Impact of Nonuniform Electron Density on Plasmonic Properties of Metal Nanoparticles

The interfacial nonuniformity of the electron density that occurs in metals as a result of atomic imperfections can strongly affect the plasmonic properties of metallic nanostructures. Under certain conditions, it induces the bulk plasmon resonance in the transition area and can significantly change scattering and absorption of light by metallic nanostructures in a broad frequency range. This effect is numerically demonstrated for radially nonuniform spherical silver nanoparticles and analytically investigated with respect to the resonant coupling with the dipolar surface plasmons of the metal core.     

Plasmonic Scattering by Metal Nanoparticles for Solar Cells

We investigate on absorption and scattering from metal nanoparticles in view of possible applications to photovoltaic cells. The analysis, accounting for most of the parameters involved in the physical mechanism of scattering, is split into two parts. In the first part, scattering from a metallic sphere is treated analytically to investigate the dependence on sphere size, sphere metal, and surrounding medium. In the second part, scattering from a metallic particle is investigated as a function of particle shape (spheroids, hemispheres, and cylinders) via numerical simulations based on the finite-difference time-domain method. The aim of the work is to provide a systematic study on scattering and absorption by metal nanoparticles, exploring several combinations of material and geometrical parameters in order to identify those combinations that could play a key role in solar cell efficiency improvement.     

Enhanced Light Harvesting in Organic Solar Cells Featuring a Biomimetic Active Layer and a Self‐Cleaning Antireflective Coating

Advanced light manipulation is extremely attractive for applications in organic optoelectronics to enhance light harvesting efficiency. A novel method of fabricating high‐efficiency organic solar cells (OSCs) is proposed using biomimetic moth eye nanostructures in a quasi‐periodic gradient shape active layer and an antireflective coating. A 24.3% increase in photocurrent is realized without sacrificing dark electrical properties, yielding a 22.2% enhancement in power conversion efficiency to a record of 7.86% for OSCs with a poly(3‐hexylthiophene‐2,5‐diyl):indene‐C60 bis‐adduct (P3HT:ICBA) active layer. The experimental and theoretical characterizations verify that the substantial improvement of OSCs is mainly ascribed to the self‐enhanced absorption resulting from the broadband polarization‐insensitive light trapping in biomimetic nanostructured active layer, the reduction in reflectance by the antireflective coating, and surface plasmonic effect excited by corrugated metallic electrode. It is noteworthy that the pathway described here is promising for opening up opportunities to realize high‐performance OSCs towards the future photovoltaic applications.     

Optical Absorption in Overcoats of Nanoparticle Arrays on a Metallic Substrate

Surface plasma oscillations in metallic particles as well as in thin metallic films have been studied extensively in the past decades. New features regarding surface plasma excitations are, however, constantly discovered, leading, for example, to surface-enhanced Raman scattering studies and enhanced optical transmission though metal films with nanohole arrays. In the present work, the role of a metallic substrate is examined in two cases, one involving an overcoat of dielectric nanoparticles and the other an overcoat of metallic nanoparticles. Theoretical results are obtained by modeling the nanoparticles as forming a two-dimensional, hexagonal lattice of spheres. The scattered electromagnetic field is then calculated using a variant of the Green function method. Comparison with experimental results is made for nanoparticles of tungsten oxide and tin oxide deposited on either gold or silver substrates, giving qualitative agreement on the extra absorption observed when the dielectric nanoparticles are added to the metallic surfaces. Such absorption would be attributed to the mirror image effects between the particles and the substrate. On the other hand, calculations of the optical properties of silver or gold nanoparticle arrays on a gold or a silver substrate demonstrate very interesting features in the spectral region from 400 to 1,000 nm. Interactions between the nanoparticle arrays surface plasmons and their images in the metallic substrate would be responsible for the red shift observed in the absorption resonance. Moreover, effects of particle size and ambient index of refraction are studied, showing a great potential for applications in biosensing with structures consisting of metallic nanoparticle arrays on metallic substrates.     

