Surface Plasmon Resonance Frequency Research Articles

Equivalent RLC Ladder Circuit for Scattering by Graphene-Coated Nanospheres

In this paper, an equivalent RLC ladder circuit is derived for a graphene-coated nanosphere. Using this model, it is feasible to tune the resonance frequency of graphene surface plasmons by means of closed-form formulas. To this end, the spherical Bessel and Hankel functions of the modified Mie–Lorenz coefficients are expanded in terms of polynomials and then they are expressed as simple continued fraction forms from which the parameters of the equivalent circuit are derived. It is observed that due to the presence of graphene complex surface conductivity, the impedance of each circuit element is complex, and the real part represents a positive- or negative-valued resistor and the imaginary part modeling an inductor or capacitor. The emergence of the negative resistor which results in the field enhancement is discussed in detail.

One-Pot Synthesis of Multi-Branch Gold Nanoparticles and Investigation of Their SERS Performance.

Gold nanoparticles with multiple branches have attracted intensive studies for their application in sensing of low trace molecules. A large number of the merits found on the gold nanoparticles for the above applications are attributed to the strong localized surface plasmon resonance excited by the incident radiation. However, a facile and flexible way of synthesizing the multi-branch gold nanoparticles with tunable localized surface plasmon resonance frequency is still a challenge for the plasmonic research field. Herein, we report an efficient one-pot synthesis of multi-branch gold nanoparticles method that resembles a seed-medicated approach while using no further chemicals except chloroauric acid, ascorbic acid and 4-(2-Hydroxyethyl)-1-piperazinyl]-ethanesulfonic acid. By controlling the amounts of ascorbic acid volumes in the reaction mixture, the morphology and the localized surface plasmon resonance frequency of the synthesized multi-branch gold nanoparticles can be manipulated conveniently. Moreover, using the 4-Mercaptobenzoic acid as the Raman reporter, the multi-branch gold nanoparticles show superior surface-enhanced Raman spectroscopy characteristics that can be potentially used in chemical and biological sensing.

Green synthesis of silver/silica nanocomposite in seconds at room temperature using reducing agents obtained from silicon nanoparticles

In this work, we present a single step, highly stable, energy efficient, green and economically scalable room temperature method for the synthesis of silver/silica (Ag/SiO2) nanocomposite. The Ag/SiO2 nanocomposites are synthesized from silver nitrate solution using extracts of silicon nanoparticles. The silicon nanoparticles, which are prepared through the laser ablation of silicon in ethanol with polyvinylpyrrolidone,are used as the reducing agent. The nanocomposites have been characterized by ultraviolet–visible spectroscopy, x-ray diffraction pattern and Fourier transform infrared spectrum. The ultraviolet–visible absorption spectrum of the colloidal nanocomposites shows a surface plasmon peak at 424 nm. The x-ray diffraction pattern of Ag/SiO2 nanocomposite powder only contains silver structures, and thus SiO2 nanoparticles are amorphous. The nonlinear properties of Ag/SiO2 colloidal nanocomposite solution are studied using the Z-scan method. The value of the nonlinear absorption is measured as 17 × 10−3 cm W−1 with a positive sign, which illustrates the two photon absorption phenomena. The Ag/SiO2 nanocomposite shows a large nonlinear absorption near the surface plasmon resonance frequency of Ag nanoparticles.

Plasmon coupling between complex gold nanostructures and a dielectric substrate

Intercoupling of an incident electric field in metal nanoparticles causes asymmetric distribution of surface charges, which eventuates in shifting of the surface plasmon resonance frequency. This feature can be used in tuning the surface plasmon resonance and controlling the light absorption in a desired wavelength. This work provides a theoretical study of the plasmonic properties of complex gold nanostructures on a dielectric substrate where the nanoparticles have different morphologies. For analysis, we have developed a discrete dipole approximation with surface interactions-z, which is the third version of the MATLAB-based DDA-SI toolbox. In this version, lower-upper decomposition of the interaction matrix is used as a preconditioning of the LSQR iterative solver. This method accelerates the DDA-SI calculations by decreasing the total number of iteration steps and decreases the relative residual to achieve more accurate results. In the analysis, nanostructures are assumed to be gold dimers, trimers, and quadrumers with different sizes and elongations of cubical or spherical geometries on a BK7 substrate. The results show that absorption spectra exhibit both red- and blueshifted plasmon resonances in array, depending on the particle shape and elongation. The cubic structure of gold array provides the highest absorption efficiency, while the spherical structures give wider bandwidth; the combination of these structures could be used to design a system with intended features. We demonstrate that the geometrical symmetry plays an important role in the plasmon resonance of gold arrays, and it is shifted when the symmetry of the array is broken.

