Metal Nanoparticle Dimer Research Articles

Enhancement and quenching of plasmon-enhanced spectroscopy of single molecule confined in metallic nanoparticle dimers

We present a theoretical analysis of plasmon-enhanced fluorescence (PEF) and Raman scattering (PERS) spectroscopy of a single molecule confined in the laser-irradiated metallic nanoparticles (NPs) dimer, focusing on the origin of the spectral enhancement and quenching effects. The theoretical method of D-parameters has been used to calculate the dimer distance-dependent nonlocal dielectric effect in Ag and Au NPs. Meanwhile, other damping rates and electric field enhancements are quantitatively computed by finite element method. Moreover, PEF and PERS spectra of rhodamine 6G are obtained within the density-functional theory. Our calculated results show that the PERS mainly depend on the excitation and emission field enhancements, and thus it occurs at the narrower dimer gap due to the stronger localized plasmon coupling. The PEF is related to fluorescence rate caused by the competition between excitation electric field and quantum efficiency, and the increase of former may enhance the fluorescence intensity while the lower latter lead to reduce the intensity as decreasing the dimer distance. The contribution of nonlocal dielectric effect can significantly reduce the quantum efficiency at smaller distance so that it overcomes the excitation field enhancement, leading to the fluorescence quenching for Au NPs dimer. Furthermore, by optimizing the dimer distance and NPs size, the maximum PERS and PEF cross sections reach 10−14 and 10−15 under 2.45 eV laser excitation for Ag NPs dimer, and 10−18 for Au NPs. Our study finely explains the experiment results showed either fluorescence enhancement or quenching with the change of molecule-NPs distance, and better guidance for optimizing the experiments.

Multifunctional charge transfer plasmon resonance sensors

Abstract Charge transfer plasmon (CTP) modes arise when metallic nanoparticle dimers are connected by a conductive junction. This offers a unique opportunity to explore electron transport at optical frequencies as well as to attain narrow plasmon resonances that can be broadly modulated from visible to IR regimes, implying their potentials for applications in single-molecule electronics and sensing. This article showcases recent developments in theory and applications of charge transfer plasmon resonances (CTPRs) in various configurations of conductively linked plasmonic nanodimers. In particular, we give a due attention to exploiting ultratunable spectral features of charge transfer plasmon resonances for developing multifunctional plasmonic sensors for bulk, surface, gas and molecular sensing applications. We also discuss the implications of the charge and energy transfer between two plasmonic nanoparticles linked by sub-nanometer thick self-assembled monolayers for single-molecule conductance sensing and molecular electronics. In addition to the well-established plasmonic sensing schemes based on propagating and localized surface plasmon resonances, charge transfer plasmon resonance sensors may open up a new route in efforts to develop multifunctional sensing technologies.

SERS and RI sensing properties of heterogeneous dimers of Au and Si nanospheres

Heterogeneous dimers of Au and Si nanoparticles are expected to exhibit different plasmon properties from that of homogeneous noble metal nanoparticle dimers. It is crucial to unveil the potential applications in surface-enhanced Raman scattering (SERS) and refractive-index (RI) sensing of the prototype dimer of Au and Si nanospheres. The near-field coupling between the two components within the dimer is revealed to not affect the resonance energy of Si mode in the extinction spectra, but decrease that of Au mode. It also accounts for the plasmon ruler behavior of the fractional shifts of both its dipolar peak wavelength of localized surface plasmon resonance (LSPR) and corresponding RI sensitivity factor [Formula: see text], which provide another kind of substitute to estimate the gap distance in between components within the dimer as that of noble metal nanoparticle dimers. Additionally, by tracking the inflection point shift of the corresponding extinction spectra, its [Formula: see text] is revealed to improve 36% than that of traditional one. The maximum [Formula: see text] and SERS enhancement factor [Formula: see text] at 2 nm gap distance are demonstrated to reach 336 nm RIU[Formula: see text] and [Formula: see text], respectively. This work paves a new way for developing efficient SERS and RI sensing substrates by combining noble metal and dielectric nanoparticles.

