Metal Surface Plasmon Resonance Research Articles

Intensity and phase sensitivities in metal/dielectric thin film systems exhibiting the coupling of surface plasmon and waveguide modes.

In this paper, we discuss four different configurations of metal/dielectric thin films systems: surface plasmon resonance, coupled plasmon-waveguide resonance, metallic waveguide-coupled surface plasmon resonance, and dielectric waveguide-coupled surface plasmon resonance. For the waveguide-coupled surface plasmon resonance thin film systems, we explore several waveguide thicknesses that produce different resonant line shapes, including plasmon-induced transparency and waveguide-induced transparency. This paper presents a theoretical analysis and comparison of the intensity and phase sensitivities to changes in the index of refraction in a sensing layer external to the thin film system. We discuss the material parameters and the variations that led to the sensitivity variations in each thin film system. We show that when the surface plasmon polariton mode is coupled with dielectric waveguides, there is an enhancement of the sensitivity. The plasmon-induced transparency is used to increase the dynamic range of the system and shows a monotonically increasing intensity in the range of 0.4 refractive index units.

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.

Optical characteristics of ultra-thin metallic films excited at visible range

An extensive research has been carried out in synthesizing and characterizing single layer Ag films and double layer Ag/Au films of different thicknesses including ultrathin films. AFM measurements confirmed the thickness and smoothness of the prepared films. Moreover, the refractive indices of the metal films and surface plasmon resonance (SPR) parameters like height, full width at half maximum (FWHM) of SPR dip, average inclinations of the slopes have been determined successfully. SPR field enhancement of the film samples has been calculated as well. It has been revealed that the refractive index of Ag single layer decreases with increasing film thickness at different exciting wavelengths. While in case of double layer, the refractive index of Au ultra-thin layer (metal clusters) increases with increasing thickness (5–9nm). The optical measurements divulged that the light of wavelength 633nm is the best for measuring SPR thickness below 10nm metal thin films.

A symmetrical surface plasmon resonance sensing structure excited by a stripe waveguide

A symmetrical metal–medium–metal surface plasmon resonance (SPR) structure was excited by a stripe waveguide. The wave-vector matching condition is adjusted by changing the metal-film spacing, which in turn allows a variation of the refractive-index testing range. This structure overcomes the range limitations encountered in the intensity SPR detection method using waveguides and makes the matching condition of wave vectors more flexible. A single-mode stripe waveguide was designed and fabricated to excite the symmetrical sensing structure based on a planar waveguide using the complementary error function as the refractive-index distribution. The stripe waveguide is analyzed more conveniently and is easily cured with optical fibers for remote online measurement. The coupling testing system of the optical fiber with the symmetrical structure excited by the stripe waveguide was constructed to facilitate the excitation of surface plasmon waves. The impacts of the metal type, the metal film thickness, and the medium thickness in the symmetrical structure were investigated to optimize the sensing structure. The resonance condition is achieved much more easily in the symmetrical than in the traditional structure, and the detection range is adjustable via the medium thickness. Online and adjustable measurements were realized simultaneously by the detection system.

Biocompatible silver nanoparticles embedded in a PEG–PLA polymeric matrix for stimulated laser light drug release

The laser-induced release of a well-known hepatoprotective drug (silibinin, SLB) from a temperature-sensitive polymeric composite loaded with silver nanoparticles (Ag NPs) was investigated. The surface chemistry tuning and the specific design of Ag NPs are fundamental in view of the engineering of specific stimuli-responsive systems, able to control drug release in response to external stimuli. The release profiles of SLB from the newly synthesized PEG–PLA@Ag composite show strong dependences on laser wavelength and Ag NPs’ Surface Plasmon Resonance (SPR). The resonant laser light excites the SPR of the NPs and the absorbed energy is converted into heat due to electron–photon collisions. The heat generated from the nanometer-sized metal particles embedded within the polymer is efficient and strongly localized. The nanovector, irradiated by a relatively low-intensity laser but tuned specifically to the metal NPs’ SPR, releases the encapsulated drug with a higher efficiency than that not irradiated or irradiated with a laser wavelength far from the metal SPR. A combination of analytical techniques including UV–Vis, NMR, and FT-IR spectroscopy and scanning/transmission electron microscopy has been used to study the structural and morphological properties of the composite. The controllable specificity of this approach and the possibility of the SPR-mediated localized photothermal effect to be usefully applied in aqueous environments are the relevant advances of the proposed system for photothermal therapies that make use of visible optical radiation or for the drug delivery in proximity of the tumor cells.

