Nanoparticle Dimers Research Articles

Analytical Approach to the Surface Plasmon Resonance Characteristic of Metal Nanoparticle Dimer in Dipole-Dipole Approximation

This theoretical study deals with the effect of bi-particle interaction on the surface plasmon resonance (SPR) in a dimer which includes two identical metal nanoparticles (NPs). Considering the dipole-dipole interaction in a Drude-like model, an appropriate equation is derived for the permittivity of each NP. The restoration force related to the classical confinement originating from the finite size of NPs is considered, and an appropriate adjustment coefficient is considered for this term through analyzing experimental data. Two different polarizations are considered for the laser beam electric field, and it is shown that the orientation of the electric field has an essential role in the linear optical properties of a dimer. Numerical investigation is accomplished for a dimer of gold NPs with two different diameters of 4 nm and 20 nm. For the parallel polarization, dipole-dipole interaction leads to the redshift of SPR wavelength and increase in its peak value, while for the perpendicular polarization, the absolute opposite results are derived. For all cases, it is shown that SPR wavelength functionality with respect to the geometric factor a/d (NP radius to the separation) can be presented by a cubic equation that fits better than an exponential one suggested by the earlier studies which demonstrates the dipole-dipole characteristic of the interaction. Qualitatively, our results are in good agreement with the other experimental studies.

Discrete pH-Responsive Plasmonic Actuators via Site-Selective Encoding of Nanoparticles with DNA Triple Helix Motif

The synthesis of discrete nanoparticle self-assemblies has challenged the nanotechnology field for decades and has limited the study of nanoparticle interactions and the advance of nanomachinery. Here, we describe the preparation of discrete nanoparticle dimers from spherical gold nanoparticles and gold nanorods with stimuli-responsive optical properties as a result of site-specific DNA functionalization with an active triple helix motif. We present discrete assemblies using anisotropic nanoparticles that dynamically respond to changes in pH. Our approach to creating building blocks for nanoparticle self-assembly capable of creating discrete structures may allow for the future study of nanoparticle-nanoparticle interactions, plasmonic sensors, and nanoparticle-DNA hybrid nanomachinery.

Alkyne‐Monofunctionalized Gold Nanoparticles as Massive Molecular Building Blocks

Using a tripodal, thioether‐based ligand comprising a remote protected acetylene, we efficiently stabilize small gold nanoparticles (Ø ≈ 1.2 nm) which are isolated and purified by chromatography. The 1:1 ligand‐to‐particle ratio is obtained by comparing the particles' dimensions measured by transmission electron microscopy with the weight fraction of the coating ligand determined by thermogravimetric analysis. The single ligand coating of the gold particle guarantees the presence of a single masked alkyne per particle. It can be addressed by wet chemical protocols providing the particles with the properties of “massive molecules”. The “massive molecule” nature of the particles is demonstrated by involving them in wet chemical coupling protocols like oxidative acetylene coupling providing gold nanoparticle dimers (34 % isolated yield) or alkyne‐azide “click”‐chemistry with a suitable triazide, giving trimeric particle architectures (30 % determined by transmission electron microscopy). The particle stabilization obtained by the coating ligand allows, for the first time, to treat these multi‐particle architectures by size exclusion chromatography.

Charge Transfer Plasmons in Dimeric Electron Clusters

The tunability of the optical response of dimers of metal clusters and nanoparticles makes them ideal for many applications from sensing and imaging to inducing chemical reactions. We have studied ...

Surface‐enhanced Raman spectroscopy with gold nanoparticle dimers created by sacrificial DNA origami technique

In a previously proposed method for molecular detection based on surface-enhanced Raman spectroscopy (SERS), gold nanoparticle (AuNP) dimers are formed on a substrate using DNA origami with a nanometre-scale gap between them. In this sacrificial DNA origami technique, a conjugate of two AuNPs and a DNA origami is deposited on a silicon chip, and then the DNA origami is selectively removed using vacuum ultraviolet light and ultrapure water rinsing to form surface-clean AuNP dimers. The performance of SERS-based molecular detection of AuNP dimers created using the proposed technique with a 30 nm AuNP diameter has now been evaluated. The Raman signals from the target molecules (4,4′-bipyridine) were greatly enhanced and thus successfully detected.

