Nanotransplantation Printing of Crystallographic-Orientation-Controlled Single-Crystalline Nanowire Arrays on Diverse Surfaces

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.

Ultrafine Filteration of Metal Catalysts Enables Fabrication of Silicon Nanowires with Diameter Approaching a Unit Cell Size

Advanced functionalities of silicon nanowires are size-dependent and downscaling of the nanostructure often leads to higher device performances. Single-crystal silicon nanowires with diameters approaching a single unit cell are fabricated using membrane-filtrated catalyst assisted chemical etching. Atomically filtrated gold is used as uniform pattern to direct anisotropic etching of dense silicon nanowire arrays. The size of the nanowires can be controlled by engineering the molecular weight of Poly(methyl methacrylate) used to fabricate the polymer globule membranes. The smallest silicon nanowires with 0.9 nm diameters exhibit direct, and wide band gap of 3.55 eV and establishes a new record. The experimentally obtained silicon nanowires in this size fill the valuable gap below the few-nanometer regime where to date only theoretical predictions have been available. This fabrication approach could provide facile access to atomic-scale silicon, which can bring further advancement to next generation nanodevices.

Synergistic Integration of Chemo-Resistive and SERS Sensing for Label-Free Multiplex Gas Detection

Practical sensing applications such as real-time safety alerts and clinical diagnoses require sensor devices to differentiate between various target molecules with high sensitivity and selectivity, yet conventional devices such as oxide-based chemo-resistive sensors and metal-based surface-enhanced Raman spectroscopy (SERS) sensors usually do not satisfy such requirements. Here, a label-free, chemo-resistive/SERS multimodal sensor based on a systematically assembled 3D cross-point multifunctional nanoarchitecture (3D-CMA), which has unusually strong enhancements in both "chemo-resistive" and "SERS" sensing characteristics is introduced. 3D-CMA combines several sensing mechanisms and sensing elements via 3D integration of semiconducting SnO₂ nanowire frameworks and dual-functioning Au metallic nanoparticles. It is shown that the multimodal sensor can successfully estimate mixed-gas compositions selectively and quantitatively at the sub-100 ppm level, even for mixtures of gaseous aromatic compounds (nitrobenzene and toluene) with very similar molecular structures. This is enabled by combined chemo-resistive and SERS multimodal sensing providing complementary information.

Unconventional grain growth suppression in oxygen-rich metal oxide nanoribbons

Nanograined metal oxides are requisite for diverse applications that use large surface area, such as gas sensors and catalysts. However, nanoscale grains are thermodynamically unstable and tend to coarsen at elevated temperatures. Here, we report effective grain growth suppression in metal oxide nanoribbons annealed at high temperature (900°C) by tuning the metal-to-oxygen ratio and confining the nanoribbons. Despite the high annealing temperatures, the average grain size was maintained at ~6 nm, which also retained their structural integrity. We observe that excess oxygen in amorphous tin oxide nanoribbons prevents merging of small grains during crystallization, leading to suppressed grain growth. As an exemplary application, we demonstrate a gas sensor using grain growth–suppressed tin oxide nanoribbons, which exhibited both high sensitivity and unusual long-term operation stability. Our findings provide a previously unknown pathway to simultaneously achieve high performance and excellent thermal stability in nanograined metal oxide nanostructures.

Optically transparent vertical silicon nanowire arrays for live-cell imaging

Programmable nano-bio interfaces driven by tuneable vertically configured nanostructures have recently emerged as a powerful tool for cellular manipulations and interrogations. Such interfaces have strong potential for ground-breaking advances, particularly in cellular nanobiotechnology and mechanobiology. However, the opaque nature of many nanostructured surfaces makes non-destructive, live-cell characterization of cellular behavior on vertically aligned nanostructures challenging to observe. Here, a new nanofabrication route is proposed that enables harvesting of vertically aligned silicon (Si) nanowires and their subsequent transfer onto an optically transparent substrate, with high efficiency and without artefacts. We demonstrate the potential of this route for efficient live-cell phase contrast imaging and subsequent characterization of cells growing on vertically aligned Si nanowires. This approach provides the first opportunity to understand dynamic cellular responses to a cell-nanowire interface, and thus has the potential to inform the design of future nanoscale cellular manipulation technologies.

