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SEM images of 30 nm PEG-DA dot patterns of orthogonal and hexagonal arrays formed with ((a), (b)) PFPE and ((c), (d)) PUA molds. As shown, no demolding problems were observed.
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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 coeffi...
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... calculated values were as follows: W PFPE / PMMA = 32 . 36 mJ m − 2 , W PUA / PMMA = 45 . 23 mJ m − 2 , W PFPE / PS = 32 . 02 mJ m − 2 and W PUA / PS = 26 . 67 mJ m − 2 . Several notable findings are derived from the calculated values. In the case of PMMA film, the work of adhesion of the PFPE mold was smaller than that of the PUA mold, in good agreement with the experimental results shown in figures 3 and 4. In particular, the contribution from the polar component in equation (2) was 41 and 67% for the PFPE and PUA molds, respectively, suggesting that the work of adhesion of the PUA mold to PMMA film is mainly determined by the polar component. In the case of PS film, on the other hand, the works of adhesion for both molds were smaller than those for PMMA film, corresponding to the results shown in figure 5. The polar component took up approximately 18 and 29% for the PFPE and PUA molds, respectively, which is readily understood by considering the relatively non-polar, hydrophobic properties of PS film. Although it is difficult to set a lower limit of the work of adhesion, it seems that the onset of pattern failure might occur when the fractional contribution from the polar component is higher than ∼ 60% for the conditions used in our experiment. Obviously, this criterion is subject to change depending on various conditions such as substrate cleaning method (i.e. the work of adhesion at the film/substrate interface), molecular weight of polymer, imprinting parameters (pressure and temperature), mold thickness, etc. Nonetheless, the fractional contribution from the polar component would be useful as an index to predict the clean release of the mold. In conjunction with these findings, we also measured macroscopic adhesion force of the mold with an imprint resist. The macroscopic peeling force was measured by a hanging test, yielding the force in the range of 0.1–0.3 kg cm − 2 for both polymer molds. Interestingly, the measured forces were nearly the same within the error range. We also attempted to measure nanoscopic adhesion force by measuring the pull-off forces of flat polymer molds with AFM setup in air. The pull-off force was obtained in force–distance mode using AFM software. The data were averaged at least over 80 locations on each polymer mold. The RMS surface roughness was 0.31 nm (PFPE mold) and 0.43 nm (PUA mold). It turned out that the measured values were 1.19 nN and 1.27 nN for the PFPE and PUA molds, respectively, indicating that the macroscopic adhesion force between the mold and the imprint resist would be similar. This further indicates that the demolding behaviors would be governed by the local adhesion energy at the structure/mold interface, especially by the polar component of the surface energy as described earlier. In addition to the adhesion force, the peeling mode (e.g. peeling direction/speed and mold thickness) is important to generate patterns without defects. With the help of flexible properties, the two polymer molds used in our study can be used to investigate the various peeling parameters mentioned above with manual control. It turns out that the demolding behavior was not notably affected by the peeling direction with a range of peeling speeds (0.1–1 cm s − 1 ). It seems that the isotropic geometry of dot arrays lends themselves to be relatively inert with the peeling direction. In the case of an anisotropic geometry, however, the peeling direction would be a key parameter to obtain high pattern fidelity. To demonstrate this, we fabricated 400 nm line patterns with a patterned field of 2 × 2 cm 2 and found that the replicated lines were distorted slightly towards the peeling direction (supplemental information, figure S1 available at stacks.iop.org/Nano/23/ 235303/mmedia). Regarding the effect of mold thickness, we did not observe significant changes in the demolding behavior with different thickness of PFPE and PUA layers. It seems that the mold’s flexibility is solely determined by the thickness of the supporting PET layer ( ∼ 50 μ m). Although a more systematic study would be required to elucidate the detailed demolding mechanism, these experiments suggest that at least the dot arrays could be formed without the need of careful control of the peeling mode. For the temperature-assisted imprinting process, polymer molds are inevitably expanded and then shrunk with