Direct Nanoimprint Research Articles

Metallic nanostructures arrays by direct nanoimprinting of silver nanoparticles

The direct nanoimprinting of silver nanoparticles (Ag NPs) generated a plasmonic metallic nanostructure. The Ag NPs used in this study have been synthesized by a phase transfer method. The procedure comprised the utilisation of sodium borohydride to reduce silver nitrate in the presence of a stabilizing agent, which facilitated the transfer of the NPs from the aqueous phase to an organic phase, resulting in uniform and stable particles. The size and shape of the Ag NPs were characterized using electron microscopy, revealing spherical particles with an average diameter of 9 nm. The XRD examination revealed that the nanoparticles exhibit a high degree of crystallinity and possess a cubic geometric phase. The UV-visible absorption spectra demonstrate a significant level of homogeneity in the size. The silicon surface imprinted with Ag NPs exhibits hydrophobic behaviour towards water molecules. This approach allows for precise control of Ag NPs on the substrate, resulting in a robust and effective platform for future applications in sensor technology. Furthermore, the hydrophobic nature of the imprinted surface suggests potential applications in water-repellent coatings or self-cleaning materials.

Chemically amplified molecular resins for shrinkage-controlled direct nanoimprint lithography of functional oxides: an application towards dark-light dual-mode antibacterial surfaces

The imprinting studies using epoxy/oxetane-based bifunctional monomers of Ti, Zr and Nb showed a reduced feature size shrinkage as low as ∼50%. The TiO2/AgBr nanocomposites displayed high antibacterial efficacy under dark-light dual-mode conditions.

Sol-Gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal.

The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here,wedemonstrate a nanoimprinted nonlinear barium titanate2D nanohole array that shows the optical properties of a 2D photonic crystal and a metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal-oxide is resolved by direct soft nanoimprint lithography with sol-gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of five. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis, while lattice resonant states are observed in out-of-plane transmission configuration. The enhanced light-matter interaction from the resonant structure enables to increase in the second-harmonic generation in the near-ultraviolet by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonicdevices.

Refractive Index Tuning of All-Inorganic TiO2 Nanocrystal-Based Films and High Aspect Ratio Nanostructures Using Atomic Layer Deposition: Implications for High-Throughput Fabrication of Metalenses

High refractive index (RI) components and nanostructures are of great interest for compact optics, waveguides, photonics, and metamaterials. Herein, we demonstrate that atomic layer deposition (ALD) can be used as a short postfabrication step to dramatically increase the RI of nanocrystal (NC)-based films and patterned nanostructures in conjunction with a single-step, direct nanoimprint lithography (NIL). The initial RI of TiO2 NC-based films was n = 1.95 at 543 nm which can then be increased to 2.00 by post-calcination and further up to 2.15 by TiO2 ALD. Fifteen cycles of ALD were sufficient to achieve this rapid increase of RI, and any intermediate RI value can be tuned by adjusting the number of ALD cycles accordingly. Nanoscale interstitial gaps between TiO2 NCs allowed a uniform diffusion of ALD precursors, resulting in a much denser structure and the RI increase. An array of 4 mm-sized metalenses were fabricated to demonstrate the effects of the RI tuning on the performance of optical devices. The focusing efficiencies of the as-imprinted metalenses (smallest dimension ∼ 80 nm, highest aspect ratio ∼ 8) were 61% on average, but the post-treatments including calcination and ALD significantly increased the average efficiency to 67% and up to 75% for the best-performing lens. This approach combines the fast, scalable, and versatile solvent-assisted NIL method to pattern optical nanostructures with a short post-patterning deposition and densification step that significantly enhances optical performance.

Consolidation and performance gains in plasma-sintered printed nanoelectrodes.

We report on the unusual, advantageous ageing of flexible transparent electrodes (FTEs) that were self-assembled from oleylamine-capped gold nanospheres (AuNPs) by direct nanoimprinting of inks with different particle concentrations (cAu = 3 mg mL-1 to 30 mg mL-1). The resulting lines were less than 2.5 μm wide and consisted of disordered particle assemblies. Small-Angle X-ray Scattering confirmed that particle packing did not change with ink concentration. Plasma sintering converted the printed structures into lines with a thin, electrically conductive metal shell and a less conductive hybrid core. We studied the opto-electronic performance directly after plasma sintering and after fourteen days of storage at 22 °C and 55% rH in the dark. The mean optical transmittance T̄400-800 in the range from 400 nm to 800 nm increased by up to ≈ 3%, while the sheet resistance Rsh strongly decreased by up to ≈ 82% at all concentrations. We correlated the changes with morphological changes visible in scanning and transmission electron microscopy and identified two sequential ageing stages: (I) post-plasma relaxation effects in and consolidation of the shell, and (II) particle re-organization, de-mixing, coarsening, and densification of the core with plating of Au from the core onto the shell, followed by solid-state de-wetting (ink concentrations cAu < 15 mg mL-1) or stability (cAu ≥ 15 mg mL-1). The plating of Au from the hybrid core improved the FTEs' Figure of Merit FOM = T̄400-800·Rsh-1 by up to ≈ 5.8 times and explains the stable value of ≈ 3.3%·Ωsq-1 reached after 7 days of ageing at cAu = 30 mg mL-1.

