The SERS activity of a supported Ag nanocube strongly depends on its orientation relative to laser polarization

Metal nanoparticles with gain toward single-molecule detection by surface-enhanced Raman scattering

Single-molecule detection via surface-enhanced Raman scattering (SERS) has raised great interest over the past decade. The usual approach toward this goal is to harness the strong surface plasmon resonance of light with complex metallic nanostructures, such as particle aggregates, two-particle gaps, sharp tips, or particles with sharp apexes. Here we propose another route toward the goal by introducing gain medium into single metal nanoparticles with simple geometry. Our calculations show that cubic gold nanobox particles that contain a gain material within the core can create an extremely high enhancement factor of local field intensity larger than 10(8) and a SERS enhancement factor on the order of 10(16)-10(17).

Measuring the SERS Enhancement Factors of Dimers with Different Structures Constructed from Silver Nanocubes

We describe a systematic investigation on the SERS enhancement factors of individual dimers (EF(dimer)) constructed from two Ag nanocubes that display a face-to-face, edge-to-face, or edge-to-edge structure. The highest field-enhancements were obtained for the dimers displaying a face-to-face and edge-to-face configuration. In these two systems, EF(dimer) was insensitive the dimer geometry and corresponded to 2.0×10(7) and 1.5×10(7), respectively. However, EF(dimer) was decreased to 5.6×10(6) for the edge-to-edge structure. These variations in the detected field-enhancements could be explained based on the relative orientation of the nanocubes and the number of probe molecules enclosed in the hot-spot region for each dimer configuration.

Etching and dimerization: a simple and versatile route to dimers of silver nanospheres with a range of sizes

This paper describes a facile method that generates dimers of Ag nanospheres by etching Ag nanocubes with Fe(NO3)3 in ethanol with the assistance of poly(vinyl pyrrolidone) (PVP). During the etching process, the corners and edges of the Ag nanocubes were truncated off to generate spherical particles, accompanied by dimerization as a result of reduction in colloidal stability due to the addition of ionic species. Both ethanol and PVP play an important role in the etching and dimerization processes. By starting with Ag nanocubes of different sizes, we obtained well-defined dimers of Ag spheres 40, 63, and 80 nm in diameter with percentages of dimerization >60%. Since this approach can be used to fabricate dimers of Ag nanospheres with a range of different sizes, it allows for a systematic study of the hot-spot phenomenon in SERS. By correlating with SEM imaging, we measured the SERS enhancement factors for individual dimers from the three different samples, and an average value of 3.9×107, 9.3×107, and 1.7×108 was obtained, respectively.

Gold nanoparticles with externally controlled, reversible shifts of local surface plasmon resonance bands

We have achieved reversible tunability of local surface plasmon resonance in conjugated polymer functionalized gold nanoparticles. This property was facilitated by the preparation of 3,4-ethylenedioxythiophene (EDOT) containing polynorbornene brushes on gold nanoparticles via surface-initiated ring-opening metathesis polymerization. Reversible tuning of the surface plasmon band was achieved by electrochemically switching the EDOT polymer between its reduced and oxidized states.

Strong polarization dependence of plasmon-enhanced fluorescence on single gold nanorods

We report on the strong polarization dependence of the plasmon-enhanced fluorescence on single gold nanorods. The fluorescence from the organic fluorophores that are embedded in a mesostructured silica shell around individual gold nanorods is enhanced by the longitudinal plasmon resonance of the nanorods. Our electrostatic calculations show that under an off-resonance excitation, the electric field intensity contour around a nanorod rotates away from the length axis as the excitation polarization is varied. The polarization dependence of the plasmon-enhanced fluorescence is ascribed to the dependence of the averaged electric field intensity enhancement within the silica shell on the excitation polarization. The measured fluorescence enhancement factor is in very good agreement with that obtained from the electrostatic calculations. The fluorescence enhancement factor increases as the longitudinal plasmon wavelength is synthetically tuned close to the excitation wavelength. In addition, the polarization dependence is used to determine the orientation angle of the gold nanorods. The results are in excellent agreement with the actual measurements. Furthermore, the emission spectrum of the fluorophore is modified by the longitudinal plasmon resonance of the gold nanorods. A linear correlation between the emission peak wavelength and the longitudinal plasmon wavelength is obtained.

