Repeatable and metal-independent nanotransfer printing based on metal oxidation for plasmonic color filters

Tip-based Lithography with a Sacrificial Layer

The fabrication of a highly controlled gold (Au) nanohole (NH) array via tip-based lithography is improved by incorporating a sacrificial layer-a tip-crash buffer layer. This inclusion mitigates scratches during the nano-indentation process by employing a 300 nm thick poly(methyl methacrylate) layer as a sacrificial layer on top of the Au film. Such a precaution ensures minimal scratches on the Au film, facilitating the creation of sub-50 nm Au NHs with a 15 nm gap between the Au NHs. The precision of this method exceeds that of fabricating Au NHs without a sacrificial layer. Demonstrating its versatility, this Au NH array is utilized in two distinct applications: as a dry etching mask to form a molybdenum disulfide hole array and as a catalyst in metal-assisted chemical etching, resulting in conical-shaped silicon nanostructures. Additionally, a significant electric field is generated when Au nanoparticles (NPs) are placed within the Au NHs. This effect arises from coupling electromagnetic waves, concentrated by the Au NHs and amplified by the Au NPs. A notable result of this configuration is the enhancement factor of surface-enhanced Raman scattering, which is an order of magnitude greater than that observed with just Au NHs and Au NPs alone.

Illuminating Recent Progress in Nanotransfer Printing: Core Principles, Emerging Applications, and Future Perspectives

As the demand for diverse nanostructures in physical/chemical devices continues to rise, the development of nanotransfer printing (nTP) technology is receiving significant attention due to its exceptional throughput and ease of use. Over the past decade, researchers have attempted to enhance the diversity of materials and substrates used in transfer processes as well as to improve the resolution, reliability, and scalability of nTP. Recent research on nTP has made continuous progress, particularly using the control of the interfacial adhesion force between the donor mold, target material, and receiver substrate, and numerous practical nTP methods with niche applications have been demonstrated. This review article offers a comprehensive analysis of the chronological advancements in nTP technology and categorizes recent strategies targeted for high-yield and versatile printing based on controlling the relative adhesion force depending on interfacial layers. In detail, the advantages and challenges of various nTP approaches are discussed based on their working mechanisms, and several promising solutions to improve morphological/material diversity are presented. Furthermore, this review provides a summary of potential applications of nanostructured devices, along with perspectives on the outlook and remaining challenges, which are expected to facilitate the continued progress of nTP technology and to inspire future innovations.

Advances in lithographic techniques for precision nanostructure fabrication in biomedical applications

Nano-fabrication techniques have demonstrated their vital importance in technological innovation. However, low-throughput, high-cost and intrinsic resolution limits pose significant restrictions, it is, therefore, paramount to continue improving existing methods as well as developing new techniques to overcome these challenges. This is particularly applicable within the area of biomedical research, which focuses on sensing, increasingly at the point-of-care, as a way to improve patient outcomes. Within this context, this review focuses on the latest advances in the main emerging patterning methods including the two-photon, stereo, electrohydrodynamic, near-field electrospinning-assisted, magneto, magnetorheological drawing, nanoimprint, capillary force, nanosphere, edge, nano transfer printing and block copolymer lithographic technologies for micro- and nanofabrication. Emerging methods enabling structural and chemical nano fabrication are categorised along with prospective chemical and physical patterning techniques. Established lithographic techniques are briefly outlined and the novel lithographic technologies are compared to these, summarising the specific advantages and shortfalls alongside the current lateral resolution limits and the amenability to mass production, evaluated in terms of process scalability and cost. Particular attention is drawn to the potential breakthrough application areas, predominantly within biomedical studies, laying the platform for the tangible paths towards the adoption of alternative developing lithographic technologies or their combination with the established patterning techniques, which depends on the needs of the end-user including, for instance, tolerance of inherent limits, fidelity and reproducibility.

All-Dielectric Gratings with High-Quality Structural Colors

We present a dual-layer hafnium dioxide (HfO2) grating capable of full-color modulation in the visible spectrum by leveraging the magnetic dipole resonance induced by the lower-layer HfO2 grating, while the upper-layer HfO2 grating serves as a refractive index matching layer to effectively suppress high-order Mie resonances at shorter wavelengths. The HfO2/HfO2 grating exhibits a significantly larger distribution area in the CIE 1931 chromaticity diagram compared to the HfO2 grating. Furthermore, the structural color saturation closely approximates that of monochromatic light. Under varying background refractive index environments, this structure consistently exhibits high-quality structural color. However, the hue of the structural color undergoes alterations. When the polarization angle is below 20°, the saturation of the acquired structural color remains remarkably consistent. However, exceeding 20° results in a significant degradation in the quality of the structural color. This study demonstrates the promising potential for diverse applications, encompassing fields such as imaging and displays.

