Plasmonic Behavior Research Articles

Surface plasmon resonance and coloration in stainless steel with a 2D periodic texture

Although stainless steel (STS) is an alloy commonly used in both daily life and the industrial field, little is known about its plasmonic behavior. In this study, we investigated the surface plasmon resonance (SPR) phenomenon in two-dimensionally (2D) textured STS. A 2D periodic grating with a 500 nm pitch was fabricated by imprinting combined with electrochemical etching on the surface of 316L STS plates. Since the fabricated surface texture gave rise to SPR absorption peaks and structural colors, its resonance behavior was characterized in terms of light incident direction and polarization state; the dependence of the SPR wavelengths on these two parameters was theoretically calculated based on the grating-assisted light-coupling mechanism. The experimental results were in good agreement with the theoretical calculations. Grating-coupled SPR can be an effective tool to generate structural colors in STS and may be used in many applications including surface decoration, product identification, and anti-counterfeiting.

Photodeposition of Pd onto Colloidal Au Nanorods by Surface Plasmon Excitation.

A protocol is described to photocatalytically guide Pd deposition onto Au nanorods (AuNR) using surface plasmon resonance (SPR). Excited plasmonic hot electrons upon SPR irradiation drive reductive deposition of Pd on colloidal AuNR in the presence of [PdCl4]2-. Plasmon-driven reduction of secondary metals potentiates covalent, sub-wavelength deposition at targeted locations coinciding with electric field "hot-spots" of the plasmonic substrate using an external field (e.g., laser). The process described herein details a solution-phase deposition of a catalytically-active noble metal (Pd) from a transition metal halide salt (H2PdCl4) onto aqueously-suspended, anisotropic plasmonic structures (AuNR). The solution-phase process is amenable to making other bimetallic architectures. Transmission UV-vis monitoring of the photochemical reaction, coupled with ex situ XPS and statistical TEM analysis, provide immediate experimental feedback to evaluate properties of the bimetallic structures as they evolve during the photocatalytic reaction. Resonant plasmon irradiation of AuNR in the presence of [PdCl4]2- creates a thin, covalently-bound Pd0 shell without any significant dampening effect on its plasmonic behavior in this representative experiment/batch. Overall, plasmonic photodeposition offers an alternative route for high-volume, economical synthesis of optoelectronic materials with sub-5 nm features (e.g., heterometallic photocatalysts or optoelectronic interconnects).

Hybrid One-Dimensional Plasmonic-Photonic Crystals for Optical Detection of Bacterial Contaminants.

Photonic crystal-based biosensors hold great promise as low-cost devices for real-time monitoring of a variety of biotargets, for example, bacterial contaminants in food. Here, we report the proof-of-concept for a new colorimetric sensor of bacterial contamination, which is based on a novel hybrid plasmonic-photonic device. Our system consists of a layer of silver, a plasmonic metal exhibiting a well-known bioactivity, on top of a one-dimensional photonic crystal. We attribute the bioresponsivity to the formation of polarization charges at the Ag/bacterium interface within a sort of "bio-doping" mechanism. Interestingly, this triggers a blue shift in the photonic response. As an example, we assessed the validity of our approach by detecting one of the most hazardous contaminants, Escherichia coli. This work demonstrates that our device can be a low-cost and portable platform for the detection of common bacterial contaminants.

Study on the doping effect of spin coated Al and In doped and (Al/In) co-doped ZnO thin films for near-infrared plasmonic applications

Al and In doped and (Al/In) co-doped ZnO films were fabricated by spin coating for near infrared plasmonic applications. The total dopant concentration was fixed at 3 at.%, in which In and Al were varied sequentially. The as-coated films were annealed in air at 400 °C followed by second annealing at 300 °C in reduced atmosphere. Films selected with respect to carrier concentration (n) were further investigated for plasmonic applications. Films with a particular ratio of Al and In only exhibited n~1020/cm3, necessary for plasmonic behaviour in near infrared. Structural studies were carried out by X-Ray diffractometer and band gap variations by calculating Urbach energy. Plasma frequency calculated from reflectance–transmittance spectra was theoretically verified using Drude-Lorentz equation. Al and In doped films and Al:In = 3:7 film exhibited metallic behaviour near to the wavelength 2000 nm. The films possessed minimum loss compared to those prepared by vacuum techniques. The properties obtained confirmed suitability of the films for plasmonic applications in the near infrared.

