Fluorinated low-viscosity copolymer with enhanced release property for roll-to-plate UV nanoimprint lithography.

A new fluorinated low-viscosity copolymer resin with enhanced release properties has been developed to provide higher fidelity for roll-based ultraviolet (UV) nanoimprint lithography (NIL) and simplify the process. The properties are optimized by blending (2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,9-heptadecafluorononyl) oxirane (EP-F8) and 1,4-butanediol divinyl ether (BDDVE) with 3,4-epoxycyclohexylmethyl-3,4-epoxy cyclohexane carboxylate. The cured resin renders the hydrophobicity and allows demolding from the stamp mold without an anti-stick layer (ASL), as further verified by the contact angle and the work of adhesion measurements. Due to the addition of BDDVE, our resin also has much lower viscosity compared to commercially available low-viscosity resins. These properties enable the transfer of high-density dot patterns by roll-to-plate (R2P) UV NIL in the newly developed resin without adding solvent or coating it with an ASL. These results demonstrate that our resin is a good option for high-quality, low-cost R2P UV NIL, as its use eliminates several steps in the imprinting process, making it applicable to the scalable fabrication of flexible devices.

Atomic Migration Induced Crystal Structure Transformation and Core-Centered Phase Transition in Single Crystal Ge2Sb2Te5 Nanowires.

A phase change nanowire holds a promise for nonvolatile memory applications, but its transition mechanism has remained unclear due to the analytical difficulties at atomic resolution. Here we obtain a deeper understanding on the phase transition of a single crystalline Ge2Sb2Te5 nanowire (GST NW) using atomic scale imaging, diffraction, and chemical analysis. Our cross-sectional analysis has shown that the as-grown hexagonal close-packed structure of the single crystal GST NW transforms to a metastable face-centered cubic structure due to the atomic migration to the pre-existing vacancy layers in the hcp structure going through iterative electrical switching. We call this crystal structure transformation "metastabilization", which is also confirmed by the increase of set-resistance during the switching operation. For the set to reset transition between crystalline and amorphous phases, high-resolution imaging indicates that the longitudinal center of the nanowire mainly undergoes phase transition. According to the atomic scale analysis of the GST NW after repeated electrical switching, partial crystallites are distributed around the core-centered amorphous region of the nanowire where atomic migration is mainly induced, thus potentially leading to low power electrical switching. These results provide a novel understanding of phase change nanowires, and can be applied to enhance the design of nanowire phase change memory devices for improved electrical performance.

Enhanced detection of broadband incoherent light with nanoridge plasmonics.

Emerging photonic integrated circuit technologies require integrative functionality at ultrahigh speed and dimensional compatibility with ultrasmall electronics. Plasmonics offers a promise of addressing these challenges with novel nanophotonic approaches for on-chip information processing or sensing applications. Short communication range and strong light-matter interaction enabled by on-chip plasmonics allow us to extend beyond a conventional approach of integrating coherent and narrowband light source. Such hybrid electronic and photonic interconnection desires a on-chip photodetector that is highly responsive to broadband incoherent light, yet provides elegant design for nanoscale integration. Here we demonstrate an ultracompact broadband photodetection with greatly enhanced photoresponsivity using plasmonic nanoridge geometry. The nanoridge photodetector confines a wide spectrum of electromagnetic energy in a nanostructure through the excitation of multiple plasmons, which thus enables the detection of weak and broadband light. With nanoscale design, material, and dimensional compatibility for the integration, the nanoridge photodetector opens up a new possibility of highly sensitive on-chip photodetection for future integrated circuits and sensing applications.

Synthesis of wafer-scale hexagonal boron nitride monolayers free of aminoborane nanoparticles by chemical vapor deposition.

