Metal Nanoparticle Chains Research Articles

Full Analytical Model for Obtaining Surface Plasmon Resonance Modes of Metal Nanoparticle Structures Embedded in Layered Media

This work addresses the need for a fully-retarded theoretical model for surface plasmons on metal nanoparticle chains and arrays embedded in a multilayered medium. The proposed method uses dyadic layered medium Green's functions not only to obtain the electric field created by an oscillating electric dipole but also to modify the polarizability of nanoparticles in a multilayered medium appropriately. Theoretically calculated resonance frequencies show a very good agreement with the experimental results found in the literature. Theoretical results suggest that surface plasmon propagation lengths of 1 micrometer are possible using silver or gold nanoparticles embedded in a multilayered medium.

Tunable localization and oscillation of coupled plasmon waves in graded plasmonic chains

The localization (confinement) of coupled plasmon modes, named as gradons, has been studied in metal nanoparticle chains immersed in a graded dielectric host. We exploited the time evolution of various initial wavepackets formed by the linear combination of the coupled modes. We found an important interplay between the localization of plasmonic gradons and the oscillation in such graded plasmonic chains. Unlike in optical superlattices, gradient cannot always lead to Bloch oscillations, which can only occur for wavepackets consisting of particular types of gradons. Moreover, the wavepackets undergo different forms of oscillations. The correspondence can be applied to design a variety of optical devices by steering among various oscillations.

Metal nanoparticle chains embedded in TiO 2 nanotubes prepared by one-step electrodeposition

Arrays of Ni nanoparticle chains embedded in TiO 2 nanotubes (TiO 2/Ni) were fabricated by one-step electrodeposition in anodic aluminum oxide membranes. The formation of spacing Ni nanoparticles or continuous nanorods in the TiO 2 nanotubes depends on the deposition potential, the potential waveform, and the pH value in the electrolyte. The growth mechanism is attributed to the generation of H 2 bubbles and their periodical evolution outside of the TiO 2 nanotubes.

On the Surface Plasmon Resonance Modes of Metal Nanoparticle Chains and Arrays

A method for estimating the surface plasmon resonance modes of metal nanoparticle chains and arrays within a multilayered medium is proposed. In this fully retarded point-dipole method, an inhomogeneous background is replaced with a homogeneous one, based on an effective refractive index approximation. The proposed method includes the effects of retardation, radiative damping, and dynamic depolarization due to the finite size of the nanoparticles. The use of diagonal terms of dyadic Green’s functions and different polarizability coefficients along the semi-axes of ellipsoidal nanoparticles provides a complete set of both longitudinal and transverse resonance modes. Numerical results are compared with experimental results found in the literature.

Narrowband optical interactions in a plasmonic nanoparticle chain coupled to a metallic film

We study the coupling of localized surface plasmon (LSP) and surface-plasmon polariton (SPP) modes in a system composed of a metallic nanoparticle chain separated from a thin metallic film by a dielectric spacer. The thickness of such a spacer influences the level of interaction between LSP and SPP modes and controls the electromagnetic enhancement in this system. An enhancement with narrow resonances can be observed for appropriate parameters. The high-resonance quality factor and tunability of this system make it a very promising candidate for biosensing and surface-enhanced spectroscopy applications.

Optical waves on nanoparticle chains coupled with surfaces

Linear chains of metal nanoparticles coupled with dielectric surfaces support a variety of optical phenomena including traveling and leaky waves of several types. We investigate the chain-surface interactions and show that traveling waves can remain bound to the chain, radiate into surface wave beams, or radiate into space and surface wave beams. Radiation into surface waves may be exploited to create a leaky surface wave antenna with potential applications to surface wave microscopy.

