hASCs with 1.2‐fold and 2‐fold increases, respectively.

Replication of Mesoscale Pore One-dimensional Nanostructures: Surface-induced Phase Separation of Polystyrene/Poly(vinyl alcohol) (PS/PVA) Blends.

Mesoscale pore one–dimensional (1D) nanostructures, or vertically aligned porous nanostructures (VAPNs), have attracted attention with their excellent hydrophobic properties, ultra−high surface area, and high friction coefficient, compared to conventional vertically aligned nanostructures (VANs). In this study, we investigate the replication of VAPNs produced by the thermal nanoimprint process using anodic aluminum oxide (AAO2) templates (100 nm diameter). Polystyrene/poly(vinyl alcohol) (PS1/PVA) blends, prepared by the advanced melt–mixing process with an ultra–high shear rate, are used to investigate the formation of porosity at the nanometer scale. The results reveal that domain size and mass ratios of PVA precursors in the PS matrix play a dominant role in the interfacial interaction behavior between PS1–PVA–AAO2, on the obtained morphologies of the imprinted nanostructures. With a PVA nanodomain precursor (PS1/PVA 90/10 wt%), the integration of PVA nanodroplets on the AAO2 wall due to the hydrogen bonding that induces the phase separation between PS1–PVA results in the formation of VAPNs after removal of the PVA segment. However, in the case of PVA microdomain precursors (PS1/PVA 70/30 wt%), the structure transformation behavior of PS1 is induced by the Rayleigh instability between PVA encapsulated around the PS1 surfaces, resulting in the PS1 nanocolumns transforming into nanopeapods composed of nanorods and nanospheres.

A Rapid Thermal Nanoimprint Apparatus through Induction Heating of Nickel Mold.

Thermal nanoimprint lithography is playing a vital role in fabricating micro/nanostructures on polymer materials by the advantages of low cost, high throughput, and high resolution. However, a typical thermal nanoimprint process usually takes tens of minutes due to the relatively low heating and cooling rate in the thermal imprint cycle. In this study, we developed an induction heating apparatus for the thermal imprint with a mold made of ferromagnetic material, nickel. By applying an external high-frequency alternating magnetic field, heat was generated by the eddy currents and magnetic hysteresis losses of the ferromagnetic nickel mold at high speed. Once the external alternating magnetic field was cut off, the system would cool down fast owe to the small thermal capacity of the nickel mold; thus, providing a high heating and cooling rate for the thermal nanoimprint process. In this paper, nanostructures were successfully replicated onto polymer sheets with the scale of 4-inch diameter within 5 min.

Fabrication of Line & Space Structure and Wiring on 3D Surface Features with Imprinting Process

In this study, we have investigated the wiring on the curved surfaces with nanoimprint technology using the flexible resin mold. The maximum transfer ratios of line & space structures of polyetherimide (PEI) and cyclic olefin copolymer (COC) were higher than 93% by using thermal nanoimprint process. It found that the silver nano-ink can be transferred to the curved surfaces using the UV curable resin as the adhesive material. The wired on curved surfaces using a flexible polymer mold, silver nano-ink, and UV curable resin as an adhesive showed the electrical conduction indicated. Therefore, the possibility to apply nanoimprint technology to wiring on three-dimensional surface features was indicated.

