Nanohole Diameter Research Articles

Visible Light-Illuminated Gold Nanohole Arrays With Tunable On-Chip Plasmonic Sensing Properties

Since the discovery of the extraordinary optical transmission phenomenon, nanohole arrays have attracted much attention and been widely applied in sensing. However, their typical fabrication process, utilizing photolithographic top-down manufacturing technologies, has intrinsic drawbacks including the high costs, time consumption, small footprint, and low throughput. This study presented a low-cost, high-throughput, and scalable method for fabricating centimeter-scale (1×2 cm2) nanohole arrays using the improved nanosphere lithography. The large-scale close-packed polystyrene monolayers obtained by the hemispherical-depression-assisted self-assembly method were employed as colloidal masks for the nanosphere lithography, and the nanohole diameter was tuned from 233 nm to 346 nm with a fixed period of 420 nm via plasma etching. The optical properties and sensing performance of the nanohole arrays were investigated, and two transmission dips were observed due to the resonant coupling of plasmonic modes. Both dips were found to be sensitive to the surrounding environment, and the maximum bulk refractive index sensitivity was up to 162.1 nm/RIU with a 233 nm hole diameter. This study offered a promising approach for fabricating large-scale highly ordered nanohole arrays with various periods and nanohole diameters that could be used for the development of low-cost and high-throughput on-chip plasmonic sensors.

GdFe-based nanostructured thin films with large perpendicular magnetic anisotropy for spintronic applications

In this study, we investigated the impact of geometric factors on the magnetic anisotropy of Gd-Fe alloy thin films deposited on nanoporous alumina membranes. By synthesizing Gd-Fe alloy nanostructure thin films with different hole diameters (ranging from 45 to 90 nm) and keeping the layer thickness and lattice parameters fixed at 45 nm and 105 nm, respectively, we observed a significant perpendicular magnetic anisotropy (PMA) in samples with hole diameter above 65 nm. The transition from in-plane to out-of-plane magnetization in Gd-Fe alloy nanostructure thin films occurred at a critical antidot hole diameter of 75 nm. The observed variations in coercivity and remanence with the nanohole diameter are attributed to substantial changes in the magnetization mechanisms induced by the nanoholes. This novel induction of PMA in Gd-Fe alloy nanostructure thin films through the manipulation of geometric parameters in the antidot arrays opens new possibilities for tailoring the magnetic behavior of ferromagnetic metals with pronounced PMA.

Plasmon-Enhanced Fluorescence of Single Quantum Dots Immobilized in Optically Coupled Aluminum Nanoholes.

Fluorescence-based optical sensing techniques have continually been explored for single-molecule detection targeting myriad biomedical applications. Improving signal-to-noise ratio remains a prioritized effort to enable unambiguous detection at single-molecule level. Here, we report a systematic simulation-assisted optimization of plasmon-enhanced fluorescence of single quantum dots based on nanohole arrays in ultrathin aluminum films. The simulation is first calibrated by referring to the measured transmittance in nanohole arrays and subsequently used for guiding their design. With an optimized combination of nanohole diameter and depth, the variation of the square of simulated average volumetric electric field enhancement agrees excellently with that of experimental photoluminescence enhancement over a large range of nanohole periods. A maximum 5-fold photoluminescence enhancement is statistically achieved experimentally for the single quantum dots immobilized at the bottom of simulation-optimized nanoholes in comparison to those cast-deposited on bare glass substrate. Hence, boosting photoluminescence with optimized nanohole arrays holds promises for single-fluorophore-based biosensing.

Enhancement of Exchange Bias and Perpendicular Magnetic Anisotropy in CoO/Co Multilayer Thin Films by Tuning the Alumina Template Nanohole Size.