temperature cycle, potentially posing a problem to mold removal. To understand the interplay between surface energy and thermal expansion, the CTEs of PFPE and PUA molds were measured, along with their elastic moduli. Also, the CTE and elastic modulus of hard PDMS (h-PDMS) material were used for comparison, which are available from the literature [27]. As summarized in table 2, the CTEs of three different mold materials are on the same order of 10 − 4 ◦ C − 1 , with the minimum being observed from the PUA mold ( ∼ 1 / 3 of hard PDMS). The fact that the CTE of PFPE is higher than that of PUA indicates that the surface tension is more involved in the demolding of the high-temperature imprinting process. To investigate the combined effects of surface tension and CTE, we created 30 nm dot arrays of PEG-DA, which was performed at ambient conditions by UV exposure. When curing such a UV-curable material, a significant volumetric shrinkage occurs ( ∼ 6%) in the course of photo-crosslinking, which would result in a clean release of the mold [28]. As shown in figure 6, 30 nm dot patterns of orthogonal and hexagonal arrays were fabricated with better pattern fidelity, without appreciable delamination or breakage of pillars. Strikingly, the surface tension of PEG-DA (73.27 mJ m − 2 ) [29] is much higher than those of PMMA and PS, suggesting that the CTE indeed comes into play when replicating high-density nanopatterns. We have investigated the combined effects of surface tension and thermal expansion coefficient that determine the demolding characteristic of two UV-curable, flexible polymer molds of PFPE and PUA for 30 nm scale nanoimprinting. It was found that the effect of surface tension is dominant over thermal expansion for the high-temperature imprinting process with PMMA or PS films. In particular, the fractional contribution from the polar component was used as a measure to predict pattern failure; when the fraction was higher than ∼ 60%, delamination or breakage of replicated features was observed. For a low-temperature imprinting process with UV-curable PEG-DA films, a clean release of the mold was found in spite of high surface tension of PEG-DA (i.e. high work of adhesion), indicating the importance of thermal expansion in the demolding step. However, it is not possible at this stage to decouple the combined effects of surface tension and CTE, for which a further study would be required. Taken together, our results suggest that very small features down to 30 nm scale can be fabricated with flexible polymer molds without appreciable failures, but the mold material needs to be carefully chosen for ensuring high pattern fidelity. Also, these results would be useful to understand and control the demolding step in other contact-based lithographic methods. This work was supported by the Center for Nanoscale Mechatronics and Manufacturing (08K1401-00210), one of the 21st Century Frontier Research Programs in Korea, and supported in part by a Korea Research Foundation grant (KRF-J03003), WCU (World Class University) programs (R31-2008-000-10083-0, R31-2008-000-10075-0) and the Global Frontier R&D Program on Center for Multiscale Energy ...
Citations
... The thermal nanoimprinting temperatures must be greater than the glass transition temperature (T g ) and melting temperature (T m ) of polymers to ensure the sufficient movement of the polymer chain (Shibata et al. 2008;Muanchan et al. 2017Muanchan et al. , 2019b. Moreover, viscoelastic properties, thermal expansion, and adhesion between the film and mold affect the transferability and replication quality of the imprinting structure (Scheer et al. 2008;Kim et al. 2012;Muanchan et al. 2017;Li et al. 2021). Hence, optimizing the temperature, pressure, and time in the thermal imprinting process as well as surface tension between the polymer and imprinted mold is crucial for preventing defects of replicated thermal imprinting micro-and nanopatterns. ...
... The surface properties of PVA and the PVA/CNF composites were comparable. Hence, the high surface free energy and low friction coefficient of the films would yield fewer defects and improve demolding in the thermal imprinting process by using the fluoropolymer surface-treated Ni-P mold (Kim et al. 2012;Muanchan et al. 2019a;Li et al. 2021). ...
... At a temperature of greater than 140 °C, the thermal dimensional stability of the PVA film drastically decreased, with high CTE values as shown in Fig. 6a and b. The decrease in the thermal dimensional stability would restrict the thermal imprinting of the PVA film at high temperatures (Kim et al. 2012;Biswas et al. 2019). The addition of CNFs improved the thermal dimensional stability and CTE of the PVA/CNF composite films (Biswas et al. 2017(Biswas et al. , 2019. ...