Nib-Assisted Coaxial Electrohydrodynamic Jet Printing for Nanowires Deposition

This paper presents the concrete design of nanowires under the precise size and morphology that play a crucial role in the practical operation of the micro/nano devices. A straightforward and operative method termed as nib-assistance coaxial electrohydrodynamic (CEHD) printing technology was proposed. It extracts the essence of a nib-assistance electric field intensity to enhance and lessen the internal fluid reflux of the CEHD jet. The experiments were performed to add microparticles into the inner liquid to indicate the liquid flow consistency within the coaxial jet. The reflux in the coaxial jet was observed for the first time in experiments. The nanowires with a minimum size of 70 nm were printed under optimum experimental conditions. The nanopatterns contained aligned nanowires structures with diameters much smaller than the inner diameter of nozzle, relying on the coaxial nib-assisted technique. The printed results revealed that the nib-assisted CEHD printing technique offers a certain level high quality for application of NEMS system.

Nanotransfer-on-Things: From Rigid to Stretchable Nanophotonic Devices

The growing demand for nanophotonic devices has driven the advancement of nanotransfer printing (nTP) technology. Currently, the scope of nTP is limited to certain materials and substrates owing to the temperature, pressure, and chemical bonding requirements. In this study, we developed a universal nTP technique utilizing covalent bonding-based adhesives to improve the adhesion between the target material and substrate. Additionally, the technique employed plasma-based selective etching to weaken the adhesion between the mold and target material, thereby enabling the reliable modulation of the relative adhesion forces, regardless of the material or substrate. The technique was evaluated by printing four optical materials on nine substrates, including rigid, flexible, and stretchable substrates. Finally, its applicability was demonstrated by fabricating a ring hologram, a flexible plasmonic color filter, and extraordinary optical transmission-based strain sensors. The high accuracy and reliability of the proposed nTP method were verified by the performance of nanophotonic devices that closely matched numerical simulation results.

Nanoscale three-dimensional fabrication based on mechanically guided assembly

The growing demand for complex three-dimensional (3D) micro-/nanostructures has inspired the development of the corresponding manufacturing techniques. Among these techniques, 3D fabrication based on mechanically guided assembly offers the advantages of broad material compatibility, high designability, and structural reversibility under strain but is not applicable for nanoscale device printing because of the bottleneck at nanofabrication and design technique. Herein, a configuration-designable nanoscale 3D fabrication is suggested through a robust nanotransfer methodology and design of substrate's mechanical characteristics. Covalent bonding-based two-dimensional nanotransfer allowing for nanostructure printing on elastomer substrates is used to address fabrication problems, while the feasibility of configuration design through the modulation of substrate's mechanical characteristics is examined using analytical calculations and numerical simulations, allowing printing of various 3D nanostructures. The printed nanostructures exhibit strain-independent electrical properties and are therefore used to fabricate stretchable H₂ and NO₂ sensors with high performances stable under external strains of 30%.

High-fidelity and clean nanotransfer lithography using structure-embedded and electrostatic-adhesive carriers

Metallic nanostructures are becoming increasingly important for both fundamental research and practical devices. Many emerging applications employing metallic nanostructures often involve unconventional substrates that are flexible or nonplanar, making direct lithographic fabrication very difficult. An alternative approach is to transfer prefabricated structures from a conventional substrate; however, it is still challenging to maintain high fidelity and a high yield in the transfer process. In this paper, we propose a high-fidelity, clean nanotransfer lithography method that addresses the above challenges by employing a polyvinyl acetate (PVA) film as the transferring carrier and promoting electrostatic adhesion through triboelectric charging. The PVA film embeds the transferred metallic nanostructures and maintains their spacing with a remarkably low variation of <1%. When separating the PVA film from the donor substrate, electrostatic charges are generated due to triboelectric charging and facilitate adhesion to the receiver substrate, resulting in a high large-area transfer yield of up to 99.93%. We successfully transferred the metallic structures of a variety of materials (Au, Cu, Pd, etc.) with different geometries with a <50-nm spacing, high aspect ratio (>2), and complex 3D structures. Moreover, the thin and flexible carrier film enables transfer on highly curved surfaces, such as a single-mode optical fiber with a curvature radius of 62.5 μm. With this strategy, we demonstrate the transfer of metallic nanostructures for a compact spectrometer with Cu nanogratings transferred on a convex lens and for surface-enhanced Raman spectroscopy (SERS) characterization on graphene with reliable responsiveness.

