Silver nanoparticle array structures that produce remarkably narrow plasmon lineshapes

Using electrodynamics calculations, we have discovered one dimensional array structures built from spherical silver nanoparticles that produce remarkably narrow ( approximately meV or less) plasmon resonance spectra upon irradiation with light that is polarized perpendicular to the array axis. The narrow lines require a minimum particle radius of about 30 nm to achieve. Variations of the plasmon resonance wavelength, extinction efficiency and width with particle size, array structure, interparticle distance and polarization direction are examined, and conditions which lead to the smallest widths are demonstrated. A simple analytical expression valid for infinite lattices shows that the sharp resonance arises from cancellation between the single particle width and the imaginary part of the radiative dipolar interaction.     

A unified view of propagating and localized surface plasmon resonance biosensors

The intense colors of noble metal nanoparticles have inspired artists and fascinated scientists for hundreds of years. In this review, we describe refractive index sensing platforms based on the tunability of the localized surface plasmon resonance (LSPR) of arrays of silver nanoparticles and of single nanoparticles. Specifically, the color associated with single nanoparticles and surface-confined nanoparticle arrays will be shown to be tunable and useful as platforms for chemical and biological sensing. Finally, the LSPR nanosensor will be compared to traditional, flat surface, propagating surface plasmon resonance sensors.     

Second harmonic excitation spectroscopy of silver nanoparticle arrays

Frequency-scanned excitation profiles of coherent second harmonic generation (SHG) were measured for silver nanoparticle arrays prepared by nanosphere lithography. The frequency of the fundamental beam did not coincide with the localized surface plasmon resonance (LSPR) of the nanoparticles and was tuned so that the coherent second harmonic (SH) emission was in the region of the LSPR at 720-750 nm. The SH emission from the arrays was compared with a smooth silver film to identify an enhancement of SH emission efficiency that peaks near approximately 650 nm for nanoparticles 50 nm in height. The polarization and orientation dependence of this enhancement suggests that it is related to a dipolar LSPR mode polarized normal to the plane of the substrate. Linear extinction spectra are dominated by in-plane dipoles and do not show this weak out-of-plane LSPR mode. The nanoparticle arrays are truncated tetrahedrons symmetrically oriented by nanosphere lithography to cancel SH from in-plane dipoles which allows observation of the weak out-of-plane component.     

Automated preparation method for colloidal crystal arrays of monodisperse and binary colloid mixtures by contact printing with a pintool plotter

Photonic crystals and photonic band gap materials with periodic variation of the dielectric constant in the submicrometer range exhibit unique optical properties such as opalescence, optical stop bands, and photonic band gaps. As such, they represent attractive materials for the active elements in sensor arrays. Colloidal crystals, which are 3D gratings leading to Bragg diffraction, are one potential precursor of such optical materials. They have gained particular interest in many technological areas as a result of their specific properties and ease of fabrication. Although basic techniques for the preparation of regular patterns of colloidal crystals on structured substrates by self-assembly of mesoscopic particles are known, the efficient fabrication of colloidal crystal arrays by simple contact printing has not yet been reported. In this article, we present a spotting technique used to produce a microarray comprising up to 9600 single addressable sensor fields of colloidal crystal structures with dimensions down to 100 mum on a microfabricated substrate in different formats. Both monodisperse colloidal crystals and binary colloidal crystal systems were prepared by contact printing of polystyrene particles in aqueous suspension. The array morphology was characterized by optical light microscopy and scanning electron microscopy, which revealed regularly ordered crystalline structures for both systems. In the case of binary crystals, the influence of the concentration ratio of the large and small particles in the printing suspension on the obtained crystal structure was investigated. The optical properties of the colloidal crystal arrays were characterized by reflection spectroscopy. To examine the stop bands of the colloidal crystal arrays in a high-throughput fashion, an optical setup based on a CCD camera was realized that allowed the simultaneous readout of all of the reflection spectra of several thousand sensor fields per array in parallel. In agreement with Bragg's relation, the investigated arrays exhibited strong opalescence and stop bands in the expected wavelength range, confirming the successful formation of highly ordered colloidal crystals. Furthermore, a narrow distribution of wavelength-dependent stop bands across the sensor array was achieved, demonstrating the capability of producing highly reproducible crystal spots by the contact printing method with a pintool plotter.     

2-D array photonic crystal sensing motif

We have developed the first high-diffraction-efficiency two-dimensional (2-D) photonic crystals for molecular recognition and chemical sensing applications. We prepared close-packed 2-D polystyrene particle arrays by self-assembly of spreading particle monolayers on mercury surfaces. The 2-D particle arrays amazingly diffract 80% of the incident light. When a 2-D array was transferred onto a hydrogel thin film showing a hydrogel volume change in response to a specific analyte, the array spacing was altered, shifting the 2-D array diffraction wavelength. These 2-D array photonic crystals exhibit ultrahigh diffraction efficiencies that enable them to be used for visual determination of analyte concentrations.     

