Au Nanodisks Research Articles

Fano Resonances in Complex Plasmonic Necklaces Composed of Gold Nanodisks Clusters for Enhanced LSPR Sensing

Plasmonic nanoparticles in complex clusters and in specific molecular orientations are able to support strong plasmon and Fano resonances in their nanoscale geometries. In this paper, we examine the spectral response of a symmetric necklace shape nanostructure composed of Au nanodisk heptamers that are located in a close proximity to each other. Determining the appropriate geometrical parameters for the proposed necklace, we analyzed the effect of geometrical variations on the Fano resonance position and quality by calculating the scattering cross-sectional profile, numerically. Considering the strong localization of surface plasmon resonances (LSPR) in the heptamer clusters, the LSPR sensitivity for the studied necklace has been determined. Moreover, we evaluated the performance of the structure for different medium conditions by plotting corresponding figure of merit (FoM). To this end, we measured the plasmon resonance energy differences (Δ E eV) over the refractive index (n) alterations, we quantified the corresponding FoM for the final necklace as 13.7, which can be utilized in designing precise and highly sensitive sensors. Ultimately, we proved that highly complex structures composed of metal nanoparticles clusters yield high sensitivity to the environmental perturbations.

Plasmon enhanced organic devices utilizing highly ordered nanoimprinted gold nanodisks and nitrogen doped graphene.

High performance organic devices including polymer solar cells (PSCs) and light emitting diodes (PLEDs) were successfully demonstrated with the presence of highly ordered nanoimprinted Au nanodisks (Au NDs) in their solution-processed active/emissive layers, respectively. PSCs and PLEDs were fabricated using a low bandgap polymer and acceptor, nitrogen doped multiwalled carbon nanotubes poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]-thiophenediyl] (n-MWCNTs:PTB7), and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and (4,4-N,N-dicarbazole) biphenyl (CBP) doped with tris(2-phenylpyridine) iridium(iii) (Ir(ppy)3) as active/emissive layers, respectively. We synthesized nitrogen doped graphene and used it as anodic buffer layer in both devices. The localized surface plasmon resonance (LSPR) effect from Au NDs clearly contributed to the increase in light absorption/emission in the active layers from electromagnetic field enhancement, which originated from the excited LSPR in PSCs and PLEDs. In addition to the high density of LSPR and strong exciton-SP coupling, the electroluminescent (EL) enhancement is ascribed to enhanced spontaneous emission rates. This is due to the plasmonic near-field effect induced by Au NDs. The PSCs and PLEDs exhibited 14.98% (8.08% to 9.29%) under one sun of simulated air mass 1.5 global (AM1.5G) illumination (100 mW cm(-2)) and 19.18% (8.24 to 9.82 lm W(-1)) enhancement in the power conversion efficiencies (PCEs) compared to the control devices without Au NDs.

Guided formation of sub-5 nm interstitial gaps between plasmonic nanodisks.

To achieve a reliable formation of a surface-enhanced Raman scattering (SERS) sensor with evenly distributed hot spots on a wafer scale substrate, we propose a hybrid approach combining physical nanolithography for preparing Au nanodisks and chemical Au reduction for growing them. During the chemical growth, the interstitial distance between the nanodisks decreased from 60 nm to sub-5 nm. The resulting patterns of the nanogap-rich Au nanodisks successfully enhance the SERS signal, and its intensity map shows only a 5% or less signal variation on the entire sample.

Engineering of parallel plasmonic-photonic interactions for on-chip refractive index sensors.