Electrically-assisted nanoimprint of block copolymers

Guiding of the phase separation of a block copolymer (BCP) by an electric field perpendicular to the substrate is investigated in order to obtain vertical structures that can provide a mask for subsequent etching. Because of practical aspects, the substrate is bare Si without any neutral brush and the process time is limited to 1 h. A polystyrene-block polymethylmethacrylate lamellar material is employed in the study. For a unique guiding of the lamellar phase, an ordering mechanism orthogonal to the electric field is introduced by the interaction with the stamp in a thermal nanoimprint process. The naturally low surface energy of the stamp shall induce the formation of lamellae along the sidewalls of linear cavities. In order to fully utilize these two ordering mechanisms, the stamp sidewalls and the electric field, the imprint process is conducted in such a way that no residual layer remains below the stamp structures and the whole BCP is accumulated inside the cavities which are just partly filled. The electrically-assisted imprint process is studied analytically, considering the capacitive effects due to the local electric field in the cavity and in particular in the BCP. In addition, a numerical simulation is performed for the actual experimental conditions to compute the electric vector field in the BCP. In this way, an extensive understanding of the situation is gained which is the basis for choosing optimal experimental conditions for electrically-assisted thermal nanoimprint. Furthermore, the ambiguity of the electric field in a thermal nanoimprint process with partly filled cavities is addressed. The field shall induce vertical phase separation but, due to instabilities, it also may induce capillary bridges that represent replication defects. An improvement of the vertical phase separation by applying an electric field as high as 25 V/μm could be identified under specific experimental conditions. However, the guiding effect within the cavities and thus the long-range order of the lamellae remained limited. This may be due to a field strength too low in the BCP; in the present configuration, higher field strengths are prohibited by an electrical breakthrough.

Nanostructured Free-Form Objects via a Synergy of 3D Printing and Thermal Nanoimprinting.

High‐resolution surface patterning has garnered interests as a nonchemical‐based surface engineering approach for creating functional surfaces. Applications in consumer products, parts for transportation vehicles, optics, and biomedical technologies demand topographic patterning on 3D net shape objects. Through a hybrid approach, high‐resolution surface texture is incorporated onto 3D‐printed polymers via direct thermal nanoimprinting process. The synergy of geometry design freedom in 3D printing and the high spatial resolution in nanoimprinting is demonstrated to be a versatile fabrication of high‐fidelity surface pattern (from 2 µm to 200 nm resolution) on convex, concave semicylindrical, and hemispherical objects spanning a range of surface curvatures. The novel hybrid fabrication is further extended to achieve a high‐resolution curved mold insert for rapid prototyping via injection molding. The versatility of the fabrication strategies reported here not only provides a post‐3D printing process that enhances the surface properties of 3D‐printed objects but also opens a new pathway to enable future study on the effects of combining microscale and nanoscale surface texture with macroscopic curvature. Both have been known, individually, as an effective approach to tune surface functionalities.

Fabrication of Line & Space Structure and Wiring on 3D Surface Features with Imprinting Process

In recent years, electronic devices made by polymers with wiring materials are widely used, such as FPC (Flexible Printed Circuit) in a PC (Personal Computer), and smartphones. Fine Pitch Wiring technologies on 3D objects are needed for coming IoT(Internet of Things) trends. Therefore, in this research, we conducted a fundamental study of the low-cost replication process with etching less wiring method in order to replicate microstructure on the curved polymer surfaces by using the hybrid nanoimprint technology. The first imprinting step was carried out to produce the intermediate mold on COP (Cyclic Olefin Polymer) surface by thermal nanoimprint process. The aluminum mold with a concavo-convex structure called L/S (Line & Space; concave width of 500 µm, concave depth of 100 µm) was used. The obtained intermediate structure was coated with UV (Ultraviolet) curable resin and it was applied to the 3D substrate (curved surface region: lens shape) having planar region and curved surface region as shown in Fig. 1. In this step, UV imprinting was applied with the pressure of 5 MPa and UV irradiation time of 300 seconds to transfer the L/S structure to the 3D substrate. Thereafter, the silver nanoink was dropped and filled in the concave portion of the L/S structure using a dispenser and sintered at 160 oC for 1 hour. SEM (Scanning Electron Microscope) image revealed that the 3D structure can be precisely transferred on the curved surface by using this hybrid nanoimprint technology (See Fig. 2). In Fig. 3, the transfer rates in the planar region (flat) and curved region were 100 and 60%, respectively. It is considered that the liquid UV curable resin may flow downward from the upper part to the lower part along the inclination of the substrate during the crimping and curing steps, resulting in the decrease in filling amount from the intermediate mold structure. The L/S patterned wired with silver nanoink is shown in Fig. 4. This result proposes that the hybrid nanoimprint technology is considered to be an effective method for the replication of wiring on the curved surface. Figure 1

Large-area flexible infrared nanowire grid polarizer fabricated using nanoimprint lithography.

