Substrate-induced interfacial plasmonics for photovoltaic conversion

Raman Scattering Enhancements Due to Super- and Subradiant Collective Plasmon Modes on Large-Area 2D-Au Arrays

Ordered metal nanoparticle (MNP) arrays with ultrasmall interparticle gaps S exhibit strong enhancement of the electromagnetic (EM) near-field, known as hotspots, for surface-enhanced Raman spectroscopy (SERS) sensing. These arrays, with uniform gap sizes, are also essential for studying nonlinear Raman scattering effects and surface selection rules. Optical characterization of the fabricated large-area Au arrays with S ≪ r, where r is the MNP radius, revealed the excitation of hybridized-collective plasmon modes with giant EM near-field enhancement. We found that the SERS enhancement associated with a subradiant plasmon mode depends primarily on the interparticle gap distance S, rather than on the ordering of MNPs into arrays. However, arranging MNPs in the form of arrays influenced the far-field scattering of the super-radiant mode excited at a longer wavelength, resulting in lower but highly anisotropic SERS enhancements that depend on the far-field excitation polarization angle σ. This study on ordered MNP arrays with ultrasmall interparticle gaps S ≪ r highlights the roles of S and MNP ordering in SERS enhancement of an analyte. This understanding is pivotal for designing SERS substrates with very small interparticle gaps, as they generate a large number of intense and well-distributed SERS hotspots. Furthermore, an anisotropy-induced SERS dichroism effect was observed. Polarization-dependent SERS intensities varied based on the excitation wavelength λexc and its corresponding Stokes wavelength positions related to the excited plasmon mode. As a result, the SERS dichroism of lower-frequency Stokes-shifted peaks exhibited a cos2(σ) dependence, whereas higher-frequency Stokes-shifted peaks exhibited a sin2(σ) dependence. This observation validates the EM near-field mechanism of SERS. The fabricated large-area 2D-Au arrays meet most of the essential requirements for efficient, robust, and reliable large-area SERS sensing.

Transparent graphene electrodes based hybrid perovskites photodetectors with broad spectral response from UV-visible to near-infrared

A new class of transparent graphene electrode based organic-inorganic halide perovskite photodetectors with broad spectral response is developed. These ultrasensitive devices exhibit high ON/OFF current ratio, high linear dynamic range, broad spectral range, excellent detection for weak light and easy fabrication with low-cost. Their semi-transparent feature and distinct photodetecting function for both sides would provide new applications affecting our daily lives.

The Influence of Size, Shape, and Twin Boundaries on Heat-Induced Alloying in Individual Au@Ag Core-Shell Nanoparticles

Environmental conditions during real-world application of bimetallic core-shell nanoparticles (NPs) often include the use of elevated temperatures, which are known to cause elemental redistribution, in turn significantly altering the properties of these nanomaterials. Therefore, a thorough understanding of such processes is of great importance. The recently developed combination of fast electron tomography with in situ heating holders is a powerful approach to investigate heat-induced processes at the single NP level, with high spatial resolution in 3D. In combination with 3D finite-difference diffusion simulations, this method can be used to disclose the influence of various NP parameters on the diffusion dynamics in Au@Ag core-shell systems. A detailed study of the influence of heating on atomic diffusion and alloying for Au@Ag NPs with varying core morphology and crystallographic details is carried out. Whereas the core shape and aspect ratio of the NPs play a minor role, twin boundaries are found to have a strong influence on the elemental diffusion.

Diverse Substrate-Mediated Local Electric Field Enhancement of Metal Nanoparticles for Nanogap-Enhanced Raman Scattering