Second harmonic generation from metal nanoparticle dimer: an analytical approach in dipole approximation

In this theoretical study, the problem of Second Harmonic Generation (SHG) in the interaction of laser beam with a Metallic Nanoparticle (MNP) dimer is considered. Using a classical electrodynamics approach, the nonlinear interaction of laser beam fields with Nanoparticles (NPs) is considered taking into account the dipole-dipole interparticle interactions. Analytical formulae are derived for the effect of dipole-dipole interaction on the Second Harmonic (SH) radiation power for two different polarizations of laser beam. It is found that the interaction causes the substantial enhancement of the SH radiation power while for the case when the laser beam field is parallel to the dimer axis, this enhancement is larger. Additionally, the dipole-dipole interaction of NPs leads to the redshift of the peak value with respect to the individual NP radiation. The resonance frequency displacement of the parallel case is more than that of the perpendicular one. The effect of particles size and interparticle separation on the SHG is studied.

Effect of Symmetry Breaking on Plasmonic Coupling in Nanoring Dimers

Breaking the morphological and compositional symmetries of metallic nanoparticle (NP) dimers provides a novel approach to modulate plasmon coupling between the NPs. In this study, we theoretically investigate the effect of symmetry breaking on the plasmonic coupling in morphologically asymmetric Au nanoring-nanodisk (NR-ND) dimer, compositionally heterogeneous Au-Ag nanoring dimer, and compositionally as well as morphologically asymmetric Au-Ag NR-ND heterodimers. It has been found that when the inner radii of symmetrical Au NR dimer is decreased, the scattering spectral intensity of coupled bonding mode and higher-order coupled bonding mode drastically decreases and increases, respectively. Besides, the effect of aspect ratio on the plasmon resonance modes is investigated. Furthermore, with fixed geometric parameters of one NR and by morphing the shape of the other NR into ND through inner radius, we show the generation and modification of a Fano resonance. As a result of introducing morphological and compositional asymmetry separately, single Fano-type spectral feature is observed in the scattering spectra. We demonstrate that the double Fano resonances can be easily achieved in compositionally and morphologically asymmetric heterodimer with double symmetry breaking. These dual modes are caused by the interference of dipolar bright mode (localized LSPR modes) of Ag NP with the quadrupole and hexapole modes (interband transitions) of the Au NP. Finally, at optimum asymmetric heterodimers, we discussed the effect of refractive index on the Fano-type resonance. These new and simple designs provide a novel insight to modulate optical response and the generation of higher-order Fano resonances, which have many potential applications such as in photonics and refractive index sensing.

Topology Optimization‐Based Inverse Design of Plasmonic Nanodimer with Maximum Near‐Field Enhancement

AbstractThe near‐field enhancement factor is one of the most significant parameters to evaluate the performance of plasmonic nanostructures. Numerous efforts have been made to maximize the enhancement factor through optimizing the size, shape, and spatial arrangement of metallic nanostructures with simple geometries, such as disk, triangle, and rod. This work implements topology optimization to inversely design a metallic nanoparticle dimer with the goal of optimizing the near‐field enhancement factor in its sub‐10 nm gap. By optimizing the material layout within a given design space, the topology optimization algorithm results in a plasmonic nanodimer of two heart‐shaped particles having both convex and concave features. Full‐wave electromagnetic analysis reveals that the largest near‐field enhancement in the heart‐shaped nanoparticle dimer is originated from the greatest concentration of surface charges at the nano‐heart apex. Inversely designed heart‐, bowtie‐, and disk‐shaped nanodimers are fabricated by using focused helium ion beam milling with a “sketch and peel” strategy, and their near‐field enhancement performances are characterized with nonlinear optical spectroscopies at the single‐particle level. Indeed, the heart‐shaped nanodimer exhibits much stronger signal intensities than the other two structures. The present work corroborates the validity and effectiveness of topology optimization‐based inverse design in achieving desired plasmonic functionalities.

Modified plasmonic response of dimer nanoantennas with nonlocal effects: From near-field enhancement to optical force

The metallic nanoparticle dimer is a fundamental model system for enhancing and tuning localized surface plasmon resonances. In the past, it had been found that the far- and near-field optical properties of dimer antennas can be regulated by many parameters (e.g., gap, size, orientation, materials, and surrounding medium). In recent years, the quantum mechanical effects such as nonlocal screening and electron tunneling have been achieved when the gap distance in a dimer approaches 1 nm and subnanometer. In this communication, both the near-field enhancement and optical force in dimer are fully investigated and compared between classical and nonlocal models. Compared with classical theory, we found that both the resonant wavelength and peak intensity have smaller changes in nonlocal model when geometrical or material parameters changes. Besides, the extent of parameter-induced spectral changes is slightly different between near-field enhancement and optical force. These results make possible the quantitative analysis of nonlocal effects in surface-enhanced spectroscopy, nanoantennas, refractive-index sensing, surface-enhanced optical force, and quantum plasmonics.