Plasmon dispersion in graphite: A comparison of current ab initio methods

We perform a systematic study of the macroscopic dielectric function and electron energy loss (EEL) spectra for graphite. We obtain the dispersion behavior for the $\ensuremath{\pi}$ plasmon, as a function of the momentum transfer $q$ for two nonequivalent paths that traverse the first four Brillouin zones. We carry out these calculations within both time-dependent density functional theory (with two exchange-correlation functionals) and the Bethe-Salpeter equation. Additionally, we explore the effects of using the complete excitonic Hamiltonian (with all electron-hole pairs and antipairs), and within the Tamm-Dancoff approximation (neglecting antipairs). By analyzing the behavior of the macroscopic dielectric function, we are able to determine which peaks are predominantly from plasmonic behavior or only interband transitions. We compare the calculated spectra against several experiments that span almost five decades; our results present clear trends that follow the physical origins of the observed peaks. We carry out this study over a large range of momentum transfer in order to better evaluate the different theoretical methods compared to experiment, and predict the plasmonic behavior beyond available experimental data. Our results indicate that including the complete Hamiltonian with the exciton coupling included is essential for accurately describing the observed EEL spectra and plasmon dispersion of graphite, particularly for low values of momentum transfer. However, the solution of the Bethe-Salpeter equation is computationally intensive, so time-dependent density functional theory methods used in conjunction with the complete Hamiltonian may be an attractive alternative.

Solvothermally-synthesized tin-doped indium oxide plasmonic nanocrystals spray-deposited onto glass as near-infrared electrochromic films

Nanocrystals of semiconducting, highly-doped metal oxides present localized surface plasmon resonance properties responsible for intense optical absorption in the near-infrared (NIR) spectral range. These features make them strong candidates for designing “new generation”, spectrally-selective electrochromic smart windows and other optoelectronic devices. In the present study, nanocrystals of tin-doped indium oxide (ITO) are solvothermally-synthesized in benzyl alcohol. Their structure, morphology and composition are assessed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, while their optical plasmonic behavior is evidenced by means of VIS-NIR spectrometry and electron energy loss spectroscopy coupled to scanning transmission electron microscopy. Remarkably, the latter setup allows for the value-added acquisition of high-resolution mappings and quantifications of the nanocrystals individual plasmon responses at both surface and bulk levels. Inks of ITO nanocrysals are then spray-coated onto glass to act as NIR-modulating plasmonic electrochromic layers. ITO dispersions are straightforwardly deposited from low toxic media (isopropanol) according to a room-temperature procedure implying no harsh chemical or thermal post-treatment – only a mild drying step at 100 °C for 10 min. Finally, in situ spectroelectrochemical measurements completed on the films immersed in liquid electrolyte show appreciable NIR contrasts (up to 40%) upon electrochemical charge/discharge, together with a >80% VIS transmittance.

Anomalous phonon relaxation in Au333(SR)79 nanoparticles with nascent plasmons

Research on plasmons of gold nanoparticles has gained broad interest in nanoscience. However, ultrasmall sizes near the metal-to-nonmetal transition regime have not been explored until recently due to major synthetic difficulties. Herein, intriguing electron dynamics in this size regime is observed in atomically precise Au333(SR)79 nanoparticles. Femtosecond transient-absorption spectroscopy reveals an unprecedented relaxation process of 4-5 ps-a fast phonon-phonon relaxation process, together with electron-phonon coupling (∼1 ps) and normal phonon-phonon coupling (>100 ps) processes. Three types of -R capped Au333(SR)79 all exhibit two plasmon-bleaching signals independent of the -R group as well as solvent, indicating plasmon splitting and quantum effect in the ultrasmall core of Au333(SR)79 This work is expected to stimulate future work on the transition-size regime of nanometals and discovery of behavior of nascent plasmons.

Plasmonic Core-Shell Silicon Carbide-Graphene Nanoparticles.

We demonstrate the synthesis of silicon carbide nanoparticles exhibiting monolayer to few-layer graphene coatings and characterize their optical response to confirm their plasmonic behavior. A multistep, low-temperature plasma process is used to nucleate silicon particles, carbonize them in-flight to give small silicon carbide nanocrystals, and coat them in-flight with a graphene shell. These particles show surface plasmon resonance in the infrared region. Tuning of the plasma parameters allows control over the nanoparticle size and consequently over the absorption peak position. A simplified equivalent dielectric permittivity model shows excellent agreement with the experimental data. In addition, optical characterization at high temperatures confirms the stability of their optical properties, making this material attractive for a broad range of applications.

