Investigating the Effect of Ag and Au Nanostructures with Spherical and Rod Shapes on the Emission Wavelength of OLED

Noble metals, especially Ag and Au nanostructures, have unique and adjustable optical attributes in terms of surface plasmon resonance. In this research, the effect of Ag and Au nanoparticles with spherical and rod shapes on the light extraction efficiency and the FWHM of OLED structures was investigated using the finite difference time domain (FDTD) method. The simulation results displayed that by changing the shape and size of Ag and Au nanostructures, the emission wavelength can be adjusted, and the FWHM can be reduced. The presence of Ag and Au nanoparticles in the OLEDs showed a blue and red shift of the emission wavelength, respectively. Also, the Ag and Au nanorods caused a significant reduction in the FWHM and a shift to the longer wavelengths in the structures. The structures containing Ag nanorods showed the narrowest FWHM and longer emission wavelength than the other structures.

     

Localized Surface Plasmon Resonance and Refractive Index Sensitivity of Metal–Dielectric–Metal Multilayered Nanostructures

The localized surface plasmon resonances of multilayered nanostructures are studied using finite difference time domain simulations and plasmon hybridization method. Concentric metal–dielectric–metal (MDM) structure with metal core and nanoshell separated by a thin dielectric layer exhibits a strong coupling between the core and nanoshell plasmon resonance modes. The coupled resonance mode wavelengths show dependence on the dielectric layer thickness and composition of core and outer layer metal. The aluminum-based MDM structures show lower plasmon wavelength compared with Ag- and Au-based MDM nanostructures. The calculated refractive index sensitivity (RIS) factor is in the order Ag–Air–Ag>Au–Air–Au>Al–Air–Al for monometallic multilayered nanostructures. Bimetallic multilayered nanostructures support strong and tunable plasmon resonance wavelengths as well as high RIS factor of 510 nm/refractive index unit (RIU) and 470 nm/RIU for Al–Air–Au and Ag-Air-Au, respectively. The MDM structures not only exhibit higher index sensitivity but also cover a wide ultraviolet–near-infrared wavelengths, making these structures very promising for index sensing, biomolecule sensing, and surface-enhanced Raman spectroscopy.     

Numerical Simulation of Broadband Scattering by Coated and Noncoated Metal Nanostructures Using Discrete Dipole Approximation Method

We suggest numerical method to study the optical response of metal nanostructures. The analysis of optical properties such as scattering and absorption by coated and noncoated nanogeometry has been done using discrete dipole approximation (DDA) method. The core-shell nanogeometry supports surface plasmon resonances, which are highly tunable from 400 to 1100 nm. The tunability of surface plasmon resonance (SPR) highly depends on the structural anisotropy and chosen core-shell material. Further, we have observed that aspect ratio is one of the key parameter to decide the nature and position of the plasmonic peaks and magnitude of optical cross section. We have also shown that coated nanospheroid is a more appropriate geometry as compared to coated nanosphere and noncoated nanospheroid in terms of wide tunability of surface plasmon resonance. The wide tunability in SPR is observed for the effective radii 90 nm core-shell (Au@SiO₂) nanospheroid with aspect ratio 0.1.     

Biosynthesis of fluorescent gold nanoparticles using an edible freshwater red alga,Lemanea fluviatilis (L.) C.Ag. and antioxidant activity of biomatrix loaded nanoparticles

Biosynthesis of gold nanoparticles has been accomplished via reduction of an aqueous chloroauric acid solution with the dried biomass of an edible freshwater epilithic red alga, Lemanea fluviatilis (L.) C.Ag., as both reductant and stabilizer. The synthesized nanoparticles were characterized by UV–visible, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), and dynamic light scattering (DLS) studies. The UV–visible spectrum of the synthesized gold nanoparticles showed the surface plasmon resonance (SPR) at around 530 nm. The powder XRD pattern furnished evidence for the formation of face-centered cubic structure of gold having average crystallite size 5.9 nm. The TEM images showed the nanoparticles to be polydispersed, nearly spherical in shape and have sizes in the range 5–15 nm. The photoluminescence spectrum of the gold nanoparticles excited at 300 nm showed blue emission at around 440 nm. Gold nanoparticles loaded within the biomatrix studied using a modified 2,2-diphenyl-1-picrylhydrazyl (DPPH) method exhibited pronounced antioxidant activity.     