Role of aperiodic order in the spectral, localization, and scaling properties of plasmon modes for the design of nanoparticle arrays

In this paper, we propose a general and efficient method to analyze the dipolar modes of aperiodic arrays of metal nanoparticles with ellipsoidal shapes and their electromagnetic coupling with external fields. We reduce the study of the spectral and localization properties of dipolar modes to the understanding of the spectral properties of an operator L expressing the electric field along the chain in terms of the electric-dipole moments within the electric quasistatic approximation. We show that, in general, the spectral properties of the L operator are at the origin of the formation of pseudoband gaps and localized modes in aperiodic chains. These modal properties are therefore uniquely determined by the aperiodic geometry of the arrays for a given shape of the nanoparticles. The proposed method, which can be easily extended in order to incorporate retardation effects and higher multipolar orders, explains in very clear terms the role of aperiodicity in the particle arrangement, the effect of particle shapes, incoming field polarization, material dispersion, and optical losses. Our method is applied to the simple case of linear arrays generated according to the Fibonacci sequence, which is the chief example of deterministic quasiperiodic order. The conditions for the resonant excitation of dipolar modes in Fibonacci chains are systematically investigated. In particular, we study the scaling of localized dipolar modes, the enhancement of near fields, and the formation of Fibonacci pseudodispersion diagrams for chains with different interparticle separations and particle numbers. Far-field scattering cross sections are also discussed in detail. All results are compared with the well-known case of periodic linear chains of metal nanoparticles, which can be derived as a special application of our general model. Our theory enables the quantitative and predictive understanding of band-gap positions, field enhancement, scattering, and localization properties of aperiodic arrays of resonant nanoparticles in terms of their geometry. This is central to the design of metallic resonant arrays that, when excited by an external electromagnetic wave, manifest strongly localized and enhanced near fields.

Microplasma Array Serving as Photonic Crystals and Plasmon Chains

An array of microplasmas with sizes ranging from a millimeter to a micrometer, has potential for novel and promising electromagnetic-wave media, especially when the wave frequency is below the electron plasma frequency. Photonic crystals or band-gap materials composed of microplasmas have unique properties arising from their loss term, and they can become band-pass filters instead of the band-stop filters usually observed in photonic crystals of dielectrics. Such behavior is well understood using the dispersion relation in a three-dimensional space of frequency and complex wavenumber with real and imaginary parts. Another functional array is a simple one-dimensional (1D) array; it can conduct microwaves for a wide frequency range below the electron plasma frequency. The propagating modes are similar to the coupling of localized surface plasmon polaritons observed along a metallic nanoparticle chain in the photon range; however a 1D microplasma array features differ from those of a metallic sphere array, leading to a dynamic wide-band waveguide.

Effective Refractive Index Approximation and Surface Plasmon Resonance Modes of Metal Nanoparticle Chains and Arrays

The discrete-dipole approximation method is implemented with an efiective re- fractive index approximation to obtain surface plasmon resonance modes of metal nanoparticle chains and arrays in a multilayered medium. This fully retarded theoretical model includes the efiects of retardation, radiative damping and dynamic depolarization due to the flnite size of the nanoparticles. The use of diagonal terms of dyadic Green's functions and difierent polarizability coe-cients along the semi-axes of ellipsoidal nanoparticles provide the complete set of resonance modes. Numerical results show a reasonable agreement with experiment results.

General Assembly Method for Linear Metal Nanoparticle Chains Embedded in Nanotubes

We demonstrate a flexible assembly method for producing linear metal nanoparticle chains embedded in nanotubes. The chain formation is based on the Rayleigh instability after annealing metal nanowires confined in nanotubes. Beginning with metal nanowires from arbitrary synthesis methods, atomic layer deposition (ALD) was applied to coat the wires first with a sacrificial layer then with a shell layer. Subsequently, the sacrificial layer was removed leading to confined wires in nanotubes with a free volume. Finally, embedded nanoparticle chains were produced inducing the Rayleigh instability by annealing the confined nanowires. This method is quite general not only for different metals but also for different shell materials. We are able to tune the particle spacing and diameter, the shape of the nanochains, the tube diameter and the shell thickness by ALD significantly.

Tailor-Made Inorganic Nanopeapods: Structural Design of Linear Noble Metal Nanoparticle Chains

