Plasmonic Metal Nanoparticles Research Articles

A Systematic Study on Au-Capped Si Nanowhiskers for Size-Dependent Improved Biosensing Applications

Reducing the distance between the fluorescence molecules and noble metal (resonant) nanostructures is known to advance the process of electromagnetic coupling and energy transfer, which in return yields fluorescence enhancement particularly exploited for improved biomedical applications. In this study, Au-capped Si nanowhiskers (NWs) at various sizes were fabricated using a vapor–liquid–solid (VLS) mechanism for systematically investigating the dependence of the size of the Au-capped Si NWs on the fluorescence enhancement factor with respect to the fluorescence emission from Rhodamine 6G (Rh-6G) fluorophore. Opposite to what is anticipated from the literature, the maximum enhancement was obtained for the sample for which the Au-nanoparticle (NP) capping is well isolated from the fluorophore and the vertical distance between the fluorophore and the plasmonic metal nanoparticle is largest. Numerical simulations using the finite element method (FEM) were shown to support the experimental optical response results. Four-point probe I-V measurements also show that the Schottky ideality factor of Au-capped Si NWs decays exponentially upon the rise in the fluorescence enhancement factor. Graphical abstract

Hybrid Porous Silicon Biosensors Using Plasmonic and Fluorescent Nanomaterials: A Mini Review.

During the last two decades, porous silicon (PSi) has been proposed as a high-performance biosensing platform due to its biocompatibility, surface tailorability, and reproducibility. This review focuses on the recent developments and progress in the area related to hybrid PSi biosensors using plasmonic metal nanoparticles (MNPs), fluorescent quantum dots (QDs), or a combination of both MNPs and QDs for creating hybrid nanostructured architectures for ultrasensitive detection of biomolecules. The review discusses the mechanisms of sensitivity enhancement based on Localized Surface Plasmon Resonance (LSPR) of MNPs, Fluorescence Resonance Energy Transfer (FRET) in the case of MNPs/QDs donor-acceptor interactions, and photoluminescence/fluorescence enhancement resulting from the embedded fluorescent QDs inside the PSi microcavity. The review highlights the key features of hybrid PSi/MNPs/QDs biosensors for dual-mode detection applications.

Plasma Enhanced Wet Chemical Surface Activation of TiO2 for the Synthesis of High Performance Photocatalytic Au/TiO2 Nanocomposites

To enhance the effectiveness of TiO2 as a photocatalyst, it was believed that the drawbacks of the large bandgap and the rapid electron-hole recombination can be overcome by coupling TiO2 with plasmonic metal nanoparticles. The incorporation of the nanoparticles onto the TiO2 surface requires a suitable procedure to achieve the proper particle adhesion. In this work, we propose a simple, clean, and effective surface activation of TiO2 using plasma enhanced wet chemical surface treatment. Under only 5 min of plasma treatment in a 3% NH3/3% H2O2 solution, gold nanoparticles were found better adhered onto the TiO2 surface. Hence, the methylene blue degradation rate of the Au/TiO2 under sunlight treated was improved by a factor of 3.25 times in comparison to non-treated Au/TiO2 and by 13 times in comparison to the bare rutile TiO2.

Plasmon-enhanced organic and perovskite solar cells with metal nanoparticles

Abstract Solution-processing thin-film solar techniques, such as organic solar cells (OSCs) and perovskite solar cells (PeSCs), hold great promise as cost-effective renewable energy sources with feasible large-scale manufacturing. However, these devices are suffering from the incomplete photon absorption and thereby cannot unlock the full potential of device efficiency despite their rapid development in recent decades. Incorporation of plasmonic metal nanoparticles (NPs) into the thin active layers has been considered as a breakthrough strategy to solve this inherent limit and represent an imperative milestone toward the highly efficient OSCs and PeSCs, arising from the significantly enhanced light absorption and electrical characteristics in fundamental. Herein, the recent advances in fabrication and incorporation strategies of plasmonic NPs are reviewed. The in-depth efficiency and stability enhancement mechanisms are investigated and highlighted. Meanwhile, potential strategies and perspectives for their further development of NP-based solution-processing OSCs and PeSCs are presented.

