Nanoparticle Dimers Research Articles

Tip-Enhanced Raman Excitation Spectroscopy (TERES): Direct Spectral Characterization of the Gap-Mode Plasmon.

The plasmonic properties of tip-substrate composite systems are of vital importance to near-field optical spectroscopy, in particular tip-enhanced Raman spectroscopy (TERS), which enables operando studies of nanoscale chemistry at a single molecule level. The nanocavities formed in the tip-substrate junction also offer a highly tunable platform for studying field-matter interactions at the nanoscale. While the coupled nanoparticle dimer model offers a correct qualitative description of gap-mode plasmon effects, it ignores the full spectrum of multipolar tip plasmon modes and their interaction with surface plasmon polariton (SPP) excitation in the substrate. Herein, we perform the first tip-enhanced Raman excitation spectroscopy (TERES) experiment and use the results, both in ambient and aqueous media, in combination with electrodynamics simulations, to explore the plasmonic response of a Au tip-Au substrate composite system. The gap-mode plasmon features a wide spectral window corresponding to a host of tip plasmon modes interacting with the plasmonic substrate. Simulations of the electric field confinement demonstrate that optimal spatial resolution is achieved when a hybrid plasmon mode that combines a multipolar tip plasmon and a substrate SPP is excited. Nevertheless, a wide spectral window over 1000 nm is available for exciting the tip plasmon with high spatial resolution, which enables the simultaneous resonant detection of different molecular species. This window is robust as a function of tip-substrate distance and tip radius of curvature, indicating that many choices of tips will work, but it is restricted to wavelengths longer than ∼600 nm for the Au tip-Au substrate combination. Other combinations, such as Ag tip-Ag substrate, can access wavelengths as low as 350 nm.

A Plasmonic Approach to Study Protein Interaction Kinetics through the Dimerization of Functionalized Ag Nanoparticles

Understanding the kinetics of protein interactions plays a key role in biology with significant implications for the design of analytical methods for disease monitoring and diagnosis in medical care, research and industrial applications. Herein, we introduce a novel plasmonic approach to study the binding kinetics of protein-ligand interactions following the formation of silver nanoparticles (Ag NPs) dimers by UV-Vis spectroscopy that can be used as probes for antigen detection and quantification. To illustrate and test the method, the kinetics of the prototype biotin-streptavidin (Biot-STV) pair interaction was studied. Controlled aggregates (dimers) of STV functionalized Ag NPs were produced by adding stoichiometric quantities of gliadin-specific biotinylated antibodies (IgG-Biot). The dimerization kinetics was studied in a systematic way as a function of Ag NPs size and at different concentrations of IgG-Biot. The kinetics data have shown to be consistent with a complex reaction mechanism in which only the Ag NPs attached to the IgG-Biot located in a specific STV site are able to form dimers. These results help in elucidating a complex reaction mechanism involved in the dimerization kinetics of functionalized Ag NPs, which can serve as probes in surface plasmon resonance-based bioassays for the detection and quantification of different biomarkers or analytes of interest.

Colloidal SERS Substrate for the Ultrasensitive Detection of Biotinylated Antibodies Based on Near-Field Gradient within the Gap of Au Nanoparticle Dimers

Surface-enhanced Raman spectroscopy (SERS) has demonstrated to be a powerful technique for the ultrasensitive detection of different types of analytes and particularly biomolecules, with the ration...

Collective multipole oscillations direct the plasmonic coupling at the nanojunction interfaces.

We present a systematic study of the effect of higher-multipolar order plasmon modes on the spectral response and plasmonic coupling of silver nanoparticle dimers at nanojunction separation and introduce a coupling mechanism. The most prominent plasmonic band within the extinction spectra of coupled resonators is the dipolar coupling band. A detailed calculation of the plasmonic coupling between equivalent particles suggests that the coupling is not limited to the overlap between the main bands of individual particles but can also be affected by the contribution of the higher-order modes in the multipolar region. This requires an appropriate description of the mechanism that goes beyond the general coupling phenomenon introduced as the plasmonic ruler equation in 2007. In the present work, we found that the plasmonic coupling of nearby Ag nanocubes does not only depend on the plasmonic properties of the main band. The results suggest the decay length of the higher-order plasmon mode is more sensitive to changes in the magnitude of the interparticle axis and is a function of the gap size. For cubic particles, the contribution of the higher-order modes becomes significant due to the high density of oscillating dipoles localized on the corners. This gives rise to changes in the decay length of the plasmonic ruler equation. For spherical particles, as the size of the particle increases (i.e., ≥80 nm), the number of dipoles increases, which results in higher dipole-multipole interactions. This exhibits a strong impact on the plasmonic coupling, even at long separation distances (20 nm).

