Incorporation Of Plasmonic Nanostructures Research Articles

Nano/Micromotor‐Driven SERS for Highly Sensitive and Spatially Controlled Sensing

AbstractNano/micromotors (NMs) are tiny structures capable of converting various forms of energy into mechanical motion at the micro and nanoscale. These motors operate in environments characterized by low inertia and low Reynolds numbers. The potential applications of NMs are vast, particularly in the fields of biomedicine and environmental science. One of the most intriguing developments in this field is the integration of NMs with surface‐enhanced Raman scattering (SERS) spectroscopy. SERS is a powerful analytical technique that enhances the Raman intensity of molecules, allowing for highly sensitive detection and analysis of trace amounts of substances. This integration offers highly precise and localized ultrasensing capabilities. The combination of NMs with SERS can also facilitate real‐time imaging inside living organisms. This has immense potential in chemical and cell biology and medical diagnostics and prognosis. Herein this review describes the types of NMs and their fabrication, the incorporation of plasmonic nanostructures, capable of creating strong electromagnetic fields when illuminated by light, which in turn enhances the Raman signals significantly, their applications, and their future prospects in areas such as precision medicine, environmental monitoring, and possibly even in new realms like microscale robotics and targeted therapeutics.

Efficient discontinuous Galerkin scheme for analyzing nanostructured photoconductive devices.

Incorporation of plasmonic nanostructures in the design of photoconductive devices (PCDs) has significantly improved their optical-to-terahertz conversion efficiency. However, this improvement comes at the cost of increased complexity for the design and simulation of these devices. Indeed, accurate and efficient modeling of multiphysics processes and intricate device geometries of nanostructured PCDs is challenging due to the high computational cost resulting from multiple characteristic scales in time and space. In this work, a discontinuous Galerkin (DG)-based unit-cell scheme for efficient simulation of PCDs with periodic nanostructures is proposed. The scheme considers two physical stages of the device and models them using two coupled systems: a system of Poisson and drift-diffusion equations describing the nonequilibrium steady state, and a system of Maxwell and drift-diffusion equations describing the transient stage. A "potential-drop" boundary condition is enforced on the opposing boundaries of the unit cell to mimic the effect of the bias voltage. Periodic boundary conditions are used for carrier densities and electromagnetic fields. The unit-cell model described by these coupled equations and boundary conditions is discretized using DG methods. Numerical results demonstrate that the proposed DG-based unit-cell scheme has the same accuracy in predicting the THz photocurrent as the DG framework that takes into account the whole device, while it significantly reduces the computational cost.

(Invited) Charge Carriers Behaviour in Solar Energy Harvesting & Converting Semiconducting Systems.

Solar energy conversion to different types of energy which might be subsequently released by divers’ ways remains an unsaturated need, since the global demand for energy is continuously growing and will continue to rise. However, currently the field of renewable energy suffers from lack of an ideal material able to drive conversion of the solar energy with high efficiency and being stable enough to provide a long-term productivity. Therefore, multi-directional efforts including development of new materials and catalysts, incorporation of plasmonic nanostructures or creation of low-level sub- stoichiometry through different approaches including combination of intrinsic and surface modifications are continuously undertaken and devoted to minimisation of the required bias voltage, improvement of light capture or charge transport properties. Our recent achievements regrading employment of different in structure polyoxometalates as catalysts, sub-stochiometric semiconducting oxides, newly designed and engineered crystal structures in solar driven arrangements as well as their performance for photo induced processes such as oxidation or reduction will be presented and discussed in detail. Besides the extensive characterization of the above-mentioned systems, the kinetic assessment of the transient absorption phenomena will be approached. Use of transient absorption spectroscopy to study charge carrier behaviour in semiconductors-based systems allows to relate a photoelectrode architecture to the charge carrier origin, separation, collection, trapping, lifetime and therefore final efficiency. Presentation and comparison of the charge carrier dynamics occurring in semiconducting systems: bare, modified as well as mixed oxides, are particularly useful in clarifying the observed differences in in their photoelectrochemical performance.

Boosting the Efficiency of Photoelectrolysis by the Addition of Non-Noble Plasmonic Metals: Al & Cu.

Solar water splitting by semiconductor based photoanodes and photocathodes is one of the most promising strategies to convert solar energy to chemical energy to meet the high demand for energy consumption in modern society. However, the state-of-the-art efficiency is too low to fulfill the demand. To overcome this challenge and thus enable the industrial realization of a solar water splitting device, different approaches have been taken to enhance the overall device efficiency, one of which is the incorporation of plasmonic nanostructures. Photoanodes and photocathodes coupled to the optimized plasmonic nanostructures, matching the absorption wavelength of the semiconductors, can exhibit a significantly increased efficiency. So far, gold and silver have been extensively explored to plasmonically enhance water splitting efficiency, with disadvantages of high cost and low enhancement. Instead, non-noble plasmonic metals such as aluminum and copper, are earth-abundant and low cost. In this article, we review their potentials in photoelectrolysis, towards scalable applications.

