Au Nanodisks Research Articles

Controllable Dual Hybrid Tamm Plasmon Modes in Binary Gold Nanodisk Arrays and Distributed Bragg Reflector Structure

Optical Tamm plasmon (TP) can be excited at the boundary of photonic crystal and metal film. In this work, we propose a composite structure consisting of binary Au nanodisk arrays on top of a distributed Bragg reflector (DBR) of TiO2/SiO2 1D photonic crystal; the structure supports the confined dual hybrid TP modes that benefited from the excitation of a localized lattice collective resonance on the vicinity of the nanoparticles array. The hybrid TP modes possess enhanced confinement, controllable hybrid TP frequency, and broadened spectral width compared with that of TP mode within the gold film/DBR structure. Moreover, the hybrid TP mode that due to the in-phased localized lattice resonance is dominated when the nanodisk radius increases, while the hybrid TP mode resulted from weak coupling of in-phased and anti-phased localized lattice resonance is weakened, especially when the radii of binary arrays approach identical. Thus, the tunable dual confined TP states can be realized through adjusting the arrays radii rather than changing the properties of DBR. Importantly, the enhancement of confined hybrid TP modes with controllable resonant frequency has a potential application in perfect absorption.

Plasmonic hot electron transfer in anisotropic Pt-Au nanodisks boosts electrochemical reactions in the visible-NIR region.

Anisotropic plasmonic metals have attracted significant attention in enhancing the catalytic performance of catalysts due to their broad light-harnessing capabilities and active hot electrons; however, limited investigations have been dedicated towards improving their electrochemical reaction performance in the visible and near infrared (NIR) regions. Herein, anisotropic Pt-edged Au nanodisks (NDs) were synthesized by controlling the preferential loading of Pt and used as catalysts for plasmon-enhanced electrochemical methanol oxidation reactions (MORs) under visible-NIR light irradiation by, and the light-enhanced electric current over the Pt-edged Au NDs was found to be 3-fold higher than that under dark conditions. Wavelength-dependent electric current over the Pt-edged Au NDs for the MOR in the visible-NIR light region demonstrates that the light-induced enhancement of the electric current is due to surface plasmon resonance (SPR) of the Au NDs. Furthermore, plasmonic hot electron transfer was studied by the single-particle photoluminescence images and spectra of Au NDs and Pt-Au NDs, and the dipole surface plasmon resonance (DSPR) mode was proved to be the main channel for hot electron transfer. During the electrochemical reaction under visible-NIR light irradiation, a plasmonic hot electron is transferred to the electrode, and a "hot hole" is left on the surface, boosting the MOR.

The effect of size of Au nanodisc on the localized surface plasmon resonance simulated using finite-difference time-domain (FDTD) method

This work studies the effect of different diameter sizes of Au nanodisc on the plasmonic surface plasmon resonance simulated using FDTD method. The extinction efficiency and the local field intensity enhancement have been observed via manipulating diameter sizes of the Au nanodisc whilst fixing its thickness to 5nm. This plasmonic effect tailoring steps can provide for many potential applications for design and fabrication of plasmonic devices. Both the extinction efficiency and electric field intensity at the center of the Au nanodisc show significant enhancement with the increase of diameter sizes. This observation of localized surface plasmon effect on the different size of the Au nanodisc are expected to provide understanding on the design of plasmonic hot carrier generation.

Resonant Plasmon-Enhanced Upconversion in Monolayers of Core-Shell Nanocrystals: Role of Shell Thickness.

The upconversion luminescence (UCL) of colloidal lanthanide-doped upconversion nanocrystals (UCNCs) can be improved either by precise encapsulation of the surface by optically inert shells around the core, by an alteration of the nearby environment via metal nanoparticles, or by a combination of both. Considering their potential importance in crystalline silicon photovoltaics, the present study investigates both effects for two-dimensional arrangements of UCNCs. Using excitation light of 1500 nm wavelength, we study the variation in the upconversion luminescence from an Er3+-doped NaYF4 core as a function of the thickness of a NaLuF4 shell in colloidal solutions as well as in spin-cast-assisted self-assembled monolayers of UCNCs. The observed UCL yields and decay times of Er3+ ions of the UCNCs increase with increasing shell thickness in both cases, and nearly no variation in decay times is observed in the transition of the UCNCs from solution to film configurations. The luminescence efficiency of the UCNC monolayers is further enhanced by electron-beam-lithographic-designed Au nanodiscs deposited either on top of or buried within the monolayer. It is observed that the improvement by the nanocrystal shells is greater than that of the Au nanodiscs.