Self-focusing and self-bending of surface plasmons in longitudinally modulated metasurfaces

We propose an effective method to manipulate the flow of surface plasmons propagating in a longitudinally modulated metasurface formed by periodic silver ridge gratings. We show that band structures with elliptical, flat and hyperbolic dispersion profiles can be achieved in surface ridge grating metasurface with different ridge height and width, therefore, the propagation behaviors of the surface plasmons in the metasurface can be actively molded in longitudinally modulated ridge grating metasurface. Both self-focusing and self-bending propagation of surface plasmons are demonstrated. Such longitudinally modulated metasurfaces provide a good platform to steer or mold the flow of surface plasmons, which may have many potential applications in nanoplasmonics such as on-chip directional routing of surface plasmons, subwavelength beam steering and on-chip signal processing.

Structural and light trapping properties of nanoporous silicon micro-pyramid patterns encrusted with silver nanoparticles

Surface modification to improve light trapping properties is one of the choices that can be used to improve the efficiency of silicon (Si)-based solar cells. In this work, a simple combined treatment is used to modify Si surfaces to improve their overall light trapping properties. Silver (Ag) nanoparticles were deposited on porous-Si micro-pyramid structures to investigate the effects of combining the three mechanisms on light trapping properties of the treated-Si surfaces. The surface modifications were introduced for a simple large-scale chemical treatment of the Si surfaces. It was found that the etching current is a limiting factor that controls the size of the etched nanopores. The size of the deposited-Ag nanoparticle depends on both the nanopore sizes of the porous pyramids and on the AgNO3 dipping time. The average Ag particle size increased from 20.5 nm, for samples with smaller nanopores etched with 10 mA/cm2 current, to 24.0 nm for samples with larger nanopores etched with 20 mA/cm2 current. From scanning electron microscopy (SEM) and as verified by X-ray diffraction (XRD), the average Ag particle size increased from 48.25 nm to about 64.09 nm as the AgNO3 dipping time increased from 10 to 40 min. The reflectivity measurements showed a considerable 98.44% reduction in the average reflectance of the treated samples in the wavelength range of 400–840 nm. It is believed that the Ag nanoparticles’ plasmonic behavior and the nanoporous pyramid patterns are responsible for this substantial reflectivity reduction which leads to an increased efficiency of light trapping of the treated-Si samples.

Anisotropic Plasmonic CuS Nanocrystals as a Natural Electronic Material with Hyperbolic Optical Dispersion

Copper sulfide nanocrystals have recently been studied due to their metal-like behavior and strong plasmonic response, which make them an attractive material for nanophotonic applications in the near-infrared spectral range; however, the nature of the plasmonic response remains unclear. We have performed a combined experimental and theoretical study of the optical properties of copper sulfide colloidal nanocrystals and show that bulk CuS resembles a heavily doped p-type semiconductor with a very anisotropic energy band structure. As a consequence, CuS nanoparticles possess key properties of relevance to nanophotonics applications: they exhibit anisotropic plasmonic behavior in the infrared and support optical modes with hyperbolic dispersion in the 670-1050 nm spectral range. We also predict that the ohmic loss is low compared to conventional plasmonic materials such as noble metals in the NIR. The plasmonic resonances can be tuned by controlling the size and shape of the nanocrystals, providing a playground for future nanophotonic applications in the near-infrared.

Surface Plasmon Resonances and Local Field Enhancement in Aluminum Nanoparticles Embedded in Silicon Nitride

We investigate the plasmonic response of periodic chains of flat aluminum nanoparticles capped with a transparent protective silicon nitride layer. The elaboration method is based on the nucleation and growth of nanoparticles by physical vapor deposition of Al at glancing incidence on a prepatterned silicon nitride surface. A detailed structural characterization by transmission electron microscopy evidences a core@shell Al@AlN structure, with an in-plane particle size below 15 nm and a chain periodicity of ∼30 nm. The nanoparticle assembly exhibits surface plasmon resonances whose spectral positions depend on the polarization of the electric field with respect to the particle chains, as revealed by absorbance measurements. Near-field calculations highlight the possible overlap between the surface plasmon resonance and the interband transitions of aluminum. The plasmonic behavior of such nanoparticle arrays suggests a potential for polarization-selective near-field enhancement over a broad spectral range.

A plasma‐based gas‐phase method for synthesis of gold nanoparticles

AbstractThere are many applications for gold nanoparticles (AuNPs) due to their interesting optoelectronic properties such as tunable optical absorption and plasmonic resonance behavior. Although synthesis and stabilization of colloidal AuNPs are well established, new synthesis routes can lead to enhanced versatility of applications for AuNPs, particularly if the methods allow avoidance of solution processes or surfactants. Here, we introduce a plasma‐based synthesis of AuNPs, using a consumable gold wire and a radiofrequency power source. The AuNPs are monodisperse, with an average diameter of 4 nm. Although production yield is low, the narrow size distribution of the AuNPs and the avoidance of solution processing in this method are promising for future syntheses of metal NPs based on plasmas.