In this study, cellulose nanofibers (CNFs) were treated in an ultrahigh-pressure homogenizer (UHPH) in the thermal imprinting process. After 20 passes, UHPH-treated CNFs with concentrations of 1, 2, and 5 wt% were used to reinforce polyvinyl alcohol (PVA), and PVA/CNF composite films were fabricated by solvent casting. The composite films were subjected to thermal imprinting using a fluoropolymer-treated nickel–phosphorus mold with various line/space micropatterns at different mold temperatures. In addition, the effect of the CNF contents on the viscoelastic behavior, thermomechanical properties, thermal dimensional stability, surface properties, and replication performance of the PVA/CNF composites were investigated. The viscoelastic properties and induced stiffness were improved as a result of the solid-like behavior and CNF network structure, and the coefficient of thermal expansion (CTE) decreased with the increase in the CNF content. The low modulus of the film at thermal imprinting temperatures increased transferability, while the high stiffness, molecular recovery, and low CTE of the composites secured the micropatterns and increased the replication quality of the thermal imprinted PVA/CNF composite films. Results suggested that the addition of CNFs can improve the processability of the thermal imprinting process. Notably, the high replication of films for all micropatterns was observed at a CNF content of 5 wt%. Nevertheless, the narrow micropattern of 5 wt% CNFs could be thermally imprinted at a temperature of less than 120 °C.
... PMMA was frequently adopted as a good candidate polymer for comparison with PS because it has similar glass-transition temperature (105 and 100°C, respectively), molecular weight of monomer (100 and 104 g mol −1 ), surface tension (40.2 and 42.0 mN/m), and solubility parameter (18.47 and 19.26 MPa 1/2 ). 63 Meanwhile, PMMA has distinguish difference with PS due to ester groups instead of benzene groups, resulting in higher water adsorption, lower CA, and higher adhesion with polar molecules. 63 Results of the solvent category of PMMA films are summarized in Table 4. Compared with that of PS films, PMMA films from acetone (20), cyclohexanone (12), and DMSO (15) that contain polarized carbonyl or sulfoxide groups are categorized as "rare holes" and from toluene (9) and m-xylene (19) that less polar molecules turned to be the categories of "several holes" or "many holes." ...
... 63 Meanwhile, PMMA has distinguish difference with PS due to ester groups instead of benzene groups, resulting in higher water adsorption, lower CA, and higher adhesion with polar molecules. 63 Results of the solvent category of PMMA films are summarized in Table 4. Compared with that of PS films, PMMA films from acetone (20), cyclohexanone (12), and DMSO (15) that contain polarized carbonyl or sulfoxide groups are categorized as "rare holes" and from toluene (9) and m-xylene (19) that less polar molecules turned to be the categories of "several holes" or "many holes." Morphologies of prepared films are summarized in Figure S10. ...
... The molding of surfaces that are covered in nano-and micron-scale features has already been achieved using various techniques such as soft lithography, photolithography, injection molding, hot embossing, atomic layer deposition, capillary force lithography and ultraviolet nanoimprint lithography (NIL) [26][27][28][29][30][31][32][33][34][35]. Depending on the molding polymer and technique employed, feature resolutions of 100 nm can be achieved, with reports of features as small as 6 nm in diameter [36][37][38]. Zhang et al. used wings as templates for nanoimprint lithography to make a negative mold in polymethyl methacrylate (PMMA), followed by a deposition of a thick gold layer, to produce a gold replica of the wing itself [39]. Similarly, Wang et al. reported on a rapid fabrication method for producing a PMMA replica of a cicada wing and studied its anti-reflection properties. ...
... There are advantages and disadvantages to the above reported techniques, but the main drawbacks are related to the time consumed due to increased steps in the procedure and overall fabrication costs. This is especially true if expensive instrumentation is required to create molding templates using ion-based lithography methods, high pressures required for NIL, or resists for photolithography [37,[42][43][44][45]. While the long-term development of these types of surfaces will rely on scalable processes, and not molding of natural materials, we saw the need to better understand the resolution of molding processes on various structures, as it was clear that the role topography played, and the development of surfaces with varied topographies, would be an important next step in this field. ...