Hexagonal boron nitride (h-BN) has gained great attention as a two-dimensional material, along with graphene. In this work, high-quality h-BN monolayers were grown in wafer scale (7 × 7 cm(2)) on Cu substrates by using low-pressure chemical vapor deposition (LPCVD). We created h-BN monolayers that were free of polymeric aminoborane (BH2NH2) nanoparticles, which are undesirable by-products of the ammonia borane precursor, by employing a simple filtering system in the CVD process. The optical band gap of 6.06 eV and sharp and symmetric Raman peak measured at 1371 cm(-1) indicate the synthesis of monolayer h-BN. In addition, spherical aberration (CS)-corrected high-resolution transmission electron microscopic images confirm the production of a single-layer hexagonal array of boron and nitrogen atoms.

How keratin cortex thickness affects iridescent feather colours.

The bright, saturated iridescent colours of feathers are commonly produced by single and multi-layers of nanostructured melanin granules (melanosomes), air and keratin matrices, surrounded by an outer keratin cortex of varying thicknesses. The role of the keratin cortex in colour production remains unclear, despite its potential to act as a thin film or absorbing layer. We use electron microscopy, optical simulations and oxygen plasma-mediated experimental cortex removal to show that differences in keratin cortex thickness play a significant role in producing colours. The results indicate that keratin cortex thickness determines the position of the major reflectance peak (hue) from nanostructured melanosomes of common pheasant (Phasianus colchicus) feathers. Specifically, the common pheasant has appropriate keratin cortex thickness to produce blue and green structural colours. This finding identifies a general principle of structural colour production and sheds light on the processes that shaped the evolution of brilliant iridescent colours in the common pheasant.

Broadband Enhancement of Anti-reflectivity for a High Angle of Incidence Using Nanocone Geometry.

The broadband antireflective (AR) effect for wide incident angles has a significant effect on the photoconversion efficiency of photovoltaics and visibility of large-format display panels. The fabrication of surface nanostructures has continued to attract research interest as an effective way to provide such optical performance. However, the effects of different nanostructure geometries are not fully understood, especially for wide-angle AR effects. In this work, we conduct a systematic analysis of the effect of periodic nanostructures such as nanocones (NCs) and inverted nanocones (INCs) on anti-reflectivity at high angles of incidence (AOIs) in terms of light scattering, guided-mode resonance (GMR), and internal reflections. NCs provide good coupling of light scattering and GMR because of their protruding geometry; hence, reduced reflectance can be obtained in the short wavelength region. Further, NCs exhibit relatively weaker GMR intensities and internal reflections, resulting in low reflectance in the long wavelength region. Therefore, NCs offer a superior broadband AR effect for high AOIs compared with INCs. Based on this analysis, we demonstrate an extremely low average reflectance (5.4%) compared to that of the bare substrate (34.7%) for the entire visible range at an AOI of 75° by fabricating NCs on both sides of the substrate.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap.

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Melanin-based structural coloration of birds and its biomimetic applications.

Melanin has been a widely researched pigment by scientists for decades as it is undoubtedly the most ubiquitous and ancient pigment found in nature. Melanin plays very significant roles in structural plumage colors in birds: it has visible light-absorbing capabilities, and nanoscale structures can be formed by self-assembling melanin granules. Herein, we review recent progress on melanin-based structural coloration research. We hope that this review will provide current understanding of melanin's structural and optical properties, natural coloration mechanisms, and biomimetic methods to implement artificial melanin-based structural colors.

Advanced atomic force microscopy-based techniques for nanoscale characterization of switching devices for emerging neuromorphic applications.

Neuromorphic systems require integrated structures with high-density memory and selector devices to avoid interference and recognition errors between neighboring memory cells. To improve the performance of a selector device, it is important to understand the characteristics of the switching process. As changes by switching cycle occur at local nanoscale areas, a high-resolution analysis method is needed to investigate this phenomenon. Atomic force microscopy (AFM) is used to analyze the local changes because it offers nanoscale detection with high-resolution capabilities. This review introduces various types of AFM such as conductive AFM (C-AFM), electrostatic force microscopy (EFM), and Kelvin probe force microscopy (KPFM) to study switching behaviors.

Morphogenesis of Iridescent Feathers in Anna's Hummingbird Calypte anna.