The interest in magnetic nanostructures exhibiting perpendicular magnetic anisotropy and exchange bias (EB) effect has increased in recent years owing to their applications in a new generation of spintronic devices that combine several functionalities. We present a nanofabrication process used to induce a significant out-of-plane component of the magnetic easy axis and EB. In this study, 30 nm thick CoO/Co multilayers were deposited on nanostructured alumina templates with a broad range of pore diameters, 34 nm ≤ Dp ≤ 96 nm, maintaining the hexagonal lattice parameter at 107 nm. Increase of the exchange bias field (HEB) and the coercivity (HC) (12 times and 27 times, respectively) was observed in the nanostructured films compared to the non-patterned film. The marked dependence of HEB and HC with antidot hole diameters pinpoints an in-plane to out-of-plane changeover of the magnetic anisotropy at a nanohole diameter of ∼75 nm. Micromagnetic simulation shows the existence of antiferromagnetic layers that generate an exceptional magnetic configuration around the holes, named as antivortex-state. This configuration induces extra high-energy superdomain walls for edge-to-edge distance >27 nm and high-energy stripe magnetic domains below 27 nm, which could play an important role in the change of the magnetic easy axis towards the perpendicular direction.

Simulation and Experimental Studies of Size Effects on Plasmonic Gas Sensing with Nanohole Arrays

Simulation and Experimental Studies of Size Effects on Plasmonic Gas Sensing with Nanohole Arrays

Double-Langmuir model for optimized nanohole array-based plasmonic biosensors

The sensing mechanism of plasmonic nanohole arrays is investigated and a novel model is proposed to interpret their optical response over a wide dynamic range of concentrations (10-13-10-5 M), based on a double-Langmuir model. This model describes the signal response of the analyte binding as the sum of two independent contributions which are related to two different surface regions of the biosensor, namely the top gold surface of the nanohole array and the lateral gold area inside the nanoholes. Numerical simulations highlight the different near-field behaviour of these two regions and their very different refractive index sensitivities, which both support the double-Langmuir model. This is corroborated by experimental biosensing measurements with gold nanohole arrays with hexagonal symmetry, synthesized by nanosphere lithography. Their sensing performances are optimized by numerical simulations by changing their geometrical parameters (i.e., lattice constant, nanohole diameter and height) in order to achieve a maximum sensitivity. For the biosensing experiments, the biotin-streptavidin complex is used as a benchmark test for the optimized nanohole array and a robust calibration is provided by the double-Langmuir model obtaining a limit of detection of 0.3 ng/mL, which corresponds to an absolute analyte quantity of 0.02 fmol.

Characterization of Bulk Nanobubbles Formed by Using a Porous Alumina Film with Ordered Nanopores.

Nanobubbles (NBs), with their unique physicochemical properties and promising applications, have become an important research topic. Generation of monodispersed bulk NBs with specified gas content remains a challenge. We developed a simple method for generating bulk NBs, using porous alumina films with ordered straight nanoscaled holes. Different techniques, such as nanoparticle tracking analysis (NTA), atomic force microscopy (AFM), and infrared absorption spectroscopy (IRAS), are used to confirm NB formation. The NTA data demonstrate that the minimum size of the NBs formed is less than 100 nm, which is comparable to the diameter of nanoholes in the porous alumina film. By generating NBs with different gases, including CO2, O2, N2, Ar, and He, we discovered that the minimum size of NBs negatively correlated with the solubility of encapsulated gases in water. Due to the monodispersed size of NBs generated from the highly ordered porous alumina, we determined that NB size is distributed discretely with a uniform increment factor of [Formula: see text]. To explain the observed characteristic size distribution of NBs, we propose a simple model in which two NBs of the same size are assumed to preferentially coalesce. This characteristic bubble size distribution is useful for elucidating the basic characteristics of nanobubbles, such as the long-term stability of NBs. This distribution can also be used to develop new applications of NBs, for example, nanoscaled reaction fields through bubble coalescence.

Simulated and Experimental Studies of Size Effects on Plasmonic Gas Sensing with Nanohole Arrays

Simulated and Experimental Studies of Size Effects on Plasmonic Gas Sensing with Nanohole Arrays

Influence of the reactor environment on the selective area thermal etching of GaN nanohole arrays

