Plasmons in strongly coupled metallic nanostructures.

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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Present and Future of Surface-Enhanced Raman Scattering

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the preparation of their hybrid structures. The plasmonic properties and the associated applications of Au nanorods will also be discussed in detail.

Gold nanorods and their plasmonic properties

When light interacts with a metal nanoparticle (NP), its conduction electrons can be driven by the incident electric field in collective oscillations known as localized surface plasmon resonances (LSPRs). These give rise to a drastic alteration of the incident radiation pattern and to striking effects such as the subwavelength localization of electromagnetic (EM) energy, the formation of high intensity hot spots at the NP surface, or the directional scattering of light out of the structure. LSPRs can also couple to the EM fields emitted by molecules, atoms, or quantum dots placed in the vicinity of the NP, leading in turn to a strong modification of the radiative and nonradiative properties of the emitter. Since LSPRs enable an efficient transfer of EM energy from the near to the far-field of metal NPs and vice versa, we can consider plasmonic nanostructures as nanoantennas, because they operate in a similar way to radio antennas but at higher frequencies. Typically, plasmonic nanoantennas at optical frequencies are made of gold and silver due to their goodmetallic properties and low absorption. Controlling and guiding light has been one of science’s most influential achievements. It affects everyday life in many ways, such as the development of telescopes, microscopes, spectrometers, and optical fibers, to name but a few. These examples exploit the wave nature of light and are based on the reflection, refraction and diffraction of light by optical elements such as mirrors, lenses or gratings. However, the wave nature of light limits the resolution to which an object can be imaged, as well as the size of the transverse cross section of efficient guiding structures to the wavelength dimension. Plasmonic resonances in nanoantennas overcome these constraints, allowing unprecedented control of light-matter interactions within subwavelength volumes (i.e., within the nanoscale at optical frequencies). Such properties have attracted much interest lately, due to the implications they have both in fundamental research and in technological applications. Metal NPs have been used since antiquity. Due to their strong scattering properties in the visible range, they show attractive colors. One of their first applications, dating back to the Roman Empire more than 2000 years ago, was as a colorant for clothing. In art, they were used to stain window glass and ceramics. Obviously, it was not known then that the colorants being used contained metal NPs or that the spectacular colors were due to the excitation of LSPRs. The first reported intentional production of metal NPs dates from 1857, when Faraday synthesized gold colloids. However, at the time there was not much interest in understanding the physics behind the optical properties of colloids due to the impossibility of synthesizing NPs with well-controlled shapes and sizes, as well as the lack of accurate detection techniques. The first theoretical work on the scattering of light by particles smaller than the incident wavelength was carried out by Lord Rayleigh at the end of the 19th century. He analyzed the diffusion of light by diluted gases, and his theory explained physical phenomena such as the blueness of the sky, the redness of the sunset, or the yellow color of the sun. Mie took the next step forward by deriving an analytical solution to Maxwell’s equations to describe the interaction of light with spheres of arbitrary radius and composition. Subsequently, based on the results of Rayleigh and Mie, Gans considered elliptical geometries. He demonstrated that the optical response of metal NPs is

Plasmonic nanoantennas: fundamentals and their use in controlling the radiative properties of nanoemitters.

We demonstrate that coupling between grating diffraction and localized surface plasmons in two-dimensional gold nanoparticle arrays in water leads to narrow near-infrared resonance peaks in measured far field extinction spectra. Good agreement is obtained between finite difference time domain (FDTD) calculations and experimental extinction spectra. The FDTD calculations predict that the gold nanoparticle arrays exhibit near-field electric field intensity (E2) enhancements approximately one order of magnitude greater than those of single isolated gold nanoparticles.

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Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays

We report a surface-enhanced Raman scattering (SERS) substrate with plasmon resonances at both excitation and Stokes frequencies. This multilayer structure combines localized surface plasmons on the nanoparticles with surface plasmon polaritons excited on a gold film. The largest SERS enhancement factor for a gold device is measured to be 7.2 x 10(7), which is more than 2 orders of magnitude larger than that measured on a gold nanoparticle array on a glass substrate. The largest SERS enhancement for a silver device is measured to be 8.4 x 10(8).