Ultra-narrow linewidth in the extinction spectrum of noble metal nanoparticle arrays induced by the lattice plasmon resonances (LPRs) is of great significance for applications in plasmonic lasers and plasmonic sensors. However, the challenge of sustaining LPRs in an asymmetric environment greatly restricts their practical applications, especially for high-performance on-chip plasmonic sensors. Herein, we fully study the parallel plasmonic-photonic interactions in both the Au nanodisk arrays (NDAs) and the core/shell SiO2/Au nanocylinder arrays (NCAs). Different from the dipolar interactions in the conventionally studied orthogonal coupling, the horizontal propagating electric field introduces the out-of-plane "hot spots" and results in electric field delocalization. Through controlling the aspect ratio to manipulate the "hot spot" distributions of the localized surface plasmon resonances (LSPRs) in the NCAs, we demonstrate a high-performance refractive index sensor with a wide dynamic range of refractive indexes ranging from 1.0 to 1.5. Both high figure of merit (FOM) and high signal-to-noise ratio (SNR) can be maintained under these detectable refractive indices. Furthermore, the electromagnetic field distributions confirm that the high FOM in the wide dynamic range is attributed to the parallel coupling between the superstrate diffraction orders and the height-induced LSPR modes. Our study on the near-field "hot-spot" engineering and far-field parallel coupling paves the way towards improved understanding of the parallel LPRs and the design of high-performance on-chip refractive index sensors.

Plasmon-enhanced Raman scattering by suspended carbon nanotubes

We report plasmon-enhanced Raman scattering of the order of 103 by a metallic carbon nanotube partially suspended inside a near-field cavity. The tube is part of a small bundle, and is interfaced with an Au nanodisc dimer using a recently developed assembly scheme based on dielectrophoretic deposition. Spatially resolved Raman measurements with two excitation wavelengths and two orthogonal polarizations confirm that the enhancement arises from a 65 nm long suspended tube segment. We show that the orientation of the tube inside the cavity can be as effective for generating enhancement as placing the nanotube precisely in a plasmonic hotspot. Position and shape of the G-peak show that the suspended part of the tube is free of strain and doped with a Fermi energy shift ≤40 meV. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Wet etching-assisted colloidal lithography: a general strategy toward nanodisk and nanohole arrays on arbitrary substrates.

A simple and facile strategy is presented to fabricate the metal nanodisk and nanohole arrays based on a wet etching-assisted polystyrene colloidal lithography. Gold is chosen to demonstrate the validity of such a strategy. The hexagonally arranged Au nanodisk and nanohole arrays are thus fabricated with large area and good uniformity. The structural parameters of the arrays, such as thicknesses, diameters, and spacings of the nanodisks or nanoholes, are facilely tunable and controllable by predeposition conditions, etching conditions and colloidal monolayer structure. More importantly, these arrays can be produced on any supporting substrates, such as conductive or nonconductive and even flexible substrates with flat, rough, or even curved surfaces. In general, the presented strategy is low in cost, simple in operation and arbitrary in substrate, and the as-prepared arrays could find potential devices' applications with nice compatibility in the fields of optics, surface-enhanced Raman spectroscopy, biosensing, and so forth.

STUDY ON PL ENHANCEMENT OF InGaAs/GaAs QUANTUM WELL EMISSION BY GOLD NANOPARTICLE ARRAYS

In this paper, enhancement of photoluminescence (PL) intensity from InGaAs / GaAs quantum well (QW) caused by coupling effect of surface plasmon (SP) resonance generated by Au nanodisk arrays with QW emission is demonstrated experimentally. To increase the coupling efficiency, a thin layer of SiO 2 is introduced between Au nanodisk and GaAs surface. To match SP wavelength with QW emission wavelength (945 nm), Au nanodisk arrays with periodicity of 290 nm, radius of 70 nm, and thickness of 10 nm is fabricated. 2.17-fold enhancement is observed.