Plasmoelectric Potential Mapping of a Single Nanoparticle

The plasmoelectric potential effect, based on all-metal nanostructures, is a newly discovered plasmon-induced photoelectric converting mechanism. In this work, we have experimentally demonstrated that gold and silver nanoparticles exhibit opposite plasmoelectric potentials under the same excitation source, depending on their distinct surface plasmon resonance (SPR) frequencies. Three-dimensional mapping of plasmoelectric potentials on gold and silver nanoparticles depicts height-related surface potential information as well as the charge density evolution process down to the single nanoparticle level with Kelvin Probe Force Microscopy (KPFM). Upon employing a statistical method, the quantitative plasmoelectric potential is obtained. Under the power intensity of 5.4 mW/cm2, the maximum value of the plasmoelectric potential reaches −79 mV on the gold nanoparticle and 54 mV on silver nanoparticles. Finally, our work may provide guidance to the understanding of plasmoelectric potential on the single nanoparti...

Narrowing of Plasmon Resonance Peaks as an Ensemble Effect

The frequency of localized surface plasmon resonance (LSPR) displayed by gold nanoparticles (AuNPs) redshifts as a function of their local refractive index, which renders them valuable transducers for sensing applications. An ensemble hypothesis is presented herein, along with spectroscopic evidence, using the biotin–streptavidin system on immobilized AuNPs to interpret the decrease in ensemble line width (ELW) consistently observed upon functionalization of plasmonic nanoparticles and the subsequent analyte binding. These results demonstrate that ELW can be used to monitor recognition reactions, providing spectral details and a possible sensitivity enhancement to the conventional wavelength sensing. A novel sensing platform allowing the simultaneous measurement of both LSPR wavelength and ELW is proposed, which not only combines the advantages of both parameters but also permits real-time measurement and miniaturization.

Highly Tunable Hollow Gold Nanospheres: Gaining Size Control and Uniform Galvanic Exchange of Sacrificial Cobalt Boride Scaffolds.

In principle, the diameter and surface plasmon resonance (SPR) frequency of hollow metal nanostructures can be independently adjusted, allowing the formation of targeted photoactivated structures of specific size and optical functionality. Although tunable SPRs have been reported for various systems, the shift in SPR is usually concomitant with a change in particle size. As such, more advanced tunability, including constant diameter with varying SPR or constant SPR with varying diameter, has not been properly achieved experimentally. Herein, we demonstrate this advanced tunability with hollow gold nanospheres (HGNs). HGNs were synthesized through galvanic exchange using cobalt-based nanoparticles (NPs) as sacrificial scaffolds. Co2B NPscaffolds were prepared by sodium borohydride nucleation of aqueous cobalt chloride and characterized using UV-vis, dynamic light scattering, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Careful control over the size of the Co2B scaffold and its galvanic conversion is essential to realize fine control of the resultant HGN diameter and shell thickness. In pursuit of size control, we introduce B(OH)4- (the final product of NaBH4 hydrolysis) as a growth agent to obtain hydrodynamic diametersrangingfrom ∼17-85 nm with relative standard deviation <3%. The highly monodisperse Co2B NPs were then used as scaffolds for the formation of HGNs. In controlling HGN shell thickness and uniformity, environmental oxygen was shown to affect both the structural and optical properties of the resultant gold shells. With careful control of these key factors, we demonstrate an HGN synthesis that enables independent variation of diameter and shell thickness, and thereby SPR, with unprecedented uniformity. The new synthesis method creates a truly tunable plasmonic nanostructure platform highly desirable for a wide range of applications, including sensing, catalysis, and photothermal therapy.

Au Nanoparticle Sub-Monolayers Sandwiched between Sol-Gel Oxide Thin Films.

Sub-monolayers of monodisperse Au colloids with different surface coverage have been embedded in between two different metal oxide thin films, combining sol-gel depositions and proper substrates functionalization processes. The synthetized films were TiO2, ZnO, and NiO. X-ray diffraction shows the crystallinity of all the oxides and verifies the nominal surface coverage of Au colloids. The surface plasmon resonance (SPR) of the metal nanoparticles is affected by both bottom and top oxides: in fact, the SPR peak of Au that is sandwiched between two different oxides is centered between the SPR frequencies of Au sub-monolayers covered with only one oxide, suggesting that Au colloids effectively lay in between the two oxide layers. The desired organization of Au nanoparticles and the morphological structure of the prepared multi-layered structures has been confirmed by Rutherford backscattering spectrometry (RBS), Secondary Ion Mass Spectrometry (SIMS), and Scanning Electron Microscopy (SEM) analyses that show a high quality sandwich structure. The multi-layered structures have been also tested as optical gas sensors.