Selective area thermal etching (SATE) of gallium nitride is a simple subtractive process for creating novel device architectures and improving the structural and optical quality of III-nitride-based devices. In contrast to plasma etching, it allows, for example, the creation of enclosed features with extremely high aspect ratios without introducing ion-related etch damage. We report how SATE can create uniform and organized GaN nanohole arrays from c-plane and (11–22) semi-polar GaN in a conventional MOVPE reactor. The morphology, etching anisotropy and etch depth of the nanoholes were investigated by scanning electron microscopy for a broad range of etching parameters, including the temperature, the pressure, the NH3 flow rate and the carrier gas mixture. The supply of NH3 during SATE plays a crucial role in obtaining a highly anisotropic thermal etching process with the formation of hexagonal non-polar-faceted nanoholes. Changing other parameters affects the formation, or not, of non-polar sidewalls, the uniformity of the nanohole diameter, and the etch rate, which reaches 6 µm per hour. Finally, the paper discusses the SATE mechanism within a MOVPE environment, which can be applied to other mask configurations, such as dots, rings or lines, along with other crystallographic orientations.

Highly Compact and Efficient Four-Mode Multiplexer Based on Pixelated Waveguides

An ultra-compact mode multiplexer for TE0, TE1, TE2 and TE3 modes is proposed and demonstrated experimentally. The device is realized via pixelated waveguides with a footprint of only $5.4\times 6~\mu \,\,\text{m}^{2}$ . The measured insertion loss of the multiplexer is less than 1.5 dB. The inter-mode crosstalk is ~−20 dB at 1550 nm and below −14.6 dB from 1510 nm – 1570 nm. The fabrication and temperature tolerance of the device are investigated. The device can well tolerate the nanohole diameter variation of ±10 nm and the temperature variation of 100 K. The compactness of the proposed mode multiplexer enables potential applications in densely integrated multimode photonic circuits.

MEA Recordings and Cell-Substrate Investigations with Plasmonic and Transparent, Tunable Holey Gold.

Microelectrode arrays are widely used in different fields such as neurobiology or biomedicine to read out electrical signals from cells or biomolecules. One way to improve microelectrode applications is the development of novel electrode materials with enhanced or additional functionality. In this study, we fabricated macroelectrodes and microelectrode arrays containing gold penetrated by nanohole arrays as a conductive layer. We used this holey gold to optically excite surface plasmon polaritons, which lead to a strong increase in transparency, an effect that is further enhanced by the plasmon's interaction with cell culture medium. By varying the nanohole diameter in finite-difference time domain simulations, we demonstrate that the transmission can be increased to above 70% with its peak at a wavelength depending on the holey gold's lattice constant. Further, we demonstrate that the novel transparent microelectrode arrays are as suitable for recording cellular electrical activity as standard devices. Moreover, we prove using spectral measurements and finite-difference time domain simulations that plasmonically induced transmission peaks of holey gold red-shift upon sensing medium or cells in close vicinity (<30 nm) to the substrate. Thus, we establish plasmonic and transparent holey gold as a tunable material suitable for cellular electrical recordings and biosensing applications.

Finite-difference time-domain analysis of local electric field enhancement by nanoholes of varying shape and diameter in metal with different thicknesses

This work reports changes in surface plasmons according to the shape and diameter of nanoholes in thin metal films of varying thickness. Cylindrical and reversed trapezoidal nanoholes were simulated using the finite-difference time-domain (FDTD) method. Nanoholes of 10, 50, and 100 nm diameters were modelled in 40-, 60-, and 80-nm-thick metal films. The strongest local electric field enhancement occurred with a reversed trapezoidal hole of 10 nm size in a 100-nm-thick metal film. The resonance wavelength was red shifted by decreasing the hole size and/or increasing the metal film thickness.

Design of an Aluminum/Polymer Plasmonic 2D Crystal for Label-Free Optical Biosensing.

A design study of a nanostructured two-dimensional plasmonic crystal based on aluminum and polymeric material for label-free optical biosensing is presented. The structure is formed of Al nanohole and nanodisk array layers physically separated by a polymeric film. The photonic configuration was analyzed through finite-difference time-domain (FDTD) simulations. The calculated spectral reflectance of the device exhibits a surface plasmon polariton (SPP) resonance feature sensitive to the presence of a modeled biolayer adhered onto the metal surfaces. Simulations also reveal that the Al disks suppress an undesired SPP resonance, improving the device performance in terms of resolution as compared to that of a similar configuration without Al disks. On the basis of manufacturability issues, nanohole diameter and depth were considered as design parameters, and a multi-objective optimization process was employed to determine the optimum dimensional values from both performance and fabrication points of view. The effect of Al oxidation, which is expected to occur in an actual device, was also studied.

