On the Plasmonic Properties of Ag@SiO2@Graphene Core-Shell Nanostructures

Plasmonics - This work reports on a study regarding the plasmonic properties of the Ag@SiO2@Graphene core-shell nanostructures, in the wavelength range of 0.3–2 μ m. Spherical and...     

Plasmonic Focusing in Nanostructures

In this review, we show that by designing the metallic nanostructures, the surface plasmon (SP) focusing has been achieved, with the focusing spot at a subwavelength scale. The central idea is based on the principle of optical interference that the constructive superposition of SPs with phase matching can result in a considerable electric-field enhancement of SPs in the near field, exhibiting a pronounced focusing spot. We first reviewed several new designs for surface plasmon focusing by controlling the metallic geometry or incident light polarization: We made an in-plane plasmonic Fresnel zone plates, a counterpart in optics, which produces an obvious SP focusing effect; We also fabricated the symmetry broken nanocorrals which can provide the spatial phase difference for SPs, and then we propose another plasmon focusing approach by using semicircular nanoslits, which gives rise to the phase difference through changing refractive index of the medium in the nanoslits. Further, we showed that the spiral metallic nanostructure can be severed as plasmonic lens to control the plasmon focusing under a linearly polarized light with different angles.

     

Photothermal Enhancement in Core-Shell Structured Plasmonic Nanoparticles

Plasmonic nanoparticles (NPs) with photothermal effects can be exploited as efficient heat sources in various applications. Here, the photothermal properties in core-shell structured plasmonic NPs, including metal/silica NP, silica/metal NP, and metal/silica/metal NP, are investigated. Compared with bare metal NPs, the core-shell plasmonic NPs not only exhibit extremely agile tunability in the surface plasmon resonances but also show considerably enhanced photothermal effects in terms of the maximum temperature rise. For metal/silica NPs and metal/silica/metal NPs, the SiO₂ shells function as effective thermal-protective layers for enhanced photothermal effect. For silica/metal NPs, the SiO₂ core and the metal shell show uniform temperature rise. These findings are essential for applying the core-shell structured plasmonic NPs on photothermal imaging, nanofluidics, etc.     

Surface-Assisted Carrier Excitation in Plasmonic Nanostructures

We present a quantum-mechanical model for surface-assisted carrier excitation by optical fields in plasmonic nanostructures of arbitrary shape. We derive an explicit expression, in terms of local fields inside the metal structure, for surface absorbed power and surface scattering rate that determine the enhancement of carrier excitation efficiency near the metal-dielectric interface. We show that surface scattering is highly sensitive to the local field polarization and can be incorporated into metal-dielectric function along with phonon and impurity scattering. We also show that the obtained surface scattering rate describes surface-assisted plasmon decay (Landau damping) in nanostructures larger than the nonlocality scale. Our model can be used for calculations of plasmon-assisted hot carrier generation rates in photovoltaics and photochemistry applications.     

Plasmonic Optical Properties and Applications of Metal Nanostructures

In this review article, we provide an overview of recent research activities in the study of plasmonic optical properties of metal nanostructures with emphasis on understanding the relation between surface plasmon absorption and structure. Both experimental results and theoretical calculations have indicated that the plasmonic absorption strongly depends on the detailed structure of the nanomaterials. Examples discussed include spherical nanoparticles, nanorods, nanowires, hollow nanospheres, aggregates, and nanocages. Plasmon–phonon coupling measured from dynamic studies as a function of particle size, shape, and aggregation state is also reviewed. The fascinating optical properties of metal nanostructures find important applications in a number of technological areas including surface plasmon resonance, surface-enhanced Raman scattering, and photothermal imaging and therapy. Their novel optical properties and emerging applications are illustrated using specific examples from recent literature. The case of hollow nanosphere structures is highlighted to illustrate their unique features and advantages for some of these applications.     

