Manipulating the optical properties of pyramidal nanoparticle arrays

This paper reports the orientation-dependent optical properties of two-dimensional arrays of anisotropic metallic nanoparticles. These studies were made possible by our simple procedure to encapsulate and manipulate aligned particles having complex three-dimensional (3D) shapes inside a uniform dielectric environment. Using dark field or scattering spectroscopy, we investigated the plasmon resonances of 250-nm Au pyramidal shells embedded in a poly(dimethylsiloxane) (PDMS) matrix. Interestingly, we discovered that the scattering spectra of these particle arrays depended sensitively on the direction and polarization of the incident white light relative to the orientation of the pyramidal shells. Theoretical calculations using the discrete dipole approximation support the experimentally observed dependence on particle orientation with respect to incident field. This work presents an approach to manipulate--by hand--ordered arrays of particles over cm(2) areas and provides new insight into the relationship between the shape of well-defined, 3D particles and their supported plasmon resonance modes.     

Plasmon hybridization in spherical nanoparticles

We show that the plasmon resonances in single metallic nanoshells and multiple concentric metallic shell particles can be understood in terms of interaction between the bare plasmon modes of the individual surfaces of the metallic shells. The interaction of these elementary plasmons results in hybridized plasmons whose energy can be tuned over a wide range of optical and infrared wavelengths. The approach can easily be generalized to more complex systems, such as dimers and small nanoparticle aggregates.     

Plasmon modes of curvilinear metallic core/shell particles

The plasmon hybridization method is generalized to calculate the plasmon modes and optical properties of solid and dielectric-core/metallic-shell particles of geometrical structures that can be described using separable curvilinear coordinates. The authors present a detailed discussion of the plasmonic properties of hollow metallic nanowires with dielectric cores and core/shell structures of oblate and prolate spheroidal shapes. They show that the plasmon frequencies of these particles can be expressed in a common form and that the plasmon modes of the core/shell structures can be viewed as resulting from the hybridization of the solid particle plasmons associated with the outer surface of the shell and of the cavity plasmons associated with the inner surface.     

Plasmon hybridization in nanoshell dimers

We extend the plasmon hybridization method to investigate the plasmon modes of metallic nanoshell dimers. The formalism is also generalized to include the effects of dielectric backgrounds. It is shown that the presence of dielectrics shifts the plasmon resonances of the individual nanoparticles to lower energies and screens their interaction in the dimer configuration. The net result is a redshift of dimer energies compared to the system without dielectrics and a weaker dependence of the dimer plasmon energies on dimer separation. We calculate the plasmon energies and optical absorption of nanoshell dimers as a function of dimer separation. The results are in excellent agreement with the results of finite difference time domain simulations.     

A tunable Fano resonance in silver nanoshell with a spherically anisotropic core

The influences of the anisotropic permittivity and permeability in inner core on the Fano resonance have been investigated in Ag nanoshell by means of Mie scattering theory. The decreased inner core radius can enhance the coupling between superradiant and subradiant dipole modes and hence a distinct Fano profile. With increasing the tangential permittivity or permeability of inner core, the Fano resonance shows a redshift and the magnitude of Fano profile increases. The variation of Fano resonance with anisotropic permeability of the core is much weaker than that induced by anisotropic permittivity. We further find that the combined action of the increased tangential permittivity and permeability of inner core can induce a significant enhancement of Fano resonance in Ag nanoshell.     

Interference of surface plasmon resonances causes enhanced depolarized light scattering from metal nanoparticles

We show that the strongly depolarized light scattering from noble metal particles is a result of interference of two surface plasmon resonances on the same particle. The maximum depolarization occurs between two resonances. Under favorable conditions the anisotropy of the scattering light can be much lower than what is possible for dielectric particles. This explanation is discussed in relation to earlier published experimental measurements. Comparison of experimental results with theoretical calculations provides information on the shape distribution of metallic particles in the suspension.     

