Metal Nanostructure Arrays Research Articles

Near-field enhancement of light by higher-order multipole excitations in a metal nanodisc trimer

Near-field enhancement of light by dipole excitations in plasmonic nanoparticles plays an important role in many applications of optical nanotechnology, including solar cells, plasmonic sensors, and nonlinear optical devices. Recently, we have shown that a seemingly weak octupole resonance in a pair of metal nanospheres can provide a higher near-field enhancement than the dipole resonance. Being motivated by this discovery, we now design a plasmonic nanodisc trimer that supports hybridized higher-order excitations and simultaneously suppresses the dipole excitation. We show that, under these conditions, the near-field enhancement can reach a high level, exceeding the value achievable with a corresponding dimer structure. The interference of the electric currents belonging to different multipole moments is found to play an important role in the enhancement. We believe that arrays of similar metal nanostructures can be designed to enhance optical fields via higher-order resonances for many applications, e.g., in nonlinear optics and optical sensing based on surface-enhanced fluorescence or Raman scattering.

Inverse design of ultranarrow and high-efficiency color filters based on tandem convolutional neural networks

In this paper, an ultranarrow, high-efficiency color filter based on a two-dimensional grating nanostructure array is realized by a tandem inverse design network. The tandem inverse designed network is generated by connecting an inverse network with a pre-trained forward network. The tandem convolutional neural network successfully overcame the non-uniqueness problem of the training data. The performance of the networks is verified by a test set of 2592 data points, and the average MSE of the prediction results of the tandem network is 1.01 × 10−3. Extremely low error indicates that the tandem network can learn and capture the distribution characteristics of the transmission spectrum and give a reasonable design of the structural parameters. Finally, we obtained multiple inverse-designed transmissive color filters with ultranarrow linewidths and high efficiency, with central wavelengths between 480 nm and 630 nm. The linewidth and efficiency of each kind of color filter are less than 10.6 nm and higher than 60%. Narrow linewidths and wide wavelength tunability result in high color accuracy and a broad color gamut of the filters. The filters with two-dimensional metallic nanostructure array also show polarization-independent characteristics. Compared with the traditional method, this method can quickly and accurately design the structural parameters corresponding to the spectral response, which provides a new method for the nanostructure design.

Fabrication of silver nanostructure array patterns (SNAPs) on silicon wafer for highly sensitive and reliable SERS substrates

Metal nanostructure arrays with large amounts of nano-gaps are important for surface enhanced Raman scattering applications, though the fabrications of such nanostructures are difficult due to the complex and multiple synthetic steps. In this research, we report silver nanostructure array patterns (SNAPs) on silicon wafer, which is fabricated with semiconductor manufacturing technology, Cu2O electrochemistry deposition, and Ag In-situ oxidation–reduction growth. Benefiting from the dense and uniform distribution of Ag nanowires, the fabricated SNAPs demonstrate a very strong and uniform surface-enhanced Raman scattering (SERS) effect. The efficiency of SNAPs was investigated by using rhodamine 6G (R6G) dye as an analyte molecule. The results show that the minimum detectable concentration of R6G can reach as low as 10−11 M, and the Raman signals in the random region show good signal homogeneity with a low relative standard deviation (RSD) of 4.77 %. These results indicate that the SNAPs perform a great sensitivity and uniformity as a SERS substrate. Furthermore, we used the SNAPs substrate to detect antibiotic sulfadiazine. The main peaks in sulfadiazine Raman and vibration modes assignments were obtained and the quantitative analysis model was established by principal component analysis (PCA). The detection and application results of sulfadiazine indicate that the SNAPs substrate can be applied for trace detection of antibiotics. In addition, we have cited the application of the SNAPs substrate in anti-counterfeiting labels. These practical applications demonstrate that the fabricated SNAPs can potentially provide a way to develop low-cost SERS platforms for environmental detections, biomedicine analysis, and commodities anti-counterfeiting.

Normal Incidence Excitation of Out-of-Plane Lattice Resonances in Bipartite Arrays of Metallic Nanostructures.

