Plasmonic Properties Research Articles

(Invited) Photophysics and Photochemistry of Individual Plasmonic Nanostructures

A surface plasmon in a metal nanoparticle is the coherent oscillation of the conduction band electrons leading to both absorption and scattering as well as strong local electromagnetic fields. These fundamental properties have been exploited in many different ways, including surface enhanced spectroscopy and sensing, photocatalysis, photothermal cancer therapy, and color display generation. Chemical synthesis and assembly of nanostructures are able to tailor plasmonic properties that are, however, typically broadened by ensemble averaging. Single particle spectroscopy together with correlated imaging is capable of removing heterogeneity in size, shape, and assembly geometry and furthermore allows one to separate absorption and scattering contributions. In this talk I will discuss our recent work on distinguishing the different contributions that cause plasmon decay as probed by the homogeneous single-particle linewidth.[1-4] In particular, I will focus on plasmon damping due to energy and charge transfer from the metal to its environment with the goal to mechanistically understand how energy conversion from an incident photon to a plasmon and then a localized excitation can be maximized while circumventing fast metal-intrinsic internal conversion that simply leads to heat generation.

Design of Biopolymer-Coated Gold Nanorods as Biorelevant Photothermal Agents.

Gold nanorods (AuNRs) are emerging metallic nanoparticles utilized to generate heat for photothermal therapy (PTT) in cancer. The tunable plasmonic properties of AuNRs make them a remarkable candidate for hyperthermia. However, the cytotoxicity of AuNRs limits its biological applicability due to the existence of cetyltrimethylammonium bromide (CTAB) on the surface as a common surfactant. In this study, AuNRs are synthesized by seed-mediated growth and then the optical properties are optimized by altering AgNO3 concentration. Afterward, CTAB is replaced with biopolymers which are BSA:Dextran and BSA:Guar Gum conjugates resulting in enhanced cellular viability, enabling to use of them as biologically relevant photothermal agents. The biocompatibility of AuNRs is improved to utilize them at high concentrations for laser studies, in which similar heat generation success of CTAB- and biopolymer-coated AuNRs are shown for potential PTT applications. CTAB and biopolymer-coated AuNRs in concentrations of 0.5 and 1mg mL-1 are irradiated under NIR light at 808nm laser at 0.5, 0.75, and 1W cm-2 for 300 s. The biopolymer-coated gold nanorods with different coatings preserve photothermal properties while reducing the cytotoxicity effects of CTAB and thus they are promising photothermal agents for potential PTT.

Localized Surface Plasmon Resonances of AgAl Alloy Nanoparticles Self-Organized by Annealing of Ag/Al Bilayer Films

Plasmonic applications have traditionally relied on noble metals such as gold (Au) and silver (Ag) for their excellent plasmonic performance in the visible and near-infrared spectrum. However, these metals are costly, scarce, and have limitations such as low stability (Ag) and interband transition losses, which restrict their spectral range. To address these issues, alternative plasmonic materials have been explored. One such material is aluminum (Al), which is inexpensive, abundant, and exhibits remarkable plasmonic properties in the UV region as well as wide tunability. Al is also compatible with complementary metal–oxide semiconductor (CMOS) fabrication processes and is very stable due to its ultrathin native oxide layer. Alloying different metals can combine their advantageous properties, resulting in enhanced tunable optical characteristics. This study investigates the LSPR properties of AgAl alloy nanoparticles grown after the annealing of precursor AgAl bilayer films. Interestingly, LSPRs were also observed in some cases for the as-deposited bilayers. The experimental results were complemented with simulations conducted via the rigorous coupled-wave analysis (RCWA) method. The investigated materials could be potentially useful for applications in energy harvesting or color printing.

Self-Assembled TiN-Metal Nanocomposites Integrated on Flexible Mica Substrates towards Flexible Devices.

