Optical Response Of Metal Nanoparticles Research Articles

Colloidal Synthesis of Crystalline Aluminum Nanoparticles for UV Plasmonics

Numerous applications of nanotechnologies rely on the wide availability of high-quality crystalline nanoparticles (NPs). Although the chemical synthesis of noble metal nanoparticles (gold and silver) is well mastered, pushing the optical response of metallic nanoparticles towards the ultraviolet requires other materials, such as aluminum. Although a few demonstrations of chemical synthesis of Al nanoparticles have been reported so far, an elaboration path allowing for a wide range of NP size that is compatible with mass production has yet to be demonstrated. In this article, we report on the production of spherical, size-controlled crystalline Al NPs starting from commercial Al foils and without the use of a catalyst. The proposed method combines sonochemistry with solvochemistry and is fully up-scalable. The obtained NPs have a 10-100 nm crystalline Al core surrounded by a thin alumina shell, allowing long-term stability in ethanol. Well-defined plasmonic resonances in the UV and visible ranges are experimentally evidenced on single nanoparticles.

Gain-driven singular resonances in active core-shell and nano-shell plasmonic particles

Within the frame of a simple, long-wavelength, quasi-static description, we present a theoretical characterization of the optical response of metal nanoparticles doped with active gain elements in a core-shell (metallic core within an active dielectric shell) and nano-shell (active dielectric core within a metallic shell) configurations. The common feature of these structures is that, adding gain to the system produces an increase of the quality of the plasmon resonance, which becomes sharper and sharper until a singular point, after which, the system switches from absorptive to emissive (nanolaser). We use this aforementioned simple model to develop a general method allowing us to calculate both the expected singular plasmon frequency and the gain level needed to realize it and to discuss the spectral deformation occurring before and after this singular point. Finally, we propose a way to calculate if the singular behavior is reachable using realistic amounts of gain.

Lattice plasmon mode excitation via near-field coupling

The optical response of metal nanoparticles can be modified through near-field or far-field interaction, yet the lattice plasmon modes (LPMs) considered can only be excited from the latter. Here instead, we present a theoretical evaluation for LPM excitation via the near-field coupling process. The sample is an arrayed structure with specific units composed of upper metal disks, a lower metal hole and a sandwiched dielectric post. The excitation process and underlying mechanism of the LPM and the influence of the structure parameters on the optical properties have been investigated in detail by using a finite-difference time-domain (FDTD) numerical method. Our investigation presented here should advance the understanding of near-field interaction of plasmon modes for LPM excitation, and LPMs could find some potential applications, such as in near-field optical microscopes, biosensors, optical filters and plasmonic lasers.

First-Principles Insights into Plasmon-Induced Catalysis.

The size- and shape-controlled enhanced optical response of metal nanoparticles (NPs) is referred to as a localized surface plasmon resonance (LSPR). LSPRs result in amplified surface and interparticle electric fields, which then enhance light absorption of the molecules or other materials coupled to the metallic NPs and/or generate hot carriers within the NPs themselves. When mediated by metallic NPs, photocatalysis can take advantage of this unique optical phenomenon. This review highlights the contributions of quantum mechanical modeling in understanding and guiding current attempts to incorporate plasmonic excitations to improve the kinetics of heterogeneously catalyzed reactions. A range of first-principles quantum mechanics techniques has offered insights, from ground-state density functional theory (DFT) to excited-state theories such as multireference correlated wavefunction methods. Here we discuss the advantages and limitations of these methods in the context of accurately capturing plasmonic effects, with accompanying examples.