As a result of their coherent interaction, two-dimensional periodic arrays of metallic nanostructures support collective modes commonly known as lattice resonances. Among them, out-of-plane lattice resonances, for which the nanostructures are polarized in the direction perpendicular to the array, are particularly interesting since their unique configuration minimizes radiative losses. Consequently, these modes present extremely high quality factors and field enhancements that make them ideal for a wide range of applications. However, for the same reasons, their excitation is very challenging and has only been achieved at oblique incidence, which adds a layer of complexity to experiments and poses some limitations on their usage. Here, we present an approach to excite out-of-plane lattice resonances in bipartite arrays under normal incidence. Our method is based on exploiting the electric-magnetic coupling between the nanostructures, which has been traditionally neglected in the characterization of arrays made of metallic nanostructures. Using a rigorous coupled dipole model, we demonstrate that this coupling provides a general mechanism to excite out-of-plane lattice resonances under normal incidence conditions. We complete our study with a comprehensive analysis of a potential implementation of our results using an array of nanodisks with the inclusion of a substrate and a coating. This work provides an efficient approach for the excitation of out-of-plane lattice resonances at normal incidence, thus paving the way for the leverage of the extraordinary properties of these optical modes in a wide range of applications.

Morphological investigation and 3D simulation of plasmonic nanostructures to improve the efficiency of perovskite solar cells

The light absorption process is a key factor in improving the performance of perovskite solar cells (PSCs). Using arrays of metal nanostructures on semiconductors such as perovskite (CH3NH3PbI3), the amount of light absorption in these layers is significantly increased. Metal nanostructures have been considered for their ability to excite plasmons (collective oscillations of free electrons). Noble metal nanoparticles placed inside solar cells, by increasing the scattering of the incident light, effectively increase the optical absorption inside PSCs; this in turn increases the electric current generated in the photovoltaic device. In this work, by calculating the cross-sectional area of dispersion and absorption on gold (Au) nanoparticles, the effects of the position of nanoparticles in the active layer (AL) and their morphology on the increase of absorption within the PSC are investigated. The optimal position of the plasmonic nanoparticle was obtained in the middle of the AL using a three-dimensional simulation method. Then, three different morphologies of nano-sphere, nano-star and nano-cubes were investigated, where the short-circuit currents (Jsc) for these three nanostructures were obtained equal to 19.01, 18.66 and 20.03 mA/cm2, respectively. In our study, the best morphology of the nanostructure according to the Jsc value was related to the nano-cube, in which the device power conversion efficiency was equal to 16.20%, which is about 15% better than the PSC with the planar architecture.

Enhanced THz Radiation from Spatially Inhomogeneous Fields

Nonlinear terahertz (THz) radiation from gas media usually relies on the asymmetric laser-induced current produced by ultra-intense two-color laser fields with a specific phase delay. Here a new scheme is proposed and theoretically investigated, in which the radiation is generated by spatially inhomogeneous fields induced by relatively low-intensity monochromatic lasers and an array of single triangular metallic nanostructures. Our simulations are based on the classical photocurrent model and the time-dependent Schrödinger equations separately. It is found that the collective motion of the ionized electrons can be efficiently controlled by the inhomogeneous field, resulting in strong residual currents. The intensity of the THz radiation could be enhanced by about two orders of magnitude by increasing the spatial inhomogeneity of the field.

Fine Tuning of Plasmonic Metal Array Structures for Effective Light Manipulation

Improvements in light manipulation is the key for effective photoenergy conversions. Herein, we propose a method to improve the quality factor of metal nanostructure array using a relatively low temperature annealing. The method leads to the change in the extinction spectra of Au nanoprisms showing a narrow full width at half-maximum while maintaining structural anisotropy. X-ray diffraction and electrochemical measurements proved the improvement of the crystallinity of the Au nanoprisms as the origin of relatively high quality factor. Moreover, the method was also applied to the well-defined Au lattice structure prepared using electron-beam lithography, which resulted in a 5-fold improvement of the quality factors in the surface lattice resonance mode. We applied these structures to construct strong coupling systems with dye excitons. Ultra-strong coupling states was achieved by the optimization of the lattice modes to the energy of the excitons. The present systems could be applied to create controlled light-matter hybridized materials.

