Nanosphere Lithography Research Articles

Antireflection Structures for VIS and NIR on Arbitrarily Shaped Fused Silica Substrates with Colloidal Polystyrene Nanosphere Lithography.

Antireflective (AR) nanostructures offer an effective, broadband alternative to conventional AR coatings that could be used even under extreme conditions. In this publication, a possible fabrication process based on colloidal polystyrene (PS) nanosphere lithography for the fabrication of such AR structures on arbitrarily shaped fused silica substrates is presented and evaluated. Special emphasis is placed on the involved manufacturing steps in order to be able to produce tailored and effective structures. An improved Langmuir-Blodgett self-assembly lithography technique enabled the deposition of 200 nm PS spheres on curved surfaces, independent of shape or material-specific characteristics such as hydrophobicity. The AR structures were fabricated on planar fused silica wafers and aspherical planoconvex lenses. Broadband AR structures with losses (reflection + transmissive scattering) of <1% per surface in the spectral range of 750-2000 nm were produced. At the best performance level, losses were less than 0.5%, which corresponds to an improvement factor of 6.7 compared to unstructured reference substrates.

Biomimetic moth-eye nanostructures fabricated by nanosphere lithography in ZnSe enhancing transmission in the mid-infrared

ZnSe is a highly regarded material for optical windows in the infrared spectrum, where transmission efficiency is a critical factor. Traditional surface antireflective coatings have limitations that can be overcome with biomimetic moth-eye nanostructures. In this paper, we present a novel method using nanosphere lithography with polystyrene (PS) nanosphere to fabricate moth-eye nanostructures (MENS) on ZnSe surface with an exceptional antireflection effect. The parabolic profile MENS, which is challenging to fabricate through conventional methods, was directly obtained using our dry etch process. This profile outperformed other shapes like circular cones, resulting in better antireflective performance. Our experimental results matched perfectly with the modeling and demonstrated that the MENS had broadband antireflection performance in the 2–5 μm wavelength range. The maximum transmission of MENS was 82.6%, which was a significant improvement over the flat ZnSe with 71.28% transmission, achieving a 17.2% transmittance enhancement that is close to the ZnSe transmission limit. Moreover, the MENS exhibited a highly desirable hydrophobicity with a wetting angle of 114° instead of 71° on flat ZnSe. Our work displays great potential in optoelectronic devices and high-power laser applications.

Broadband, angle-dependent optical characterization of asymmetric self-assembled nanohole arrays in silver

Plasmonic nanostructured materials made of nanohole arrays in metal are significant plasmonic devices exhibiting resonances and strong electromagnetic confinement in the visible and near-infrared range. As such, they have been proposed for use in many applications such as biosensing and communications. In this work, we introduce the asymmetry in nanoholes, and investigate its influence on the electromagnetic response by means of broadband experimental characterization and numerical simulations. As a low-cost fabrication process, we use nanosphere lithography, combined with tilted silver evaporation, to obtain a 2D hexagonal array of asymmetric nanoholes in Ag. Our experimental set-up is based on a laser, widely tunable in the near-infrared range, with precise polarization control in the input and in the output. We next resolve the circular polarization degree of the transmitted light when the nanohole array is excited with linear polarization. We attribute the disbalance of left and right transmitted light to the asymmetry of the nanohole, which we support by numerical simulations. We believe that the optimization of such simple plasmonic geometry could lead to multifunctional flat-optic devices.

Photofabrication of Chiral Plasmonic Nanostructure Arrays

AbstractChiral plasmonic nanostructures can be potentially applied to chiroptical materials and devices, as well as sensors for chiral molecules. When those are applied to metasurfaces or metamaterials, those would be appropriately oriented and aligned. We therefore prepared arrays of chiral plasmonic nanostructures on the basis of bottom‐up, self‐assembling methods. A triangular Au nanoplate array as an achiral precursor was prepared on a TiO2‐coated substrate by nanosphere lithography. The array was irradiated with right‐ or left‐circularly polarized light (CPL) in a solution containing Pb2+ for site‐selective deposition of PbO2 in a chiral geometry by plasmon‐induced charge separation. The well‐aligned Au−PbO2 nanostructure array thus obtained exhibits circular dichroism, and the handedness of the array can be inverted by changing the rotation direction of the irradiated CPL.

