Optimisation of absorption efficiency for varying dielectric spherical nanoparticles

Abstract-In this paper we compare the optical absorption for nanospheres made from a range of transition and alkali metals from Li (A=3) to Au (A=79). Numerical solutions to Mie theory were used to calculate the absorption efficiency, Q abs , for nanospheres varying in radii between 5 nm and 100 nm in vacuum. We show that, although gold is the most commonly used nanoparticle material, its absorption efficiency at the plasmon resonance is not as strong as materials such as the alkali metals. Of all the materials tried, potassium spheres with a radius of 21 nm have an optimum absorption efficiency of 14.7. In addition we also show that, unlike gold, the wavelength of the plasmon peak in other materials is sensitive to the sphere radius. In potassium the peak position shifts by 100 nm for spheres ranging from 5 nm to 65 nm, the shift is less than 10 nm for gold spheres

Phase Stability Effects on Hydrogen Embrittlement Resistance in Martensite–Reverted Austenite Steels

Earlier studies have shown that interlath austenite in martensitic steels can enhance hydrogen embrittlement (HE) resistance. However, the improvements were limited due to microcrack nucleation and growth. A novel microstructural design approach is investigated, based on enhancing austenite stability to reduce crack nucleation and growth. Our findings from mechanical tests, X-ray diffraction, and scanning electron microscopy reveal that this strategy is successful. However, the improvements are limited due to intrinsic microstructural heterogeneity effects

Combined Quantum Mechanics (TDDFT) and Classical Electrodynamics (Mie Theory) Methods for Calculating Surface Enhanced Raman and Hyper-Raman Spectra

Multiscale models that combine quantum mechanics and classical electrodynamics are presented, which allow for the evaluation of surface-enhanced Raman (SERS) and hyper-Raman scattering spectra (SEHRS) for both chemical (CHEM) and electrodynamic (EM) enhancement mechanisms. In these models, time-dependent density functional theory (TDDFT) for a system consisting of the adsorbed molecule and a metal cluster fragment of the metal particle is coupled to Mie theory for the metal particle, with the surface of the cluster being overlaid with the surface of the metal particle. In model A, the electromagnetic enhancement from plasmon-excitation of the metal particle is combined with the chemical enhancement associated with a static treatment of the molecule–metal structure to determine overall spectra. In model B, the frequency dependence of the Raman spectrum of the isolated molecule is combined with the enhancements determined in model A to refine the enhancement estimate. An equivalent theory at the level of model A is developed for hyper-Raman spectra calculations. Application to pyridine interacting with a 20 nm diameter silver sphere is presented, including comparisons with an earlier model (denoted G), which combines plasmon enhanced fields with gas-phase Raman (or hyper-Raman) spectra. The EM enhancement factor for spherical particles at 357 nm is found to be 10⁴ and 10⁶ for SERS and SEHRS, respectively. Including both chemical and electromagnetic mechanisms at the level of model A leads to enhancements on the order of 10⁴ and 10⁹ for SERS and SEHRS

Plasmon-Sampled Surface-Enhanced Raman Excitation Spectroscopy on Silver Immobilized Nanorod Assemblies and Optimization for Near Infrared (λ_ex = 1064 nm) Studies

For many Surface-Enhanced Raman Spectroscopy (SERS) applications, the enhancing substrate must exhibit a number of critical properties that include low cost, robustness, and reproducibly high enhancement over large areas of the substrate. In this study we investigate the SERS fundamental enhancement factor of silver Immobilized Nanorod Assembly (AgINRA) substrates as a function of both the dielectric sphere diameter (310–780 nm) and the input laser wavelength (633–1064 nm) with a technique called plasmon-sampled surface-enhanced Raman excitation spectroscopy (PS-SERES). The nonresonant molecule benzenethiol was chosen as the probe molecule. Higher enhancement factors (EFs) were measured as the plasmon resonance and excitation wavelength’s relative separation were optimized and both moved toward the infrared region, ultimately eclipsing the 10⁸ mark. This is the highest EF to date measured on this type of large-area substrate. The enhancement factors reported here are the result of efficient coupling between free space photons and the surface plasmon states in the metal INRA substrate. Coupled with their robustness and ease of fabrication, these results further underscore the value and versatility of metal INRA substrates in the field of surface-enhanced Raman spectroscopy

Immobilized Nanorod Assemblies: Fabrication and Understanding of Large Area Surface-Enhanced Raman Spectroscopy Substrates

We describe the fabrication of optimized plasmonic substrates in the form of immobilized nanorod assemblies (INRA) for surface-enhanced Raman spectroscopy (SERS). Included are high-resolution scanning electron micrograph (SEM) images of the surface structures, along with a mechanistic description of their growth. It is shown that, by varying the size of support microspheres, the surface plasmon resonance is tuned between 330 and 1840 nm. Notably, there are predicted optimal microsphere sizes for each of the commonly used SERS laser wavelengths of 532, 633, 785, and 1064 nm

