Metal Nanoshells Research Articles

Seeded growth of ultrathin gold nanoshells using polymer additives and microwave radiation

Nanoshells made of a silica core and a gold shell possess an optical response that is sensitive to nanometer-scale variations in shell thickness. The exponential red shift of the plasmon resonance with decreasing shell thickness makes ultrathin nanoshells (less than 10 nm) particularly interesting for broad and tuneable ranges of optical properties. Nanoshells are generally synthesised by coating gold onto seed-covered silica particles, producing continuous shells with a lower limit of 15 nm, due to an inhomogeneous droplet formation on the silica surface during the seed regrowth. In this paper, we investigate the effects of three variations of the synthesis protocol to favour ultrathin nanoshells: seed density, polymer additives and microwave treatment. We first maximised gold seed density around the silica core, but surprisingly its effect is limited. However, we found that the addition of polyvinylpyrrolidone during the shell synthesis leads to higher homogeneity and a thinner shell and that a post-synthetic thermal treatment using microwaves can further smooth the particle surface. This study brings new insights into the synthesis of metallic nanoshells, pushing the limits of ultrathin shell synthesis.

Power transfer due to Kerr nonlinearity in plasmonic nanohybrids

We have studied the energy transfer using Kerr nonlinearity for nanohybrids made of an ensemble of interacting quantum dots and metallic nanoshells. The energy transfer in quantum dots has been calculated by using the quantum density matrix method in the dipole-dipole coupling between quantum dots. We showed that the power transfer occurs from the metallic nanoshell to the quantum dot by four processes. In the first process, three probe photons are transferred to the quantum dot. In the second process, two probe photons and one polariton from the metallic nanoshell are transferred to the quantum dot. Similarly, in the third process, one probe photon and two polaritons from the metallic nanoshell are transferred to the quantum dot. Finally, three polaritons from the metallic nanoshell are transferred to the quantum dot. We found an enhancement in the energy transfer in the quantum dot due to the last three processes. We have also predicted that the power spectrum peak would split into two peaks due to the dipole-dipole coupling. The present findings can be used to fabricate nano-power devices for nanotechnology and nanomedical applications.

Ultra-High-Speed Dynamics in Surface-Enhanced Raman Scattering

In this perspective, we discuss recent observations related to the temporal dynamics of intensities in surface-enhanced Raman scattering (SERS) experiments. SERS is a well-established and highly active research field, driven by the potential of the technique in analytical and bioanalytical applications. However, there are several fundamental aspects of the effect that still challenge and fascinate researchers in the area. Here we will focus on the recent observation that strong SERS intensity fluctuations (SIFs) are seen when experiments are performed at fast acquisition rates, even when the metal surface is completely covered by an adsorbate. Interestingly, the SIF dynamics indicate bursts of SERS activities that last only a few hundreds of microseconds, followed by a longer period of inactivity. This type of behavior has been observed from several systems and configurations, including single metallic nanoshells and nanostars, immobilized colloidal aggregates, nanoparticle-on-a-mirror configurations, and metal-coated microspheres. This diversity suggests that the dynamical behavior is a fundamental characteristic of the SERS effect. Time-dependent atomic rearrangements within the confined environment of the SERS hotspots are suggested as the main mechanism driving these fluctuations. The dynamical SERS behavior, revealed with high-speed acquisitions, should provide a new direction for the study of atomic reconstruction and single molecule interactions with metallic nanoenvironments with an unprecedented level of detail.

Dipole emission characteristics near a topological insulator sphere coated with a metallic nanoshell

The modified emission characteristics of a source dipole near a metal-coated topological insulator sphere are studied with the application of axion electrodynamics in the long wavelength limit. Our focus is on the effect due to the topological magneto-electric (TME) response of the sphere and its influence on the multipolar plasmonic resonance of the metallic nanoshell. It is found that the TME response mainly affects the bonding modes of the metal shell plasmons with the lower order multipole resonances experiencing appreciable red-shifts. In addition, the TME effects can lead to either greater or smaller decay rates and lower emission frequencies, depending crucially on the frequency of the emitting dipole; but otherwise rather insensitive to the orientation of the dipole moments. These findings thus provide an alternative approach to probing the TME effects via molecular fluorescence experiments, as well as new possibility of controlling the emission properties of molecules using these novel nanoparticles.