Linear noble-metal nanoparticle (NP) chains have been demonstrated both theoretically and experimentally to be promising candidates for applications in one-dimensional nano-optical devices (e.g. plasmonic waveguides, plasmonic printing). The ability of metal NP chains to transport electromagnetic energy below the diffraction limit has major advantages for scaling down the size of optical devices and components to the nanometer scale. Precise control of particle size, shape and separation in metal NP chains is an important issue for constructing nano-optical devices. Conventional electron-beam lithographic techniques and scanning-probe manipulation have enabled excellent control over the size and position of metal NPs but are time-consuming and costly. Self-assembly routes enable metal NPs to be incorporated into preformed grooves but can control neither the separation between NPs nor their densities. Other approaches to the fabrication of metal NP chains, including wet-chemistry etching and exploitation of the Rayleigh instability of metal nanowires (NWs), suffer from low throughput and limited controllability. Herein, we show a facile and controllable route to the fabrication of linear noble-metal NP chains, in which the size and the separation of optically interesting NPs can be controlled easily. As an example, Pt@CoAl2O4 inorganic peapod nanostructures, which consist of well-defined Pt NPs encapsulated in continuous CoAl2O4 nanoshells, were realized by electrodeposition of cobalt/platinum multilayered (ML) NWs into nanoporous anodic aluminum oxide (AAO) membranes and subsequent solid-state reaction at high temperature. Fabrication of Pt@CoAl2O4 inorganic nanopeapods is schematically illustrated in Figure 1a. First, Co/Pt ML NWs were electrodeposited into the nanoporous AAO template by pulsed potential electrodeposition. As-prepared Co/Pt ML NWs with alternately distributed Co and Pt segments served as the precursors and the backbones for subsequent generation of Pt@CoAl2O4 inorganic nanopeapods (see Figure S1 in the Supporting Information). The size (diameter, DPt) and separation (center-to-center distance, Dc-c) of Pt “peas” in peapod nanostructures were also defined in this step by the lengths of Pt (LPt) and Co segments (LCo), respectively. After electrodeposition, the Co/Pt ML NWs/AAO composite membrane was annealed at 700 8C in an ambient atmosphere for 1–5 h. During heat treatment, Co segments reacted with alumina to form continuous CoAl2O4 nanoshells, that is, the “pods”, while most Pt segments agglomerated into a spherical shape to minimize their surface energy, forming the peas. Thus, the peapod nanostructures were obtained. As-prepared Pt@CoAl2O4 inorganic nanopeapods can either be kept in the porous AAO template or be easily released from the template by selectively removing the surrounding alumina (see Figure S2a in the Supporting Information). Recently, exploitation of the Rayleigh instability of metal NWs has been considered as an effective “bottom-up” approach to create linear metal NP chains. However, this method only allows very limited control of the separation between metal NPs. Its controlling capability is strictly confined by the maximum perturbation wavelength lmax, which is directly proportional to the radius R0 of the metal NWs used, as shown in Figure 1c. The proportionality Figure 1. Fabrication of peapod nanostructures. a,b) Fabrication of Pt@CoAl2O4 inorganic nanopeapods (Pt nanoparticle chains encapsulated in CoAl2O4 nanoshells) by template-based pulsed electrodeposition and high-temperature solid-state reaction. The separation between Pt nanoparticles (Dc-c) is proportional to the pulse duration for Co electrodeposition, tCo, and can be changed at will. c) Metal nanoparticle chains fabricated on the basis of the Rayleigh instability of metal nanowires.

Heat transport through plasmonic interactions in closely spaced metallic nanoparticle chains

We report a numerical investigation on the heat transfer through one dimensional arrays of metallic nanoparticles closely spaced in a host material. Our simulations show that the multipolar interactions play a crucial role in the heat transport via collective plasmons. Calculations of the plasmonic thermal conductance and of the thermal conductivity in ballistic and diffusive regime, respectively have been carried out. (a) Using the Landauer-Buttiker formalism we have found that, when the host material dielectric constant takes positive values, the multipolar interactions drastically enhance by several order of magnitude the ballistic thermal conductance of collective plasmons compared with that of a classical dipolar chain. On the contrary, when the host material dielectric constant takes negative values, we have demonstrated the existence of non-ballistic multipolar modes which annihilate the heat transfer through the chains. (b) Using the kinetic theory we have also examined the thermal behavior of chains in the diffusion approximation. We have shown that the plasmonic thermal conductivity of metallic nanoparticle chains can reach 1% of the bulk metal thermal conductivity . This result could explain the anomalously high thermal conductivity observed in many colloidal suspensions, the so called nanofluids.