Measuring the surface-enhanced Raman scattering enhancement factors of hot spots formed between an individual Ag nanowire and a single Ag nanocube

This paper describes a systematic study of the surface-enhanced Raman scattering (SERS) activity of hot spots formed between a Ag nanowire and a Ag nanocube with sharp corners. We investigated two distinct dimer structures: (i) a nanocube having one side face nearly touching the side face of a nanowire, and (ii) a nanocube having one edge nearly touching the side face of a nanowire. The field enhancements for the dimers displayed a strong dependence on laser polarization, and the strongest SERS intensities were observed for polarization along the hot-spot axis. Moreover, the detected SERS intensities were dependent on the hot-spot structure, i.e., the relative orientation of the Ag nanocube with respect to the nanowire's side face. When the dimer had a face-to-face configuration, the enhancement factor EF(dimer) was 1.4 x 10(7). This corresponds to 22-fold and 24-fold increases compared to those for individual Ag nanowires and nanocubes, respectively. Conversely, when the dimer had an edge-to-face configuration, EF(dimer) was 4.3 x 10(6). These results demonstrated that the number of probe molecules adsorbed at the hot spot played an important role in determining the detected SERS intensities. EF(dimer) was maximized when the dimer configuration allowed for a larger number of probe molecules to be trapped within the hot-spot region.

Real-space mapping of the strongly coupled plasmons of nanoparticle dimers

We carried out the near-field optical imaging of isolated and dimerized gold nanocubes to directly investigate the strong coupling between two adjacent nanoparticles. The high-resolution (approximately 10 nm) local field maps (intensities and phases) of self-assembled nanocube dimers reveal antisymmetric plasmon modes that are starkly different from a simple superposition of two monomeric dipole plasmons, which is fully reproduced by the electrodynamics simulations. The result decisively proves that, for the closely spaced pair of nanoparticles (interparticle distance/particle size approximately 0.04), the strong Coulombic attraction between the charges at the interparticle gap dominates over the intraparticle charge oscillations, resulting in a hybridized dimer plasmon mode that is qualitatively different from those expected from a simple dipole-dipole coupling model.

Chemical synthesis of novel plasmonic nanoparticles

Under the irradiation of light, the free electrons in a plasmonic nanoparticle are driven by the alternating electric field to collectively oscillate at a resonant frequency in a phenomenon known as surface plasmon resonance. Both calculations and measurements have shown that the frequency and amplitude of the resonance are sensitive to particle shape, which determines how the free electrons are polarized and distributed on the surface. As a result, controlling the shape of a plasmonic nanoparticle represents the most powerful means of tailoring and fine-tuning its optical resonance properties. In a solution-phase synthesis, the shape displayed by a nanoparticle is determined by the crystalline structure of the initial seed produced and the interaction of different seed facets with capping agents. Using polyol synthesis as a typical example, we illustrate how oxidative etching and kinetic control can be employed to manipulate the shapes and optical responses of plasmonic nanoparticles made of either Ag or Pd. We conclude by highlighting a few fundamental studies and applications enabled by plasmonic nanoparticles having well-defined and controllable shapes.

Surface-enhanced Raman scattering on periodic metal nanotips with tunable sharpness

This paper reports on a scalable bottom-up technology for producing periodic gold nanotips with tunable sharpness as surface-enhanced Raman scattering (SERS) substrates. Inverted silicon pyramidal pits, which are templated from non-close-packed colloidal crystals prepared by a spin-coating technology, are used as structural templates to replicate arrays of polymer nanopyramids with nanoscale sharp tips. The deposition of a thin layer of gold on the polymer nanopyramids leads to the formation of SERS-active substrates with a high enhancement factor (up to 10(8)). The thickness of the deposited metal determines the sharpness of the nanotips and the resulting Raman enhancement factor. Finite-element electromagnetic modeling shows that the nanotips can significantly enhance the local electromagnetic field and the sharpness of nanotips greatly affects the SERS enhancement.