We fabricated a 4-in large-area flexible infrared nanowire grid polarizer using a nanoimprint and metal thermal evaporation process. To protect the Si master template, as well as to prolong the service life of it, we first fabricated a nickel template as an alternative by an electroforming process. Then, the nanowire grid structure was transferred from this template to IPS substrate by a thermal nanoimprint process. Finally, Al was deposited on the IPS nanowire structure by vertical thermal evaporation technology. The results of the infrared optical test reveal that the TM transmittance of the polarizer is greater than 60% in the 4-5.71μm and 5.73-6.7μm wavelength ranges, and, especially, it is greater than 70% in the wavelength ranges of 4.70-5.69μm and 5.75-6.59μm. The extinction ratio is more than 20dB in the wavelength range of 3.6-6.7μm, proving that the polarizer has good polarization characteristics. The flexible infrared nanowire grid polarizer has potential applications in the fields of curved surface monitoring equipment and polarized imaging equipment.

Fabrication of Energy-Boosted Triboelectric Nanogenerator Via Electric Field Assisted Thermal Nanoimprinting Process

Energy harvesting technologies, which scavenge and capture various types of abandoned energies from an ambient environment, have grown more comprehensive and crucial as they play a role to power the emerging mobile electronic systems and micro devices. Lots of energy harvesting technologies have been successfully demonstrated and explored based on photovoltaic effect, piezoelectric effect, electromagnetic effect and so forth. Recently, the eminent concept of the triboelectric nanogenerator (TENG), where operation mechanism is coupling of triboelectricity and electrostatic induction, has been proposed. The TENG is mainly composed of 1) the contact layer, where the triboelectric effect occurs, and 2) the electrode layer inducing movement of the electrical charges to generate electric current and power. Since its first demonstration, the TENG has been proven as one of the promising energy harvesting platforms due to its high accessibility, low manufacturing cost and high energy conversion efficiency compared with the other existing energy harvesting platforms. With those advantageous aspects, so far, the TENG has been widely applied to versatile applications such as biomedical monitoring system, self-powered sensor and portable power supply to drive wearable electronics. As above mentioned, although lots of potential applications of TENGs have been successfully demonstrated, for their reliable utilization as a practical power source, the electrical output performance of TENGs still needs to be further increased. From this point of view, enhancing the electrical output performance of the TENG has been one of the important issues in research fields from its first proposal to present. The simplest and most powerful way to enhance the electrical output performance of the TENG is based on formation of micro/nanoscale surface topographies on its contact layer inducing localized concentration of contact forces, which generates higher electrical charge amounts on its surface. As well as formation of topographies on its external surface, recently, the concept of modifying internal molecular arrangement has been suggested. For example, the synthesis of material where important derivatives are carbon increased the interface for charge storage by acting charge trapping sites. On top of that, the alignment of positive and negative charges referred to as poling process has been reported. During this poling or dipolar alignment, a large electric potential is applied across the material which induces the alignment of dipole and production of net positive charge. To be detailed, the maximized surface charge and charge difference between upper and bottom surface of material can produce the developed electrical energy compared with that of non-aligned material. In this respect, it has been considered as a potent strategy to increase the energy output of the TENG. Previously, we reported the one-step simple method to fabricate the advanced energy output TENG by using nanoimprinting process. Through thermal nanoimprint process, there are obvious advantages in terms of increasing energy output of the TENG, and practical commercialization. Firstly, the energy output of the TENG can be enhanced due to the formation of nano topographies on the contact layer of thermoplastic fluoropolymer. Secondly, in viewpoint of commercialization, the thermal nanoimprint is one-step fabrication, with coincidently replication of nano structure and integration of contact later with the electrode layer which are essential prerequisite components of TENG. Besides these merits, low cost and mass producible nanofabrication process present the application of practical commercialization. In this study, we first introduced the innovative strategy which is coupling process between nano imprint and poling process to maximize the electrical energy output performance of TENG based on the formation of nano-topographical surface on contact layer and modulation of the internal dipole moments of ferroelectric thermoplastic polymer. The most remarkable novelties are shown that energy output of TENG can be enhanced by achieving the alignment of dipole moments of polymer while taking advantages of fabricating the TENG by using the nanoimprint process. In aspects of the modulation of internal moments, the degree of alignment dipole moments is higher than when the conventional poling method is used. Compared with the conventional poling process, the method we suggested has distinct advantages. Given that the flexibility of thermoplastic polymer increased due to the elevated temperature, the alignment of dipole moments can be achieved more effectively. In addition, the electric field concentrated on each nano structure contributes to obtain the high portion of dipole moments alignment due to their higher electric field in comparison with when the electrode which has no nano-topograpies on surface is used to poling process. Lastly, based on these effects, the overall poling period is much shorter than that of conventional poling. Figure 1