Narrower Nanoribbon Biosensors Fabricated by Chemical Lift-off Lithography Show Higher Sensitivity

Wafer-scale nanoribbon field-effect transistor (FET) biosensors fabricated by straightforward top-down processes are demonstrated as sensing platforms with high sensitivity to a broad range of biological targets. Nanoribbons with 350 nm widths (700 nm pitch) were patterned by chemical lift-off lithography using high-throughput, low-cost commercial digital versatile disks (DVDs) as masters. Lift-off lithography was also used to pattern ribbons with 2 μm or 20 μm widths (4 or 40 μm pitches, respectively) using masters fabricated by photolithography. For all widths, highly aligned, quasi-one-dimensional (1D) ribbon arrays were produced over centimeter length scales by sputtering to deposit 20 nm thin-film In₂O₃ as the semiconductor. Compared to 20 μm wide microribbons, FET sensors with 350 nm wide nanoribbons showed higher sensitivity to pH over a broad range (pH 5 to 10). Nanoribbon FETs functionalized with a serotonin-specific aptamer demonstrated larger responses to equimolar serotonin in high ionic strength buffer than those of microribbon FETs. Field-effect transistors with 350 nm wide nanoribbons functionalized with single-stranded DNA showed greater sensitivity to detecting complementary DNA hybridization vs 20 μm microribbon FETs. In all, we illustrate facile fabrication and use of large-area, uniform In₂O₃ nanoribbon FETs for ion, small-molecule, and oligonucleotide detection where higher surface-to-volume ratios translate to better detection sensitivities.

Creation of Curved Nanostructures Using Soft-Materials-Derived Lithography

In this study, curved nanostructures, which are difficult to obtain, were created on an Si substrate through the bonding, swelling, and breaking processes of the polymer and silicone substrate. This method can be utilized to obtain convex nanostructures over large areas. The method is simpler than typical semiconductor processing with photolithography or compared to wet- or vacuum-based dry etching processes. The polymer bonding, swelling (or no swelling), and breaking processes that are performed in this process were theoretically analyzed through a numerical analysis of permeability and modeling. Through this process, we designed a convex nanostructure that can be produced experimentally in an accurate manner.

Geometrically Structured Nanomaterials for Nanosensors, NEMS, and Nanosieves

Recently, geometrically structured nanomaterials have received great attention due to their unique physical and chemical properties, which originate from the geometric variation in such materials. Indeed, the use of various geometrically structured nanomaterials has been actively reported in enhanced-performance devices in a wide range of applications. Recent significant progress in the development of geometrically structured nanomaterials and associated devices is summarized. First, a brief introduction of advanced nanofabrication methods that enable the fabrication of various geometrically structured nanomaterials is given, and then the performance enhancements achieved in devices utilizing these nanomaterials, namely, i) physical and gas nanosensors, ii) nanoelectromechanical devices, and iii) nanosieves are described. For the device applications, a systematic summary of their structures, working mechanisms, fabrication methods, and output performance is provided. Particular focus is given to how device performance can be enhanced through the geometric structures of the nanomaterials. Finally, perspectives on the development of novel nanomaterial structures and associated devices are presented.

Desolvation-Triggered Versatile Transfer-Printing of Pure BN Films with Thermal-Optical Dual Functionality

Although hexagonal boron nitride (BN) nanostructures have recently received significant attention due to their unique physical and chemical properties, their applications have been limited by a lack of processability and poor film quality. In this study, a versatile method to transfer-print high-quality BN films composed of densely stacked BN nanosheets based on a desolvation-induced adhesion switching (DIAS) mechanism is developed. It is shown that edge functionalization of BN sheets and rational selection of membrane surface energy combined with systematic control of solvation and desolvation status enable extensive tunability of interfacial interactions at BN-BN, BN-membrane, and BN-substrate boundaries. Therefore, without incorporating any additives in the BN film and applying any surface treatment on target substrates, DIAS achieves a near 100% transfer yield of pure BN films on diverse substrates, including substrates containing significant surface irregularities. The printed BNs demonstrate high optical transparency (>90%) and excellent thermal conductivity (>167 W m^-1 K^-1) for few-micrometer-thick films due to their dense and well-ordered microstructures. In addition to outstanding heat dissipation capability, substantial optical enhancement effects are confirmed for light-emitting, photoluminescent, and photovoltaic devices, demonstrating their remarkable promise for next-generation optoelectronic device platforms.