均一取向等离子体二聚体的制备及光学机理

以单层SiO₂胶体微球为模板, 利用Au/SiO₂/Au交替沉积结合后退火处理的方法, 制备了一种垂直堆叠且均一取向的等离子体二聚体结构. 该方法具有很大的自由度, 可以通过调节实验流程来制备大面积取向相同的同质或异质纳米粒子二聚体. 所制备的纳米粒子的等离子体杂化效应明显, 在消光光谱中可以观察到成键及反键共振峰. 由于所得纳米粒子二聚体具有垂直堆叠的特殊规整取向, 还可以观察到所得样品等离子体吸收峰的角度依赖特性. 此外, 还探讨了Au/SiO₂/Au同质二聚体和Au/SiO₂/Ag异质二聚体的光学特性差异, 发现与Au/SiO₂/Au同质二聚体相比, Au/SiO₂/Ag异质二聚体由于Ag偶极等离子体模式与Au带间吸收的耦合而呈现Fano共振峰. 所得结果提供了一个调节贵金属等离子体光学共振峰位、 强度和波形的策略, 在纳米光子学领域有着广阔的应用前景, 对今后的实验和理论研究具有重要参考价值.     

Grating-induced plasmon mode in gold nanoparticle arrays

We study the dipolar coupling of gold nanoparticles arranged in regular two-dimensional arrays by extinction micro-spectroscopy. When the interparticle spacing approaches the plasmon resonance wavelength of the individual particles, an additional band of very narrow width emerges in the extinction spectrum. By systematically changing the particles dielectric environment, the particles shape, the grating constant and angle of incidence, we show how this band associated to a grating induced-resonance can be influenced in strength and spectral position. The spectral position can be qualitatively understood by considering the conditions for grazing grating orders whereas the strength can be related to the strength of dipolar scattering from the individual particles.     

Light trapping and guidance in plasmonic nanocrystals

We illustrate the possibility of light trapping and funneling in periodic arrays of metallic nanoparticles. A controllable minimum in the transmission spectra of such constructs arises from a collective plasmon resonance phenomenon, where an incident plane wave sharply localizes in the vertical direction, remaining delocalized in the direction parallel to the crystal plane. Using hybrid arrays of different structures or different materials, we apply the trapping effect to structure the eigenmode spectrum, introduce overlapping resonances, and hence direct the light in space in a wavelength-sensitive fashion.     

Surface plasmon mapping of dumbbell-shaped gold nanorods: the effect of silver coating

We report on the identification of surface plasmons in individual gold dumbbell-shaped nanoparticles (AuDBs), as well as AuDBs coated with silver. We use spatially resolved electron energy-loss spectroscopy in a scanning electron microscope, which allows us to map plasmon-energy and intensity spatial distributions. Two dominant plasmon resonances are experimentally resolved in both AuDBs and silver-coated AuDBs. The intensity of these features is peaked either at the tips or at the sides of the nanoparticles. We present boundary element method simulations in good agreement with the experiment, allowing us to elucidate the nature of such modes. While the lower-energy, tip-focused plasmon is of longitudinal character for all dumbbells under consideration, the second side-bound plasmon has a more involved symmetry, starting as a longitudinal quadrupole in homogeneous AuDBs and picking up transversal components when silver coating is added. The longitudinal dipolar mode energy is found to blue-shift upon coating with silver. We find that the substrate produces sizable shifts in the plasmons of silver-coated AuDBs. Our analysis portraits a complex plasmonic scenario in metal nanoparticles coated with silver, including a transition from the original homogeneous gold dumbbell plasmons to the modes of homogeneous silver rods. We believe that these findings can have potential application to plasmon engineering.     

Broken symmetry in valence molecular region within Hartree-Fock calculations

Symmetry restricted and unrestricted Hartree-Fock calculations at theab initio LCAO-MO-SCF level have been carried out on the ground, core and valence hole states of N₂ at various N-N distances. A one-particle criterion for symmetry breaking is discussed. Strong broken-symmetry effects in the inner valence molecular region of N₂ have been found at larger N-N distances. This breaking of symmetry accompanying the symmetry unrestricted Hartree-Fock calculations of the inner valence hole states at large internuclear separations can be considered to be a common phenomenon with all highly symmetric molecules. The outer valence broken-symmetry effects with N₂ have showed some deviations as compared with these effects in the inner valence and core molecular regions.     