Investigation of Gold and Silver Nanoparticles on Absorption Heating and Scattering Imaging

Efficient conversion of absorbed light to heat energy and strong scattering by gold and silver nanoparticles suggest these nanoparticles as the agents of heating and imaging. Absorption efficiency and scattering efficiency of gold and silver nanoparticles were studied through numerical simulation using the discrete dipole approximation method. This study shows that the size of gold and silver nanoparticles can effect gold and silver nanoparticles’ absorption efficiency and scattering efficiency. The gold nanoparticle is found to possess the maximum absorption efficiency when the size of gold nanoparticle is 50 nm and the incident wavelength is 540 nm, and the increasing scattering efficiency with the increasing size of gold nanoparticle in the medium, and refractive index of the medium is around 1.33. However, the silver nanoparticle owns the maximum absorption efficiency when the size of silver nanoparticle is 20 nm and the incident wavelength is 396 nm, and the maximum scattering efficiency when the size of silver nanoparticle is 30 nm and the incident wavelength is 410 nm in the same medium. The conditions for achieving the maximum adsorption efficiency and scattering efficiency of gold and silver nanoparticle can be used for heating and imaging using visible and near-infrared light.     

Design of Plasmonic Nanoparticles for Efficient Subwavelength Light Trapping in Thin-Film Solar Cells

This paper explores geometry-sensitive scattering from plasmonic nanoparticles deposited on top of a thin-film amorphous silicon solar cell to enhance light trapping in the photo-active layer. Considering the nanoparticles as ideal spheroids, the broadband optical absorption by the silicon layer is analyzed and optimized with respect to the nanoparticle aspect ratio in both the cases of resonant (silver) and nonresonant (aluminum) plasmonic nanostructures. It is demonstrated how the coupling of sunlight with the semiconductor can be improved through tuning the nanoparticle shape in both the dipolar and multi-polar scattering regimes, as well as discussed how the native oxide shell formed on the nanospheroid surface after the prolonged action of air and moisture affects the light trapping in the active layer and changes the photocurrent generation by the solar cell.     

Triple helical polysaccharide-induced good dispersion of silver nanoparticles in water

Silver nanoparticles were constructed by using triple helical polysaccharide (lentinan) dissolved in water as matrix for the first time. The structure, morphology, and size of the nanocomposites in the polysaccharide aqueous solutions were investigated with UV-visible spectroscopy (UV-vis), transmission electron microscopy (TEM), and dynamic laser light scattering (DLS). The results revealed that the silver nanoparticles were attached to the polysaccharide chains through the strong noncovalent interactions, leading to the good dispersion of silver nanoparticles with mean radius of 6 nm in water. The silver nanoparticles were stable in the lentinan aqueous solution for 9 months. However, with an addition of NaOH, the polysaccharide with the imperfect helical structure broken partially by NaOH could aggregate in the alkali aqueous solution. The aggregation of the lentinan-bonded silver nanoparticles increased with an increase in the NaOH concentration, whereas the size of the silver nanoparticles barely changed, further confirming that the Ag nanoparticles were stable in this system. The aggregation was related to the conformation transition of the polysaccharide from the triple helix to random coil in the solution. A new method to detect the aggregates and aggregation rate was established according to the intensity of the maximum absorption peaks of the polysaccharide labeled by Ag nanoparticles in the UV spectrum.     

Strategies for the purification and characterization of protein scaffolds for the production of hybrid nanobiomaterials

Rotavirus VP6 self-assembles into high order macrostructures useful as novel scaffolds for the construction of multifunctional hybrid nanobiomaterials. This application requires large quantities of high quality pure material with strict structural consistency. Strategies for obtaining high quality recombinant VP6 and different characterization techniques are explored and compared in this work. VP6 was expressed in the insect cell-baculovirus system. VP6 assemblies were selectively purified utilizing an ion exchange and size exclusion (SE) chromatography. Purification steps were monitored and characterized by dynamic light scattering (DLS), ELISA, SDS-PAGE, HPLC and Western blot. DLS showed that the initial ultrafiltration step removed small particles, the intermediate anion exchange chromatographic step completely removed the baculovirus, whereas the final size exclusion chromatography permitted the selective recovery of correctly assembled VP6 nanotubes and discrimination of non-assembled VP6, as confirmed by transmission electron microscopy. VP6 assembled into tubular structures with diameter of 75 nm and several nanometers in length. The purification yield was 20% of multimeric assemblies with a purity >98%. The resulting material was suitable for the production of functionalized hybrid nanobiomaterials through in situ synthesis of metallic nanoparticles.     