Gamma–Radiation-Assisted Synthesis of Luminescent ZnO/Ag Heterostructure Core–Shell Nanocomposites

There is increasing interest in tuning the physical properties of semiconductor nanostructures using metal nanoparticles. In this work, ZnO nanosphere covered with Ag nanoparticles were synthesized using gamma–radiation-assisted method. The amount of deposited Ag nanoparticles is controlled by changing irradiation dose in the range of 30–100 kGy in order to tune the semiconductor–metal interaction. The successful deposition of Ag on the ZnO nanoparticles is examined by analyzing the morphology, microstructure, optical, and magnetic properties of ZnO/Ag nanoparticles through field emission scanning electron (FESEM), microscopy X-ray diffraction spectra, UV-visible absorption, photoluminescence measurement, and vibrating sample magnetometer. FESEM and elemental mapping results confirmed that Ag nanoparticles have been concentrated at the surface of spherical ZnO particles. Moreover, formation of pure metallic Ag nanoparticles has been confirmed by XRD analysis. UV-visible absorption spectra of obtained ZnO/Ag showed two combined peaks, a weak peak at the shoulder around 360 nm corresponds to ZnO and a sharp absorption at 420 nm refers to spherical Ag nanoparticles. Obtained results from photoluminescence revealed that the near-band-edge emission and defect-related visible emission bands of ZnO could be enhanced dramatically at the same time by deposition of Ag nanoparticles, which was ascribed to localized surface plasmon–exciton coupling and surface plasmon scattering. Controlling the semiconductor and metal coupling effect is interesting because of its application in highly efficient optoelectronic devices and biosensor.     

Optimized Synthesis of PEG-Encapsulated Gold Nanorods for Improved Stability and Its Application in OCT Imaging with Enhanced Contrast

Gold nanorods (GNRs) are synthesized with a surfactant template, which often poses toxicity issues for biomedical applications. In addition, blue shift of longitudinal surface plasmon resonance (LSPR) peak of GNR is an inherent problem that needs to be addressed for time-course studies. In this work, we resolve these issues by optimizing the encapsulation of GNRs with polyethylene glycol (PEG) where biocompatibility is improved by ～20 % and blue shift over a period of 8 days is reduced from 20 nm in the case of CTAB-GNR to 2 nm for PEG-encapsulated GNR. The encapsulated GNRs were then bioconjugated for targeted dark-field imaging of cancer cells. As an application, we also demonstrate the contrast-enhancing capability of GNRs in optical coherence tomography (OCT) imaging of tumor xenograft where the LSPR closely matches the OCT excitation wavelength. Our study proves that incorporating GNRs enhances the contrast of tumor tissue interfaces along with a considerable broadening in OCT depth profile by six times.     

Multiple Surface Plasmon Resonances in a Metal–Insulator–Metal Structure Assembled Via Silver–Copper Nanoparticle Arrays

The Ag–Cu nanoparticle arrays, prepared using the electrochemical deposition method, were assembled into the metal–insulator–metal (MIM) structure with polyvinyl alcohol acting as insulating layer, the transmission spectrum of the MIM structure was observed to support the multiple surface plasmon resonances in the wavelength range 1,000 to 2,600 nm. The multiple peaks were formed due to the superposition and coupling of the surface plasmon resonance of nanoparticles with various sizes in the metal layers. The newly found MIM structure in which multiple resonances exist has a potential application in multiband-pass filters and optical magnetic metamaterials at the resonance wavelength.     

The Tunable and Well-Controlled Surface Plasmon Resonances of Au Hollow Nanostructures by a Chemical Route

Au plasmonic hollow spherical nanostructures were synthesized by electrochemical reduction (GRR, the Galvanic Replacement Reaction) using Ag nanoparticles as templates. From UV-visible absorption spectroscopy, it was found that the surface plasmon resonance (SPR) of gold hollow spherical nanostructures first showed red shift and then blue shift. However, further addition of gold precursor (HAuCl₄) resulted into a red shift of SPR peak. The morphological changes from Ag nanoparticles to Au hollow nanostructures were assessed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX)analysis. The Mie Scattering theory based simulations of SPR of Au hollow nanostructures were performed which are in good agreement with the experimental observations. Based on the experimental observations and theoretical calculations, a complete growth mechanism for Au hollow nanostructures is proposed.     