Hierarchical self-assembly of silver nanocluster arrays on triblock copolymer templates

Poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer templates which present in-plane cylinders of polystyrene (PS) aligned parallel to the plane of the substrate have been prepared by a solvent-induced order-disorder phase transition method. Silver nanoclusters have been obliquely deposited onto the SBS copolymer templates at low coverage, utilizing the directed low-energy cluster beam deposition (LECBD) method. The morphology of the samples has been characterized by a tapping-mode AFM. It is shown that the silver nanoclusters form ordered linear arrays and the intercluster distance within each individual linear array is comparable to the cluster size. Optical absorption spectra indicate that the surface plasmon resonance (SPR) of the silver nanocluster linear arrays occurs at about 444.5 nm, manifesting a red shift of approximately 21.4 nm compared to the SPR absorption of silver nanoclusters deposited on a fused quartz substrate. This is attributed mainly to the near-field electrodynamic interactions between the silver nanoclusters. This hierarchical approach to create ordered nanostructures transcends the spatial limits of lithography and provides a promising route to achieve well-ordered cluster-based nanostructures.     

硅烯量子点二聚物的等离激元激发

基于含时密度泛函理论,研究了随着间距改变时硅烯量子点二聚物的等离激元激发特性。沿垂直于硅烯所在平面方向激发时,在一定间距范围内,硅烯量子点二聚物中形成了长程电荷转移激发模式。参与长程电荷转移激发的π电子主要在两个量子点之间运动。该等离激元模式随着间隙的减小发生蓝移。此外,在不同间距时,体系中还有两个等离激元共振带,分别位于7和15 eV附近。沿平行于硅烯所在平面方向激发时,由于两个量子点之间的耦合,在低能     

Polarized surface enhanced Raman and absorbance spectra of aligned silver nanorod arrays

Polarized surface-enhanced Raman scattering (SERS) and UV-vis absorbance spectra were measured for a nonplanar Ag nanorod array substrate prepared by oblique angle vapor deposition. The anisotropy of the SERS polarization was shown to differ from that of the polarized UV-vis absorbance. The maximum SERS intensity was observed in the polarization direction perpendicular to the long axis of the Ag nanorods, while the UV-vis absorbance was strongly polarized along the direction of the long axis of the nanorod array. Analysis of the polarization data showed that molecular orientation was not the cause of the anisotropic SERS scattering. Rather, the SERS anisotropy was primarily attributed to the lateral arrangement of the three-dimensional tilted nanorod lattice in which highly localized plasmon modes are created by strong electromagnetic coupling between adjacent metallic nanorods.     

Chemical sensing and imaging with metallic nanorods

In this Feature Article, we examine recent advances in chemical analyte detection and optical imaging applications using gold and silver nanoparticles, with a primary focus on our own work. Noble metal nanoparticles have exciting physical and chemical properties that are entirely different from the bulk. For chemical sensing and imaging, the optical properties of metallic nanoparticles provide a wide range of opportunities, all of which ultimately arise from the collective oscillations of conduction band electrons ("plasmons") in response to external electromagnetic radiation. Nanorods have multiple plasmon bands compared to nanospheres. We identify four optical sensing and imaging modalities for metallic nanoparticles: (1) aggregation-dependent shifts in plasmon frequency; (2) local refractive index-dependent shifts in plasmon frequency; (3) inelastic (surface-enhanced Raman) light scattering; and (4) elastic (Rayleigh) light scattering. The surface chemistry of the nanoparticles must be tunable to create chemical specificity, and is a key requirement for successful sensing and imaging platforms.     

Molecularly Imprinted Photonic Polymers as Sensing Elements for the Creation of Cross‐Reactive Sensor Arrays

By combining molecular imprinting and colloidal crystal templating, molecularly imprinted inverse‐opal photonic polymers (MIPPs) acting as sensing elements have been exploited to create sensor arrays for the first time. With this new strategy, abundant sensing elements with differential sensing abilities were easily accessible. Because of the unique hierarchical porous structure integrated in each sensing element, high sensitivity and selectivity, fast response and self‐reporting (label‐free) detection could be simultaneously achieved. All these fascinating features indicate that MIPPs are ideal sensing elements for creating sensor arrays. By integrating the individual sensing elements on a substrate, the formed array chip delivers better portability and high‐throughput capability. As a demonstration, six kinds of contaminants were selected as analytes. The detection and discrimination of these analytes and even their mixtures in a wide range of concentrations, particularly trace amounts of analyte against a high background of other components, could be achieved, indicating the powerful capability of MIPPs‐based sensor array for sensing. These results suggest that the described strategy opens a new route for sensor array creation and should find important applications in a wide range of areas.     

