Showing papers on "Nanosphere lithography published in 2022"

Selective Area Epitaxy of GaN Nanowires on Si Substrates Using Microsphere Lithography: Experiment and Theory

Abstract: GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiOx/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature–Ga/N2 flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N2 flux ratios. A model was developed that explains the selective growth range, which appeared to be highly sensitive to the growth temperature and Ga flux, as well as to the radius and pitch of the patterned pinholes. High crystal quality in the GaN nanowires was confirmed through low-temperature photoluminescence measurements.

Plasmonic Metasurfaces for Specific SERS Detection of Shiga Toxins

Abstract: The interest in the development of nanoscale plasmonic technologies has dramatically increased in recent years. The photonic properties of plasmonic nanopatterns can be controlled and tuned via their size, shape, or the arrangement of their constituents. In this work, we propose a 2D hybrid metallic polymeric nanostructure based on the octupolar framework with enhanced sensing property. We analyze its plasmonic features both numerically and experimentally, demonstrating the higher values of their relevant figures of merit: we estimated a surface-enhanced Raman spectroscopy (SERS) enhancement factor of 9 × 107 and a SPR bulk sensitivity of 430 nm/RIU. In addition, our nanostructure exhibits a dual resonance in the visible and near-infrared region, enabling our system toward multispectral plasmonic analysis. Finally, we illustrate our design engineering strategy as enabled by electron beam lithography by the outstanding performance of a SERS-based biosensor that targets the Shiga toxin 2a, a clinically relevant bacterial toxin. To the best of our knowledge, this is the first time that a SERS fingerprint of this toxin has been evidenced.

Enhanced Electrochemiluminescence from 3D Nanocavity Electrode Arrays

Abstract: 3D arrays comprising interconnected voids have been created using nanosphere lithography (2000≤diameter≤240 nm) followed by electrodeposition of gold. The top surface was blocked with 11-mercaptoundecanoic acid and then the templating polystyrenes were dissolved using THF. The interior surfaces were then functionalized with DNA capture strands that are complementary for part of a target sequence associated with methicillin-resistant Staphylococcus aureus, MRSA, genomic DNA. Once the target is hybridised, a probe sequence that is functionalised with [Ru(bpy)2 PIC]2+, is hybridised to the overhang of the target, bpy is 2,2’-bipyridyl and PIC is (2,2′-bipyridyl)-2(4-carboxy phenyl) imidazo [4,5][1,10] phenanthroline. The electrochemiluminescence, ECL, intensity generated from the hybridised ruthenium dye in the presence of tri-propyl amine and normalised for the charge passed depends on the size of the nanocavity. Significantly, the normalised ECL intensity is approximately 7-fold higher for the 820 nm array than either the 2000 nm array or a planar electrode. The array that is functionalised with the capture-target-Ru probe can be used as a 3D bipolar electrode to wirelessly generate ECL. The peak shape and intensity of the wireless ECL spectra depend on the cavity size with the brightest ECL being observed for the 820 nm cavity array. The results are consistent with a fraction of the ECL light activating the broad cavity plasmon of the array and enhancing the overall ECL intensity. The 820 nm array has been used to develop an assay for the detection of MRSA DNA giving a linear dynamic range from 10 nM to 30 µM with an LOD of 1 µM and an LOQ of 3.2 µM. Significantly, the analytical sensitivity of the wireless ECL assay is approximately 7 times higher for the 820 nm array than that observed for a planar electrode.

Self-Assembled Metal Nanohole Arrays with Tunable Plasmonic Properties for SERS Single-Molecule Detection

Abstract: Arrays of metal nano-holes have proved to be among of the most promising structures for applications in the field of nano-photonics and optoelectronics. Supporting both localized and propagating surface plasmons resonances, they are characterized by very high versatility thanks to the tunability of these modes, by means of the change of their periodicity, the size of the holes and metal composition. The interaction between different optical features can be exploited to modulate electromagnetic field distribution leading various hot-spots excitations on the metal surfaces. In this work, long range ordered arrays of nano-holes in thin gold films, with different geometrical characteristics, were fabricated by a modified nano-sphere lithography protocol, which allows precise control on holes’ dimensions together with the preservation of the order and of the pristine periodicity of the array. An in-depth analysis of the correlation between surface plasmon modes interference and its effect on electromagnetic field distribution is proposed, both by numerical simulations and experimentally. Finally, metal nano-holes arrays are exploited for surface enhanced Raman experiments, evaluating and comparing their performances by the estimation of the enhancement factor. Values close to the single molecule detection are obtained for most of the samples, proving their potentialities in surface enhanced spectroscopy applications.

