Plasmonic Metal Nanoparticles Research Articles

Mechanistic Studies of Plasmon Chemistry on Metal Catalysts.

Chemical reactions induced by the localized surface plasmon (LSP) of metal nanostructures could be important for a sustainable society to achieve highly efficient conversion from solar energy to chemical energy. However, the reaction mechanism of plasmon chemistry in metal catalysis is still controversial. Mechanistic studies of plasmon chemistry involving direct interactions between the LSP and molecules are reviewed and discussed in terms of the excitation mechanisms of the molecules. We focus on the studies performed using plasmonic metal nanoparticles and highlight the recent progress in plasmon chemistry investigated using scanning probe microscopy with high spatial resolution to obtain mechanistic insights that cannot be obtained by macroscopic analytical methods. This Minireview delivers an overview of the mechanistic understanding of plasmon chemistry in metal catalysis at the current stage, and provides guidance for future studies with respect to clarifying reaction mechanisms.

Mechanistic Studies of Plasmon Chemistry on Metal Catalysts

AbstractChemical reactions induced by the localized surface plasmon (LSP) of metal nanostructures could be important for a sustainable society to achieve highly efficient conversion from solar energy to chemical energy. However, the reaction mechanism of plasmon chemistry in metal catalysis is still controversial. Mechanistic studies of plasmon chemistry involving direct interactions between the LSP and molecules are reviewed and discussed in terms of the excitation mechanisms of the molecules. We focus on the studies performed using plasmonic metal nanoparticles and highlight the recent progress in plasmon chemistry investigated using scanning probe microscopy with high spatial resolution to obtain mechanistic insights that cannot be obtained by macroscopic analytical methods. This Minireview delivers an overview of the mechanistic understanding of plasmon chemistry in metal catalysis at the current stage, and provides guidance for future studies with respect to clarifying reaction mechanisms.

Plasmonic metal nanoparticles as efficient mass tags for ion signal amplification and ultrasensitive detection of protein markers

The development of sensitive and specific analytical methods is critical for the discovery of molecular biomarkers, which assists disease diagnosis and understanding biological processes. Herein, a highly sensitive method is developed using antibody-conjugated plasmonic metal nanoparticles for the detection of targeted biomarkers down to low attomole level via coupling of immunoassay techniques with laser ionization mass spectrometry (LI-MS). The conjugated antibodies target specific antigens, while the metal nanoparticles act as mass tags and ion reservoirs for the signal amplification. With the characteristic localized surface plasmon resonance (LSPR) properties, gold (AuNPs) and silver nanoparticles (AgNPs) undergo explosive ionization upon laser irradiation to generate abundant characteristic mass reporter ions for strong MS signal amplification. With the antibody-conjugated NPs, detection of trace proteins in various biological samples with complex matrix environment, including urine, cell lysates, and animal tissues was demonstrated.

Hot Holes Assist Plasmonic Nanoelectrode Dissolution.

Strong light-absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. We demonstrate the role of photogenerated hot holes in the oxidative dissolution of individual gold nanorods with millisecond time resolution while tuning charge-carrier density and photon energy using snapshot hyperspectral imaging. We show that light-induced hot charge carriers enhance the rate of gold oxidation and subsequent electrodissolution. Importantly, we distinguish how hot holes generated from interband transitions versus hot holes around the Fermi level contribute to photooxidative dissolution. The results provide new insights into hot-hole-driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of nonequilibrium processes on innately heterogeneous nanoparticle supports.