Smart Systems for Material and Process Designing in Direct Nanoimprint Lithography Using Hybrid Deep Learning.

A hybrid smart process and material design system for nanoimprinting is proposed, which is combined with a learning system based on experimental and numerical simulation results. Instead of carrying out extensive learning experiments for various conditions, the simulation learning results are partially complimented when the results can theoretically be predicted by numerical simulation. In other words, the data that are lacking in experimental learning are complimented by simulation-based learning results. Therefore, the prediction of nanoimprint results without experimental learning could be realized under various conditions, even for unknown materials. In this study, material and process designs are demonstrated for a low-temperature nanoimprint process using glycerol-containing polyvinyl alcohol. The experimental results under limited conditions were learned to investigate the optimum glycerol concentrations and process temperatures. Simulation-based learning was used to predict the dependence on press pressure and shape parameters. The prediction results for unknown glycerol concentrations agreed well with the follow-up experiments.

Direct nanoimprint of chalcogenide glasses with optical functionalities via solvent-based surface softening.

Chalcogenide glasses are attractive materials for optical applications. However, these applications often require patterning of the surface with functional micro-/ nanostructures. Such patterning is challenging by traditional microfabrication methods. Here, we present a new, to the best of our knowledge, approach of direct imprint via solvent-based surface softening, for the patterning of As2Se3 surface. Our approach is based on an elastomeric stamp soaked in an organic solvent. During the imprint, the solvent diffuses into the imprinted substrate, plasticizes its surface, and thereby allows its imprint at the temperature below its glass transition point. Thus, our approach combines the full pattern transfer with the maintenance of the shape of the imprinted substrate, which is necessary for optical devices. By using this approach, we demonstrated functional antireflective microstructures directly imprinted on As2Se3 surface. Furthermore, we showed that our approach can produce imprinted features sized down to 20 nm scale. We believe that our new approach paves the way for more future applications of chalcogenide glasses.

Replacing Metals with Oxides in Metal-Assisted Chemical Etching Enables Direct Fabrication of Silicon Nanowires by Solution Processing.

Metal-assisted chemical etching (MACE) has emerged as an effective method to fabricate high aspect ratio nanostructures. This method requires a catalytic mask that is generally composed of a metal. Here, we challenge the general view that the catalyst needs to be a metal by introducing oxide-assisted chemical etching (OACE). We perform etching with metal oxides such as RuO2 and IrO2 by transposing materials used in electrocatalysis to nanofabrication. These oxides can be solution-processed as polymers exhibiting similar capabilities of metals for MACE. Nanopatterned oxides can be obtained by direct nanoimprint lithography or block-copolymer lithography from chemical solution on a large scale. High aspect ratio silicon nanostructures were obtained at the sub-20 nm scale exclusively by cost-effective solution processing by halving the number of fabrication steps compared to MACE. In general, OACE is expected to stimulate new fundamental research on chemical etching assisted by other materials, providing new possibilities for device fabrication.

Surface plasticizing of chalcogenide glasses: a route for direct nanoimprint with multifunctional antireflective and highly hydrophobic structures.

Chalcogenide glasses are attractive materials for optical applications. However, these applications often require pattering of the surface with functional micro-/ nanostructures, which is challenging by traditional microfabrication. Here, we present a novel, robust, and scalable approach for the direct patterning of chalcogenide glasses, based on soft imprinting of a solvent-plasticized glass layer formed on the glass surface. We established a methodology for surfaces plasticizing, through tuning of its glass transition temperature by process conditions, without compromising on the chemical composition, structure, and optical properties of the plasticized layer. This control over the glass transition temperature allowed to imprint the surface of chalcogenide glass with features sized down to 20 nm, and achieve an unprecedented combination of full pattern transfer and complete maintenance of the shape of the imprinted substrate. We demonstrated two applications of our patterning approach: a diffraction grating, and a multifunctional pattern with both antireflective and highly hydrophobic water-repellent functionalities - a combination that has never been demonstrated for chalcogenide glasses. This work opens a new route for the nanofabrication of optical devices based on chalcogenide glasses and paves the way to numerous future applications for these important optical materials.