Effective Dielectric Constant of Plasmonic Nanofluid Containing Core-Shell Nanoparticles

This paper focuses on the effective dielectric constant of water-based plasmonic nanofluid containing SiO₂/Ag core/shell nanoparticles (NPs). Two effective models, based on S-parameter retrieval method and Maxwell-Garnett effective medium theory, are employed. The effective dielectric constants predicted by the two effective models are compared and the applicability is evaluated by comparing the reflectance and absorptance. Three influence factors, including volume fraction, core-shell ratio, and size of NPs, are considered. Results show both of the two effective models can predict reliable effective dielectric constants when the volume fraction, size, and core-shell ratio of nanoparticles are 5%, 25 nm, and 4:1 respectively. Only small deviations appear in the resonant region under this condition. With the increase of volume fraction, shell proportion, or size, deviations in the resonant region become larger for both of the two effective models. Therefore, the predicted effective dielectric constants are not suitable for the prediction of optical properties, because the resonant region is the key region of the solar conversion for plasmonic nanofluids. Hence, the parameters of NPs need to be changed to make the effective models applicable. Moreover, the effective model based on S-parameter retrieval can predict more reliable dielectric constant than the effective model based on Maxwell-Garnett theory.

     

Plasmonic Properties of Gold Nanostructures on Gold Film

This paper reports on a systematic study of the plasmonic properties of periodic arrays of gold cylindrical nanoparticles in contact with a gold thin film. Depending on the gold film thickness, it observes several plasmon bands. Using a simple analytical model, it is able to assign all these modes and determine that they are due to the coupling of the grating diffraction orders with the propagating surface plasmons travelling along the film. With finite difference time domain (FDTD) simulations, it demonstrates that large field enhancement occurs at the surface of the nanocylinders due to the resonant excitation of these modes. By tilting the sample, it also observes the evolution of the spectral position of these modes and their tuning through nearly the whole visible range is possible. Such plasmonic substrates combining both advantages of the propagative and localised surface plasmons could have large applications in enhanced spectroscopies.

     

Localized Surface Plasmon Resonances of Simple Tunable Plasmonic Nanostructures

Plasmonics - We derive and present systematic relationships between the analytical formulas for calculation of the localized surface plasmon resonances (LSPR) of some plasmonic nanostructures which...     

Near- and Far-Field Plasmonic Properties of Different Types of Eccentric Core-Shell Nanodimers

Finite element method (FEM) simulations have been carried out on free-standing and finite dielectric substrate-supported eccentric (i) silica core-gold nanoshell dimers and (ii) gold core-silica nanoshell dimers for understanding their near- and far-field plasmonic properties. In the case of eccentric silica core-gold nanoshell dimers, multiple peaks are observed in the near- and far-field spectra due to the plasmon hybridization. The number of peaks is found to be sensitive to the core offset parameters of the nanoshells forming nanodimer. The wavelength locations of the peaks due to the constructive coupling of the lower order modes found relatively more sensitive to the dielectric substrate. The number of peaks in the near- and far-field spectra found the same presence and absence of the dielectric substrate. The values of full width at half maximum (FWHM) of the peaks observed in the near-field spectra are found larger as compared to those observed in the far-field spectra. In contrast, in the case of eccentric gold core-silica nanoshell dimers, multiple peaks have not been observed. The FWHM of the observed peak is found sensitive to the core offset parameters of the nanoshells, and the number of peaks in the near field- and far-field spectra found not same in the presence and absence of the dielectric substrate. Moreover, the differences in near- and far-field spectra of plasmonically coupled (i) concentric nanoshells, (ii) eccentric nanoshells, and (iii) concentric and eccentric nanoshells also investigated numerically.

     

Plasmonic Nanostructures as Accelerators for Nanoparticles: Optical Nanocannon

We suggest a model of an optical structure that allows to accelerate nanoparticles to velocities on the order of tens of centimeters per second using low-intensity external optical fields. The nano-accelerator system employs metallic V-grooves which concentrate the electric field in the vicinity of their bottoms and creates large optical gradient forces for the nanoparticles in that groove. The conditions are found when this optical force tends to eject particles away from the groove.     

Orientation Effects on Plasmonic Heating of Near-Infrared Colloidal Gold Nanostructures

Plasmonics - Photothermal therapy assisted by plasmonic nanostructure relies on the absorption of light energy by the metallic nanoparticle. The manifestation of a rational use of...     

Synthesis and Plasmonic Property of Ag Nanorods

Plasmonics - In this paper, Ag nanorods (AgNRs) with different aspect ratios (ARs) were prepared by a seed-mediated fast growth approach. The possible growth mechanism of Ag nanostructures was...     