Multipolar Resonances of Ag Nanoparticle Arrays in Anodic Aluminum Oxide Nanochannels for Enhanced Hot Spot Intensity and Signal-to-Background Ratio in Surface-Enhanced Raman Scattering

Hot spots generated through the electrodynamic coupling of the local plasmons in metallic nanoparticles play an important role in enhancing surface-enhanced Raman spectroscopy (SERS) signal. Howeve...

Analytical modeling for plasmonic graphene/GaAs solar cells with random distribution of nonuniform-sized nanoparticles

Numerical simulations based on solving the Maxwell equations provide a robust and efficient approach to obtaining optical properties of plasmonic metal nanoparticles (NPs), but they have high computational costs when considering the real conditions of incorporated NPs such as random distribution and nonuniform size. Therefore, an effective medium theory, taking into account light scattering on the surface of NPs and light concentration by localized surface plasmon resonance, has been employed to obtain optical properties of randomly embedded spherical NPs with nonuniform radii into GaAs. The results are input into an optoelectrical model of graphene/GaAs Schottky barrier solar cells (SCs), calibrated by the experimental data, to compare the performance of plasmonic graphene/GaAs SCs with the incorporation of different metal NPs of Au, Ag, Al, and Cu. Furthermore, the effects of the number of graphene layers, the radius of NPs, and the amount of size dispersion on the photovoltaic parameters of plasmonic graphene/GaAs SCs have been investigated. It is demonstrated that increasing the radius and size dispersion of NPs, by inducing an inhomogeneous broadening and an increase of plasmon band amplitude, enhances the efficiency of plasmonic graphene/GaAs SCs.

Two-Dimensional Van der Waals Heterostructures for Synergistically Improved Surface-Enhanced Raman Spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) is a precise and noninvasive analytical technique that is widely used in chemical analysis, environmental protection, food processing, pharmaceutics, and diagnostic biology. However, it is still a challenge to produce highly sensitive and reusable SERS substrates with a minimum fluorescence background. In this work, we propose the use of van der Waals heterostructures of two-dimensional materials to cover plasmonic metal nanoparticles to solve this challenge. The heterostructures of atomically thin boron nitride (BN) and graphene provide synergistic effects: (1) electrons could tunnel through the atomically thin BN, allowing the charge transfer between graphene and probe molecules to suppress the fluorescence background; (2) the SERS sensitivity is enhanced by graphene via a chemical enhancement mechanism in addition to an electromagnetic field mechanism; and (3) the atomically thin BN protects the underlying graphene and Ag nanoparticles from oxidation during heating for regeneration at 360 °C in the air so that the SERS substrates could be reused. These advances will facilitate wider applications of SERS especially on the detection of fluorescent molecules with higher sensitivity.

Theoretical investigation of broadband absorption enhancement in a-Si thin-film solar cell with nanoparticles

Thin-film solar cells have attracted increasing attention due to its low material cost and large flexibility, but they also face the challenge of low solar absorption due to reduced active layer thickness. Through exciting surface plasmon resonance, plasmonic metal nanoparticles are usually placed on the cell front surface to enhance solar absorption. However, if eliminating the unuseful intrinsic absorption in nanoparticles, we find that dielectric ones are better choices to enhance a-Si thin-film solar cell absorption efficiency. Moreover, a composite light trapping structure with dielectric nanoparticles on the front surface and metal hemispheres on the rear surface is proposed to achieve broadband absorption enhancement in both short and long wavelengths, with the aim to get a higher conversion efficiency. The finite-difference-time-domain simulation results show that, compared with bare 100-nm-thick amorphous silicon solar cell, the short-circuit current density and photoelectric conversion efficiency could be respectively improved by 21% and 18% with addition of optimized composite light trapping structure. The general method proposed in this study could provide valuable guidance to light trapping structure design for various kinds of thin-film solar cells.