Surface enhanced Raman scattering of some nanotechnological material systems: A computational approach

Surface enhanced Raman spectroscopy or scattering, is an old analytical technique having multiple applications, but it still remains with great challenges. The complex physical and chemical processes involved results in difficulties for modelling and experimentation. In this work, numerical results of surface enhanced Raman spectroscopy spectra for some systems of interest in nanoscience and nanotechnology, monochlorobiphenyl isomers, graphene, magnetite and graphene oxide, are presented and discussed. A single gold nanoparticle and a dimer gold nanoparticle were utilized as surface-enhanced Raman spectroscopy substrates. The possibility of predicting structural properties of graphene oxide from comparison between its experimental and simulated Raman spectra is discussed. A relation between the enhancement factor for the G band of graphene oxide and the radius of a spherical nanoparticle dimer of gold, is suggested.

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.

Light-Induced Coalescence of Plasmonic Dimers and Clusters.

The properties of nanoplasmonic structures depend strongly on their geometry, creating the need for high-precision control and characterization. Here, by exploiting the low activation energy of gold atoms on nanoparticle surfaces, we show how laser irradiation reshapes nanoparticle dimers. Time-course dark-field microspectroscopy allows this process to be studied in detail for individual nanostructures. Three regimes are identified: facet growth, formation of a conductive bridge between particles, and bridge growth. Electromagnetic simulations confirm the growth dynamics and allow measurement of bridge diameter, found to be highly reproducible and also self-limiting. Correlations in spectral resonances for the initial and final states give insight into the energy barriers for bridge growth. Dark-field microscopy shows that coalescence of multiple gaps in nanoparticle clusters can be digitally triggered, with each gap closing after discrete increases in irradiation power. Such control is important for light-induced nanowire formation or trimming of electronic and optoelectronic devices.

Surface-enhanced Raman spectroscopic–based aptasensor for Shigella sonnei using a dual-functional metal complex-ligated gold nanoparticles dimer

Herein, we constructed an aptamer-based sensor for the sensitive and highly specific detection of Shigella sonnei via surface enhanced Raman spectroscopy (SERS) analysis. A composite material integrated of the Raman active 4-MBA ligand of the Eu-complex and citrate-stabilized Au nanoparticles (cit-Au NPs) was synthesized and served as both active substrate and Raman reporter. Aptamers targeted to S. Sonnei was then modified onto the surface of this dual-functional material. With the introduction of S. Sonnei, aptamer bound with target with high affinity and specificity, leaving the dual-functional material onto the bacteria. The SERS intensity response showed a strong positive linear correlation (R = 0.9956) with increasing concentrations of S. sonnei (ranging from 10 to 106 cfu/mL). High specificity was achieved at Shigella species (S. dysenteriae, S. flexneri, S. boydii) and other common bacteria (Salmonella typhimurium, Staphylococcus aureus and Escherichia coli). When applied in real samples, the approach showed recoveries from 92.6 to 103.8 %. The designed approach holds great potential for the construction of various aptasensors for the effective and convenient detection of different food hazards.

Anti-crossing property of strong coupling system of silver nanoparticle dimers coated with thin dye molecular films analyzed by electromagnetism.

Evidence of strong coupling between plasmons and molecular excitons for plasmonic nanoparticle (NP) dimers exhibiting ultra-sensitive surface enhanced resonant Raman scattering is the observation of anti-crossing in the coupled resonance. However, experimentally tuning the plasmon resonance of such dimers for the observation is difficult. In this work, we calculate the anti-crossing property of dimers coated with thin dye films according to the classical electromagnetism. This property is quantitatively evaluated according to the coupled oscillator model composed of a plasmon and a molecular exciton representing the molecular multi-level system. A comparison of the film thickness dependences of dimer spectral changes with those of silver ellipsoidal NPs indicates that the dipole plasmons localized in the dimer gap are coupled with molecular excitons of the film much stronger than the dipole plasmons of ellipsoidal NPs. Furthermore, the anti-crossing behavior of coupled resonances is investigated while tuning plasmon resonance by changing the morphology and refractive index of the surrounding medium. The spectral changes observed for ellipsoidal NPs clearly exhibit anti-crossing property; however, the anti-crossing behavior of dimers is more complex due to the strong coupling of dipoles and higher-order plasmons with multiple molecular excitons. We find that the anti-crossing for dimers is clearly confirmed by the refractive index dependence of coupled resonance.