SERS assisted sandwich immunoassay platforms for ultrasensitive and selective detection of human Thyroglobulin

We developed an immunoassay platform for the detection of human Thyroglobulin (Tg) to be integrated with fine-needle aspiration biopsy for early detection of lymph node metastases in thyroid cancer patients. The sensing platform detects Tg by a sandwich immunoassay involving a self-assembled surface-enhanced Raman scattering (SERS) substrate assisted by functionalized gold nanoparticles that provide additional Raman signal amplification and improved molecular specificity. Specifically, the SERS-active substrates were functionalized with Tg Capture antibodies and fabricated either on-chip or on optical fiber tips by nanosphere lithography. Gold nanoparticles were functionalized with Detection antibodies and conjugated with 4-mercaptobenzoic acid, which serves as a Raman reporter. The sandwich assay platform was validated in the planar configuration and a detection limit as low as 7 pg/mL was successfully achieved. Careful morphological examination of the SERS substrates before and after Tg measurements further assessed the effective capture of nanoparticles and correlated the average nanoparticle coverage with the Tg concentration obtained by SERS measurements. The sandwich assay was successfully demonstrated on washout fluids of fine needle aspiration biopsies from cancer patients and confirmed the high specificity of the proposed methodology when complex biological matrices are considered. Finally, SERS optrodes were fabricated and successfully used to detect Tg concentration by applying the same bio-recognition strategy and Raman interrogation through an optical fiber. This opens the possibility of transferring the Tg detection approach to the optical fiber tip to develop point-of-care platforms that can be directly integrated into fine needle aspiration biopsies.

Miniature Transparent Dopamine Sensor Based on Nanosphere Lithography

AbstractDetection of neurotransmitters for in vitro analysis systems is an essential component in evaluating the integrity and viability of in vitro models of neurodegenerative disorders and the cytotoxicity of drug candidates. Currently, electrochemical analysis such as cyclic voltammetry (CV) is the most widely used method for the detection of neurotransmitters. However, while most in vitro analysis systems are based on optical imaging, electrochemical sensors are non‐transparent and hinder in situ simultaneous cell monitoring. In this work, a highly sensitive miniaturized electrochemical sensor that is also transparent is proposed, which enables simultaneous dopamine detection and in vitro optical assays. The microfabrication process and nanosphere lithography are utilized to fabricate a transparent working electrode. The transparent electrode is functionalized with the overoxidized polypyrrole/sodium dodecyl sulfate‐modified carbon nanotube (OPPy/SDS‐CNT). The detection of DA is successfully demonstrated using the miniaturized transparent sensor. In addition, the PC‐12 cell is cultured in the sensor‐integrated chamber and the cell viability is confirmed on the sensor over 3 days. The proposed miniature transparent sensor is a great candidate for next‐generational multi‐modal in vitro analysis systems.

Does the chemical contribution have a secondary role in SERS?