Effect of Ar-plasma treatment and annealing on thermally evaporated β-In2S3 thin films

In the present study, the effect of annealing and Ar-plasma treatment on structural, morphological and optical properties of thermally evaporated β-In2S3 thin films has been investigated. During Ar-plasma treatment, some interesting results were observed that an array of metallic indium nanostructures was formed over In2S3 film surface with quasi-spherical or spread droplet shapes of an average size of 20–100 nm in the lateral direction and a height of less than 70 nm. Here, the Ar-plasma treatment serves as a new strategy for the self-formation of metallic indium nanostructures over the film surface. Further, the optical absorption of In2S3 films has been enhanced from 104 to 107 cm−1 while the optical band gap energy decreased from 2.71 eV to 2.50 eV after Ar-plasma treatment. The metallic nanostructures loaded on semiconductor surface can act as an electron trap that can effectively prevent the recombination of photo-generated electron-hole pairs.

Chiral Lattice Resonances in 2.5-Dimensional Periodic Arrays with Achiral Unit Cells.

Lattice resonances are collective electromagnetic modes supported by periodic arrays of metallic nanostructures. These excitations arise from the coherent multiple scattering between the elements of the array and, thanks to their collective origin, produce very strong and spectrally narrow optical responses. In recent years, there has been significant effort dedicated to characterizing the lattice resonances supported by arrays built from complex unit cells containing multiple nanostructures. Simultaneously, periodic arrays with chiral unit cells, made of either an individual nanostructure with a chiral morphology or a group of nanostructures placed in a chiral arrangement, have been shown to exhibit lattice resonances with different responses to right- and left-handed circularly polarized light. Motivated by this, here, we investigate the lattice resonances supported by square bipartite arrays in which the relative positions of the nanostructures can vary in all three spatial dimensions, effectively functioning as 2.5-dimensional arrays. We find that these systems can support lattice resonances with almost perfect chiral responses and very large quality factors, despite the achirality of the unit cell. Furthermore, we show that the chiral response of the lattice resonances originates from the constructive and destructive interference between the electric and magnetic dipoles induced in the two nanostructures of the unit cell. Our results serve to establish a theoretical framework to describe the optical response of 2.5-dimensional arrays and provide an approach to obtain chiral lattice resonances in periodic arrays with achiral unit cells.

Sub-10 nm metal nanogaps with high SERS performance via a novel conformal coating process

Metallic nanogaps enables inducing the strong coupling between neighboring metal structures, resulting in a great field enhancement in the gap. Therefore, it is crucial fundamental building blocks for nanoelectronics and nanophotonics. Moreover, the electromagnetic field enhancement of the tiny gap increases rapidly with the decrease of gaps size. Naturally, enables better device performance with higher field enhancements. However, as the size of nanogaps reduced to the sub-10 nm scale, the fabrication of metallic nanogaps faced with great challenges, including the decrease in uniformity, repeatability and stability of the fabricating process due to various manufacturing and physical constraints. Thus, how to further reduce the size of metal nanogaps for higher device performance has become a key issue in this field. In this work, we propose and demonstrate a novel approach to fabricate sub-10 nm metal nanogaps by coating a conformal ultrathin oxide film via atomic-layer deposition. The method can fabricate sub-10 nm gap in dense metal-nanostructure arrays with high stability, reliability and uniformity, which provides a reliable technical for the fabrication and application of high-performed metallic nanogap structures and devices.

Microfluidic device using metallic nanostructure arrays for the isolation, detection, and purification of exosomes

