Nanomachining by colloidal lithography

Controlled Growth of Large SiO2 Shells onto Semiconductor Colloidal Nanocrystals: A Pathway Toward Photonic Integration

The growth of SiO2 shells on semiconductor nanocrystals is an established procedure and it is widely employed to provide dispersibility in polar solvents, and increased stability or biocompatibility. However, to exploit this shell to integrate photonic components on semiconductor nanocrystals, the growth procedure must be finely tunable and able to reach large particle sizes (around 100 nm or above). Here, we demonstrate that these goals are achievable through a design of experiment approach. Indeed, the use of a sequential full-factorial design allows us to carefully tune the growth of SiO2 shells to large values while maintaining a reduced size dispersion. Moreover, we show that the growth of a dielectric shell alone can be beneficial in terms of emission efficiency for the nanocrystal. We also demonstrate that, according to our modeling, the subsequent growth of two shells with increasing refractive index leads to an improved emission efficiency already at a reduced SiO2 sphere radius.

Grain Crystallinity, Anisotropy, and Boundaries Govern Microscale Hydrodynamic Transport in Semicrystalline Porous Media

Polycrystallinity is often an unintended consequence of real manufacturing processes used to produce designer porous media with deterministic and periodic architectures. Porous media are widely employed as high-surface conduits for fluid transport; unfortunately, even small concentrations of defects in the long-range order become the dominant impediment to hydrodynamic transport. In this study, we isolate the effects of these defects using a microfluidic analogy to energy transport in atomic polycrystals by directly tracking capillary transport through polycrystalline inverse opals. We reveal─using high-fidelity florescent microscopy─the boundary-limited nature of flow motions, along with nonlinear impedance elements introduced by the presence of "grain boundaries" that are separating the well-ordered "crystalline grains". Coupled crystallinity, anisotropy, and linear defect density contribute to direction-dominated flow characteristics in a discretized manner rather than traditional diffusive-like flow patterns. Separating individual crystal grains' transport properties from polycrystals along with new probabilistic data sets enables demonstrating statistical predictive models. These results provide fundamental insight into transport phenomena in (poly)crystalline porous media beyond the deterministic properties of an idealized unit cell and bridge the gap between engineering models and the ubiquitous imperfections found in manufactured porous materials.

Modeling the co-assembly of binary nanoparticles

In this work, we present a binary assembly model that can predict the co-assembly structure and spatial frequency spectra of monodispersed nanoparticles with two different particle sizes. The approach relies on an iterative algorithm based on geometric constraints, which can simulate the assembly patterns of particles with two distinct diameters, size distributions, and at various mixture ratios on a planar surface. The two-dimensional spatial-frequency spectra of the modeled assembles can be analyzed using fast Fourier transform analysis to examine their frequency content. The simulated co-assembly structures and spectra are compared with assembled nanoparticles fabricated using transfer coating method are in qualitative agreement with the experimental results. The co-assembly model can also be used to predict the peak spatial frequency and the full-width at half-maximum bandwidth, which can lead to the design of the structure spectra by selection of different monodispersed particles. This work can find applications in fabrication of non-periodic nanostructures for functional surfaces, light extraction structures, and broadband nanophotonics.

Directional Assembly of Large-Area Silica Nanorod Film Using the Electric-Field-Assisted Capillary Channel Method

The self-assembly of colloidal particles, especially colloidal particles with anisotropic geometry, is important for applications in the construction of many functional materials. Compared with the self-assembly of colloidal particles with isotropic geometries, not only does the geometric orientation among neighboring anisotropic particles need to be considered for the reduction of Gibbs free energy, the orientations of the particles are best to be externally influenced. Because of this, the preparation of assembled nanorod arrays with uniform alignment across a large area is still a significant challenge. In this work, an electric-field-assisted capillary channel method is reported, using an external electric field to influence the orientation of silica nanorods or FeOOH ellipsoids during assembly. By application of an external electric field, the alignment of the nanorods is effectively controlled. The capillary channel method provides continuous replenishment of a colloidal solution containing nanorods or spheres for assembly of large-area films. The area of the formed films was influenced by the assembly temperature, channel width, colloidal solution concentration, and solvent surface tension. The competition between the thermal Brownian motion and torque generated by the external electric field impacted the nanorod array quality in the film. While increasing the intensity of the electric field improved nanorod alignment, applying a potential greater than 6 V also produced a heating effect, negatively affecting the quality of the nanorod arrays. The nematic order parameter S which characterizes the degree of alignment of FeOOH ellipsoids with smaller length is significantly lower than the one for silica nanorods due to the higher critical field strength and the increased susceptibility to the effects of thermal motion. The assembly of silica nanorods at 35 °C under an effective potential of 4-6 V provides a compromise between achieving uniform nanorod orientation and maximizing the coverage area of the colloidal film.

Electrostatically Driven Vertical Combinatorial Patterning of Colloidal Nano-Objects

The hierarchically directed assembly of multiple types of colloidal nano-objects on surfaces is of interest for developing disruptive applications combining their original properties. We propose herein a versatile, electrostatically driven strategy to arrange various kinds of colloids vertically in the shape of 3D micropatterns by nanoxerography. We made the proof of concept of this vertical combinatorial nano-object patterning using two types of photoluminescent CdSe(S)/CdZnS core/shell nanoplatelets emitting in the red and green wavelengths as model colloidal nanoparticles. The key experimental parameters were investigated to tune the thickness of each independent level of nanoplatelets within the vertical stack. We finally applied such a concept to make dual-colored nanoplatelet patterns. Interestingly, we proved numerically that the relatively high index of the nanoplatelet level is responsible for the partially directed emissions observed in photoluminescence experiments.

