Preparation and Optical Characterization of Core–Shell Bimetal Nanoparticles

Chemical approaches allow for the synthesis of highly defined metal heteronanostructures, such as core–shell nanospheres. Because the material in the metal nanoparticles determines the plasmon resonance-induced absorption band, control of particle composition results in control of the position of the absorption band. Metal deposition on gold or silver nanoparticles yielded core–shell particles with modified optical properties. UV–vis spectroscopy on solution-grown, as well as surface-grown, particles was conducted and provided ensemble measurements in solution. Increasing the layers of a second metal leads to a shift in the absorption band. A shell diameter comparable to the original particle diameter leads to a predominant influence by the shell material. Extent of shell growth could be controlled by reaction time or the concentration of metal salt or reducing agent. Besides optical characterization, the utilization of atomic force microscopy, scanning electron microscopy, and transmission electron microscopy yielded important information about the ultrastructure of nanoparticle complexes. Surface-grown core–shell particles were superior in terms of achievable shell thickness, because of difficulties encountered with solution-grown particles due to salt-induced aggregation.     

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.

     

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.     

Plasmonic Biosensor Based on Triangular Au/Ag and Au/Ag/Au Core/Shell Nanoprisms onto Indium Tin Oxide Glass

Gold–silver core–shell triangular nanoprisms (Au/AgTNPs) were grown onto transparent indium tin oxide (ITO) thin film-coated glass substrate through a seed-mediated growth method without using peculiar binder molecules. The resulting Au/AgTNPs were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–vis spectroscopy, and cyclic voltammograms. The peak of dipolar plasmonic resonance was located at near infrared region of ～700 nm, which showed the refractive index (RI) sensitivity of 248 nm/RIU. Moreover, thin gold shells were electrodeposited onto the surface of Au/AgTNPs in order to stabilize nanoparticles. Compared with the Au/AgTNPs, this peak of localized surface plasmon resonance (LSPR) was a little red-shift and decreased slightly in intensity. The refractive index sensitivity was estimated to be 287 nm/RIU, which showed high sensitivity as a LSPR sensing platform. Those triangular nanoprisms deposited on the ITO substrate could be further functionalized to fabricate LSPR biosensors. Results of this research show a possibility of improving LSPR sensor by using core–shell nanostructures.     

Comparative Theoretical Study of the Optical Properties of Silicon/Gold,Silica/Gold Core/Shell and Gold Spherical Nanoparticles

The scattering and absorption efficiencies of light by individual silicon/gold core/shell spherical nanoparticles in air are analysed theoretically in the framework of Lorenz-Mie formalism. We have addressed the influence of particle-diameter and gold-shell thickness on the scattering and absorption efficiencies of such nano-heterostructures. For comparison, we also considered the famous silica/gold core/shell nanoparticle and pure gold nanoparticle. Our simulation clearly shows that the optical response of the illuminated Si/Au core/shell nanoparticle differs markedly from that of the famous SiO₂/Au heterostructure which in turn does not show a significant difference with that of the pure gold nanoparticle. This difference is clearly evident for shell thickness to outer particle radius ratio of less than 0.5. It manifests itself essentially by the occurrence of a strong and sharp absorption resonance beyond the wavelength of 600 nm where the silica/gold and the pure gold nanoparticles never absorb. The characteristics of this resonance are found to be sensitive to the particle diameter and the shell thickness. In particular, its spectral position can be adjusted over a wide spectral range from the visible to the mid-IR by varying the particle diameter and/or the shell thickness.     

Simulation of the Nonlinear Optical Properties of Colloids Containing Metallic Core–Dielectric Shell Nanoparticles

Nonlinear (NL) optical properties of composite materials containing metallic core–dielectric shell nanoparticles in aqueous solution were investigated numerically using the Maxwell–Garnett model and the degenerate electron gas model. Influence of geometry and excitation laser intensity was considered to describe the local field factor and the third-order NL susceptibility.     

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.     

Analytical Modeling Based on Modified Effective Medium Theories for Optical Properties of Photovoltaic Material-Incorporated Plasmonic Nanoparticles

Theoretical guidance on the optical properties of plasmonic nanoparticles (NPs) is of significant importance in tremendous numbers of fields like photovoltaics. The incorporation of plasmonic NPs into photovoltaic material can promote optical absorption either via the excitation of localized surface plasmon resonance (LSPR) modes or due to multiple light scattering. Since most fabrication techniques for the incorporation of NPs into photovoltaic material result in a random array of NPs with various sizes, numerical simulations based on solving the Maxwell equations are computationally expensive and prohibitively slow for this large number of NPs. Therefore, in this paper, based on modified effective medium theories, taking into account finite size of NPs, size dispersion for NPs, extrinsic dynamic effect, and intrinsic confinement effect, fast and cost-effective analytical modeling, considering both LSPR and scattering effects, is presented to obtain the optical properties of photovoltaic material incorporated by spherical NPs with nonuniform size and random distribution. Then, by means of presented analytical modeling, considering reasonably low and high volume fractions of NPs in addition to small and large size of NPs, the effect of different parameters of embedded NPs into organic and inorganic photovoltaic materials is explored.