Rayleigh-Instability-Induced Metal Nanoparticle Chains Encapsulated in Nanotubes Produced by Atomic Layer Deposition

Cu nanoparticle chains encapsulated in Al2O3 nanotubes were successfully generated in a controlled manner by reduction of CuO nanowires embedded in Al2O3 at a sufficiently high temperature. The Al2O3 coating was deposited by atomic layer deposition (ALD). The particles mainly show a rodlike shape and are regularly distributed. The particle diameters and chain lengths corresponding to the inner diameters and lengths of the tubes, respectively, are controlled by the size of the CuO nanowire templates. Rayleigh instability, assisted by the uniform volume shrinkage created by the reduction of oxide to metal, is proposed to induce the formation of the nanochains. This method may potentially be extended to the synthesis of nanochains of other metals by reducing corresponding oxide nanowires embedded in ALD shells.

Heat transport through plasmonic interactions in closely spaced metallic nanoparticle chains

We report a numerical investigation on the near-field heat transfer through one-dimensional arrays of metallic nanoparticles closely spaced in a host material. Using the Landauer-Buttiker formalism we study the thermal behavior of chains in ballistic regime. It is shown that that the heat transport through plasmonic interactions can be several order of magnitude more efficient than in classical dipolar chains. This result could explain the anomalously high thermal conductivity observed in many nanoparticles based composite media such as nanofluids and could find broad practical applications in nanoscale thermal management.

A computational study of the optical response of strongly coupled metal nanoparticle chains

We study the optical response of strongly coupled metal nanoparticle chains using rigorous multiple scattering calculations. The collective resonant frequency of silver nanosphere chains and the coupling between chains are considered. The coupling between silver nanoparticle chains are understood by the transmission and reflection calculations of 2D periodic arrays of nanospheres. The results are in agreement with recent experiments. The splitting of plasmon resonance modes for different polarizations of the incident light are explored. Our results show that the transverse mode resonant wavelength is very sensitive to the inter-chain distance. Results on the effect of disorder are also presented.

Dipole-dipole interaction between molecules mediated by a chain of silver nanoparticles

We study the dipole-dipole coupling between two fluorescent molecules in the presence of a chain of metallic nanoparticles. We analyze the spectral behavior of the coupling strength and its dependence on the molecular orientation. Our results show that for certain resonant wavelengths the coupling strength between the molecules is greatly enhanced and is strongly polarization sensitive. We also demonstrate how metallic nanoparticles can be utilized in implementing a polarization-sensitive coupler.

Optical wave properties of nano-particle chains coupled with a metal surface

Optical phenomena supported by ordered and disordered chains of metal nano-particles on a metal surface are investigated by considering a particular example of gold nano-bumps on a gold surface. The TWs supported by these structures are analyzed by studying the frequency-wavenumber spectra of the fields excited by localized sources placed near the chain. Periodic nano-bump chains support traveling waves (TWs) that propagate without radiation loss along, and are confined to the region near, the chain. These TWs are slow waves with respect to both space fields and surface plasmon polaritons supported by the metal surface. For nearly resonant nano-bumps, the TWs are well confined and can be excited efficiently by a localized source placed near the chain but the TW propagation length is short. For non-resonant nano-bumps, the TWs have large propagation lengths but are not well confined and are excited less efficiently. The TWs supported by nano-bump chains were shown to have larger propagation lengths than free-standing chains of the same dimension/size and cross-sectional confinement. TWs also are supported by disordered chains and chains with sharp bends. Perturbations in nano-bump positions are shown to reduce the TW propagation length much less significantly than perturbations in their sizes. Transmission through sharp chain bends is much stronger for nearly resonant nano-bumps than for nonresonant ones. In addition to their ability to support TWs, nano-bump chains can be used to manipulate (excite/reflect/refract) SPPs on the metal surface.

Necklace‐like Noble‐Metal Hollow Nanoparticle Chains: Synthesis and Tunable Optical Properties