Direct Chemisorption-Assisted Nanotransfer Printing with Wafer-Scale Uniformity and Controllability

Nanotransfer printing techniques have attracted significant attention due to their outstanding simplicity, cost-effectiveness, and high throughput. However, conventional methods via a chemical medium hamper the efficient fabrication with large-area uniformity and rapid development of electronic and photonic devices. Herein, we report a direct chemisorption-assisted nanotransfer printing technique based on the nanoscale lower melting effect, which is an enabling technology for two- or three-dimensional nanostructures with feature sizes ranging from tens of nanometers up to a 6 in. wafer-scale. The method solves the major bottleneck (large-scale uniform metal catalysts with nanopatterns) encountered by metal-assisted chemical etching. It also achieves wafer-scale, uniform, and controllable nanostructures with extremely high aspect ratios. We further demonstrate excellent uniformity and high performance of the resultant devices by fabricating 100 photodetectors on a 6 in. Si wafer. Therefore, our method can create a viable route for next-generation, wafer-scale, uniformly ordered, and controllable nanofabrication, leading to significant advances in various applications, such as energy harvesting, quantum, electronic, and photonic devices.

Reverse-Offset Printing of Polymer Resist Ink for Micrometer-Level Patterning of Metal and Metal-Oxide Layers

Printed electronics has advanced during the recent decades in applications such as organic photovoltaic cells and biosensors. However, the main limiting factors preventing the more widespread use of printing in flexible electronics manufacturing are (i) the poor attainable linewidths via conventional printing methods (≫10 μm), (ii) the limited availability of printable materials (e.g., low work function metals), and (iii) the inferior performance of many printed materials when compared to vacuum-processed materials (e.g., printed vs sputtered ITO). Here, we report a printing-based, low-temperature, low-cost, and scalable patterning method that can be used to fabricate high-resolution, high-performance patterned layers with linewidths down to ∼1 μm from various materials. The method is based on sequential steps of reverse-offset printing (ROP) of a sacrificial polymer resist, vacuum deposition, and lift-off. The sharp vertical sidewalls of the ROP resist layer allow the patterning of evaporated metals (Al) and dielectrics (SiO) as well as sputtered conductive oxides (ITO), where the list is expandable also to other vacuum-deposited materials. The resulting patterned layers have sharp sidewalls, low line-edge roughness, and uniform thickness and are free from imperfections such as edge ears occurring with other printed lift-off methods. The applicability of the method is demonstrated with highly conductive Al (∼5 × 10^-8 Ωm resistivity) utilized as transparent metal mesh conductors with ∼35 Ω_□ at 85% transparent area percentage and source/drain electrodes for solution-processed metal-oxide (In₂O₃) thin-film transistors with ∼1 cm²/(Vs) mobility. Moreover, the method is expected to be compatible with other printing methods and applicable in other flexible electronics applications, such as biosensors, resistive random access memories, touch screens, displays, photonics, and metamaterials, where the selection of current printable materials falls short.

Topographically designed hybrid nanostructures via nanotransfer printing and block copolymer self-assembly

Nanotransfer printing (nTP) has attracted much attention due to its high pattern resolution, simple process, and low processing cost for useful nanofabrication. Here, we introduce a thermally assisted nTP (T-nTP) process for the effective fabrication of various periodic three-dimensional (3D) nanosheets, such as concavo-convex lines, spine lines, square domes, and complex multi-line patterns. The T-nTP method allows continuous nanoscale 3D patterns with functionality to be transferred onto both rigid and flexible substrates by heat without any collapse of uniform convex nanostructures with nanochannels. We also show the pattern formation of multi-layered hybrid structures consisting of two or more materials by T-nTP. Furthermore, the formation of silicon oxide nanodots (0D) within a printed metallic nanowave structure (3D) can be achieved by the combined method of T-nTP and the self-assembly of poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) block copolymers. Moreover, we demonstrate how to obtain well-defined oxide-metal hybrid nanostructures (0D-in-3D) through the spontaneous accommodation of PDMS spheres in the confined spaces of an Au-wave nanotemplate. This approach is applicable during the nanofabrication of various high-resolution devices with complex geometrical nanopatterns.