Microfluidic devices are widely used in disease detection and specimen separation due to their low cost, rapid processing, and the ability to use even minuscule samples. This study reports on the embedding of metallic nanostructure arrays (MeNTAs) with tube-like features in microfluidic devices for the immunoaffinity-based detection and efficient isolation of exosomes. We assessed MeNTA candidates in terms of their ability to withstand the mechanical stress of microfluidic operations (indentation analysis) as well as X-ray diffraction, zeta potential, and electrostatic interactions involving antibody coatings. Note that Zr60Cu25Al10Ni5 thin film metallic glass (Zr-TFMG) features outstanding mechanical properties and a negative zeta potential exceeding those of other common materials (e.g., Cu, Bronze, Ag, 7075Al, Ti64, 718Ni, SS316, Cu-TFMG, W-TFMG, and Al-TFMG). The resulting microfluidic device featured Zr-based MeNTAs with an interdigital electrode embedded in a microchannel. When applied to MCF-7 derived exosomes using a liquid biopsy of only 500 μL, the proposed device (∼6.25 million nanostructures/cm2) achieved an exosome recovery rate of 95.3% within 1 h, while resisting nonspecific binding to HeLa-derived exosomes (recovery rate of<0.1%). The device enabled the isolation of 1 × 108 exosome particles per mL–1 for detection via electrochemical impedance spectroscopy. It also enabled the efficient release of captured exosomes via cyclic voltammetry (CV) operations over a potential range of –0.8 to +0.8 V. The proposed Zr-MeNTA microfluidic device has considerable potential for the isolation, detection, and purification of exosomes in liquid biopsy samples for cancer diagnosis.

Systematic analysis of the Rayleigh–Wood anomalies and the symmetry relations of 2D‐periodic nanostructures by imaging spectroscopic ellipsometry

AbstractPeriodic arrays of metallic nanostructures were fabricated by electron beam lithography and studied by means of spectroscopic imaging ellipsometry in the near infrared to ultraviolet spectral ranges. The sample consists of gold nanodisc and nanorod gratings on a silicon substrate. Spectroscopic imaging ellipsometry, allowing for the simultaneous observation of all gratings with microscopic resolution, was used to systematically analyze the varying grating and nanostructure parameters. The ellipsometric spectra obtained for a full in‐plane sample rotation proved to be a highly sensitive measure of the gratings’ Rayleigh–Wood anomalies. Their dependence on the sample azimuth was in excellent agreement with the Rayleigh lines calculated from the grating parameters and could be tracked for both the ambient‐to‐grating and the grating–substrate interfaces. Contrary to other studies on similarly sized nanoparticle arrays, we found no indication of localized plasmon resonance in the visible range of the spectrum, but only a weak NIR response truncated by the Rayleigh lines. Finally, the symmetry of the structures was investigated by imaging Mueller‐matrix measurements.

Nonlinear-optical microscopy of asymmetric-shaped nanoantennas

Arrays of metal nanostructures have attracted much interest due to their unique potential as optical nanoantennas and nanosensors. Here we use the second harmonic generation (SHG) microscopy technique for the studies of the nonlinear optical (NLO) response of cobalt nanoparticles of triangular and trapezoid shapes separated from a Py/Si3N4 film by a 1.5 nm thick MgO spacer. We demonstrate that the nonsymmetric elongated shape of planar nanoparticles along with the strong light localization effects result in the enhancement of the NLO response, including SHG and two-photon fluorescence. We also demonstrate that the efficiency of the SHG in nanoparticles is rather sensitive to the polarization of the incident laser beam, the visibility of the nanostructures in the nonlinear microscopy images being the highest for the linear polarization of the laser beam.

Metallic Plasmonic Nanostructure Arrays for Enhanced Solar Photocatalysis

AbstractPlasmon‐enhanced photocatalysis has emerged as a promising technology for solar‐to‐chemical energy conversion. Compared to isolated or disordered metal nanostructures, by controlling the morphology, composition, size, spacing, and dispersion of individual nanocomponents, plasmonic nanostructure arrays with coupling architectures yield strong broadband light‐harvesting capability, efficient charge transfer, enhanced local electromagnetic fields, and large contact interfaces. Although metallic nanostructure arrays are extensively studied for various applications, such as refractive index sensing, surface‐enhanced spectroscopy, plasmon‐enhanced luminescence, plasmon nanolasing, and perfect light absorption, the connection between surface plasmon resonance and enhanced photocatalysis remains relatively unexplored. In this study, an overview of plasmonic nanostructure arrays over a broad range, from 0D to 3D, for efficient photocatalysis is presented. By reviewing the fundamental mechanisms, recent applications, and latest developments of plasmonic nanostructure arrays in solar‐driven chemical conversion, this study reports on the latest guidance toward the integration of plasmonic nanostructures for functional devices in the fields of plasmonic, photonics, photodetection, and solar‐energy harvesting.

