Silver Surface Plasmon Resonance Research Articles

Investigation of cladding thicknesses on silver SPR based side-polished optical fiber refractive-index sensor

Abstract A single mode optical fiber modified using side-polishing method is applied as a sensor based on surface plasmon resonance (SPR) principles. The SPR sensor was designed using side-polished optical fiber of which the cladding was symmetrically removed and coated with different thicknesses of embedded silver film. The amount of cladding removed was based on the insertion power loss during the polishing process, where losses were recorded at 0.65 dB corresponding to 20 μm thickness of remaining cladding and 1.8 dB refers to no cladding respectively. Finite Difference Time Domain (FDTD) simulation was used to investigate the effects of this configuration. The system has been constructed using different refractive indices of liquid. Silver thin layer thickness of 40 nm found to be the most desirable after it display better sensitivity in sensing mechanism. The application of 40 nm-thick Ag has been also coated on the fiber with no cladding, which shows higher sensitivity of ∼2166 nm/refractive index unit (RIU) and 208.333 nm/refractive index unit (RIU) using distilled water (n = 1.333) and alcohol (n = 1.345), respectively. The SPR dip transmission wavelength was recorded as ∼460 nm and ∼530 nm for both fiber conditions at active sensing area as 3 mm length operating at wavelength range of 300–1100 nm. The system has the advantages of being low-cost and applicable in bio-sensors.

Effects of TiO2 on the performance of silver coated on side-polished optical fiber for alcohol sensing applications

Abstract This work reports the experimental and numerical investigations on the effect of titanium oxide (TiO2) integrated on the optical fiber silver (Ag) surface plasmon resonance (SPR) sensors. In this paper, the numerical simulation is also analyzed using the finite element method (FEM) which shows good agreement. Results are compared with the experimental findings, focusing on the SPR phenomena for refractive index sensing using side-polished optical fiber, where the characteristics of the Ag layer with the integration of TiO2 are optimized. The essential parameters include the thickness of Ag namely 20 nm, 30 nm and 40 nm for optimization studies of SPR properties. TiO2 was then coated on the Ag thin layer to sense isopropanol at different concentration. It is shown that the combination of a 30 nm thickness of Ag layer with a dielectric TiO2 improves sensor performance in terms of sensitivity reading of 268 n m / R I U with a detection limit of 0.012. The main interest is to develop the device with cost effective, ease fabrication and less cumbersome of sensors using TiO2 which one of an alternative material to defeat the oxidation process and avoid charge recombination. If the sensitivity increases at a higher wavelength, it indicates that the TiO2 holds great potential in photonic applications.

Geranium wallichianum Leaf Extract Mediated Synthesis of Silver Nanoparticles: Characterization and its Antimicrobial Activity

In present study, the Geranium wallichianum leaf extract (aqueous) and 1 mM silver nitrate solution (aqueous) were amalgamated to synthesize silver particles within the nanometer range. Initial colour changes and surface-plasmon-resonance (SPR) absorbance band observed in UV-visible spectroscopic study gave support to configuration of silver nano-particles. Silver nanoparticles were characterized by XRD, TEM and FTIR spectroscopy. Then antimicrobial (antibacterial and antifungal) activities of silver nanoparticles against selected microbial strains were studied by well diffusion method. During this work, silver SPR absorption bands between 405-425 nm range were shown by Geranium wallichianum leaf extract mediated synthesized silver nanoparticles sample solutions at different intervals. X-ray diffraction pattern displayed the formation of face centred cubic phase silver nano-structures. TEM measurements confirmed that silver nanoparticles are spherically shaped and maximum particles in 9-16 nm size range having average diameter 12.5 nm. Important FT-IR peaks at 3314.11, 1710.60, 1587.11 and 1347.77 cm-1 were predicted for hydroxyl, carbonyls, unsaturated C-C bonds and phenolic groups respectively. Further from antimicrobial results, it has been found that values of diameter of zone of inhibition (mm) of synthesized silver nanoparticles against B. subtilis, S. aureus, L. plantarum, P. aeruginosa, A. niger and C. albicans strains were 32, 28, 25, 25 35 and not active, respectively. MIC-MLC (μL) values were also determined. At last, it can be concluded that antimicrobial agents of 9-16 nm size range and stabilized by polyhydroxy-bioactive-components present in extract, in the form of silver nanoparticles has been productively synthesized.