The integration of nanocomposite thin films with combined multifunctionalities on flexible substrates is desired for flexible device design and applications. For example, combined plasmonic and magnetic properties could lead to unique optical switchable magnetic devices and sensors. In this work, a multiphase TiN-Au-Ni nanocomposite system with core-shell-like Au-Ni nanopillars embedded in a TiN matrix has been demonstrated on flexible mica substrates. The three-phase nanocomposite film has been compared with its single metal nanocomposite counterparts, i.e., TiN-Au and TiN-Ni. Magnetic measurement results suggest that both TiN-Au-Ni/mica and TiN-Ni/mica present room-temperature ferromagnetic property. Tunable plasmonic property has been achieved by varying the metallic component of the nanocomposite films. The cyclic bending test was performed to verify the property reliability of the flexible nanocomposite thin films upon bending. This work opens a new path for integrating complex nitride-based nanocomposite designs on mica towards multifunctional flexible nanodevice applications.

Improvement of directivity in plasmonic nanoantennas based on structured cubic gold nanoparticles

An array of metallic nanoparticles can diffract or concentrate the incident electromagnetic wave and behave as an antenna. In this paper, the effects of the inner sub-wavelength structure of nanoparticles are studied on the directivity of the plasmonic nanoantenna, which is coated on the output of a waveguide. Three 5*5 element configurations are analyzed: nanocubes, nanoshells, and nanoframes array. Numerical results are obtained using the 3D FDTD technique. The results show that structured nanoantennas can improve the antenna's directivity due to the plasmonic properties and hybridization mechanism. Between the three configurations investigated in the 250–800 nm wavelength range, the nanoshell array exhibits maximum and minimum amounts of its directivity at 321.5 nm and 591 nm, respectively. At 558 nm, nanoframes and nanoshells' arrays show the same amount of directivity, and from the wavelength greater than 558 nm, the nanoframe array has the best performance. The results may help design and fabricate directive optical fiber endcaps.

One‐Pot Synthesis of Gold Nanoparticles and Aluminum Hydroxide Hydrogels‐Based Nanocomposites with Modulated Optical Properties

AbstractIn this work, the one‐pot synthesis of composites constituted by gold nanoparticles (AuNPs) and aluminum hydroxide hydrogels (AlHG) by employing the Epoxide Route is presented. To modulate the optical properties of the final composites, different anions (X= , , and ) were used as nucleophile, complexing and growth directing agents of the AuNPs. In addition, the concentration of the reactants, e. g., the X : Cl ratio, was set in such a way to preserve the alkalization rate, the transparency of the hydrogels supporting the AuNPs, and the stability of the final composites. Consequently, the composites exhibit different plasmonic properties, resulting from the AuNPs with different sizes and morphologies, as confirmed through transmission electron microscopy, depending on the nature of the employed anion, exclusively. Furthermore, this versatile one‐pot synthesis strategy was employed to design new composites with different I : Cl ratio and synthesize stable colloidal AuNPs within an aluminum hydroxide sol (AuNP@Alsol) without adding any conventional capping agent. This AuNP@Alsol composite can be used as seed to accelerate the extremely slow AuNPs formation kinetics in AuNP@AlHG(SCN), demonstrating the potential of this synthesis method to create composites susceptible to be applied in the photonic and catalysis areas.

Gold nanoparticle microemulsion films with tunable surface plasmon resonance signal

Composite films were prepared by the dip-coating method. An organosol of Au nanoparticles (6.3±0.4 nm) was used as the reaction mixture. The nanoparticles were stabilized with sodium bis(2-ethylhexyl) sulfosuccinate (AOT). The prepared films were formed of an AOT layer with encapsulated Au nanoparticles (i.e. Au@AOT films). The films demonstrated a stable plasmon effect. The variation of the immersion number of the glass substrate from 1 to 10 allowed the adjustment of the plasmonic properties. The absorption intensity and maximum wavelength of the surface plasmon resonance were found to change from 0.183 to 0.245 and from 529±2 to 539±2 nm, respectively. The plasmonic properties were found to correlate with the film thickness, roughness, and morphology. An increase in the concentration of Au nanoparticles improves the film’s mechanical strength and wettability.