Carbon Nanodots as Electron Transport Materials in Organic Light Emitting Diodes and Solar Cells

Charge injection and transport interlayers play a crucial role in many classes of optoelectronics, including organic and perovskite ones. Here, we demonstrate the beneficial role of carbon nanodots, both pristine and nitrogen-functionalized, as electron transport materials in organic light emitting diodes (OLEDs) and organic solar cells (OSCs). Pristine (referred to as C-dots) and nitrogen-functionalized (referred to as NC-dots) carbon dots are systematically studied regarding their properties by using cyclic voltammetry, Fourier-transform infrared (FTIR) and UV-Vis absorption spectroscopy in order to reveal their energetic alignment and possible interaction with the organic semiconductor's emissive layer. Atomic force microscopy unravels the ultra-thin nature of the interlayers. They are next applied as interlayers between an Al metal cathode and a conventional green-yellow copolymer-in particular, (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1',3}-thiadiazole)], F8BT)-used as an emissive layer in fluorescent OLEDs. Electrical measurements indicate that both the C-dot- and NC-dot-based OLED devices present significant improvements in their current and luminescent characteristics, mainly due to a decrease in electron injection barrier. Both C-dots and NC-dots are also used as cathode interfacial layers in OSCs with an inverted architecture. An increase of nearly 10% in power conversion efficiency (PCE) for the devices using the C-dots and NC-dots compared to the reference one is achieved. The application of low-cost solution-processed materials in OLEDs and OSCs may contribute to their wide implementation in large-area applications.

Reflow transfer for conformal three-dimensional microprinting

From microcircuits to metamaterials, the micropatterning of surfaces adds valuable functionality. For nonplanar surfaces, incompatibility with conventional microlithography requires the transfer of originally planar micropatterns onto those surfaces; however, existing approaches accommodate only limited curvatures. A microtransfer approach was developed using reflowable materials that transform between solid and liquid on demand, freely stretching to yield transfers that naturally conform down to nanoscale radii of curvature and arbitrarily complex topographies. Such reflow transfer helps generalize microprinting, extending the reach of precision planar microlithography to highly nonplanar substrates and microstructures. With gentle water-based processing, reflow transfer can be applied to a range of materials, with microprinting demonstrated onto metal, plastic, paper, glass, polystyrene, semiconductor, elastomer, hydrogel, and multiple biological surfaces.

Infrared Plasmonic Metamaterials Based on Transparent Nanoparticle Films of In2O3:Sn for Solar-Thermal Shielding Applications

Three-dimensional nanoparticle (NP) assemblies show interesting optical responses that differ from naturally occurring materials, such as metals, oxides, and semiconductors. In this study, we investigate the optical response of thin films comprising Sn:In₂O₃ NPs (ITO NP films) based on the correlation between complex permittivity and infrared (IR) reflectance for solar-thermal shielding applications. IR ellipsometry measurements are conducted to clarify the presence of Lorentz resonances in plasmonic metamaterials. The Lorentz resonances are correlated to the electric field strength at interparticle gaps by varying the Sn dopant concentration, as confirmed using finite-difference time-domain (FDTD) simulations. High solar-thermal shielding performance was obtained owing to selective near-IR reflection based on strong Lorentz resonances as the ITO NP films were electrically polarizable but magnetically inactive. Thermal shielding efficiency was demonstrated via a comparison of the air temperature change in a simulated box used as a model house. Additionally, we demonstrate the significance of NP packing density on the enhancement of the near-IR reflectance. The role of interparticle spacing for high near-IR reflectance was revealed by comparing effective medium approximation analyses and FDTD simulations. This relationship was also demonstrated by the reduction of solar-thermal shielding performance when using aggregated ITO NPs. Our work confirmed that the control of complex permittivity in plasmonic metamaterials must be considered in the structural design of transparent and reflective materials for solar-thermal shielding applications.

Versatile Surface Patterning with Low Molecular Weight Photoswitches

Surface patterning of functional materials is a key technology in various fields such as microelectronics, optics, and photonics. In micro- and nanofabrication, polymers are frequently employed either as photoreactive or thermoresponsive resists that enable further fabrication steps, or as functional adlayers in electronic and optical devices. In this article, a method is presented for imprint lithography using low molecular weight arylazoisoxazoles photoswitches instead of polymer resists. These photoswitches exhibit a rapid and reversible solid-to-liquid phase transition upon photo-isomerization at room temperature, making them highly suitable for reversible surface functionalization at ambient conditions. Beyond photo-induced imprint lithography with multiple write-and-erase cycles, prospective applications as patterned matrix for nanoparticles and etch resist on gold surfaces are demonstrated.

Sub-5 nm Anisotropic Pattern Transfer via Colloidal Lithography of a Self-Assembled GdF3 Nanocrystal Monolayer

Patterning materials with nanoscale features opens many research opportunities ranging from fundamental science to technological applications. However, current nanofabrication methods are ill-suited for sub-5 nm patterning and pattern transfer. We demonstrate the use of colloidal lithography to transfer an anisotropic pattern of discrete features into substrates with a critical dimension below 5 nm. The assembly of monodisperse, anisotropic nanocrystals (NCs) with a rhombic-plate morphology spaced by dendrimer ligands results in a well-ordered monolayer that serves as a 2D anisotropic hard mask pattern. This pattern is transferred into the underlying substrate using dry etching followed by removal of the NC mask. We exemplify this approach by fabricating an array of pillars with a rhombic cross-section and edge-to-edge spacing of 4.4 ± 1.1 nm. The fabrication approach enables broader access to patterning materials at the deep nanoscale by implementing innovative processes into well-established fabrication methods while minimizing process complexity.

Polymer-based self-assembled photonic crystals to tune light transport and emission

The advent of photonic crystals has made possible the idea of controlling flows of light, which has revolutionized photonics-based technology. Photonic crystals are constructed based on periodic refractive index variations in one-, two-, or three-spatial dimensions on the optical-wavelength scale. Photonic crystals show inherent photonic stop gaps or band gaps depending upon the crystal symmetry and refractive index contrast. Showing ease of fabrication, polymer-based self-assembled photonic crystals with stop gaps have been widely explored. We discuss angle- and polarization-dependent stop gap creation and splitting at higher angles of incidence. The observed stop gaps in self-assembled photonic crystals often deviate from theoretical predictions due to experimental constraints, such as finite size and fabrication disorders associated with samples. We perform micro-reflectivity experiments on a single domain, showing minimal disorder, with nearly 100% reflectivity, which is in agreement with theory. We obtain more than 75% emission intensity suppression and a 30% increase in the emission lifetime at the stop gap using micro-emission experiments in a single domain. This enables us to study the role of finite-size effects in photonic crystals in modifying the emission properties. We observe the linear scaling of the emission intensity suppression and the emission rate with the finite size of the crystal. Our single-domain experimental studies reveal that the use of low index-contrast self-assembled photonic crystals is a potential platform for strategically modifying light transport and emission properties.