Size dependent optical properties of ZnO@Ag core/shell nanostructures

In this paper, we studied the effect of size and thickness variation on the optical properties of a system that consists of spherical ZnO@Ag core/shell composite nanostructures embedded in a dielectric host matrix. The effective dielectric function, polarizability, refractive index, and absorbance of the composite nanostructures are determined using the Maxwell-Garnett effective medium theory within the framework of the electrostatic approximation. The numerical simulations using nanoinclusions of radii 20 nm show interesting behavior in the optical responses of the ensemble. In particular, it is shown that for different values of metal fraction and filling factor, the polarizability, refractive index, and optical absorbance of the ensemble exhibit two sets of resonance peaks in the UV (around 300 nm) and visible (between 400 and 640 nm) spectral regions. These peaks are attributed to the surface plasmon resonance of silver at the ZnO/Ag and Ag/host-matrix interface. Moreover, when the Ag shell thickness is increased, the observed resonance peaks are enhanced; accompanied with slight red shifts in the UV and blue shifts in the visible regions. The results obtained may be used in various applications such as sensors and nano-optoelectronics devices in optimizing material parameters to the ‘desired’ values.

Effect of Additives of Pt, Pd, Ag, and Y in Thin Nanocrystalline SnO2 Films on the Characteristics of Resistive Hydrogen Sensors

The results of studying electrical and gas sensitive characteristics of H2 sensors based on thin nanocrystalline SnO2 films with the Pt, Pd, and Ag dispersed layers deposited on the surface and Ag, Y, and Ag + Y additives in the volume are presented. It is shown that various combinations of catalysts on the surface and in the volume have a significant effect on the microstructure of films and density of oxygen adsorption sites on the surface of tin dioxide. As a result, the resistance values of sensors in clean air R0, activation energies of the temperature dependences of R0, responses to hydrogen in the concentration range of 50–2000 ppm are different. Studies performed by the method of UV-visible spectroscopy revealed in the absorption spectra of films with a silver addition in the volume a band of surface plasmon resonance of silver indicating that the additive is present in the form of metallic Ag nanoparticles. Particular attention was paid to the influence of long-term tests on the properties of sensors with the listed additives. It was established that the joint introduction of Ag+Y into the volume of films prevents the increase of the resistance and responses during prolonged exposure to hydrogen, which is observed during the operation of all other samples studied. Possible mechanisms for changing the sensors’ properties during testing and the role of additives in their stabilization are considered.

On the Ability of Low Molecular Weight Chitosan Enzymatically Depolymerized to Produce and Stabilize Silver Nanoparticles.

Silver nanoparticles (AgNPs) are of great interest due to their antimicrobial, optical and catalytical properties. Green synthesis of AgNPs is fundamental for some applications such as biomedicine and catalysis. Natural polymers, such as chitosan, have been proposed as reducing and stabilizing agents in the green synthesis of AgNPs. Physico-chemical properties of chitosan have a great impact on its technological and biological properties. In this paper, we explore the effect of chitosan molecular weight (Mw) on the thermal AgNPs production using two sample sets of low Mw chitosans (F1 > 30 kDa, F2: 30–10 kDa and F3: 10–5 kDa) produced by enzymatic depolymerization of a parent chitosan with chitosanase and lysozyme. Both polymer sets were able to effectively reduce Ag+ to Ag0 as the presence of the silver surface plasmon resonance (SRP) demonstrated. However, the ability to stabilize the nanoparticles depended not only on the Mw of the polymer but particularly on the polymer pattern which was determined by the enzyme used to depolymerize the parent chitosan. Low Mw chitosan samples produced by lysozyme were more effective than those produced by chitosanase to stabilize the AgNPs and smaller and less polydisperse nanoparticles were visualized by transmission electron microscopy (TEM). With some polymer sets, more than 80% of the AgNPs produced were lower than 10 nm which correspond to quantum dots. The preparation method described in this paper is general and therefore, it may be extended to other noble metals, such as palladium, gold or platinum.