Improved photovoltaic performance of dye-sensitized solar cells by Au-ion implantation of titania film electrodes

Plasmonic metal nanoparticles are known to work as light-harvesting antennae and to enhance photocurrents of photovoltaic cells. Therefore, in this work, the effect of gold (Au) ions distribution on the efficiency of dye sensitized solar cells (DSSCs) has been investigated. Thin films of TiO2 have been deposited on FTO glass substrates by the doctor tape method. Au ions were then implanted on these films with different fluence rates (i.e., 2 × 1014, 4 × 1014 and 6 × 1014 ions/cm2). XRD results confirmed that TiO2 is present in the anatase phase up to a 4 × 104 ions/cm2 fluence rate. At a 6 × 1014 ions/cm2 fluence rate, one peak of rutile is obtained which confirmed that the film has mixed phases of TiO2. UV results show that dye is adsorbed into all photoanodes. Maximum dye absorption is seen at a fluence rate of 4 × 1014 ions/cm2. The energy conversion efficiency of DSSC is highly dependent on the fluence rate of Au. At a fluence rate of 4 × 1014 ions/cm2, the cell has a high short circuit current density (JSC) of 7.21 mA/cm2, resulting in a high efficiency (×) of 2.92%. Impedance spectroscopy shows that the cell formed by Au ions fluence rate of 4 × 1014 ions/cm2 has a low recombination rate of electron/hole pairs.

Conductometric Response-Triggered Surface-Enhanced Raman Spectroscopy for Accurate Gas Recognition and Monitoring Based on Oxide-wrapped Metal Nanoparticles.

Accurate and efficient gas monitoring is still a challenge because the existing sensing techniques mostly lack specific identification of gases or hardly meet the requirement of real-time readout. Herein, we present a strategy of conductometric response-triggered surface-enhanced Raman spectroscopy (SERS) for such gas monitoring, via designing and using ultrathin oxide-wrapped plasmonic metal nanoparticles (NPs). The oxide wrapping layer can interact with and capture target gaseous molecules and produce the conductometric response, while the plasmonic metal NPs possess strong SERS activity. In this strategy, the conductometric gas sensing is performed throughout the whole monitoring process, and once a conductometric response is generated, it will trigger SERS measurements, which can accurately recognize molecules and hence realize gas monitoring. The feasibility of this strategy has been demonstrated via using ultrathin SnO2 layer-wrapped Au NP films to monitor gaseous 2-phenylethanethiol molecules. It has been shown that the monitoring is rapid, accurate, and quantifiable. There exist optimal values of working temperature and SnO2 layer thickness, which are about 100 °C and 2.5 nm, respectively, for monitoring gaseous 2-phenylethanethiol. The monitoring signal intensity has a linear relation with the gas concentration in the range from 1 to 100 ppm on a logarithmic scale. Furthermore, the monitoring limits are at the ppm level for some typical gases, such as 2-phenylethanethiol, cyclohexanethiol, 1-dodecanethiol, and toluene. This study establishes the conductometric response-triggered SERS, which enables accurate gas recognition and real-time monitoring.

Promoting reactivity of photoexcited hot electrons in small-sized plasmonic metal nanoparticles that are supported on dielectric nanospheres.

