Blue-Shifted SPR of Au Nanoparticles with Ordering of Carbon by Dense Ionization and Thermal Treatment

Thin films of carbon-containing Au nanoparticles (NPs), prepared by the co-sputtering using a neutral Ar atom beam, were irradiated by 120 MeV Ag ions and also annealed, separately, at increasing temperatures in inert atmosphere. The surface plasmon resonance (SPR) band of the nanocomposite film was observed to be blue shifted (～50 nm) in both cases, with increasing fluence and temperature. The structural changes of Au NPs embedded in amorphous carbon matrix were investigated using X-ray diffraction and transmission electron microscopy. A growth of Au NPs was observed with increasing fluence and also with increasing temperature. A percolation of Au NPs was observed at 500 °C. A growth of Au NPs with ion irradiation is explained in the framework of a thermal spike model. Raman spectroscopy revealed the ordering of a-C thin films with increasing fluence and temperature, which is ascribed to a change of refractive index and the blue shift of the SPR band.     

Modulation of Visible and Near-Infrared Surface Plasmon Resonance of Au Nanoparticles Based on Highly Doped Graphene

We study an active modulation of surface plasmon resonance (SPR) of Au nanoparticles based on highly doped graphene in visible and near-infrared regions. We find that compared to the traditional metal SPR, the SPR of Au nanoparticles based on graphene causes a remarkable blue shift. The field intensity in the gap is redistributed to standing wave. The field intensity of standing wave is about one order of magnitude higher than the traditional model. Moreover, the SPR of Au nanoparticles can be actively modulated by varying the graphene Fermi energy. We find the maximum modulation of field intensity of absorption spectra is more than 21.6 % at λ?=?822?nm and the amount of blue shift is 17.4 nm, which is about 2.14 % of the initial wavelength λ ₀?=?813.4?nm, with increasing monolayer graphene Fermi energy from 1.0 to 1.5 ev. We find that the SPR sensitivity to the refractive index n of the environment is about 642 nm per refractive index unit (RIU). The SPR wavelengths have a big blue shift, which is about 33 nm, with increasing number of graphene layers from 1 to 3, and some shoulders on the absorption spectra are observed in the models with multilayer graphene. Finally, we study the Au nanorod array based on monolayer graphene. We find that the blue shift caused by the graphene increases from 14 to 24 nm, with increasing gap g _y from 10 to 20 nm. Then, it decreases from 24 to 14 nm, with increasing gap g _y from 20 to 50 nm. This study provides a new way for actively modulating the optical and optoelectronic devices.     

Colloidal Synthesis of Plasmonic Metallic Nanoparticles

Solutions of Ag and Au nanoparticles are strongly colored because of localized surface plasmon resonance in the UV/visible spectral region. The optical properties of these nanoparticles may be tuned to suit the needs of the application. This article summarizes our work in recent years on the solution synthesis of nanoparticles with tunable optical properties. The systems of interest include zero-dimensional bimetallic Ag–Au nanoparticles with different structures, one-, two-, and three-dimensional anisotropic monometallic Ag or Au nanoparticles. All of these nanosystems were prepared from colloidal synthesis through simple changes in the synthesis conditions. This is a demonstration of the versatility of colloidal synthesis as a convenient scalable technique for tuning the properties of metallic nanoparticles. Zhang, Tan, and Xie contributed equally to this article     

Study of Broadband Tunable Properties of Surface Plasmon Resonances of Noble Metal Nanoparticles Using Mie Scattering Theory: Plasmonic Perovskite Interaction

We suggest semi-analytical approach to study the optical properties of noble metal nanoparticles and their interaction to the perovskite material (methyl ammonia lead halide: CH₃NH₃PbI₃). Metal nanoparticles embedded in perovskite matrix exhibits broadband surface plasmon resonances, and the tunability of these plasmonic resonances is highly sensitive to particle size. The calculation of optical cross section have been done using Mie scattering theory which is applicable to arbitrary size and spherical-shape metal nanoparticles. We have taken five different radii ranging from 15 to 100 nm to understand the plasmonic resonances and its spectral width in the wavelength range 300 to 800 nm. Out of these noble metal nanoparticles, silver have highest scattering efficiency nearly of the order of 18 for the case of 15 nm radii at resonance wavelength 613 nm. Our finding reveals a new concept to understand the applications of plasmonic resonances in order to enhance the photon absorption inside the thin film of perovskite.     