     

Optical Detection of Thiol Drugs by Core–Shell Luminous Carbon Dots—Gold Nanoparticles System

Plasmonics - Nowadays, carbon quantum dots (CQDs) with size less than 10 nm have emerged as one of the most exciting areas of chemical research in the class of inorganic nanomaterials....     

Influence of Anisotropy on Optical Bistability in Plasmonic Nanoparticles with Cylindrical Symmetry

In this paper, the effect of radial anisotropy on optical bistability in the cylindrical nanoshells is theoretically investigated within the quasi-static approximation. We consider two cases: when the shell is anisotropic and the core is nonlinear metal and when the core is anisotropic and the shell is a nonlinear metal. The dependence of optical bistability on the size of the nonlinear/anisotropic shell or core, the embedding medium, the anisotropy parameter, and the type of noble metals as candidates for plasmonics is studied and demonstrated. We show that by changing the type of the plasmonic metal, the switching threshold field changes can be used to design nanoparticle-based all-optical sensors. It is also shown that significant optical bistability and all-optical switching behavior can be obtained in the cylindrical nanoshells due to nonlinearity enhancement via the plasmonic structure.     

The Study of Surface Plasmon in Au/Ag Core/Shell Compound Nanoparticles

Au/Ag core/shell nanoparticles are fabricated by laser-ablating Ag plates in Au colloid solution. The absorption band is found to blue shift with increasing ablation time. Mie theory calculations show that the shift is caused by the increase of the Ag shell thickness. The average Ag shell thickness can be determined from the measured absorption peak. Using the plasmon hybridization approach, we show that the absorption band around 510 nm originates from an anti-bonding mode ω _?+ caused by the interaction between a bonding Ag shell mode ω _?? and Au sphere mode ω _S-Au. The blue shift of the ω _?+ mode with the increase of Ag shell thickness is also well predicted by the hybridization theory.     

Nonlinear Dependences of Optical Properties of Spherical Core–Shell Silver–Gold and Gold–Silver Nanoparticles on Their Parameters

Modeling of nonlinear optical properties of spherical core–shell gold–silver and silver–gold nanoparticles (NPs) placed in water was carried out on the base of extended Mie theory. Efficiency cross sections of absorption σ _abs, scattering σ _sca, and extinction σ _ext of radiation with wavelengths λ?=?400 and 532 nm for core–shell NPs with constant core radii r ₀₀?=?5, 10, 20, and 40 nm and in the range of relative radii r ₁/r ₀₀?=?1–8 were calculated (r ₁ is the radius of shell). Dependences of optical properties of gold–silver and silver–gold NPs on increasing of core radius r ₀ in the range 0???r ₁ under condition r ₁?=?const and increasing of r ₀ under r ₁???r ₀?=?const were investigated. Results show the nonlinear behavior of optical properties of core–shell gold–silver and silver–gold NPs on radiation wavelengths (optical indexes of metals), different core and shell radii, and their correlation, on relative NP radii r ₁/r ₀₀. An increase and decrease of absorption, scattering, and extinction efficiency cross sections of core–shell NPs with changing of wavelengths, core and shell radii, and relative NP radii r ₁/r ₀₀ are established. These dependences can be used for experimental investigation of the interesting first stages of shell formation on core and optical determination of core–shell NP parameters.     

Optical Modeling of Plasmonic Nanoparticles Enhanced Light Emission of Silicon Light-Emitting Diodes

Significant enhancement of radiative efficiency of thin-film silicon light-emitting diodes achieved by placing the active layer in close proximity to silver (Ag) nanoparticles has been observed. In this paper, optical properties including transmission, reflection, and absorption of a random assembly of Ag nanoparticles are theoretically investigated using the effective medium model. Furthermore, the influence of Ag nanoparticles on light emission of silicon light-emitting diodes is studied by an improved effective mode volume model we propose here. The normalized line shape of dipole oscillation is calculated directly using Lorentz–Drude model without using any approximation. Thus, it results in more accurate calculation of the enhanced Purcell factor in comparison with the conventional approach. We show that an enhancement of radiative efficiency of silicon light-emitting diodes can be achieved by localized surface plasmons on metal nanoparticles. The calculated result of optimal Ag nanoparticle size to enhance light emission of silicon light-emitting diodes at 900 nm wavelength is in very good agreement with those obtained from the experimental result. The model is useful for the design of metallic nanoparticles enhanced light emitters.     