Numerical Simulation of Extinction Spectra of Plasmonically Coupled Nanospheres Using Discrete Dipole Approximation: Influence of Compositional Asymmetry

We demonstrate plasmon coupling phenomenon between equivalent (homodimer) and non-equivalent (heterodimer) spherical shape noble metal nanoparticle (Ag, Au and Al). A systematic comparison of surface plasmon resonance (SPR) and extinction properties of various configurations (monomer, homodimer and heterodimer) has been investigated to observe the effect of compositional asymmetry. Numerical simulation has been done by using discrete dipole approximation method to study the optical properties of plasmonically coupled metal nanoparticles (MNPs). Plasmon coupling between similar nanoparticles allows only higher wavelength bonding plasmon mode while both the plasmon modes lower wavelength antibonding mode as well as higher wavelength bonding mode in the case of heterodimer. Au monomer of radius 50 nm shows resonance peak at 518 nm while plasmon coupling between Au-Au homodimer results in a spectral red shift around 609 nm. Au-Ag plasmonic heterodimer (radius 50 nm) reveals two resonant modes corresponding to higher energy antibonding mode (422 nm) as well as lower energy bonding mode (533 nm). Further, we have shown that interparticle edge-to-edge separation is the most significant parameter affecting the surface plasmon resonances of MNPs. As the inter particle separation decreases, resonance wavelength shows red spectral shift which is maximum for the touching condition. It is shown that plasmon coupling is a reliable strategy to tune the SPR.

     

Optimal Dimensions of Gold Nanorod for Plasmonic Nanosensors

Localized surface plasmon resonance (LSPR) for longitudinal mode of gold nanorod is simulated by using Gans theory. The parameters like surface scattering, radiation damping, and dynamic depolarization of radiation across the surface of nanorod affecting response of free electrons towards optical excitation are considered. Simulation results show that refractive index sensitivity linearly rises with size and aspect ratio, whereas this leads to the broadening of resonant line width also. Therefore, to optimize the size of nanorod, figure of merit (FOM) is calculated and observed that optimized width is 15 nm for an aspect ratio of 2, whereas it is 12 nm for aspect ratios 3 and 4. Further, optimization by using newly modified figure of merit (MFOM) shows that optimized width is 39 nm for aspect ratio of 2 and 24 nm for 3 and 4 aspect ratios. It is also found that at aspect ratio 2, both FOM and MFOM are higher than the aspect ratios 3 and 4. The quality factor calculation for LSPR response of nanorod explains its dependence with aspect ratio and optimized dimensions.     

Blue-Shifted SPR of Au Nanoparticles with Ordering of Carbon by Dense Ionization and Thermal Treatment

Thin films of carbon-containing Au nanoparticles (NPs), prepared by the co-sputtering using a neutral Ar atom beam, were irradiated by 120 MeV Ag ions and also annealed, separately, at increasing temperatures in inert atmosphere. The surface plasmon resonance (SPR) band of the nanocomposite film was observed to be blue shifted (～50 nm) in both cases, with increasing fluence and temperature. The structural changes of Au NPs embedded in amorphous carbon matrix were investigated using X-ray diffraction and transmission electron microscopy. A growth of Au NPs was observed with increasing fluence and also with increasing temperature. A percolation of Au NPs was observed at 500 °C. A growth of Au NPs with ion irradiation is explained in the framework of a thermal spike model. Raman spectroscopy revealed the ordering of a-C thin films with increasing fluence and temperature, which is ascribed to a change of refractive index and the blue shift of the SPR band.     