Indium silicon oxide thin films for infrared metaphotonics

We demonstrate silicon-doped indium oxide (ISO) with tunable epsilon near-zero and plasmonic behavior and propose it as a Si-compatible alternative material for metaphotonic applications in the mid-infrared (MIR) spectral range. ISO thin films were grown using DC and RF magnetron cosputtering deposition. Post-deposition thermal annealing was performed that resulted in large tunability of the optical dispersion data, as measured directly by variable-angle ellipsometry. Screened plasma wavelengths in the range 1.7 μm–5.8 μm were experimentally obtained depending on the annealing conditions. Moreover, ISO films with 350 nm thickness were investigated by atomic force microscopy that reveals a maximum rms roughness of 1.9 nm, enabling high-quality nanofabrication. In order to demonstrate the suitability of ISO thin films for the fabrication of plasmonic structures, we have produced ISO nanoparticle disk arrays using electron-beam lithography on Al2O3 substrates and shown, using both finite element method simulations and Fourier transform infrared spectroscopy, distinctive plasmonic resonances at MIR wavelengths tuned by the nanoparticles' spacing and aspect ratio. A figure of merit based on Mie theory is computed for spheres whose radii are either λ/10 or λ/3 over a broad spectral range that enables quantitative comparison with alternative plasmonic materials and reveals the full potential of ISO for engineering localized surface plasmons across the MIR spectral range. The development of fully Si-compatible plasmonic materials with engineered optical dispersion is important to enable cost-effective photonic-plasmonic high-density device integration for metaphotonic research.

Phase Separation in Liquid Metal Nanoparticles

Nanoparticles produced from gallium-based liquid metal alloys have been explored for developing applications in the fields of electronics, catalysis, and biomedicine. Nonetheless, physical properties, such as phase behavior at micro-/nanosize scale, are still significantly underexplored for such nanoparticles. Here, we conduct an in situ investigation of phase behavior for gallium-based liquid metal nanoparticles and discover the unprecedented coexistence of solid particles in spherical liquid metal shells without the support of a crystalline substrate. The particles can also transform into solid Janus nanoparticles after temperature cycling. In addition, we investigate the optical properties of the nanoparticles before and after phase separation using in situ electron energy-loss spectroscopy. Most importantly, we discover that increasing the content of indium within the nanoparticle can stabilize the solid-core/liquid-shell structure at room temperature. This study provides a foundation to engineer liquid metal nanoparticles for developing new applications in nanoscale optical platforms and shape-configurable transformers.Although various electronic, chemical, and biomedical applications have been demonstrated for nanoparticles made from gallium-based liquid metal alloys, fundamental physical properties such as phase behavior of such nanoparticles are still significantly underexplored. Here, Tang and coworkers present the in situ investigation of phase separation in binary and ternary spherical liquid metal nanoparticles upon cooling, and demonstrate the coexistence of solid core/liquid shell without the support of a crystalline substrate. This study provides insight into engineering such nanoparticles for the development of new applications.Despite gallium-based liquid metal alloys attracting extensive attention for various applications, their phase behavior at the nanoscale is still underexplored. Understanding the impact of phase separation in nanoparticles can be extremely useful for developing new structures and exploring suitable applications. Here, we report on the investigation of phase behavior for spherical nanoparticles made from gallium-based liquid metal alloys upon cooling. We discover the thermally stable coexistence of solid cores in spherical liquid metal shells without the support of a crystalline substrate at room temperature. Given the unique properties of liquid metal, together with the facile process for producing core-shell and Janus nanoparticles, this work will encourage further investigation of the properties of such nanoparticles for developing applications in the fields of electronics, catalysis, nanomedicine, and beyond.