A study of light induced surface reactions of sildenafil citrate on hybrid AgCl/Ag nanoparticle dimers by surface enhanced Raman scattering and pulse radiolysis techniques

AgCl/Ag nanoparticle dimers (NPD) are important from photocatalysis point of view as it is reported to show photocatalytic activity in visible region. In this work, AgCl/Ag NPDs were synthesized by controlling the concentration of available chloride ions as well as the capping agent, sildenafil citrate (SC). Transmission electron microscopy (TEM), selected area electron diffraction (SAED) and UV–vis absorption property was used to probe morphology, lattice structure and optical property of as synthesized AgCl/Ag NPDs. SERS spectrum of SC adsorbed on the AgCl/Ag NPD excited at 632 nm, which is energetically closer to the coupled plasmon peak at 605 nm, showed some unexplained vibrational peaks. The above system was probed further by using pulse radiolysis technique and time-dependent density functional theory (TD-DFT) calculations. The results suggest that a direct electron transfer reaction from AgCl/Ag NPD to SC is involved in appearance of anomalous features in SERS spectrum.

Hierarchical Hybridization in Plasmonic Honeycomb Lattices.

This paper reports hierarchical hybridization as a mode-mixing scheme to account for the unique optical properties of non-Bravais lattices of plasmonic nanoparticles (NPs). The formation of surface lattice resonances (SLRs) mediated by localized surface plasmons (LSPs) of different multipolar orders (dipole and quadrupole) can result in asymmetric electric near-field distributions surrounding the NPs. This asymmetry is because of LSP hybridization at the individual NP level from LSPs of different multipole order and at the unit cell level (NP dimer) from LSPs of the same multipole order. Fabricated honeycomb lattices of silver NPs exhibit ultrasharp SLRs at the Γ point that can also facilitate nanolasing. Modeling of the stimulated emission process revealed that the multipolar component of the lattice plasmon mode was responsible for feedback for lasing. By leveraging multipolar LSP responses in Al NP lattices, we achieved two distinct Γ point band-edge modes from a single honeycomb lattice. This work highlights how multipolar LSP coupling in plasmonic lattices with a non-Bravais symmetry has important implications for the design of SLRs and their associated plasmonic near-field distributions. These relatively unexplored degrees of freedom can decrease both ohmic and radiative losses in nanoscale systems and enable SLRs to build unanticipated connections among photonics and nanochemistry.

Well-ordered self-assembled plasmonic dimers for surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) using structures of metal nanoparticles (NPs) is extremely sensitive to the relative orientation of the polarization of the incident light with the interparticle axis. Self-assembly of metal nanoparticles is a well-known approach to fabricate nanostructures. However, controlling the assembly direction of the NPs in this method is difficult. In this Letter, we propose a controlled self-assembly method using vacuum ultraviolet (VUV) light. By illuminating the surface-functionalized NPs with VUV light, the functionality is cancelled out, and therefore, we can select desirable direction of assembly. In the light of this method, well-ordered dimers (WODs) of silver NPs are fabricated. The Raman enhancement factor of WODs is about twice larger than randomly ordered ones.

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.

Directed Microwave‐Assisted Self‐Assembly of Au–Graphene–Au Plasmonic Dimers for SERS Applications

AbstractNoble metal dimers with sub‐nanometer separation support strong electromagnetic field enhancement which has practical applications in surface enhanced Raman scattering (SERS), photodetection, and photocatalysis. Monolayer graphene is an excellent spacer material to practically realize uniform separation between the dimers. Here, directed microwave‐assisted self‐assembly of Au nanoparticle dimers is reported, separated by graphene monolayer over 1 cm2 substrates. Detailed analytical models of Au particle formation kinetics explain the experimentally observed control of the density and selectivity of the dimer formation. SERS substrates with 7 × 106 cm−2 of Au–Graphene–Au dimers are obtained which yield a 35‐fold increase in the Raman spectral signal of graphene from a single dimer, and an enhancement factor in intensity per molecule of 107 allows ppb level detection of Rhodamine 6G. A system of such dimers can provide an efficient, reliable, and inexpensive solution for many nanophotonic applications that require ultrahigh field confinement, such as SERS and photodetection.