Evolution of High Symmetry Points of Photonic Alumina Superlattices in a Lithography-Free Approach

Ceaselessly increasing demands for elaborate nanostructures prompt advanced structure fabrication with good practicability, especially, subwavelength ordered structures in simple lattices even in superlattices over a large area, namely, large-scale photonic lattices, in which lattice arrangement, geometry, and components of unit cells are key factors for their macroscopic optical properties. Moreover, exciting properties always occur at high symmetry points of the lattice; therefore, straightforward modulation of symmetry points over a large area is very important for the investigation and application of photonic lattices. Here, this work establishes a lithography-free approach of undervoltage oxidation (UVO) for regulating high symmetry points in the reciprocal space of a dielectric alumina superlattice. Embedding subunit cells at high symmetry points Γ (M) result in the degenerate energy changing from 1.34 eV (924.6 nm) to 1.87 eV (662.6 nm) under normal excitation at the Γ point, and the degeneracy lifting under off-normal excitation along the Γ-X high symmetry orientation. Furthermore, systematic characterizations of the alumina membrane are presented to learn its dynamic evolution of the morphology on a centimeter scale, and the pore array changes from a hierarchical period to a form of hexagonal close packing, especially at Γ and M points of the square lattice. Therefore, the reported lithography-free alumina-based nanofabrication offers an ability for varying the spatial structure at high symmetry points of photonic lattices, which is of great significance in the fields of nanomanufacturing and has great potential to bring about preferable performances in nanodevices.

Nanoplasmonic Sensor Approaches for Sensitive Detection of Disease-Associated Exosomes

Exosomes are abundantly secreted by most cells that carry membrane and cytosolic factors that can reflect the physiologic state of their source cells and thus have strong potential to serve as biomarkers for early diagnosis, disease staging, and treatment monitoring. However, traditional diagnostic or prognostic applications that might use exosomes are hindered by the lack of rapid and sensitive assays that can exploit their biological information. An array of assay approaches have been developed to address this deficit, including those that integrate immunoassays with nanoplasmonic sensors to measure changes in optical refractive indexes in response to the binding of low concentrations of their targeted molecules. These sensors take advantage of enhanced and tunable interactions between the electron clouds of nanoplasmonic particles and structures and incident electromagnetic radiation to enable isolation-free and ultrasensitive quantification of disease-associated exosome biomarkers present in complex biological samples. These unique advantages make nanoplasmonic sensing one of the most competitive approaches available for clinical applications and point-of-care tests that evaluate exosome-based biomarkers. This review will briefly summarize the origin and clinical utility of exosomes and the limitations of current isolation and analysis approaches before reviewing the specific advantages and limitations of nanoplasmonic sensing devices and indicating what additional developments are necessary to allow the translation of these approaches into clinical applications.

A wafer-scale fabrication method for three-dimensional plasmonic hollow nanopillars

Access to nanofabrication strategies for crafting three-dimensional plasmonic structures is limited. In this work, a fabrication strategy to produce 3D plasmonic hollow nanopillars (HNPs) using Talbot lithography and I-line photolithography is introduced. This method is named subtractive hybrid lithography (SHL), and permits intermixed usage of nano-and-macroscale patterns. Sputter-redeposition of gold (Au) on the SHL resist pattern yields large areas of dense periodic Au-HNPs. These Au-HNPs are arranged in a square unit cell with a 250 nm pitch. The carefully controlled fabrication process resulted in Au-HNPs with nanoscale dimensions over the Au-HNP dimensions such as an 80 ± 2 nm thick solid base with a 133 ± 4 nm diameter, and a 170 ± 10 nm high nano-rim with a 14 ± 3 nm sidewall rim-thickness. The plasmonic optical response is assessed with FDTD-modeling and reveals that the highest field enhancement is at the top of the hollow nanopillar rim. The modeled field enhancement factor (EF) is compared to the experimental analytical field enhancement factor, which shows to pair up with ca. 103 < EF < 104 and ca. 103 < EF < 105 for excitation wavelengths of 633 and 785 nm. From a broader perspective, our results can stimulate the use of Au-HNPs in the fields of plasmonic sensors and spectroscopy.

Recent Advances of Nanospheres Lithography in Organic Electronics

Nanospheres lithography (NSL) is an economical technique, which makes use of highly monodispersed nanospheres such as deposition or etch masks for generating patterns with nanoscale features. Embedding nanostructures into organic electronic devices can endow them with unique capabilities and enhanced performance, which have greatly advanced the development of organic electronics. In this review, a brief summary of the methods for the preparation of monodispersed nanospheres is presented. Afterward, the authors highlight the recent advances of a wide variety of applications of nanospheres lithography in organic electronic devices. Finally, the challenges in this field are pointed out, and the future development of this field is discussed.

Metal-Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

Electroless etching of semiconductors has been elevated to an advanced micromachining process by the addition of a structured metal catalyst. Patterning of the catalyst by lithographic techniques facilitated the patterning of crystalline and polycrystalline wafer substrates. Galvanic deposition of metals on semiconductors has a natural tendency to produce nanoparticles rather than flat uniform films. This characteristic makes possible the etching of wafers and particles with arbitrary shape and size. While it has been widely recognized that spontaneous deposition of metal nanoparticles can be used in connection with etching to porosify wafers, it is also possible to produced nanostructured powders. Metal-assisted catalytic etching (MACE) can be controlled to produce (1) etch track pores with shapes and sizes closely related to the shape and size of the metal nanoparticle, (2) hierarchically porosified substrates exhibiting combinations of large etch track pores and mesopores, and (3) nanowires with either solid or mesoporous cores. This review discussed the mechanisms of porosification, processing advances, and the properties of the etch product with special emphasis on the etching of silicon powders.