Non-close-packed plasmonic Bravais lattices through a fluid interface-assisted colloidal assembly and transfer process

AbstractThe assembly of colloids at fluid interfaces followed by their transfer to solid substrates represents a robust bottom-up strategy for creating colloidal monolayers over large, macroscopic areas. In this study, we showcase how subtle adjustments in the transfer process, such as varying the contact angle of the substrate and controlling deposition speed and direction, enable the realization of all five two-dimensional Bravais lattices. Leveraging plasmonic core–shell microgels as the building blocks, we successfully engineered non-close-packed plasmonic lattices exhibiting hexagonal, square, rectangular, centered rectangular, and oblique symmetries. Beyond characterizing the monolayer structures and their long-range order, we employed extinction spectroscopy alongside finite difference time domain simulations to comprehensively investigate and interpret the plasmonic response of these monolayers. Additionally, we probed the influence of the refractive index environment on the plasmonic properties by two methods: first, by plasma treatment to remove the microgel shells, and second, by overcoating the resulting gold nanoparticle lattices with a homogeneous refractive index polymer film. Graphical Abstract

MXene Supported Surface Plasmon Polaritons for Optical Microfiber Ammonia Sensing.

The properties of surface plasmons are notoriously dependent on the supporting materials system. However, new capabilities cannot be obtained until the technique of surface plasmon enabled by advanced two-dimensional materials is well understood. Herein, we present the experimental demonstration of surface plasmon polaritons (SPPs) supported by single-layered MXene flakes (Ti3C2Tx) coating on an optical microfiber and its application as an ammonia gas sensor. Enabled by its high controllability of chemical composition, unique atomistically thin layered structure, and metallic-level conductivity, MXene is capable of supporting not only plasmon resonances across a wide range of wavelengths but also a selective sensing mechanism through frequency modulation. Theoretical modeling and optics experiments reveal that, upon adsorbing ammonia molecules, the free electron motion at the interface between the SiO2 microfiber and the MXene coating is modulated (i.e., the modulation of the SPPs under applied light), thus inducing a variation in the evanescent field. Consequently, a wavelength shift is produced, effectively realizing a selective and highly sensitive ammonia sensor with a 100 ppm detection limit. The MXene supported SPPs open a promising path for the application of advanced optical techniques toward gas and chemical analysis.

Charge transfer plasmons in nanoparticle arrays on graphene: Theoretical development

The properties of charge transfer plasmons (CTPs) in periodic metallic nanoparticle arrays (PMNPAs) on the single-layer graphene surface are studied within a computationally efficient original hybrid quantum-classical model. The model is based on the proven assumption that the carrier charge density in doped graphene remains unchanged under plasmon oscillations. Calculated CTP frequencies for two PMNPA geometries are shown to lie within the THz range and to be factorized, i.e., presented as a product of two independent factors determined by the graphene charge density and the PMNPA geometry. Equations are derived for describing the CTP frequencies and eigenvectors, i.e., oscillating nanoparticle charge values. It is shown that the CTP plasmons having a band structure containing a wave vector and a band number, like to phonons in periodic media, can be divided into an acoustic mode and optical CTP modes. For the acoustic modes, the CTP group velocity tends to zero at k→0, but reaches a value of ∼VFermi in graphene inside the Brillouin zone, while for the optical modes, the group velocity dispersion is extremely weak, although their energy is higher than the acoustic plasmon energies. It is shown that the calculated dependence of CTP frequencies on the carrier concentration in graphene is in good agreement with experimental data. We believe that the proposed model can help in designing various graphene-based terahertz nanoplasmonic devices of complex geometry due to very high computational efficiency.

Welded Gold Nanoparticle Assemblies Defined Plasmonic Coupling.

Nanoparticle assemblies with interparticle ohmic contacts are crucial for nanodevice fabrication. Despite tremendous progress in DNA-programmable nanoparticle assemblies, seamlessly welding discrete components into welded continuous three-dimensional (3D) configurations remains challenging. Here, we introduce a single-stranded DNA-encoded strategy to customize welded metal nanostructures with tunable morphologies and plasmonic properties. We demonstrate the precise welding of gold nanoparticle assemblies into continuous metal nanostructures with interparticle ohmic contacts through chemical welding in solution. We find that the welded gold nanoparticle assemblies show a consistent morphology with welded efficiency over 90%, such as the rod-like, triangular, and tetrahedral metal nanostructures. Next, we show the versatility of this strategy by welding gold nanoparticle assemblies of varied sizes and shapes. Furthermore, the experiment and simulation show that the welded gold nanoparticle assemblies exhibit defined plasmonic coupling. This single-stranded DNA encoded welding system may provide a new route for accurately building functional plasmonic nanomaterials and devices.