... The high surface energy (62 mN·m −1 ) and high modulus of the PEG primary also played a role in the difficulties with mold separation [58,59]. Kim et al. highlighted that the polar component of surface energy may play a critical role in the clean release of the molds while replicating polymeric nanopillar arrays [37]. ...
Recent studies have shown that insect wings have evolved to have micro- and nanoscale structures on the wing surface, and biomimetic research aims to transfer such structures to application- specific materials. Herein, we describe a simple and cost-effective method of replica molding the wing topographies of four cicada species using UV-curable polymers. Different polymer blends of polyethylene glycol diacrylate and polypropylene glycol diacrylate were used as molding materials and a molding chamber was designed to precisely control the x, y, and z dimensions. Analysis by scanning electron microscopy showed that structures ranged from 148 to 854 nm in diameter, with a height range of 191–2368 nm, and wing patterns were transferred with high fidelity to the crosslinked polymer. Finally, bacterial cell studies show that the wing replicas possess the same antibacterial effect as the cicada wing from which they were molded. Overall, this work shows a quick and simple method for patterning UV-curable polymers without the use of expensive equipment, making it a highly accessible means of producing microstructured materials with biological properties.
... That is, PDMS has a low Young's modulus (∼2.0 Mpa) and low surface energy (∼16 mJ/m 2 ) that allows for conformal contact and easy release from both a master mold and imprinted patterns. Furthermore, PDMS demonstrates relatively high toughness with a high elongation at break (>150%) which provides great degree of flexibility during patterning conditions [53,56,74,75]. Furthermore, it has high gas permeability, which allows the air trapped between the soft mold and the imprinted substrate to be released through the soft mold itself. ...
Nanoimprint lithography has attracted considerable attention in academic and industrial fields as one of the most prominent lithographic techniques for the fabrication of the nanoscale devices. Effectively controllable shapes of fabricated elements, extremely high resolution, and cost-effectiveness of this especial lithographic system have shown unlimited potential to be utilized for practical applications. In the past decade, many different lithographic techniques have been developed such as electron beam lithography, photolithography, and nanoimprint lithography. Among them, nanoimprint lithography has proven to have not only various advantages that other lithographic techniques have but also potential to minimize the limitations of current lithographic techniques. In this review, we summarize current lithography techniques and, furthermore, investigate the nanoimprint lithography in detail in particular focusing on the types of molds. Nanoimprint lithography can be categorized into three different techniques (hard-mold, soft-mold, and hybrid nanoimprint) depending upon the molds for imprint with different advantages and disadvantages. With numerous studies and improvements, nanoimprint lithography has shown great potential which maximizes its effectiveness in patterning by minimizing its limitations. This technique will surely be the next generation lithographic technique which will open the new paradigm for the patterning and fabrication in nanoscale devices in industry.
... Finally, the cured PDMS mold was carefully detached from silicon wafer. [31,32] Fabrication of photovoltaic devices: 30-40 nm thick PEDOT:PSS (AI4083, clevious) films wer spin-coated on the mold and directly stamped onto a precleaned ITO substrate at 90 8C under 0.1 kgf (1 kg f = 9.81 N) for 20 s. After peeling off the mold from the transferred PEDOT:PSS film at about 2 mm s À1 , the transferred film was then thermally annealed at 150 8C for 15 min. ...
Polymer solar cells with enhanced initial cell performances and long-term stability were fabricated by performing a simple dry transfer of a hole extraction layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] onto an indium tin oxide (ITO) substrate. Due to the very flat surface of the polyurethane acrylate/polycarbonate (PUA/PC) film, which was used as a mold and resembled the surface of the original substrate (silicon wafer), the transferred layer had a very smooth surface morphology, resulting in enhancement of the interfacial characteristics. The work function of the PEDOT:PSS layer and the morphology of bulk hetero junction (BHJ) layer were tuned by controlling the position of PSS enrichment in the PEDOT:PSS layer using the dry transfer. The power conversion efficiency of PTB7:PC71 BM BHJ device prepared by the dry transfer was 8.06 %, which was significantly higher than that of the spin-cast device (7.32 %). By avoiding direct contact between the ITO substrate and the PEDOT:PSS solution in the dry transfer system, etching and diffusion of indium in the ITO substrate were greatly reduced, thereby improving the stability.