Surface enhanced resonant Raman scattering in hybrid MoSe2@Au nanostructures.

We report on the surface enhanced resonant Raman scattering (SERRS) in hybrid MoSe2@Au plasmonic-excitonic nanostructures, focusing on the situation where the localized surface plasmon resonance of Au nanodisks is finely tuned to the exciton absorption of monolayer MoSe2. Using a resonant excitation, we investigate the SERRS in MoSe2@Au and the resonant Raman scattering (RRS) in a MoSe2@SiO2 reference. Both optical responses are compared to the non-resonant Raman scattering signal, thus providing an estimate of the relative contributions from the localized surface plasmons and the confined excitons to the Raman scattering enhancement. We determine a SERRS/RRS enhancement factor exceeding one order of magnitude. Furthermore, using numerical simulations, we explore the optical near-field properties of the hybrid MoSe2@Au nanostructure and investigate the SERRS efficiency dependence on the nanodisk surface morphology and on the excitation wavelength. We demonstrate that a photothermal effect, due to the resonant plasmonic pumping of electron-hole pairs into the MoSe2 layer, and the surface roughness of the metallic nanostructures are the main limiting factors of the SERRS efficiency.

Subwavelength asymmetric Au–VO2 nanodisk dimer for switchable directional scattering**Project supported by the National Key R&D Program of China (Grant No. 2017YFA0303800), the National Natural Science Foundation of China (Grant Nos. 11634010, 61675170, and 11874050), the Natural Science Basic Research Plan in Shaanxi Province, China (Grant No. 2017JM6022), and the Fundamental Research Funds for the Central Universities, China (Grant No. 3102017zy017).

We propose an asymmetric Au–VO2 nanodisk dimer for realizing a switchable directional scattering. Specifically, the directional scattering can be triggered on/off through controlling the phase transition of the VO2 nanodisk from metallic to semiconductor state. More strikingly, an obvious directional scattering with the directivity of ∼40 dB is achieved under the metallic state of VO2 nanodisk. This tunable directional scattering is further explained with an interference model where the Au and VO2 nanodisks are treated as two weakly interacting electric dipoles. The phase transition controlled scattering patterns of asymmetric Au–VO2 nanodisk dimer are then well interpreted from the phase difference between these two dipoles.

First Principles Calculations Toward Understanding SERS of 2,2'-Bipyridyl Adsorbed on Au, Ag, and Au-Ag Nanoalloy.

First principles electrodyanmics and quantum chemical simulations are performed to gain insights into the underlying mechanisms of the surface enhanced Raman spectra of 22BPY adsorbed on pure Au and Ag as well as on Au-Ag alloy nanodiscs. Experimental SERS spectra from Au and Ag nanodiscs show similar peaks, whereas those from Au-Ag alloy reveal new spectral features. The physical enhancement factors due to surface nano-texture were considered by numerical FDTD simulations of light intensity distribution for the nano-textured Au, Ag, and Au-Ag alloy and compared with experimental results. For the chemical insights of the enhancement, the DFT calculations with the dispersion interaction were performed using Au20 , Ag20 , and Au10 Ag10 clusters of a pyramidal structure for SERS modeling. Binding of 22BPY to the clusters was simulated by considering possible arrangements of vertex and planar physical as well as chemical adsorption models. The DFT results indicate that 22BPY prefers a coplanar adsorption on a (111) face with trans-conformation having close energy difference to cis-conformation. Binding to pure Au cluster is stronger than to pure Ag or Au-Ag alloy clusters and adsorption onto the alloy surface can deform the surface. The computed Raman spectra are compared with experimental data and assignments for pure Au and Ag models are well matching, indicating the need of dispersion interaction to reproduce strong Raman signal at around 800 cm-1 . This work provides insight into 3D character of SERS on nanorough surfaces due to different binding energies and bond length of nanoalloys. © 2018 Wiley Periodicals, Inc.

Optical-Force-Dominated Directional Reshaping of Au Nanodisks in Al-Au Heterodimers.