Periodic Nanohole Arrays with Enhanced Lasing and Spontaneous Emissions for Low-Cost Plasmonic Devices

Abstract: Periodic arrays of air nanoholes in thin metal films that support surface plasmon resonances can provide an alternative approach for boosting the light–matter interactions at the nanoscale. Nanohole arrays have garnered great interest in recent years for their use in biosensing, light emission enhancement, and spectroscopy. Here, we employ a simple technique to fabricate nanohole arrays and examine their photonic applications including enhanced lasing and spontaneous emission of novel nanomaterials. In contrast to the complicated and most commonly used electron-beam lithography technique, hexagonal arrays of nanoholes are fabricated by using a simple combination of shadowing nanosphere lithography technique and electron-beam deposition. Through spectral and temporal characterizations, it was shown that these arrays offer an enhancement in the lasing emission of an organic dye liquid gain medium with a quality factor above 150 as well as an accelerated decay rate for CdSe quantum dots. The simple fabrication of nanohole arrays together with their excellent optical responses can therefore offer a great potential in the industrialization of plasmonic devices for use in various realms of emerging technologies such as gas sensing, biomedical imaging, and ultrafast on-chip coherent light sources.

Graphene Oxide-Coated Metal–Insulator–Metal SERS Substrates for Trace Melamine Detection

Abstract: Surface-enhanced Raman spectroscopy (SERS) has long been an ultrasensitive technique for trace molecule detection. However, the development of a sensitive, stable, and reproducible SERS substrate is still a challenge for practical applications. Here, we demonstrate a cost-effective, centimeter-sized, and highly reproducible SERS substrate using the nanosphere lithography technique. It consists of a hexagonally packed Ag metasurface on a SiO2/Au/Si substrate. A seconds-lasting etching process of a self-assembled nanosphere mask manipulates the geometry of the deposited Ag metasurface on the SiO2/Au/Si substrate, which attains the wavelength matching between the optical absorbance of the Ag/SiO2/Au/Si substrate and the excitation laser wavelength as well as the enhancement of Raman signals. By spin-coating a thin layer of graphene oxide on the substrate, a SERS performance with 1.1 × 105 analytical enhancement factor and a limit of detection of 10−9 M for melamine is achieved. Experimental results reveal that our proposed strategy could provide a promising platform for SERS-based rapid trace detection in food safety control and environmental monitoring.

Magnetic nanoarrays on flexible substrates

Abstract: Abstract In this work, we used nanosphere lithography to fabricate large area 2-D magnetic nanoparticle (MNP) arrays on a flexible polyimide substrate (Kapton). Samples were fabricated by assembling polystyrene (PS) spheres on thin films of Co capped with Au. Etched PS spheres were used to mask Co–Au particle arrays. The MNP arrays were subjected to superconducting quantum interference device measurements; flat samples (10 nm Co coated with 10 nm Au) exhibited an M _s of 117.3 emu g^−1, which was lower than the reported literature value for bulk Co (162.7 emu g^−1). When compared to the flat film, coercivity, H _c, increased in a linear fashion with respect to particle size. These preliminary results reveal that future investigations of the magnetic properties on flexible substrates should account for residual Co remaining in the polymeric material, the unique MNP shape, the effect of order (or lack or order) of the 2D array, and positioning with respect to the direction of the magnetic field. Graphical Abstract

Magnetic nanoarrays on flexible substrates

Abstract: In this work, we used nanosphere lithography to fabricate large area 2-D magnetic nanoparticle (MNP) arrays on a flexible polyimide substrate (Kapton). Samples were fabricated by assembling polystyrene (PS) spheres on thin films of Co capped with Au. Etched PS spheres were used to mask Co–Au particle arrays. The MNP arrays were subjected to superconducting quantum interference device measurements; flat samples (10 nm Co coated with 10 nm Au) exhibited an Ms of 117.3 emu g−1, which was lower than the reported literature value for bulk Co (162.7 emu g−1). When compared to the flat film, coercivity, Hc, increased in a linear fashion with respect to particle size. These preliminary results reveal that future investigations of the magnetic properties on flexible substrates should account for residual Co remaining in the polymeric material, the unique MNP shape, the effect of order (or lack or order) of the 2D array, and positioning with respect to the direction of the magnetic field.