Laplacian-Level Quantum Hydrodynamic Theory for Plasmonics

An accurate description of the optical response of subwavelength metallic particles and nanogap structures is a key problem of plasmonics. Quantum hydrodynamic theory (QHT) has emerged as a powerful method to calculate the optical response of metallic nanoparticles (NPs) since it takes into account nonlocality and spill-out effects. Nevertheless, the absorption spectra of metallic NPs obtained with conventional QHT, i.e., incorporating Thomas-Fermi (TF) and von Weizsäcker (vW) kinetic energy (KE) contributions, can be affected by several spurious resonances at energies higher than the main localized surface plasmon (LSP). These peaks are not present in reference time-dependent density-functional-theory spectra, where, instead, only a broad shoulder exists. Moreover, we show here that these peaks incorrectly reduce the LSP peak intensity and have a strong dependence on the simulation domain size so that a proper calculation of QHT absorption spectra can be problematic. In this article, we introduce a more general QHT method accounting for KE contributions depending on the Laplacian of the electronic density (q), thus, beyond the gradient-only dependence of the TFvW functional. We show that employing a KE functional with a term proportional to q2 results in an absorption spectrum free of spurious peaks, with LSP resonance of correct intensity and numerically stable Bennett state. Finally, we present a novel Laplacian-level KE functional that is very accurate for the description of the optical properties of NPs of different sizes as well as for dimers. Thus, the Laplacian-level QHT represents a novel, efficient, and accurate platform to study plasmonic systems.4 MoreReceived 5 June 2020Revised 21 October 2020Accepted 24 December 2020DOI:https://doi.org/10.1103/PhysRevX.11.011049Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDensity functional theoryPlasmonicsPlasmonsSurface plasmonsPhysical SystemsNanoparticlesTechniquesClassical electromagnetismDensity functional approximationsDensity functional calculationsFree-electron modelGGACondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Plasmonic Resonances of Metal Nanoparticles: Atomistic vs. Continuum Approaches.

The fully atomistic model, ωFQ, based on textbook concepts (Drude theory, electrostatics, quantum tunneling) and recently developed by some of the present authors in Nanoscale, 11, 6004-6015 is applied to the calculation of the optical properties of complex Na, Ag, and Au nanostructures. In ωFQ, each atom of the nanostructures is endowed with an electric charge that can vary according to the external electric field. The electric conductivity between nearest atoms is modeled by adopting the Drude model, which is reformulated in terms of electric charges. Quantum tunneling effects are considered by letting the dielectric response of the system arise from atom-atom conductivity. ωFQ is challenged to reproduce the optical response of metal nanoparticles of different sizes and shapes, and its performance is compared with continuum Boundary Element Method (BEM) calculations.

Quantitative optical microspectroscopy, electron microscopy, and modelling of individual silver nanocubes reveal surface compositional changes at the nanoscale.

The optical response of metal nanoparticles is governed by plasmonic resonances, which depend often intricately on the geometry and composition of the particle and its environment. In this work we describe a method and analysis pipeline unravelling these relations at the single nanoparticle level through a quantitative characterization of the optical and structural properties. It is based on correlating electron microscopy with microspectroscopy measurements of the same particle immersed in media of different refractive indices. The optical measurements quantify the magnitude of both the scattering and the absorption cross sections, while the geometry measured in electron microscopy is used for numerical simulations of the cross section spectra accounting for the experimental conditions. We showcase the method on silver nanocubes of nominal 75 nm edge size. The large amount of information afforded by the quantitative cross section spectra and measuring the same particle in two environments, allows us to identify a specific degradation of the cube surface. We find a layer of tarnish, only a few nanometers thick, a fine surface compositional change of the studied system which would be hardly quantifiable otherwise.

Significance of the nonlocal optical response of metal nanoparticles in describing the operation of plasmonic lasers

A metal nanoparticle (MNP) coupled to a quantum emitter (QE) is a versatile composite nanostructure with unique chemical and physical properties which has been studied intensively, owing to its vast range of promising applications in nanoscience and nanotechnology. When pumped into a higher gain level, an MNP-QE composite nanostructure functions as a nanoplasmonic counterpart of a conventional laser, which is capable of operating at subwavelengths. The theory of plasmonic lasers is hitherto developed on the local optical response of the MNP, disregarding the nanoscale effects of its free electrons. In this paper, we perform a comprehensive quantum mechanical analysis of a complex MNP-QE composite nanostructure, capturing the size-dependent nonclassical effects through the nonlocal optical response of the MNP. Our study reveals that the nonlocal correction introduces significant deviations to the plasmon statistics of the hybrid particle suggested by the local calculations, becoming more prominent when the number of QEs coupled to the MNP increases. Furthermore, for the typical material parameter values used in the literature, we observed the initiation of quenching effects at lower pumping rates than suggested by the local response formalism. In essence, nonlocally assessed plasmon statistics of MNP-QE composite nanostructures demand the concerted coupling of an even higher number of QEs to compensate the deterioration in coherence and to sustain lasing.

Tailoring a periodic metal nanoantenna array using low cost template-assisted lithography

Tailoring the optical response of metal nanoparticles by controlling their morphology is a key topic in the field of nano-optics. Here, a simple approach for the fabrication of tunable plasmonic nanostructures by nanosphere lithography is presented.