Near‐Infrared Broadband Polymer‐Dot Modulator with High Optical Nonlinearity for Ultrafast Pulsed Lasers

AbstractOptical modulators hold great promise for nonlinear optical and photonic devices. However, current optical modulators are exclusively based on inorganic saturable absorbers such as semiconductor thin films, 2D materials, and plasmonic nanocrystals. Here, nonlinear optical modulation is demonstrated by designing a series of organic semiconducting polymer dots (Pdots) for passive Q‐switching and mode‐locking lasers in the near‐infrared (NIR) region. Combining density functional theory and judicious polymer design, Pdots with large extinction coefficient and broadband absorption are developed which span the entire spectrum from ultraviolet (UV) to NIR region (300 to 2500 nm). The Pdots exhibit strong saturable absorption (β = −1.2 × 10−3 cm W−1) and optical nonlinearity (Imχ(3) = −8.4 × 10−7 esu), which are of orders higher than other nonlinear optical materials such as carbon‐based materials, plasmonic metal nanoparticle, and semiconductor nanocrystals. The Pdot modulators are integrated into the fiber laser systems, and demonstrate passively mode‐locked lasers at 1036, 1038, 1945, 1950 nm and Q‐switched lasers at wavelengths of 1559 and 1957 nm, highlighting the potential of Pdots for advanced nonlinear optical and photonic applications.

An Experimental and Theoretical Comparison of Plasmonic Properties of Metallic Nano-Particles Have Different Morphological Properties

Abstract In this study, we have grown some plasmonic metal-nanoparticles have different morphological properties on the microscope slide and silicon wafer as substrates depending on an ambient background gas pressure and number of laser pulses by using Pulse Laser Deposition (PLD) technique in connection with a nanosecond Nd:YAG laser system. The morphology of these nanoparticles thin film was investigated by Atomic Force Microscopy. UV-vis spectra have shown that plasmonic metal-nanoparticles present a Localised Surface Plasmon Resonance band in visible and near-infrared (NIR) regions depending on the morphologic structure of the nanoparticles thin film. The LSPR of the plasmonic metal nanoparticles was calculated theoretically using Boundary Element Method simulation programme. The reliability of experimental results was verified by comparing the theoretical results with their experimental counterparts produced by using PLD, both were obtained in this work. Both theoretical and experimental results and their comparison have been presented and discussed in this study, and a conclusion has been made that the theoretical and experimental results for Cu were in all good agreement.

Highly efficient plasmon-mediated electron injection into cerium oxide from embedded silver nanoparticles.

The coupling with plasmonic metal nanoparticles (NPs) represents a promising opportunity to sensitize wide band gap oxides to visible light. The processes which come into play after the excitation of localized surface plasmon resonances (LSPRs) in the NPs largely determine the efficiency of the charge/energy transfer from the metal NP to the oxide. We report a study of plasmon-mediated energy transfer from mass-selected silver NPs into the cerium oxide matrix in which they are embedded. Femtosecond transient absorption spectroscopy is used to probe the dynamics of charge carrier relaxation after the excitation of the LSPR of the silver nanoparticles and to evaluate the plasmon-mediated electron transfer efficiency from the silver nanoparticles to the cerium oxide. High injection efficiencies in the 6-16% range have been identified for excitation between 400 and 600 nm. These high values have been explained in terms of plasmon-mediated direct electron injection as well as indirect hot electron injection from the NPs to the oxide. The information obtained provides an important contribution towards a knowledge-driven design of efficient cerium oxide based nanostructured materials for solar to chemical energy conversion.

Femtosecond Laser Ablation Synthesis of Nanoparticles and Nano-Hybrides in Ethanol Medium

Abstract The nanoparticle production has a significant importance because of its unique optical properties in molecular biology and medicine. Plasmonic metallic nanoparticles are widely used in cancer monitoring studies as contrast agents on the account of their surface plasmon resonance (SPR) effect. Beside, silicon (Si) semiconductor nanoparticles can provide several beneficial properties that they are used commonly intensifying the detecting signals. Nanoalloys/Nanohybides/Nanocomposites are more functional materials than a single material structure and having many restrictions. At that point, the application of Pulsed Laser Ablation (PLA) technique has many advantages if applied in the liquid. Since no any chemical technique is applied, this procedure promises a clean synthesis process; it is a simple and economical technique in the production process. It provides some possibilities to generate particles in the desired size, shape and density, and these properties of nanoparticles produced can be controlled by controlling the laser parameters. In this work, we have focused on the production of nanoparticles to rid off chemical processes using femtosecond laser ablation technique, and this technique presents a process to reach the fast and reliable results for production of nanoshells, nanocomposites and all nanoforms of particles. We have produced Si@Au nanohybrids in ethanol medium using femtosecond laser ablation depending on laser pulse energy. Absorption spectra were recorded day by day to determine the optical characteristics of produced nanoparticles within these time durations, Transmission Electron Microscopy (TEM) images were obtained for monitoring and determining the size/structure of nanoparticles, energy dispersive spectroscopy (EDS) for material compositions were produced, these results obtained are presented in the scope of this paper