The localized surface plasmon resonance of plasmonic nanoparticles (NPs) can be coupled with a noble metal substrate (S) to induce a localized augmented electric field (E-field) concentrated at the NP-S gap. Herein, we analyzed the fundamental near-field properties of metal NPs on diverse substrates numerically (using the 3D finite-difference time-domain method) and experimentally [using surface-enhanced Raman scattering (SERS)]. We systematically examined the effects of plasmonic NPs on noble metals (Ag and Au), non-noble metals (Al, Ti, Cu, Fe, and Ni), semiconductors (Si and Ge), and dielectrics (TiO₂, ZnO, and SiO₂) as substrates. For the AgNPs, the Al (11,664 times) and Si (3969 times) substrates produced considerable E-field enhancements, with Al in particular generating a tremendous E-field enhancement comparable in intensity to that induced by a Ag (28,224 times) substrate. Notably, we found that a superior metallic character of the substrate gave rise to easier induction of image charges within the metal substrate, resulting in a greater E-field at the NP-S gap; on the other hand, the larger the permittivity of the nonmetal substrate, the greater the ability of the substrate to store an image charge distribution, resulting in stronger coupling to the charges of localized surface plasmon resonance oscillation on the metal NP. Furthermore, we measured the SERS spectra of rhodamine 6G (a commonly used Raman spectral probe), histamine (a biogenic amine used as a food freshness indicator), creatinine (a kidney health indicator), and tert-butylbenzene [an extreme ultraviolet (EUV) lithography contaminant] on AgNP-immobilized Al and Si substrates to demonstrate the wide range of potential applications. Finally, the NP-S gap hotspots appear to be widely applicable as an ultrasensitive SERS platform (∼single-molecule level), especially when used as a powerful analytical tool for the detection of residual contaminants on versatile substrates.

Electro-Ionic Control of Surface Plasmons in Graphene-Layered Heterostructures

Precise control of light is indispensable to modern optical communication devices especially as the size of such devices approaches the subwavelength scale. Plasmonic devices are suitable for the development of these optical devices due to the extreme field confinement and its ability to be controlled by tuning the carrier density at the metal/dielectric interface. Here, an electro-ionic controlled plasmonic device consisting of Au/graphene/ion-gel is demonstrated as an optical switch, where an external electric field modulates the real part of the electrical conductivity. The graphene layer enhances charge penetration and charge separation at the Au/graphene interface resulting in an increased photoinduced voltage. The ion-gel immobilized on the Au/graphene further enables the electrical tunability of plasmons which modulates the intensity of the reflected laser light. This work paves the way for developing novel plasmonic electro-optic switches for potential applications such as integrated optical devices.

Bimetallic Implanted Plasmonic Photoanodes for TiO2 Sensitized Third Generation Solar Cells

An auspicious way to enhance the power conversion efficiency (PCE) of third generation sensitized solar cells is to improve the light harvesting ability of TiO2 sensitizer and inhibition of back recombination reactions. In the present work, we have simultaneously comprehended both the factors using stable bimetallic Au and Ag metal nanoparticles (Mnps) embedded in TiO2 with ion implantation technique at lower fluence range; and explored them in third generation dye sensitized solar cells (DSSCs). The best performing Au-Ag implanted DSSC (Fluence- 6 × 1015 ions cm-2) revealed 87.97% enhancement in its PCE relative to unimplanted DSSC; due to plasmon induced optical and electrical effects of Mnps. Here, optimized bimetallic Au-Ag Mnps embedded in TiO2 improves light harvesting of N719 dye; due to the well matched localized surface plasmon resonance (LSPR) absorption band of Au and Ag with low and high energy absorption bands of N719 dye molecules, respectively. Furthermore, Au and Ag acts as charge separation centers in TiO2 that inhibit the recombination reactions occurring at photoanode/electrolyte interface via prolonging photo-generated electron lifetime; resulting in efficient inter-facial charge transportation in DSSCs.

