Thermal Nanoimprint Lithography Research Articles

Development of the Soaking Method for Manufacturing Polymer Electrolyte Fuel Cells with a High-Aspect-Ratio Microstructure

1. IntroductionPolymer electrolyte fuel cells (PEFCs) are one of the major devices utilizing hydrogen. Owing to its small size and capacity of low-temperature operation, PEFCs are used in practical applications such as fuel cell vehicles.The power generation efficiency of PEFC is relatively low. Heinzmann et al. suggested that the main cause of low efficiency is the chemical reactions at the cathode [1]. Therefore, having a large triple phase boundary (TPB), where an oxygen reduction reaction occurs, is crucial. Bea et al. reported that the performance of PEFCs was improved by introducing micro-patterns on the surface of a Nafion membrane [2]. Organizing a high-aspect-ratio microstructure on its surface is more effective for increasing the size of the TPB.In the powder molding field, a typical problem exists: particles in solution form bridge structures because Van der Waals forces are dominant at the micrometer scale. Thus, fabricating a high-aspect-ratio microstructure on the surface of a Nafion membrane is challenging.2. ExperimentLine-and-space patterns with three different dimensions—width/spacing/height of 1/1/2 μm, respectively—were introduced onto the cathode surface via thermal nanoimprint lithography (120 °C; 5 MPa). The catalyst ink was prepared using Pt/C (TEC10V50E, Tanaka Kikinzoku Kogyo K.K.), deionized water, and 10 wt.% ionomer solution. Two samples (samples A and B) were fabricated by spraying the catalyst ink on the micro-patterned surface with a 1-cm2 mask attached at 80 °C and 30 °C, respectively. Moreover, the time between one push and another was controlled at 5 s and 0.5 s, respectively. The same amount of catalyst ink was sprayed on both samples under the same relative pressure (0.1 MPa).After drying, the cross-sections of both samples were investigated using scanning electron microscopy (SEM).3. Results and discussionFigure 1 shows the appearance of each sample immediately after spraying. As shown in Figure 1 (a), the ink on sample A had already dried, whereas, as shown in Figure 1 (b), the Nafion membrane of sample B was soaked in the ink. Figure 2 shows the SEM images of the cross-sections of both samples. As shown in Figure 2 (a), the catalyst layer formed a bridge structure in sample A. On the contrary, as shown in Figure 2 (b), the catalyst layer goes deep into the spaces of line-and-space patterns without forming a bridge structure in sample B.Figure 3 shows the mechanism of bridge structure formation. Water accounted for 99 wt.% of the catalyst ink; therefore, the ink is incompatible with the water-repellent Nafion membrane. The reason for forming a bridge structure when normally sprayed as sample A can be that water, which is a solvent, rapidly evaporates before the catalyst ink enters the spaces of line-and-space patterns. On the contrary, the reason for preventing bridge structure from forming when soaking can be that water evaporates after the catalyst ink goes deep into the spaces.4. ConclusionsBy using the original soaking method, a high-aspect-ratio microstructure was successfully arranged in the cathode without the formation of a bridge structure. When a cell is manufactured, the surface of its cathode with micro patterns will first be soaked and dried as the process of making sample B. Subsequently, catalyst ink was normally sprayed on both the cathode and anode surfaces as much as needed. Optimization of the soaking process parameters is required to manufacture a higher-aspect-ratio microstructure and to increase the size of the TPB.

Enhanced Mechanical and Thermal Resistances of Nanoimprinted Antireflective Moth‐Eye Surfaces Based on Poly Vinylidene Fluoride/TiO2 Surface Nanocomposites

Herein, the fabrication of bioinspired moth‐eye antireflective (AR) films based on surface nanocomposites of poly vinylidene fluoride (PVDF) and TiO2 nanoparticles produced in a single thermal nanoimprint lithography step is described. The incorporation of nanoparticles enhances the mechanical and thermal stability of the AR topography as demonstrated by nanoindentation tests and in situ temperature‐dependent grazing incidence X‐ray scattering measurements using synchrotron radiation. The effect of thermal annealing and UV radiation on the degradation in the optical performance and durability of the AR films is also evaluated in weathering tests. Improving the thermal and mechanical behavior as well as the long‐term durability of nanoimprinted polymer AR films will significantly expand their potential for implementation in solar devices.