Plasmonic and Energy Studies of Ag Nanoparticles in Silica-Titania Hosts

The present work reports an investigation of surface plasmon resonance (SPR) of silver nanoparticles in SiO₂–TiO₂ hosts. The surface plasmon resonance of silver nanoparticles was observed in the wavelength range 300–400 nm. Numerical calculation of SPR of silver nanoparticles with spherical morphology was done on the basis of discrete dipole approximation (DDA) method. The observed fluorescence spectrum fits well with the theoretically calculated one. The luminescence enhancement is attributed to the strong local electric field which increases the exciting and emitting photons coupled to SPR. An effort has been made to study the surface plasmon mediated excitation energy transfer (EET) between two spherical metal nanoparticles. The van der Waals (vdW) energy between plasmonic silver nanoparticles in the present hosts has been estimated.     

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.     

Substrate Temperature Effect on Microstructure,Optical, and Glucose Sensing Characteristics of Pulsed Laser Deposited Silver Nanoparticles

This work reports the substrate temperature-influenced change in the structural, morphological, optical, and glucose sensing properties of silver (Ag) nanoparticles (NPs) deposited on p-type Si (100) wafers. AgNP films grown at temperatures ranging from RT to 600 °C clearly show a dependence of orientation texture and surface morphology on substrate temperature (T _s). As T _s increases from RT towards 600 °C, the preferred orientation of AgNP film changes from (111) to (200). The AgNPs size, that is T _s-dependent, reaches the maximum value at T _s = 300 °C. This result is attributed to restructuring of AgNPs texture. Moreover, the AgNP shape also changes from ellipsoid to sphere as T _s increases from RT to 600 °C. Surface plasmon enhancement in photoluminescence intensity is observed with increase in T _s. It is found also that the AgNP film deposited at 300 °C has considerable reflectance reduction relative to the silicon substrate, in wavelength range of 300–800 nm and a progressive red shift of localized surface plasmon resonances caused by the adding of increasing quantities of glucose has been observed. As a proof of concept, we also demonstrate the capability of grown AgNP substrates for glucose detection based on surface enhanced Raman spectroscopy in physiological concentration range with short integration time 10 s, varying with T _s.     

Rational Selection of Nanorod Plasmons: Material, Size, and Shape Dependence Mechanism for Optical Sensors

The localized surface plasmon resonance dependence on surrounding medium refractive index of Ag, Al, Au, and Cu nanoparticles is examined by electrodynamic approach. The refractive index sensitivity and sensing figure of merit (FOM) dependence of selected metal nanoparticles with similar geometry shows that although, sensing relevant parameters are shape (i.e., aspect ratio), and material dependent below the width 20 nm, but above this size these parameters are material independent under similar geometrical conditions. We have concluded that at optimum size, however, Al shows much higher refractive index sensitivity (RIS) in comparison to Au, Cu, and Ag, but FOM is higher for Ag in comparison to other metals. The observed sensing behavior is expected due to parameters like surface scattering, dynamic depolarization, radiation damping, and interband transitions, which may influence the nanorod plasmons.     