Resonant Broadband Field Enhancement in Cylindrical Plasmonic Structure Surrounded by Perovskite Environment

We demonstrate the optical response of metal nanoparticles and their interaction with organic-inorganic perovskite (methyl ammonia lead halide (CH3NH3PbI3)) environment using discrete dipole approximation (DDA) simulation technique. Important optical properties like absorption, scattering, and electric field calculations for metal nanoparticle using different geometry have been analyzed. The metal nanoparticles embedded in the perovskite media strongly support surface plasmon resonances (SPRs). The plasmonic interaction of metal nanoparticles with perovskite matrix is a strong function of MNP’s shape, size, and surrounding environment that can manipulate the optical properties considerably. The cylindrical shape of MNPs embedded in perovskite environment supports the SPR which is highly tunable to subwavelength range of 400–800 nm. Wide range of particle sizes has been selected for Ag, Au, and Al spherical and cylindrical nanostructures surrounded by perovskite matrix for simulation. The chosen hybrid material and anisotropy of structure together make a complex function for resonance shape and width. Among all MNPs, 70-nm spherical silver nanoparticle (NP) and cylindrical Ag NP having diameter of 50 nm and length of 70 nm (aspect ratio 1.4) generate strong electric field intensity that facilitates increased photon absorption. The plasmonic perovskite interaction plays an important role to improve the absorption of photon inside the thin film perovskite environment that may be applicable to photovoltaics and photonics.

Plasmonic Nanoparticles as a Physically Unclonable Function for Responsive Anti‐Counterfeit Nanofingerprints

Far‐field scattering of randomly deposited Au nanoparticles (NPs) is demonstrated as a physically unclonable optical function for anti‐counterfeit applications in which the scattering patterns are easily produced yet impractical to replicate. Colloidal metal NPs are superb components for nanoscale labels owing to their small dimensions and intense far‐field scattering visible at wavelengths that depend on colloidal size, shape, composition, and their local environment. The feasibility of Au NP depositions as nanofingerprints is presented using a simple pattern matching algorithm. These NPs offer extended functionality as environmental sensors. Taking advantage of the local refractive index dependent scattering wavelengths of metal NPs, a detectable color change is also demonstrated from a nanofingerprint comprised of Au and Ag NPs when placed in media with different refractive index. The facile deposition method coupled with the intense scattering and optical response of metal NPs provides physically unclonable tags (nanofingerprints) with the ability to serve as tamper‐evident and aging labels.

Axis-selective excitation of gold nanoparticle resonances

Designing the optical response of metallic nanoparticles (NPs) is crucial for applications based on localized surface plasmons. In order to gain insight into the optical properties of metallic NPs and their experimental control, a well-defined, highly flexible excitation of single NPs is needed. In this study, we report on the realization of a measurement setup that allows 3D axis-selective excitation of single NPs with high accuracy, facilitating the study of the resulting scattering cross sections in the visible wavelength range. The experimental setup combines the concepts of dark-field optical microscopy and spectroscopy and objective-type total internal reflection fluorescence microscopy. Its functionality is validated by the study of nanocylinders with elliptical footprints. A retardation-influenced size range of moderate aspect ratios is addressed by choosing axis lengths between 70 and 200 nm. We experimentally separate three different polarization states of dipolar character, one for an electric field along each of the three principal axes, and show that the overall scattering spectrum of such a nanocylinder follows the expected superposition principle. Based on this separation, the scaling of the three resonances with the length of the principal axes is analyzed. It is found that each resonance scales exclusively with the corresponding principal axis in a linear way with slopes of the order of 1 to 2. Reasons for this scaling behavior and for its limited validity are discussed in terms of particle depolarization in the quasi-static limit and retardation.

Evaluation of E. M. Fields and Energy Transport in Metallic Nanoparticles with Near Field Excitation

We compare two ways of calculating the optical response of metallic nanoparticles illuminated by near field dipole sources. We develop tests to determine the accuracy of the calculations of internal and scattered fields of metallic nanoparticles at the boundary of the particles and in the far field. We verify the correct transport of energy by checking that the evaluation of the energy flux agrees at the surface of the particles and in the far field. A new test is introduced to check that the surface fields fulfill Maxwell's equations allowing evaluation of the validity of the internal field. Calculations of the scattering cross section show a faster rate of convergence for the principal mode theory. We show that for metallic particles the internal field is the most significant source of error.