The optical reshaping of metallic nanostructures typically requires intense laser pulses to first approach or achieve melting, followed by surface-tension-dominated reshaping, transforming the original nanostructures into more spherical morphologies. Here, we report the directional optical reshaping of the Au nanodisk of an Al-Au heterodimer in the illuminated junction of an atomic force microscope (AFM). Both the heightening and the repositioning of the Au nanodisk component are induced, reducing the gap between the two nanodisks. There are three contributors to this process: the photothermal softening of the Au lattice, the optical force applied to the Au nanodisk by the Al nanodisk, and the optical force from the nearby AFM tip. The asymmetric reshaping of the heterodimer is observable structurally, through electron microscopic imaging, and through changes in the heterodimer optical response. This optical-force-directed shape manipulation may have potential applications in nanofabrication, optically induced nanomanufacturing, sensing, and quality control.

Enhancement of Gold Nanoparticle Coupling with a 2D Plasmonic Crystal at High Incidence Angles.

2D nanoplasmonic substrates excited in transmission spectroscopy are ideal for several biosensing, metamaterial, and optical applications. We show that their excellent properties can be further improved with plasmonic coupling of Au nanoparticles (AuNPs) on gold-coated nanodisk arrays excited at large incidence angles of up to 50°. The Bragg modes (BM) thereby strongly couple to AuNP immobilized on the plasmonic substrate due to shorter decay length of the plasmon at higher incidence angles, leading to a further enhanced field between the AuNP and the plasmonic substrate. The field was highest and two hotspots were created at orthogonal positions for AuNP located close to the corner of the Au film and Au nanodisk, which was also observed for AuNP dimers. Hybridization between single-stranded DNA (ssDNA) immobilized on the surface of the AuNPs and the capture ssDNA on the gold-coated nanodisk arrays led to at least a 5-fold signal improvement and a 7-fold lower limit of detection at 7 pM for ssDNA-functionalized AuNPs at large incident angles. Thus, we demonstrate that higher field strength can be accessed and the significant advantages of working with high incidence angles with AuNP on a 2D plasmonic crystal in plasmonic sensing.

Electrochemical Control of Plasmonic Metal Nanogap for Ultra-Small Light Confinement

Ultra-small nanogap of plasmonic metal dimer can confine light into molecular size. In this study, we have tried to establish the method for the control of the gap distance of Au nanodimer structures using electrochemical method. Au nanodimer structures were fabricated on a conductive glass substrate by the electron beam lithography method. Electrochemical dissolution of bridged Au nanodisk dimer was conducted under in-situ dark-field microscopy observation. The electrode potential was kept at the potential for Au dissolution. SEM images proved the formation of the gap between Au nanoparticles. From the time series scattering spectra of Au nanodimer, the drastic optical property changes has been observed at the gap formation. This optical property change can be assigned to the plamon mode change from the charge transfer mode to bonding dipolar mode due to the formation of the sub-nm gap at Au nanodimer. The present method is a useful tool for controlling the shape and the gap distance between Au nanoparticles in a single nanometer scale.

Correlation between Gas Bubble Formation and Hydrogen Evolution Reaction Kinetics at Nanoelectrodes.

We report the correlation between H2 gas bubble formation potential and hydrogen evolution reaction (HER) activity for Au and Pt nanodisk electrodes (NEs). Microkinetic models were formulated to obtain the HER kinetic information for individual Au and Pt NEs. We found that the rate-determining steps for the HER at Au and Pt NEs were the Volmer step and the Heyrovsky step, respectively. More interestingly, the standard rate constant ( k0) of the rate-determining step was found to vary over 2 orders of magnitude for the same type of NEs. The observed variations indicate the HER activity heterogeneity at the nanoscale. Furthermore, we discovered a linear relationship between bubble formation potential ( Ebubble) and log( k0) with a slope of 125 mV/decade for both Au and Pt NEs. As log ( k0) increases, Ebubble shifts linearly to more positive potentials, meaning NEs with higher HER activities form H2 bubbles at less negative potentials. Our theoretical model suggests that such linear relationship is caused by the similar critical bubble formation condition for Au and Pt NEs with varied sizes. Our results have potential implications for using gas bubble formation to evaluate the HER activity distribution of nanoparticles in an ensemble.