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.

Hollow PtFe Alloy Nanoparticles Derived from Pt-Fe3 O4 Dimers through a Silica-Protection Reduction Strategy as Efficient Oxygen Reduction Electrocatalysts.

The development of efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) is critical for the large-scale production of fuel cells. Platinum (Pt) nanoparticle catalysts show excellent performance for ORR, though the high cost of Pt is a limiting factor that directly impacts fuel cell production costs. Alloying Pt with other transition metals is an effective strategy to reduce Pt utilization whilst maintaining good ORR performance. In this work, novel hollow PtFe alloy catalysts were successfully synthesized by high-temperature pyrolysis of SiO2 -coated Pt-Fe3 O4 nanoparticle dimers supported on carbon at 900 °C, followed by SiO2 shell removal and partial dealloying of the PtFe nanoparticles formed using HF. The obtained hollow PtFe nanoparticle catalysts (denoted herein as PtFe-900) showed a 2.3-fold enhancement in ORR mass activity compared to PtFe nanoparticles synthesized without SiO2 protection, and a remarkable 7.8-fold enhancement relative to a commercial Pt/C catalyst. Further, after 10 000 potential cycles, the ORR mass activity of PtFe-900 remained very high (90.9 % of the initial mass activity). The outstanding ORR performance of PtFe-900 can be attributed to the modification of Pt lattice and electronic structure by alloying with Fe at high temperature under the protection of the SiO2 coating. This work guides the development of improved, highly dispersed Pt-based alloy nanoparticle catalysts for ORR and fuel cell applications.

Constructing a library of metal and metal–oxide nanoparticle heterodimers through colloidal assembly

Nanoparticle dimers composed of different metals or metal oxides, as well as different shapes and sizes, are of wide interest for applications ranging from nanoplasmonic sensing to nanooptics to biomedical engineering. Shaped nanoparticles, like triangles and nanorods, can be particularly useful in applications due to the strong localized plasmonic hot-spot that forms at the tips or corners. By placing catalytic, but traditionally weakly- or non-plasmonic nanoparticles, such as metal oxides and metals like palladium, in these hot-spots, an enhanced function for sensing, photocatalysis or optical use is predicted. Here, we present an electrostatic colloidal assembly strategy for nanoparticles, incorporating different sizes, shapes and metal or metal oxide compositions into heterodimers with smaller gaps than are achievable using nanofabrication techniques. This versatile method is demonstrated on 14 combinations, including a variety of shaped gold nanoparticles as well as palladium, iron oxide, and titanium oxide nanoparticles. These colloidal nanoparticles are stabilized with traditional surfactants, such as citrate, CTAB, PVP and oleic acid/oleylamines, indicating the wide applicability of our approach. Heterodimers of gold and palladium are further analyzed using cathodoluminescence to demonstrate the tunability of these "plasmonic molecules". Since systematically altering the absorption and emission of the plasmonic nanoparticles dimers is crucial to extending their functionality, and small gap sizes produce the strongest hot-spots, this method indicates that the electrostatic approach to heterodimer assembly can be useful in creating new nanoparticle dimers for many applications.

Independent engineering of individual plasmon modes in plasmonic dimers with conductive and capacitive coupling

Abstract We revisit plasmon modes in nanoparticle dimers with conductive or insulating junction resulting in conductive or capacitive coupling. In our study, which combines electron energy loss spectroscopy, optical spectroscopy, and numerical simulations, we show the coexistence of strongly and weakly hybridised modes. While the properties of the former ones strongly depend on the nature of the junction, the properties of the latter ones are nearly unaffected. This opens up a prospect for independent engineering of individual plasmon modes in a single plasmonic antenna. In addition, we show that Babinet’s principle allows to engineer the near field of plasmon modes independent of their energy. Finally, we demonstrate that combined electron energy loss imaging of a plasmonic antenna and its Babinet-complementary counterpart allows to reconstruct the distribution of both electric and magnetic near fields of localised plasmon resonances supported by the antenna, as well as charge and current antinodes of related charge oscillations.