Plasmon effects in the two-photon Rabi oscillations in a strongly coupled semiconductor quantum dot–metal nanoshell composite

We theoretically study two-photon Rabi oscillations (TPRO) in a nanocomposite comprising a semiconductor quantum dot (SQD) strongly coupled to a metallic nanoshell (MNS), which undergoes a pulsed excitation. The SQD is modeled as a three-level ladder-like quantum system (ground, one-exciton, and bi-exciton states). Its optical dynamics is described quantum-mechanically within the density matrix formalism framework, while the MNS is treated classically through its frequency-dependent polarizability. We find two effects of the presence of a MNS in close proximity to the SQD: (a) - a sufficient increase of the number of Rabi cycles as compared to the case of an isolated SQD and (b) - incoherent population of the one-exciton state giving rise to the destruction of the TPRO.

A simple derivation of the shell polarizability formula and investigation of the plasmonic behavior of aluminum nanoshells with the Mie theory

Shell polarizability is derived in detail to clarify this formula, which is not uniform in popular monographs. Different from a previous study, new surface plasmon resonances of aluminum metal nanoshells are obtained based on the Mie theory.

Optical second harmonic generation from plasmonic nanoshells using the nonlocal hydrodynamic model

In this paper, we investigate the linear and nonlinear optical response of spherical plasmonic nanoshells within the nonlocal hydrodynamical model for conduction electrons. The optical absorption and surface second-harmonic generation (SHG) associated with metallic core-shell nanoparticles is studied on the basis of the hydrodynamic Drude model. The influence of nonlocality on the optical SHG is investigated. We show that nonlocal effects can be applied to boost SHG from plasmonic nanoshells. It is found that the surface SHG radiation from the surface of nanoshells can be achieved by symmetry-breaking and is sensitive to nonlocality of the electron response. It is also shown that nonlocal effects lead to enhancement and shift of the resonant wavelengths of the optical absorption and SHG in metal nanoshells. Keywords: Optical absorption, Second harmonic generation, plasmonic nanoshell, nonlocal effects.

Multifunctional hollow porous Au/Pt nanoshells for simultaneous surface-enhanced Raman scattering and catalysis

Multifunctional nanostructures based on noble metal nanoparticles (NPs) have attracted great interest owing to their unique properties and potential for various applications. However, sophisticated control of the structural design through synthesis still remains challenging. Here we report a simple approach to synthesizing hollow porous metal nanoshells (HP metal NSs) comprising different metal NPs sintered together, with a structure and composition that can be readily controlled. The hybrid HP metal NSs can have multifunctionality in its single entity depending on their constituent metal NPs. As a demonstration, we prepared HP metal NSs comprising Au, Pt, and their hybrids, and measured their catalytic and surface-enhanced Raman scattering (SERS) activities in a model catalytic reduction of 4-nitrothiophenol to 4-aminothiophenol by sodium borohydride. The HP metal NSs comprising homogeneous Au or Pt NPs had high SERS activity and catalytic activity, respectively. In contrast, the HP Au/Pt hybrid NSs showed excellent multifunctional features with sensitive SERS performance and high catalytic activity owing to synergistic effects of the exposed Au and Pt NPs sites. Our simple strategy for fabricating HP metal NSs with a unique structure enables easy control of the NPs composition, and the resulting materials hold great promise for practical applications in various fields.

Pd decorated TiO2 nanotube array Schottky barrier diodes for efficient hydrogen sensing application

Well aligned TiO2 nanotube arrays have been synthesized via electrochemical anodization, and Pd-based nanoparticles (NPs) were decorated by electrodeposition with the aid of UV irradiation. A new back-to-back Schottky barrier diode (SBD) was formed by connecting with Ag–(Pd decorated TiO2)–Ti. We found that such an SBD sensor has excellent hydrogen (H2) sensing performances at a working temperature of 80 °C, with a detection limit as low as 1 ppm as well as a quick response time within 10 s at a low bias voltage (0.025 V). I–V curves and electrochemical impedance spectroscopy analysis demonstrate a typical rectification characteristic of SBD as well as a dramatic decrease of the barrier height of SBD during the H2 sensing process. The high H2 sensing performance of a Pd-based NP decorated TiO2 nanotube array film SBD could be attributed to the synergism of the highly ordered nano-porous configurations and the Pd-based NP (with a high catalytic active metallic Pd nanoshell layer) decoration providing the enhancement of H2 spillover effect (and/or electronic sensitization) related redox processes.