Shape Tailored TiO₂ Nanostructures and Their Hybrids for Advanced Energy and Environmental Applications: A Review.

Shape tailored TiO₂ nanostructures with various dimensionality (zero to three dimension) have unique physicochemical and functional properties that facilitates its efficient energy and environment applications, e.g., solar light driven photocatalytic hydrogen generation and decontamination of organic/inorganic toxic pollutants, CO₂ reduction into the hydrocarbon fuels, solar cells, supercapacitors and lithium-ion batteries etc. However, the wide band gap nature and the fast recombination of the photogenerated charge carriers in TiO₂ usually limit its overall performance under solar light illumination. In this review, we present a state of the art on the fabrication techniques of shape tailored TiO₂ nanostructures and the strategies employed to make the system catalytically more efficient. Though shape tailored TiO₂ nanostructures with large specific surface area and highly energetic (001) facet exposed TiO₂ nanostructures (2D and 3D) can enhance the photocatalytic efficiency to a reasonable extent, further surface engineering is needed for the modification of the electronic band arrangement, visible light sensitization and efficient charge separation. Herein, TiO₂ heterostructures (HSs) with metal/non-metal doping, surface fluorination, plasmonic noble metal nanoparticles (NPs) and coupling with the narrow band gap suitable semiconductor (type-II) are discussed in details covering from zero dimensional to three dimensional heterostructures. The synthesis strategies, charge transfer mechanism and their participation in the photocatalysis are elaborated. Though one dimensional TiO₂ HSs have been widely studied, we present the recent development of critical surface engineering strategies of two and three dimensional systems, which give rise to the excellent properties including the enlargement of surface area, light absorption capability and efficient separation of electrons/holes resulting in the superior performance in advanced applications. Based on recent breakthroughs in the field, future directions and outlook of the field are presented at the end.

Visible Light Plasmonic Heating-Enhanced Electrochemical Current in Nanoporous Gold Cathodes

Taking advantage of the localized surface resonance effect to boost the performance of electrochemical cells has rarely been demonstrated using nanoporous metal films as photoactive electrodes. Rather, studies on plasmon-enhanced electrochemical processes use plasmonic metal nanoparticles loaded onto semiconductor or conductor substrates. Here we present experimental evidence for the use of visible light to significantly enhance the kinetics of a redox reaction involving the oxidation of aluminum at the anode, and the reduction of hydrogen ions to hydrogen gas at the cathode. This reduction reaction is allowed to take place on a nanoporous Au film used as plasmonic photocathode. A 20-fold increase in the electrochemical current density was recorded upon exposure of the nanoporous Au cathode to visible light. We demonstrate that this significant current enhancement is associated with local heat generated in Au during localized surface plasmon resonance. These results could be exploited to develop high-performance heterogeneous photocatalysts based on plasmonic heating, for the conversion of renewable energy resources into fuels and value-added chemicals.

Sandwiching analytes with structurally diverse plasmonic nanoparticles on paper substrates for surface enhanced Raman spectroscopy.