Systematics of plexciton formation in linear chains of quantum emitters and metal nanoparticles

We present systematic numerical calculations of plexcitonic properties of a linear chain comprised of quantum emitters - metal nanoparticle dimers interacting in the strong coupling regime. The calculations are based on a recently developed semi-analytical technique based on a multiple-scattering polaritonic-operator formalism in conjunction with an electromagnetic coupled dipole method. Based on this method, we calculate the two main features corresponding to the formation of plexcitonic chain modes: the primary plexcitonic resonance as well as the corresponding Rabi splitting associated with the generation of an avoided-crossing area. We apply an exponential model to the numerical results obtained via the above technique to describe the plexciton redshift induced with increasing chain length. We also identify an asymptotic plexciton frequency at a chain length of approximately 20 dimers. A model Hamiltonian is employed to assess the energy splitting defined by the avoided crossing area of the main plexciton resonance which is found to scale as where N is the number of dimers in the chain.

Optical characteristics of nanospace of silver nanowire on a silver mirror

AbstractLocalized surface plasmons in subnanometer gaps defined by metal nanoparticle dimer has recently attracted attention as a high‐sensitivity sensor with a small volume. We demonstrated multiple gap plasmons by a single silver nanowire with a spacer layer on silver film, that is, nanowire on mirror (NWoM) in the dark‐field with polarization‐dependent.

Dark Plasmon Modes in Symmetric Gold Nanoparticle Dimers Illuminated by Focused Cylindrical Vector Beams

The plasmon hybridization model of electromagnetic coupling between plasmonic nanoparticles predicts the formation of lower energy “bonding” and higher energy “antibonding” modes in analogy with the quantum mechanical description of chemical bonding. For a symmetric metallic nanoparticle dimer excited by linearly polarized light, the hybridization picture predicts that in-phase coupling of the dipole moments is optically allowed, creating bright “modes”, whereas the out-of-phase coupling is dark due to the cancellation of the oppositely oriented dipole moments (in the quasistatic approximation). These bright modes are electric dipolar in nature and readily couple to scalar (i.e., linearly or circularly polarized) beams of light. We show that focused cylindrical vector beams, specifically azimuthally and radially polarized beams, directly excite dark plasmon modes in symmetric gold nanoparticle (AuNP) dimers at normal incidence. We use single-particle spectroscopy and electrodynamics simulations to study the resonance scattering of AuNP dimers illuminated by azimuthally and radially polarized light. The electric field distributions of the focused azimuthal or radial beams are locally polarized perpendicular or parallel to the AuNP dimer axis, but with opposite directions at each particle. Therefore, the associated combinations of single-particle dipole moments are out-of-phase, and the excitation (resonance) is of so-called “dark modes”. In addition, multipole expansion of the fields associated with each scattering spectrum shows that the vector beam excitation involves driving multipolar, e.g., magnetic dipolar and electric quadrupolar, modes, and that they even dominate the scattering spectra (vs electric dipole). This work opens new opportunities for investigating dark plasmon modes in nanostructures, which are difficult to selectively excite by conventional polarized light.

Bonding state energy of metal nanoparticle dimer and its dependence on nanosphere size and interparticle separation

A study is conducted regarding the effects of particle size [Formula: see text] and interparticle separation [Formula: see text] on the electromagnetic (plasmon) coupling in a dimer of two identical metal nanospheres. The dimer states are modeled as the hybridized bonding and antibonding states of two isolated plasmon states, with the associated energies given in terms of the isolated plasmon energy ([Formula: see text], the coupling energy ([Formula: see text] and the overlap integral ([Formula: see text] of the constituent plasmonic fields. The resonance absorption energies of the isolated plasmon and the dimer in certain dielectric medium are calculated according to the Mie theory for incident light of parallel polarization along the dimer axis. The results are fitted with the bonding state energies of both Au and Ag nanosphere dimers for [Formula: see text] ranging within 10–20[Formula: see text]nm and x varied within [Formula: see text]–200[Formula: see text]nm in compliance with the restricted consideration of dipole absorption spectra. The excellent fits of the bonding state energies [Formula: see text] for the ranges of [Formula: see text] and [Formula: see text] variations are consistently achieved with [Formula: see text] around 0.99 by a single function of the form [Formula: see text] where [Formula: see text] and [Formula: see text] vary with the nanosphere materials and the surrounding media considered. This result suggests the possible relation of the best fitted functional form [Formula: see text] with the underlying physical mechanism.