Resist-free nanoimprinting on optical fibers for plasmonic optrodes

Nanostructure patterning on optical fibers enables miniaturized optrodes for photonic and plasmonic applications. Here we report a direct nanoimprint technique to produce high-quality nanostructure arrays on optical fiber endfaces. It has only one single step: imprinting optical fiber tips against a mold with nanostructures at the elevated temperature. This new method abandons resist used in traditional fiber-imprinting methods. Hundreds of fibers can be shaped simultaneously with one mold within minutes. The imprinted nanostructure arrays on optical fibers are transformed into plasmonic optrodes through metal deposition. Variation of imprint depths and mold patterns allows tailoring of the plasmonic resonances of these nanostructure arrays for high-performance refractometric sensing and on-fiber polarization. The sensitivity of 690 nm/RIU and figure of merit of 50 are both among the highest values for similar plasmonic nanostructure arrays. This resist-free nanoimprint paves the way towards a low-cost and high-throughput realization of plasmonic optrodes and their wide applications.

Atomistic investigation of process parameter variations on material deformation behavior in nanoimprint lithography of gold

Direct metal nanoimprint (NIL) of the gold substrate by silicon mold has been investigated through molecular dynamics simulations. The effects of different process parameters such as imprint temperature, mold velocity, and mold profile on the contact pressure, von Mises stresses, and dislocation behavior of the lattice structure was studied during the nanoimprinting process. Higher contact pressures were observed during the molding phases which further declined during the relaxation and demolding phases. The narrow mold geometry (aspect ratio = 2) had the highest contact pressure (6.61 GPa) at 1300 K and 50 m/s due to the lower cross-sectional area and incompressible nature of the gold atoms. The maximum von Mises stress (1.8 × 107 GPa) was observed under the mold tooth during the insertion phase at 298 K and 473 K. Further, the highest von Mises stress (1.32 × 107 GPa) was tracked at 1300 K due to the restraining effect of the mold on the expanding molten gold atoms. The lattice dislocation analysis using the polyhedral template matching (PTM) method revealed that the gold substrate maintains its face centered cubic (FCC) structure and consequent ductile nature after the NIL process at 298 K and 473 K, respectively. At 1300 K, for both mold profiles more than 60% of gold atoms remained with distorted structure after completion of NIL process due to the melting and rapid crystallization of the gold atoms. High-resolution imprints were achieved at the recrystallization temperature (473 K) of gold for both the mold geometries.

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.

Direct nanoimprinting of nanoporous organosilica films consisting of covalently crosslinked photofunctional frameworks.

Nanoimprinting methods have been used widely to prepare various patterned or nanostructured thin films from inorganic or organic components. However, the accumulation of large functional aromatic groups in covalently crosslinked nanoimprints is challenging, due to the difficulty in controlling the fluidity and reactivity of the precursor films. In this work, nanoimprinting of naphthalimide-silica sol-gel films results in vertically oriented nanoporous structures consisting of covalently crosslinked UV-absorbing frameworks. The nanoimprinted films demonstrate potential as robust analytical substrates for laser desorption/ionization mass spectrometry (LDI-MS). The sol-gel polycondensation behavior of the precursors is examined using 29Si NMR spectroscopy to determine reaction conditions suitable for nanoimprinting. The inorganic-organic hybrid frameworks containing a high density of naphthalimide groups exhibit small volume shrinkage during the polycondensation reactions, which leads to desired nanoimprinting. Various bio-related compounds on the order of picomole to femtomole quantities are detectable by LDI-MS measurements using the nanoimprinted substrates. To improve their user-friendliness and signal intensity in LDI-MS analysis, the nanoimprinted substrates are patterned with surface-modified silica nanoparticles. The direct formation of surface nanostructures by nanoimprinting of functional organosilica films may open a new path to developing optically and electronically functional materials, thereby widening their utility.

Direct Resistless Soft Nanopatterning of Freeform Surfaces.

Nanoimprint is broadly used to pattern thin polymer films on rigid substrates. The resulted patterns can be used either as functional nanostructures or as masks for a pattern transfer. Also, nanoimprint could, in principle, be used for the direct patterning of thermoformable substrates with functional nanostructures; however, the resulted global substrate deformation makes this approach unpractical. Here, we present a new approach for the direct nanoimprint of thermoformable substrates with functional nanostructures through precise maintaining of the substrate shape. Our approach is based on an elastomeric stam soaked in organic solvent, which diffuses into the imprinted substrate, plasticizes its surface, and thereby allows its imprint at the temperature below its glass transition point. Using this approach, we imprinted features at the 20 nm scale, which are comparable to those demonstrated by conventional nanoimprint techniques. We illustrated the applicability of our approach by producing functional antireflective nanostructures onto flat and curved optical substrates. In both cases, we achieved full pattern transfer and maintained the shape of the imprinted substrates, a combination that has not been demonstrated so far. Our approach substantially expands the capabilities of nanoimprint and paves the way to its numerous applications, which have been impossible by existing nanopatterning technologies.