Lithography Technology for Micro- and Nanofabrication

Micro and nanofabrication technologies are integral to the development of miniaturized systems. Lithography plays a key role in micro and nanofabrication techniques. Since high functional miniaturized systems are required in various fields, such as the development of a semiconductor, chemical and biological analysis, and biomedical researches, lithography techniques have been developed and applied for their appropriate purpose. Lithography can be classified into conventional and unconventional lithography, or top-down and bottom-up, or with mask and mask-less approaches. In this chapter, various lithography techniques are categorized and classified into conventional and unconventional lithography. In the first part, photolithography, electron beam, and focused-ion beam lithography are introduced as conventional lithography techniques. The second part introduces nanoimprint lithography, deformation lithography, and colloidal lithography as unconventional lithography techniques. In the last part, the pros and cons of each lithography are discussed for an appropriate design of fabrication processes.

Short-Pulse Lasers: A Versatile Tool in Creating Novel Nano-/Micro-Structures and Compositional Analysis for Healthcare and Wellbeing Challenges

Driven by flexibility, precision, repeatability and eco-friendliness, laser-based technologies have attracted great interest to engineer or to analyze materials in various fields including energy, environment, biology and medicine. A major advantage of laser processing relies on the ability to directly structure matter at different scales and to prepare novel materials with unique physical and chemical properties. It is also a contact-free approach that makes it possible to work in inert or reactive liquid or gaseous environment. This leads today to a unique opportunity for designing, fabricating and even analyzing novel complex bio-systems. To illustrate this potential, in this paper, we gather our recent research on four types of laser-based methods relevant for nano-/micro-scale applications. First, we present and discuss pulsed laser ablation in liquid, exploited today for synthetizing ultraclean "bare" nanoparticles attractive for medicine and tissue engineering applications. Second, we discuss robust methods for rapid surface and bulk machining (subtractive manufacturing) at different scales by laser ablation. Among them, the microsphere-assisted laser surface engineering is detailed for its appropriateness to design structured substrates with hierarchically periodic patterns at nano-/micro-scale without chemical treatments. Third, we address the laser-induced forward transfer, a technology based on direct laser printing, to transfer and assemble a multitude of materials (additive structuring), including biological moiety without alteration of functionality. Finally, the fourth method is about chemical analysis: we present the potential of laser-induced breakdown spectroscopy, providing a unique tool for contact-free and space-resolved elemental analysis of organic materials. Overall, we present and discuss the prospect and complementarity of emerging reliable laser technologies, to address challenges in materials' preparation relevant for the development of innovative multi-scale and multi-material platforms for bio-applications.

Molecular Mechanisms of Topography Sensing by Osteoblasts: An Update

Bone is a specialized tissue formed by different cell types and a multiscale, complex mineralized matrix. The architecture and the surface chemistry of this microenvironment can be factors of considerable influence on cell biology, and can affect cell proliferation, commitment to differentiation, gene expression, matrix production and/or composition. It has been shown that osteoblasts encounter natural motifs in vivo, with various topographies (shapes, sizes, organization), and that cell cultures on flat surfaces do not reflect the total potential of the tissue. Therefore, studies investigating the role of topographies on cell behavior are important in order to better understand the interaction between cells and surfaces, to improve osseointegration processes in vivo between tissues and biomaterials, and to find a better topographic surface to enhance bone repair. In this review, we evaluate the main available data about surface topographies, techniques for topographies’ production, mechanical signal transduction from surfaces to cells and the impact of cell–surface interactions on osteoblasts or preosteoblasts’ behavior.

Breaking the symmetry of nanosphere lithography with anisotropic plasma etching induced by temperature gradients

We report a novel anisotropic process, termed plasma etching induced by temperature gradients (PE-TG), which we use to modify the 3D morphology of a hexagonally close-packed polystyrene sphere array. Specifically, we combined an isotropic oxygen plasma (generated by a plasma cleaner) and a vertical temperature gradient applied from the bottom to the top of a colloidal mask to create an anisotropic etching process. As a result, an ordered array of well-defined and separated nano mushrooms is obtained. We demonstrate that the features of the mushrooms, namely the hat size and their intrinsic undercut, as well as the pillar diameter and height, can be easily tuned by adjusting the main parameters of the process i.e. the temperature gradient and etching time, or the spheres' size. We show that PS mushroom arrays can be used as nanostructured templates to fabricate plasmonic arrays, such as gold-capped nano mushrooms and ultra-small nanoapertures, by using vertical and oblique gold sputtering deposition respectively. PE-TG reveals a new, cheap and facile approach to produce plasmonic nanostructures of great interest in the fields of molecular sensing, surface-enhanced Raman scattering (SERS), energy harvesting and optoelectronics. We study the optical properties of the Au-capped nano mushroom arrays and their performance as biosensing platforms by performing SERS measurements.

Mechanical and Physical Regulation of Fibroblast-Myofibroblast Transition: From Cellular Mechanoresponse to Tissue Pathology

Fibroblasts are cells present throughout the human body that are primarily responsible for the production and maintenance of the extracellular matrix (ECM) within the tissues. They have the capability to modify the mechanical properties of the ECM within the tissue and transition into myofibroblasts, a cell type that is associated with the development of fibrotic tissue through an acute increase of cell density and protein deposition. This transition from fibroblast to myofibroblast-a well-known cellular hallmark of the pathological state of tissues-and the environmental stimuli that can induce this transition have received a lot of attention, for example in the contexts of asthma and cardiac fibrosis. Recent efforts in understanding how cells sense their physical environment at the micro- and nano-scales have ushered in a new appreciation that the substrates on which the cells adhere provide not only passive influence, but also active stimulus that can affect fibroblast activation. These studies suggest that mechanical interactions at the cell-substrate interface play a key role in regulating this phenotype transition by changing the mechanical and morphological properties of the cells. Here, we briefly summarize the reported chemical and physical cues regulating fibroblast phenotype. We then argue that a better understanding of how cells mechanically interact with the substrate (mechanosensing) and how this influences cell behaviors (mechanotransduction) using well-defined platforms that decouple the physical stimuli from the chemical ones can provide a powerful tool to control the balance between physiological tissue regeneration and pathological fibrotic response.