Plasmonic metal nanoparticles (NPs) represent a promising class of photocatalysts to drive chemical transformations by the photoexcited hot electrons in the NPs. In this work, the dependence of photon-to-chemical conversion efficiency on the size of plasmonic silver nanoparticles (Ag NPs) has been comprehensively studied with the use of the photocatalytic degradation of methylene blue as a probe reaction. Comparison of Ag NPs with two different sizes (6 nm and 13 nm in diameter) highlights that the smaller sized Ag NPs favor the photocatalytic activity by positively translating the high efficiency of hot electron generation to the hot-electron-driven chemical reaction on the surface of the Ag NPs. Loading the small Ag NPs to the dielectric silica nanospheres (SiOX NSs, average diameter of 400 nm) with high surface coverage increases the light absorption power in the Ag NPs due to the surface light scattering resonances of the SiOX NSs and interparticle plasmon coupling of the adjacent Ag NPs. The enhanced light absorption can also be rendered to the improved photocatalytic activity. This design principle of plasmonic photocatalysts provides a promise of utilizing solar energy to drive desirable chemical reactions with high photon-to-chemical conversion efficiency.

Electronic structures associated with enhanced photocatalytic activity in nanogap-engineered g-C3N4/Ag@SiO2 hybrid nanostructures

Graphitic carbon nitride (g-C3N4) has attracted considerable attention with regard to its use in photocatalytic solar hydrogen production by the splitting of water. High charge carrier recombination critically limits the photocatalytic activity of g-C3N4. Plasmonic metal nanoparticles that can generate localized surface plasmon resonance (LSPR) have been suggested to enhance the harvesting of visible light and to improve water splitting efficiency. However, direct contact between metal nanoparticles and g-C3N4 reduces the hydrogen generation efficiency owing to energy loss by Förster resonance energy transfer (FRET), which competes with plasmon resonance energy transfer (PRET). Decorating g-C3N4 with Ag@SiO2 core-shell plasmonic nanoparticles increases its photocatalytic ability. Tuning the size of the SiO2 nanogap can optimize the photocatalytic performance of g-C3N4/Ag@SiO2, which involves a trade-off between PRET and FRET. X-ray absorption spectroscopy (XAS) is utilized to investigate the electronic structure of g-C3N4 and its modulation with Ag@SiO2. In situ XAS reveals the dynamics of the charge carriers under solar illumination. Analytic results suggest charge redistribution, shifting of the conduction band, modification of the unoccupied states, and consequent improvement in photocatalytic activity by solar illumination. This work sheds light on the effect of LSPR on this photocatalyst with reference to its electronic structure.

Silver thin film deposited 3-dimensional gold nanorod arrays for plasmonic catalysis

Plasmonic metal nanoparticles are of great importance in heterogeneous catalysis applications including partial oxidations, dehydrogenations, and hydrocarbon reforming. Although utilization of such nanoparticulate metals seems to be a simple and cheap approach for catalysis, their low surface area, un-controlled aggregation, and limited recovery and recycling abilities prevent their further applications. Herein, to overcome these limitations, vertically aligned, highly ordered, and unidirectional 3-dimensional (3D) gold nanorod (AuNR) arrays were fabricated through an oblique angle vapor deposition method. The plasmonic features and catalytic efficiencies of 3D AuNR arrays were further improved by combining them with a thin layer of silver having varying thicknesses. The catalytic activity of fabricated samples was evaluated by the reduction of 4-nitrophenol to 4-aminophenol with NaBH4 in the absence and presence of laser illumination with different wavelengths. All the results indicate that the current Ag@AuNR substrates are expected to have a broad impact in a range of fields including plasmonic catalysis, sensing, and many others.

Correlative Study of Enhanced Excitonic Emission in ZnO Coated with Al Nanoparticles using Electron and Laser Excitation