Plasmon-enhanced Electrically Light-emitting from ZnO Nanorod Arrays/p-GaN Heterostructure Devices.

Effective and bright light-emitting-diodes (LEDs) have attracted broad interests in fundamental research and industrial application, especially on short wavelength LEDs. In this paper, a well aligned ZnO nanorod arrays grown on the p-GaN substrate to form a heterostructured light-emitting diode and Al nanoparticles (NPs) were decorated to improve the electroluminescence performance. More than 30-folds enhancement of the electroluminescence intensity was obtained compared with the device without Al NPs decoration. The investigation on the stable and transient photoluminescence spectraof the ZnO nanorod arrays before and after Al NPs decoration demonstrated that the metal surface plasmon resonance coupling with excitons of ZnO leads to the enhancement of the internal quantum efficiency (IQE). Our results provide aneffective approach to design novel optoelectronic devices such as light-emitting diodes and plasmonic nanolasers.

Direct observation of electronic transition–plasmon coupling for enhanced electron injection in dye-sensitized solar cells

We illustrate experimental evidence of the effect of surface plasmon resonance (SPR) of a noble metal on the ultrafast-electron injection efficiencies of a sensitizing dye in proximity of a wide band gap semiconductor.

Optical integrated chips with micro and nanostructures for refractive index and SERS-based optical label-free sensing

Abstract: Label-free optical biosensing technologies have superior abilities of quantitative analysis, unmodified targets, and ultrasmall sample volume, compared to conventional fluorescence-label-based sensing techniques, in detecting various biomolecules. In this review article, we introduce our recent results in the field of evanescent-wavebased refractive index sensing and surface enhanced Raman scattering (SERS)-based sensing, both of which are promising platforms for label-free optical biosensors. First, silicon-on-insulator (SOI) nanowire waveguide and metallic surface plasmon resonance (SPR)-based refractive index sensing are discussed. In order to improve the detection limit, phase interrogation techniques are introduced to these types of sensors based on prism-coupled SPR and SOI microring resonators. A detection limit in the order of 10−6 refractive index unit is achieved. Detection of 16.7 pM anti-IgG is also demonstrated based on the SPR devices. Second, SERS substrates based on various nanometallic structures are discussed. Metallic nanowire arrays and inverted nanopyramids and grooves with a thin metal surface are fabricated based on anisotropic wetetching of silicon substrates. Both structures have demonstrated a Raman signal enhancement on the order of 107. In order to improve the extraction efficiency of the Raman signal at a high wave number, a nano-bowtie array substrate is fabricated, which exhibits double resonances at both the excitation wavelength and the desired Raman scattering wavelength. Experimental results have shown that this double-resonance structure can further enhance the received Raman signal, as compared to conventional SERS substrates with only one resonance at the excitation wavelength.

Synthesis of a Polyhistidine-bearing Amphipol and its Use for Immobilizing Membrane Proteins.