Solvent-Induced Morphological Changes of Polyhedral Silver Nanoparticles in Epoxy Resin

The use of just one material as reducing and capping agent during the synthesis of noble metal nanoparticles is of great interest for potential applications. This paper reports a simple method to prepare polyhedral silver nanoparticles at 80 °C using an epoxy resin (Araldite 506) as both reducing and capping agent. The formation of metal nanoparticles was investigated by Fourier-transform infrared spectroscopy, atomic force microscopy, transmission electronic microscopy, high-resolution transmission electronic microscopy and UV-vis spectroscopy. The proposed mechanism for the reduction of silver ions involves radicals as precursors of ketones and other products originated by thermo-oxidation of the resin. The nucleation of small spherical and polyhedral seeds (～7 nm) of polycrystalline silver and twin planes lead to big polyhedral silver nanoparticles of average size, 68 nm. On the other hand, the polyhedral silver nanoparticles dispersed in toluene changed to prolate-like particles, and their dispersion in dimethyl-sulphoxide and formamide originated elongated polyhedrons and concave nanostructures, respectively. These structural changes lead to unusual solvent-induced optical properties. For instance, the polyhedral nanoparticles dispersed in toluene red-shifted their surface plasmon resonance from 425 to 540 nm, in dimethyl-sulphoxide the spectrum exhibited a peak at 418 nm and a shoulder at 520 nm, and for the silver nanoparticles in formamide a broad band with maximum peak at 420 nm was observed. It is showed that the solvent/resin system works itself as structure-directing agent of silver nanoparticles. These results open the doors to achieve silver nanostructures highly sensitive to the dielectric environment, an ideal condition for applications in colorimetric sensors of molecules of biological or chemical interest.     

Refractive Index Sensitivity Analysis of Ag, Au, and Cu Nanoparticles

The localized surface plasmon resonance (LSPR) spectrum of noble metal nanoparticles is studied by quasi-static approximation. Taking the sensitivity of LSPR shape to the size and shape of nanoparticle along with surrounding refractive index, parameters like refractive index sensitivity and sensing figure of merit have been determined. In the present analysis from the sensing relevant parameters, it is concluded that Ag represents a better sensing behavior than Au and Cu over the entire visible to infrared regime of EM spectrum.     

Fabrication of Monodispersed Silver Nanoparticles and their Collective Sharp Plasmonic Response

Monodispersed silver (Ag) nanoparticles (NPs) were obtained by applying an electric field on unipolar-charged Ag NPs fabricated using a heterogeneous condensation technique in gas media. Well defined and charged Ag NPs were separated based on their sizes and were collected on quartz substrates. Thin films consisting of monodispersed Ag NPs with size ranging from 35 to 120 nm were prepared by varying an applied electric field during the fabrication process. Scanning electron microscope results showed that the samples have uniform size distribution. Coherent oscillations of conduction band electrons in gas medium induced by electromagnetic field and coupling of all similar plasmon resonances due to uniform Ag NPs size produced unique and interesting optical properties. Narrow extinction widths (～41 to ～69 nm) were observed compared to the width of polydispersed Ag sample. The ability to prepare samples in gas media and tune the plasmon resonance by merely varying an electric field during fabrication makes the method simple, fast, and highly economical.     

The Effect of Ag Nanoparticles on Surface-Enhanced Luminescence from Au Nanovoid Arrays

Studies comparing the effect of adding two different nanoparticle compositions on the plasmonic properties of Au nanovoid arrays were undertaken. Surface-enhanced resonance luminescence and surface-enhanced resonance Raman studies comparing dispersed Ag nanoparticles and Ag nanoparticle aggregates on gold nanovoid arrays were undertaken. These studies showed that using Ag nanoparticle aggregates increased both luminescence and Raman efficiency relative to when dispersed nanoparticles were used; in addition, these studies also showed that adding dispersed Ag nanoparticles supported a more reproducible enhancement in luminescence and Raman across the substrate compared to using Ag nanoparticle aggregates. Finite element analysis simulations indicated that surface plasmon polariton distribution in the sample was affected by the presence of the Ag nanoparticles on the Au nanovoid array.     