Plasmonic Solar Fuels Based on Nanostructured Oxide Photocatalysts

Solar energy can be directly converted to electrical energy via photovoltaic devices. Photocatalysis provides an alternative route to the conversion of solar energy that can be stored in chemical fuels through photoelectrochemical cells or particulate photocatalytic systems. While a number of strategies were developed to overcome the deficiencies of existing semiconductors, incorporation of plasmonic nanostructures with semiconductors has gained increasing attention as a promising paradigm to improve the solar energy conversion efficiency. In this presentation, we introduce diverse approaches for the fabrication of nanostructured semiconductors integrated with plasmonic structures that can be exploited to exhibit enhanced photo-conversion efficiencies.First, a surface plasmon (SP) induced visible light active photocatalyst system composed of silica-titania core-shell (SiO2@TiO2) nanostructures decorated with Au nanoparticles (Au NPs) was developed. The influence of size and distribution of Au NPs on photocatalysis, its fabrication methods and exploration of the mechanism of visible light activity were investigated. A favorable architecture of SiO2 beads with a thin layer of TiO2 was decorated with Au NP arrays having different size and areal density. Surface modification of SiO2@TiO2 leads to a viable and homogenous loading of Au NPs on the surface of TiO2, which renders visible light-induced photocatalytic activity on the whole TiO2 surface. An optimized system employing Au NP arrays with 15 nm size and 700/µm2 density showed best catalytic efficiency, due to a synergistic effect of the firm contact between Au NPs and TiO2 and efficiently coupled SPR excitation. A brief mechanism relating the electron transfer from surface plasmon stimulated Au NPs to the conduction band of TiO2 is proposed. The same system was further tailor-modified to exhibit far-field scattering effect, to lead to maximized performance toward solar water splitting.Next, we focus on our design principle for the plasmon-enhanced oxide-based photocatalytic systems based on inverse opal structures and demonstrate the SP coupling effect on the efficiency of water splitting. Inverse opals of representative semiconductor oxide photocatalysts were fabricated by well-known colloid templating, and the surface was decorated with Au NPs. The effect of size and areal density of Au NP arrays were explored in terms of oxidation reactions and plasmon-enhancement effect was supported by FDTD analysis. An advance strategy to improve the efficiency of solar fuel generation includes the combined block-copolymer-based self-assembly, leading to hierarchically organized inverse opals with tailored surface area and nano-textured surface morphology. Further, we show that carbon-doped analogue inverse opal structures could be readily obtained by direct carbonization of block copolymer templates, based on which simultaneous carbon-doping and plasmon-enhancement effect are synergistically realized.

Cascading metallic gratings for broadband absorption enhancement in ultrathin plasmonic solar cells

The incorporation of plasmonic nanostructures in the thin-film solar cells (TFSCs) is a promising route to harvest light into the nanoscale active layer. However, the light trapping scheme based on the plasmonic effects intrinsically presents narrow-band resonant enhancement of light absorption. Here we demonstrate that by cascading metal nanogratings with different sizes atop the TFSCs, broadband absorption enhancement can be realized by simultaneously exciting multiple localized surface plasmon resonances and inducing strong coupling between the plasmonic modes and photonic modes. As a proof of concept, we demonstrate of 66.5% in the photocurrent in an ultrathin amorphous silicon TFSC with two-dimensional cascaded gratings over the reference cell without gratings.

Silver nanowires enhance absorption of poly(3-hexylthiophene)

Results of optical spectroscopy reveal strong influence of plasmon excitations in silver nanowires on the fluorescence properties of poly(3-hexylthiophene) (P3HT), which is one of the building blocks of organic solar cells. For the structure where a conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) was used as a spacer in order to minimize effects associated with non-radiative energy transfer from P3HT to metallic nanoparticles, we demonstrate over two-fold increase of the fluorescence intensity. Results of time-resolved fluorescence indicate that the enhancement of emission intensity can be attributed to increased absorption of P3HT. Our findings are a step towards improving the efficiency of organic solar cells through incorporation of plasmonic nanostructures.

Plasmon-Enhanced Solar Energy Harvesting

Solar energy can be directly converted to electrical energy via photovoltaics. Alternatively, solar energy can be converted and stored in chemical fuels through photoelectrochemical cells and photocatalysis (see front cover image), allowing continued power production when the sky is cloudy or dark. The conversion of solar energy is regulated by four processes: light absorption, charge separation, charge migration, and charge recombination. An individual material cannot be optimized for all four processes. Incorporation of plasmonic nanostructures with semiconductors offers an alternative route to improve the solar energy conversion efficiency. One enhancement mechanism is a so called photonic enhancement, the other one is plasmonic energy transfer enhancement. Two different mechanisms of plasmonic energy transfer enhancement are discussed: plasmon-induced resonant energy transfer (PIRET) from the metal to the semiconductor and direct electron transfer (DET). Plasmonic nanostructures can be designed with orders of magnitude larger EM field enhancements, creating the possibility of highly efficient photocatalysts and solar cells.

Design Considerations for Plasmonic Photovoltaics

This paper reviews the recent research progress in the incorporation of plasmonic nanostructures with photovoltaic devices and the potential for surface plasmon enhanced absorption. We first outline a variety of cell architectures incorporating metal nanostructures. We then review the experimental fabrication methods and measurements to date, as well as systematic theoretical studies of the optimal nanostructure shapes. Finally we discuss photovoltaic absorber materials that could benefit from surface plasmon enhanced absorption.