Controllable fabrication of polymeric nanowires by NIL technique and self-assembled AAO template for SERS application

A controllable strategy to fabricate the polymeric nanowires with high throughput and low cost is developed by using the thermal nanoimprint lithography (NIL) technique and self-assembled anodic aluminum oxide (AAO) template. The length of polymeric nanowires can be controlled by adjusting the duration of thermal NIL. A fill mechanism of thermoplastic intermediate polymer stamp (IPS) polymer pressed into the AAO nanopores is closely studied. The as-prepared IPS polymeric nanowire-based Surface-Enhanced Raman Scattering (SERS)-active substrate exhibits a remarkable reproducibility. The effective adsorption of the R6G as probe molecule near to hotspots generated at 3D vertically aligned polymeric nanowire SERS active substrates shows extraordinary enhancement of Raman signal with an enhancement factor (EF) of 105–106. The present strategy is of great guiding significance to broaden the use of thermal NIL technique and AAO template for the fabrication of other nanomaterials, especially for the flexible and transparent polymer-based nanomaterials.

Proposal of hybrid deep learning systems for process and material design in thermal nanoimprint lithography

Proposal of hybrid deep learning systems for process and material design in thermal nanoimprint lithography

Bottom-Up Development of Nanoimprinted PLLA Composite Films with Enhanced Antibacterial Properties for Smart Packaging Applications

In this work, polymer nanocomposite films based on poly(L-lactic acid) (PLLA) were reinforced with mesoporous silica nanoparticles, mesoporous cellular foam (MCF) and Santa Barbara amorphous-15 (SBA). PLLA is a biobased aliphatic polyester, that possesses excellent thermomechanical properties, and has already been commercialized for packaging applications. The aim was to utilize nanoparticles that have already been established as nanocarriers to enhance the mechanical and thermal properties of PLLA. Since the introduction of antibacterial properties has become an emerging trend in packaging applications, to achieve an effective antimicrobial activity, micro/nano 3D micropillars decorated with cone- and needle-shaped nanostructures were implemented on the surface of the films by means of thermal nanoimprint lithography (t-NIL), a novel and feasible fabrication technique with multiple industrial applications. The materials were characterized regarding their composition and crystallinity using Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD), respectively, and their thermal properties using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Their mechanical properties were examined by the nanoindentation technique, while the films’ antimicrobial activity against the bacteria Escherichia coli and Staphylococcus aureus strains was tested in vitro. The results demonstrated the successful production of nanocomposite PLLA films, which exhibited improved mechanical and thermal properties compared to the pristine material, as well as notable antibacterial activity, setting new groundwork for the potential development of biobased smart packaging materials.

Bioinspired antireflective flexible films with optimized mechanical resistance fabricated by roll to roll thermal nanoimprint

This work describes the fabrication process of moth eye antireflective poly (methyl methacrylate) transparent films via roll to roll thermal nanoimprint lithography. The process parameters are investigated and adjusted in order to obtain from a single moth-eye structured mold, a range of antireflective topographies that gradually vary their geometry from protruding to intruding nanocones. A correlation between the process parameters with the optical and mechanical properties of the films is established to illustrate the influence of the processing parameters and serve as guideline to produce antireflective flexible films with balanced properties and optimized performance adequate to the application environment. A finite element model is described predicting the mechanical behavior of the moth-eye PMMA imprinted nanostructures.

Nanoimprinting for High-Throughput Fabrication of Geometrically Precise Pillars to Regulate Cell Behavior

Developing high-throughput nanopatterning techniques that also allow for precise control over the dimensions of the fabricated features is essential for the study of cell-nanopattern interactions. Here, we developed a process that fulfills both of these criteria. Firstly, we used electron-beam lithography (EBL) to fabricate precisely controlled arrays of high aspect ratio submicron pillars with varying values of interspacing on a large area (3 × 5 mm2) of fused silica substrates. Two types of etching procedures with two different systems were developed to etch the fused silica and create the final desired height. We then studied the interactions of preosteoblasts (MC3T3-E1) with these pillars. Varying interspacing was observed to significantly affect the morphological characteristics of the cell, the organization of actin fibers, and the formation of focal adhesions. The expression of osteopontin (OPN) significantly increased on the patterns, indicating the potential of the pillars for inducing osteogenic differentiation. The EBL arrays of pillars were thereafter used as master molds in two subsequent processing steps, namely soft lithography (using hybrid PDMS) and thermal nanoimprint lithography for high-fidelity replication of the pillars on the substrates of interest. The molding parameters were optimized to maximize the fidelity of the generated patterns and minimize the wear and tear of the master mold. Comparing the replicated feature with those present on the original mold confirmed that the geometry and dimensions of the replicated pillars closely resemble those of the original ones (i.e., 1% deviation in pillar interspacing, < 20% deviation in pillar diameter). The method proposed in this study, therefore, enables the precise fabrication of submicron- and nanopatterns on a wide variety of substrate materials that are relevant for systematic cell studies.