It is an established understanding that the electromagnetic contribution (plasmon-mediated enhancement of a laser and scattered local electromagnetic fields) is the main actor in surface enhanced Raman scattering (SERS), with the so-called chemical (molecule-related) contribution assuming only, if any, a supporting role. The conclusion of our comprehensive experimental resonant study of a broad range of nanosphere lithography based metallic substrates, with covalently attached 4-mercaptobenzoic acid monolayers used as a probe (standard molecules that are non-resonant in solution), is that this accepted understanding needs to be revised. We present a detailed resonant SERS study of metal-film-over-nanosphere (MFON) substrates that is done by both scanning the laser wavelength and tuning the plasmon response through the nanosphere diameter, which is varied from 500 to 900 nm. Far and local field properties are characterized through measures of optical reflectivity and SERS efficiency, respectively, and are supported by numerical simulations. We demonstrate that SERS intensity depends indeed on the electromagnetic mechanism, determined by the plasmonic response of the system, but we observe that it is also strongly defined by a chemical resonant contribution related to a metal-to-ligand electronic transition of the covalently bound probe molecule. Optimum amplification occurs when the plasmon modes intersect with the ligand-to-metal chemical resonance, contributing synergically both mechanisms together. Quite notably, however, the largest SERS signal is observed when the laser is tuned with the metal-to-ligand transition, and typically does not follow the wavelength dependence of the plasmon modes when varying the nanosphere size. The same general trend is observed for other nanosphere lithography based substrates, including sphere segment void cavities and hexagonally ordered triangular nanoparticles, using either Ag or Au as the plasmonic metal, and also with a commercial substrate (Klarite). Interestingly, this extensive comparative investigation shows in addition that MFON substrates are significantly better than these other studied plasmonic substrates in terms of Raman intensity and homogeneity. We conclude that a deep understanding of both electromagnetic and chemical mechanisms is necessary to fully exploit these substrates for analytical applications.

Cross-fertilized biomimetic structures achieved through nanosphere lithography on an ultrathin wafer for flexible black c-Si SHJ solar cells

The millions of years of history of evolution are practically a history of optimization of all the biological designs and structures so as to cope up with nature and to get the best out of it. These natural designs can be exhaustive sources of readily available solutions for engineering and technological problems. Moth eye-like nanostructured anti-reflection coatings are an innovative concept for the fabrication of advanced and efficient solar cells due to its low reflection attributes when applied on the top of a cell. However, owing to its high surface area and low aspect ratio, the minority carrier life time and wide-angle light collection may be compromised, and these are treated as the major barriers of using such moth eye-like nanostructured anti-reflection coatingsin real cases. To overcome this problem, in this work, it has been aimed to introduce a ‘cross fertilized’ biomimetic structure between the ‘moth eye’ and ‘cacti’, so that maximum light can be utilized without much compromising the electrical properties. Detailed experimental and theoretical analyses on the optical and structural properties of such cross-fertilized nanostructures were carried out, giving emphasis on some crucial parameters like wide angle light collection, unified photon-electron harvesting capabilities, and surface recombination velocity. Finally, practical implementation of this optimized geometry has been carried out by the formation of ‘nano-cone’-based black c-Si structures through a metal-assisted chemical etching technique, followed by the fabrication of black p-c-Si/n-a-Si:H flexible solar cells.

Flexible and Speedy Preparation of Large-Scale Polystyrene Monolayer through Hemispherical-Depression-Assisted Self-Assembling and Vertical Lifting

Liquid interfacial self-assembled two-dimensional nanomonolayers are promising as colloidal crystal templates or lithographic masks for the mass production of periodic nanoarrays. However, the interfacial self-assembled monolayers usually suffer various defects such as grain boundaries, line defects, vacancy defects, and multilayers due to imperfect process technology. Here, we demonstrated a series of optimized strategies including the hemispherical-depression-assisted self-assembly, dot coating approach, and vertical lifting transfer protocol to eliminate surface defects especially multilayers jointly. After stabilizing the hemispherical depression height to 1.5 mm with a 40° tilted silicon slide, the self-assembly time was significantly shortened to 10 min for a 1 × 2 cm2 area of PS monolayer. The series of strategies were successfully implemented not only on glass and silicon flat substrates but also on the side and end face of optical fiber, even on flexible substrates, while maintaining great morphologies. Besides, in association with nanospherical lens photolithography and nanosphere lithography technologies, various two-dimensional nanoarrays such as nanotriangles and nanoholes could be successfully obtained for subsequent optical properties research, which present broad application prospects in biochemical sensing, medicine, environmental protection, and other fields.