Development of a surface plasmon resonance acetone sensor for noninvasive screening and monitoring of diabetes

Diabetes can cause many complications in the human body. The disease is commonly diagnoses invasively using blood glucose test. A good correlation between blood glucose and exhale breathe acetone has opened a way for non-invasive diagnosis through the detection of the breath acetone from the exhale breathe. However, the conventional means of detecting exhale breathe acetone (e.g. GC-MS) are bulky, require technical knowhow, do not offer real time measurement and found only at advanced institutions that can afford the equipment. This work is aimed at developing a real time, sensitive and selective optical based Surface Plasmon Resonance (SPR) breath acetone biosensor for non-invasive monitoring and screening of diabetes. The motivation of this work arises from the promising advantages of optical based biosensors over other biosensors. Additionally, a novel doped polyaniline (PANI) conducting polymer would be the SPR metallic layer while Chitosan would be the selective sensing layer. Mathematical modeling and SPR simulation has been conducted. The result has shown that the replacement of conventional gold or silver SPR layer with doped Polyaniline (PANI) SPR layer provides better promising sensitivity at mid infrared wavelength range. Therefore, additional inclusion of chitosan layer on the proposed SPR based biosensor would give rise to a sensitive and selective breathe acetone biosensor for easy monitoring and screening of diabetes.

Biomedical Potentialities of Taraxacum officinale-based Nanoparticles Biosynthesized Using Methanolic Leaf Extract.

In the present study, the potential of methanolic leaf extract of Taraxacum officinale plant as a function of bio-inspired green synthesis for the fabrication of silver nanoparticles (AgNPs) has been explored. The bio-reduction of aqueous silver nitrate (AgNO3) solution was confirmed by visually detecting the color change from pale yellow to blackish-brown. Maximum absorbance was observed at 420 nm due to the presence of characteristic surface Plasmon resonance of nano silver by UV-visible spectroscopy. The role of various functional groups in the bio-reduction of silver and chemical transformation was verified by Fourier transform infrared spectroscopy (FTIR). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) predict the shape (rocky, flack type, ellipsoidal, etc.), size (68 nm) and elemental composition (Ag as a major constituent) of the biosynthesized AgNPs, respectively. Transmission electron microscopy (TEM) analysis further corroborated the morphology of the AgNPs. Color mapping and atomic force microscopy (AFM) confirmed the nano-sized topography. The dynamic light scattering (DLS) showed the charge, stability, and size of the AgNPs. The generated AgNPs presented potential antibacterial activities against Gram-positive and Gram-negative bacterial strains including Staphylococcus aureus, Escherichia coli, and Haemophilus influenzae. The biosynthesized AgNPs also showed antiproliferative activity against MCF-7 breast cancer cell line in a dosedependent manner. In conclusion, results clearly indicate that biosynthesized AgNPs could be used as effective nano drug for treating infectious diseases caused by multidrug resistant bacterial strains in the near future.

Tuning local surface plasmon resonance of silver and photoluminescence intensity enhancement by adding copper

In order to match the local surface plasmon resonance (LSPR) frequency of Ag with the frequency of light that needs enhancement, the relationship between the free carrier concentration and LSPR frequency must be considered in order to design noble alloy nanoparticles (NPs) to substitute for pure Ag. In this study, Ag, Cu, and Ag-Cu continuous films were prepared by magnetron sputtering to investigate the relationship between the free carrier concentration and LSPR wavelength, which were tested with a Hall effect detector and ultraviolet-visible spectrophotometer, respectively. Ag and Ag-Cu NP films were deposited by short-time magnetron sputtering on an organic-YAG:Ce3+ substrate to test and compare their effects on the photoluminescence (PL) intensity. The thickness of the metal films was determined using an optical thin film analysis system. The morphology was observed for the continuous and NP films by scanning electron microscopy. Compared with the pure Ag films, the Ag-Cu films had a lower free carrier concentration and longer peak wavelength, which was closer to the emission wavelength of YAG:Ce3+. The enhancement ratio for the PL intensity reached a maximum of 1.16 with the Ag NP films. By contrast, for the Ag-Cu NP films with a shorter deposition time, the PL enhancement ratio reached 1.125, where the optimal ratio of Ag to Cu for the product of sputtering power and time was 2:1. The addition of Cu to Ag is beneficial for industrial production because the cost of the materials and the preparation time for Ag-Cu alloy NP films are more than 50% lower compared with pure Ag NP films.