Recently, metal nanoparticle surface coatings have been found to significantly enhance the ultra-violet luminescence intensity from ZnO, providing a viable means to mitigate optical losses and improve LED performance. Although there is general agreement that resonantly excited Localized Surface Plasmons (LSPs) in metal nanoparticles can directly couple to excitons in the semiconductor increasing their spontaneous emission rate, the exact mechanisms involved in this phenomenon are currently not fully understood. In this work, LSP-exciton coupling in bulk and nanostructured ZnO coated with a 2 nm Al nanoparticle layer is investigated using correlative photoluminescence and depth-resolved cathodoluminescence and time-resolved photoluminescence spectroscopy. Temperature-resolved cathodoluminescence and photoluminescence measurements from 10 K to 250 K show free exciton (FX) emission enhancement factors up to 12x at 80 K, and reveal that the FX couple more efficiently to the LSPs compared to the localized donor-bound excitons. A strong polarization dependence between the LSPs and FX is observed where FX transitions are more strongly enhanced when polarized in the same direction as the electric field of the incident excitation, which is different for laser and electron beam sources. This result indicates that selective enhancement of the excitonic emission peaks in the ZnO coated with Al nanoparticles can be achieved by choosing the appropriate ZnO substrate orientation.

A Multi‐Functional Highly Efficient Upconversion Luminescent Film with an Array of Dielectric Microbeads Decorated with Metal Nanoparticles

AbstractUpconversion nanoparticles (UCNPs) have been integrated with photonic platforms to overcome the intrinsically low quantum efficiency limit of upconversion luminescence (UCL). However, platforms based on thin films lack transferability and flexibility, which hinders their broader and more practical application. A plasmonic structure is developed that works as a multi‐functional platform for flexible, transparent, and washable near‐infrared (NIR)‐to‐visible UCL films with ultra‐strong UCL intensity. The platform consists of dielectric microbeads decorated with plasmonic metal nanoparticles on an insulator/metal substrate. Distinct improvements in NIR confinement, visible light extraction, and boosted plasmonic effects for upconversion are observed. With weak NIR excitation, the UCL intensity is higher by three orders of magnitude relative to the reference platform. When the microbeads are organized in a square lattice array, the functionality of the platform can be expanded to wearable and washable UCL films. The platform can be transferred to transparent, flexible, and foldable films and still emit strong UCL with a wide viewing angle.

Interaction of ZnO nanorods with plasmonic metal nanoparticles and semiconductor quantum dots

We model the enhancement of near band edge emission from ZnO nanorods using plasmonic metal nanoparticles and compare it with emission enhancement from ZnO with semiconducting quantum dots. Selected CdSe quantum dots with absorption energies close to those of Ag and Au nanoparticles are chosen to construct model systems with ZnO to comprehend the role of ZnO's intrinsic defects and plasmonic excitation in realizing the spectrally selective luminescence enhancement. Excitation wavelength dependent photoluminescence spectra along with theoretical models quantifying the related transitions and plasmonic absorption reveal that a complex mechanism of charge transfer between the ZnO nanorods and metal nanoparticles or quantum dots is essential along with an optimal energy band alignment for realizing emission enhancement. The theoretical model presented also provides a direct method of quantifying the relative transition rate constants associated with various electronic transitions in ZnO and their change upon the incorporation of plasmonic nanoparticles. The results indicate that, while the presence of deep level defect states may facilitate the essential charge transfer process between ZnO and the plasmonic nanoparticles, their presence alone does not guarantee UV emission enhancement and strong plasmonic coupling between the two systems. The results offer clues to designing novel multicomponent systems with coupled plasmonic and charge transfer effects for applications in charge localization, energy harvesting, and luminescence enhancement, especially in electrically triggered nanophotonic applications.

Enhanced visible-light-driven photoelectrochemical and photocatalytic performance of Au-SnO2 quantum dot-anchored g-C3N4 nanosheets