Reducing the loss of electric field enhancement for plasmonic core–shell nanoparticle dimers by high refractive index dielectric coating

Plasmonic nanoparticle (NP) dimers, generating highly intense areas of electric field enhancement named hot spots, have been playing a vital role in various applications like surface enhanced Raman scattering. For stabilization and functionalization, such metallic NPs are often coated with dielectric shells, yet suffer from a rapid degeneration of the hot spot with the increase of the shell thickness. Herein, it is demonstrated that the use of appropriately high refractive dielectric coatings can greatly reduce the loss of local electric field enhancement, maintaining usable hot spots. Two mechianisms work synergistically. Firstly, the high refractive index dielectric coating enables a great leap of the local electric fields reaching the gap, which follows the boundary conditions at the interface within electrodynamics. Secondly, owing to its strong Mie resonances that can participate in the plasmon hybridization, the high refractive index dielectric coating contributes to a strong light coupling effect in terms of improving the light absorption. Taking advantage of the proposed physical process decomposition, both the resonance shift and local electric field enhancement can be elaborated. These findings should be of significant importance in extended applications of surface enhanced spectroscopies and related plasmonic devices based on hot spots.

Ag Ion Soldering: An Emerging Tool for Sub-nanomeric Plasmon Coupling and Beyond.

Self-assembly represents probably the most flexible way to construct metastructured materials and devices from a wealth of colloidal building blocks with synthetically controllable sizes, shapes, and elemental compositions. In principle, surface capping is unavoidable during the synthesis of nanomaterials with well-defined geometry and stability. The ligand layer also endows inorganic building blocks with molecular recognition ability responsible for their assembly into desired structures. In the case of plasmonic nanounits, precise positioning of them in a nanomolecule or an ordered nanoarray provides a chance to shape their electrodynamic behaviors and thereby assists experimental demonstration of modern nanoplasmonics toward practical uses. Despite previous achievements in bottom-up nanofabrication, a big challenge exists toward strong coupling and facile charge transfer between adjacent nanounits in an assembly. This difficulty has impeded a functional development of plasmonic nanoassemblies. The weakened interparticle coupling originates from the electrostatic and steric barriers of ionic/molecular adsorbates to guarantee a good colloidal stability. Such a dilemma is rooted in fundamental colloidal science, which lacks an effective solution. During the past several years, a chemical tool termed Ag ion soldering (AIS) has been developed to overcome the above situation toward functional colloidal nanotechnology. In particular, a dimeric assembly of plasmonic nanoparticles has been taken as an ideal model to study plasmonic coupling and interparticle charge transfer. This Account starts with a demonstration of the chemical mechanism of AIS, followed by a verification of its workability in various self-assembly systems. A further use of AIS to realize postsynthetic coupling of DNA-directed nanoparticle clusters evidences its compatibility with DNA nanotechnology. Benefiting from the sub-nanometer interparticle gap achieved by AIS, a conductive pathway is established between two nanoparticles in an assembly. Accordingly, light-driven charge transfer between the conductively bridged plasmonic units is realized with highly tunable resonance frequencies. These situations have been demonstrated by thermal/photothermal sintering of silica-isolated nanoparticle dimers as well as gap-specific electroless gold/silver deposition. The regioselective silver deposition is then combined with galvanic replacement to obtain catalytically active nanofoci (plasmonic nanogaps). The resulting structures are useful for real time and on-site Raman spectroscopic tracking of chemical reactions in the plasmonic hotspots (nanogaps) as well as for study of plasmon-mediated/field-enhanced catalysis. The Account is concluded by a deeper insight into the chemical mechanism of AIS and its adaption to conformation-rich structures. Finally, AIS-enabled functional pursuits are suggested for self-assembled materials with strongly coupled and easily reshapable physicochemical properties.

Competitive aptasensor with gold nanoparticle dimers and magnetite nanoparticles for SERS-based determination of thrombin.