The formation of visible light-driven Ag/Ag2O photocatalyst with excellent property of photocatalytic activity and photocorrosion inhibition

Ag/Ag2O composites were prepared via an extremely simple solvent-free chemical technique. The obtained Ag/Ag2O samples exhibited superior visible-light degradation efficiency for methyl orange removal. The degradation rate constant of as-obtained composite is almost 2 times as that of commercial Ag2O, and has only little change after five cycles of utilization. The excellent photocatalytic properties of Ag/Ag2O materials could be ascribed to desirable absorption property and surface plasmon resonance of metal silver on the surface of semiconductor Ag2O. Meanwhile, the unstable and photo-sensitive disadvantages of commercial Ag2O due to the photocorrosion could be largely suppressed in the presence of metallic Ag for Ag/Ag2O composites, which plays an important role as an electron sink to receive the photo-generated electrons from Ag2O, accordingly restraining the reduction of Ag+ in Ag2O and improving the recyclability of Ag2O. With the assistance of scavengers and ESR technique, the h+ and O2− were proved to be crucial reactive species in the pathways of photocatalytic degradation reaction.

Microwave-assisted green synthesis and antimicrobial activity of silver nanoparticles derived from a supercritical carbon dioxide extract of the fresh aerial parts of <i>Phyllanthus niruri</i> L

Purpose: To synthesize and evaluate the antimicrobial activity of silver nanoparticles (AgNPs) derived from a supercritical carbon dioxide extract of the fresh aerial parts of Phyllanthus niruri. Methods: The synthesis of AgNPs of a P. niruri extract was carried out in a microwave oven. The extraction was carried out using a supercritical fluid extractor. The AgNPs were characterized by the Ultraviolet-visible (UV-vis) spectral analysis, Dynamic Light Scattering (DLS) zetasizer analysis, Transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis and Fourier transform infrared (FT-IR) spectroscopy. The antimicrobial assays of AgNPs were carried out against different bacterial and fungal strains. Results: Results of various analytical techniques confirmed the synthesis of AgNPs of a P. niruri extract. The UV–vis spectroscopy showed an intense silver surface plasmon resonance band at 415 NM. The AgNPs had a mean size of 110 nm in the Zetasizer analysis. TEM images illustrated spherical AgNPs having a mean particle size of 110 nm. The X-ray diffractograms showed peaks at 38.17°, 44.28°, and 64.52°. The average crystallite size of Ag-NPs was found to be 110 nm. FT-IR spectra confirmed the stability of the AgNPs. The AgNPs demonstrated good antimicrobial effects against several tested pathogenic microbes. Conclusion: An efficiently synthesized AgNPs of P. niruri (SC-CO2) extract has been prepared by a simple, eco-friendly, cost-effective, rapid green chemistry methodology. The AgNPs of P. niruri extract possesses significant antimicrobial properties against the tested bacterial and fungal strains. Keywords: Nanoparticles, Phyllanthus niruri, Supercritical fluid extraction, Microwave, Antimicrobial activity

The development of the anti-biofouling coating agent using metal nanoparticles and analysis by Raman spectroscopy and FIB system

Thus far, biofouling on material surfaces has been investigated from various viewpoints, and concrete methods for its evaluation have been proposed. Biofilms, which are formed by bacterial activities, are essential components of biofouling. They play an important role in the occurrence of biofouling on the materials of structures, which leads to serious economic loss. Therefore, the establishment of countermeasures for biofilm formation and its control are needed urgently. In addition, appropriate evaluation techniques to measure biofilm formation are required. Thus far, we have used scientific evaluation devices and techniques such as an optical microscope and Raman spectroscopy to evaluate biofouling/biofilms. In this study, we used the silane coating of dispersed silver and copper nanopowder rather than that of dispersed organometals. Specimens were immersed into a laboratory biofilm reactor. Then, the biofouling behavior on the sheets was investigated using an optical microscope, Raman spectroscopy, a focused ion beam system (FIB), and scanning electron microscopy (SEM). The optical-microscope observations showed many deposits on the specimens without any metallic dispersions. Although the amount of deposits on the specimens decreased with copper dispersions, many deposits were still observed on them with silver dispersion. These deposits were not confirmed by Raman spectroscopy because of the surface plasmon resonance of silver. Next, we observed and analyzed the specimens using the FIB system (combination of FIB–SEM and EDX) instead of Raman spectroscopy. The SEM image showed that the deposition had formed pores and irregularities on the specimens. FIB–SEM confirmed the existence of deposition on coating layers and revealed that it originated from biooraganisms. Based on these experiments, we could conclude that the structure of antifouling coating could be observed and analyzed using a combination of FIB–SEM and EDX even when other analysis methods are not useful because of various reasons.