Simultaneous Enhancement of Three Parameters of P3HT‐Based Organic Solar Cells with One Oxygen Atom

Poly(3‐hexylthiophene) (P3HT)‐based organic solar cells (OSCs) have attracted much attention due to their advantages of low‐cost production and matured roll‐to‐roll manufacture. However, the efficiency of P3HT‐based OSCs lag much behind the non‐P3HT ones due to their negligible absorption of long wavelengths of light over 650 nm, high‐lying highest occupied molecular orbitals (HOMO), and difficulty of controlling morphology. In this study, the alkyl chains of the nonfullerene acceptors are replaced with alkoxy chains to achieve synergistic enhancement of all three parameters ( short circuit current density (J_SC), open circuit voltage (V_OC), and fill factor (FF)) and thus significant increase of power conversion efficiency for P3HT‐based OSCs. As a result, the OSCs exhibit a maxima efficiency of 6.6%. The P3HT‐based systems are systematically studied with optical spectroscopy, photoluminescence, cyclic voltametry, space charge limit current, grazing incident wide‐angle X‐ray scattering, transient absorption spectroscopy, transmission electron microscope, and atomic force microscopy to probe the mechanism, which reveal that introducing alkoxy chains simultaneously increases the energy levels of the HOMO and the lowest unoccupied molecular orbitals, enhances the light absorption, improves the rigidity of the backbone and charge transport mobility, and tunes the molecular orientation and film morphology, thus improving the photovoltaic performance. This contribution provides an important guidance in the design of novel nonfullerene acceptors for high‐performance P3HT‐based OSCs.     

Synthesis of silver nanoparticles by endosymbiont Pseudomonas fluorescens CA 417 and their bactericidal activity

The present study emphasizes on biogenic synthesis of silver nanoparticles and their bactericidal activity against human and phytopathogens. Nanoparticle synthesis was performed using endosymbiont Pseudomonas fluorescens CA 417 inhabiting Coffea arabica L. Synthesized nanoparticles were characterized using hyphenated spectroscopic techniques such as UV–vis spectroscopy which revealed maximum absorption 425 nm. Fourier transform infrared spectroscopy (FTIR) analysis revealed the possible functional groups mediating and stabilizing silver nanoparticles with predominant peaks occurring at 3346 corresponding to hydroxyl group, 1635 corresponding carbonyl group and 680 to aromatic group. X-ray diffraction (XRD) analysis revealed the Bragg’s diffraction pattern with distinct peaks at 38° 44°, 64° and 78° revealing the face-centered cubic (fcc) metallic crystal corresponding to the (111), (200), (220) and (311) facets of the crystal planes at 2θ angle. The energy dispersive X-ray spectroscopy (EDS) analysis revealed presence of high intense absorption peak at 3 keV is a typical characteristic of nano-crystalline silver which confirmed the presence of elemental silver. TEM analysis revealed the size of the nanoparticles to be in the range 5–50 nm with polydisperse nature of synthesized nanoparticles bearing myriad shapes. The particle size determined by Dynamic light scattering (DLS) method revealed average size to be 20.66 nm. The synthesized silver nanoparticles exhibited significant antibacterial activity against panel of test pathogens. The results showed Klebsiella pneumoniae (MTCC 7407) and Xanthomonas campestris to be more sensitive among the test human pathogen and phyto-pathogen respectively. The study also reports synergistic effect of silver nanoparticles in combination with kanamycin which displayed increased fold activity up to 58.3% against Klebsiella pneumoniae (MTCC 7407). The results of the present investigation are promising enough and attribute towards growing scientific knowledge on development of new antimicrobial agents to combat drug resistant microorganisms. The study provides insight on emerging role of endophytes towards reduction of metal salts to synthesize nanoparticles.     

Silver Nanoparticles in Epoxy Resin Deposited on Silicon Substrates for Light Trapping

Metallic silver nanoparticles were prepared in epoxy resin using N,N-dimethylformamide (DMF) as reducing agent of silver ions at room temperature. They were characterized by UV–Vis spectroscopy, atomic force microscopy (AFM), transmission electronic microscopy (TEM), and high-resolution transmission electronic microscopy (HRTEM). The silver nanoparticles showed broadband absorption spectra attributed to high-order plasmonic resonances. The morphology of the metallic particles corresponds to elongated particles and their aggregates with a size above 30 nm. These silver nanoparticles were deposited by the spin-coating process on crystalline silicon at room temperature. Then, the antireflective properties of these samples were measured. According to the observed results, it is inferred that the films of coalesced silver nanoparticles decrease the reflectance of crystalline silicon better than particles separated by large distances. These results are discussed in terms of forward scattering of large metallic nanoparticles where higher order multipolar modes are dominant and the retardation effects are very important.     