SERS-Active Substrate with Collective Amplification Design for Trace Analysis of Pesticides

Health risks posed by the exposure to trace amounts of pesticide residue in agricultural products have gained a lot of concerns, due to their neurotoxic nature. The applications of surface-enhanced Raman Scattering (SERS) as a detection technique have consistently shown its potential as a rapid and sensitive means with minimal sample preparation. In this study, gold nanoparticles (Au NPs) in elliptical shapes were collected into a layer of ordered zirconia concave pores. The porous zirconia layer (pZrO₂) was then deposited with Au NPs, denoted as Au NPs (x)/pZrO₂, where x indicates the deposition thickness of Au NPs in nm. In the concave structure of pZrO₂, Au-ZrO₂ and Au-Au interactions provide a synergistic and physical mechanism of SERS, which is anticipated to collect and amplify SERS signals and thereafter improve the enhancement factor (EF) of Au NPs/pZrO₂. By taking Rhodamine 6G (R6G) as the test molecule, EF of Au NPs/pZrO₂ might reach to 7.0 × 10⁷. Au NPs (3.0)/pZrO₂ was then optimized and competent to detect pesticides, e.g., phosmet and carbaryl at very low concentrations, corresponding to the maximum residue limits of each, i.e., 0.3 ppm and 0.2 ppm, respectively. Au NPs (3.0)/pZrO₂ also showed the effectiveness of distinguishing between phosmet and carbaryl under mixed conditions. Due to the strong affinities of the phosphoric groups and sulfur in phosmet to the Au NPs (3.0)/pZrO₂, the substrate exhibited selective detection to this particular pesticide. In this study, Au NPs (3.0)/pZrO₂ has thus demonstrated trace detection of residual pesticides, due to the substrate design that intended to provide collective amplification of SERS.

Purely Visible-Light-Induced Photochromism in Ag-TiO2 Nanoheterostructures

We report titania nanoheterostructures decorated with silver, exhibiting tuneable photochromic properties for the first time when stimulated only by visible white light (domestic indoor lamp), with no UV wavelengths. Photochromic materials show reversible color changes under light exposure. However, all inorganic photochromic nanoparticles (NPs) require UV light to operate. Conventionally, multicolor photochromism in Ag-TiO₂ films involves a change in color to brownish-gray during UV-light irradiation (i.e., reduction of Ag⁺ to Ag⁰) and a (re)bleaching (i.e., (re)oxidation of Ag⁰ to colorless Ag⁺) upon visible-light exposure. In this work, on the contrary, we demonstrate visible-light-induced photochromism (ranging from yellow to violet) of 1-10 mol % Ag-modified titania NPs using both spectroscopic and colorimetric CIEL*a*b* analyses. This is not a bleaching of the UV-induced color but a change in color itself under exposure to visible light, and it is shown to be a completely different mechanism-driven by the interfacial charge transfer of an electron from the valence band of TiO₂ to that of the Ag_xO clusters that surround the titania-to the usual UV-triggered photochromism reported in titania-based materials. The quantity of Ag or irradiation time dictated the magnitude and degree of tuneability of the color change, from pale yellow to dark blue, with a rapid change visible only after a few seconds, and the intensity and red shift of surface plasmon resonance induced under visible light also increased. This effect was reversible after annealing in the dark at 100 °C/15 min. Photocatalytic activity under visible light was also assessed against the abatement of nitrogen oxide pollutants, for interior use, therefore showing the coexistence of photochromism and photocatalysis-both triggered by the same wavelength-in the same material, making it a multifunctional material. Moreover, we also demonstrate and explain why X-ray photoelectron spectroscopy is an unreliable technique with such materials.

Broadband absorption enhancement in plasmonic nanoshells-based ultrathin microcrystalline-Si solar cells

With the objective to conceive a plasmonic solar cell with enhanced photocurrent, we investigate the role of plasmonic nanoshells, embedded within a ultrathin microcrystalline silicon solar cell, in enhancing broadband light trapping capability of the cell and, at the same time, to reduce the parasitic loss. The thickness of the considered microcrystalline silicon (μc-Si) layer is only ~1/6 of conventional μc-Si based solar cells while the plasmonic nanoshells are formed by a combination of silica and gold, respectively core and shell. We analyze the cell optical response by varying both the geometrical and optical parameters of the overall device. In particular, the nanoshells core radius and metal thickness, the periodicity, the incident angle of the solar radiation and its wavelength are varied in the widest meaningful ranges. We further explain the reason for the absorption enhancement by calculating the electric field distribution associated to resonances of the device. We argue that both Fabry-Pérot-like and localized plasmon modes play an important role in this regard.