Large-scale uniform Au nanodisk arrays fabricated via x-ray interference lithography for reproducible and sensitive SERS substrate

Large-scale Au nanodisk arrays on Si substrate are successfully fabricated via x-ray interference lithography and followed by electron-beam vapor deposition. The Au nanodisk arrays exhibit a significant, uniform, and reproducible surface enhancement on Raman scattering signal, which enables the detection of R6G as low as 10−8 M with an enhancement factor of 106. Importantly, the Au nanodisk arrays SERS-active substrates with uniformly high sensitivity also have high reproducibility and stability. The diameters of the nanodisks and the inter-disk distance can be simply optimized to obtain high enhancement in Raman signal by varying exposure time and development time in XIL process. The electric fields of the Au nanodisks with various diameters and inter-disk distance simulated by the finite difference time domain (FDTD) techniques further confirm that the Raman signal enhancement of Au nanodisks is determined by the diameters of nanodisks and the inter-disk distance of nanodisks. The Au/Ag double-layer bimetal nanodisk arrays are also fabricated which show a significant increase in the Raman signal enhancement than that of the Au nanodisk arrays. XIL nanofabrication appears to be a feasible approach to prepare uniform and reproducible SERS-active substrates with high sensitivity for practical SERS applications.

Radioactive 198Au-doped nanostructures with different shapes for in vivo analyses of their biodistribution, tumor uptake, and intratumoral distribution.

With Au nanocages as an example, we recently demonstrated that radioactive 198Au could be incorporated into the crystal lattice of Au nanostructures for simple and reliable quantification of their in vivo biodistribution by measuring the γ radiation from 198Au decay and for optical imaging by detecting the Cerenkov radiation. Here we extend the capability of this strategy to synthesize radioactive 198Au nanostructures with a similar size but different shapes and then compare their biodistribution, tumor uptake, and intratumoral distribution using a murine EMT6 breast cancer model. Specifically, we investigated Au nanospheres, nanodisks, nanorods, and cubic nanocages. After PEGylation, an aqueous suspension of the radioactive Au nanostructures was injected into a tumor-bearing mouse intravenously, and their biodistribution was measured from the γ radiation while their tumor uptake was directly imaged using the Cerenkov radiation. Significantly higher tumor uptake was observed for the Au nanospheres and nanodisks relative to the Au nanorods and nanocages at 24 h postinjection. Furthermore, autoradiographic imaging was performed on thin slices of the tumor after excision to resolve the intratumoral distributions of the nanostructures. While both the Au nanospheres and nanodisks were only observed on the surfaces of the tumors, the Au nanorods and nanocages were distributed throughout the tumors.

Au Nanodisk-Core Multishell Nanoparticles: Synthetic Method for Controlling Number of Shells and Intershell Distance

Au nanodisk-core multishell nanoparticles (NDCMS-NPs) were synthesized using a Au nanodisk core as a seed to deposit a controlled number of surrounding Ag layers, followed by galvanic replacement reactions with Au ions in water. Uniform Au NDCMS-NPs with controllable shell numbers and intershell distances exhibited unique surface plasmon mode shifts, which were found to be dependent on the thickness of the Ag layer. The number of shells was tuned by controlling the multistep reactions from one to five. The Au core played a critical role in terms of holding the multishell structure during the experimental procedures. We systematically followed their shape evolution by monitoring the surface plasmon resonance bands, as well as by viewing these physical changes with electron microscopy.

Enhancement of GaAs/InGaAs quantum well emission by disordered gold nanoparticle arrays

Coupling effect of surface plasmon (SP) with InGaAs/GaAs QW emission is demonstrated experimentally. The SP resonance is generated by disordered arrays of Au nanodisks on the InGaAs/GaAs QW surface. More than twofold enhancement in QW PL is observed. Theoretical simulations also indicated that the disordered arrays of Au structures enlarged the cone angle for which light can be radiated out. The larger angle enhances the PL intensity.