Spectral Investigations on the Fluorescence Quenching of 1,4-dihydroxy-2,3-dimethylanthracene-9,10-dione by Plasmonic Silver Nanoparticles

Silver nanoparticles (Ag NPs) of different sizes have been prepared by Lee and Meisel’s method using trisodium citrate as reducing agent under ultra sonication. Optical absorption and fluorescence emission techniques were employed to investigate the interaction of 1,4-dihydroxy-2,3-dimethyl anthracene-9,10-dione (DHDMAD) with silver nanoparticles. In fluorescence spectroscopic study, we used the DHDMAD and Ag NPs as component molecules for construction of Förster Resonance Energy Transfer (FRET), whereas DHDMAD serve as donor and Ag NPs as acceptor. The surface plasmon resonance (SPR) peak of the prepared silver colloidal solution was observed from 419 nm to 437 nm. The synthesized silver nanoparticles at different heating time intervals were spherical in shape about the size of 25 nm and 55 nm. The fluorescence interaction between silver nanoparticles and DHDMAD confirms the FRET mechanism. According to Förster theory, the distance between silver nanoparticles and DHDMAD and the critical energy transfer distance were calculated and it is increase with heating time.     

Sensitive and Robust Colorimetric Sensing of Sulfide Anion by Plasmonic Nanosensors Based on Quick Crystal Growth

A highly sensitive and selective method for colorimetric sensing of sulfide anions in aqueous solutions is illustrated. The sensing mechanism is based on quick crystallization from Ag to Ag₂S in the presence of sulfide anions which alter the dielectric properties of the Au/Ag core/shell nanorods. The longitudinal surface plasmon resonance peak of the Au/Ag nanorods at about 686 nm undergoes a redshift and the color of the nanorod solution also changes from light green to purple. Sulfide ions at a concentration of 4.0 μM (1.3 ppb) can be detected visually and a sensitivity of 0.5 μM (167 ppt) is achieved by Vis–near-infrared spectrophotometry. Compared to other plasmonic sensors, our Au/Ag nanorod probe does not require surface modification while exhibiting high stability and robustness under different pH conditions. This simple and cost-effective sensing platform provides a rapid and convenient detection for sulfide anions at concentrations far below the hazardous limit in aqueous media.     

Optical and Nonlinear Optical Limiting Properties of AgNi Alloy Nanostructures

Silver-nickel alloy nanoparticles with varying size were synthesized by reducing the metal precursors chemically using a single-step solution-based synthesis route. The structural, optical, and nonlinear optical properties of the prepared samples were investigated. The synthesized samples having highly agglomerated, interconnected nature and found to exhibit dipole and multipole surface plasmon resonance related optical absorption bands. Nonlinear optical and optical limiting properties were investigated using a single beam open aperture z-scan technique with the use of 532 nm, 5-ns laser pulses. The nonlinearity observed was found to have contributions from saturable absorption (SA) and excited state absorption (ESA) related to free carriers. The effective nonlinear optical absorption was enhanced in AgNi alloy compared to pure Ag nanostructures.     

Rational Selection of Nanorod Plasmons: Material, Size, and Shape Dependence Mechanism for Optical Sensors

The localized surface plasmon resonance dependence on surrounding medium refractive index of Ag, Al, Au, and Cu nanoparticles is examined by electrodynamic approach. The refractive index sensitivity and sensing figure of merit (FOM) dependence of selected metal nanoparticles with similar geometry shows that although, sensing relevant parameters are shape (i.e., aspect ratio), and material dependent below the width 20 nm, but above this size these parameters are material independent under similar geometrical conditions. We have concluded that at optimum size, however, Al shows much higher refractive index sensitivity (RIS) in comparison to Au, Cu, and Ag, but FOM is higher for Ag in comparison to other metals. The observed sensing behavior is expected due to parameters like surface scattering, dynamic depolarization, radiation damping, and interband transitions, which may influence the nanorod plasmons.     