Plasmonic Perovskite Solar Cells Utilizing Au@SiO<Subscript>2</Subscript> Core-Shell Nanoparticles

The role of Au@SiO₂ core-shell nanoparticles on optical properties of perovskite solar cells has been explored using both the theoretical computations and the experiments. A quasi-static model is used to study the surface plasmon resonances (SPRs) of Au@SiO₂ core-shell nanospheres. Au@SiO₂ core-shell nanoparticles, with varying shell thickness and core radius, were assumed to be embedded in methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite active layer. Enhanced absorption in the active layer is obtained due to the near-field plasmonic effect of the embedded core-shell nanoparticles. Theoretical modelling shows that a shell thickness of 1 nm and core diameter of 20 nm provide absorption enhancement in the orange-red region of the electromagnetic spectrum. Experiments performed using ～20-nm-sized Au@SiO₂ core-shell nanoparticles (with a shell thickness of ～1 nm) clearly demonstrate the enhanced absorption and the resulting enhancement in photocurrent due to the plasmonic effects. An efficiency enhancement of over 18 % is obtained for the best plasmonic perovskite solar cell containing Au@SiO₂ nanoparticles in Au@SiO₂-TiO₂ weight ratio of ～1 %. Incident photon-to-current conversion efficiency (IPCE) data also showed enhancement in photocurrent for the plasmonic device. The quasi-static modelling approach provides a good correlation between theory and experiment.     

Finite and Boundary Element Methods for Simulating Optical Properties of Plasmonic Nanostructures

In this study, a numerical investigation was done on the optical properties of silver nanostructures using the boundary element method (BEM) and finite element method (FEM). The BEM simulation was done using a freely available code called MNBEM in MATLAB with minor modifications. The FEM simulation was done by Comsol Multiphysics, a commercial software package. Silver nanostructures in the sphere, rod, and triangle geometries and the presence of different polarization angles were compared between these two methods. According to the obtained results, the absorption cross-sections for both BEM and FEM were consistent with their actual optical properties. For instance, both longitudinal and transverse resonance modes were observed in the case of nanorods, and all three in–plane dipole, in–plane quadrupole, and out–plane quadrupole plasmon resonances were observed successfully obtained for triangular nanostructures. Although both BEM and FEM results were similar to each other (from the number and position of the peaks in the final spectra), this similarity was decreased as the anisotropy was increased in the structure. For example, nearly 40 nm difference was observed between the BEM and FEM results in the triangular nanostructures, even though the trends and shape of the peaks were similar. It was revealed that specific points should be considered in the discretization process (especially the corner fillets) to close the gap in the obtained results from BEM and FEM. According to the obtained results, BEM significantly reduces the computational cost and time by discretizing only the boundary of the domain. A self-written software was developed to predict the optical cross-section of a plasmonic-ensemble consisting of spherical, rod-shaped, and triangular nanostructures, which can be used in different disciplines such as plasmon-enhanced solar cells, plasmon-enhanced photocatalysis, and plasmon-enhanced fluorescence.

Graphical Abstract

     

Fabrication of Gold Nanostructures and Studies of Their Morphological and Surface Plasmonic Properties

We report fabrication of gold nanostructures on glass and indium tin oxide (ITO)-coated glass substrates using high fluence and highly energetic gold ions generated by hot, dense, and strongly non-equilibrium plasma. Nanodots and nanorods are observed in scanning electron microscopy (SEM) of nanostructures grown on glass substrate with single and double shots of gold ions which is in conformity with the transmission electron microscopy image. SEM images for single and double shots of gold ions on ITO-coated glass substrate show only nanodots. The mean diameter of nanodots obtained on both glass and ITO-coated glass is found to increase with increase in the number of gold ions shot from one to two. The gold nanostructures exhibit red shift in surface plasmon resonance with increased interaction which is in agreement with other reported work.     