Electrochemical Cell Chips Based on Functionalized Nanometals

The electrochemical technique is one of the most accurate, rapid, and sensitive analytical assays, which becomes promising techniques for biological assays at a single-cell scale. Nanometals have been widely used for modification of the traditional electrodes to develop highly sensitive electrochemical cell chips. The electrochemical cell chips based on the nanostructured surface have been used as label-free, simple, and non-destructive techniques for in vitro monitoring of the effects of different anticancer drugs at the cellular level. Here, we will provide the recent progress in fabrication of nanopatterned surface and cell-based nanoarray, and discuss their applications based on electrochemical techniques such as detection of cellular states and chemicals, and non-destructive monitoring of stem cell differentiation.

Route to Cost-Effective Fabrication of Wafer-Scale Nanostructure through Self-Priming Nanoimprint

Nanoimprint technology is powerful for fabricating nanostructures in a large area. However, expensive equipment, high cost, and complex process conditions hinder the application of nano-imprinting technology. Therefore, double-layer self-priming nanoimprint technology was proposed to fabricate ordered metal nanostructures uniformly on 4-inch soft and hard substrates without the aid of expensive instruments. Different nanostructure (gratings, nanoholes and nanoparticles) and different materials (metal and MoS2) were patterned, which shows wide application of double-layer self-priming nanoimprint technology. Moreover, by a double-layer system, the width and the height of metal can be adjusted through the photoresist thickness and developing condition, which provide a programmable way to fabricate different nanostructures using a single mold. The double-layer self-priming nanoimprint method can be applied in poor condition without equipment and be programmable in nanostructure parameters using a single mold, which reduces the cost of instruments and molds.

3D Layer-By-Layer Pd-Containing Nanocomposite Platforms for Enhancing the Performance of Hydrogen Sensors

Herein, a nanowelding technique is adopted to fabricate three-dimensional layer-by-layer Pd-containing nanocomposite structures with special properties. Nanowires fabricated from noble metals (Pd, Pt, Au, and Ag) were used to prepare Pd-Pd nanostructures and Pd-Au, Pd-Pt, Pd-Ag, and Pd-Pt-Au nanocomposite structures by controlling the welding temperature. The recrystallization behavior of the welded composite materials was observed and analyzed. In addition, their excellent mechanical and electrical properties were confirmed by performing 10,000 bending test cycles and measuring the resistances. Finally, flexible and wearable nanoheaters and gas sensors were fabricated using our proposed method. In comparison with conventional techniques, our proposed method can not only easily achieve sensors with a large surface area and flexibility but also improve their performance through the addition of catalyst metals. A gas sensor fabricated using the Pd-Au nanocomposites demonstrated 3.9-fold and 1.1-fold faster H₂ recovery and response, respectively, than a pure Pd-Pd gas sensor device. Moreover, the Pd-Ag nanocomposite exhibited a high sensitivity of 5.5% (better than that of other fabricated gas sensors) for 1.6% H₂ concentration. Therefore, we believe that the fabricated nanocomposites appear promising for wide applications in wearable gas sensors, flexible optical devices, and flexible catalytic devices.

Adhesive-Layer-Free and Double-Faced Nanotransfer Lithography for a Flexible Large-Area MetaSurface Hologram

Herein, we develop an adhesive-free double-faced nanotransfer lithography (ADNT) technique based on the surface deformation of flexible substrates under the conditions of temperature and pressure control and thus address the challenge of realizing the mass production of large-area nanodevices in the fields of optics, metasurfaces, and holograms. During ADNT, which is conducted on a flexible polymer substrate above its glass transition temperature in the absence of adhesive materials and chemical bonding agents, nanostructures from the polymer stamp are attached to the deformed polymer substrate. Various silicon masters are employed to prove our method applicable to arbitrary nanopatterns, and diverse Ag and Au nanostructures are deposited on polymer molds to demonstrate the wide scope of useable metals. Finally, ADNT is used to (i) produce a flexible large-area hologram on the defect-free poly(methyl methacrylate) (PMMA) film and (ii) fabricate a metasurface hologram and a color filter on the front and back surfaces of the PMMA film, respectively, to realize dual functionality. Thus, it is concluded that the use of ADNT can decrease the fabrication time and cost of high-density nanodevices and facilitate their commercialization.