BIOMIMETIC SYNTHESIS, CHARACTERIZATION AND EVALUATION OF ANTIOXIDANT, ANTIMICROBIAL EFFICACY OF SILVER NANOPARTICLES USING ANDREDERA CORDIFOLIA LEAF EXTRACT

Objective: The present study is focussed on the biosynthesis of silver nanoparticles using aqueous extract of Andredera cordifolia and to investigate the free radical scavenging potential, antimicrobial activity of the nanoparticles against different human pathogens.Methods: The formation of silver nanoparticles was indicated by the colour change from colourless to reddish brown. Biosynthesized nanoparticles was characterised using several techniques, viz- UV-Vis spectroscopy, FT-IR, XRD, TEM, SEM and EDAX analysis. The free radical scavenging potential was measured by DPPH, FRAP assay, antimicrobial activity against six microorganisms was tested using disc diffusion method. Results: UV-Vis spectral analysis showed silver Surface Plasmon Resonance band at 426 nm. The crystalline morphology and size of the nanoparticles were determined by TEM, SEM, and X-ray diffraction studies which showed the average size of the nanoparticles in the range 40 –60nm. The biologically synthesized nanoparticles efficiently inhibited pathogenic organisms such as E. coli, Staphylococcus aureus, Klebsiella Pneumonia, Pseudomonas aeruginosa and Proteus vulgaris. The biosynthesized nanoparticles might serve as a potent antioxidant as revealed by DPPH assay and FRAP assay. Conclusion: The biosynthesis of silver nanoparticles had several advantages in pharmaceutical applications as well as large scale commercial production.Key Words: Silver nanoparticle, Andredera cordifolia leaf, SEM,TEM,XRD,antioxidant, antimicrobial,etc.

Effect of the Al2O3 addition on the formation of silver nanoparticles in heat treated soda-lime silicate glasses

We investigate the formation and evolution of silver nanoparticles in the non-irradiated thermally treated glass system Ag:(37–x)Na2O–10CaO–xAl2O3–53SiO2mol%, where x=0.0, 0.5, 1, 2 and 5, doped with 0.2mol% AgNO3. Using optical absorption and transmission electron microscopy we studied the effect of the gradual addition of Al2O3 to the chemical composition of the soda aluminosilicate glasses. A quantitative method for the evaluation of the relative nanoparticles concentration, proportional to the optical absorption intensity of the silver Surface Plasmon Resonance band, was developed and applied to the study of the behavior of the number of particles precipitated in samples thermally treated at temperatures of 400, 500, and 600°C. Three mechanisms were found, as consequence of the substitution of SiO4 by AlO4 glass forming local tetrahedral structural units, resulting in the gradual suppression of the negatively charged NBOs−, which are the only reducing sources available in the present non-irradiated glass system. The effects produced by the glass chemical and structural changes on the Ag nanoparticles production are discussed.

Biosynthesis of Silver Nanoparticles using Cinnamon zeylanicum Plants Bark Extract

Nanoparticles are a special group of materials with unique features and extensive applications in diverse fields. The use of nanoparticles of some metals is common and useful in several fields due to the good properties of these nanoparticles. In this work, silver nanoparticles synthesized by extract of Cinnamon zeylanicum plants barks were investigated. 1ml of Cinnamon zeylanicum plants barks extract was added to 50 ml of 1mM aqueous silver nitrate (AgNO3) solution at the room temperature for (1hr -8hrs) to produce silver nanoparticles. A change in the color was observed. The color intensity increased with increasing reaction time. After 2 hrs, the color intensity was even higher; and giving a darker looks after 8 hrs. Separation by centrifugation, characterization using UV-VIS spectrophotometry and electron microscopy analysis were performed. The centrifugation lead to formation of 0.35 gm silver nanoparticles by 100 ml of Cinnamon zeylanicum plants barks extract. The UV-Vis spectral analysis showed silver surface plasmon resonance band at 400 nm. Electron microscopy showed that the particles were crystalline in nature, most of the nanoparticles were roughly spherical or circular in shape with smooth edges were observed, while the average size of the nanoparticles ranged between 8 and 20 nm. The approach of green synthesis seems to be cost effective, eco-friendly and easy alternative to conventional methods of silver nanoparticles synthesis.

Green Extracellular Synthesis of the Silver Nanoparticles Using Thermophilic Bacillus Sp. AZ1 and its Antimicrobial Activity Against Several Human Pathogenetic Bacteria.