Vibrational Fano resonances in dipole-bound anions

This paper explores Fano resonances due to non-adiabatic coupling of vibrational modes and the electron continuum in dipole-bound anions. We adopt a simple one-electron model consisting of a point dipole and an auxiliary potential to represent the electron interaction with the neutral core. Nuclear motion is added by assuming that harmonic vibrations modulate the dipole moment. When the model is parameterized to simulate key features of the water tetramer anion, the resultant photodetachment lineshape closely resembles that observed experimentally and analyzed as a Fano resonance with a parameter q close to -1. Other parameterizations are explored for the model and it is found that large changes in the auxiliary potential are required to change the sign of q. This is consistent with the experimental finding that q is negative for all water cluster sizes studied.     

Breaking the mode degeneracy of surface plasmon resonances in a triangular system

In this paper, we present a systematic investigation of symmetry-breaking in the plasmonic modes of triangular gold nanoprisms. Their geometrical C(3v) symmetry is one of the simplest possible that allows degeneracy in the particle's mode spectrum. It is reduced to the nondegenerate symmetries C(v) or E by positioning additional, smaller gold nanoprisms in close proximity, either in a lateral or a vertical configuration. Corresponding to the lower symmetry of the system, its eigenmodes also feature lower symmetries (C(v)), or preserve only the identity (E) as symmetry. We discuss how breaking the symmetry of the plasmonic system not only breaks the degeneracy of some lower order modes, but also how it alters the damping and eigenenergies of the observed Fano-type resonances.     

Vibronic coupling and symmetry breaking in core electron ionization

It is shown that for highly symmetric molecules the ionization of a core electron leads quite generally to a lowering of the symmetry. The breaking of the symmetry is a consequence of the vibronic coupling between nearly degenerate core orbitals of different symmetry. The vibronic coupling leads to strong excitation of non-totally symmetric vibrational modes in addition to the usually observed excitation of totally symmetric modes. As an example, the vibrational structure of the Ols line of the CO₂ molecule is computed on the one-particle level.     

Multimode resonances in silver nanocuboids

A rich variety of dipolar and higher order plasmon resonances have been predicted for nanoscale cubes and parallopipeds of silver, in contrast to the simple dipolar modes found on silver nanospheres or nanorods. However, in general, these multimode resonances are not readily detected in experimental colloidal ensembles, due primarily to the usual variation of size and shape of the particles obscuring or blending the individual extinction peaks. Recently, methods have been found to prepare silver parallopipeds with unprecedented shape control by nucleating the silver onto a tightly controlled suspension of gold nanorods (Okuno, Y.; Nishioka, K.; Kiya, A.; Nakashima, N.; Ishibashi, A.; Niidome, Y. Uniform and Controllable Preparation of Au-Ag Core-Shell Nanorods Using Anisotropic Silver Shell Formation on Gold Nanorods. Nanoscale 2010, 2, 1489-1493). The optical extinction spectra of suspensions of such monodisperse particles are found to contain multiple extinction peaks, which we show here to be due to the multimode resonances predicted by theoretical studies. Control of the radius of the nanoparticle edges is found to be an effective way to turn some of these modes on or off. These nanoparticles provide a flexible platform for the excitation, manipulation, and exploration of higher order plasmon resonances.     

Understanding nascent plasmons and metallic bonding in atomically precise gold nanoclusters

The metallic bond is arguably the most intriguing one among the three types of chemical bonds, and the resultant plasmon excitation (e.g. in gold nanoparticles) has garnered wide interest. Recent progress in nanochemistry has led to success in obtaining atomically precise nanoclusters (NCs) of hundreds of atoms per core. In this work, thiolate-protected Au₂₇₉(SR)₈₄ and Au₃₃₃(SR)₇₉ NCs, both in the nascent metallic state are investigated by cryogenic optical spectroscopy down to 2.5 K. At room temperature, both NCs exhibit distinct plasmon resonances, albeit the NCs possess a gap (estimated 0.02–0.03 eV, comparable to thermal energy). Interestingly, we observe no effect on plasmons with the transition from the metallic state at r.t. to the insulating state at cryogenic temperatures (down to 2.5 K), indicating a nonthermal origin for electron-gas formation. The electronic screening-induced birth of metallic state/bonding is discussed. The obtained insights offer deeper understanding of the nascent metallic state and covalent-to-metallic bonding evolution, as well as plasmon birth from concerted excitonic transitions.