Modulation of Visible and Near-Infrared Surface Plasmon Resonance of Au Nanoparticles Based on Highly Doped Graphene

We study an active modulation of surface plasmon resonance (SPR) of Au nanoparticles based on highly doped graphene in visible and near-infrared regions. We find that compared to the traditional metal SPR, the SPR of Au nanoparticles based on graphene causes a remarkable blue shift. The field intensity in the gap is redistributed to standing wave. The field intensity of standing wave is about one order of magnitude higher than the traditional model. Moreover, the SPR of Au nanoparticles can be actively modulated by varying the graphene Fermi energy. We find the maximum modulation of field intensity of absorption spectra is more than 21.6 % at λ?=?822?nm and the amount of blue shift is 17.4 nm, which is about 2.14 % of the initial wavelength λ ₀?=?813.4?nm, with increasing monolayer graphene Fermi energy from 1.0 to 1.5 ev. We find that the SPR sensitivity to the refractive index n of the environment is about 642 nm per refractive index unit (RIU). The SPR wavelengths have a big blue shift, which is about 33 nm, with increasing number of graphene layers from 1 to 3, and some shoulders on the absorption spectra are observed in the models with multilayer graphene. Finally, we study the Au nanorod array based on monolayer graphene. We find that the blue shift caused by the graphene increases from 14 to 24 nm, with increasing gap g _y from 10 to 20 nm. Then, it decreases from 24 to 14 nm, with increasing gap g _y from 20 to 50 nm. This study provides a new way for actively modulating the optical and optoelectronic devices.     

Enhancing LSPR Sensitivity of Au Gratings through Graphene Coupling to Au Film

A particular interesting plasmonic system is that of metallic nanostructures interacting with metal films. As the localized surface plasmon resonance (LSPR) behavior of gold nanostructures (Au NPs) on the top of a gold thin film is exquisitely sensitive to the spacer distance of the film-Au NPs, we investigate in the present work the influence of a few-layered graphene spacer on the LSPR behavior of the NPs. The idea is to evidence the role of few-layered graphene as one of the thinnest possible spacer. We first show that the coupling to the Au film induces a strong lowering at around 507 nm and sharpening of the main LSPR of the Au NPs. Moreover, a blue shift in the main LSP resonance of about 13 nm is observed in the presence of a few-layered graphene spacer when compared to the case where gold nanostructures are directly linked to a gold thin film. Numerical simulations suggest that this LSP mode is dipolar and that the hot spots of the electric field are pushed to the top corners of the NPs, which makes it very sensitive to surrounding medium optical index changes and thus appealing for sensing applications. A figure of merit of such a system (gold/graphene/Au NPs) is 2.8, as compared to 2.1 for gold/Au NPs. This represents a 33 % gain in sensitivity and opens-up new sensing strategies.     

Fabrication of Monodispersed Silver Nanoparticles and their Collective Sharp Plasmonic Response

Monodispersed silver (Ag) nanoparticles (NPs) were obtained by applying an electric field on unipolar-charged Ag NPs fabricated using a heterogeneous condensation technique in gas media. Well defined and charged Ag NPs were separated based on their sizes and were collected on quartz substrates. Thin films consisting of monodispersed Ag NPs with size ranging from 35 to 120 nm were prepared by varying an applied electric field during the fabrication process. Scanning electron microscope results showed that the samples have uniform size distribution. Coherent oscillations of conduction band electrons in gas medium induced by electromagnetic field and coupling of all similar plasmon resonances due to uniform Ag NPs size produced unique and interesting optical properties. Narrow extinction widths (～41 to ～69 nm) were observed compared to the width of polydispersed Ag sample. The ability to prepare samples in gas media and tune the plasmon resonance by merely varying an electric field during fabrication makes the method simple, fast, and highly economical.     

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n ₂?=?2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ?=?2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.     

A Polarization Filter Based on Photonic Crystal Fiber with Symmetry Around Gold-Coated Holes

We propose a modified design for a photonic crystal fiber (PCF) polarization filter based on surface plasmon resonance (SPR). The air holes are arrayed in diamond lattices, and the diameter of the holes around the gold-coated holes are different that can separate the refractive index of the x-polarization and y-polarization second order surface plasmon polariton (SPP) modes. The influences of structural parameters of the photonic crystal fiber (PCF) on the filter characteristics are studied using the finite element method (FEM). Great changes have taken place in the results of numerical simulation by changing the thickness of the gold film and air hole diameter. Simulation results show that the resonance wavelength is communication wavelength 1550 mm, the loss of the y-polarization mode is 43,126.7 dB/m. When the length of the fiber is 500 μm, extinction ratio is more than 20 dB at the communication wavelength, and bandwidth achieve to 190 nm. It is an important property of PCF polarization filter in production.