Hyperspectral dark-field optical microscopy correlated to atomic force microscopy for the analysis of single plasmonic nanoparticles: tutorial

Plasmonic nanostructures are actively investigated for their optical properties and for a wide range of applications in nanophotonics, biosensing, photocatalysis, hot carrier physics, and advanced cancer therapies. The localized surface plasmon resonance (LSPR) can be excited in gold or silver nanoparticles or in more complex nanostructures and gives rise to a wide range of unique optical properties. It is often critical to be able to localize individual plasmonic nanoparticles and simultaneously measure their spectrum. This is known as hyperspectral microscopy. In this tutorial, we describe and carefully explain how to achieve this goal with an optical microscope equipped with a dark-field objective and an optical spectrometer. The images and the scattering spectra of spherical gold nanoparticles with diameters of 90, 70, 50, and 25 nm are recorded. We compare them with the scattering spectra predicted with the Mie formula (LSPR peaks measured at 553, 541, 535, and 534 nm, respectively). The optical images are limited by the diffraction, and this is discussed in the framework of the Abbe equation. We also describe a strategy to easily correlate the optical images with atomic force microscope images of the samples. This allows us to precisely relate the morphology of the nanoparticles with their optical images, their color, and their optical spectrum. The case of non-spherical nanostructures, namely, dimers of nanoparticles, is also discussed. This approach allows a relatively low-cost setup and efficient characterization method that will be helpful for teachers who want to introduce their students to the wide topics of plasmonics. This will also be useful for labs seeking an affordable method to investigate the plasmonic properties of single nanostructures.

Unleashing 2D MXene's Plasmonic Effect for Advanced Photonic Device Applications

AbstractMXenes, the largest family of 2D transition metal carbides/nitrides, have emerged as promising materials for various applications due to their exceptionally fascinating properties. In this study, the potential of MXenes is investigated in plasmonic applications, particularly focusing on surface‐enhanced Raman scattering (SERS) and broadband photodetection. The study explored a broader range of light‐matter interactions based on edge plasmons that emerge at sub‐monolayer coverage of MXene, while existing studies have primarily focused on thicknesses exceeding tens of nanometers. Additionally, by incorporating MXenes onto 3D trench nanostructures, To maximize their plasmonic properties is aimed by enhancing the manifestation of edge plasmons, leading to intensified light amplification. The fabricated opto‐electronic devices exhibit remarkable sensitivity in SERS detection, achieving a detection limit as low as ≈0.1 nm for the target dye molecule, significantly surpassing typical thresholds for non‐metal‐based detection. Additionally, the enhancement factor exceeds that of commercially available Au‐based SERS substrates. Furthermore, MXene‐based photodetectors demonstrate competitive photoresponsivity and response time, particularly toward the near‐infrared (NIR) wavelength bands, which are challenging to achieve with MXene‐based photodetectors. Through a comprehensive analysis, including electric field calculations, the mechanisms driving the observed enhancements are unveiled, primarily attributing them to edge plasmons.

Fast and scalable fabrication of Ag/TiO2 nanostructured substrates for enhanced plasmonic sensing and photocatalytic applications

Here we propose a fast and scalable (up to 3 min for 6 × 1 cm size substrate) innovative approach to fabricate coral-shaped TiO2 nanostructured substrates based on thermal oxidation of titanium foil followed by deposition of Ag nanoparticles for bifunctional sensing and photocatalysis. Comprehensive studies on the morphology, crystal structure, stoichiometry, optical, and electronic properties of these nanostructures reveal the formation of a Schottky barrier at the Ag/TiO2 interface. This interface alters the electronic structure of the Ag/TiO2 nanosystem, retaining the plasmonic properties of Ag nanoparticles while enhancing their catalytic activity. The SERS performance and photocatalytic activity of Ag/TiO2 nanostructures are demonstrated by the test analyte Rhodamine 6G detection down to concentrations of 10-12 M and acceleration of its degradation under simulated sunlight by 5.8 times. Tests with tetracycline water solutions indicate a nanomolar detection limit and 4.4-fold increase in degradation rate. The proposed Ag/TiO2 nanostructures are prospective for creation of low-cost bifunctional platforms for the sensitive detection of organic residues in water as well as their fast neutralization under solar light.