It is known that the intensity of surface-enhanced Raman scattering (SERS) of monomeric gold nanoparticles (GNPs) is insufficient for ultrasensitive analysis. The authors describe dimeric GNPs for use in a competitive SERS and aptamer based assay for thrombin. The reagent 1,2-bis(4-pyridyl) ethylene serves as both the coupling agent and the Raman reporter on the GNP dimers. In the presence of thrombin, the hybridization of two aptamers, one attached to the GNP dimers, the other to magnetic nanoparticles, is competitively prevented. This method takes advantage of the unique "hot spots" of the GNP dimers to amplify the Raman signal. This results in an ultra-sensitive thrombin assay when compared to assays using GNP monomers. The limit of detection is as low as 1 fM of thrombin. The Raman intensity, best measured at 1612cm-1, increases linearly in the 1 fM to 10nM thrombin concentration range. The method was applied to the determinaiton of thrombin in spiked simulated body fluid and human serum. Graphical abstract This method takes advantage of the unique "hot spots" of the gold nanoparticle dimers to amplify the Raman signal. The dimers are linked to the magnetic nanoparticles via an aptamer. The use of both competitive displacementand magnetic separation greatly improves the sensitivity of the thrombin assay.

Full-wave electromagnetic modes and hybridization in nanoparticle dimers

The plasmon hybridization theory is based on a quasi-electrostatic approximation of the Maxwell’s equations. It does not take into account magnetic interactions, retardation effects, and radiation losses. Magnetic interactions play a dominant role in the scattering from dielectric nanoparticles. The retardation effects play a fundamental role in the coupling of the modes with the incident radiation and in determining their radiative strength; their exclusion may lead to erroneous predictions of the excited modes and of the scattered power spectra. Radiation losses may lead to a significant broadening of the scattering resonances. We propose a hybridization theory for non-Hermitian composite systems based on the full-Maxwell equations that, overcoming all the limitations of the plasmon hybridization theory, unlocks the description of dielectric dimers. As an example, we decompose the scattered field from silicon and silver dimers, under different excitation conditions and gap-sizes, in terms of dimer modes, pinpointing the hybridizing isolated-sphere modes behind them.

Plasmonic properties and optimization of ultraviolet surface-enhanced Raman spectroscopy

The investigation of plasmonic properties and optimization of ultraviolet surface-enhanced Raman spectroscopy (UV-SERS) are essential for improving the detective capability in terms of high sensitivity and application range. In this work, the finite element method (FEM) are performed for the UV-SERS using Al nanoparticles (NPs) dimer. We theoretically explore the influence of incident angle, radius and separation of NPs dimer on the electric enhancement. The simulation results show that the optimized radius and separation of NPs dimer are 30 nm and 1 nm for UV-SERS measurements under the normal incidence condition, where the strongest Raman enhancement factor reach as high as 9 orders with excitation wavelength of 215 nm. Our results may help to find the optimal geometric factor for further improving the performance of the UV-SERS system.

Arrays of Plasmonic Nanoparticle Dimers with Defined Nanogap Spacers.

Plasmonic molecules are building blocks of metallic nanostructures that give rise to intriguing optical phenomena with similarities to those seen in molecular systems. The ability to design plasmonic hybrid structures and molecules with nanometric resolution would enable applications in optical metamaterials and sensing that presently cannot be demonstrated, because of a lack of suitable fabrication methods allowing the structural control of the plasmonic atoms on a large scale. Here we demonstrate a wafer-scale "lithography-free" parallel fabrication scheme to realize nanogap plasmonic meta-molecules with precise control over their size, shape, material, and orientation. We demonstrate how we can tune the corresponding coupled resonances through the entire visible spectrum. Our fabrication method, based on glancing angle physical vapor deposition with gradient shadowing, permits critical parameters to be varied across the wafer and thus is ideally suited to screen potential structures. We obtain billions of aligned dimer structures with controlled variation of the spectral properties across the wafer. We spectroscopically map the plasmonic resonances of gold dimer structures and show that they not only are in good agreement with numerically modeled spectra, but also remain functional, at least for a year, in ambient conditions.