This report describes the systematic combination of structurally diverse plasmonic metal nanoparticles (AgNPs, AuNPs, Ag core–Au shell NPs, and anisotropic AuNPs) on flexible paper-based materials to induce signal-enhancing environments for surface enhanced Raman spectroscopy (SERS) applications. The anisotropic AuNP-modified paper exhibits the highest SERS response due to the surface area and the nature of the broad surface plasmon resonance (SPR) neighboring the Raman excitation wavelength. The subsequent addition of a second layer with these four NPs (e.g., sandwich arrangement) leads to the notable increase of the SERS signals by inducing a high probability of electromagnetic field environments associated with the interparticle SPR coupling and hot spots. After examining sixteen total combinations, the highest SERS response is obtained from the second layer with AgNPs on the anisotropic AuNP paper substrate, which allows for a higher calibration sensitivity and wider dynamic range than those of typical AuNP–AuNP arrangement. The variation of the SERS signals is also found to be below 20% based on multiple measurements (both intra-sample and inter-sample). Furthermore, the degree of SERS signal reductions for the sandwiched analytes is notably slow, indicating their increased long-term stability. The optimized combination is then employed in the detection of let-7f microRNA to demonstrate their practicability as SERS substrates. Precisely introducing interparticle coupling and hot spots with readily available plasmonic NPs still allows for the design of inexpensive and practical signal enhancing substrates that are capable of increasing the calibration sensitivity, extending the dynamic range, and lowering the detection limit of various organic and biological molecules.

Configurational resonances in absorption of metal nanoparticles seeded onto a semiconductor surface

Abstract In the present study, the optical absorption of a semiconductor surface covered with metallic spheroidal nanoparticles is considered theoretically. The theoretical approach used for calculation of light absorption in such structures is based on the concept of effective susceptibility and made in the frame of Green’s functions theory. The effective susceptibility is taken in an analytical form. The absorption spectra were calculated for coverages with nanoparticles having different shapes and surface densities at different angles of external light incidence. The obtained results predict strong configurational resonances in the spectral range where the absorption is maximum, i.e. resonant enhancement of absorption by nanoparticles of certain shape and surface density. The developed technique of calculation can be further applied to calculation of various effects caused by the excitation of surface plasmons in metallic nanoparticles under applied external fields yielding a wide range of applications of such structures in many fields, such as biomedicine, solar energy, environment protection, and information storage technology.

Harvesting hot electrons on Au nanoparticle monolayer by efficient compensation of holes

Plasmonic metal nanoparticles (NPs), due to its unique optical properties, has been employed in various fields, including photocatalysis and surface-enhanced Raman scattering (SERS). Hot electrons generated from non-radiative decay of plasmons can be transferred to reactant molecules adsorbed on the metal surface and thus greatly facilitate photocatalytic chemical conversions under far milder conditions than conventional thermal catalysis. However, due to the ultra-fast relaxation or recombination, the number of hot electrons that can be effectively utilized is very limited. Herein, we report the efficient compensation of holes and harvesting hot electrons by adding NaBH4 or acidic Na2SO3 solution as holes scavenger to contribute for the lasting generation of hot electrons as well as active hydrogen species. A six-electron photocatalytic reduction of nitroarenes is achieved under laser illumination even on the catalytically inert ∼80nm Au NP self-assembled monolayer (SAM) after NaBH4 or acidic Na2SO3 solution are added. The role of NaBH4 or acidic Na2SO3 solution is elucidated to compensate holes rather than directly reduce the nitroarene. This concept of harvesting hot electrons by quick compensation of holes offers new opportunities for driving efficient light-to-energy conversion.

On Modeling of Plasmon-Induced Enhancement of the Efficiency of Solar Cells Modified by Metallic Nano-Particles.