Enhanced visible light photocatalytic hydrogen evolution of Ta2O5 nanohoneycomb induced by plasmonic Au nanoparticle array

Polystyrene nanospheres in the sizes of 400, 200, and 100 nm were employed as a template to form hexagonal close-packed Ta2O5 nanohoneycombs (nHCs) by means of solution-based nanosphere lithography. Moreover, Au nanoparticle arrays with various diameters were further deposited on the Ta2O5nHCs to achieve surface plasmon resonance (SPR) and enhance visible-light photocatalytic hydrogen evolution. Finite difference time domain (FDTD) simulation was performed and the result showed that the 100-nm Ta2O5nHCs coupled with smaller Au nanoparticles exhibited more effective localized SPR effect to enclose the Ta2O5 photocatalyst. The photocatalytic hydrogen evolution results were consistent with those of FDTD simulation and photoelectrochemical tests. A concept of effective yield ratio is proposed to explain how the Au nanoparticle size and spacing affect the hydrogen evolution efficiency.

Development of controlled nanosphere lithography technology

This work is devoted to the development of nanosphere lithography (NSL) technology, which is a low-cost and efficient method to form nanostructures for nanoelectronics, as well as optoelectronic, plasmonic and photovoltaic applications. Creating a nanosphere mask by spin-coating is a promising, but not sufficiently studied method, requiring a large experimental base for different sizes of nanospheres. So, in this work, we investigated the influence of the technological parameters of NSL by spin-coating on the substrate coverage area by a monolayer of nanospheres with a diameter of 300 nm. It was found that the coverage area increases with decreasing spin speed and time, isopropyl and propylene glycol content, and with increasing the content of nanospheres in solution. Moreover, the process of controllably reducing the size of nanospheres in inductively coupled oxygen plasma was studied in detail. It was determined that increasing the oxygen flow rate from 9 to 15 sccm does not change the polystyrene etching rate, whereas changing the high-frequency power from 250 to 500 W increases the etching rate and allows us to control the decreasing diameter with high accuracy. Based on the experimental data, the optimal technological parameters of NSL were selected and the nanosphere mask on Si substrate was created with coverage area of 97.8% and process reproducibility of 98.6%. Subsequently reducing the nanosphere diameter lets us obtain nanoneedles of various sizes, which can be used in field emission cathodes. In this work, the reduction of nanosphere size, silicon etching, and removal of polystyrene residues occurred in unified continuous process of plasma etching without sample unloading to atmosphere.

Wafer-scale low-cost complementary vertically coupled plasmonic structure for surface-enhanced infrared absorption

A scalable nanofabrication approach for the fabrication of hexagonal packed complementary vertically coupled plasmonic (CVCP) structures for surface-enhanced infrared absorption (SEIRA) applications is reported in this study. This fabrication approach consists of nanosphere lithography, reactive ion etching, and metal deposition and can be implemented on a four-inch wafer scale with low cost and high uniformity. The CVCP structure contains dense plasmonic nanoparticles inside a three-dimensional cavity nanoantenna array. The localized surface plasmon resonance between the top-bottom metallic nanoantenna couple generates a solid localized electric field inside the cavity. A high simulated and experimental SEIRA electric-field enhancement was achieved by optimizing the height of the silicon nanopedestals. Using a representative monolayer octadecanethiol, an enhancement factor of 2.1 × 103 was demonstrated with an optimized structure geometry. When the detection area was extended to square millimeters, the limit of detection of monolayer octadecanethiol on the CVCP substrate reached 10 nM, which is four orders of magnitude lower than that on a flat gold substrate. This study demonstrates a feasible fabrication approach for producing three-dimensional nanoantenna structures with high enhancement factors in a scalable fashion, empowering the practical application of SEIRA substrates.