Antiviral Activity of Gold/Copper Sulfide Core/Shell Nanoparticles against Human Norovirus Virus-Like Particles

Human norovirus is a leading cause of acute gastroenteritis worldwide in a plethora of residential and commercial settings, including restaurants, schools, and hospitals. Methods for easily detecting the virus and for treating and preventing infection are critical to stopping norovirus outbreaks, and inactivation via nanoparticles (NPs) is a more universal and attractive alternative to other physical and chemical approaches. Using norovirus GI.1 (Norwalk) virus-like particles (VLPs) as a model viral system, this study characterized the antiviral activity of Au/CuS core/shell nanoparticles (NPs) against GI.1 VLPs for the rapid inactivation of HuNoV. Inactivation of VLPs (GI.1) by Au/CuS NPs evaluated using an absorbance-based ELISA indicated that treatment with 0.083 μM NPs for 10 min inactivated ~50% VLPs in a 0.37 μg/ml VLP solution and 0.83 μM NPs for 10 min completely inactivated the VLPs. Increasing nanoparticle concentration and/or VLP-NP contact time significantly increased the virucidal efficacy of Au/CuS NPs. Changes to the VLP particle morphology, size, and capsid protein were characterized using dynamic light scattering, transmission electron microscopy, and Western blot analysis. The strategy reported here provides the first reported proof-of-concept Au/CuS NPs-based virucide for rapidly inactivating human norovirus.     

Synthesis,Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.     

Enhancement of Extraordinary Optical Transmission in a Double Heterostructure Plasmonic Bandgap Cavity

In this paper, a novel plasmonic bandgap cavity inducing the enhancement of extraordinary optical transmission is presented. Numerical simulations have been performed to model a free-standing structure made of a one-dimensional periodic arrangement of gold strips. Two different values of the lattice constant have been properly chosen to realize a double heterostructure-like cavity to accomplish extraordinary optical transmission assisted by the formation of a plasmonic bandgap in the adjacent regions. Numerical results prove the capability of this optical device to efficiently transmit input light beams with far-field transmission values close to 100% due to the excitation of surface plasmon polariton resonant modes.     

Plasmonic Sensor Based on Silver Nanoparticles for the Detection of Glucose

Plasmonics - Sensors for detecting glucose concentrations are crucial to medical testing. Here, we introduce silver nanoparticles (Ag NPs) uniformly distributed in space to investigate the sensing...     

The Effect of Negative Curvature on the Plasmonic Coupling of Concentric Core–Shell Metallic Nanostructures

Negative curvature-dependent localized surface plasmon resonance (LSPR) properties of concentric core–shell metallic nanostructure have been studied using quasistatic approach and plasmon hybridization theory. Whether in single-layered gold nanoshell or double gold nanoshells, the oscillating surface charges always concentrate close to the poles of the metal surface with negative curvature, which results in the anisotropic local electric field distribution and affects both the inter-surface plasmonic coupling and inter-shell plasmonic coupling. Therefore, the change of the radius of the gold surface with negative curvature could modulate the plasmon hybridization and lead to the LSPR shifting. The physical mechanism of the negative curvature-dependent LSPR presents a potential for design and fabrication of nanoscale optical device based on core–shell type metallic nanostructures.     

Efficiency Enhancement of Ultra-thin CIGS Solar Cells Using Bandgap Grading and Embedding Au Plasmonic Nanoparticles

The objective of this study is to enhance the efficiency of copper indium gallium selenide (CIGS) solar cells. To accomplish that, composition grading of absorber layer was carried out by using SILVACO’s technology aided computer design (TCAD) ATLAS program. Results showed a meaningful improvement of output parameters including open-circuit voltage (V_oc), short-circuit current (I_sc), fill factor (FF), and power conversion efficiency (η). For further performance improvement of the cell, Au plasmonic scattering nanoparticles were loaded on the top of the ZnO window layer. Plasmonic nanoparticles can restrict, absorb, navigate, or scatter the incident light. By using the spherical Au nanoparticles, a very good increase in the light absorption in the cell over the reference planar CIGS solar cell was observed. The highest η = 19.01% was achieved for the designed ultra-thin bandgap-graded CIGS solar cell decorated by Au nanoparticles.

     

Broadband Tunable and Double Dipole Surface Plasmon Resonance by TiO2 Core/Ag Shell Nanoparticles

A design of a TiO₂ core and Ag shell spherical nanoparticle is theoretically presented. The nanoparticles display double dipole plasmonic resonance peaks: one located at the ultraviolet range, the other is widely tunable from the visible to the near infrared region. The tunability can be easily controlled by varying the sizes of the core and the shell. The near field patterns of the double plasmonic resonance peaks are analyzed, and the dipole resonance modes for those two peaks are confirmed for the suitable core–shell sizes.