Amphipols (APols) are short amphipathic polymers that stabilize membrane proteins (MPs) in aqueous solutions. In the present study, A8-35, a polyacrylate-based APol, was grafted with hexahistidine tags (His6-tags). The synthesis and characterization of this novel functionalized APol, named HistAPol, are described. Its ability to immobilize MPs on nickel ion-bearing surfaces was tested using two complementary methods, immobilized metal affinity chromatography (IMAC) and surface plasmon resonance (SPR). Compared to a single His6-tag fused at one extremity of a MP, the presence of several His6-tags carried by the APol belt surrounding the transmembrane domain of a MP increases remarkably the affinity of the protein/APol complex for nickel ion-bearing SPR chips, whereas it does not show such a strong effect on an IMAC resin. HistAPol-mediated immobilization, which allows reversibility of the interaction and easy regeneration of the supports and dispenses with any genetic modification of the target protein, provides a novel, promising tool for attaching MPs onto solid supports while stabilizing them.

Tip-enhanced Raman spectroscopy of bradykinin and its B2 receptor antagonists adsorbed onto colloidal suspended Ag nanowires.

The tip-enhanced Raman scattering (TERS) spectra of bradykinin (BK) and its potent B2 BK receptor antagonists, [d-Arg(0),Hyp(3),Thi(5,8),l-Pip(7)]BK and [d-Arg(0),Hyp(3),Thi(5),d-Phe(7),l-Pip(8)]BK, approximately with a size of about 40 nm, adsorbed onto colloidal suspended Ag nanowires with diameter in the range of 350-500 nm and length of 2-50 μm were recorded. The metal surface plasmon resonance and morphology of the Ag nanowires were studied by ultraviolet-visible (UV-Vis) spectroscopy and scanning electron microscopy (SEM). Briefly, it was shown that two C-terminal amino acids of BK and [d-Arg(0),Hyp(3),Thi(5,8),l-Pip(7)]BK are involved in the interaction with the colloidal suspended Ag nanowire surface, whereas three last amino acids of the [d-Arg(0),Hyp(3),Thi(5),d-Phe(7),l-Pip(8)]BK sequence attached the Ag surface. Thus, BK adsorbs on the colloidal suspended Ag nanowires mainly through the Phe(5/8) ring (tilted orientation) and the one oxygen atom of the carboxylate group and the H2N-C-NH-CH2- fragment of Arg(9). In the case of [d-Arg(0),Hyp(3),Thi(5,8),l-Pip(7)]BK, the Thi(8) ring (through the lone electron pair on the sulfur atom) and the both oxygen atoms of the carboxylate group and the amine group of Arg(9) mainly participated in the interaction with the Ag nanowire surface. For [d-Arg(0),Hyp(3),Thi(5),d-Phe(7),l-Pip(8)]BK, the d-Phe(7) ring, the Pip(8) ring, and the Arg(9) side-chain assisted in the peptide interaction with the Ag surface. The obtained results emphasize the importance of the C-terminal part of these peptides in the adsorption process onto the colloidal suspended Ag nanowires.

Self-corrected sensing based on a metal–dielectric–metal surface plasmon resonance structure for disturbance-free detection

A self-corrected sensing method is proposed and studied using a surface plasmon resonance scheme with metal–dielectric–metal (MDM) structure. The surface plasmon wave (SPW) modes supported by the MDM structure couple further to the SPW at sensing interface to form two kinds of hybridized SPWs. One of them is highly sensitive to the change in the optical constant of analyte solution, whereas the other is barely influenced by the change and can then be a reference signal to remove any drift. By using an intensity interrogation, the entirely different change in the intensity of reflected light at the corresponding resonance angles of the two hybridized SPWs were demonstrated both numerically and experimentally.

Direct resonant coupling of Al surface plasmon for ultraviolet photoluminescence enhancement of ZnO microrods.

More than 170-fold ultraviolet emission enhancement was obtained from the hexagonal ZnO microrods synthesized by a simple vapor-phase transport process and decorated by Al nanoparticles (NPs). Based on the stable and transient photoluminescence (PL) spectra of the ZnO microrods sputtered with Al NPs for different times, the underlying mechanism was deduced and can be attributed to the metal surface plasmon resonance (SPR) coupling with ZnO. Interestingly, with increasing of the sputtering time, the ratio of the band gap emission to the defect-related emission was increased from 0.1 to 42.7. Our results demonstrated that ZnO microrods decorated with Al NPs shed light on developing stable and high-efficiency excitonic optoelectronic devices such as light-emitting diodes and lasers.