Gradient wettability induced by deterministically patterned nanostructures

We report a large-scale surface with continuously varying wettability induced by ordered gradient nanostructures. The gradient pattern is generated from nonuniform interference lithography by utilizing the Gaussian-shaped intensity distribution of two coherent laser beams. We also develop a facile fabrication method to directly transfer a photoresist pattern into an ultraviolet (UV)-cured high-strength replication molding material, which eliminates the need for high-cost reactive ion etching and e-beam evaporation during the mold fabrication process. This facile mold is then used for the reproducible production of surfaces with gradient wettability using thermal-nanoimprint lithography (NIL). In addition, the wetting behavior of water droplets on the surface with the gradient nanostructures and therefore gradient wettability is investigated. A hybrid wetting model is proposed and theoretically captures the contact angle measurement results, shedding light on the wetting behavior of a liquid on structures patterned at the nanoscale.

Atomistic Investigation of Material Deformation Behavior of Polystyrene in Nanoimprint Lithography

This research investigates deformation behavior of polystyrene (PS) as a thermoplastic resist material for the thermal nanoimprint lithography (T-NIL) process. Molecular dynamics modeling was conducted on a PS substrate with dimensions 58 × 65 × 61 Å that was imprinted with a rigid, spherical indenter. The effect of indenter size, force, and imprinting duration were evaluated in terms of indentation depth, penetration depth, recovery depth, and recovery percentage of the polymer. The results show that the largest indenter, regardless of force, has the most significant impact on deformation behavior. The 40 Å indenter with a 1 µN of force caused the surface molecules to descend to the lowest point compared to the other indenters. An increase in indenter size resulted in higher penetration depth, recovery depth, and recovery percentage. Higher durations of imprint cycle (400 fs) resulted in plastic deformation of the PS material with minimal recovery (4 Å). The results of this research lay the foundation for explaining the effect of several T-NIL process parameters on virgin PS thermoplastic resist material.

Patterned dextran ester films as a tailorable cell culture platform

The elucidation of cell-surface interactions and the development of model platforms to help uncover their underlying mechanisms remains vital to the design of effective biomaterials. To this end, dextran palmitates with varying degrees of substitution were synthesised with a multipurpose functionality: an ability to modulate surface energy through surface chemistry, and an ideal thermal behaviour for patterning. Herein, dextran palmitate films are produced by spin coating, and patterned by thermal nanoimprint lithography with nano-to-microscale topographies. These films of moderately hydrophobic polysaccharide esters with low nanoscale roughness performed as well as fibronectin coatings in the culture of bovine aortic endothelial cells. Upon patterning, they display distinct regions of roughness, restricting cell adhesion to the smoothest surfaces, while guiding multicellular arrangements in the patterned topographies. The development of biomaterial interfaces through topochemical fabrication such as this could prove useful in understanding protein and cell-surface interactions.

Molecular dynamics investigation of material deformation behavior of PMMA in nanoimprint lithography

Computational analysis of polymeric materials plays a key role in defining their tribological characteristics. This research investigates the deformation behavior of poly(methylmethacrylate) (PMMA) as a thermoplastic resist material for the thermal nanoimprint lithography (T-NIL) process. Molecular dynamics modeling was conducted on a PMMA substrate imprinted with a rigid spherical indenter. The effect of indenter size, force, and imprinting duration on the indentation depth, penetration depth, recovery depth, and recovery percentage of the polymer was evaluated. The results showed that the largest indenter, regardless of force, had the most significant impact on deformation behavior. The 40-Å indenter with a 1 μN of force caused the surface molecules to descend to the lowest point compared to the other indenters. An increase in the indenter size resulted in a higher penetration depth, recovery depth, and recovery percentage. Higher durations of imprint cycle (400 fs) resulted in plastic deformation of the PMMA material with minimal recovery (30 Å). The results of this research lay foundation for explaining the effect of several T-NIL process parameters on the virgin PMMA thermoplastic resist material.

Nanoelectrode Arrays Fabricated by Thermal Nanoimprint Lithography for Biosensing Application.