Quantum optical response of metallic nanoparticles and dimers

The optical properties of metallic nanoparticles (NPs) can be described with analytical models based on fundamental quantum mechanical principles, of which the Drude model constitutes the classical limit. Here, we examine the plasmonic properties of silver and gold nanospheres and dimers, with radii ranging from 10 to 1 nm, extending from the classically described regime to the quantum size regime. We have studied the spectral extinction cross section by using the T-matrix method. The results indicate an increasingly substantial change in NP permittivity as the radius is reduced below 5 nm, showing a clear blueshift and weakening of the plasmon resonances for both silver and gold. As a consequence, we observe a dramatic change in the interaction of dimers, especially in the case of gold, where the introduction of quantum mechanically corrected optical properties quenches the plasmonic resonance and predicts an absence of the expected associated redshift.

Theory for collective dynamics and optical response of metallic nanoparticles under light-induced force and fluctuations

Due to the strong optical response of localized surface plasmon (LSP) in metallic nanoparticles (NPs), the light-induced force (LIF) is also strong and can be used for the control of their dynamics even at room temperature. However, properties of LIF are still unclear under the collective effects of LSP in multiple NPs. In this article, I discuss the fundamental properties of LIF exerted on metallic NPs taking into account photomediated interaction between NPs, and light-induced dynamics of NPs in fluid medium (for example, water) in the presence of the thermal fluctuations. Remarkably, it has been clarified that the collective optical response of LSP can be greatly modulated through the dynamical pattern formation process of NPs by LIF.

Spatial Nonlocality in the Optical Response of Metal Nanoparticles

Spatial nonlocality is known to play an important role in nano-optics when small nanometer-sized structures are involved, but few efforts have been made to assess nonlocal effects in a rigorous way. We present two different approaches to account for nonlocality in metal nanoparticles: (i) the nonretarded specular reflection model and (ii) the retarded hydrodynamical model. Excellent agreement with available experiments is obtained from our parameter-free simulations, which lead to dramatic differences with respect to local theory. Both models predict sizable plasmon blue shifts and broadenings in individual metal nanoparticles, nanoshells, particle dimers, and Yagi–Uda antennas. An analysis of plasmon resonances for varying particle size and spacing allows us to separate nonlocal and retardation effects within the hydrodynamical model. We find a wide range of geometrical parameters for which nonlocal effects coexist with significant retardation. This study is particularly relevant for broad, active areas involving applications of local field enhancement to biosensing and nonlinear optics in plasmonics.

Atomic structure, electronic structure and optical response of metal nanoparticles

Atomic structure, electronic structure and optical response of metal nanoparticles

Spectral dependence of the ultrafast optical response of nonspherical gold nanoparticles

The extension of the discrete dipole approximation (DDA) model to the calculation of the ultrafast transient optical response of metal nanoparticles, whatever their shape, is presented. For this, the DDA is combined with the resolution of the Boltzmann equation for the conduction electrons within the low-perturbation limit and the Rosei model for the metal band structure. This approach thus takes into account the athermal regime for the electron gas. It is then illustrated in the cases of gold nanospheres and nanopeanuts. The spectral dependence of the ultrafast response is shown to be very sensitive to the particle morphology, the surface plasmon resonance enhancing the transient modification of the interband transitions. These results are used to interpret spectrally resolved pump-probe measurements performed on a dense nanoparticle assembly, demonstrating selective-excitation and selective-detection processes.

Inside Front Cover: Modeling the Optical Response of Highly Faceted Metal Nanoparticles with a Fully 3D Boundary Element Method (Adv. Mater. 22/2008)

The shape and optical properties of gold nanoparticles can be tailored to yield intriguing reflection and transmission colors. The color of the particles shown on the inside cover results from light scattering, whereas the background of the image corresponds to the color of the light transmitted through a colloidal dispersion. Further details on the modeling of the optical response of metal nanoparticles can be found in the article by Luis Liz-Marzán and co-workers on page 4288.

Size Control of Au Nanoparticles on TiO2 and Al2O3 by DP Urea: Optical Absorption and Electron Microscopy as Control Probes

Gold nanoparticles supported on TiO2 and Al2O3 were prepared by using the deposition-precipitation with urea (DP Urea) method. The control of the particle size was achieved by varying both, the stirring time during the deposition-precipitation (DP) procedure and the conditions of thermal treatment. We focused mainly on the stirring time and the treatment temperature, although gas flow and type of atmosphere also influence importantly the particle's size and shape, as we shall show. The optical response of metallic nanoparticles is given by its surface plasmon resonance and its position and shape depends strongly on the size and the shape of the nanoparticle, as well as on its surrounding. Then, we followed the control of the nanoparticles size by using mainly optical absorption measurements, which gave us account of the size and the shape of the nanoparticles and the effect of the support on their optical response. These optical results were compared to, and supported with, TEM micrographs of our samples.