Photocontrolled nanoparticle delivery systems for biomedical applications.

"Smart" stimuli-responsive nanomaterials are becoming popular as targeted delivery systems because they allow the use of internal or external stimuli to achieve spatial or temporal control over the delivery process. Among the stimuli that have been used, light is of special interest because it is not only noninvasive but also controllable both spatially and temporally, thus allowing unprecedented control over the delivery of bioactive molecules such as nucleic acids, proteins, drugs, etc. This is particularly advantageous for biomedical applications where specificity and selectivity are highly desired. Several strategies have evolved under the umbrella of light based delivery systems and can be classified into three main groups. The first strategy involves "caging" of the bioactive molecule using photolabile groups, loading these caged molecules onto a carrier and then "uncaging" or activating them at the targeted site upon irradiation with light of a particular wavelength. The second strategy makes use of nanocarriers that themselves are made photoresponsive either through modification with photosensitive groups or through the attachment of photolinkers on the carrier surface. These nanoparticles upon irradiation dissociate, releasing the cargo encapsulated within, or the photolinkers attaching the cargo to the surface get cleaved, resulting in release. The third approach makes use of the surface plasmon resonance of noble metal based nanoparticles. Upon irradiation with light at the plasmon resonant frequency, the resulting thermal or nonthermal field enhancement effects facilitate the release of bioactive molecules loaded onto the nanoparticles. In addition, other materials, certain metal sulfides, graphene oxide, etc., also exhibit photothermal transduction that can be exploited for targeted delivery. These approaches, though effective, are constrained by their predominant use of UV or visible light to which most photolabile groups are sensitive. Near infrared (NIR) excitation is preferred because NIR light is safer and can penetrate deeper in biological tissues. However, most photolabile groups cannot be excited by NIR light directly. So light conversion from NIR to UV/visible is required. Nanomaterials that display upconversion or two-photon-excitation properties have been developed that can serve as nanotransducers, converting NIR to UV/visible light to which the aforementioned photoresponsive moieties are sensitive. This Account will review the existing light-based nanoparticle delivery systems, their applications, the limitations they face, and the technologies that have emerged in an effort to overcome these limitations.

Evaluation of Anisotropic Biaxial Stress in Thin Strained-SiGe Layer Using Surface Enhanced Raman Spectroscopy

We applied surface-enhanced Raman spectroscopy (SERS) to the excitation of transversal optical (TO) phonos in strained SiGe. The SERS technique can greatly enhance the Raman signal owing to metal-surface plasmon resonance. Furthermore, the electrical field includes a large amount of z-polarization, which can excite TO phonons. In this study, we evaluated anisotropic biaxial stress state in thin strained-SiGe layer on a Si substrate with the SERS technique.

Performance evaluation of a metal-insulator-metal surface plasmon resonance optical gas sensor under the effect of Gaussian beams.

In this work, the performance of a nonconventional IR surface plasmon resonance (SPR) gas sensor structure based on the use of a metal-insulator-metal (MIM) structure is studied. This MIM-based sensor structure gives enhanced performance five times better than the conventional MI SPR optical gas sensors. The performance of the SPR gas sensors is studied under the effect of oblique incident Gaussian beams with different spot sizes, and the performance enhancement of the MIM structure is confirmed for different spot sizes. The simulation technique used to generate the results is also verified by comparing them to actual experimental results available in the literature.