Shape Modulation of Plasmonic Nanostructures by Unconventional Lithographic Technique

Abstract: Conventional nano-sphere lithography techniques have been extended to the fabrication of highly periodic arrays of sub-wavelength nanoholes in a thin metal film. By combining the dry etching processes of self-assembled monolayers of polystyrene colloids with metal physical deposition, the complete transition from increasing size triangular nanoprism to hexagonally distributed nanoholes array onto thin metal film has been gradually explored. The investigated nano-structured materials exhibit interesting plasmonic properties which can be precisely modulated in a desired optical spectral region. An interesting approach based on optical absorbance measurements has been adopted for rapid and non-invasive inspections of the nano-sphere monolayer after the ion etching process. By enabling an indirect and accurate evaluation of colloid dimensions in a large area, this approach allows the low-cost and reproducible fabrication of plasmonic materials with specifically modulated optical properties suitable for many application in biosensing devices or Raman enhanced effects.

Large-Area Periodic Arrays of Atomically Flat Single-Crystal Gold Nanotriangles Formed Directly on Substrate Surfaces.

Abstract: The advancement of nanoenabled wafer-based devices requires the establishment of core competencies related to the deterministic positioning of nanometric building blocks over large areas. Within this realm, plasmonic single-crystal gold nanotriangles represent one of the most attractive nanoscale components but where the formation of addressable arrays at scale has heretofore proven impracticable. Herein, a benchtop process is presented for the formation of large-area periodic arrays of gold nanotriangles. The devised growth pathway sees the formation of an array of defect-laden seeds using lithographic and vapor-phase assembly processes followed by their placement in a growth solution promoting planar growth and threefold symmetric side-faceting. The nanotriangles formed in this high-yield synthesis distinguish themselves in that they are epitaxially aligned with the underlying substrate, grown to thicknesses that are not readily obtainable in colloidal syntheses, and present atomically flat pristine surfaces exhibiting gold atoms with a close-packed structure. As such, they express crisp and unambiguous plasmonic modes and form photoactive surfaces with highly tunable and readily modeled plasmon resonances. The devised methods, hence, advance the integration of single-crystal gold nanotriangles into device platforms and provide an overall fabrication strategy that is adaptable to other nanomaterials.

Transparent ultrathin Ag nanomesh electrode fabricated by nanosphere lithography for organic light-emitting devices

Abstract: We report a transparent ultrathin Ag nanomesh electrode fabricated by nanosphere lithography combined with a dielectric wetting layer, which is simple and cost-effective. Compared with the continuous ultrathin film Ag electrode, the nanomesh electrode exhibits a higher figure of merit (FOM = T10/Rs, T is the transmittance and Rs is the sheet resistance) of 3.01 × 10−3 Ω−1, which is due to the retained high electrical conductivity and nanopores as optical windows. The maximum luminance and current efficiency enhancement of organic light emitting devices (OLEDs) with an ultrathin Ag nanomesh electrode are up to 56.2% and 41.4% compared to that of conventional devices with an ITO anode. It is worth noting that the ultrathin Ag nanomesh electrode has been proved able to resolve the emission intensity roll-off of OLEDs with dual metal electrodes as the enlargement of viewing angle due to the redistribution of the light emission direction derived from strong scattering. More light emission at larger angles is of great value for lighting panel applications.

Large Area Patterning of Highly Reproducible and Sensitive SERS Sensors Based on 10-nm Annular Gap Arrays

Abstract: Applicable surface-enhanced Raman scattering (SERS) active substrates typically require low-cost patterning methodology, high reproducibility, and a high enhancement factor (EF) over a large area. However, the lack of reproducible, reliable fabrication for large area SERS substrates in a low-cost manner remains a challenge. Here, a patterning method based on nanosphere lithography and adhesion lithography is reported that allows massively parallel fabrication of 10-nm annular gap arrays on large areas. The arrays exhibit excellent reproducibility and high SERS performance, with an EF of up to 107. An effective wearable SERS contact lens for glucose detection is further demonstrated. The technique described here extends the range of SERS-active substrates that can be fabricated over large areas, and holds exciting potential for SERS-based chemical and biomedical detection.

Honeycomb-structured copper indium sulfide thin films obtained via a nanosphere colloidal lithography method

Abstract: Honeycomb structured copper indium sulfide layers are successfully realized via a nanosphere lithography route employing polystyrene nanosphere array templates and metal xanthates or a nanocrystal ink.