Tuning local surface plasmon resonance of silver and photoluminescence intensity enhancement by adding copper

In order to match the local surface plasmon resonance (LSPR) frequency of Ag with the frequency of light that needs enhancement, the relationship between the free carrier concentration and LSPR frequency must be considered in order to design noble alloy nanoparticles (NPs) to substitute for pure Ag. In this study, Ag, Cu, and Ag-Cu continuous films were prepared by magnetron sputtering to investigate the relationship between the free carrier concentration and LSPR wavelength, which were tested with a Hall effect detector and ultraviolet-visible spectrophotometer, respectively. Ag and Ag-Cu NP films were deposited by short-time magnetron sputtering on an organic-YAG:Ce3+ substrate to test and compare their effects on the photoluminescence (PL) intensity. The thickness of the metal films was determined using an optical thin film analysis system. The morphology was observed for the continuous and NP films by scanning electron microscopy. Compared with the pure Ag films, the Ag-Cu films had a lower free carrier concentration and longer peak wavelength, which was closer to the emission wavelength of YAG:Ce3+. The enhancement ratio for the PL intensity reached a maximum of 1.16 with the Ag NP films. By contrast, for the Ag-Cu NP films with a shorter deposition time, the PL enhancement ratio reached 1.125, where the optimal ratio of Ag to Cu for the product of sputtering power and time was 2:1. The addition of Cu to Ag is beneficial for industrial production because the cost of the materials and the preparation time for Ag-Cu alloy NP films are more than 50% lower compared with pure Ag NP films.

Impact of Mg layer thickness on the performance of the Mg/Bi2O3 plasmonic interfaces

In this work, the effects of the Mg layer thickness on the structural, morphological, optical and plasmon-dielectric spectral interactions in the Mg/Bi2O3 films are investigated by means of X-ray diffraction, scanning electron microscopy, optical absorbance and reflectance spectroscopies. It was observed that, the polycrystalline Bi2O3 become amorphous and recrystallizes when deposited onto 50 and 100 nm and 200 nm thick Mg layer, respectively. The optical absorption coefficient and dielectric spectroscopic analysis and modeling revealed a remarkable shrinkage in the energy band gap from 2.49 to 2.15 eV associated with a pronounced enhancement in the dielectric properties of the Bi2O3 upon Mg interfacing and thickness controlling. In addition, while the hole scattering time and hole drift mobility increased with increasing Mg layer thickness, the free holes concentration and hole bounded plasmon frequency significantly decreased. Particularly, as the Mg substrate thickness increases from 50 to 100 and reaches 200 nm, the plasmon frequency decreased from 13.9 to 11.1 and reaches 6.54 GHz, respectively. This wide tunability of the surface plasmon resonance frequency, which was achieved via Mg thickness layer control, are promising for the use of the Mg/Bi2O3 layer as plasmonic surface stabilizers and as microcavities.

Interplay of Nanoparticle Resonance Frequency and Array Surface Coverage in Live-Cell Plasmon-Enhanced Single-Molecule Imaging

Super-resolution imaging has provided new insights into nanoscale optics. Plasmonic gold nanotriangle arrays created by nanosphere lithography can enhance single-molecule fluorescence intensity to further improve imaging. Here, gold nanotriangle arrays on glass coverslips were used as inexpensive, facile, and broadly applicable imaging substrates for living Vibrio cholerae cells expressing photoactivatable fluorescent proteins—the red PAmCherry or the green PAGFP—and resulted in fluorescence enhancements upon coupling living cells to nanotriangle arrays. Within the requirements for this wide-field coupling geometry, we analyze and optimize the coupling as a function of local surface plasmon resonance frequency and particle coverage.