Local Heating Control of Plasmonic Nanoparticles for Different Incident Lights and Nanoparticles

This paper investigates the nanoscale control of heating processes in the Au nanostructures from the aspects of light sources, nanoparticle morphologies, multi-layer Au shells, and nanoparticle dimers. The spatiotemporal evolution of the temperature profile inside and around the Au nanoparticles is computed using a numerical framework based on the finite element method. One-temperature model and two-temperature model are used for the calculation of continuous-wave and picosecond pulse laser, respectively. Results show that the maximum temperature increase is linear with the laser energy density, the slopes of which are various for continuous-wave laser and picosecond pulse laser. For the Au cube, ellipsoid, ring, and sphere with the same volume, the maximum resonance wavelengths locate in ~ 580 nm, 610 nm, 500 nm, and 530 nm, respectively. The trend of the maximum temperature increase agrees well with the absorption cross section. With increasing the number of the shell, a red shift occurs from 665 to 690 nm and the total absorption cross section also increases. What needs to be emphasized is that it does not need to heat the surrounding environment while the core region remains a very high temperature, which is very interesting and suitable for photothermal applications such as photothermal catalysis and nanoreaction oven. For the nanoshell dimers, it can be heated selectively by adjusting the incident angle and light wavelength for this dimer system.

Ultrasensitive Real-Time Rolling Circle Amplification Detection Enhanced by Nicking-Induced Tandem-Acting Polymerases.

Padlock probe ligation-based rolling circle amplification (RCA) can distinguish single-nucleotide variants, which is promising for the detection of drug-resistance mutations in, e.g., Mycobacterium tuberculosis (Mtb). However, the clinical application of conventional linear RCA is restricted by its unsatisfactory picomolar-level limit of detection (LOD). Herein, we demonstrate the mechanism of a nicking-enhanced RCA (NickRCA) strategy that allows several polymerases to act simultaneously on the same looped template, generating single-stranded amplicon monomers. Limiting factors of NickRCA are investigated and controlled for higher amplification efficiency. Thereafter, we describe a NickRCA-based magnetic nanoparticle (MNP) dimer formation strategy combined with a real-time optomagnetic sensor monitoring MNP dimers. The proposed methodology is applied for the detection of a common Mtb rifampicin-resistance mutation, rpoB 531 (TCG/TTG). Without additional operation steps, an LOD of 15 fM target DNA is achieved with a total assay time of ca. 100 min. Moreover, the proposed biosensor holds the advantages of single-nucleotide mutation discrimination and the robustness to quantify targets in 10% serum samples. NickRCA produces short single-stranded monomers instead of the DNA coils produced in conventional RCA, which makes it more convenient for downstream operation, immobilization or detection, thus being applicable with different molecular tools and biosensors.

Absorption cross-section spectroscopy of a single strong-coupling system between plasmon and molecular exciton resonance using a single silver nanoparticle dimer generating surface-enhanced resonant Raman scattering

This study investigated spectral changes in the absorption cross-sections of single strong coupling systems composed of single silver nanoparticle dimers and a few dye molecules during the quenching of surface-enhanced resonant Raman scattering (SERRS). The absorption cross-section was obtained by subtracting the scattering cross-section from an extinction cross-section. The spectral changes in these cross-sections were evaluated using a classical hybridization model composed of a plasmon and a molecular exciton including a molecular multi-level property. The changes in the scattering and extinction cross-sections exhibit blue-shifts in their peak energy and increased peak intensities, respectively, during SERRS quenching. These properties are effectively reproduced in the model by decreasing the coupling energy. In particular, the peaks in the scattering and extinction cross-sections appear as peaks or dips in the absorption cross-sections depending on the degree of scattering loss, which reflects the dimer sizes. These results are useful for optimizing photophysical and photochemical effects mediated by the electronic excited states of strong coupling systems.

Comparison of Octahedral and Spherical Nanoparticles for Plasmonics

Monocrystalline metallic nanoparticles are preferable for plasmonic applications, but grow only in polyhedral shapes, which deviate from the predominantly employed spherical shape. Through numerical simulations we have investigated possibilities for using octahedral instead of spherical Au nanoparticles in plasmonics. Our results show that the dimers of octahedral Au nanoparticles can provide higher field enhancement compared to spherical nanoparticles and that placing the octahedrons close to a metal plate can also have certain advantages compared to spheres.

New coupling mechanism of titanium nitride nanosphere dimers at short separation distances

The coupled localized plasmon modes generated by a metal plasmonic nanoparticle (NP) dimer induces a stronger plasmon enhanced electromagnetic field as well as a stronger optical response compared to that of a single NP. Owing to the small Drude damping factor, however, the absorption bandwidth of noble metallic NPs is insufficiently broad. Herein, the near-field wide band coupling absorption for 25 nm diameter TiN nanospheres is investigated in a homo-dimer arrangement for various separation distances using the finite element method. An enhancement of the wide band coupling absorption and a red-shift is found, which can be explained by an uncomplicated dipole–dipole coupling model at interparticle distances greater than 5 nm. At short separation distances, the coupling absorption of the TiN dimer exhibits a tremendous change, which is diametrically opposite the results found for a Au dimer. This unexpected change phenomenon is shown by calculation and analysis to be owing to the change of the charge distribution approach at short separation distances, which is demonstrated to play a key role in the wide band coupling characteristic variation. With decreasing separation gap, a new coupling mechanism caused by surface charge properties is responsible for the decline in coupling absorption as well as the break in the ruler equations for both plasmon shift and temperature enhancement.