An Expanded Surface-Enhanced Raman Scattering Tags Library by Combinatorial Encapsulation of Reporter Molecules in Metal Nanoshells.

Raman-encoded gold nanoparticles (NPs) have been widely employed as photostable multifunctional probes for sensing, bioimaging, multiplex diagnostics, and surface-enhanced Raman scattering (SERS)-guided tumor therapy. We report a strategy toward obtaining a particularly large library of Au nanocapsules encoded with Raman codes defined by the combination of different thiol-free Raman reporters, encapsulated at defined molar ratios. The fabrication of SERS tags with tailored size and predefined codes is based on the in situ incorporation of Raman reporter molecules inside Au nanocapsules during their formation via galvanic replacement coupled to seeded growth on Ag NPs. The hole-free closed-shell structure of the nanocapsules is confirmed by electron tomography. The unusually wide encoding possibilities of the obtained SERS tags are investigated by means of either wavenumber-based encoding or Raman frequency combined with signal intensity, leading to an outstanding performance as exemplified by 26 and 54 different codes, respectively. We additionally demonstrate that encoded nanocapsules can be readily bioconjugated with antibodies for applications such as SERS-based targeted cell imaging and phenotyping.

Hybridization of plasmon modes in multishell bimetallic nanoparticles: a numerical study

We study the near-infrared properties of spherical multishell nanoparticles comprising a loss-less dielectric core enclosed by a concentric layering of metallic–dielectric–metallic nanoshells. The coupling between the metallic shells induces plasmon resonance redshifts and peak splitting in the absorption spectra of the layered particle relative those of the metallic constituents. We use full-wave electromagnetic analysis to investigate changes in the absorption spectra as a function of key parameters including the material properties of the inner and outer metallic shells and the aspect ratio of their inner and outer radii. We systematically vary the aspect ratios and quantify the degree of plasmonic coupling between the metallic nanoshells. Our analysis reveals conditions under which the spectral resonance peaks blueshift and/or redshift. We consider bimetallic particles with gold and silver nanoshells and determine the dependency of plasmon resonance peak shifting and splitting as a function of the order of these material layers, i.e., as inner or outer shells.

High efficiency metallic nanoshells for improving polymer solar cells performance: An opto-electrical study

Metallic nanoshells have attracted more attention compared to their solid counterparts as a result of producing high-yield plasmonic effects, such as large scattering and absorption cross-sections, potent near-field, strong optical trapping and long fluorescence time, during the plasmon hybridization mechanism. However, no focused study has been performed on optimizing this type of nanoparticles to improve the performance of polymer solar cells (PSCs) because they are difficult to synthesize. Our coupled optical-electrical simulation results show that the effort to fabricate may be worth it. In this work, we propose SiO2@Ag@SiO2 nanoparticles and optimize their size, material, and shells thickness. We show that their optimization makes full use of plasmonic effects possible, providing the PSCs with improvements in light concentration and optical absorption. Subsequently, we investigate the effect of nanoparticles’ concentration, their array and shape on the performance of PSCs based on PTB7:PC71BM, P3HT:PC60BM and PCDTBT:PC70BM. For the first time, we show that spherical nanoparticles can more improve the performance of PSCs compared to cubic ones, despite the fact that cubic nanoparticles have better plasmonic properties than their spherical counterparts. The results show significant improvement in electrical performance of the PSCs which results in efficiency enhancement of ~63%, ~106% and ~55%, respectively.