Utilization of Plasmonic and Photonic Crystal Nanostructures for Enhanced Micro- and Nanoparticle Manipulation

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological research. Perhaps the most widely used physical force to achieve noninvasive manipulation of small particles has been dielectrophoresis(DEP).¹ However, DEP on its own lacks the versatility and precision that are desired when manipulating cells since it is traditionally done with stationary electrodes. Optical tweezers, which utilize a three dimensional electromagnetic field gradient to exert forces on small particles, achieve this desired versatility and precision.² However, a major drawback of this approach is the high radiation intensity required to achieve the necessary force to trap a particle which can damage biological samples.³ A solution that allows trapping and sorting with lower optical intensities are optoelectronic tweezers (OET) but OET''s have limitations with fine manipulation of small particles; being DEP-based technology also puts constraint on the property of the solution.^4,5This video article will describe two methods that decrease the intensity of the radiation needed for optical manipulation of living cells and also describe a method for orientation control. The first method is plasmonic tweezers which use a random gold nanoparticle (AuNP) array as a substrate for the sample as shown in Figure 1. The AuNP array converts the incident photons into localized surface plasmons (LSP) which consist of resonant dipole moments that radiate and generate a patterned radiation field with a large gradient in the cell solution. Initial work on surface plasmon enhanced trapping by Righini et al and our own modeling have shown the fields generated by the plasmonic substrate reduce the initial intensity required by enhancing the gradient field that traps the particle.^6,7,8 The plasmonic approach allows for fine orientation control of ellipsoidal particles and cells with low optical intensities because of more efficient optical energy conversion into mechanical energy and a dipole-dependent radiation field. These fields are shown in figure 2 and the low trapping intensities are detailed in figures 4 and 5. The main problems with plasmonic tweezers are that the LSP''s generate a considerable amount of heat and the trapping is only two dimensional. This heat generates convective flows and thermophoresis which can be powerful enough to expel submicron particles from the trap.^9,10 The second approach that we will describe is utilizing periodic dielectric nanostructures to scatter incident light very efficiently into diffraction modes, as shown in figure 6.¹¹ Ideally, one would make this structure out of a dielectric material to avoid the same heating problems experienced with the plasmonic tweezers but in our approach an aluminum-coated diffraction grating is used as a one-dimensional periodic dielectric nanostructure. Although it is not a semiconductor, it did not experience significant heating and effectively trapped small particles with low trapping intensities, as shown in figure 7. Alignment of particles with the grating substrate conceptually validates the proposition that a 2-D photonic crystal could allow precise rotation of non-spherical micron sized particles.¹⁰ The efficiencies of these optical traps are increased due to the enhanced fields produced by the nanostructures described in this paper.Download video file.^{(57M, mov)}     

Luminescence Enhancement and SERS by Self-Assembled Plasmonic Silver Nanostructures in Nanoporous Glasses

It was shown experimentally that the action of continuous electric field on nanoporous silicate glasses with interconnecting pores, containing silver nanoparticles, leads to the spatial redistribution of nanoparticles. The concentration of nanoparticles near the negative electrode increases and results in silver nano- and microdendrite structure growth. The main mechanisms of the described effects are the field emission of silver ions from silver nanoparticles near negative electrode, migration of silver ions in the external electric field to the negative electrode, reduction of silver ions by free electrons, and new silver nanoparticle formation. The experiments have shown that at the ends of microdendrites, local field enhancement appears, which results in luminescence enhancement and in SERS.

     

Shell Thickness-Dependent Plasmon Coupling and Creation of SERS Hot Spots in Au@Ag Core-Shell Nanostructures

The main objective of the present study is to investigate the shell thickness-dependent Raman enhancement activity of silver-coated gold nanoparticles (Au@Ag NPs) when bound to a model analyte 2-mercaptobenzoic acid (2-MBA). With an optimized Ag:Au ratio, dimeric and trimeric Au@Ag nanostructures were prepared in the presence of 2-MBA and are characterized by spectroscopic and microscopic techniques. These dimeric junctions act as hot spots and the molecules trapped at these junctions showed higher Raman signal enhancements due to the presence of amplified electric field.     

Novel Apolar Plasmonic Nanostructures with Extended Optical Tunability for Sensing Applications

This paper outlines the design of complex nanostructures with apolar behavior which pave the way to a wider range of plasmon resonance tuning and applications requiring higher enhancement. These new nanostructure families are simply defined by symmetry considerations. An irreducible decomposition of optical response tensor demonstrates that nanoparticles which belong to C _n , with n?≥?3, symmetry point group for at least one scale have an optical response insensitive on the light polarization. This is experimentally confirmed by extinction and surface-enhanced Raman-scattering measurements.