We demonstrate that the direct application of numerical packets like Comsol to plasmonic effect in solar cells metallically modified in nano-scale may be strongly inaccurate if quantum corrections are neglected. The near-field coupling of surface plasmons in metallic nanoparticles deposited on the top of a solar cell with band electrons in a semiconductor substrate strongly enhances the damping of plasmons in metallic components, which is not accounted for in standard numerical packets using the Drude type dielectric function for metal (taken from measurements in bulk or in thin layers) as the prerequisite for the numerical e-m field calculus. Inclusion of the proper corrections to plasmon damping causes additional enhancement of the plasmon-induced photo-effect efficiency growth of a metalized photo-diode by ten percent, at least, in comparison to only effect induced by the electric field concentration near metallic nanoparticles. This happens to be consistent with the experimental observations which cannot be explained by only local increases of the electrical field near the curvature of metallic nanoparticles determined by a finite-element solution of the Maxwell–Fresnel boundary problem as given by a numerical system like Comsol. The proper damping rate for plasmons can be identified by application of the Fermi Golden Rule approach to the plasmon-band electron coupling. We demonstrate this effect including the material and size dependence in two types of solar cells, multi-crystalline Si and CIGS (copper-indium-gallium-diselenide) as idealized photo-diode semiconductor substrate modified by various metallic nano-particles, in comparison to the experimental data and Comsol simulation.

Mechanistic View of the Main Current Issues in Photocatalytic CO2 Reduction.

After 40 years of research on photocatalytic CO2 reduction, there are still many unknowns about its mechanistic aspects even for the most common TiO2-based photocatalytic systems. These uncertainties include the pathways inducing visible-light activity in wide-band gap semiconductors, the charge transfer between semiconductors and plasmonic metal nanoparticles, the unambiguous determination of the origin of C-bearing products, the very first step in the activation of the CO2 molecule, the factors determining the selectivity, the reasons for photocatalyst deactivation, the closure of the catalytic cycle by the hole-scavenging reagent, and the detailed reaction pathways and the most suitable techniques for their determination. This Perspective discusses these controversial issues based on the most relevant investigations reported so far. For that purpose, we have tried to view the complex CO2 reduction in a holistic manner, considering today's state-of-the-art approaches, strategies, and techniques for the study of one of the hottest topics in energy research.

Chemical Requirement for Extracting Energetic Charge Carriers from Plasmonic Metal Nanoparticles to Perform Electron-Transfer Reactions.

Performing electron-transfer reactions on metal nanoparticles requires separation of charge carriers at the nanoparticle and their transfer to the reacting molecules. Inducing these reactions using light is challenging due to the exceedingly short lifetimes of energetic charge carriers formed in metal nanoparticles under light illumination. The results described here show that certain conditions must be met to drive these electron-transfer reactions on plasmonic nanoparticles. One critical requirement is that the process of electronic excitation takes place at the nanoparticle/molecule interface. This is accomplished by high plasmonic electric fields at the surface of plasmonic nanoparticles. Furthermore, it is also evident from our study that the electron (or hole)-donating capacity of the hole (or electron) scavengers needs to be high enough to allow for the extraction of holes (or electrons) from the nanoparticle/molecule complex, therefore completing the catalytic cycle. We discuss these findings through a case study of the conversion of methylene blue (MB) into a reduced MB ion radical on the surface of plasmonic Ag and Ag-Pt core-shell nanoparticles. To directly monitor the reduction reaction of MB on the nanoparticle surfaces, we have used time-dependent in situ surface-enhanced Raman scattering measurement, which also informs us about the underlying mechanistic details of plasmon-driven charge transfer.

Unravelling the effect of charge dynamics at the plasmonic metal/semiconductor interface for CO2 photoreduction

Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO2 reduction by artificial photosynthesis is one of the cornerstones to produce renewable fuels and environmentally friendly chemicals. Interface interactions between plasmonic metal nanoparticles and semiconductors exhibit improved photoactivities under a wide range of the solar spectrum. However, the photo-induced charge transfer processes and their influence on photocatalysis with these materials are still under debate, mainly due to the complexity of the involved routes occurring at different timescales. Here, we use a combination of advanced in situ and time-resolved spectroscopies covering different timescales, combined with theoretical calculations, to unravel the overall mechanism of photocatalytic CO2 reduction by Ag/TiO2 catalysts. Our findings provide evidence of the key factors determining the enhancement of photoactivity under ultraviolet and visible irradiation, which have important implications for the design of solar energy conversion materials.