Electrochemical sensors are devices capable of detecting molecules and biomolecules in solutions and determining the concentration through direct electrical measurements. These systems can be miniaturized to a size less than 1 µm through the creation of small-size arrays of nanoelectrodes (NEA), offering advantages in terms of increased sensitivity and compactness. In this work, we present the fabrication of an electrochemical platform based on an array of nanoelectrodes (NEA) and its possible use for the detection of antigens of interest. NEAs were fabricated by forming arrays of nanoholes on a thin film of polycarbonate (PC) deposited on boron-doped diamond (BDD) macroelectrodes by thermal nanoimprint lithography (TNIL), which demonstrated to be a highly reliable and reproducible process. As proof of principle, gliadin protein fragments were physisorbed on the polycarbonate surface of NEAs and detected by immuno-indirect assay using a secondary antibody labelled with horseradish peroxidase (HRP). This method allows a successful detection of gliadin, in the range of concentration of 0.5–10 μg/mL, by cyclic voltammetry taking advantage from the properties of NEAs to strongly suppress the capacitive background signal. We demonstrate that the characteristics of the TNIL technology in the fabrication of high-resolution nanostructures together with their low-cost production, may allow to scale up the production of NEAs-based electrochemical sensing platform to monitor biochemical molecules for both food and biomedical applications.

Magnetic force assisted thermal nanoimprint lithography (MF-TNIL) for cost-effective fabrication of 2D nanosquare array

The work presented here describes a simple, low-cost, and unconventional technique to fabricate a 2D nanosquare array using magnetic force assisted thermal nanoimprint lithography (MF-TNIL). The nanofabrication process involves two steps: (i) fabrication of a 2D nanosquare array template on a laminated plastic sheet via sequential thermal nanoimprinting of linear nanograting polydimethylsiloxane (PDMS) stamp, and (ii) reversal imprinting of template on UV curable polymer using soft UV-nanoimprint lithography. Without using an expensive nanofabrication tool, our proposed technique can fabricate nanosquare array over an area of 1 × 1 cm2 with individual nanosquare having a feature size of about 383 nm × 354 nm × 70 nm. We believe that our proposed MF-TNIL represents a promising nanofabrication technique that will allow fabricating various types of nanostructures for their applications in developing sensors, anti-reflective surfaces, self-cleaning surfaces, etc.

3D Micro/Nanopatterning of a Vinylferrocene Copolymer.

In nanoimprint lithography (NIL), a pattern is created by mechanical deformation of an imprint resist via embossing with a stamp, where the adhesion behavior during the filling of the imprint stamp and its subsequent detachment may impose some practical challenges. Here we explored thermal and reverse NIL patterning of polyvinylferrocene and vinylferrocene-methyl methacrylate copolymers to prepare complex non-spherical objects and patterns. While neat polyvinylferrocene was found to be unsuitable for NIL, freshly-prepared vinylferrocene-methyl methacrylate copolymers, for which identity and purity were established, have been structured into 3D-micro/nano-patterns using NIL. The cross-, square-, and circle-shaped columnar structures form a 3 × 3 mm arrangement with periodicity of 3 µm, 1 µm, 542 nm, and 506 nm. According to our findings, vinylferrocene-methyl methacrylate copolymers can be imprinted without further additives in NIL processes, which opens the way for redox-responsive 3D-nano/micro-objects and patterns via NIL to be explored in the future.

Fabrication of a Metal Roller Mold with Nanoimprinted Pattern Using Thermal Nanoimprint Lithography

In this work, fabrication of a metal roller mold with nanoimprinted pattern was demonstrated. To get metal nanopattern on a metal roller mold, thermal nanoimprint lithography (TNIL) and subsequent electroforming process were conducted. A poly(methyl methacrylate) (PMMA) nanopattern was fabricated by TNIL process using a polydimethylsiloxane (PDMS) soft stamp on a bare PMMA film. An optimal experimental condition of TNIL process was investigated for large area, uniform PMMA nanopatterning. As a result, large area PMMA line patterns with 200 nm linewidth were fabricated by large-area TNIL process. Electroforming process on the PMMA nanopatterned film resulted in nickel (Ni) nanopattern with a linewidth of 200 nm from the PMMA line pattern. A large area (360 mm by 730 mm in width and length) Ni stamp for a roller mold was fabricated by laser cutting and tiling process of the multiple electroformed Ni stamps. We successfully fabricated a Ni roller mold with feature size of 200 nm in linewidth by attaching the large area Ni stamp to the surface of a roller body.

Direct Nanoimprint Lithography of Polyethersulfone Using Cellulose‐Based Mold

AbstractPolyethersulfone (PES) with a glass transition temperature of 224 °C is a class of high‐performance super engineering plastics with excellent heat resistance, as well as mechanical, electrical, and optical properties. However, the nanopatterning of PES using thermal nanoimprint lithography at a high temperature can induce technical difficulties. High‐accuracy 900‐nm line patterning of PES is fabricated 50 times using both a solvent‐permeable cellulose‐based mold and dimethylformamide as a dilution solvent in direct thermal nanoimprint lithography. The use of dilution solvents in PES has yet been limited in the nanoimprint lithography, and the developed processes expand the surface patterning of super engineering plastic.