Tunable Nanoplasmonic Transducers: Performance Analysis and Potential Application

Plasmonic nanostructures with tunable optical properties can have many different applications, including high-sensitivity optical sensing for biological and chemical analyses in different field such as medical, environmental and food safety. The realization of an optimized sensing platform is closely related to the ability to finely control optical properties of nanostructures, which are, in turn, intimately linked to their geometrical and compositional characteristics. In this paper, an efficient and reproducible fabrication protocol, based on nanosphere lithography, for the realization of metal nanostructures with tunable plasmonic features is presented. In particular, the relationships between the geometric characteristics of different types of nanostructures with related optical phenomena such as enhanced absorption or extraordinary transmission are investigated in detail. These properties, together with electric field enhancement and confinement, are characterized and optimized in view of the employment of the fabricated nanostructures as optical transducers in nanoplasmonic chemosensor platforms working in the UV-VIS spectral range.

Pyramidal hyperbolic metasurfaces enhance spontaneous emission of nitrogen-vacancy centers in nanodiamond.

Nitrogen-vacancy (NV) centers in nanodiamond hold great promise for creating superior biological labels and quantum sensing methods. Yet, inefficient photon generation and extraction from excited NV centers restricts the achievable sensitivity and temporal resolution. Herein, we report an entirely complementary route featuring pyramidal hyperbolic metasurface to modify the spontaneous emission of NV centers. Fabricated using nanosphere lithography, the metasurface consists of alternatively stacked silica-silver thin films configured in a pyramidal fashion, and supports both spectrally broadband Purcell enhancement and spatially extended intense local fields owing to the hyperbolic dispersion and plasmonic coupling. The enhanced photophysical properties are manifested as a simultaneous amplification to the spontaneous decay rate and emission intensity of NV centers. We envision the reported pyramidal metasurface could serve as a versatile platform for creating chip-based ultrafast single-photon sources and spin-enhanced quantum biosensing strategies, as well as aiding in further fundamental understanding of photoexcited species in condensed phases.

Ordered non-close packed colloidal array with morphology control.

Non-close packed (NCP) colloidal arrays find wide applications in the fields of photonics, optical chip fabrication, nano sphere lithography and so on. However, unlike their close packed counterparts, such arrays cannot be obtained by direct self-organization of colloidal particles and require specialized techniques involving plasma/reactive ion etching, electric field driven assembly, substrate stretching or precise positioning of the particles. In this article, we present a facile template guided approach for fabricating ordered NCP arrays of colloidal particles. First, we employ soft lithography to replicate self-assembled hexagonal close packed (HCP) arrays of 'larger colloidal particles' (LPs) to obtain a topographically patterned positive and/or negative replica of the initial array. These replicas are then used as templates to spin coat 'smaller colloidal particles' (SPs), which may even have some degree of poly-dispersity, to obtain ordered NCP arrays. We further show that pattern morphology can be modulated based on whether a single or a double replicated template is used to confine the SPs, the concentration (Cn) of the SPs in the casting solution as well as the relative commensuration of the diameter of the SPs (ds) with that of the LPs (dL). Finally, we show that such NCP arrays can be transferred onto any flat surface by UVO mediated colloidal transfer printing.

Polarized coherent emission outside high-symmetry points of dye-coupled plasmonic lattices.

Developing intense, coherent and ultra-fast light sources with nanoscale dimensions is a crucial issue for many applications in nanophotonics. To date, plasmonic nanolasers represent one of the most promising nanophotonic devices capable of this remarkable feature. In the present work we report on the emission properties of two-dimensional Au hexagonal nanodome arrays, fabricated by nanosphere lithography, coupled with a dye liquid solution used as the gain medium. Low-threshold stimulated emission at room temperature is demonstrated by spectral and angle-resolved photoluminescence measurements performed as a function of the pump fluence. The emission arises with narrow angular divergence in off-normal direction, out of high-symmetry points of the plasmonic lattice. The polarization properties of the stimulated emission are investigated, revealing a strong linear polarization character controlled by the polarization orientation of the pumping beam, while the first-order temporal coherence properties are measured by using a tilted-mirrors Michelson interferometer. Finally, by comparing the results obtained for the plasmonic Au nanodomes arrays with those of purely dielectric nanoarrays, the role of the plasmonic modes and the photonic lattice modes in the emission process is highlighted.