Magnesium Nanoparticles for Surface-Enhanced Raman Scattering and Plasmon-Driven Catalysis.

Nanostructures of some metals can sustain localized surface plasmon resonances, collective oscillations of free electrons excited by incident light. This effect results in wavelength-dependent absorption and scattering, enhancement of the incident electric field at the metal surface, and generation of hot carriers as a decay product. The enhanced electric field can be utilized to amplify the spectroscopic signal in surface-enhanced Raman scattering (SERS), while hot carriers can be exploited for catalytic applications. In recent years, cheaper and more earth abundant alternatives to traditional plasmonic Au and Ag have gained growing attention. Here, we demonstrate the ability of plasmonic Mg nanoparticles to enhance Raman scattering and drive chemical transformations upon laser irradiation. The plasmonic properties of Mg nanoparticles are characterized at the bulk and single particle level by optical spectroscopy and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy and supported by numerical simulations. SERS enhancement factors of ∼102 at 532 and 633 nm are obtained using 4-mercaptobenzoic acid and 4-nitrobenzenethiol. Furthermore, the reductive coupling of 4-nitrobenzenethiol to 4,4'-dimercaptoazobenzene is observed on the surface of Mg nanoparticles under 532 nm excitation in the absence of reducing agents, indicating a plasmon-driven catalytic process. Once decorated with Pd, Mg nanostructures display an enhancement factor of 103 along with an increase in the rate of catalytic coupling. The results of this study demonstrate the successful application of plasmonic Mg nanoparticles in sensing and plasmon-enhanced catalysis.

Reuseable Fe3O4@PEI-DTC-Au@Ag magnetic nanocomposites: A versatile and sensitive SERS substrate for food safety assessment

A novel recyclable SERS substrate with magnetic manipulation capability and surface-enhanced Raman scattering (SERS) effect based on Fe3O4@PEI-DTC-Au@Ag (FPDAA) core-shell nanocomposites has been successfully prepared by engineered hydrothermal deposition and seed deposition techniques. The uniformity and SERS efficacy of these substrates, incorporating various Au@Ag core-shell nanocomposites with differing Ag shell thicknesses, were evaluated using 4-aminothiophenol (4-ATP) molecules. The optimal SERS substrate, Fe3O4@PEI-DTC-Au@Ag-3 (FPDAA-3), exhibited an enhancement factor (EF) of up to 1.121×106. This remarkable performance can be attributed to the synergistic effect between the magnetic plasmon of the Fe3O4 nanoparticles (NPs) core and the surface plasmonic resonance properties of Au@Ag core-shell nanocomposites, which create abundant interparticle hotspots within the FPDAA nanocomposites. Furthermore, the FPDAA-3 nanocomposite was employed as a SERS substrate for rapid thiram detection, achieving a minimum detectable concentration of 1×10−9 M. Notably, the relative standard deviation of SERS measurements from 50 randomly selected points was found to be less than 12.13 %, and the SERS spectra intensity showed minimal variation after 5 cycling tests, underscoring the excellent uniformity and recyclability of the FPDAA-3 substrate. The highly reusable and reliable FPDAA-3 nanocomposites enable direct on-site detection of thiram pesticide residues on fruit skins, thereby offering significant implications for food safety assessment and spectroscopic identification.