Aligned silver nanoparticles anchored on pyrrolic and pyridinic-nitrogen induced carbon nanotubes for enhanced oxygen reduction reaction

Comprehension of the metal-nanostructure arrays is of great significance for the rational design of the cathode catalyst for the oxygen reduction reaction (ORR). Therefore, we fabricated a cathode catalyst consisting of the small-size, aligned silver nanoparticles (Ag NPs) embedded into the pyrrolic and pyridinic-Nitrogen functionalized carbon nanotubes. The N-defected structure was prepared by acidification of carbon nanotubes (CNTs) to avoid inherited stacking of its layers followed by the introduction of 2,7 diaminophenazine (DPA) using a hydrothermal process, while small-size Ag NPs were aligned through electrodeposition onto the N-defected carbon nanotube (CNT-DPA-Ag). The individual role of pyrrolic and pyridinic -N in the electrocatalytic ORR performance, synthesis of small size, and distribution behavior of Ag NPs were evaluated. To maximize the ORR performance of the catalyst, the advantage was taken from the presence of carbon atoms next to the pyridinic-N which is a Lewis base and acts as an adsorption and protonation site for the oxygen (O2) molecule and eventually ensures a direct 4e− transfer route for the electrochemical conversion of O2 into water (H2O). Transmission electron microscopy (TEM) analysis of CNT-DPA-Ag revealed a small size, aligned Ag NPs (average size 2 nm) resulting in a large surface area and rich electroactive sites for adsorption of O2. The CNT-DPA-Ag showed a remarkable ORR performance in terms of positive onset potential (-0.052 V), high diffusion-limiting current density (-5.54 mAcm−2), and noteworthy methanol tolerance, and long-term durability in comparison to the commercial 20 wt% Pt/C.

Lattice Resonances Excited by Finite-Width Light Beams.

Periodic arrays of metallic nanostructures support collective lattice resonances, which give rise to optical responses that are, at the same time, stronger and more spectrally narrow than those of the localized plasmons of the individual nanostructures. Despite the extensive research effort devoted to investigating the optical properties of lattice resonances, the majority of theoretical studies have analyzed them under plane-wave excitation conditions. Such analysis not only constitutes an approximation to realistic experimental conditions, which require the use of finite-width light beams, but also misses a rich variety of interesting behaviors. Here, we provide a comprehensive study of the response of periodic arrays of metallic nanostructures when excited by finite-width light beams under both paraxial and nonparaxial conditions. We show how as the width of the light beam increases, the response of the array becomes more collective and converges to the plane-wave limit. Furthermore, we analyze the spatial extent of the lattice resonance and identify the optimum values of the light beam width to achieve the strongest optical responses. We also investigate the impact that the combination of finite-size effects in the array and the finite width of the light beam has on the response of the system. Our results provide a solid theoretical framework to understand the excitation of lattice resonances by finite-width light beams and uncover a set of behaviors that do not take place under plane-wave excitation.

Preserving High‐Q Lattice Plasmon Resonances for Poor Spatial Coherence of Light: Application in Enhanced Second Harmonic Generation

AbstractSurface lattice plasmon resonances (SLPR) in periodic arrays of metal nanostructures can feature a narrow spectral linewidth in case of the spectral overlap between the localized plasmons and Rayleigh anomalies. However, because of strong angular dispersion in experimental realizations, the excitation of SLPR requires collimated light, which has high spatial coherence. Here, excellent quality (Q)‐factor preservation in 1D all‐metallic nanograting metasurfaces due to the unique angular dispersion is reported. A high Q‐factor around 400 is obtained even when poor collimation, a numerical aperture (NA) objective with NA = 0.4, is employed for focusing the excitation light. With the advantages of highly focused excitation and high Q‐factor of the SLPR, a strong enhancement of second harmonic generation is realized from the proposed metasurface integrated with a few‐layers of GaSe flake. The results demonstrate that a 1D all‐metallic nanograting metasurface can overcome the demand for collimated light for the SLP resonances and manipulate focused light beams. The avenue of research proposed in this paper can open the door for the use of plasmonic metamaterials with SLPRs in many practical applications involving poorly coherent light sources, like the optoelectronic devices utilizing solar and daylight illuminations.