The sizeand shape-dependent physical properties of inorganic nanoparticles provide tunable materials with broad potential applications, and the fabrication of complex nanostructures further enhances their functionality. For example, noble metal nanoparticles exhibit localized surface plasmon resonances (LSPRs), resulting in strong absorbance at visible wavelengths. The LSPRs enable applications such as biological and chemical sensing, biological imaging labels, and nanometer-scale optical waveguides. The formation of more complex metal nanostructures, such as metallodielectric gold nanoshells and gold nanocages, shifts the LSPR to the near-infrared (NIR), enabling significant diagnostic and therapeutic biomedical applications. On the other hand, self-assembly, an attractive and practical methodology, allows the formation of a wide range of nanostructures for promising applications such as nanoparticle arrays for new optical band-gap materials, high-density magnetic recording media, self-assembled monolayers for nanometer-thick films on a variety of substrates, and nanofibers of self-organized small molecules or oligopeptides for drug delivery and tissue engineering. 1D chains of metal nanoparticles exhibit unique, coherent optical properties when the distance between the particles is short enough to allow for nearfield coupling of their surface plasmon resonances. For example, 1D nanoparticle chains can serve as a plasmon waveguide to overcome the diffraction limit of the optical image. Surface lithography, solution-phase templated assembly, oriented aggregation, micelle transition, electric–dipolar interactions, and magnetic dipole moments have all been used to fabricate plasmon-coupled nanoparticle chains, with the goal of controlling precisely the shape and size of the particles as well as the chain geometry. Despite extensive investigations on 1D systems, there are very few reports of the fabrication of 1D chains of structurally complex nanoparticles, such as hollow nanoparticles. Therefore, we describe in this Communication the synthesis methodology and the tunable optical properties of necklace-like noble-metal hollow nanoparticle chains (HNPCs) from templated Co nanoparticle chains formed under a magnetic field. We will demonstrate that the magnetic-field-guiding and -dependent strategy, which is seldom used in nanostructure fabrication, may provide a useful toolkit for the design of advanced nanoparticle devices. Herein, we show that an in situ magnetic field can induce the formation of a sacrificial necklace-like chain template composed of multicrystalline Co nanoparticles with diameter of ca. 15–20 nm. That Co chains could be found on a substrate after removal of solvent from a dispersion demonstrates that they are sufficiently stable to be used as “hard templates” for the production of advanced 1D nanostructures in solution. By using a novel and potentially general galvanic displacement reaction based on the template, similar to the recently reported formation of hollow metal nanoparticles, nanocages, and nanotubes, Au, Pt, and Pd HNPCs can be formed without loss of the preassembled 1D structure of the template. Because the hollow nanostructures exhibit novel properties that differ from their solid counterparts, we demonstrate that the morphology, as well as the LSPR properties, of Au HNPCs can be tuned by the Co-templated chain through symmetrical variation of the magnetic fields. The one-pot synthesis of noble-metal HNPCs is straightforward. Jiang and co-workers have successfully prepared isolated Au hollow nanoparticles using Co nanoparticles as sacrificial templates. Here, we use the same starting materials to synthesize noble-metal HNPCs while an external field is applied. CoCl2·6 H2O and poly(vinyl pyrrolidone) were dissolved in ultrapure H2O (18.2 MX), sonicated for 15 min, and purged with Ar for 15 min. After placing the system into a defined symmetrical magnetic field, a freshly prepared solution of NaBH4 was added dropwise with stirring. Immediately after all of the NaBH4 had been added, an aqueous solution of HAuCl4/KOH (pH = 7.0) was added dropwise with stirring. After 30 min, the product was collected by centrifugation, C O M M U N IC A TI O N

Plasmonic modes in periodic metal nanoparticle chains: a direct dynamic eigenmode analysis

We demonstrate an efficient eigendecomposition method for analyzing the guided modes in metal nanoparticle chains. The proposed method has the advantage of simultaneously showing the dispersion relation and the mode quality. It can also separate the material properties from the geometrical properties. Its efficiency therefore does not depend on the complexity of the material polarizability. We used the method to analyze the guided modes of a single and a pair of metal nanoparticle chains. The rigorous dynamic dipole polarizability typically gives a redshift compared with the results obtained from the broadly used quasi-static dipole polarizability with radiation correction.

Spectral gaps and mode localization in Fibonacci chains of metal nanoparticles

In this paper we study the spectral, localization and dispersion properties of dipolar modes in quasi-periodically modulated nanoparticle chains based on the Fibonacci sequence. By developing a transfer matrix approach for the calculation of resonant frequencies, oscillation eigenvectors and integrated density of states (IDS) of spatially-modulated dipole chains, we demonstrate the presence of large spectral gaps and calculate the pseudo-dispersion diagram of Fibonacci plasmonic chains. The presence of plasmonic band-gaps and localized states in metal nanoparticle chains based on quasi-periodic order can have a large impact in the design and fabrication of novel nanophotonics devices.