Surface-enhanced Raman scattering: comparison of three different molecules on single-crystal nanocubes and nanospheres of silver

We have investigated the surface-enhanced Raman scattering (SERS) of chemically prepared single-crystal nanocubes and nanospheres of Ag with three different molecules to quantitatively understand the effect of sharp features on the SERS enhancement factor. Both experimental measurements and theoretical calculations confirmed a higher SERS activity for the nanocubes as a result of sharp features on their surfaces. We also found major discrepancies between the measured SERS intensities and those predicted from the electromagnetic mechanism. Through analysis of SERS bands, we concluded that sharp features on the Ag nanocubes could greatly increase the contribution of the chemical enhancement to the SERS intensity.

Pushing nanocrystal synthesis toward nanomanufacturing

The potential of using nanocrystals in applications within the fields of catalysis, electronics, medicine, and others has fueled research into the preparation and assembly of these materials. For most applications, it is necessary to have nanocrystal samples in which the size, shape, composition, and structure are tightly controlled within a narrow distribution. This need has motivated researchers to explore different synthesis protocols, including a method featured in this issue of ACS Nano by Kitaev and co-workers, where decahedral silver nanoparticles were used as seed particles for the growth of faceted silver rods. In this Perspective, we describe recent advances in seeded growth as the ultimate approach to producing metal nanocrystals with precisely controlled sizes, shapes, and compositionsthe necessary first step toward their use and assembly for large-scale applications.

Isolating and probing the hot spot formed between two silver nanocubes

Out of the frying pan: Hot spots can greatly increase the sensitivity of surface-enhanced Raman scattering, but they remain poorly understood. A new strategy based on plasma etching (see picture) can be used to isolate and exclusively probe the SERS-active molecules adsorbed in the hot-spot region between two silver nanocubes.

Etching and growth: an intertwined pathway to silver nanocrystals with exotic shapes

Two-faced nanocrystals: Rapid addition of a second aliquot of silver nitrate during a polyol synthesis led to the formation of anisotropically truncated octahedrons as a result of oxidative etching and overgrowth of silver nanocubes. Three adjacent faces of the nanocube grew more rapidly than the three other faces, generating a non-centrosymmetric structure (see picture).

Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics?

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Synthesis and Optical Properties of Cubic Gold Nanoframes

This paper describes a facile method of preparing cubic Au nanoframes with open structures via the galvanic replacement reaction between Ag nanocubes and AuCl(2) (-). A mechanistic study of the reaction revealed that the formation of Au nanoframes relies on the diffusion of both Au and Ag atoms. The effect of the edge length and ridge thickness of the nanoframes on the localized surface plasmon resonance peak was explored by a combination of discrete dipole approximation calculations and single nanoparticle spectroscopy. With their hollow and open structures, the Au nanoframes represent a novel class of substrates for applications including surface plasmonics and surface-enhanced Raman scattering.

A facile, water-based synthesis of highly branched nanostructures of silver

We report a facile and environmentally friendly method of preparing highly branched silver nanostructures. By reducing AgNO 3 with l-ascorbic acid in an aqueous solution, silver particles having a coral-like morphology were formed in a few minutes. A mechanistic study of the growth process revealed that the silver branches grew from a bulbous seed formed through aggregation, and that by changing the concentrations of the reagents, the degree of particle branching could be altered. With their potentially high surface areas, these branched structures could find use as catalysts or as substrates for surface-enhanced Raman scattering applications.

Nanostructured surfaces and assemblies as SERS media

Metallic nanostructures attract much interest as an efficient media for surface-enhanced Raman scattering (SERS). Significant progress has been made on the synthesis of metal nanoparticles with various shapes, composition, and controlled plasmonic properties, all critical for an efficient SERS response. For practical applications, efficient strategies of assembling metal nanoparticles into organized nanostructures are paramount for the fabrication of reproducible, stable, and highly active SERS substrates. Recent progress in the synthesis of novel plasmonic nanoparticles, fabrication of highly ordered one-, two-, and three-dimensional SERS substrates, and some applications of corresponding SERS effects are discussed.

Local electric field enhancement and polarization effects in a surface-enhanced Raman scattering fiber sensor with chessboard nanostructure

A surface-enhanced Raman scattering fiber sensor with chessboard nanostructure on a cleaved fiber facet is studied by finite-difference time-domain method. Surface plasmons at the metal coated nanostructured fiber facet can be effectively excited and strong local electric field enhancement is obtained. Studies on the influence of light polarization demonstrate a large polarization dependence of the field enhancement factor while the polarization effects on the plasmon resonance wavelength are relatively small.