... After separation of the blank layer, the exposed PUA pillars are clearly observed with the background of the PFPE polymer. In our experiment, the hydrophilic PUA 34,35 was mostly used as upper and lower moulds in order to replicate the hydrophobic PFPE resins [38][39][40] ; the materials can be swapped to obtain the exposed PFPE pillars with the background of the dewetted PUA domain (Fig. 1b). Although not shown, the PFPE mould could even dewet a highly viscous (140 cps) Norland Optical Adhesive (NOA73) 41 and other UV-curable resins in the same one-step moulding process. ...
Membranes with nano-apertures are versatile templates that possess a wide range of electronic, optical and biomedical applications. However, such membranes have been limited to silicon-based inorganic materials to utilize standard semiconductor processes. Here we report a new type of flexible and free-standing polymeric membrane with nano-apertures by exploiting high-wettability difference and geometrical reinforcement via multiscale, multilevel architecture. In the method, polymeric membranes with various pore sizes (50-800 nm) and shapes (dots, lines) are fabricated by a hierarchical mould-based dewetting of ultraviolet-curable resins. In particular, the nano-pores are monolithically integrated on a two-level hierarchical supporting layer, allowing for the rapid (<5 min) and robust formation of multiscale and multilevel nano-apertures over large areas (2 × 2 cm(2)).
We examined the effect of environmental temperature (–40 to 70 °C) on the hydrogen sensing performance of a Pd nanogap sensor supported on an elastomeric polydimethylsiloxane (PDMS) substrate. Sensing tests were conducted using various sensors with different gap widths, prepared with different tensile strains. All sensors operated in an “On-Off” mode with a rapid response time of ~1 s at the operating temperatures. The sensing performance was significantly influenced by the high thermal contraction/expansion properties of the PDMS substrate, depending on the environmental temperature. At subzero temperatures (0, –20, and –40 °C), the sensing performance was improved owing to the gap narrowing. At high temperatures (above 40 °C), it decreased owing to the gap broadening. The detection limit was 300 ppm at subzero temperatures but >1% at high temperatures. The results reveal that the Pd nanogap sensor on PDMS is more suitable for detecting H2 at subzero temperatures. In addition, the sensor showed excellent reproducibility for hundreds of sensing cycles at 25 °C and –40 °C. Furthermore, the sensor was stable to high humidity (above 70 RH%) at 25 °C. Our study demonstrated that the Pd nanogap sensor on a PDMS substrate is a possible candidate for use as an on-board safety H2 sensor in hydrogen-electric vehicles.
High aspect ratio (HAR) nanoneedle arrays can be used to tune the intrinsic properties of substrates such as their wettability and reflectivity. Here, a simple and scalable fabrication method for producing dense arrays of freestanding polyethylene glycol (PEG) nanoneedles with sub 50 nm tips and surface coverage up to 83 needles per µm2 is presented. Two distinct sets of silicon nanoneedle master arrays with base diameters between 15 and 265 nm and heights between 146 and 613 nm are fabricated using block copolymer micelle lithography. Replication of selected silicon masters using photocurable polymers produces HAR PEG nanoneedle arrays with feature base diameters ranging between 15 and 292 nm and heights between 133 and 656 nm. At their maximum, the aspect ratio of the pillars is 4.6. PEG nanoneedle arrays are produced using polymers with two different molecular weights as well as two different photoinitiators, showing the versatility of the process. A versatile and accessible approach enabling patterning of Si and polyethylene glycol nanoneedle arrays across centimetre‐scale surface areas is discussed. Block copolymer micelle lithography is applied as a straightforward method for Si nanoneedle master array fabrication while replica molding using photocurable acrylate‐terminated polymers is explored as a direct means of translating these structures onto soft biocompatible materials.
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-nanoimprint 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-/nano-cavities 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 the suspended-template electric-assisted-nanoimprint, 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.