Optically controllable deformation and phase change in VO2/Si3N4/Au hybrid nanostructures with polarization selectivity

Optically spatial displacement and material modification hold great potential for the appealing applications in nanofabrication and reconfiguration of functional optical devices. Here, we propose and demonstrate a scheme to achieve simultaneous deformation and phase change in vanadium dioxide (VO2)/Si3N4/Au hybrid nanostructures by laser stimuli. Low triggering threshold and significant deformation characteristics of VO2, based on controllable phase transition, are demonstrated in microscale cantilevers. The plasmonic properties of the nanostructure array are further utilized to achieve a polarization-selective dynamic response. The persistence of deformation and dynamical optical modulation are further demonstrated. Such high-precision fabrication methods and non-contact reconfiguration methods are useful for future applications in dynamic optical manipulation.

Plasmonic magnesium arrays with nanosphere lithography

Magnesium is a rising alternative plasmonic metal that is potentially cheaper, more biocompatible, and less lossy in the ultraviolet-blue region of the visible spectrum than the commonly used gold and silver. Recent studies of colloidal magnesium nanoparticles demonstrated the plasmonic resonances of a variety of faceted shapes. However, applications such as refractive index sensing benefit from well-defined arrays, which have been developed for all other plasmonic metals. Here, we implement nanosphere lithography to fabricate metallic magnesium arrays that display attractive plasmonic properties. The deposition process was found to be highly vulnerable to oxidation, recrystallization, kinetic energy of the metal vapor, and substrate properties. The resulting structures obtained with 350, 500, and 750 nm hexagonally packed nanosphere masks exhibit the hallmark light–matter interactions of plasmonic metals, including strong extinction and resonance energy dependence on feature size, further securing Mg’s place as an alternative plasmonic metal.

Polydopamine-mediated facile Silver grown on ZnO Thin Films as High Performance SERS Substrates for R6G Detection

Surface-enhanced Raman spectroscopy (SERS) is a widely known technique that uses plasmonic structures (silver, gold, etc.) to detect low-concentration molecules. However, the limited number of metallic elements with plasmonic properties leads to limitations in their application. The qualitative detection method of SERS holds considerable promise in providing novel platforms for diverse applications, owing to its utilization of hybrid structures. Mussel-inspired polydopamine offers a promising avenue for the fabrication and integration of hybrid structures suitable for the SERS platform. Here, sputtered ZnO thin films were modified with Ag Nanostructures using polydopamine to fabricate a homogeneous Ag@ZnO hybrid high-performance SERS substrate. Ag/ZnO hybrid nanostructures were analyzed by FESEM and XRD to investigate their morphological and structural characterizations. SERS measurements were performed for all silver growth times to understand the effect of the silver growth process and hybrid structure synergy on SERS performance. The Ag@ZnO hybrid structure, cultivated with 24-hour silver growth, exhibited remarkable detectability even at an ultra-low R6G concentration of 10 pM

Janus Fe3O4-Au@Pt nanozymes based lateral flow assay for enhanced-sensitive colorimetric detection of influenza A virus

The traditional colloidal gold-based lateral flow assay (LFA) is widely applied in point-of-care testing (POCT) for pathogens. However, its disadvantage of low sensitivity greatly limits its effectiveness in early detection. In this work, we developed an enhanced-sensitive colorimetric nucleic acid lateral flow assay (NALFA) based on Janus Fe3O4-Au@Pt nanoparticles (Fe3O4-Au@Pt NPs) for H1N1 influenza virus detection. Dumbbell-like heterodimer Fe3O4-Au@Pt NPs were prepared by a seed-mediated method, and their unique Janus structure endowed them with both good plasmonic property and high peroxidase-like activity. With the Fe3O4-Au@Pt NPs as NALFA signal labels, qualitative detection was achieved by observing the color change with the naked eyes, and quantitative results were obtained by measuring the gray values on test strips. Due to the strong peroxidase-like activity of Fe3O4-Au@Pt NPs, they could quickly catalyze 3,3′,5,5′-tetramethylbenzidine (TMB) to oxidize to the dark blue chelate (oxTMB), thus resulting in greatly enhanced color on the strips. The limit of detection (LOD) of NALFA reached as low as 50 pM, which was 20-fold lower than that of before catalysis. Therefore, the constructed Janus Fe3O4-Au@Pt NPs could effectively improve the performance of traditional colloidal gold LFA and provide a potential tool for the development of sensitive sensing platforms.