Double-Resonance Plasmon Substrates for Surface-Enhanced Raman Scattering with Enhancement at Excitation and Stokes Frequencies

The interaction between localized and propagating surface plasmons is investigated in a structure consisting of a two-dimensional periodic gold nanoparticle array, an SiO2 spacer, and a gold film. The resonance wavelengths of the two types of surface plasmons supported by the structure are tailored by changing the gold nanoparticle size and the array period. An anticrossing of the resonance positions is observed in the reflection spectra, demonstrating the strong coupling between localized and propagating surface plasmons.

Experimental study of the interaction between localized and propagating surface plasmons.

Metal nanostructures that effi ciently capture or radiate electromagnetic waves at optical frequencies offer a means to concentrate electromagnetic energy into deep subwavelength regions. Wessel noted that these structures can therefore be considered antennas. [ 1 ] Recent work has focused on more effi cient designs, termed ‘optical antennas’, which employ small gaps or very sharp tips. [ 2‐4 ] Optical antennas present opportunities for ultrasensitive spectroscopy, near-fi eld scanning optical microscopy, and compact subwavelength light sources. [ 5‐7 ] However, the achievable feature sizes are usually determined by fabrication, being approximately given by the gap size or tip sharpness. Here, we report a top-down fabrication procedure to fabricate pairs of nanoparticles separated by a controllable gap size that can be as small as 3 nm. As an application, we show that the enhancement factors of surfaceenhanced Raman scattering (SERS) increase signifi cantly for smaller gap sizes, indicating greatly enhanced electromagnetic fi elds within the gaps. We anticipate that the fabrication method we introduce here for nanoparticle pairs with nanoscale gaps would be useful not only for SERS, where it could potentially enable single-molecule sensitivity, but also for other applications in plasmonics. Raman spectroscopy is a powerful analytical method, enabling molecules to be identifi ed through their characteristic vibrational spectra. Through SERS, the Raman cross-section of molecules adsorbed to nanostructures can be increased by orders of magnitude. SERS has attracted renewed attention since the fi rst demonstrations of singlemolecule sensitivity. [ 8 , 9 ] Increasing the adoption of the SERS technique further, however, requires fabrication methods capable of routinely delivering reproducible substrates with high enhancement factors. Dimer structures, consisting of two metallic nanoparticles closely placed together, are the simplest optical antenna structures that are confi rmed to be single-molecule SERS active. [ 10 , 11 ] It is believed that substantial electromagnetic fi elds generated in dimer gaps are one of the main enhancement mechanisms in single-molecule SERS. [ 12 , 13 ] This motivates the development of a reproducible

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Lithographically Fabricated Optical Antennas with Gaps Well Below 10 nm

Ramansignalsfrommoleculesadsorbedonanoblemetalsurfaceare enhanced by many orders of magnitude due to the plasmon resonances of the substrate.Additionally,theenhancedspectraaremodifiedcomparedtothespectra ofneatmolecules;manyvibrationalfrequenciesareshifted,andrelativeintensities undergo significant changes upon attachment to the metal. With the goal of devising an effective scheme for separating the electromagnetic and chemical effects, we explore the origin of the Raman spectra modification of benzenethiol adsorbed on nanostructured gold surfaces. The spectral modifications are attrib- uted to the frequencydependence of the electromagnetic enhancement and to the effect of chemical binding. The latter contribution can be reproduced computa- tionally using molecule-metal cluster models. We present evidence that the effect of chemical binding is mostly due to changes in the electronic structure of the molecule rather than to the fixed orientation of molecules relative to the substrate. SECTION Kinetics, Spectroscopy

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Yizhuo Chu

Papers

Experimental observation of narrow surface plasmon resonances in gold nanoparticle arrays

Double-Resonance Plasmon Substrates for Surface-Enhanced Raman Scattering with Enhancement at Excitation and Stokes Frequencies

Experimental study of the interaction between localized and propagating surface plasmons.

Lithographically Fabricated Optical Antennas with Gaps Well Below 10 nm

Separation of Electromagnetic and Chemical Contributions to Surface-Enhanced Raman Spectra on Nanoengineered Plasmonic Substrates