Circular Gold Nanodisks with Synthetically Tunable Diameters and Thicknesses

Abstract2D or pseudo‐2D plasmonic Au nanocrystals, such as circular Au nanodisks, possess unique plasmonic properties. Circular Au nanodisks not only possess two large surfaces with circular symmetry but also exhibit the wide tunability for their plasmon resonance. However, the lack of effective synthetic methods for producing size‐tunable and monodispersed circular Au nanodisks hinders further studies on their properties and applications. Herein, the synthesis of uniformly sized circular Au nanodisks with synthetically tunable diameters and thicknesses is reported. By performing mild anisotropic oxidation on pregrown Au nanoplates with different thicknesses, the thicknesses of the obtained nanodisks are varied from ≈10 nm to ≈50 nm. The nanodisk diameters are tailored from ≈50 nm to ≈150 nm by controlling the oxidation time. Moreover, both homodimers and heterodimers made of circular Au nanodisks are constructed using molecular linkers. They exhibit rich plasmon modes. In particular, dark multipolar plasmon resonance modes can be excited and observed in the asymmetric heterodimers. Such circular Au nanodisks with controllable sizes, large atomically flat surfaces, and a dominant dipolar plasmon mode are ideal building blocks for constructing plasmonic assemblies and plasmon‐coupled systems with desired plasmonic properties and functions.

Large-Aperture and Grain-Boundary Engineering through Template-Assisted Metal Dewetting for Resonances in the Short Wave Infrared

We extend the fabrication method of template-assisted metal dewetting (TeAMeD) to create near-infrared resonant nanostructures in an Au film without the need for etching or lift-off. TeAMeD has previously been used to generate high aspect-ratio sub-10 nm apertures, but struggles to generate larger apertures (>100 nm). In this work, we introduce a method to create larger apertures using templates consisting of fin-like patterns with radial symmetry. We also report evidence of grain boundary engineering, through the template pinning effect. Our three-dimensional phase field model of TeAMeD predicts both the grain-boundary pinning and aperture opening effects that agree well with experiments. Combined with simulation design, TeAMeD can be established as a grain engineering platform, allowing grain shape and boundary position to be controlled. Variations of template motif produce larger grains and numerous possible outcomes, including suspended Au nanodisks and triangular apertures.

XPS characterization of a plasmonic sensor for catalysis studies by controlled differential charging

The use of external bias during XPS measurements has been successfully extended to characterization of a plasmonic Au nanostructured catalyst support, with and without deposition of a TiO2 film. Application of pulsed bias voltage indicates that the Au nanoparticles and the silica base are not in complete electrical contact as they do not exhibit the same frequency dependence. This effect is substrate dependent and is not present in a sample of Au nanodiscs on a silicon/SiO2 substrate. Upon deposition of a TiO2 film, the charging capacity of the Au nanodiscs as well as the silica substrate decrease significantly. Finally, we show that controlled differential charging in XPS can help identify defects caused by sample manufacturing.

Exclusive and efficient excitation of plasmonic breathing modes of a metallic nanodisc with the radially polarized optical beams

BackgroundThe plasmonic breathing modes of a metallic nanodisc are dark plasmonic modes and thus also have the advantage of much smaller radiation loss. These modes emerged previously in electron energy loss spectroscopy experiments and were then pursued with optical excitation methods.ResultsIn this paper, through the finite element method type numerical simulations, we show evidence of the pure excitation of these modes under the illumination of two counter-propagating, radially-polarized optical beams. The obtained near-field spectrum of a single Au or Ag nanodisc shows the plasmonic resonant peaks at which the electric field distributions, as well as the plasmonic dispersion relationship both clearly exhibit the characteristics of the plasmonic breathing modes.ConclusionWe expect that the method that we have proposed, due to operating conveniently in the manner of far field excitation, will open many possibilities in practical applications based on the interaction between a single metal nanodisc with the radially polarized optical beam or between a metal nanodisc array with the beam array.

Single-Nanoparticle Plasmonic Electro-optic Modulator Based on MoS2 Monolayers.

The manipulation of light in an integrated circuit is crucial for the development of high-speed electro-optic devices. Recently, molybdenum disulfide (MoS2) monolayers generated broad interest for the optoelectronics because of their huge exciton binding energy, tunable optical emission, direct electronic band-gap structure, etc. Miniaturization and multifunctionality of electro-optic devices further require the manipulation of light-matter interaction at the single-nanoparticle level. The strong exciton-plasmon interaction that is generated between the MoS2 monolayers and metallic nanostructures may be a possible solution for compact electro-optic devices at the nanoscale. Here, we demonstrate a nanoplasmonic modulator in the visible spectral region by combining the MoS2 monolayers with a single Au nanodisk. The narrow MoS2 excitons coupled with broad Au plasmons result in a deep Fano resonance, which can be switched on and off by applying different gate voltages on the MoS2 monolayers. A reversible display device that is based on this single-nanoparticle modulator is demonstrated with a heptamer pattern that is actively controlled by the external gates. Our work provides a potential application for electro-optic modulation on the nanoscale and promotes the development of gate-tunable nanoplasmonic devices in the future.