Theory of all-optical switching based on the Kerr nonlinearity in metallic nanohybrids

We have developed a theory for the Kerr nonlinearity in nanohybrids made of an ensemble of metallic nanoshells and low concentration of quantum emitters. A metallic nanoshell is made of a metallic core sphere and dielectric shell. We consider that quantum emitters are four-level quantum systems. When a probe laser light falls in the metallic nanoshells, the surface plasmon polariton electric field is produced at the interface between the metal sphere and dielectric shell. This electric field along with the probe field induces dipoles in metallic nanoshells. These dipoles interact with each other via the dipole-dipole interaction. The Kerr nonlinearity has been calculated by using the quantum density matrix method in the presence of the dipole-dipole interaction (coupling). We found that in the weak-coupling limit there is an enhancement in the Kerr nonlinearity. On the other hand, in the strong-coupling limit, the peaks in the Kerr coefficient split from two peaks to four peaks when the frequency of the dipole electric field is in the resonance with the exciton frequency. The splitting in the spectrum is due to the presence of the dressed sates created in the system. We showed that heights and locations of peaks are very sensitive to the strength of the dipole-dipole interaction. Physics of the enhancement can be used fabricate Kerr nanosensors. On the other hand, physics of the splitting from two peaks (ON) to four peaks (OFF) can be used to fabricate Kerr nanoswitches.

Plasmon-assisted two-photon absorption in a semiconductor quantum dot–metallic nanoshell composite

Tho-photon absorption holds potential for many practical applications. We theoretically investigate the onset of this phenomenon in a semiconductor quantum dot -- metallic nanoshell composite subjected to a resonant CW excitation. Two-photon absorption in this system may occur in two ways: incoherent -- due to a consecutive ground-to-one-exciton-to-biexciton transition and coherent -- due to a coherent two-photon process, involving the direct ground-to-biexciton transition in the quantum dot. The presence of the nanoshell nearby the quantum dot gives rise to two principal effects: (i) -- renormalization of the applied field amplitude and (ii) -- renormalization of the resonance frequencies and radiation relaxation rates of the quantum dot, both depending on the the quantum dot level populations. We show that in the perturbation regime, when the excitonic levels are only slightly populated, each of these factors may give rise to either suppression or enhancement of the two-photon absorption. The complicated interplay of the two determines the final effect. Beyond the perturbation regime, it is found that the two-photon absorption experiences a drastic enhancement, which occurs independently of the type of excitation, either into the one-exciton resonance or into the two-photon resonance. Other characteristic features of the two-photon absorption of the composite, emerging from the coupling between both nanoparticles, are bistability and self-oscillations.

Switching and Sensing Using Kerr Nonlinearity in Quantum Dots Doped in Metallic Nanoshells

A theory of a switching and sensing mechanisms using the Kerr nonlinearity has been developed for nanohybrids made of ensemble of three-level quantum emitters and metallic nanoshells. We have calcu...

A study of two-photon florescence in metallic nanoshells

A theory of the two-photon florescence for a metallic nanoshell in the presence of quantum emitters has been developed. The metallic nanoshell is made of a metallic nanosphere as a core and a dielectric material as a shell. An ensemble of quantum emitters is deposited on the surface of the dielectric shell. A probe field is applied to study the two-photon process in the metallic nanoshell. Surface plasmon polaritons are created at the interface between the core and shell due to coupling between probe photons and surface plasmons present at the surface of the metallic nanosphere. The intensity of the surface plasmon polariton field is huge when the probe photon energy is in resonance with the polariton resonance energy. Induced electric dipoles are created in each quantum emitter due to the surface plasmon polariton field and the probe field. Dipoles in quantum emitters interact with each other via the dipole-dipole interaction. The dipole-dipole interaction is calculated using the many-body theory and mean field approximation. It is found that the dipole-dipole interaction has new term which is induced by the surface plasmon polariton field. An analytical expression of the two-photon florescence is derived in the presence the dipole-dipole interaction. Our theory predicts that the intensity of the two-photon florescence is enhanced in the presence of quantum emitters relative to the florescence of the metallic nanoshell in isolation. Physics behind the enhancement is the presence of the dipole-dipole interaction between the ensemble of quantum emitters. It is also found that as the concentration of quantum emitters increases, the dipole-dipole field also increases. This in turn, increases the two-photon florescence as function of the concentration. Finally, we have compared our theory with experiments of a metallic nanoshell which is made for Au nanosphere core and the SiO2 shell. The metallic nanoshell is surrounded by various concentrations of Cadmium-Selenium quantum dots as quantum emitters. A good agreement between theory and experiment is found.