Fabrication of highly sensitive QCM sensor using AAO nanoholes and its application in biosensing

To detect biomolecules with low molecular weight by quartz crystal microbalance (QCM) method, it is necessary to improve its sensitivity. Here, we have investigated the performance of a QCM based biosensor coated with anodic aluminum oxide (AAO). AAO nanostructures were fabricated on a quartz crystal substrate by anodizing thin aluminum film in an oxalic acid solution. We obtain AAO nanostructures with nanohole diameter ranging from 40 to 80 nm by changing anodic conditions, where the pitch of the nanohole was maintained about 100 nm. The AAO thicknesses ranged from 200 to 1000 nm by changing the deposition time of the Al thin film. We demonstrated that the QCM-based sensor with an AAO nanostructure increased sensitivity with the surface area due to AAO nanostructure on the measuring an antigen-antibody interaction. Additionally, the calibration curve corresponding to the antigen-antibody interaction showed that the detection limits of the AAO-nanostructured and Au-thin-film QCM-based sensors were 0.20 and 0.48μg/mL, respectively.

On the Adsorption of DNA Origami Nanostructures in Nanohole Arrays.

DNA origami nanostructures are versatile substrates for the controlled arrangement of molecular capture sites with nanometer precision and thus have many promising applications in single-molecule bioanalysis. Here, we investigate the adsorption of DNA origami nanostructures in nanohole arrays which represent an important class of biosensors and may benefit from the incorporation of DNA origami-based molecular probes. Nanoholes with well-defined diameter that enable the adsorption of single DNA origami triangles are fabricated in Au films on Si wafers by nanosphere lithography. The efficiency of directed DNA origami adsorption on the exposed SiO2 areas at the bottoms of the nanoholes is evaluated in dependence of various parameters, i.e., Mg2+ and DNA origami concentrations, buffer strength, adsorption time, and nanohole diameter. We observe that the buffer strength has a surprisingly strong effect on DNA origami adsorption in the nanoholes and that multiple DNA origami triangles with 120 nm edge length can adsorb in nanoholes as small as 120 nm in diameter. We attribute the latter observation to the low lateral mobility of once adsorbed DNA origami on the SiO2 surface, in combination with parasitic adsorption to the Au film. Although parasitic adsorption can be suppressed by modifying the Au film with a hydrophobic self-assembled monolayer, the limited surface mobility of the adsorbed DNA origami still leads to poor localization accuracy in the nanoholes and results in many DNA origami crossing the boundary to the Au film even under optimized conditions. We discuss possible ways to minimize this effect by varying the composition of the adsorption buffer, employing different fabrication conditions, or using other substrate materials for nanohole array fabrication.

Design optimization and antireflection of silicon nanowire arrays fabricated by Au-assisted chemical etching

In this work, we have synthesized a highly periodic silicon nanowire (SiNW) arrays with the spacing from 300 nm to 1000 nm by combining nanosphere lithography (NSL) and metal-assisted chemical etching (MACE). It has been demonstrated that it is effective to produce large-area, well-separated and controlled SiNW arrays. We also present a simple method to solve the problem of distorting spherical shape by angled-deposition method according to shadowing effect during NSL process. So we do not need to reduce the diameter of polystyrene (PS) spheres heavily and avoid making spherical shape distorted and rough. The large-area Au meshes can be further optimized using angled-deposition method and have a small nanohole diameter and relatively large nanohole spacings. And the center-center distance of PSs used as mask merely keeps invariable with 820 nm and 1300 nm pitch after reactive ion etching (RIE) process. The method help improve the distortion of spherical shape heavily during NSL process so that we can obtain SiNW arrays with controlled diameter, length and density using the non-close-packed PSs as the mask. Furthermore, the SiNW arrays obtained by MACE can improve antireflection properties of silicon substrate. The high surface area of SiNW arrays makes the reflectance below 10% or less over a wavelength range of 300–1000 nm. The antireflection structure (ARS) is great of importance in application of nanoscale optical devices.