Optimal design of nanoplasmonic materials using genetic algorithms as a multiparameter optimization tool

An optimal control approach based on multiple parameter genetic algorithms is applied to the design of plasmonic nanoconstructs with predetermined optical properties and functionalities. We first develop nanoscale metallic lenses that focus an incident plane wave onto a prespecified, spatially confined spot. Our results illustrate the mechanism of energy flow through wires and cavities. Next we design a periodic array of silver particles to modify the polarization of an incident, linearly polarized plane wave in a desired fashion while localizing the light in space. The results provide insight into the structural features that determine the birefringence properties of metal nanoparticles and their arrays. Of the variety of potential applications that may be envisioned, we note the design of nanoscale light sources with controllable coherence and polarization properties that could serve for coherent control of molecular, electronic, or electromechanical dynamics in the nanoscale.     

Ellipsometric identification of collective optical properties of silver nanocrystal arrays

The optical properties of silver nanocrystal arrays are investigated using spectroscopic ellipsometry in combination with polarized reflection measurements. Analysis of the ellipsometry and reflectometry spectra in terms of the "thin island film" theory enables a transparent identification of the contribution of collective effects to the optical response. Negligible image charge effects imply that only dipole contributions have to be considered. The interactions between the hexagonally ordered silver nanocrystals give rise to an effective modification of the spherical response to oblate entities with different polarizabilities parallel and perpendicular to the substrate, expressed in terms of corresponding depolarization factors. The effect of nanocrystal ordering, nearest-neighbor distance, size distribution, surrounding ambient, and the optical properties of the single nanocrystals on the optical response are analyzed. The extent of plasmon resonance peak splitting as a function of surface coverage is discussed.     

Plasmon resonances on metal tips: understanding tip-enhanced Raman scattering

Calculations of the electric-field enhancements in the vicinity of an illuminated silver tip, modeled using a Drude dielectric response, have been performed using the finite difference time domain method. Tip-induced field enhancements, of application in "apertureless" Raman scanning near-field optical microscopy (SNOM), result from the resonant excitation of plasmons on the metal tip. The sharpness of the plasmon resonance spectrum and the highly localized nature of these modes impose conditions to better exploit tip plasmons in tip-enhanced apertureless SNOM. The effect of tip-to-substrate separation and polarization on the resolution and enhancement are analyzed, with emphasis on the different field components parallel and perpendicular to the substrate.     

THE ORIENTATION OF CHLOROPHYLL AND BACTERIOCHLOROPHYLL MOLECULES IN AN ORIENTED LECITHIN MULTILAYER

Abstract— Chlorophyll a and b and bacteriochlorophyll a have been incorporated in an oriented phospholipid multilayer. The orientation of the chlorophyll molecules in this layer has been measured by polarization spectroscopy. Formulas are given expressing the dichroic ratio R as a function of the angle of tilt θ between the molecule and the plane of the multilayer and the angle μ between this plane and the transition dipole moment measured in the plane of the molecule. The possibility of overlapping bands of mutually perpendicular polarization has been taken into account. Analysis of the experimental results gives the angles θ and μ as well as an estimate for the fraction of polarization of some bands along the dihydro axis in the plane of the chlorophyll molecule.     

Electromagnetic interactions in plasmonic nanoparticle arrays

Single two-dimensional planar silver arrays and one-dimensional linear gold chains of nanoparticles were investigated by dark-field surface plasmon spectroscopy and studied as a function of interparticle distance, particle size, and number of particles. In agreement with recent theoretical predictions, a red shift of the surface plasmon resonance occurring in two-dimensional arrays was found for lattice spacings below 200 nm. This red shift is associated with a significant broadening of the resonance and is attributed to the onset of near-field interactions. We found that the relative contributions of the long-range and short-range interactions in two-dimensional arrays of particles are fundamentally different to those occurring in individual linear chains.     