Label-Free Near-Infrared Plasmonic Sensing Technique for DNA Detection at Ultralow Concentrations

Biomolecular detection at a low concentration is usually the most important criterion for biological measurement and early stage disease diagnosis. In this paper, a highly sensitive nanoplasmonic biosensing approach is demonstrated by achieving near-infrared plasmonic excitation on a continuous gold-coated nanotriangular array. Near-infrared incident light at a small incident angle excites surface plasmon resonance with much higher spectral sensitivity compared with traditional configuration, due to its greater interactive volume and the stronger electric field intensity. By introducing sharp nanotriangular metallic tips, intense localization of plasmonic near-fields is realized to enhance the molecular perception ability on sensing surface. This approach with an enhanced sensitivity (42103.8 nm per RIU) and a high figure of merit (367.812) achieves a direct assay of ssDNA at nanomolar level, which is a further step in label-free ultrasensitive sensing technique. Considerable improvement is recorded in the detection limit of ssDNA as 1.2 × 10-18 m based on the coupling effect between nanotriangles and gold nanoparticles. This work combines high bulk- and surface-sensitivities, providing a simple way toward label-free ultralow-concentration biomolecular detection.

Gold Nanozymes: From Concept to Biomedical Applications

In recent years, gold nanoparticles have demonstrated excellent enzyme-mimicking activities which resemble those of peroxidase, oxidase, catalase, superoxide dismutase or reductase. This, merged with their ease of synthesis, tunability, biocompatibility and low cost, makes them excellent candidates when compared with biological enzymes for applications in biomedicine or biochemical analyses. Herein, over 200 research papers have been systematically reviewed to present the recent progress on the fundamentals of gold nanozymes and their potential applications. The review reveals that the morphology and surface chemistry of the nanoparticles play an important role in their catalytic properties, as well as external parameters such as pH or temperature. Yet, real applications often require specific biorecognition elements to be immobilized onto the nanozymes, leading to unexpected positive or negative effects on their activity. Thus, rational design of efficient nanozymes remains a challenge of paramount importance. Different implementation paths have already been explored, including the application of peroxidase-like nanozymes for the development of clinical diagnostics or the regulation of oxidative stress within cells via their catalase and superoxide dismutase activities. The review also indicates that it is essential to understand how external parameters may boost or inhibit each of these activities, as more than one of them could coexist. Likewise, further toxicity studies are required to ensure the applicability of gold nanozymes in vivo. Current challenges and future prospects of gold nanozymes are discussed in this review, whose significance can be anticipated in a diverse range of fields beyond biomedicine, such as food safety, environmental analyses or the chemical industry.

Nematic colloids at liquid crystal-air interfaces via photopolymerization

We demonstrate the preparation of colloidal crystals at nematic liquid crystal-air interfaces by simultaneous photopolymerization and assembly. Polymer colloids are produced by polymerization-induced phase separation of 2-hydroxyethyl methacrylate in the non-reactive liquid crystal (LC) 4-cyano-4'-pentylbiphenyl (5CB) using an open-cell setup. Colloids adsorbed to the nematic 5CB-air interface form non-close-packed hexagonal crystals that cover the entire interface area. We examine the mechanism of growth and assembly for the preparation of LC-templated interfacial colloidal superstructures.

Manipulation and Applications of Hotspots in Nanostructured Surfaces and Thin Films

The synthesis of nanostructured surfaces and thin films has potential applications in the field of plasmonics, including plasmon sensors, plasmon-enhanced molecular spectroscopy (PEMS), plasmon-mediated chemical reactions (PMCRs), and so on. In this article, we review various nanostructured surfaces and thin films obtained by the combination of nanosphere lithography (NSL) and physical vapor deposition. Plasmonic nanostructured surfaces and thin films can be fabricated by controlling the deposition process, etching time, transfer, fabrication routes, and their combination steps, which manipulate the formation, distribution, and evolution of hotspots. Based on these hotspots, PEMS and PMCRs can be achieved. This is especially significant for the early diagnosis of hepatocellular carcinoma (HCC) based on surface-enhanced Raman scattering (SERS) and controlling the growth locations of Ag nanoparticles (AgNPs) in nanostructured surfaces and thin films, which is expected to enhance the optical and sensing performance.

Stepwise nanosphere lithography: an alternate way of fabricating nanostructures

This work demonstrates a new nanosphere lithography technique, termed stepwise nanosphere lithography, to create matrices of novel two- and three-dimensional nanostructures. Different sets of nanostructures are placed at desired locations through step-by-step deposition during thermal evaporation onto a substrate surface. Three deposition parameters: (1) number of deposition steps; (2) angle of deposition; and (3) nanosphere mask orientation angle were investigated. By changing these parameters, the ordering, shape, and size of nanostructures were modified accordingly. Two- and three-dimensional nanostructure matrices with different arrangements and symmetries were successfully simulated and fabricated experimentally through a combination of multiple stationary deposition stages with different parameters.

Dynamic Interfacial Regulation by Photodeformable Azobenzene-Containing Liquid Crystal Polymer Micro/Nanostructures

Photoresponsive materials offer local, temporal, and remote control over their chemical or physical properties under external stimuli, giving new tools for interfacial regulation. Among all, photodeformable azobenzene-containing liquid crystal polymers (azo-LCPs) have received increasing attention because they can be processed into various micro/nanostructures and have the potential to reversibly tune the interfacial properties through chemical and/or morphological variation by light, providing effective dynamic interface regulation. In this feature article, we highlight the milestones in the dynamic regulation of different interfacial properties through micro/nanostructures made of photodeformable azobenzene-containing liquid crystal polymers (azo-LCPs). We describe the preparation of different azo-LCP micro/nanostructures from the aspects of materials and processing techniques and reveal the importance of mesogen orientation toward dynamic interfacial regulation. By introducing our recently developed linear azo-LCP (azo-LLCP) with good mechanical and photoresponsive performances, we discuss the challenge and opportunity with respect to the dynamic light regulation of two- and three-dimensional (2D/3D) micro/nanostructures to tune their related interfacial properties. We have also given our expectation toward exploring photodeformable micro/nanostructures for advanced applications such as in microfluidics, biosensors, and nanotherapeutics.