Large-Area Ordered Palladium Nanostructures by Colloidal Lithography for Hydrogen Sensing

Abstract: Reliable gas sensors are very important for hydrogen (H2) gas detection and storage. Detection methods based on palladium (Pd) metal are cost-effective and widely studied. When Pd is exposed to H2, it turns into palladium hydride with modified optical properties, which thus can be monitored for H2 sensing. Here, we fabricated large-area Pd nanostructures, including Pd nanotriangles and nanohole arrays, using colloidal lithography and systematically studied their H2-sensing performance. After hydrogen absorption, both the Pd nanoholes and nanotriangles showed clear transmittance changes in the visible–near infrared range, consistent with numerical simulation results. The influences of the structural parameters (period of the array P and diameter of the nanohole D) of the two structures are further studied, as different structural parameters can affect the hydrogen detection effect of the two structures. The nanohole arrays exhibited bigger transmittance changes than the nanotriangle arrays.

Facile fabrication of nanoapertures with tunable plasmonic resonances

Abstract: In the present work, we fabricate nano-apertures (NAs) in gold thin films using shadow sphere lithography exhibiting tunable plasmonic resonances. The control over the plasmon resonances is achieved by varying the conditions used for the shadow sphere lithography process. Two different sizes of masks have been used to realize apertures of different diameters. Furthermore, the effect of thickness on the optical response have been investigated. From single aperture scattering measurements, it is seen that on increasing the mask size, the resonance wavelength shifted towards higher wavelengths. In contrary, increasing the thickness of the gold thin films results in a blue shift of the resonance.

Surface-enhanced Raman Scattering of Au–Ag bimetallic nanopillars fabricated using surface-plasmon lithography

Abstract: Arrays of gold–silver (Au–Ag) bimetallic nanopillars were fabricated by a newly developed surface-plasmon lithography (SPL) and their enhancement properties as surface-enhanced Raman scattering (SERS) substrates have been studied. We demonstrated that the SPL is a low-cost and high efficiency method for the fabrication of SERS substrates with both high sensitivity and reproducibility. The nanopillars showed a good response in the detection of methylene blue molecules at a low concentration of 1.0 × 10–11 mol· l−1. The SERS enhancement factors (EFs) are on the orders of 107 and the relative standard deviation of SERS intensity is <8% over an area of 50 μm × 50 μm. The EFs increase fast with the height increasing from 200 to 530 nm, then increase slowly when further increase the height of the nanopillars to 1100 nm. In addition, the Au–Ag bimetallic coating has shown much higher SERS enhancement than the coatings of either the pure Au or Ag. The excellent SERS enhancement and reproducibility of the Au–Ag coated nanopillars indicated that the fabricated SERS substrates can be used for the detection of biochemical molecules at trace level and the SPL is a promising method for fabrication of SERS substrates.

Dense high aspect ratio nanostructures for cell chip applications - Fabrication, replication, and cell interactions

Abstract: Culturing human cells on the surface of a microchip brings living cells in direct contact with artificial microstructured surfaces. This work focuses on the effect of high aspect ratio nanostructures – dense nanoneedle arrays – on the mechanical response and proliferation of fibroblasts. We present a fabrication process for micropatterned chips that feature areas with hierarchical high aspect ratio nanostructures directly adjacent to flat chip areas. The chip was pre-patterned by conventional lithography. We have fabricated “black silicon” arrays of densely packed, sharp, vertical nano-needles for high aspect ratio structures by cryogenic reactive ion etching in an SF6/O2 plasma. An essential requirement for many real-life applications is to make such complex high aspect ratio 3D nanostructures available in larger areas. We have successfully demonstrated the 3D replication of black silicon by a UV- nanoimprint lithography process. This study provides insight into the extent to which such nanoneedle arrays influence the growth of human fibroblasts. We have investigated microstructured samples featuring a combination of (i) smooth surfaces and (ii) nanoneedle surfaces for the mechanical behavior and proliferation of fibroblasts. Our results show that the bonding viability and proliferation of the fibroblast on the high aspect ratio nano-needle surfaces differ significantly from flat surfaces. The nanoneedles only provide a minimum area for cell attachment compared to the neighboring flat, unstructured chip areas.