Superhydrophobic Ag‐Decorated CuO Nanowire Arrays with Analyte‐Concentrating and Self‐Cleaning Binary Functions for Ultrasensitive and Recyclable Surface‐Enhanced Raman Scattering

AbstractSurface‐enhanced Raman scattering (SERS) is considered a promising analytical technique for the detection of analytes. Considering the practical SERS detection, it is necessary to develop low‐cost, highly sensitive, and recyclable SERS‐active substrates. Here, a simple and flexible approach is proposed to fabricate a superhydrophobic Ag‐decorated CuO (named as CuO@Ag) nanowire array substrate with analyte‐concentrating and self‐cleaning binary functions for ultrasensitive and recyclable SERS detection. During this process, a superhydrophobic CuO@Ag nanowire array is made via fabrication of a CuO nanowire array by oxidizing and calcinating an ordered porous anodic aluminum oxide template on Cu foil, and following with Ag sputtering deposition on CuO nanowire array. Remarkably, this SERS substrate exhibits ultrasensitive SERS detection ability attributed to superhydrophobic plasmonic metal nanostructured arrays with high analyte‐concentrating ability and excellent photocatalytic capability due to this metal‐semiconductor composite nanostructure. These advantageous properties allow for SERS detection of malachite green with a limit of detection of 6.73 × 10−13 m and self‐cleaning via photodegradation of adsorbed analytes with visible‐light illumination. Consequently, it achieves recyclable SERS detection. This research demonstrates a simple and flexible approach for fabricating recyclable SERS‐active substrate, which expands the practical SERS applications in chemical and biological analysis, food safety, and healthcare.

Plasmonic Au loaded semiconductor-engineered large-scale metallic nanostructure arrays for SERS application

The need for a highly-sensitive and reproducible surface-enhanced Raman scattering (SERS) substrate is the key factor restricting its practical applicability. In this work, we report on a periodic three-dimensional (3D) metallic nanostructure array, referred to as MeNTA fabricated using processes developed for large-scale semiconductor engineering. Specifically, MeNTAs were fabricated using Zr-based (Zr60Cu24Al11Ni5) metallic glass, which provides high strength, high ductility, and good biocompatibility. Over this was deposited a uniform coating of gold nanoparticles (NPs) to form a high-sensitivity AuNP@MeNTAs 3D-SERS substrate. Using crystal violet as a probe molecule, the performance of an AuNP@MeNTA 3D-SERS substrate provided a minimum detection limit of 10−13 M with an enhancement factor of 1.49 × 108, thereby demonstrating its high sensitivity. The minimum relative standard deviation (RSD) for the AuNP@MeNTA substrate was 5.45%, and SERS performance did not degrade within 14 days after installation, thereby demonstrating high stability and repeatability. This research provides useful guidelines for the low-cost fabrication of SERS substrates of high sensitivity.

Green Tensor Analysis of Lattice Resonances in Periodic Arrays of Nanoparticles

When arranged in a periodic geometry, arrays of metallic nanostructures are capable of supporting collective modes known as lattice resonances. These modes, which originate from the coherent multiple scattering between the elements of the array, give rise to very strong and spectrally narrow optical responses. Here, we show that, thanks to their collective nature, the lattice resonances of a periodic array of metallic nanoparticles can mediate an efficient long-range coupling between dipole emitters placed near the array. Specifically, using a coupled dipole approach, we calculate the Green tensor of the array connecting two points and analyze its spectral and spatial characteristics. This quantity represents the electromagnetic field produced by the array at a given position when excited by a unit dipole emitter located at another one. We find that, when a lattice resonance is excited, the Green tensor is significantly larger and decays more slowly with distance than the Green tensor of vacuum. Therefore, in addition to advancing the fundamental understanding of lattice resonances, our results show that periodic arrays of nanostructures are capable of enhancing the long-range coupling between collections of dipole emitters, which makes them a promising platform for applications such as nanoscale energy transfer and quantum information processing.