Time‐Dependent Surface Plasmon Resonance Spectroscopy of Silver Nanoprisms in the Presence of Halide Ions

Herein, we find that the surface plasmon resonance (SPR) spectra of silver nanoprisms in the presence of halide ions change gradually with reaction time. The changes in the spectra correspond to the shape transformation of silver nanoprisms. There are threshold concentrations of halide ions that initiate the shape‐transformation reaction. The threshold concentrations for Cl^?, Br^?, and I^? are about 3×10^?4 M , 1×10^?6 M , and 1.5×10^?6 M , respectively. Any concentrations of the added halide ions above these thresholds can eventually etch the silver nanoprisms into nanodisks if the reaction time is long enough. The higher the concentration of the halide ions, the higher the etching rate will be. The kinetics of the shape transformation of the silver nanoprisms can be studied by recording their time‐dependent surface plasmon resonance (SPR) spectra on a commercial UV/Vis–NIR spectrometer. The peak positions of in‐plane dipole SPR bands of silver colloids in the presence of chloride and bromide ions can be fitted very well with the biexponential functions. We propose that the fast components of the biexponential behaviors should correlate to the truncating effect on the corners of silver nanoprisms, and the slow component should correlate to the redeposition of the truncated residues onto the basal plane of the nanoplates.     

Imprinted Photonic Hydrogels for the Size‐ and Shell‐Selective Recognition of Nanoparticles

Sensors based on responsive photonic hydrogels have recently attracted considerable attention for visual medical diagnostics, pharmaceutical bioassays, and environmental monitoring. However, the use of these promising materials for the detection of nanoparticles (NPs) has never been explored so far, although the sensing of nanoobjects is a rapidly evolving area of research. To address this issue, we have combined the concepts of inverse‐opal hydrogels and nanoparticle‐imprinted polymers. In this way, we could obtain a NP‐imprinted photonic hydrogel consisting of a three‐dimensional, highly ordered poly(methacrylic acid) macroporous array, in which nanocavities complementary to the target NPs, in this case colloidal quantum dots, are distributed. This novel type of NP‐imprinted photonic hydrogel sensor was shown to display high sensitivity and selectivity, thus opening new prospects for the development of equipment‐free and cost‐efficient sensing devices for NPs.     

Surface-enhanced Raman scattering system of sample molecules in silver-modified silver film

Surface-enhanced Raman scattering (SERS) spectrum of very good quality of "silver nano-particles/sample molecules/silver film" system was reported by nesting the sample molecules to the gap of silver nano-particles and silver film, indicating that "silver nano-particles/sample molecules/silver film" is a highly SERS-active system. Not only was the number of the vibrational modes increased, but also were the frequencies of Raman bands up and down shifted. It is difficult to separate the contributions of the electromagnetic and chemical mechanisms to the great enhancement of the Raman signal. The shift by 5-30cm(-1) of the SERS bands and the change in their relative intensity compared with the ordinary Raman spectrum indicate the chemisorption of the sample molecules on the silver-modified silver surface. Furthermore, the silver nano-particles modified on the rough silver film surface play an important role in magnifying the surface local electric field near the silver surface through resonant surface plasmon excitation. From the rich information, obtained from high-quality SERS of PHBA in ternary system, we inferred that PHBA molecules in ternary system adsorb onto the metal surfaces through carboxyl at a perpendicular orientation.     

银纳米线表面等离子波导研究进展

近年来,基于银纳米线表面等离子极化子的研究得到了迅猛的发展。银纳米线支持的表面电磁波可以沿金属-电介质的界面进行传导,并且能够实现二维亚波长模式受限,这种奇异特性使得银纳米线在设计集成光子信号传输系统领域具有独特的应用前景.本文中作者评述了银纳米线的制备、纳米线表面等离子成像以及纳米线光学元件和器件方面取得的最新研究进展,分析了表面等离子波导所面临的一些重要问题及其应对策略.     

Photochemical incorporation of silver quantum dots in monodisperse silica colloids for photonic crystal applications.

We developed a novel method to fabricate nanocomposite monodisperse SiO2 spheres (approximately 100 nm) containing homogeneously dispersed Ag quantum dots (approximately 2 to 5 nm). The inclusion morphology is controlled through the timing of the photochemical reduction of silver ions during hydrolysis of tetraethoxysilane in a microemulsion. Depending on the timing, Ag quantum dots can be directed to different annuli within the SiO2 spheres, as well as onto the SiO2 sphere surfaces. The embedded Ag quantum dots show a plasmon resonance absorption band at 438 nm. These Ag@SiO2 particles have significant surface charge and readily self-assemble into crystalline colloidal array (CCA) photonic crystals which Bragg-diffract light in the visible region. The magnitude of the plasmon resonance absorption depends on the CCA Bragg diffraction condition. The negative dielectric constant of the silver nanoparticles may be decreasing the silica-silver nanodot composite refractive index below that of the water medium. We may be observing an analogue of the Borrmann effect previously observed in X-ray scattering, where the incident and diffracted electric field standing wave becomes localized in regions of small CCA crystal absorption.