Study on vibration-assisted thermal nanoimprint lithography

In thermal nanoimprint lithography, polymethyl methacrylate (PMMA) with good thermoplasticity is often used as transferring media. However, due to the special properties of PMMA, the accuracy of microstructure transferred to the surface will be seriously affected. To address this problem, the micro-vibration by piezo-driven is introduced in conventional thermal nanoimprint lithography. Because of the micro-vibration will change the contact stress during the embossing process and reduce the contact time. This will improve the filling rate of PMMA during the embossing process and transfer the pattern of stamp to PMMA with excellent feature fidelity. Compared with the traditional nanoimprint lithography, the filling rate of PMMA is increased by 92% after the introduction of vibration by piezo-driven. Then, a new motion model is built to verify the rationality of introducing vibration. Besides, the effect of frequency and amplitude with filling rate is analyzed by finite element method to obtain the best frequency and amplitude for experiments. Finally, the transferred pattern is observed by experiment.

Enhancing biosensing sensitivity of metal nanostructures through site-selective binding

The localised surface plasmon resonance (LSPR) at the surface of metal nanostructures can induce a highly intense electromagnetic (EM) field, which is confined to the edges with big curvature or at narrow gaps between nanostructures. Therefore, the localisation of target molecules at these sites is crucial to achieve high sensitivity in LSPR-based biosensors. To this end, we fabricated a 40 nm high gold nano-truncated cone (GNTC) array using thermal nanoimprint lithography. As the EM field is most intense at the side surface and relatively weak at the top surface of GNTC, we improved the detection sensitivity by blocking the top surface with oxides to limit adsorption of antibodies and antigens to the top surface. We observed the difference in sensitivity by detecting α-fetoprotein (AFP) on the oxide-capped and uncapped GNTC arrays through sandwich immunoassay and enzymatic precipitation. The capped GNTC array exhibited higher detection sensitivity than the uncapped one. Particularly, six-fold enhancement of sensitivity was achieved in the serum sample. We used atomic force microscopy and electron microscopy to validate that the deposition of the oxides on the top surface of GNTC effectively blocked the adsorption of the biomolecules and the target molecules were preferentially adsorbed on the side surfaces.

Athermal and Soft Multi‐Nanopatterning of Azopolymers: Phototunable Mechanical Properties

AbstractImprinting nanopatterns on flexible substrates has diverse applications in advanced fabrication. However, the traditional thermal nanoimprint lithography (T‐NIL) often causes shrinkage upon cooling. Here, a simple yet versatile method is introduced to fabricate multiple nanopatterns on a flexible substrate coated with an azopolymer by combining athermal nanoimprint lithography (AT‐NIL) and photolithography. The azopolymer has various mechanical properties upon photoirradiation: 1) phototunable glass‐transition temperatures (Tg) and concomitantly photoinduced switch from glassy plastic to viscoplastic polymer; 2) prominent modulation of viscoplasticity under light illumination at different wavelengths. Regionally selective multiple nanopatterns are conveniently fabricated, presenting angle‐dependent structural color images on poly(ethylene terephthalate) (PET) substrates. The flexible, athermal and multiple nanopatterning method has the potential for on‐demand fabrication of complex nanopatterns.

Athermal and Soft Multi-Nanopatterning of Azopolymers: Phototunable Mechanical Properties.

Imprinting nanopatterns on flexible substrates has diverse applications in advanced fabrication. However, the traditional thermal nanoimprint lithography (T-NIL) often causes shrinkage upon cooling. Here, a simple yet versatile method is introduced to fabricate multiple nanopatterns on a flexible substrate coated with an azopolymer by combining athermal nanoimprint lithography (AT-NIL) and photolithography. The azopolymer has various mechanical properties upon photoirradiation: 1) phototunable glass-transition temperatures (Tg ) and concomitantly photoinduced switch from glassy plastic to viscoplastic polymer; 2) prominent modulation of viscoplasticity under light illumination at different wavelengths. Regionally selective multiple nanopatterns are conveniently fabricated, presenting angle-dependent structural color images on poly(ethylene terephthalate) (PET) substrates. The flexible, athermal and multiple nanopatterning method has the potential for on-demand fabrication of complex nanopatterns.