Time-resolved photoluminescence study of tris(8-hydroxyquinolinato) aluminium with surface plasmon resonance of Ag nanoparticles

Surface plasmon resonance (SPR) of Ag nanoparticles (NPs), and its emission enhancement properties are investigated by time-resolved photoluminescence spectroscopy using tris(8-hydroxyquinolinato)aluminium (Alq3), which is generally applied in organic light-emitting diode, as the emitter. 5-nm-thick Ag island-films were fabricated, and annealed at various temperatures to vary the size of the Ag NPs. The peak position of the SPR band in the absorption spectra of Ag NPs blue-shifts with increasing annealing temperature, that is, the SPR frequency is controlled by the NP size. Comparison of the photoluminescence (PL) spectra of Alq3 with and without Ag NPs annealed at 150 °C indicates 1.2-fold enhancement of PL for Alq3 with Ag NPs compared to the reference Alq3. From the temperature dependence of the PL intensity and lifetime of Alq3 with Ag NPs, we evaluated the degree of enhancement of the radiative recombination rate in the presence of the localized surface plasmon of Ag NPs, in comparison with the competitive non-radiative quenching process.

Surface-enhanced Raman scattering from bowtie nanoaperture arrays

This paper is dedicated to Professor P.R. Norton on the occasion of his 75th birthday, in honor of his profound contributions to Surface Science. Bowtie nanoaperture arrays with different tip-to-tip distances were fabricated on thin gold film by focused ion beam (FIB) milling. White light transmission spectra for all the bowtie nanoaperture arrays were collected and the relationship between the tip-to-tip distance and the surface plasmon resonance (SPR) frequency was evaluated. Finite-difference time domain (FDTD) simulations were carried out for the white light transmission and for the local field intensity with the aim of correlating the optical transmission peaks to the tip-to-tip distances. Surface-enhanced Raman scattering (SERS) was collected from bowtie nanoaperture arrays coated with rhodamine 6G. It was found that the SERS properties vary with the tip-to-tip distances of the bowties of same aperture size and same periodicity. The relationships between SERS, SPR frequency and the local field intensity are discussed.

Combined redox and plasmonic electrochromic effects in WO3/ITO double-layer films

In this paper, a double-layer coating with both redox and plasmonic electrochromic properties has been designed and deposited on fluorine doped tin oxide (FTO) substrates by sol-gel method. This coating consists of an amorphous WO3 film and a nanocrystalline indium tin oxide (ITO) film. Both the valence of W atoms in WO3 and the localized surface plasmon resonance (LSPR) frequency of ITO nanocrystals can be tuned reversibly and persistently by electrochemical process. Due to the LSPR electrochromism, the double-layer films can block near-infrared (NIR) and visible (Vis) light selectively and independently by applying different voltages, which is better and more functional than the traditional electrochromic films. High transmittance of WO3/ITO films is obtained both in Vis and NIR for positive applied voltages. When a weakly negative voltage is applied, the transmittance of WO3/ITO films in visible region is still high, while the transmittance in NIR decreases. The transmittance of the double-layer films in both Vis and NIR further decreases for increasingly more negative voltages.

Surface-enhanced Raman scattering enhancement due to localized surface plasmon resonance coupling between metallic nanoparticles and substrate

In this study, gold nanostructures (AuNSs) and silver nanoparticles (AgNPs) were integrated with a silver micro-flower-like structure deposited on a screen-printed carbon electrode (AgMF-SPCE) for enhancing surface-enhanced Raman scattering (SERS) by using 4-mercaptobenzoic acid (4-MBA) as a Raman reporter. SERS was enhanced by approximately 3.6–52.1-fold, depending on the frequency of the incident laser, the localized surface plasmon resonance frequency of metallic NPs, and particle–particle aggregation effects. Compared with AgNP/SPCE and AgMF-SPCE substrates, the AgNP/AgMF-SPCE substrate showed high temperature tolerance and long-term durability. Furthermore, the proposed substrates easily obtained hot spots for other Raman reporters such as 4-aminothiophenol, 5,5′-dithiobis-2-nitrobenzoic acid, and 4-chlorothiophenol. A linear relationship was found between the Raman signal and the concentration of Raman reporters in the range 10 nM–100 μM, with the limit of detection in the range of 6.19–77.2 nM at a signal-to-noise ratio of 3.0. These results suggest that the AgNP/AgMF-SPCE substrate will be well suited for quantitative analysis.