Development of Thermal Nanoimprinting Process for the Fabrication of Tailored-Triboelectric Nanogenerator

Recently, triboelectric nanogenerator (TENG), working based on triboelectric effect and electrical induction, emerge as one kind of energy harvesting technology. The TENG is mainly composed of dielectric layer where frictional contact occurs and conductive layer where current flows. Because of the characteristic of triboelectric effect, strategies to make efficient TENG are focused on material selection and effective contact area increment. Given materials, micro/nanostructure fabrication corresponds to the latter strategy. Several fabrication methods are already reported such as surface etching with nanoparticle and surface replication, but they require high fabrication cost. Furthermore, they need additional process such as integration micro/nanotophographical layer with conductive layer, resulting in a multistep fabrication. The fabrication environment hinders high productivity and commercialization of TENG. In this study, we used thermal nanoimprinting to fabricate a transparent and flexible TENG named a Pattern Assisted TriboElectric Replicable Nanogenerator (PATERN). From the method, we can achieve nanostructure replication and integration with conductive layer simultaneously, which are two essential steps to fabricate high performance TENG. The fabrication cycle was within 6 min. The PATERN has maximum transmittance about 88% at the 850-1600 nm range, and bending radius about 3.2 mm. From modifying the shape of electrode layer, we can also achieve a bi-directional mechanical energy harvesting ability, which is essential for efficiently harvest the energy from complex real mechanical motions. Given that TENG can be cost-effectively fabricated by utilizing conventionally available films (less than $1) with fast fabrication process, we believed the technology lead a new concept of disposable TENG and its accompanying application, such as hybrid energy harvester or biomechanical energy harvester. Figure 1

Soft thermal nanoimprint of PMMA doped with upconverter nanoparticles

Upconversion of low-energy photons into higher energy photons is a non-linear effect that can be strongly enhanced by increasing the intensity of impinging light. Such an enhanced intensity can also be achieved by near-field optical effects in photonic structures. In this paper, we investigate photonic structures in the form of linear gratings with varying periods realised in poly-methyl-methacrylate (PMMA) layers using soft stamps in a thermal nanoimprint process. Master structures were fabricated using e-beam lithography and reactive ion etching in silicon. These master structures were replicated into bi-layer polydimethylsiloxane (PDMS) stamps. PMMA layers with embedded β-NaYF4:25% Er3+ upconverter nanoparticles were applied to glass substrates using spin coating. The PDMS stamps were used to imprint the PMMA layer with very accurate pattern fidelity. The upconversion luminescence around 980nm was enhanced more than three times by the photonic structure under 1523nm laser excitation with 0.43±0.02W/cm2 irradiance, in comparison to a reference sample containing the same amount of upconverter material.

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.

Tunable resonance-domain diffraction gratings based on electrostrictive polymers.