Plasmonic Perfect Absorbers for Biosensing Applications

We present a theoretical modal investigation of plasmonic perfect absorbers (PPAs) based on the localized surface plasmon resonance (LSPR) for biosensing applications. We design the PPA geometry with a layer of periodic metallic nanoparticles on one side of a dielectric substrate and a single metallic layer on the opposite side. The electromagnetic (EM) fields confine partly in the surrounding medium above the substrate and within the substrate itself. We examine the modes of the PPA geometry for a wavelength range of 600–1500 nm. The fundamental mode of the system provides perfect absorption for a wide angle of incidence 0–70°. The second-order mode shows a strong angular dependence with a sharp resonance and exhibits perfect optical absorption when the critical coupling condition for LSPR is achieved. The coupling condition depends on the size, periodicity, dielectric spacer, and the surrounding material of the system. The strong dependence on the surrounding material makes it a promising candidate for biosensing applications. We introduce a novel approach to investigate the angular dependence of the refractive index change for the PPA system. This novel technique contributes the significant attributes of the LSPR sensors, can be used for any required resonance wavelength depending on geometric design, and it also provides sensitivity analogous to the standard surface plasmon resonance (SPR) biosensors.     

Localized Surface Plasmon Resonance and Refractive Index Sensitivity of Metal–Dielectric–Metal Multilayered Nanostructures

The localized surface plasmon resonances of multilayered nanostructures are studied using finite difference time domain simulations and plasmon hybridization method. Concentric metal–dielectric–metal (MDM) structure with metal core and nanoshell separated by a thin dielectric layer exhibits a strong coupling between the core and nanoshell plasmon resonance modes. The coupled resonance mode wavelengths show dependence on the dielectric layer thickness and composition of core and outer layer metal. The aluminum-based MDM structures show lower plasmon wavelength compared with Ag- and Au-based MDM nanostructures. The calculated refractive index sensitivity (RIS) factor is in the order Ag–Air–Ag>Au–Air–Au>Al–Air–Al for monometallic multilayered nanostructures. Bimetallic multilayered nanostructures support strong and tunable plasmon resonance wavelengths as well as high RIS factor of 510 nm/refractive index unit (RIU) and 470 nm/RIU for Al–Air–Au and Ag-Air-Au, respectively. The MDM structures not only exhibit higher index sensitivity but also cover a wide ultraviolet–near-infrared wavelengths, making these structures very promising for index sensing, biomolecule sensing, and surface-enhanced Raman spectroscopy.     

Influence of Femtosecond Laser Ablated Silver Nanoparticles on the Nonlinear Optical Properties of Basic Fuchsin dye

We investigate the linear and nonlinear optical properties of Basic Fuchsin influenced by femtosecond laser ablated silver nanoparticles in deionised water. Single beam z-scan technique using a Q-switched Nd:YAG laser (Spectra PhysicsLAB-1760, 532 nm, 7 ns, 10 Hz) is used for the present study. Quenching of fluorescence is observed in the presence of silver nanoparticles. Transmission electron microscopic observation reveals that the nanoparticles are spherical in shape, with an average size of 7 nm. The samples show self-defocusing nonlinearity and better nonlinear absorption behavior in the presence of silver sol. The nonlinear absorption varies with varying input fluence and concentration. The results show that the variations in the nonlinear parameters are also due to the surface plasmon resonance of silver nanoparticles. The nonlinearity of the dye is increased in the presence of silver nanoparticles, which makes the material suitable for various photonic and optoelectronic applications.     

Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems

Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). In this review, we discuss the SPR-enhanced optical properties of noble metal nanoparticles, with an emphasis on the recent advances in the utility of these plasmonic properties in molecular-specific imaging and sensing, photo-diagnostics, and selective photothermal therapy. The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular-specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. We also discuss the sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant, which has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs is also discussed, which forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems.     