Color is a phenotypic trait of utmost importance, particularly in birds, which are known for their diverse color signals and color-producing mechanisms including pigment-based colors, light scattering from nanostructured feather tissues and combinations thereof. Bright iridescent plumage colors of hummingbirds are caused by light scattering by an organized array of flattened, pigment organelles, containing air-filled vesicles, called melanosomes. These hollow platelets are organized in multilayer arrays that contain numerous sharp air/melanin refractive index interfaces, producing brilliant iridescent colors. Despite their ecological significance and potential for inspiration of new optical materials, how platelets form and spatially arrange in nanostructures in growing feathers remains unknown. Here, we tested the hypothesis that melanosome formation and organization occurs mostly through passive self-assembly processes by assembling a developmental time series of growing hummingbird feathers using optical and electron microscopy. We show that hummingbird platelets contain air bubbles or vesicles upon their formation in pigment-producing cells, melanocytes. When melanosomes are transferred to neighboring keratinocytes (the cells shaping barbule structure) they drastically expand in size; and variation in this enlargement appears to be driven by physical constraints caused by the placement of the melanosomes within the barbule plate and their proximity to other melanosomes. As the barbule elongates and narrows, polymerizing feather corneous beta-protein orients melanosomes unilaterally, forcing them into a stacked configuration. These results reveal potentially novel forces driving the self-assembly of the nanostructures producing some of the brightest colors in nature.

Stem cell behaviors on periodic arrays of nanopillars analyzed by high-resolution scanning electron microscope images.

The biocompatible polyurethane acrylate (PUA) nanopillars were fabricated by soft lithography using three different sizes of nanobeads (350, 500, and 1000 nm), and the human adipose-derived stem cells (hASCs) were cultured on the nanopillars. The hASCs and their various behaviors, such as cytoplasmic projections, migration, and morphology, were observed by high resolution images using a scanning electron microscope (SEM). With the accurate analysis by SEM for the controlled sizes of nanopillars, the deflections are observed at pillars fabricated with 350- and 500-nm nanobeads. These high-resolution images could offer crucial information to elucidate the complicated correlations between nanopillars and the cells, such as morphology and cytoplasmic projections.

Mechanism of Heat-Induced Fusion of Silver Nanowires.

Physical changes in arranged silver nanowires were monitored during progressive heating inside a transmission electron microscope. Using the in-situ experimental method, overall variation of silver nanowires and movement of the silver atoms could be assessed. The physical morphology of silver nanowires was rapidly transformed above 350 °C as they fused with each other, which led to extrusion of the silver atoms. Around 550 °C, silver nanowires were almost fused into one, filling a relatively large void between silver nanowires. However, above 575 °C, the united silver nanowire was completely cut off, starting from the region that was suspected to have defects. For the first time, the fusion of arranged silver nanowires and the configurational changes of silver atoms during heating were visualized, and the migration between silver atoms and the damage mechanism of silver nanowires were assessed. Moreover, the relationship of physical morphology and electrical property of silver nanowires according to the temperature were investigated using the ex-situ experimental method. As silver nanowires started to split at 300 °C, the electrical conductivity deteriorated greatly. Beyond 350 °C, the electrical conductivity was completely lost while silver nanowires disintegrated rapidly, and silver nanowires completely disappeared at 450 °C.

Quantitative Correlation of Nanotopography with Cell Spreading via Focal Adhesions Using Adipose-Derived Stem Cells.

Nanotopography mimicking extracellular environments reportedly impact cell morphological changes; however, elucidating this relationship has been challenging. To control cellular responses using nanostructures, in this study, the quantitative relationship between nanotopography and cell spreading mediated by focal adhesions (FAs) is demonstrated using adipose-derived stem cells (ASCs). The spreading of ASCs and area of FAs are analyzed for the distribution of filamentous actin and vinculin, respectively, using fluorescent images. FAs require a specific area for adhesion (herein defined as effective contact area [ECA]) to maintain cell attachment on nanopillar arrays. An ECA is the area of FAs supported by nanopillars, multiplying the area fraction (AF) of their top surface. Regarding the spreading of cells, the mean area of ASCs linearly decreases as the mean area of FAs increases. Because the area of FAs is inversely correlated to the AF of the nanopillar arrays, the spreading of cells can be quantitatively correlated with nanotopography. The results provide a conceptual framework for controlling cell behaviors to design artificial substrates for tissue-engineering applications.