Biosynthesis of copper oxide nanoparticles using Enicostemma axillare (Lam.) leaf extract

In the present study copper oxide nanoparticles (CuONPs) were synthesized via simple and eco-friendly green route using leaf extract of Enicostemma axillare (Lam.). Characterization of synthesized nanoparticles (NPs) was undertaken. The characteristic absorption peak of CuONPs was in range 264nm in UV–Vis spectrum. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed the morphological and structural character of green NPs. The mean particle size was calculated to 30nm. Energy dispersive spectroscopy (EDS) showed high intense metallic peak of copper (Cu), oxygen (O) and low intense peaks of carbon (C), sulfur (S), phosphorus (P) elements due to the capping action of biomolecules of plant extract in CuONPs formation. The X-ray diffraction (XRD) pattern showed distinctive peaks corresponding to (200), (211) and (310) planes revealing the high crystalline nature of synthesized CuONPs with a primitive phase. Zeta potential and size distribution of synthesized green NPs was concluded by Dynamic light scattering (DLS) studies.     

Optical Modeling of Plasmonic Nanoparticles Enhanced Light Emission of Silicon Light-Emitting Diodes

Significant enhancement of radiative efficiency of thin-film silicon light-emitting diodes achieved by placing the active layer in close proximity to silver (Ag) nanoparticles has been observed. In this paper, optical properties including transmission, reflection, and absorption of a random assembly of Ag nanoparticles are theoretically investigated using the effective medium model. Furthermore, the influence of Ag nanoparticles on light emission of silicon light-emitting diodes is studied by an improved effective mode volume model we propose here. The normalized line shape of dipole oscillation is calculated directly using Lorentz–Drude model without using any approximation. Thus, it results in more accurate calculation of the enhanced Purcell factor in comparison with the conventional approach. We show that an enhancement of radiative efficiency of silicon light-emitting diodes can be achieved by localized surface plasmons on metal nanoparticles. The calculated result of optimal Ag nanoparticle size to enhance light emission of silicon light-emitting diodes at 900 nm wavelength is in very good agreement with those obtained from the experimental result. The model is useful for the design of metallic nanoparticles enhanced light emitters.     

Absorption Enhancements in Plasmonic Solar Cells Coated with Metallic Nanoparticles

We find that three mechanisms lead to the absorption enhancements of light in a thin-film amorphous silicon solar cell coated with a periodic array of silver nanoparticles on the rear surface according to our simulation. They are localized surface plasmon modes of the silver nanoparticles, Fabry–Pérot resonant cavity modes and waveguide effects. Each enhancing mechanism can yield a maximum absorption enhancement of over two times at the corresponding resonant wavelengths when the nanoparticles cover 20 % of the solar cell surface, and an average absorption enhancement of up to 57 % can be achieved in the AM 1.5 G solar spectrum. The absorption enhancements can also be tuned in spectrum to optimize the total absorption in a plasmonic solar cell.     

Plasmonic Spectral Detection of Carcinoembryonic Antigen by Preventing the Direct Binding of Rhodamine 6G with Au Nanoparticles

Carcinoembryonic antigen (CEA) was used as a separator to prevent the Rhodamine 6G (R6G)-induced aggregation of colloidal gold nanoparticles. The destroyed aggregation has been monitored by measuring the absorption and resonance light scattering peaks corresponding to the longitudinal surface plasmon resonance (SPR) of the chain-like aggregated gold nanoparticles (AuNPs). It was found that the pre-adding of CEA with different concentrations to the gold colloids before mixing them with R6G could lead to the longitudinal SPR peak decrease and blue shift. By analysing the intensity changing and wavelength shifting of the absorption spectra, CEA could be detected in a linear range from 0.2 to 4 ng/mL, and the limit of detection reaches to 0.1 ng/mL. The sensitivity of the CEA concentration dependent shifting and quenching of the plasmonic absorption and scattering corresponding to the AuNPs aggregation presents a well potential application of biologic spectral sensing.     

Simultaneous Excitation of Surface Plasmon Polaritons on Both Interfaces in an Asymmetric Dielectric-Metal Corrugated Structure

The simultaneous excitation of plasmon polaritons on both surfaces of metal film was studied for asymmetric dielectric-metal-dielectric corrugated structures. Due to the small resonant absorption of the incident light on the transmission side of the structure, we investigated the enhancement of the surface plasmon polaritons on the mentioned side by controlling the structure parameters. When the illuminate light changes from normal incidence to non-normal incidence, the resonant absorption peak splits into a doublet. The simultaneous excitation of surface plasmon polaritons on both surfaces of the metal film can be achieved by controlling the incident angle. Since the wave vector matching condition is not satisfied, there is no coupling between the plasmon polaritons modes on the two surfaces of the corrugated metallic film. The excitation and control of the non-coupled surface plasmon polartions simultaneously propagating on the different interfaces of one metallic film have potential applications for designing novel compact and tunable nano-photonic devices at visible frequency.     