The study of intermolecular energy transfer in a molecule-plasmon system: The effect of an electric charge

In the present study, the transfer of energy between a pair of donor–acceptor molecules as point-like dipoles located nearby a plasmonic nanoparticle is investigated, in which an electric point charge is placed at a distance rq from the center of metallic nanoparticle or nanoshell. It is shown that the process of energy transfer between pairs of molecules is affected when an external point charge is placed next to the plasmonic nanoparticle. In addition to the contribution of the dipolar plasmon mode, the contribution of the quadrupolar and octupolar modes is taken into account because these plasmonic modes are more sensitive to changes; therefore, they can be used in sensing applications. The effect of the physical parameters of the system, such as the magnitude of the electric charge, the distance of the electric charge, the dielectric constant of the host medium, the distance of the donor molecule, and type of metal nanoparticle on the intermolecular energy transfer, is studied. However, we calculate and demonstrate that it is possible to use an electric point charge next to a hybrid plasmonic nanoparticle-molecular system to enhance the energy transfer from a donor molecule to an acceptor molecule.

Gold–silver nanoshells promote wound healing from drug-resistant bacteria infection and enable monitoring via surface-enhanced Raman scattering imaging

Chronic infections, caused by multidrug-resistant (MDR) bacteria, constitute a serious problem yet often underappreciated in clinical practice. The in situ monitoring of the bacteria-infected disease is also necessary to track and verify the therapeutic effect. Herein we present a facile approach to overcome the above challenges through a Raman tag 3,3′-diethylthiatricarbocyanine iodide (DTTC)-conjugated gold-silver nanoshells (AuAgNSs). With a strong responsive of the near-infrared laser due to surface plasmon resonance (SPR) from hybrid metallic nanoshell structure, AuAgNSs exhibits an efficient photothermal effect, and it simultaneously releases silver ions during laser irradiation to bacterial eradicate. Herein, two MDR bacteria strain, methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum β-lactamase Escherichia coli, are chosen as models and studied both in vitro and in vivo. As a result, the AuAgNSs-DTTC substrates enable surface-enhanced Raman scattering imaging to provide a non-invasive and extremely high sensitive detection (down to 300 CFU mL−1 for MRSA) and prolonged tracking (at least 8 days) of residual bacteria. In a chronic MRSA-infected wound mouse model, the AuAgNSs gel-mediated photothermal therapy/silver-release leads to a synergistic would healing with negligible toxicity or collateral damage to vital organs. These results suggest that AuAgNSs-DTTC is a promising anti-bacterial tool for clinical translation.

Topological Spaser.

We theoretically introduce a topological spaser, which consists of a hexagonal array of plasmonic metal nanoshells containing an achiral gain medium in their cores. Such a spaser can generate two mutually time-reversed chiral surface plasmon modes in the K and K^{'} valleys, which carry the opposite topological charges, ±1, and are described by a two-dimensional E^{'} representation of the D_{3h} point symmetry group. Due to the mode competition, this spaser exhibits a bistability: only one of these two modes generates, which is a spontaneous symmetry breaking. Such a spaser can be used for an ultrafast all-optical memory and information processing, and biomedical detection and sensing with chirality resolution.

Plasmonic nano-shells: atomistic discrete interaction versus classic electrodynamics models.

Using the extended discrete interaction model and Mie theory, we investigate the tunability of the optical polarizability of small metallic nano-shells. We show that the spectral positions of symmetric and antisymmetric dipolar plasmon resonances vary with the ratio of particle radius to hole radius in a manner similar to one predicted for uniform metallic nano-shells using a semiclassical approach of two coupled harmonic oscillators. We show that, according to the extended discrete interaction model, the dipolar plasmon resonances are also present for nano-shells in the 2-13 nm size region and show the same functional dependence seen for larger nano-shells. Using previously fitted data from experiment, we can predict the size-dependence of the plasma frequency for nano-shells in the 1-15 nm size region. We find that Mie theory, which utilizes the electron mean free path correction for the permittivity, is not able to reproduce the same functional form of the dipolar modes for the nano-shells of the same sizes.