Valence Electron Density-Dependent Pseudopermittivity for Nonlocal Effects in Optical Properties of Metallic Nanoparticles

The peak positions of localized surface plasmonic resonance (LSPR) are strongly dependent on the sizes of metallic nanoparticles. TDDFT calculations have shown a remarkable size effect for metallic nanoparticles smaller than 1 nm, because it could account for fully nonlocal effects. Due to the high resource consumption of TDDFT, several semiquantum approaches have been proposed to reduce the computation time while addressing nonlocal effects, and it is still desirable to introduce new ideas into this area since physical origins of related fields are not completely known yet. In this work, we took account of both spilling out of s-band electrons and the screening effect of d-band electrons in the LSPR phenomena and developed a model using pseudopermittivity to describe several quantum mechanical effects that contribute to nonlocal effects in LSPR. With incorporation of machine learning, this model is capable of calculating the optical response of large nanostructures above the nanometer scale. Besides successful prediction for different metallic nanoparticle monomers, the tunneling effect occurring in dimers can also be well described by using the concept of pseudopermittivity. The employing of pseudopermittivity and machine learning is expected to achieve both high accuracy and high efficiency in quantum plasmonics. It provides a new ideology in the simulation of wave–matter interactions

Widening the Spectral Range of Ultrahigh Field Enhancement by Efficient Coupling of Localized to Extended Plasmons and Cavity Resonances in Grating Geometry

Excitation of localized via extended plasmons was shown recently to reveal ultrahigh electromagnetic field (EM) enhancement when optimum coupling is obtained in the prism configuration. Using grating coupling, one expects several advantages over the prism scheme such as being planar, more compact, and most important the possibility of tuning the spectral range over which the enhancement occurs. In this work we show that via gratings coupling the EM field enhancement can be up to 3 orders of magnitude higher than that obtained using free space excitation of localized surface plasmons (LSPs). Furthermore, the spectral range over which the ultrahigh enhancement achieved becomes wider by tuning the grating parameters. The cavity resonances generated by thick enough gratings couple to the LSPs producing ultrahigh local enhancement and play an important role in widening the spectral range to cover the range 400–2000 nm. This is important for solar energy harvesting and improving the efficiency of infrared optoelectronic devices. Having the periodic NPs arrangement on top of the grating was found to be very significant not only under transverse magnetic (TM) polarization but also under transverse electric (TE) polarization, thus reducing the dependence on the polarization

Quantitative Prediction of Position and Orientation for Platonic Nanoparticles at Liquid/Liquid Interfaces

Because of their intrinsic geometric structure of vertices, edges, and facets, Platonic nanoparticles are promising materials in plasmonics and biosensing. Their position and orientation often play a crucial role in determining the resultant assembly structures at a liquid/liquid interface. Here, we numerically explored all possible orientations of three Platonic nanoparticles (tetrahedron, cube, and octahedron) and found that a specific orientation (vertex-up, edge-up, or facet-up) is more preferred than random orientations. We also demonstrated their positions and orientations can be quantitatively predicted when the surface tensions dominate their total interaction energies. The line tensions may affect their positions and orientations only when total interaction energies are close to each other for more than one orientation. The molecular dynamics simulation results were in excellent agreement with our theoretical predictions. Our theory will advance our ability toward predicting the final structures of Platonic nanoparticle assemblies at a liquid/liquid interface

Synergistic Effects of Water and Oxygen Molecule Co-adsorption on (001) Surfaces of Tetragonal CH₃NH₃PbI₃: A First-Principles Study

The poor environmental stability of organometallic halide perovskite solar cells presents a big challenge for its commercialization, which is mainly due to the degradation of perovskite materials in humid air. The role played by water molecules has been extensively studied in the degradation processes, where strong interactions between water molecules and perovskite surfaces are found. Using first-principles simulations, we find that oxygen molecules also have strong interactions with (001) surfaces of tetragonal CH₃NH₃PbI₃ through the formation of a chemical Pb–O bond on the PbI₂-terminated surface and a hydrogen bond on the CH₃NH₃I-terminated surface. The adsorbed oxygen molecules introduce empty states near the Fermi level of the surfaces, which can facilitate charge transfer between the surface and oxygen molecules. Furthermore, when an oxygen molecule is located atop a Pb atom on PbI₂-terminated surface, the calculated adsorption energies indicate that the surface is more attractive to water molecules, making the surface even more sensitive to humidity. These findings reveal that oxygen molecules also play an important role in the initial stage of the degradation of perovskite materials