Nanoimprinting reflow modified moth-eye structures in chalcogenide glass for enhanced broadband antireflection in the mid-infrared.

We report on the progress towards developing a new method for fabricating more efficient, broadband antireflective (AR) moth-eye structures in As2Se3 via a direct nanoimprinting technique. Thermal reflow is used during mold fabrication to reshape a conventional deep-ultraviolet lithography in order to promote a pattern transfer of "secant ogive"-like moth-eye structures. Once replicated, structures modified by reflow displayed greater AR efficiency compared to structures replicated by a conventional mold, achieving the highest spectrum-averaged transmittance improvement of 12.36% from 3.3 to 12 μm.

Multifunctional Metasurfaces Based on Direct Nanoimprint of Titania Sol–Gel Coatings

AbstractDielectric Mie resonators are taking momentum in the last years thanks to their peculiar properties in light management at visible and near‐infrared frequencies. However, their full exploitation demands for cheap materials and versatile fabrication methods, extendible over large surfaces and potentially C‐MOS compatible. Here, a sol–gel deposition and nanoimprint lithography method is used to obtain titania‐based Mie resonators over large areas (several square millimeters), showing that this platform can potentially be exploited for light management with different devices. First, their use for structural colors and efficient band pass filters covering the visible spectrum is demonstrated. Then, exploiting sharp Fano resonances in reflection, their potential for refractive index sensing is addressed obtaining a figure of merit of ≈20. Finally, when placing the resonators on porous silica, a large and reversible color tuning can be produced as a result of water adsorption within the substrate porosity. These results open the path to titania sub‐micrometric structures for applications as a multifunctional metamaterial for smart windows, displays, and all‐optical sensing.

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.

Study on induced strain in direct nanoimprint lithography

The induced shear strain distribution in a polymer film is investigated by computational study in a direct nanoimprint process. The effects of the polymer thickness, mold pattern shape such as rectangular, triangular or overcut pattern shape, and the coefficient of friction between the mold and the polymer are studied by computational work. As the coefficient of friction increases, the induced shear strain increases along the mold surface. Depending on the polymer thickness, the shear strain is induced in the residual and/or pattern area. In the triangular pattern, the strain is induced in the pattern central area. The results suggest that shear stress remains in the triangular pattern area in the direct nanoimprint process. On the other hand, the rectangular pattern is suitable for suppressing the induced strain inside the pattern.

Direct nanoimprinting of moth-eye structures in chalcogenide glass for broadband antireflection in the mid-infrared

Fresnel reflection at the boundary between two media of differing refractive indices is a major contributing factor to the overall loss in mid-infrared optical systems based on high-index materials such as chalcogenide glasses. In this paper, we present a study of broadband antireflective moth-eye structures directly nanoimprinted on the surfaces of arsenic triselenide (As2Se3)-based optical windows. Using rigorous coupled-wave analysis, we identify a relief design optimized for high transmittance ( 12%) in the 5.9–7.3 μm spectral range as well as improved omnidirectional properties. Finally, we demonstrate the adaptability of nanoimprinted surface reliefs by tailoring the nanostructure pitch and height, achieving both extremely broadband antireflective and highly efficient antireflective surface reliefs. The results and methods presented herein provide an efficient and scalable solution for improving the transmission of bulk optics, waveguides, and photonic devices in the mid-infrared.

Direct soft imprint of chalcogenide glasses

The authors explored an approach for nanostructuring of surfaces of chalcogenide glasses by direct soft nanoimprint lithography. The authors produced soft nanoimprint molds with microsized and nanosized relief features using polydimethylsiloxane. To enable the direct replication of the mold pattern on the surface of bulk chalcogenide glasses, the authors engineered a thermal imprint tool that prevents the substrate deformation using a physical confinement. The authors optimized the imprint process parameters to achieve an unprecedented full transfer micropattern onto a bulk chalcogenide glass. Furthermore, the authors explored pattern replication of nanosized structures by thermal imprint. This process paves the way for a facile and cost-effective fabrication of nanostructures on chalcogenide glasses and their numerous applications in optical and photonic devices.