SERS Properties of Different Sized and Shaped Gold Nanoparticles Biosynthesized under Different Environmental Conditions by Neurospora crassa Extract

Surface-enhanced Raman scattering (SERS) is a surface-sensitive technique that enhances Raman scattering by molecules adsorbed on rough metal surfaces. It is known that metal nanoparticles, especially gold and silver nanoparticles, exhibit great SERS properties, which make them very attractive for the development of biosensors and biocatalysts. On the other hand, the development of ecofriendly methods for the synthesis of metallic nanostructures has become the focus of research in several countries, and many microorganisms and plants have already been used to biosynthesize metallic nanostructures. However, the majority of these are pathogenic to plants or humans. Here, we report gold nanoparticles with good SERS properties, biosynthesized by Neurospora crassa extract under different environmental conditions, increasing Raman signals up to 40 times using methylene blue as a target molecule. Incubation of tetrachloroauric acid solution with the fungal extract at 60°C and a pH value of a) 3, b) 5.5, and c) 10 resulted in the formation of gold nanoparticles of a) different shapes like triangles, hexagons, pentagons etc. in a broad size range of about 10-200 nm, b) mostly quasi-spheres with some different shapes in a main size range of 6-23 nm, and c) only quasi-spheres of 3-12 nm. Analyses included TEM, HRTEM, and EDS in order to corroborate the shape and the elemental character of the gold nanoparticles, respectively. The results presented here show that these ‘green’ synthesized gold nanoparticles might have potential applicability in the field of biological sensing.     

Peptide-conjugated gold nanorods for nuclear targeting

Resonant electron oscillations on the surface of noble metal nanoparticles (Au, Ag, Cu) create the surface plasmon resonance (SPR) that greatly enhances the absorption and Rayleigh (Mie) scattering of light by these particles. By adjusting the size and shape of the particles from spheres to rods, the SPR absorption and scattering can be tuned from the visible to the near-infrared region (NIR) where biologic tissues are relatively transparent. Further, gold nanorods greatly enhance surface Raman scattering of adsorbed molecules. These unique properties make gold nanorods especially attractive as optical sensors for biological and medical applications. In the present work, gold nanorods are covalently conjugated with a nuclear localization signal peptide through a thioalkyl-triazole linker and incubated with an immortalized benign epithelial cell line and an oral cancer cell line. Dark field light SPR scattering images demonstrate that nanorods are located in both the cytoplasm and nucleus of both cell lines. Single cell micro-Raman spectra reveal enhanced Raman bands of the peptide as well as molecules in the cytoplasm and the nucleus. Further, the Raman spectra reveal a difference between benign and cancer cell lines. This work represents an important step toward both imaging and Raman-based intracellular biosensing with covalently linked ligand-nanorod probes.     

Tuning of Light Trapping and Surface Plasmon Resonance in Silver Nanoparticles/c-Si Structures for Solar Cells

In this work, we investigate silver (Ag) nanoparticle-related plasmonic effect on light absorption in Si substrate. Ag nanoparticles (Ag-NPs) deposited on top of Si were used to capture and couple incident light into these structures by forward scattering. We demonstrate that we can control nanoparticle size and shape while varying deposition time and annealing parameters. By the increase of the total time of the reaction process, morphology of Ag-NPs evolutes affecting the number and the width of surface plasmon resonance peaks, whereas for changed annealing parameters (temperature and time), the effect is more pronounced on the broadening and the position of peaks. Specific morphology of Ag-NPs can exhibit an interesting enhancement of optical properties which enables plasmon-related application in photovoltaic solar cells.     

Influence of Plasmonic Silver Nanoparticles on Fluorescence Quenching of 1,4-dihydroxy-3-methylanthracene-9,10-dione

Optical absorption and fluorescence emission techniques were employed to investigate the size effects of silver nanoparticles (Ag NPs) on 1,4-dihydroxy-3-methylanthracene-9,10-dione (DHMAD). Silver nanoparticles of different sizes were prepared by Creighton method under microwave irradiation. The prepared Ag NPs show the surface plasmon band around 400 nm. Fluorescence quenching of DHMAD by Ag NPs was found to increase with an increase in the size of Ag NPs. The fluorescence quenching is explained by resonant energy transfer mechanism between DHMAD and Ag NPs, orientation of DHMAD on silver nanoparticles through chemisorptions. The Stern–Volmer quenching constant and Benesi–Hildebrand association constant for the above system were calculated. DFT calculations were also performed to study the ground and excited state behavior of DHMAD and DHMAD + Ag system.