Surface-enhanced Raman scattering active gold nanoparticle/nanohole arrays fabricated through electron beam lithography

Effective surface-enhanced Raman scattering (SERS)-active substrates from gold nanoparticle and gold nanohole arrays were successfully fabricated through electron beam lithography with precise computer-aided control of the unit size and intergap distance. Their SERS performance was evaluated using 4-mercaptobenzoic acid (4-MBA). These gold arrays yielded strong SERS signals under 785 nm laser excitation. The enhancement factors for 4-MBA molecules on the prepared gold nanoparticle and nanohole arrays maxed at 1.08 × 107 and 8.61 × 106, respectively. The observed increase in SERS enhancement was attributed to the localized surface plasmon resonance (LSPR) wavelength shifting toward the near-infrared regime when the gold nanohole diameter increased, in agreement with the theoretical prediction in this study. The contribution of LSPR to the Raman enhancement from nanohole arrays deposited on fluorine-doped tin oxide glass was elucidated by comparing SERS and transmission spectra. This simple fabrication procedure, which entails employing electron beam lithography and the controllability of the intergap distance, suggests highly promising uses of nanohole arrays as functional components in sensing and photonic devices.

High aspect ratio nanoholes in glass generated by femtosecond laser pulses with picosecond intervals

Because of its potential uses, high aspect ratio nanostructures have been interested for last few decades. In order to generate nanostructures, various techniques have been attempted. Femtosecond laser ablation is one of techniques for generating nanostructures inside a transparent material. For generating nanostructures by femtosecond laser ablation, previous studies have been attempted beam shaping such as Bessel beam and temporal tailored beam. Both methods suppress electron excitation at near surface and initiate interference of photons at certain depth. Recent researches indicate that shape of nanostructures is related with temporal change of electron density and number of self-trapped excitons. In this study, we try to use the temporal change of electron density induced by femtosecond laser pulse for generating high aspect ratio nanoholes. In order to reveal the effect of temporal change of electron density, secondary pulses are irradiated from 100 to 1000 ps after the irradiation of first pulse. Our result shows that diameter of nanoholes is increasing and depth of nanoholes is decreasing as pulse to pulse interval is getting longer. With manipulating of pulse to pulse interval, we could generate high aspect ratio nanoholes with diameter of 250–350 nm and depth of 4∼6 µm inside a glass.

Fabrication and characterization of nano porous lattice biosensor using anodic aluminum oxide substrate

Localized surface plasmon resonance (LSPR) attracts attention for the fabrication of a biosensor because it can be used easily and is highly sensitive to the change in surface reflective index. Anodic aluminum oxide (AAO) has self-assembled nanoholes and is fabricated easily all over the Al substrate. The depth and diameter of the nanoholes are easily controlled by changing the etching solution and applied voltage. By depositing a Au thin film on the AAO substrate, the sensitivity of the sensor chip was affected by both optical interference and LSPR. In this study, the optical property of the AAO-based LSPR sensor chip was characterized on the basis of the nanostructure. We presented a biosensing application of the AAO-based LSPR sensor chip. The highest sensitivity of the sensor chip was observed at the AAO nanohole diameter of 75 nm and the AAO nanohole depth of 0.5 µm, when bovine serum albumin (BSA) adsorbed on the sensor chip.

Multiple surface plasmon resonances of square lattice nanohole arrays in Au-SiO2-Au multilayer films

The optical properties and the local electromagnetic field enhancement of a multilayer structure with square lattice nanohole arrays in Au-SiO2-Au multilayer films are numerically studied using finite-difference time domain method. Simulation results demonstrate that the multiple surface plasmons (SP) resonances consist of SP on the air/Au interface, SiO2/Au interfaces (the middle layers), Au/SiO2 interface (the lower layer) and coupling modes on the Au film and Au film. We investigated some of structure parameters that influence the SP resonances of the multilayer nanostructure. Adjustment of the thickness of SiO2 film (H2), the diameter (R) of circular nanoholes, the periods (C) of square lattice and the thickness of Au film (H1) could change the absorption intensity and the SP resonances. The simulation of the electromagnetic field distributions shows that the location of the local electromagnetic field enhancement can specify the different SP resonances patterns. Dipole, quadrupole, and twelve-pole SP resonances modes can be found in the multilayer nanostructure. These studies are important for applications using multiple SP resonances.