Structural Effects in the Electromagnetic Enhancement Mechanism of Surface-Enhanced Raman Scattering: Dipole Reradiation and Rectangular Symmetry Effects for Nanoparticle Arrays

Surface-enhanced Raman scattering (SERS) enhancement factors for Ag and Au sphere array structures are determined by rigorously including dipole reradiation in a T-matrix formalism. Comparisons are made with the more commonly used local field enhancement due to plane-wave excitation, |<b>E</b>(<b>r</b>₀;ω)|²|<b>E</b>(<b>r</b>₀;ω_<i>s</i>)|² which for zero Stokes shift is |<b>E</b>(<b>r</b>₀;ω)|⁴ to determine the errors associated with this approximation. Substantial errors (factors of 10–100) are found for the peak enhancements at a scattering angle well away from the incident direction, but for backscattering, the errors are negligible. We also present |<b>E</b>|⁴ enhancement factors using a periodic boundary discrete dipole approximation method for several metal strip array structures, and we show that a certain combination of rectangular array structure and strip properties leads to electromagnetic enhancement factors for mixed photonic-plasmonic resonances that are considerably higher than can be produced with either square arrays or 1-D arrays based on the same particles and spacings

Optimal Interparticle Gap for Ultrahigh Field Enhancement by LSP Excitation via ESPs and Confirmation Using SERS

We have predicted extremely high electromagnetic hot spots using the extended–localized coupled surface plasmon resonance configuration. With this unique configuration, we found that an array of particles shows the critical importance of the interparticle gap on the enhancement factor, which was confirmed experimentally using surface-enhanced Raman scattering (SERS). The extended plasmon wave excited in the Kretschmann–Raether configuration propagates on the silver film surface and couples with the gold nanoparticles dispersed on top through excitation of the localized plasmons. A monomolecular layer of 4-aminothiophenol sandwiched between the metal film and the nanoparticles showed an SERS enhancement factor of the order of 10¹⁰ per molecule in the hot spots. The configuration was optimized, both by simulations and experiments, with respect to the size of the nanoparticles and the interparticle distances. It is demonstrated that the ultrahigh SERS enhancement does occur only when the extended surface plasmon is coupled to the localized surface plasmon at an optimized interparticle gap. Further, highly sensitive detection of glycerol in ethanol is demonstrated using the optimum structure with a detection limit on the order of 10^–12 to the weight percentage of ethanol, which is equivalent to detection of a few molecules. This ultrahigh enhancement is useful in realizing various highly sensitive biosensors when strong enhancement is required as well as in highly efficient optoelectronic and energy devices

Dependence of Plasmonic Properties on Electron Densities for Various Coupled Au Nanostructures

Noble metallic nanostructures have great potential in optical sensing application in visible and near-infrared frequencies. Their plasmonic properties can be manipulated by <i>in situ</i> controlling their electron densities for isolated nanostructures. However, the effect of charging remains underexplored for coupled systems. In this work, we theoretically investigated the dependence of their far-field and near-field properties on their electron densities for various coupled gold structures. With increasing electron densities, their enhancement factors increase while their plasmonic resonance peaks are blue-shifted. The resonance peak position of ellipsoid-ellipsoid dimers shows the highest sensitivity in response to the charging effects with the slope of −2.87. The surface-averaged electric field of ellipsoid monomer shows largest enhancement ratio of 1.13 with 16% excess electrons. These results can be well explained by an effective dipole moment model. In addition, we also studied the sphere-on-substrate nanostructure which can be precisely fabricated. This system shows low sensitivity to the charging effect with the slope of −1.46 but remarkable enhancement ratio of 1.13 on near field response with 16% excess electrons