Surface Plasmon Coupling of Compositionally Heterogeneous Core–Satellite Nanoassemblies

Understanding plasmon coupling between compositionally heterogeneous nanoparticles in close proximity is intriguing and fundamentally important because of the energy mismatch between the localized surface plasmons of the associated nanoparticles and interactions beyond classical electrodynamics. In this Letter, we explore surface plasmon coupling between silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs), assembled in the form of core–satellite structures. A recently developed assembly method allows us to prepare ultrapure core–satellite nanoassemblies in solution, where 50 nm AgNPs are surrounded by 13 nm AuNPs via alkanedithiol linkers. We observe changes in the plasmon coupling between the AgNP core and AuNP satellites as the core-to-satellite gap distance varies from 2.3 to 0.7 nm. Comparison with theoretical studies reveals that the traditional hybridized plasmon modes are abruptly replaced by charge-transfer plasmons at a ∼1 nm gap. Changes with the number of satellites are also discussed

LSPR Imaging of Silver Triangular Nanoprisms: Correlating Scattering with Structure Using Electrodynamics for Plasmon Lifetime Analysis

Here, an investigation into the optical response of truncated and nontruncated Ag triangular nanoprisms is presented. Single particle darkfield microspectroscopy and transmission electron microscopy (TEM) are performed to obtain an overview of the relationship between plasmonic properties and nanoparticle structure. A general framework is built to describe the optical response (resonant wavelength λ_max and full width at half maximum (fwhm)) of triangle nanoprisms in a size regime (60–140 nm edge length) that has not been investigated previously at this level of detail. The discrete dipole approximation is used to infer the thickness of individual nanoprisms from λ_max measurements, thereby determining three-dimensional structures derived from two-dimensional TEM images. This additional structural information allowed the various contributions to the fwhm to be deconvoluted and analyzed. It is shown that electron-interface scattering, radiative damping, and bulk damping each make similar contributions to the plasmon width of the triangular prisms for the sizes considered in this work. Surface scattering is found to be seriously overestimated if it is assumed that electron collisions with all prism surfaces lead to dephasing. Reasonable comparisons between theory and experiment are found if it is assumed that only electron collisions involving motions parallel to the plane of the prism lead to dephasing. Substrate effects make a comparatively small contribution to the fwhm. Additionally, it is shown that the optical constants of Johnson and Christy [<i>Phys. Rev. B</i> <b>1972</b>, <i>6</i>, 4370] provide the best match to the experimental data. We conclude by presenting the design rules for producing Ag triangular nanoparticles with narrow plasmon resonances

Eutectic Liquid Alloys for Plasmonics: Theory and Experiment

We report a method based on density functional theory molecular dynamics that allows us to calculate the plasmonic properties of liquid metals and metal alloys from first principles with no a priori knowledge of the system. We show exceptional agreement between the simulated and measured optical constants of liquid Ga and the room temperature liquid In–Ga eutectic alloy (<i>T</i>_m = 289 K). We then use this method to analyze the plasmonic properties of various alloy concentrations in the In–Ga system. The plasmonic performance of the In–Ga system decreases with increasing In concentration. However, the benefits of a room-temperature plasmonic liquid are likely to outweigh the minor reduction in plasmonic performance when moving from pure Ga to the eutectic composition. Our results show that density functional theory molecular dynamics can be used as a predictive tool for studying the optical properties of liquid metal systems amenable to plasmonics

Adjusting the Metrics of 1‑D Helical Gold Nanoparticle Superstructures Using Multivalent Peptide Conjugates

The properties of nanoparticle superstructures depend on many factors, including the structural metrics of the nanoparticle superstructure (particle diameter, interparticle distances, etc.). Here, we introduce a family of gold-binding peptide conjugate molecules that can direct nanoparticle assembly, and we describe how these molecules can be systematically modified to adjust the structural metrics of linear double-helical nanoparticle superstructures. Twelve new peptide conjugates are prepared via linking a gold-binding peptide, AYSS­GAPP­MPPF (PEP_Au), to a hydrophobic aliphatic tail. The peptide conjugates have 1, 2, or 3 PEP_Au headgroups and a C₁₂, C₁₄, C₁₆, or C₁₈ aliphatic tail. The soft assembly of these peptide conjugates was studied using transmission electron microscopy (TEM), atomic force microscopy (AFM), and infrared (IR) spectroscopy. Several peptide conjugates assemble into 1-D twisted fibers having measurable structural parameters such as fiber width, thickness, and pitch that can be systematically varied by adjusting the aliphatic tail length and number of peptide headgroups. The linear soft assemblies serve as structural scaffolds for arranging gold nanoparticles into double-helical superstructures, which are examined via TEM. The pitch and interparticle distances of the gold nanoparticle double helices correspond to the underlying metrics of the peptide conjugate soft assemblies, illustrating that designed peptide conjugate molecules can be used to not only direct the assembly of gold nanoparticles but also control the metrics of the assembled structure