Critical combination of high diffraction efficiency and large diffraction angles can be delivered by resonance-domain diffractive optics with high aspect ratio and wavelength-scale grating periods. To advance from static to electrically tunable resonance-domain diffraction grating, we resorted to its replication onto 2-5 μm thick P(VDF-TrFE-CFE) electrostrictive ter-polymer membranes. Electromechanical and optical computer simulations provided higher than 90% diffraction efficiency, a large continuous deflection range exceeding 20°, and capabilities for adiabatic spatial modulation of the grating period and slant. A prototype of the tunable resonance-domain diffraction grating was fabricated in a soft-stamp thermal nanoimprinting process, characterized, optically tested, and provided experimental feasibility proof for the tunable sub-micron-period gratings on electrostrictive polymers.

Comb-shaped electrode-based triboelectric nanogenerators for bi-directional mechanical energy harvesting

Triboelectric nanogenerators (TENGs), which utilize coupling of contact electrification and electric induction to effectively harvest the mechanical energy around us, have attracted much attention due to their advantages such as simple design and high accessibility. Herein, we report new types of TENGs containing comb-shaped electrode, which are fabricated with a simple thermal nanoimprinting process where a commercially-available metal mesh was used as a stamp to simply impart microtopography on the TENGs to increase electrical output performance. The fabricated TENG with the comb-shaped electrode enables to harvest bi-directional mechanical energy (including both lateral and vertical contact/separation), which can be a new strategy to efficiently harvest the energy from complex real mechanical motions. The TENG with the comb-shaped electrode generates a short circuit current (Isc) of 85nA and an open circuit voltage (Voc) of 6.4V under the lateral contact/separation, which are increased up to 850% and 1600%, respectively, compared to the TENGs with the conventional rectangular electrode. The TENG with comb-shaped electrode is also found to harvest energy of Isc of 339nA and Voc of 31V at a pressing frequency of 0.5Hz and force of 58.8N under the vertical contact/separation without significant loss of electrical output performance compared with the TENG with the conventional rectangular electrode. The results indicate that the comb-shaped electrode would be a powerful (potential) candidate of electrode shape of the TENG to harvest the energy from real mechanical motions.

Induction-heated nanoimprint on soda-lime glass using sapphire molds

The authors developed an induction-heated high-temperature thermal nanoimprint process to directly transfer nanoscale features on sapphire molds to soda-lime glass substrates. Nanoholes of a 650-nm-period hexagonal lattice were directly imprinted in soda-lime glass using the induction-heated thermal nanoimprint configuration as an experimental demonstration. The morphology of nanostructures imprinted on glass substrates at different imprinting process parameters was characterized by scanning electron microscopy and atomic force microscopy and further numerically investigated using a finite element method. The soda-lime glass substrates imprinted with nanohole arrays were then used as secondary templates for further transferring nanostructures onto cyclic olefin copolymer films through another thermal nanoimprint process. The authors also demonstrated surface-enhanced Raman spectroscopy sensing applications on these nanostructured glass substrates after coating them with a thin layer of gold.

Flexible composite stamp for thermal nanoimprint lithography based on OrmoStamp

Flexible stamps are common for roll-to-roll processing but less common with planar processing, although they offer a number of benefits as, e.g., an improved conformal contact at reduced pressure. A simple way to realize such a flexible stamp is to use a two layer system with a structured top layer and a flexible backplane. The structured top layer is most easily obtained by molding, the backplane provides the flexibility envisaged. For use in a thermal nanoimprint process, a high thermal stability is required for both. This investigation addresses the preparation of flexible composite stamps with OrmoStamp as the structured top layer and polyimide as the flexible backplane. The process recommended for stamp preparation with OrmoStamp has to be modified to avoid bending after the hard bake that is required after ultraviolet-curing of the material to obtain a high stability of the top layer. Reduction of bending is advised, in particular, for large area stamps, where the hard bake step is in conflict with preparation of a flat stamp and limits applicability for thermal nanoimprint. A small scale almost flat flexible composite stamp is prepared and employed for low-pressure nanoimprint; it provides full area imprint at a reduced pressure of 25 bar. The replicated structures, lines of 300 nm, are of high quality and image the stamp structures. Stamps prepared in this way allow a tuning of the flexibility simply by changing the thickness of the backplane.