Origins of Charge Mobility Decreasing from Stretching–Releasing Cycles in Polymer Semiconductors

Polymer semiconductors as a key component of electronic skin need to maintain the coexistence of stretchability and electrical functionalities. However, repeated stretching–compressing cycles inevitably lead to the charge mobilities decreasing and poor working performance of polymer semiconductors. Here, a method combining molecular dynamics (MD) simulations and charge transport theory was developed to obtain the morphology–mobility relationship of amorphous poly(3-hexylthiophene) (P3HT). The simulation results show that the hole mobility decreases by 6% along the strain direction after three stretching–compressing cycles with 80% strain. These results are due to the chain alignment change caused by the mechanical operations. The stretched P3HT material presents higher charge mobility due to its better chain alignment, while the compressed P3HT shows lower charge mobility because of the poor chain alignment. Repeated stretching–compressing cycles lead to the chain alignment parameters decreasing along the deformation direction with accumulation and saturation effects. The repeated cycles also result in the primitive path length decreasing, which indicates polymer chain spatial distribution is more localized after repeated deformations. Our findings provide microscale knowledge about the dependence of molecular morphology and charge mobility on stretching–compressing cycles, which can help to guide the design of polymer semiconductors with higher charge mobility under repeated stretching–compressing cycles

Origins of Charge Mobility Decreasing from Stretching–Releasing Cycles in Polymer Semiconductors

Polymer semiconductors as a key component of electronic skin need to maintain the coexistence of stretchability and electrical functionalities. However, repeated stretching–compressing cycles inevitably lead to the charge mobilities decreasing and poor working performance of polymer semiconductors. Here, a method combining molecular dynamics (MD) simulations and charge transport theory was developed to obtain the morphology–mobility relationship of amorphous poly(3-hexylthiophene) (P3HT). The simulation results show that the hole mobility decreases by 6% along the strain direction after three stretching–compressing cycles with 80% strain. These results are due to the chain alignment change caused by the mechanical operations. The stretched P3HT material presents higher charge mobility due to its better chain alignment, while the compressed P3HT shows lower charge mobility because of the poor chain alignment. Repeated stretching–compressing cycles lead to the chain alignment parameters decreasing along the deformation direction with accumulation and saturation effects. The repeated cycles also result in the primitive path length decreasing, which indicates polymer chain spatial distribution is more localized after repeated deformations. Our findings provide microscale knowledge about the dependence of molecular morphology and charge mobility on stretching–compressing cycles, which can help to guide the design of polymer semiconductors with higher charge mobility under repeated stretching–compressing cycles

Synthesis of Spiky Ag–Au Octahedral Nanoparticles and Their Tunable Optical Properties

Spiky nanoparticles exhibit higher overall plasmonic excitation cross sections than their nonspiky peers. In this work, we demonstrate a two-step seed-mediated growth method to synthesize a new class of spiky Ag–Au octahedral nanoparticles with the aid of a high molecular weight poly­(vinylpyrrolidone) polymer. The length of the nanospikes can be controlled from 10 to 130 nm with sharp tips by varying the amount of gold precursor added and the injection rates. Spatially resolved electron energy-loss spectroscopy (EELS) study and finite-difference time-domain (FDTD) simulations on individual spiky Ag–Au nanoparticles illustrate multipolar plasmonic responses. While the octahedral core retains its intrinsic plasmon response, the spike exhibits a hybridized dipolar surface plasmon resonance at lower energy. With increasing spike length from 50 to 130 nm, the surface plasmon of the spike can be tuned from 1.16 to 0.78 eV. The electric field at the spike region increases rapidly with increasing spike length, with a 10⁴ field enhancement achieved at the tips of 130-nm spike. The results highlight that it is important to synthesize long spikes (>50 nm) on nanoparticles to achieve strong electric field enhancement. A hypothesis for the formation of sharp spikes is proposed based on our studies using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (TEM)