Nanopattern-Embedded Micropillar Structures for Security Identification

Micro-/Nanohierarchical Structures Physically Engineered on Surfaces: Analysis and Perspective

The high demand for micro-/nanohierarchical structures as components of functional substrates, bioinspired devices, energy-related electronics, and chemical/physical transducers has inspired their in-depth studies and active development of the related fabrication techniques. In particular, significant progress has been achieved in hierarchical structures physically engineered on surfaces, which offer the advantages of wide-range material compatibility, design diversity, and mechanical stability, and numerous unique structures with important niche applications have been developed. This review categorizes the basic components of hierarchical structures physically engineered on surfaces according to function/shape and comprehensively summarizes the related advances, focusing on the fabrication strategies, ways of combining basic components, potential applications, and future research directions. Moreover, the physicochemical properties of hierarchical structures physically engineered on surfaces are compared based on the function of their basic components, which may help to avoid the bottlenecks of conventional single-scale functional substrates. Thus, the present work is expected to provide a useful reference for scientists working on multicomponent functional substrates and inspire further research in this field.

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.

Recent progress in flexible pressure sensors based on multiple microstructures: from design to application

Flexible pressure sensors (FPSs) have been widely studied in the fields of wearable medical monitoring and human-machine interaction due to their high flexibility, light weight, sensitivity, and easy integration. To better meet these application requirements, key sensing properties such as sensitivity, linear sensing range, pressure detection limits, response/recovery time, and durability need to be effectively improved. Therefore, researchers have extensively and profoundly researched and innovated on the structure of sensors, and various microstructures have been designed and applied to effectively improve the sensing performance of sensors. Compared with single microstructures, multiple microstructures (MMSs) (including hierarchical, multi-layered and hybrid microstructures) can improve the sensing performance of sensors to a greater extent. This paper reviews the recent research progress in the design and application of FPSs with MMSs and systematically summarizes the types, sensing mechanisms, and preparation methods of MMSs. In addition, we summarize the applications of FPSs with MMSs in the fields of human motion detection, health monitoring, and human-computer interaction. Finally, we provide an outlook on the prospects and challenges for the development of FPSs.

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.

Solution-processable electrode-material embedding in dynamically inscribed nanopatterns (SPEEDIN) for continuous fabrication of durable flexible devices

A facile and scalable lithography-free fabrication technique, named solution-processable electrode-material embedding in dynamically inscribed nanopatterns (SPEEDIN), is developed to produce highly durable electronics. SPEEDIN uniquely utilizes a single continuous flow-line manufacturing process comprised of dynamic nanoinscribing and metal nanoparticle solution coating with selective embedding. Nano- and/or micro-trenches are inscribed into arbitrary polymers, and then an Ag nanoparticle solution is dispersed, soft-baked, doctor-bladed, and hard-baked to embed Ag micro- and nanowire structures into the trenches. Compared to lithographically embossed metal structures, the embedded SPEEDIN architectures can achieve higher durability with comparable optical and electrical properties and are robust and power-efficient even under extreme stresses such as scratching and bending. As one tangible application of SPEEDIN, we demonstrate a flexible metal electrode that can operate at 5 V at temperatures up to 300 °C even under the influence of harsh external stimuli. SPEEDIN can be applied to the scalable fabrication of diverse flexible devices that are reliable for heavy-duty operation in harsh environments involving high temperatures, mechanical deformations, and chemical hazards.

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.

Shape-Controlled and Well-Arrayed Heterogeneous Nanostructures via Melting Point Modulation at the Nanoscale

A novel method for fabricating shape-controlled and well-arrayed heterogeneous nanostructures by altering the melting point of the metal thin film at the nanoscale is proposed. Silver nanofilms (AgNFs) are transformed into silver nanoislands (AgNIs), silver nanoparticles (AgNPs), and silver nanogaps (AgNGs) that are well-ordered and repositioned inside the gold nanoholes (AuNHs) depending on the diameter of the AuNHs, the thickness of the AgNF, and the heating temperature (120-200 °C). This method demonstrates the ability to fabricate uniform, stable, and unique structures with a fast, simple, and mass-producible process. For demonstrating the diverse applicability of the developed structures, high-density AgNGs inside the AuNHs are utilized as surface-enhanced Raman spectroscopy (SERS) substrates. These AgNGs-based SERS substrates exhibit a performance enhancement, which is 1.06 × 10⁶ times greater than that of a metal film, with a relative standard deviation of 19.8%. The developed AgNP/AgNI structures are also used as nonreproducible anti-counterfeiting signs, and the anti-counterfeiting/readout system is demonstrated via image processing. Therefore, our method could play a vital role in the nanofabrication of high-demand nanostructures.

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.