Switchable plasmon-induced transparency in gold nanoarrays on vanadium dioxide film

A proof-of-concept demonstration of switchable plasmon-induced transparency is presented in gold (Au) dolmen structures on a vanadium dioxide (VO2) thin film. The dolmen structures were made by a combination of high-resolution electron-beam lithography on VO2 films deposited by electron-beam evaporation. A 100 nm (160 meV) modulation of the plasmon resonance of the dolmen structure due to the phase transition of the vanadium dioxide is observed. The plasmon resonance width is a factor 2–3 narrower than what was observed for Au nanodisks on a VO2 film in previous studies, due to the interference between the bright and dark modes of the plasmon structure. This experiment shows how one can combine a phase-change material with multielement plasmonic nanostructures to optimize the performance of optical switches and sensors based on metamaterial structures.

Behavior of plasmonic nanoparticle array in near- and far-field coupling regimes for transverse electric and transverse magnetic polarizations

We have previously reported the results of plasmonic behavior of an Au nanodisk array in the far-field coupling regime under oblique illumination with transverse electric polarization. In this paper, those results are studied in more detail. Here, results for transverse magnetic polarization are also presented and discussed. In addition to the far-field coupling regime, the results for the near-field coupling regime are also reported. Effects of different parameters, such as substrate thickness and array periodicity on the shape of plasmon spectra are discussed. It will be shown that in the far-field coupling regime, the diffractive grating orders can have a major role in the behavior of the structure.

Coupling of surface plasmon with InGaAs/GaAs quantum well emission by gold nanodisk arrays

Enhancement of photoluminescence (PL) intensity from InGaAs/GaAs quantum well (QW) is achieved experimentally by coupling surface plasmon (SP) resonance with QW emission. The SP resonance is generated by fabricating a periodic Au nanodisk array on top of InGaAs/GaAs QW structure. A thin layer of SiO(2) between Au nanodisk and GaAs surface has been employed to achieve easy adjustment of the SP resonance. A 4.16 fold enhancement of PL intensity was observed. Theoretical simulation results match well with the experimental results and confirm that the PL emission is enhanced by SP coupling with the fabricated structure.

Measurement of Interlayer Screening Length of Layered Graphene by Plasmonic Nanostructure Resonances

The variation in localized surface plasmon resonances of single Au nanodisks (diameter 100 nm and height 25 nm) on 0–13 graphene layers is investigated using dark-field scattering spectroscopy to obtain the graphene electric field screening length. For nanodisks (NDs) with and without underlying graphene layers on a SiO2 (300 nm)/Si substrate, the plasmon resonance red shifts from 604 to 620 nm with increasing graphene layers. The spectra of the plasmonic nanostructures obey an exponential saturation function versus increasing number of layers of graphene from 0 to 13. As a conducting film, the graphene layers screen the electric field generated by the plasmonic resonance of the Au NDs in the vicinity of the interface, and the red shifts of the resonance wavelength are explained in the framework of the electromagnetic field coupling between in-plane antiparallel image dipoles in the graphene layers and the ND dipole. A screening length of 1.2 ± 0.2 nm, equivalent to 3–4 graphene layers, is experimentally obtained, in good agreement with the measurement by field-effect transistors and theoretical calculation in doped graphene. The resonance shift of plasmonic nanostructures on a layered graphene system provides an alternative and convenient method for screening length measurement of graphene films.

A large-scale sub-100 nm Au nanodisk array fabricated using nanospherical-lens lithography: a low-cost localized surface plasmon resonance sensor

Large-scale Au nanodisk arrays are successfully fabricated using nanospherical-lens lithography (NLL). By incorporating both rotational oblique-angle deposition and oxygen plasma treatment, the improved NLL is capable of fabricating Au nanodisks with diameters as small as 75 nm that cover an area larger than 1 cm2. The fabricated nanodisk arrays are investigated as sensitive localized surface plasmon resonance (LSPR) sensors. The extinction spectra of the Au nanodisk arrays reveal a narrower LSPR peak when the diameter becomes smaller. The shape imperfection severely limits the minimum obtainable linewidth, especially when the nanodisk diameter is smaller than 200 nm. The imperfection is found to be improved by thermal annealing at high temperatures. The maximum theoretically predicted and experimentally obtained figure-of-merit for Au nanodisk arrays whose periodicities are 500 nm are around 15 and 9, respectively. Further optimization of the periodicity and thickness of the nanodisks will further improve their sensitivity and lead to more novel applications.