Nano- and microscale apertures in metal films fabricated by colloidal lithography with perovskite nanocrystals

We demonstrate patterning of metal surfaces based on lift-off of perovskite nanocrystals that enables the fabrication of nanometer-size features without the use of resist-based nanolithography. The perovskite nanocrystals act as templates for defining the shape of the apertures in metal layers, and we exploit the variety of sizes and shapes that can be controlled in the colloidal synthesis to demonstrate the fabrication of nanoholes, nanogaps and guides with size smaller than the wavelength of light in the visible spectrum. The process can be readily integrated with standard lithography and etching techniques for the creation of more complex structures.

Effects of Solution Flow on the Growth of Colloidal Crystals

For the versatile potential applications of colloidal crystals, precisely controlling their growth is required to achieve properties such as high crystallinity and large-area crystals. Because colloidal crystallization is a self-assembly process of dispersed particles in a solution, solution flow directly and markedly changes the behavior of particles. Thus, the effects of solution flow on the growth of colloidal crystals were investigated in the present study. We found three different effects of solution flow on the growth of colloidal crystals: enlarging the first layer, facilitating the growth of superlattice structures, and forming a new circular packing structure. Specifically, in the single-component system, because the flow speed is lower closer to the bottom of the cell, the second and further layers dissolve owing to the large flow speed, whereas the first layer remains undissolved at the appropriate flow speed. The dissolved particles (particles that are detached from the crystals and returned back into the aqueous medium) are transported near the first layer, where they facilitate the growth of the first layer. In a binary system, when colloidal crystals with different particles are neighboring each other, the flow dissolves the surface of each crystal, which forms a dense, melt-like phase between crystals, from which a superlattice structure such as AB₂ grows. The flow often moves the second layer more than the first layer because the flow speed varies with the distance from the bottom. This causes the second layer to slide above the first layer of the neighboring crystals composed of different particle sizes, which transform from the initial face-centered cubic structure of the first layer into a circular pattern with strain. These findings contribute to establishing a sophisticated control method for growing colloidal crystals.

Hybridizing Plasmonic Materials with 2D-Transition Metal Dichalcogenides toward Functional Applications

Recently, 2D transition metal dichalcogenides (TMDs) have become intriguing materials in the versatile field of photonics and optoelectronics because of their strong light-matter interaction that stems from the atomic layer thickness, broadband optical response, controllable optoelectronic properties, and high nonlinearity, as well as compatibility. Nevertheless, the low optical cross-section of 2D-TMDs inhibits the light-matter interaction, resulting in lower quantum yield. Therefore, hybridizing the 2D-TMDs with plasmonic nanomaterials has become one of the promising strategies to boost the optical absorption of thin 2D-TMDs. The appeal of plasmonics is based on their capability to localize and enhance the electromagnetic field and increase the optical path length of light by scattering and injecting hot electrons to TMDs. In this regard, recent achievements with respect to hybridization of the plasmonic effect in 2D-TMDs systems and its augmented optical and optoelectronic properties are reviewed. The phenomenon of plasmon-enhanced interaction in 2D-TMDs is briefly described and state-of-the-art hybrid device applications are comprehensively discussed. Finally, an outlook on future applications of these hybrid devices is provided.

Plasmon-enhanced linear and second-order surface nonlinear optical response of silver nanoparticles fabricated using a femtosecond pulse

We present the plasmon-enhanced linear and second-order surface nonlinear optical response of silver nanoparticles (Ag NPs) fabricated using a femtosecond pulse. Theoretical analysis indicates Ag NPs with a diameter of ∼100 nm have excellent linear response within the visible band, and the electric field intensity enhancement factor reaches ∼10⁵ under excitation of continuous light of 632.8 nm. Meanwhile, the simulation result of second-order surface nonlinear optical response shows that the second harmonic conversion efficiency of the Ag NPs dimer is two orders of magnitude higher than that of a single Ag NP, under excitation of a femtosecond pulse. In experiment, the linear response of Ag NPs is examined using surface-enhanced Raman spectroscopy (SERS) with a Raman enhancement factor of ∼1.7 × 10¹⁰, revealing the excellent linear optical response of Ag NPs. Moreover, the spectra of the second harmonic can be measured clearly under conditions of an average pump power of 40 μW, revealing the excellent second-order surface nonlinear optical response of Ag NPs.

Continuous roll-to-roll patterning of three-dimensional periodic nanostructures

In this work, we introduce a roll-to-roll system that can continuously print three-dimensional (3D) periodic nanostructures over large areas. This approach is based on Langmuir-Blodgett assembly of colloidal nanospheres, which diffract normal incident light to create a complex intensity pattern for near-field nanolithography. The geometry of the 3D nanostructure is defined by the Talbot effect and can be precisely designed by tuning the ratio of the nanosphere diameter to the exposure wavelength. Using this system, we have demonstrated patterning of 3D photonic crystals with a 500 nm period on a 50 × 200 mm² flexible substrate, with a system throughput of 3 mm/s. The patterning yield is quantitatively analyzed by an automated electron beam inspection method, demonstrating long-term repeatability of an up to 88% yield over a 4-month period. The inspection method can also be employed to examine pattern uniformity, achieving an average yield of up to 78.6% over full substrate areas. The proposed patterning method is highly versatile and scalable as a nanomanufacturing platform and can find application in nanophotonics, nanoarchitected materials, and multifunctional nanostructures.

Self assembly of nanorods on microspheres at fluid–fluid interfaces

Interfacial self-assembly of nanoparticles on curved substrates.