A novel g-C3N4/Au-SnO2 quantum dot (g-CN/Au-SQD) ternary nanocomposite was fabricated via a three-step approach for the degradation of the organic pollutant rhodamine B (RhB), and photoelectrochemical (PEC) water splitting upon visible light illumination. Au-SQDs were prepared via a one-pot chemical reduction method, and g-CN was synthesized via the thermal polymerization of urea at 550 °C. The g-CN/Au-SQD ternary nanocomposite was prepared via sonication, stirring, and finally annealing. This approach was effective for the mixing and dispersion of Au-SQDs over the entire surface of the two-dimensional (2D) g-CN nanosheets. Morphological studies revealed that the Au-SQDs were well-distributed over the nanolayers of the g-CN. The light-capturing ability was improved and optimized with the loading of different amounts of Au-SQDs. The bandgap was tuned from 2.85 eV (g-CN) to 2.58 eV (g-CN/Au-SQD). Photoluminescence analysis revealed the inhibited nature of recombination of electrons and holes in the ternary nanocomposites. Optimization yielded CNAS-20, which exhibited the best photocatalytic performance within 40 min for the degradation of the pollutant RhB. Furthermore, the CNAS-20 photoelectrode showed lower charge-transfer resistance than the other prepared samples, which was favorable for PEC water splitting. The CNAS-20 photoelectrode exhibited a significant photocurrent, which was ~3.83 times greater than that of pure g-CN. Thus, this unique design incorporates a 2D g-CN and plasmonic Au metal nanoparticles for the generation of photoexcited electrons, and SQDs receive these photogenerated electrons to increase the leave-taking of electron and holes to enhance the photocatalytic and PEC activities.

Plasmonic nanometal decorated photoanodes for efficient photoelectrochemical water splitting

Plasmonic metal nanoparticles containing photoanodes are known to exhibit stable photoelectrochemical (PEC) performance due to their optical and electronic properties. In this work, we report the application of plasmonic Bi nanoparticles supported over a g-C3N4/Bi2S3 photoanode for PEC water splitting. Typical results indicated that g-C3N4/Bi2S3/BiNPs ternary composite photoanode showed a high photo-current density of 7.11 mA cm−2 at 1.23 V under solar irradiation, which was ∼ 5 and 10 times higher than g-C3N4/Bi2S3 and g-C3N4 photoanodes, respectively. Further, the composite electrode also demonstrated superior solar to hydrogen efficiency and long-term stability. It was concluded that Bi nanoparticles play a major role in enhancing the PEC performance for hydrogen evolution reaction. Thus, g-C3N4/Bi2S3/BiNPs has superior PEC performance and proved to work as an alternative to noble metal based photo-electrodes for solar-water splitting reactions.

Rationalizing the Plasmonic Contributions to the Enhancement of Singlet Oxygen Production

The strong interaction between plasmonic metal nanoparticles and photosensitizers can significantly amplify their singlet oxygen (1O2) production. However, improving the performance of these hybrid...

The vibronic absorption spectra and exciton dynamics of plasmon-exciton hybrid systems in the regimes ranged from Fano antiresonance to Rabi-like splitting.

The complex interplay between molecules and plasmonic metal nanoparticles (MNPs) presents a set of particular characteristics in absorption/scattering spectra such as excitonic splitting, asymmetric line shapes, plasmon-induced absorption enhancement and transparencies, etc. Although the MNP-molecule systems have been intensively investigated experimentally and theoretically, the construction of a theoretical framework which can produce all the disparate experimental observations and account for the electron-phonon (e-p) coupling is still in progress. Here, we present a theoretical approach which can account for both the plasmon-exciton coupling and the e-p interaction and produce all the spectral line shapes ranging from Fano antiresonance to Rabi splitting by simply tuning the coupling strength or plasmon damping rate. Additionally, we demonstrate the evolution of vibronic spectra and exciton dynamics with the coupling strength, plasmon damping rate, and detuning energy. It is found that the vibronic structures appearing in Rabi-like spectra are worse resolved, wider, and more largely shifted than those appearing in the Fano regime, attributed to the more significant deformation of the molecular vibrational wavepacket in the Rabi-like regime than that in the Fano regime as the molecular e-p interaction increases. The positive/negative value of detuning energy can induce different degrees of the vibrational wavepacket deformation and subsequently a different effect on the spectra in different coupling regimes.