Efficient Circular Polarizer Using a Two-Layer Nanoparticle Dimer Array with Designed Chirality

Using a two-layer metal nanoparticle dimer array, we numerically show that the array can act as an efficient circular polarizer. Linearly polarized incident light can be completely split into right and left circularly polarized light. The simulation results show that the efficiency of the polarizer depends on the size of the nanoparticles, the gap distance between the two particles in the dimers, the distance between the two layers, and the relative orientation between the dimer axes in the two layers. The periodic distance between two neighboring dimers in one layer is also a crucial factor affecting the splitting efficiency. The simulation results demonstrate a simple route of using plasmonic nanomaterials for designing efficient circular polarizers.

Density-matrix evaluation of the enhancement to resonant Raman scattering and fluorescence of molecules confined in metallic nanoparticle dimers

In the present work we study the surface-enhanced resonant Raman scattering (SERRS) and fluorescence (SEF) spectra of a general model molecule confined in metallic dimers consisting of Ag, Au and hybrid AuAg nanoparticles (NPs). The electromagnetic (EM) enhancement factors were simulated by the generalized Mie scatting method and the scattering cross section of the molecules were obtained by density-matrix calculations. The influence of the size of the NPs and the separation between the dimer on the Raman scattering and fluorescence were systematically studied and analyzed in detail. It was found that the SERRS mainly related to EM enhancement and the SEF depended on the competition between EM enhancement and quantum yield, both of which could be controlled by tuning the radius and separation of the metallic dimers. The optimal radius of the NPs for SERRS were found to be around 30 nm for AgNPs, 40 nm for AuNPs and 50 nm for hybrid AuAgNPs. The strongest Raman enhancement as predicted by the theoretical simulations were 6.2 × 1010, 1.5 × 107 and 5.2 × 108 for the three types of structures, respectively. These results could offer valuable information for the design of metallic substrates for surface enhanced Raman and fluorescence measurements.

Material influence on hot spot distribution in the nanoparticle heterodimer on film

The metal nanoparticle aggregated on film, as an effective plasma enhancement pathway, has been widely used in various surface plasmon-related fields. In this study, the hot spots on the metal nanoparticle dimer composed of different materials (AgAu, AgPd, and Ag Cu) on metal (Au) film were investigated with finite element method. Based on the results, the hot spot distribution affected by the material can be confirmed by the electric field distribution of the metal nanoparticle dimer on the film. The aggregation effects of Au and Ag nanoparticles in Au Ag dimer system are not significant. However, for the PdAg dimer system, the hot spot aggregation effect is slightly larger than that of the Pd nanoparticle under the Ag nanoparticle. Besides, the non-uniform hot spots would bring about the light focusing phenomenon that the light intensity under Ag nanoparticle is almost 100 times greater than that under Cu nanoparticle in AgCu dimer system. These results were further confirmed by the surface charge distribution, and analyzed based on the plasmonic hybridization theory. The data about the nanoparticle dimer on the dielectric (Si) film demonstrate the importance of induced image charges on the film surface in such a light focusing phenomenon. Our findings can enhance the understanding of the surface plasmon coupling in different materials, which may have great application prospects in surface plasmon-related fields, such as SERS, plasmonic enhanced solar cell, and plasmonic sensoring, etc.