Composition-adjustable Ag–Au substitutional alloy microcages enabling tunable plasmon resonance for ultrasensitive SERS

Engineering the surface plasmon resonance (SPR) properties is a critical issue for improving device performance in the fields of plasmonics, nanophotonics, optoelectronics, and electrochemistry. Here, we demonstrated a programmable manipulation of the surface plasmon resonance (SPR) effect using composition-adjustable Ag-Au substitutional alloy microcages (SAMCs) through a facile NaBH4-cooperative galvanic replacement reaction. The SPR frequency of the Ag-Au SAMCs can be continuously and exquisitely manipulated without resonance damping or broadening via accurate adjustment of the elemental composition distribution at the perfect homogeneity on the atomic-level. Significantly, both the tunable SPR frequency and excellent chemical stability synergistically endow the hollow Ag-Au SAMCs with excellent SERS sensitivity and reproducibility, which lays a foundation for the realization of trace detection of thiram at an ultralow concentration of 1 × 10-12 M. This strategy is a promising candidate for efficient promotion of the SERS activity for metal-based substrates.

Plasmon coupled photoluminescence from silver coated silicon quantum dots

The surface plasmon enhanced photoluminescence (PL) emission of silver coated Si/SiO2 quantum dots (QDs) is investigated theoretically and numerically for different parameters of the QDs. Due to the interaction of radiation with the silver coat, a local surface plasmon oscillation is established which in turn results in a considerable resonant enhancement of the local field in the QDs. The local field enhancement factor inside of the silver coated spherical Si/SiO2 QDs is solved using the Laplace equation. Utilizing this enhancement factor, the plasmon enhanced radiative recombination rate, the spectral absorption, and the PL intensity of ensembles of silver coated Si/SiO2 QDs embedded in a SiO2 host matrix are studied. The induced electric field increases the overlapping of the electron and hole wave functions in the QDs leading to an increase in radiative recombination rate, spectral absorption, and the PL intensity. Moreover, by varying the thickness of silver coat and SiO2 spacer, the surface plasmon resonance frequency can be tuned to the longer wavelength regions in the visible spectrum. This enhances the coupling between surface plasmon resonance frequency of the silver coat and the energy gap of silicon QDs. It is found that the radiative recombination rate, spectral absorption and photoluminescence intensity increase up to 3 folds compared to the QDs without a metal coat.

Improved Performance of Quantum Dots Light-emitting Diodes: The Case of Au-incorporated NiO Hole Injection Layer

The performance of quantum dots light-emitting diodes (QLEDs) were enhanced by incorporating Au into NiO hole injection layer (HIL). Highly bright green QLEDs with a maximum luminance of 31210 cd m-2 and current efficiency of 6.5 cd A-1, exhibiting 50% improvement compared with device without Au NP. The improved performance may be attributed to the significant increase in the hole injection rate as a result of the introduction of Au NPs and the good matching between the resonance frequency of the localized surface Plasmon resonance (LSPR) generated by Au NPs and QDs, as well as the suppressed Auger recombination of QDs layer due to the LSPR-induced near-field enhanced radiative recombination rate of excitons.

Dispersion, propagation, and transverse spin of surface plasmon polaritons in a metal-chiral-metal waveguide

Chiral media that exist ubiquitously in both nature and artificial metamaterials have exotic optical properties. The influence of chirality on the features of surface plasmon polaritons (SPPs) in a metal-chiral-metal plasmonic waveguide is revealed under realistic material parameters. A universal dispersion relation is derived, which covers the achiral metal-insulator-metal case. When the core of a symmetric waveguide is sufficiently thin, the introduction of chirality will weaken the cutoff effect which usually occurs in the antisymmetric surface plasmon mode. It is found that in the chiral case, the surface plasmon resonance frequency is slightly raised and that the propagation is enhanced. It is also demonstrated that chirality might modulate the transverse spin effect of SPPs in the waveguide. This work may enrich the plasmonics theory which is of great importance for nanophotonic devices.

Electromagnetic radiation from filamentary sources in the presence of axially magnetized cylindrical plasma scatterers

Electromagnetic radiation from filamentary electric-dipole and magnetic-current sources of infinite length in the presence of gyrotropic cylindrical scatterers in the surrounding free space is studied. The scatterers are assumed to be infinitely long, axially magnetized circular plasma columns parallel to the axis of the filamentary source. The field and the radiation pattern of each source are calculated in the case where the source frequency is equal to one of the surface plasmon resonance frequencies of the cylindrical scatterers. It is shown that the presence of even a single resonant magnetized plasma scatterer of small electrical radius or a few such scatterers significantly affects the total fields of the filamentary sources, so that their radiation patterns become essentially different from those in the absence of scatterers or the presence of isotropic scatterers of the same shape and size. It is concluded that the radiation characteristics of the considered sources can efficiently be controlled using their resonance interaction with the neighboring gyrotropic scatterers.