Au@SiO2 core/shell nanoparticle-decorated TiO2 nanorod arrays for enhanced photoelectrochemical water splitting

To improve the separation efficiency of photoinduced charge carries, Au@SiO2 nanoparticles (NPs) with core–shell structure were loaded onto the surface of TiO2 nanorods grown on fluorine-doped tin oxide substrate by a facile two-step process. The resulted Au@SiO2/TiO2 photoanodes were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, as well as photoelectrochemical measurements. Compared with pristine TiO2 nanorod film, the Au@SiO2/TiO2 films showed remarkable enhancement in photoelectrochemical water splitting, with incident photon-to-current conversion efficiency increasing from 31 % to 37 % at 380 nm at 0.7 V versus saturated calomel electrode. This could be interpreted by the effect of metallic surface plasmon resonance of Au@SiO2 NPs, which would generate an intense electromagnetic field with spatially nonhomogenous distributed intensity. As a result, the charge carriers generated in the near-surface region of TiO2 nanorods could be easily separated. This modification method based on the effect of metallic surface plasmon resonance for promoted charge carrier separation provides a promising way to develop semiconductor photoelectrodes with high solar water-splitting performance.

High-performance SERS substrates: Advances and challenges

Abstract

Ultrafast Photoinduced Charge Separation in Metal–Semiconductor Nanohybrids

Hybrid nano-objects formed by two or more disparate materials are among the most promising and versatile nanosystems. A key parameter in their properties is interaction between their components. In this context we have investigated ultrafast charge separation in semiconductor-metal nanohybrids using a model system of gold-tipped CdS nanorods in a matchstick architecture. Experiments are performed using an optical time-resolved pump-probe technique, exciting either the semiconductor or the metal component of the particles, and probing the light-induced change of their optical response. Electron-hole pairs photoexcited in the semiconductor part of the nanohybrids are shown to undergo rapid charge separation with the electron transferred to the metal part on a sub-20 fs time scale. This ultrafast gold charging leads to a transient red-shift and broadening of the metal surface plasmon resonance, in agreement with results for free clusters but in contrast to observation for static charging of gold nanoparticles in liquid environments. Quantitative comparison with a theoretical model is in excellent agreement with the experimental results, confirming photoexcitation of one electron-hole pair per nanohybrid followed by ultrafast charge separation. The results also point to the utilization of such metal-semiconductor nanohybrids in light-harvesting applications and in photocatalysis.

Efficient synthesis of Ag/AgCl/W18O49 nanorods and their antibacterial activities

In this work, a modified photochemical method has been employed to couple the W18O49 nanorods with the Ag/AgCl nanoparticles that consist of Ag shell and AgCl core. Due to the surface treatment process, the Ag/AgCl nanoparticles were homogeneously formed on the surfaces of W18O49 nanorods. By simply changing the concentration of Ag+ ions in the photochemical reaction system, the size of the Ag/AgCl nanoparticles could be controlled. In comparison with the bare W18O49 nanorods, the photocatalytic antibacterial activity of the Ag/AgCl/W18O49 nanorods is dramatically enhanced, which can be attributed to both the surface plasmon resonance of noble metal and the synergistic effect of plasmonic photocatalyst and oxide semiconductor photocatalyst. The present modified photochemical method is highly efficient and can be extended to prepare other oxide semiconductor-based plasmonic photocatalysts.

Transformation of thin gold films morphology and tuning of surface plasmon resonance by post-growth thermal processing

The effect of vacuum heat treatment in the temperature range between 100 and 400 °C on the morphology and optical properties of thermally evaporated thin gold films with mass thickness from 8 to 80 nm was studied by AFM, TEM and optical spectroscopy in the spectral region of localized (surface) plasmons excitation. Our investigations have shown that change of film morphology induced by thermal annealing depends strongly on film thickness. Thus, optical behavior of thin metal films is modified due to being induced by recrystallization film permittivity changes as well as transformation of film morphology. The considered processes might be crucial for thin film optical properties determination and must be taken into account for the choice of adequate thin film optical model. Besides that, post-fabrication thermal annealing enables a wide modification of thin metal films morphology and surface plasmon resonance parameters, offering new possibilities for metal substrates design enhancing surface electromagnetic fields and therefore SERS, SEIRA, luminescence effects.