On the origin of nonlocal damping in plasmonic monomers and dimers

The origin and importance of nonlocal damping is discussed through simulations with the generalized nonlocal optical response (GNOR) theory, in conjunction with time-dependent density functional theory (TDDFT) calculations and equivalent circuit modeling, for some of the most typical plasmonic architectures: metal–dielectric interfaces, metal–dielectric–metal gaps, spherical nanoparticles and nanoparticle dimers. It is shown that diffusive damping, as introduced by the convective–diffusive GNOR theory, describes well the enhanced losses and plasmon broadening predicted by ab initio calculations in few-nm particles or few-to-sub-nm gaps. Through the evaluation of a local effective dielectric function, it is shown that absorptive losses appear dominantly close to the metal surface, in agreement with TDDFT and the mechanism of Landau damping due to generation of electron–hole pairs near the interface. Diffusive nonlocal theories provide therefore an efficient means to tackle plasmon damping when electron tunneling can be safely disregarded, without the need to resort to more accurate, but time-consuming fully quantum-mechanical studies.

Plasmonic surface lattice resonances in arrays of metallic nanoparticle dimers

We investigate the optical response of square arrays of metallic nanoparticles where each lattice site is occupied by two particles, a dimer. In particular we examine the surface lattice resonances arising in these structures when the in-plane dipole moments associated with the plasmon modes of the nanoparticles couple together. The addition of a second particle to the basis leads to a more complex optical response, one that is anisotropic in the plane of the array. Extinction spectra are recorded at normal incidence for different orientations of the incident electric field. We compare our experimentally derived data with those from a coupled-dipole model. We show how the separation between the particles that comprise the dimer helps determine the overall response of the system.

Fabrication and surface enhanced Raman spectroscopy of single Au@SiO2 dimers linked by benzenedithiol

Metal nanoparticle dimers with controllable gap distance have attracted considerable attention because of their promising application in plasmonics. Generally, gaps with nanometer or subnanometer dimensions generate localized surface plasmon resonance (LSPR) coupling effect, thus contributing to a strong electromagnetic field for improving surface enhanced Raman scattering (SERS) effect. Here, we developed a facile approach to fabricate Au@SiO2 dimers through the steric hindrance effect, in which the SiO2 shell functioned as a block and a rigid dithiol molecule was employed as linker. The thickness of the SiO2 shell played a critical role in improving the yield of dimers. The dimerization efficiency increased significantly as the shell thickness decreased to ~1 nm. When 1,4‐benzenedithiol was used as linker molecule, the yield of dimers was ~30%. Few dimers were obtained when mecaptobenzonic acid was used as linker. A thicker shell is associated with a low yield of dimer, whereas a thinner shell resulted in the formation of multimers and linear structures. The low number of linker molecules on the exposed area of monodisperse single nanoparticles and the lack of LSPR coupling effect (‘hot spots’) resulted in the disappearance of SERS signals of the linkers. The estimated SERS enhancement factor was about eight fold because of the strong coupling effect in the gap of the dimer with the distance of the dithiol molecular length. From the above results, SERS combined with SEM could be developed into powerful tools for monitoring the formation of dimers and positioning of single dimers. It may aid the control of assembly of Au nanoparticles and in probing key issues about SERS enhancements. Copyright © 2015 John Wiley & Sons, Ltd.

Quantized Evolution of the Plasmonic Response in a Stretched Nanorod

Quantum aspects, such as electron tunneling between closely separated metallic nanoparticles, are crucial for understanding the plasmonic response of nanoscale systems. We explore quantum effects on the response of the conductively coupled metallic nanoparticle dimer. This is realized by stretching a nanorod, which leads to the formation of a narrowing atomic contact between the two nanorod ends. Based on first-principles time-dependent density-functional-theory calculations, we find a discontinuous evolution of the plasmonic response as the nanorod is stretched. This is especially pronounced for the intensity of the main charge-transfer plasmon mode. We show the correlation between the observed discontinuities and the discrete nature of the conduction channels supported by the formed atomic-sized junction.

Erratum: Decay of dark and bright plasmonic modes in a metallic nanoparticle dimer [Phys. Rev. B91, 035431 (2015)

Received 12 November 2015DOI:https://doi.org/10.1103/PhysRevB.92.199906©2015 American Physical Society

Radiation-enhanced optical antenna based on nonperiodic metallic nanoparticle dimer chain

With the aid of multi-sphere dyadic Green's function, we present a design of optical nanoantenna which is composed of a nonperiodic nanoparticle dimer chain. By breaking the periodicity of the dimer chain, the radiative emission of the dimer chain is significantly enhanced because the strong coupling which limits radiation enhancement is inhibited when the separations between dimers are reduced. Our work clearly shows the crucial role of nonperiodicity in the design of the Yagi-Uda nanoantenna.