In vitro characterization of osteoblast cells on polyelectrolyte multilayers containing detonation nanodiamonds.

Nanoparticle-enhanced coatings of bone implants are a promising method to facilitate sustainable wound healing, leading to an increase in patient well-being. This article describes the in vitro characterization of osteoblast cells interacting with polyelectrolyte multilayers, which contain detonation nanodiamonds (NDs), as a novel class of carbon-based coating material, which presents a unique combination of photoluminescence and drug-binding properties. The cationic polyelectrolyte, namely polydiallyldimethylammonium chloride (PDDA), has been used to immobilize NDs on silica glass. The height of ND-PDDA multilayers varies from a minimum of 10 nm for one bilayer to a maximum of 90 nm for five bilayers of NDs and PDDA. Human fetal osteoblasts (hFOBs) cultured on ND-PDDA multilayers show a large number of focal adhesions, which were studied via quantitative fluorescence imaging analysis. The influence of the surface roughness on the filopodia formation was assessed via scanning electron microscopy and atomic force microscopy. The nano-rough surface of five bilayers constrained the filopodia formation. The hFOBs grown on NDs tend to show not only a similar cell morphology compared to cells cultured on extracellular matrix protein-coated silica glass substrates, but also increased cell viability by about 40%. The high biocompatibility of the ND-PDDA multilayers, indicated via high cell proliferation and sound cell adhesion, shows their potential for biomedical applications such as drug-eluting coatings and biomaterials in general.

Large-Area Nanopatterning Based on Field Alignment by the Microscale Metal Mask for the Etching Process.

Recently, researchers have dedicated efforts toward producing large-area nanostructures using advanced lithography techniques and state-of-the-art etching methods. However, these processes involve challenges such as the diffraction limit and an unintended etching profile. In this work, we demonstrate large-area nanopatterning on a silicon substrate using the microscale metal mask by meticulous optimization of the etching process. Around the vertex of a microscale metal mask, a locally induced electric field is generated by a bias voltage applied on a silicon mold. We utilize this field to change the trajectory of reactive ions and their effect flux, thus providing a controllable bowing effect. The results are analyzed by both numerical simulations and experiments. Based on the field alignment by the metal mask for the etching (FAME) process, we demonstrate the fabrication of 378 nm-size nanostructure patterns which translate to a size reduction of 63% from 1 µm-size mask patterns on a wafer by optimization of the processes. This is much higher than the undercut (∼37%) usually achieved by a typical non-Bosch process under similar etching conditions. The optimized nanostructure is used as a mold for the transfer printing of nanostructure arrays on a flexible substrate to demonstrate that it enables the functionality of FAME-processed nanostructures.

Highly Stretchable and Reliable, Transparent and Conductive Entangled Graphene Mesh Networks.

A highly stretchable and reliable, transparent and conductive entangled graphene mesh network (EGMN) exhibits an interconnected percolation network, as usually shown in 1D nanowires, but with the electrical, mechanical, and thermal properties of 2D graphene. The unique combination of the 2D material properties and the network structure of wrinkled, waved, and crumpled graphene enables the EGMN to demonstrate excellent electrical reliability, mechanical durability, and thermal stability, even under harsh environmental and external conditions such as very high temperature, humidity, bending, and stretching. Specifically, after 100 000 cycles of bending with radius of 2 mm, the EGMN maintains its resistance similar to its initial value. The EGMN shows a steady monotonic response in resistance to strain cycles of 50 000 times with nearly constant gauge factors of 0.76, 1.67, and 2.55 at 10%, 40%, and 70% strains, respectively. Moreover, the EGMN shows very little change in resistance with the temperature increasing up to 1000 °C, by in situ thermal analysis with transmission electron microscopy and also by long-term stability testing at 70 °C and 70% relative humidity for 30 d. These results demonstrate that this novel entangled graphene mesh network can significantly broaden the application areas for various types of wearable and stretchable devices.