Cryogenic spectroscopy measurements on atomically precise gold nanoclusters with nascent plasmon resonance reveal a non-thermal origin of electron-gas formation. The birth of plasmon resonance from concerted excitonic transitions is discussed.     

Plasmonic properties of single multispiked gold nanostars: correlating modeling with experiments

Gold nanostars, possessing multiple sharp spikes, have emerged as promising plasmonic particles in the field of ultrasensitive sensing. We have developed a water-based method for high-yield synthesis of size-tunable anisotropic gold nanoparticles with a varying number of spiky surface protrusions, and performed systematic experimental and theoretical analyses of the optical properties of the single gold nanostars by characterizing them simultaneously with scanning electron microscopy and dark-field scattering spectroscopy. The morphologies and corresponding scattering spectra of the individual gold nanostars have been compared with electromagnetic simulations of the plasmonic resonances utilizing the finite-difference time-domain (FDTD) method. The study provides a correlation between the experimental and calculated scattering spectra and charge distributions of the different plasmon modes in the individual gold nanostars with varying numbers and relative orientations of surface protrusions. Our results provide guidelines for choosing gold nanostars with a proper number of spikes and appropriate dimensions of the core and arms for particular plasmonic applications as well as for further developing preparation methods of multispiked metal nanoparticles.     

Colloidal synthesis of new silver-based nanostructures with tailored localized surface plasmon resonance

Silver-based nanostructures with tailored localized surface plasmon resonance are of interest for a number of practical applications. They can conventionally be divided into three main groups: (1) anisotropic silver particles, (2) particles of alloys of silver with other metals, and (3) composite particles with dielectric or magnetic cores and silver shells. Fine “tuning” of plasmon resonance of these particles is ensured by changes in their shapes, composition, and/or structure. Procedures for the colloidal synthesis of nanostructures of all these groups and some fields of their application are described, with the main attention focused on core/shell composite particles.     

The theory of surface-enhanced Raman scattering

By considering the molecule and metal to form a conjoined system, we derive an expression for the observed Raman spectrum in surface-enhanced Raman scattering. The metal levels are considered to consist of a continuum with levels filled up to the Fermi level, and empty above, while the molecule has discrete levels filled up to the highest occupied orbital, and empty above that. It is presumed that the Fermi level of the metal lies between the highest filled and the lowest unfilled level of the molecule. The molecule levels are then coupled to the metal continuum both in the filled and unfilled levels, and using the solutions to this problem provided by Fano, we derive an expression for the transition amplitude between the ground stationary state and some excited stationary state of the molecule-metal system. It is shown that three resonances contribute to the overall enhancement; namely, the surface plasmon resonance, the molecular resonances, as well as charge-transfer resonances between the molecule and metal. Furthermore, these resonances are linked by terms in the numerator, which result in SERS selection rules. These linked resonances cannot be separated, accounting for many of the observed SERS phenomena. The molecule-metal coupling is interpreted in terms of a deformation potential which is compared to the Herzberg-Teller vibronic coupling constant. We show that one term in the sum involves coupling between the surface plasmon transition dipole and the molecular transition dipole. They are coupled through the deformation potential connecting to charge-transfer states. Another term is shown to involve coupling between the charge-transfer transition and the molecular transition dipoles. These are coupled by the deformation potential connecting to plasmon resonance states. By applying the selection rules to the cases of dimer and trimer nanoparticles we show that the SERS spectrum can vary considerably with excitation wavelength, depending on which plasmon and/or charge-transfer resonance is excited.     

Surface enhanced Raman scattering arising from multipolar plasmon excitation

Visible and near infrared extinction spectra of gold nanorod regular arrays exhibit several bands assigned to high multipolar order plasmon resonances. These up to ninth order multipolar resonances generate surface enhanced Raman scattering spectra with typically 5 x 10(4) enhancement which is of similar magnitude as those obtained for dipolar excitations.