On the mechanism for nanoplasmonic enhancement of photon to electron conversion in nanoparticle sensitized hematite films

Hematite (Fe2O3) is a promising candidate for hydrogen production via water splitting despite the difference in the characteristic lengths for photon absorption and charge carrier transport. Metallic nanoparticles supporting localized surface plasmon resonances (LSPRs), i.e. collective, non-propagating oscillations of electrons excited by an external electric field, are well-suited to improve the optoelectronic properties of hematite, in particular for ultra-thin films. Several mechanisms have been proposed to explain the observed LSPR mediated performance enhancement. In this work, the improvement of incident photon-to-electron conversion efficiency (IPCE) of ultra-thin hematite photoanodes functionalized with Au nanodisks was investigated. The improvement in IPCE at wavelengths close to the bandgap in hematite was found to correlate well with the increase in optical extinction owing to the excitation of LSPR in the nanodisks. Finite-difference time-domain calculations of the near-field distribution around the nanodisks enabled us to elucidate the mechanism behind the IPCE enhancement and its variations with the position of the plasmonic resonance with respect to the bandgap of hematite. Both were attributed to an increased charge generation close to the hematite-electrolyte interface caused by the electric field enhancement in hematite. The results presented here are directly applicable to other semiconductors with similar properties to hematite and are expected to be helpful in future design of optimized photoanodes, where, for instance, functionalization with metallic nanoparticles is combined with material doping and nanostructuring.

Oscillatory Optical Response of an Amorphous Two-Dimensional Array of Gold Nanoparticles

The optical response of metallic nanoparticle arrays is dominated by localized surface plasmon excitations and is the sum of individual particle contributions modified by interparticle coupling that depends on specific array geometry. We demonstrate a so far unexplored distinct oscillatory behavior of the plasmon peak position, full width at half maximum, and extinction efficiency in large area amorphous arrays of Au nanodisks, which depend on the minimum particle center-to-center distance in the array. Amorphous arrays exhibit short-range order and are completely random at long distances. In our theoretical analysis we introduce a film of dipoles approach, within the framework of the coupled dipole approximation, which describes the array as an average particle surrounded by a continuum of dipoles with surface densities determined by the pair correlation function of the array.

Polarized Plasmonic Enhancement by Au Nanostructures Probed through Raman Scattering of Suspended Graphene

We characterize plasmonic enhancement in a hotspot between two Au nanodisks using Raman scattering of graphene. Single layer graphene is suspended across the dimer cavity and provides an ideal two-dimensional test material for the local near-field distribution. We detect a Raman enhancement of the order of 10(3) originating from the cavity. Spatially resolved Raman measurements reveal a near-field localization one order of magnitude smaller than the wavelength of the excitation, which can be turned off by rotating the polarization of the excitation. The suspended graphene is under tensile strain. The resulting phonon mode softening allows for a clear identification of the enhanced signal compared to unperturbed graphene.

Surface-Enhanced Raman Scattering of Ag–Au Nanodisk Heterodimers

We report the fabrication of Ag–Au nanodisk heterodimers based on the modified on-wire lithography method and study their surface-enhanced Raman scattering (SERS) properties experimentally and theoretically. The effects of disk thickness and gap size on SERS are explored. The SERS properties of the optimized heterodimers are compared with those of the respective homodimers and single nanodisks excited at 633 and 488 nm. We found that the enhancement factor (EF) of Ag–Au heterodimers is greater at 633 nm than that at 488 nm, while their EF is between those of the Ag–Ag and Au–Au homodimers. At 488 nm excitation, the calculation indicates that the plasmon energy can be transferred from the Ag disk to the Au disk.