Temperature-Dependent Plasmon–Exciton Interactions in Hybrid Au/MoSe2 Nanostructures

Combining localized surface plasmons and confined excitons in hybrid metallic/semiconductor nanostructures is a promising route toward the manipulation of the light–matter interaction at the nanoscale and the generation of novel technological applications. In this context, we investigate the interference between plasmonic and excitonic resonances in hybrid MoSe2@Au nanostructures consisting of monolayer MoSe2 supported by Au nanodisks. The optical properties of the nanostructures are probed by means of spatially resolved optical transmission and photoluminescence spectroscopies and interpreted using an analytical model complemented by numerical simulations. A plasmonic–excitonic interaction energy of 42 ± 8 meV is obtained, clearly setting the coupling in the Fano-type regime. On the basis of numerical simulations of the electromagnetic near-field and on calculations of the excitonic transition dipole momentum, we show that the interaction energy is concentrated in the gap region between the disks. The te...

Stacked Gold Nanodisks for Bimodal Photoacoustic and Optical Coherence Imaging.

Herein, we report on biological imaging nanoprobes: physically synthesized gold nanodisks that have inherent optical advantages-a wide range of resonant wavelengths, tunable ratio of light absorption-to-scattering, and responsiveness to random incident light-due to their two-dimensional circular nanostructure. Based on our proposed physical synthesis where gold is vacuum deposited onto a prepatterned polymer template and released from the substrate in the form of a nanodisk, monodisperse two-dimensional gold nanodisks were prepared with independent control of their diameter and thickness. The optical benefits of the Au nanodisk were successfully demonstrated by the measurement of light absorbance of the nanodisks and the application of stacked nanodisks, where a smaller sized Au nanodisk was laid atop a larger nanodisk, as bimodal contrast agents for photoacoustic microscopy and optical coherence tomography.

Nanoplasmonic Sensing at the Carbon-Bio Interface: Study of Protein Adsorption at Graphitic and Hydrogenated Carbon Surfaces.

Various forms of carbon are known to perform well as biomaterials in a variety of applications and an improved understanding of their interactions with biomolecules, cells, and tissues is of interest for improving and tailoring their performance. Nanoplasmonic sensing (NPS) has emerged as a powerful technique for studying the thermodynamics and kinetics of interfacial reactions. In this work, the in situ adsorption of two proteins, bovine serum albumin and fibrinogen, were studied at carbon surfaces with differing chemical and optical properties using nanoplasmonic sensors. The carbon material was deposited as a thin film onto NPS surfaces consisting of 100 nm Au nanodisks with a localized plasmon absorption peak in the visible region. Carbon films were fully characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Two types of material were investigated: amorphous carbon (a-C), with high graphitic content and high optical absorptivity, and hydrogenated amorphous carbon (a-C:H), with low graphitic content and high optical transparency. The optical response of the Au/carbon NPS elements was modeled using the finite difference time domain (FDTD) method, yielding simulated analytical sensitivities that compare well with those observed experimentally at the two carbon surfaces. Protein adsorption was investigated on a-C and a-C:H, and the protein layer thicknesses were obtained from FDTD simulations of the expected response, yielding values in the 1.8-3.3 nm range. A comparison of the results at a-C and a-C:H indicates that in both cases fibrinogen layers are thicker than those formed by albumin by up to 80%.

Spatial evolution of the near-field distribution on planar gold nanoparticles with the excitation wavelength across dipole and quadrupole modes

We investigate the superposition properties of the dipole and quadrupole plasmon modes in the near field both experimentally, by using photoemission electron microscopy (PEEM), and theoretically. In particular, the asymmetric near-field distributions on gold (Au) nanodisks and nanoblocks under oblique incidence with different polarizations are investigated in detail. The results of PEEM measurements show that the evolutions of the asymmetric near-field distributions are different between the excitation with s-polarized and p-polarized light. The experimental results can be reproduced very well by numerical simulations and interpreted as the superposition of the dipole and quadrupole modes with the help of analytic calculations. Moreover, we hypothesize that the electrons collected by PEEM are mainly from the plasmonic hot spots located at the plane in the interface between the Au particles and the substrate in the PEEM experiments.