Plasmonic‐Enhanced Light Harvesting and Perovskite Solar Cell Performance Using Au Biometric Dimers with Broadband Structural Darkness

Hybrid perovskites have recently attracted enormous attention for photovoltaic applications, and various strategies related to light management and photocarrier collection are developed to enhance their performance. As an effective route toward near‐field light enhancement, metal nanostructures with subwavelength dimensions can couple incident photons with conduction electrons, giving rise to localized surface plasmon resonances. However, efficiency enhancements through plasmonic routes are limited to the short wavelength range corresponding to metal extinction wavelength. Thus, the exploration of novel plasmonic nanostructures with predesigned sizes and shapes is needed to advance this field. Herein, for the first time, a bioinspired nanostructure of Au nanorod–nanoparticle dimers with structural darkness is exploited to enhance the light harvesting and performance of perovskite solar cells. Differing from conventional metallic nanoparticles, biometric nanoparticles introduce geometric singularity to the system, providing a broadband response for energy harvesting. By embedding the core–shell gold dimers in the perovskite solar cells, a notable enhancement of broadband light absorption is observed, and sequentially, the efficiency of perovskite solar cells increases by 16%.

Precision Plasmonics with Monomers and Dimers of Spherical Gold Nanoparticles: Nonequilibrium Dynamics at the Time and Space Limits

Monomers and dimers of spherical gold nanoparticles (NPs) exhibit highly uniform plasmonic properties at the single-particle level due to their high structural homogeneity (precision plasmonics). R...

Self‐Aligned Anisotropic Plasmonic Nanostructures

Great opportunities emerge not only in the generation of anisotropic plasmonic nanostructures but also in controlling their orientation relative to incident light. Herein, a stepwise seeded growth method is reported for the synthesis of rod-shaped plasmon nanostructures which are vertically self-aligned with respect to the surface of colloidal substrates. Anisotropic growth of metal nanostructure is achieved by depositing metal seeds onto the surface of colloidal substrates and then selectively passivating the seed surface to induce symmetry breaking in the subsequent seed-mediated growth process. The versatility of this method is demonstrated by producing nanoparticle dimers and linear trimers of Au, Au-Ag, Au-Pd, and Au-Cu2 O. Further, this unique method enables the automatic vertical alignment of the resulting plasmonic nanostructures to the surface of the colloidal substrate, thereby making it possible to design magnetic/plasmonic nanocomposites that allow the dynamic tuning of the plasmon excitation by controlling their orientation using an external magnetic field. The controlled anisotropic growth of colloidal plasmonic nanostructures and their dynamic modulation of plasmon excitation further allow them to be conveniently fixed in a thin polymer film with a well-controlled orientation to display polarization-dependent patterns that may find important applications in information encryption.

Tunable magneto-optical linear and circular dichroism spectra for a dimer of core–shell prolate spheroids

Searching for methods to modulate localized surface plasmons in nanoparticles is one of the challenging and important areas in plasmonic effects. Surface plasmons can be influenced in a magnetic field due to the additional magnetic force on the electrons, resulting in some magneto-optical phenomena. In this paper, we study the magneto-optical properties of core–shell spheroidal nanoparticles. We apply the coupled dipole method to calculate the absorption and linear and circular dichroism spectra of a single nanoparticle and a dimer of nanoparticles. The polarizability tensor of spherical nanoparticles is extended for spheroidal nanoparticles, and, by utilizing it, we show that the single spheroidal nanoparticle reveals circular dichroism. It is reported that core size and nanoparticles axis direction can be used to engineer the whole spectrum for both configurations.

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.

Simple and rapid method for homogeneous dimer formation of gold nanoparticles in a bulk suspension based on van der Waals interactions between alkyl chains

We developed a simple and rapid method for dimer and higher multimer formation of gold nanoparticles (AuNPs) in bulk suspension. A coupling of AuNPs modified with COOH-terminated alkanethiol by van-der-Waals interaction between alkyl chains was employed. We demonstrated the tunability of the interparticle gap by changing the alkyl chain length from C5 to C15. Efficient dimer formation that avoids unwanted aggregation was demonstrated for AuNPs with a diameter ranging from 20 nm to 80 nm. For all cases, we found that the interparticle gap is well-defined and uniform. For the shortest alkyl chain (C5), we achieved an interparticle gap as small as 1.0 nm.