Five Piconewtons: The Difference between Osteogenic and Adipogenic Fate Choice in Human Mesenchymal Stem Cells

The ability of mesenchymal stem cells to sense nanoscale variations in extracellular matrix (ECM) compositions in their local microenvironment is crucial to their survival and their fate; however, the underlying molecular mechanisms defining how such fates are temporally modulated remain poorly understood. In this work, we have utilized self-assembled block copolymer surfaces to present nanodomains of an adhesive peptide found in many ECM proteins at different lateral spacings (from 30 to 60 nm) and studied the temporal response (2 h to 14 days) of human mesenchymal stem cells (hMSCs) using a panel of real-time localization and activity biosensors. Our findings revealed that within the first 4 to 24 h postadhesion and spreading, hMSCs on smaller nanodomain spacings recruit more activated FAK and Src proteins to produce larger, longer-lived, and increased numbers of focal adhesions (FAs). The adhesions formed on smaller nanospacings rapidly recruit higher amounts of nonmuscle myosin IIA and vinculin and experience tension forces (by >5 pN/FA) significantly higher than those observed on larger nanodomain spacings. The transmission of higher levels of tension into the cytoskeleton at short times was accompanied by higher Rac1, cytosolic β-catenin, and nuclear localization of YAP/TAZ and RUNX2, which together biased the commitment of hMSCs to an osteogenic fate. This investigation provides mechanistic insights to confirm that smaller lateral spacings of adhesive nanodomains alter hMSC mechanosensing and biases mechanotransduction at short times via differential coupling of FAK/Src/Rac1/myosin IIA/YAP/TAZ signaling pathways to support longer-term changes in stem cell differentiation and state.

Numerical simulation of the pattern formation of the springtail cuticle nanostructures

Springtails (Collembola) are known to exhibit complex hierarchical nanostructures of their exoskeleton surface that repels water and other fluids with remarkable efficiency. These nanostructures were previously widely studied due to their structure, chemistry and fluid-repelling properties. These ultrastructural and chemical studies revealed the involvement of different components in different parts of the nanopattern, but the overall process of self-assembly into the complex rather regular structures observed remains unclear. Here, we model this process from a theoretical point of view partially using solutions related to the so-called Tammes problem. By using densities of three different reacting substances, we obtained a typical morphology that is highly similar to the ones observed on the cuticle of some springtail species. These results are important not only for our understanding of the formation of hierarchical nanoscale structures in nature, but also for the fabrication of novel surface coatings.

Recent progress in near-field nanolithography using light interactions with colloidal particles: from nanospheres to three-dimensional nanostructures

The advance of nanotechnology is firmly rooted in the development of cost-effective, versatile, and easily accessible nanofabrication techniques. The ability to pattern complex two-dimensional and three-dimensional nanostructured materials are particularly desirable, since they can have novel physical properties that are not found in bulk materials. This review article will report recent progress in utilizing self-assembly of colloidal particles for nanolithography. In these techniques, the near-field interactions of light and colloids are the sole mechanisms employed to generate the intensity distributions for patterning. Based on both 'bottom-up' self-assembly and 'top-down' lithography approaches, these processes are highly versatile and can take advantage of a number of optical effects, allowing the complex 3D nanostructures to be patterned using single exposures. There are several key advantages including low equipment cost, facile structure design, and patterning scalability, which will be discussed in detail. We will outline the underlying optical effects, review the geometries that can be fabricated, discuss key limitations, and highlight potential applications in nanophotonics, optoelectronic devices, and nanoarchitectured materials.

Assembly of Microparticles to Patterned Trenches Using the Depletion Volume Effect

In this paper, we demonstrate that 20 μm microbeads can be preferentially assembled into substrate trenches of similar width by employing a polymer (depletant) that induces the depletion volume effect (depletion attraction). In previous works, we proved that this strategy is useful to assemble mesoscale parts in a site-specific manner. Here, we show that it is also applicable to assemble functional parts, such as fluorescent particles, into trenches engraved on the surface of two- and three-dimensional template components. A convenient advantage of this strategy is that it is independent of material properties for assembling mesoscale functional components into desired patterns.

Colloidal lithography double-nanohole optical trapping of nanoparticles and proteins

Double-nanoholes fabricated by colloidal lithography were used for trapping single colloidal particles and single proteins. A gap separation of 60 nm between the cusps of the double-nanohole was achieved in a gold film of 70 nm thickness sputter coated onglass. The cusp separation was reduced steadily down to 10 nm by plasma etching the colloidal particles prior to sputter coating. Scanning electron microscopy was used to locate a particular double-nanohole and it was registered for later microscopy experiments. 30 nm polystyrene particles, the rubisco protein and bovine serum albumin were trapped using a laser focused through the aperture. Compared to other methods that require top-down nanofabrication, this approach is inexpensive and produces high-quality samples.

Ordered Particle Arrays via a Langmuir Transfer Process: Access to Any Two-Dimensional Bravais Lattice

We demonstrate how to directly transform a close-packed hexagonal colloidal monolayer into nonclose-packed particle arrays of any two-dimensional symmetry at the air/water interface. This major advancement in the field of nanoparticle self-assembly is based on a simple one-dimensional stretching step in combination with the particle array orientation. Our method goes far beyond existing strategies and allows access to all possible two-dimensional Bravais lattices. A key element of our work is the possibility to macroscopically stretch a particle array in a truly one-dimensional manner, which has not been possible up to now. We achieve this by stretching the nanoparticle array at an air/water interface during the transfer process. The degree of stretching is simply controlled by the wettability of the transfer substrate. To retain the symmetry of the transferred structure, the capillary forces upon drying have to be circumvented. We demonstrate two concepts based on thermal fixation for this. It allows for the first time to fabricate nonclose-packed, nonhexagonal colloidal monolayers on a macroscopic length scale.