Estimating Near Electric Field of Polyhedral Gold Nanoparticles for Plasmon-Enhanced Spectroscopies

Technological application of polyhedral plasmonic nanoparticles (NP) is closely associated with their strong ability of localizing electric field at and near the surface. In this study, we calculate the optical properties of most common regular polyhedral Au nanoparticles such as the cube, rhombic dodecahedron, pentagonal bipyramid, and octahedron and compare with the optical properties of spherical particles to investigate the plasmonic behavior of metallic nanoparticles due to the breakdown of spherical symmetry. We demonstrate that the highest electric field enhancement (1 order magnitude higher than the spherical NP) occurs in octahedral Au NPs as the consequence of strong localization of incident electromagnetic field, decreasing the diffraction limit of the optical microscopies, which depends both on their size and orientation with respect to the polarization of incident light, increasing the system resolution. Higher order modes (quadrupole, hexapole, octupole, etc.) appear along the dipole mode as the shape of the NPs deviates from their regular spherical shape, even though their size remains much smaller than the incident wavelength. Irrespective of their shape, the major contribution of small NPs comes from the light absorption. Although the scattering efficiency of Au NPs becomes significant with the increase of their size, the NPs with sharper borders and vertices scatter higher fraction of light than the NPs of spherical shape.

Plasmon Excitations in Mixed Metallic Nanoarrays.

Features of the surface plasmon from macroscopic materials emerge in molecular systems, but differentiating collective excitations from single-particle excitations in molecular systems remains elusive. The rich interactions between single-particle electron-hole and collective electron excitations produce phenomena related to the chemical physics aspects within the atomic array. We study the plasmonic properties of atomic arrays of noble (Au, Ag, and Cu) and transition-metal (Pd, Pt) homonuclear chains using time-dependent density functional theory and their Kohn-Sham transition contributions. The response to the electromagnetic radiation is related to both the geometry-dependent confinement of sp-valence electrons and the energy position of d-electrons in the different atomic species and the hybridization between d and sp electrons. It is possible to tune the position of the plasmon resonance, split it into several peaks, and eventually achieve broadband absorption of radiation. Arrays of mixed noble and transition-metal chains may have strongly attenuated plasmonic behavior. The collective nature of the excitations is ascertained using their Kohn-Sham transition contributions. To manipulate the plasmonic response and achieve the desired properties for broad applications, it is vital to understand the origins of these phenomena in atomic chains and their arrays.

Numerical Analysis of Nonlocal Optical Response of Metallic Nanoshells

Nonlocal and quantum effects play an important role in accurately modeling the optical response of nanometer-sized metallic nanoparticles. These effects cannot be described by conventional classical theories, as they neglect essential microscopic details. Quantum hydrodynamic theory (QHT) has emerged as an excellent tool to correctly predict the nonlocal and quantum effects by taking into account the spatial dependence of the charge density. In this article, we used a QHT to investigate the impact of nonlocality and electron spill-out on the plasmonic behavior of spherical Na and Au nanoshells. We adopted a self-consistent way to compute the equilibrium charge density. The results predicted by QHT were compared with those obtained with the local response approximation (LRA) and the Thomas–Fermi hydrodynamic theory (TFHT). We found that nonlocal effects have a strong impact on both the near- and far-field optical properties of nanoshells, in particular, for the antibonding resonant mode. We also investigated the optical response of these systems for different thicknesses of the shell, both for Na and Au metals.

Cryogenic characterization of titanium nitride thin films

It is well known that noble metals are not compatible with silicon fabrication processing due to their low melting point, and that their plasmonic behaviour suffers from the material losses at visible wavelengths. As an alternative, titanium nitride has been highly investigated in order to overcome these challenges. High temperature characterization of TiN films has been performed, showing its CMOS compatibility; however, information on intrinsic losses at lower temperatures is still lacking. Here we experimentally investigate the optical properties of a 100 nm TiN film under low temperatures down to 1.5 K. From the reflection measurements we retrieve the dielectric constant and analyze plasmonic applications possibilities.

Photo-induced charge density distribution in metal surfaces and its extraction with apertureless near-field optics

Electromagnetic (EM) waves impinging on finite metallic structures can induce non-uniform electrical currents and create oscillating charge densities. These local charges govern the important physical processes such as plasmonic behavior or enhanced Raman scattering. Yet the quantitative calculation and probing of the spatial distribution of the charge density still remain challenging at the subwavelength scale. This is especially the case if one considers the boundary effect, where the charge density can become divergent and conventional finite element methods fail to obtain accurate information. With an approach we recently developed, we calculate this charge density for subwavelength structures with and without sharp corners: gold disks and equilateral triangles. We also devise an independent way to extract the surface charge density distributions from experiments using scattering-type scanning near-field optical microscope (s-SNOM). We found that the charge density is related to the near field signal Sn by With no adjustable parameters, the extracted surface charge distribution from the experiments matches well with that from the theoretical prediction, both in magnitude and phase. Our work provides a quantitative study of the surface charge distributions and a systematic and rigorous treatment to extract surface charge distributions at the nanoscale, opening opportunities for mining the near-field data from s-SNOM.