Impact of thermal annealing and laser treatment on the morphology and optical responses of mono- and bi-metallic plasmonic honeycomb lattice.

Plasmonic nanoparticle arrays with a specific lattice arrangement can support surface lattice resonances (SLRs). SLR exhibits a sharp spectral peak and finds many applications including optical sensing and plasmonic lasers. To optimize SLR for application, a robust method that allows the mass production of plasmonic nanoparticle arrays with refined particle morphology and well-defined lattice arrangement is required. In this work, we combine nanosphere lithography (NSL) with thermal annealing or nanosecond-pulsed laser treatment to refine plasmonic nanoparticles in a honeycomb lattice. We comparatively study the effects of the two treatment methods on the particle morphology and lattice arrangement of mono (Ag and Pd) and bi-metallic (Ag-Pd) nanoparticle lattices. In general, thermal annealing preserves the lattice arrangement but fairly changes the particle roundness, while laser treatment produces particles with varying morphologies and spatial distribution. We also theoretically and experimentally investigate the optical responses of Ag nanoparticle lattices produced by different treatment methods. The observed difference in spectra can be attributed to the varying particle morphology, which shifts the localized surface plasmon resonance differently, resulting in a significant change in SLR. These findings provide valuable insights for optimizing plasmonic nanoparticle arrays for various applications.

Etching of TiO2 nanopillar arrays by nanosphere lithography for broadband infrared antireflection applications

Etching of TiO2 nanopillar arrays by nanosphere lithography for broadband infrared antireflection applications

Plasmonic Biosensors with Nanostructure for Healthcare Monitoring and Diseases Diagnosis

Nanophotonics has been widely utilized in enhanced molecularspectroscopy or mediated chemical reaction, which has major applications in the field of enhancing sensing and enables opportunities in developing healthcare monitoring. This review presents an updated overview of the recent exciting advances of plasmonic biosensors in the healthcare area. Manufacturing, enhancements and applications of plasmonic biosensors are discussed, with particular focus on nanolisted main preparation methods of various nanostructures, such as chemical synthesis, lithography, nanosphere lithography, nanoimprint lithography, etc., and describing their respective advances and challenges from practical applications of plasmon biosensors. Based on these sensing structures, different types of plasmonic biosensors are summarized regarding detecting cancer biomarkers, body fluid, temperature, gas and COVID-19. Last, the existing challenges and prospects of plasmonic biosensors combined with machine learning, mega data analysis and prediction are surveyed.

PtS2 metamaterials: Fabrication and physical properties

Metamaterials are electromagnetic materials with engineered sub-wavelength structures, which can be coupled with the incident electromagnetic wave to produced peculiar physical properties. Currently, many studies are being conducted to the transition metal dichalcogenides (TMDs) due to their unique structures and novel physical properties. Here, we produced honeycomb nanostructure with various sizes on few-layer PtS2 through a well-developed nanosphere lithography technology. In this way, the unique properties of PtS2 metamaterials can be tuned to explore more exotic physical characteristics. The honeycomb surface morphology of PtS2 metamaterials has been demonstrated using the scanning electron microscope and the atomic force microscope. The optical absorption spectra of Pt and PtS2 metamaterials were investigated and the results show that the absorption spectra can be modulated by changing the size of the nanostructure. In addition, the optical anisotropic properties of Pt and PtS2 metamaterials were characterized through the angle-resolved absorption spectra. Raman spectroscopy shows the surface-enhanced Raman spectroscopy (SERS) effect of Pt metamaterials. The simulation exhibits the electric field distribution of PtS2 metamaterials at different wavelengths. The results offer a new approach to develop novel optoelectronic devices based on the meta-structure of TMDs.