Fabrication of Mask by Drag Coating Convective Self-Assembly in Nanosphere Lithography for Nanohole Array

Abstract: To fabricate a nanohole array chip, a mask in nanosphere lithography was fabricated by drag-coating convective self-assembly using two glass slides. The relationship between the volume fraction of 500-nm-diameter polystyrene (PS) particles and the deposition rate under our experimental conditions was determined. Masks with a monolayer of PS particles were deposited on glass slides. Plasma etching of the PS particles was carried out to reduce their diameter. Au was sputtered over the mask, and the mask was then removed. Finally, a nanohole array was obtained using this simple procedure.

A square nano-array formed by surface aligned self-assembly of a bicontinuous lipid cubic phase

Abstract: We present the formation of oriented thin lipid films belonging to the primitive (Q_II^P) bicontinuous cubic phase, as characterized by grazing-incidence small angle X-ray scattering and atomic force microscopy. The phase shows high out-of-plane orientation with the (100) plane parallel to the substrate; this is consistent across a number of different lipid and surfactant systems. We demonstrate that the adopted orientation can be predicted using a theoretical framework, based on the most favorable facet in terms of curvature energy whilst assuming a closed bilayer. Significantly, the Primitive bicontinuous cubic phase is unique in that it contains straight water channels, with the [100] orientation presenting a square array at the surface with the straight channels running directly parallel and perpendicular to the interface. Moreover, by tuning the composition we show that we can tune the periodic pore-to-pore spacing (lattice parameter) of the structure with control over nanostructure dimensions. We present structures with lattice parameters between 13 and 23 nm. These structural features are important for applications including delivery of bioactive agents, and patterning and templating of inorganic nanomaterials. These findings represent a route to square arrays of nanofeatures with 2 3 times smaller periodicity (4-9 times higher per-area density) than those reported from block copolymer lithographic template systems.

Structure and Optical Property Prediction of 2D Plasmonic Photonic Crystals Fabricated by Shadow Sphere Lithography

Abstract: Shadow sphere lithography (SSL) is a powerful and large-scale fabrication method to produce two-dimensional (2D) plasmonic photonic crystals and three-dimensional metamaterials. Practically, one of the biggest challenges for SSL-based fabrications is that it is hard to accurately predict the physical properties of the fabricated nanostructures if the structures were only modeled by the geometric shadowing effect. A Monte Carlo (MC) simulation is developed to show that the dynamic shadowing effect due to the accumulation of materials on the template as well as the thin-film growth mechanism plays a key role in determining the structure details. For a one-to-three step-based SSL fabrication, the nanostructures predicted by MC match very well with those produced experimentally, and the plasmonic properties predicted by these MC-simulated structures are also consistent with the features obtained experimentally, both qualitative and semi-quantitative. This study indicates a possible solution to use MC simulation and numerical calculation to guide the design of the plasmonic photonic crystals and metamaterials based on SSL for optic applications.

Combining Azimuthal and Polar Angle Resolved Shadow Mask Deposition and Nanosphere Lithography to Uncover Unique Nano-Crystals

Abstract: In this article, we present a systematic investigation on a multistep nanosphere lithography technique to uncover its potential in fabricating a wide range of two- and three-dimensional nanostructures. A tilted (polar angle) electron beam shower on a nanosphere mask results in an angled shadow mask deposition. The shape of the shadow also depends on the azimuthal angle of the mask sitting on top of the substrate. We performed angled shadow mask depositions with systematic variation of these two angular parameters, giving rise to complex nanostructures (down to 50 nm), repeated over a large area without defect. In this article, nanosphere lithography with two- and four-fold azimuthal symmetry was studied at constant tilt angles followed by variations in tilt without azimuthal rotation of the substrate. Finally, both angular parameters were simultaneously varied. The structure of shadow crystals was explained using Matlab simulation. This work stretches the horizons of nanosphere lithography, opening up new scopes in plasmonic and magnonic research.

Light Extraction Efficiency Enhancement in GaN ‐Based LEDs by Colloidal Self‐assembly

Abstract: Various micro- and nanoscale structures have been incorporated into GaN-based light-emitting diodes (LEDs) by diverse techniques to improve the efficiency. Nanosphere lithography (NSL) has proved to be a low-cost and high mass production technique to make features in nanoscale. In this chapter, we review the recent advances of efficiency improvement of GaN-based LEDs by using NSL-derived nanostructures. We introduce the representative fabrication process of nanostructure by NSL method. We provide an overview of the current application of nanostructure in the efficiency enhancement of GaN-based LEDs. A new application of highly ordered nanostructures on optoelectronic devices including LEDs and lasers is also presented. Finally, a perspective on the future development of GaN-based LEDs and other optoelectronic devices by using NSL is addressed.