Photolithographic nanoseeding method for selective synthesis of metal-catalysed nanostructures

In this work we present a general method for the selective synthesis by photolithography of localised nanostructures in planar geometries. The methodology relies on the previous concept of photo-patternable metallic nanoparticle (NP)/polymer nanocomposites, which can provide a range of NP sizes, polydispersity and densities. First, a photoresist containing metallic ions is patterned by photolithography. Silver NPs are synthesised in situ after the exposure and development of the patterned thin film via the thermal-induced reduction of ions embedded in its structure. Gentle plasma ashing is used to selectively remove the polymer, which leaves NPs on the patterned areas. These NPs are used as seeds for subsequent processes. In order to demonstrate the flexibility of the method, its use to selectively produce localised nanostructures through different processes is shown here. Following a top-down approach, high aspect-ratio silicon nanograss has been produced by reactive ion etching and masking by the NPs. In a bottom-up approach, 280 nm copper clusters have been selectively grown in arrays. This method can be easily extrapolated to other metals and it provides a quick way to selectively generate hierarchical nanostructures in large planar areas that can be used for different applications, such as the fabrication of nanostructured sensor arrays.

Ordered Array of Metal Particles on Semishell Separated with Ultrathin Oxide: Fabrication and SERS Properties

Metal particles in gap cavities provide an interesting system to achieve hybrid local surface plasmon modes for local field enhancement. Here, we demonstrate a relatively simple method to fabricate Ag nanoparticles positioned on Ag semishells separated by a thin (~5 nm) dielectric layer. The obtained structure can provide strong local electric field enhancement for surface-enhanced Raman scattering (SERS). The fabrication of the ordered array structure was realized by nanosphere self-assembly, atomic layer deposition, and metal thin-film dewetting. Numerical simulation proved that, compared to the conventional metal semishell arrays, the additional Ag particles introduce extra hot spots particularly in the valley regions between adjacent Ag semishells. As a result, the SERS enhancement factor of the metal semishell-based plasmonic structure could be further improved by an order of magnitude. The developed novel plasmonic structure also shows good potential for application in plasmon-enhanced solar water-splitting devices.

Vapor Annealing and Colloid Lithography: An Effective Tool To Control Spatial Resolution of Surface Modification

Colloid lithography represents a simple and efficient method for creation of a large-scale template for subsequent surface patterning, deposition of regular metal nanostructures, or periodical surface structures. However, this method is significantly restricted by its ability to create only a limited number of structures with confined geometry and symmetry features. To overcome this limitation, different techniques, such as plasma treatment or tilting angle metal deposition, have been proposed. In this paper, an alternative method based on the vapor annealing of ordered single polystyrene (PS) microspheres layer, followed by the surface grafting with arenediazonium tosylates is proposed. Application of vapor treatment before surface grafting allows effective control of the area screened by PS microspheres. Pristine and vapor-annealed microsphere arrays on the gold substrate were electrochemically modified using ADTs. Subsequent removal of the PS microsphere mask enabled to prepare well-defined nanostructures with controllable surface features. In particular, prepared periodic arrangements were achieved by the grafting of OFGs to the empty interspaces between nanopore arrays. The process of sample preparation was controlled, and the properties of prepared structures were characterized by various techniques, including atomic force microscopy (AFM), conductive AFM, scanning electron microscopy energy-dispersive X-ray spectrometry, Raman spectroscopy, and voltammetry.

Chiral plasmonic nanocrescents: large-area fabrication and optical properties

Large-area arrays of substrate-supported plasmonic gold crescents are fabricated by using the new colloidal lithography technique, which is based on an in situ-deposited silica resistance layer. The method provides the means to control the particles' asymmetry just by changing the mutual deposition angle of gold and silica. Asymmetric crescent structures exhibit a pronounced circular dichroism in near-infrared region, with the chiral asymmetry factor reaching 0.2. According to the simulation, the optical chirality enhancement reaches between one and two orders of magnitude and is localized near the crescents' tips.

Buckling polystyrene beads with light

Colloidal transformation based on simple physicochemical processes has produced a wide variety of functional structures for different applications. But the lack of local selectivity of conventional transformation methods makes the fabrication of nanodevices with desired optical properties challenging. Here, we use a laser beam to transform spherical polystyrene (PS) beads into bull's eye-shaped nanopatterns or concentric nanorings, depending on the time of irradiation. The final morphologies are dependent on the size of the PS beads and the dielectric nature of the substrates. The simulated near field features show that it is the selective hollowing of PS beads that results in collapsing and buckling of the shells. This understanding provides a new route towards unconventional colloidal nanostructures and defect engineering in 2D photonic crystals that can be locally and selectively controlled by light.

Fabrication of Polymer Microstructures of Various Angles via Synchrotron X-Ray Lithography Using Simple Dimensional Transformation

In this paper, we developed a method of fabricating polymer microstructures at various angles on a single substrate via synchrotron X-ray lithography coupled with simple dimensional transformations. Earlier efforts to create various three-dimensional (3D) features on flat substrates focused on the exposure technology, material properties, and light sources. A few research groups have sought to create microstructures on curved substrates. We created tilted microstructures of various angles by simply deforming the substrate from 3D to two-dimensional (2D). The microstructural inclination angles changed depending on the angles of the support at particular positions. We used convex, concave, and S-shaped supports to fabricate microstructures with high aspect ratios (1:11) and high inclination angles (to 79°). The method is simple and can be extended to various 3D microstructural applications; for example, the fabrication of microarrays for optical components, and tilted micro/nanochannels for biological applications.

Three-Dimensional in Situ Electron-Beam Lithography Using Water Ice

Three-dimensional (3D) nanofabrication techniques are of paramount importance in nanoscience and nanotechnology because they are prerequisites to realizing complex, compact, and functional 3D nanodevices. Although several 3D nanofabrication methods have been proposed and developed in recent years, it is still a formidable challenge to achieve a balance among resolution, accuracy, simplicity, and adaptability. Here, we propose a 3D nanofabrication method based on electron-beam lithography using ice resists (iEBL) and fabricate 3D nanostructures by stacking layered structures and those with dose-modulated exposure, respectively. The entire process of 3D nanofabrication is realized in one vacuum system by skipping the spin-coating and developing steps required for commonly used resists. This needs far fewer processing steps and is contamination-free compared with conventional methods. With in situ alignment and correction in the iEBL process, a pattern resolution of 20 nm and an alignment error below 100 nm can be steadily achieved. This 3D nanofabrication technique using ice thus shows great potential in the fabrication of complicated 3D nanodevices.