Plasmon‐Enhanced Electrocatalytic Properties of Rationally Designed Hybrid Nanostructures at a Catalytic Interface

AbstractIn recent years, a promising role of plasmonic metal nanoparticles (NPs) has been demonstrated toward an improvement of the catalytic efficiency of well‐designed hybrid electrocatalysts. In particular, the coupling of plasmonic functionality with the metal‐based core–shell architectures in plasmon‐enhanced electrocatalysis provides a sustainable route to improve the catalytic performances of the catalysts. Herein, the rationally designed AuNPs wrapped with reduced graphene oxide (rGO) spacer along with PdNPs (AuNP@rGO@Pd) as the final composite are reported. The rGO is proposed to promote the reduction of PdO, greatly enhance the conductivity, and catalytic activity of these nanohybrid structures. The plasmon‐enhanced electrocatalytic performance of optimized AuNP@rGO(1)@Pd exhibits an ≈1.9‐ and 1.1‐fold enhanced activity for the hydrogen evolution reaction and oxygen evolution reaction, respectively. The final composite also exhibits a superior stability up to 10000 s compared with the commercial Pd/C. The mechanism of the enhanced catalytic performance is monitored through in situ X‐ray absorption spectroscopy by observing the generated electron density under light irradiation. The results demonstrate that the energetic charge carriers are concentrated in the incorporated PdNPs, allowing higher catalytic performances for the overall water‐splitting reaction. The conclusions herein drawn are expected to shed light on upcoming plasmon‐induced electrocatalytic studies with analogous hybrid nanoarchitectures.

Low-blinking SERS substrate for switchable detection of kanamycin

Metallic plasmonic nanoparticles generate surface-enhanced Raman Scattering (SERS) for wide sensing applications of biomolecules, gas and environmental contaminants. However, the metallic materials cause high thermal radiation during laser excitation in the opto-microfluidic systems and give rise to temporal intensity fluctuations, which is called blinking effects in SERS spectra. This work developed nanocomposite consisting of graphene oxide (GO) and gold nanoparticles (AuNPs) with high thermal stability and low thermal radiation, which provided plasmonic resonance while reduced blinking effect for SERS-based sensing in the microfluidic detection system. The system was applied to detect kanamycin in drinking water, orange juice, and milk by using Texas Red-dye conjugated Beacon aptamer, where the aptamer was designed to be specific and switchable in kanamycin recognition. The sensor was reusable, and its limit of detection was 0.75 nM in a linear range of 1 nM to 100 nM. The microfluidic SERS detection system can be potentially used for sensitive and reliable assays of small molecules.

Unified theory of plasmon-induced resonance energy transfer and hot electron injection processes for enhanced photocurrent efficiency.

Plasmons in metal nanoparticles (MNPs) promise to enhance solar energy conversion in semiconductors. Two essential mechanisms of enhancement in the near-field regime are hot electron injection (HEI) and plasmon-induced resonance energy transfer (PIRET). Individual studies of both mechanisms indicate that the PIRET efficiency is limited by the short lifetime of the plasmon, whereas the hot electrons result from the plasmon decay. The development of a unified theory of the coupled HEI and PIRET processes is fundamentally interesting and necessary for making reliable predictions but is complicated by the multiple interactions between various components that participate in the enhancement process. In this paper, we use the model-Hamiltonian approach to develop a combined theoretical framework including both PIRET and HEI. The coupled dynamics as well as the time evolution of hot electron energy distribution are studied. The theory further predicts an interference-induced asymmetry in the spectral dependence of PIRET, which can be used to distinguish it from HEI. As the relative contributions of PIRET and HEI strongly depend on the size of the MNPs, this presents itself as a simple route to control the strength of their contributions. The results presented here can further guide future applications of plasmonic solar energy harvesting.