Tailored Aggregate-Free Au Nanoparticle Decorations with Sharp Plasmonic Peaks on a U-Type Optical Fiber Sensor by Nanosecond Laser Irradiation

A novel experimental methodology is presented for fabricating U-shaped optical fiber probes decorated with aggregate-free Au nanoparticles exhibiting sharp localized surface plasmon resonance (LSPR) spectra. The U-type tip is coated with gold nanoparticles (AuNPs) using a simple and time-efficient dip-coating procedure, without initially taking any care to prevent the formation of nanoparticle aggregates in the coated area. In a second step, the coating was irradiated with a few tens of laser pulses of 5-ns duration at 532 nm with intensities in the range of 2–14 MW/cm², leading to the formation of aggregate-free LSPR optical fiber probes. The process was monitored and controlled in real time through the changes induced into the fiber’s extinction spectra by the laser irradiation, and the coated fibers were characterized by electron microscopy. The proposed methodology resulted into the fabrication of U-type optical fiber probes coated with AuNPs exhibiting a sharp plasmon peak, which is a perquisite for their application as sensing devices.     

Near-Field Optical Imaging of a Porous Au Film: Influences of Topographic Artifacts and Surface Plasmons

In this work, near-field scanning optical microscopy is employed to study a porous Au film and the direct observation of topographic artifacts and surface plasmon influences is revealed. Under tip illumination, topographic artifacts are found to be present in a reflection mode optical image but not in a transmission mode image. A simple algorithm is used for filtering the topographic artifacts and extracting a correct near-field optical image approximately. On the other hand, surface plasmon influences are present in both modes. By using three exciting wavelengths of 488, 647.1, and 520.8 nm, it is confirmed that a suitable wavelength should be chosen for avoiding the surface plasmon interference in a near-field optical investigation of morphological or material dielectric contrast. Finally, plasmonic or nonplasmonic regions on the porous Au film can be identified from the observed optical intensity variation in the optical images obtained at incident polarizations of 0°, 90°, and 45°.     

Plasmonic Au-MoO<Subscript>3</Subscript> Colloidal Nanoparticles by Reduction of HAuCl<Subscript>4</Subscript> by Blue MoO<Subscript>x</Subscript> Nanosheets and Observation of the Gasochromic Property

Defective colloids of blue MoO_x nanosheets were prepared by anodizing exfoliation method in water. This colloidal solution exhibits an optical plasmonic absorption band in the infrared region at about 760 nm. Merely mixing HAuCl₄ solution with the MoO_x leads to loss of the blue color, decaying of 760 nm plasmonic peak and simultaneous formation of the gold plasmon absorption peak at 550–570 nm. Some spectral variations in gold plasmonic peak and MoO_x optical band gap were observed for Mo:Au ratio of 10:1, 20:1, 30:1, and 40:1. The size of the gold nanoparticles was in the 5–6 nm range with fcc crystalline structure. X-ray photoelectron spectroscopy (XPS) revealed that the initial solution contains Mo⁵⁺ states and hydroxyl groups, which after reduction, hydroxyl groups are eliminated and the Mo⁵⁺ states converted to Mo⁶⁺. The obtained Au-MoO₃ colloids have a gasochromic property in which they are colored back to blue in the presence of hydrogen gas and the molybdenum oxide absorption peak recovered again. Furthermore, it was observed that both gold and Mo oxide plasmonic peaks redshift by insertion of hydrogen gas which is attributed to change in solution refractive index and formation of defect concentration.     

Ambiguous Refractive Index Sensitivity of Fano Resonance on an Array of Gold Nanoparticles

We investigate the optical response to refractive index changes of a Fano resonance occurring in a random array of gold nanoparticles supported on a glass substrate. The Fano resonance results from the interference between localized surface plasmon on a gold nanoparticle and the light reflected at the boundary of the glass substrate. We demonstrate that the sensitivity of the resonance to the refractive index of the surrounding medium is highly dependent on the excitation geometry and can assume either positive or negative values. We furthermore present a theoretical analysis explaining this behavior based on the rigorous coupled wave analysis (RCWA) as well as the island film theory.