Optimally Functionalized Adhesion for Contact Transfer Printing of Plasmonic Nanostructures on Flexible Substrate.

This paper demonstrates a facile method to achieve high yield and uniform fabrication for the transfer printing of nanoplasmonic structures on a flexible substrate by providing novel understanding on adhesion layers. The mercapto alkyl carboxylic acids and the alkyl dithiols are used as functionalized adhesion layers and further optimized by controlling the terminal group as well as the length and composition of the functionalization on flat and nanostructured gold surfaces. Our approach of optimized adhesion has been successfully implemented to the transfer printing of functionalized gold nanostructure arrays, thus producing much higher yield of 97.6% and uniform fabrication of nanostructures on a flexible substrate and enabling applications such as flexible nanoplasmonic devices and biosensing platforms.

Contact Transfer Printing of Side Edge Prefunctionalized Nanoplasmonic Arrays for Flexible microRNA Biosensor.

For a nanoplasmonic approach of wearable biochip platform, understanding correlation between near-field enhancement on nanostructures and sensing capability is a crucial step to improve the sensitivity in biosensing. A novel and effective method is demonstrated to increase sensitivity with the enhanced electric fields and to reduce noise with targeted functionalization enabled by transferring side edge prefunctionalized (SEPF) nanostructure arrays onto flexible substrates. Nanostructure sidewalls have selective biochemically functional terminals for the hybridization of microRNAs (miRNAs) and the immobilization of resonant nanoparticles, thus forming hetero assemblies of the nanostructure and the nanoparticles. The unique configuration has shown ultrasensitive biosensing of miRNA-21 in a 10 × 10-15 m level by a red-shift in scattering spectra induced by a plasmon coupling. This ultrasensitive SEPF nanostructure arrays are fabricated on a flexible substrate using a contact transfer printing with a release layer of trichloro(1H, 1H, 2H, 2H-perfluorooctyl)silane. The introduction of the release layer at a prefunctionalizing step has proven to provide selective functionalization only on the sidewalls of the nanostructures. This reduces a background noise caused by the scattering from nonspecifically bound nanoparticles on the substrate, thus enabling reliable and precise detection.

Hierarchical Nanoflowers on Nanograss Structure for a Non-wettable Surface and a SERS Substrate.

Hierarchical nanostructures of CuO nanoflowers on nanograss were investigated for self-cleaning and surface plasmonic applications. We achieved the hierarchical nanostructures using one-step oxidation process by controlling the formation of flower-like nanoscale residues (nanoflowers) on CuO nanograss. While the nanograss structure of CuO has a sufficient roughness for superhydrophobic characteristics, the additional hierarchy of nanoflowers on nanograss leads to a semi-reentrant structure with a high repellency even for a very small droplet (10 nL) of low surface tension liquid such as 25 % ethanol (~35 mN/m), thus providing non-wettable and self-cleaning properties. Furthermore, the CuO hierarchical nanostructure serves as a substrate for surface-enhanced Raman spectroscopy (SERS). Both of the CuO nanograss and nanoflower provide many nanoscale gaps that act as hot-spots for surface-enhanced Raman signal of 4-mercaptopyridine (4-Mpy), thus enabling a non-destructive detection in a short analysis time with relatively simple preparation of sample. Especially, the CuO nanoflower has larger number of hot-spots at the nanogaps from floral leaf-like structures, thus leading to three times higher Raman intensity than the CuO nanograss. These multifunctional results potentially provide a path toward cost-effective fabrication of a non-wettable surface for self-maintenance applications and a SERS substrate for sensing applications.

Colorimetric detection of microRNA miR-21 based on nanoplasmonic core-satellite assembly.

In order to detect microRNAs (miRNAs), we developed a colorimetric sensing method on the basis of the plasmonic coupling effect. Gold nanoplasmonic particles (GNPs) are assembled in a core-satellite configuration in the presence of target miRNA, inducing remarkable changes in the scattering color and spectra at the picomolar level with selectivity.