Siloxane-Encapsulated Upconversion Nanoparticle Hybrid Composite with Highly Stable Photoluminescence against Heat and Moisture

Herein, we report a siloxane-encapsulated upconversion nanoparticle hybrid composite (SE-UCNP), which exhibits excellent photoluminescence (PL) stability for over 40 days even at an elevated temperature, in high humidity, and in harsh chemicals. The SE-UCNP is synthesized through UV-induced free-radical polymerization of a sol-gel-derived UCNP-containing oligosiloxane resin (UCNP-oligosiloxane). The siloxane matrix with a random network structure by Si-O-Si bonds successfully encapsulates the UCNPs with chemical linkages between the siloxane matrix and organic ligands on UCNPs. This encapsulation results in surface passivation retaining the intrinsic fluorescent properties of UCNPs under severe conditions (e.g., 85 °C/85% relative humidity) and a wide range of pH (from 1 to 14). As an application example, we fabricate a two-color binary microbarcode based on SE-UCNP via a low-cost transfer printing process. Under near-infrared irradiation, the binary sequences in our barcode are readable enough to identify objects using a mobile phone camera. The hybridization of UCNPs with a siloxane matrix provides the capacity for highly stable UCNP-based applications in real environments.

Material-Independent Nanotransfer onto a Flexible Substrate Using Mechanical-Interlocking Structure

Nanowire-transfer technology has received much attention thanks to its capability to fabricate high-performance flexible nanodevices with high simplicity and throughput. However, it is still challenging to extend the conventional nanowire-transfer method to the fabrication of a wide range of devices since a chemical-adhesion-based nanowire-transfer mechanism is complex and time-consuming, hindering successful transfer of diverse nanowires made of various materials. Here, we introduce a material-independent mechanical-interlocking-based nanowire-transfer (MINT) method, fabricating ultralong and fully aligned nanowires on a large flexible substrate (2.5 × 2 cm²) in a highly robust manner. For the material-independent nanotransfer, we developed a mechanics-based nanotransfer method, which employs a dry-removable amorphous carbon (a-C) sacrificial layer between a vacuum-deposited nanowire and the underlying master mold. The controlled etching of the sacrificial layer enables the formation of a mechanical-interlocking structure under the nanowire, facilitating peeling off of the nanowire from the master mold robustly and reliably. Using the developed MINT method, we successfully fabricated various metallic and semiconductor nanowire arrays on flexible substrates. We further demonstrated that the developed method is well suited to the reliable fabrication of highly flexible and high-performance nanoelectronic devices. As examples, a fully aligned gold (Au) microheater array exhibited high bending stability (10⁶ cycling) and ultrafast (∼220 ms) heating operation up to ∼100 °C. An ultralong Au heater-embedded cuprous-oxide (Cu₂O) nanowire chemical gas sensor showed significantly improved reversible reaction kinetics toward NO₂ with 10-fold enhancement in sensitivity at 100 °C.

Highly Stretchable and Conductive Superhydrophobic Coating for Flexible Electronics

Superhydrophobic materials integrating stretchability with conductivity have huge potential in the emerging application horizons such as wearable electronic sensors, flexible power storage apparatus, and corrosion-resistant circuits. Herein, a facile spraying method is reported to fabricate a durable superhydrophobic coating with excellent stretchable and electrical performance by combing 1-octadecanethiol-modified silver nanoparticles (M-AgNPs) with polystyrene- b-poly(ethylene- co-butylene)- b-polystyrene (SEBS) on a prestretched natural rubber (NR) substrate. The embedding of M-AgNPs in elastic SEBS matrix and relaxation of prestretched NR substrate construct hierarchical rough architecture and endow the coating with dense charge-transport pathways. The fabricated coating exhibits superhydrophobicity with water contact angle larger than 160° and a high conductivity with resistance of about 10 Ω. The coating not only maintains superhydrophobicity at low/high stretch ratio for the newly generated small/large protuberances but also responds to stretching and bending with good sensitivity, broad sensing range, and stable response cycles. Moreover, the coating exhibits excellent durability to heat and strong acid/alkali and mechanical forces including droplet impact, kneading, torsion, and repetitive stretching-relaxation. The findings conceivably stand out as a new tool to fabricate multifunctional superhydrophobic materials with excellent stretchability and conductivity for flexible electronics under wet or corrosive environments.