Silver nanoparticles (AgNPs) are among the most effective antimicrobial agents that are used in the medicine and pharmaceutics. During the past decades, metal nanoparticles synthesis through application of the biological methods has increasingly been used, as the biologically synthesized particles are mostly non-toxic as well as effective. The main goal for undertaking the present investigation was to evaluate the extracellular synthesis of the AgNPs by a native thermophilic Bacillus Sp. AZ1 that was isolated from a hot spring in Ardebil province. Subsequently the antimicrobial potentials of the nanoparticle was evaluated against several human pathogenic organisms. The biosynthesized AgNPs were confirmed visually by appearance of a dark brown color formation in the mixture as well as silver surface plasmon resonance band by using UV-Visible spectroscopy. The AgNPs were further characterized by SEM, EDX and TEM. The antimicrobial activity of the AgNPs was investigated using Salmonella typhi, Escherichia coli, Staphylococcus epidermis, and Staphylococcus aureus, by applying disk diffusion method. Identification of the strain AZ1 by the 16S rRNA sequence analysis showed 99% sequence homology between this strain and B. licheniformis. The obtained UV-Visible spectrum of the aqueous medium containing silver ion, showed a peak at 425 nm which indicates a correspondence to the plasmon absorbance of the silver nanoparticles. The biosynthesized AgNPs were found to be in the size range of ~7-31 nm with spherical the shape. Studies regarding the antibacterial effect of the particles showed the highest inhibitory effect against the two strains; E. coli, and S. typhi, respectively. Our study presents a simple green synthesis process for the production of an extracellular nanoparticles which is environmental friendly. Biosynthesis of the AgNPs by a thermophilic bacillus from the hot spring (Qeynarjeh, Ardebil) in Iran with the highest similarity to Bacillus licheniformis is reported for the first time.

Optical and Vibrational Properties of Ag Nanometal-Enhanced Blue Light-Emitting Graphene Oxide-Polyvinylpyrrolidone Polymer Composite Films

We report ex situ synthesis, optical, phonon, and photoluminescence (PL) properties of silver nanoparticles (AgNPs) decorated on graphene oxide (GO) and polyvinylpyrrolidone (PVP) composite polymer matrix. Optical absorption studies show a band at 435 nm which is due to the surface plasmon resonance (SPR) of silver (Ag) and the full width half maximum (FWHM) of SPR band is found to be decreased with Ag concentration suggesting an increase in particle size. Raman spectra exhibit a D (disorder band) and a G (graphitic) band of GO with a red shift in G band with increasing Ag precursor concentration. The mechanism for the red shift is explained by the charge transfer between GO and AgNPs. PL spectra shows a shift towards longer wavelength as AgNPs size increases which is due to the overlap of interband of PVP and the SPR of AgNPs.

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.

Surface plasmon resonance of silver and gold nanoparticles in the proximity of graphene studied using the discrete dipole approximation method.

The integration of silver and gold nanoparticles with graphene is frequently sought for the realization of hybrid materials with superior optical, photoelectric and photocatalytic performances. A crucial aspect for these applications is how the surface plasmon resonance of metal nanoparticles is modified after assembly with graphene. Here, we used the discrete dipole approximation method to study the surface plasmon resonance of silver and gold nanoparticles in the proximity of a graphene flake or embedded in graphene structures. Surface plasmon resonance modifications were investigated for various shapes of metal nanoparticles and for different morphologies of the nanoparticle-graphene nanohybrids, in a step-by-step approach. Calculations show that the surface plasmon resonance of Ag nanoparticles is quenched in nanohybrids, whereas either surface plasmon quenching or enhancement can be obtained with Au nanoparticles, depending on the configuration adopted. However, graphene effects on the surface plasmon resonance are rapidly lost already at a distance of the order of 5 nm. These results provide useful indications for characterization and monitoring the synthesis of hybrid nanostructures, as well as for the development of hybrid metal nanoparticle/graphene nanomaterials with desired optical properties.

On the applicability of discrete dipole approximation for plasmonic particles

It has been recognized that the commonly used discrete dipole approximation (DDA) for calculating the optical properties of plasmonic materials may exhibit slow convergence for a certain region of the complex refractive index. In this work we investigate the quantitative accuracy of DDA for particles of different shapes, with silver as the plasmonic material. As expected, the accuracy and convergence of the method as a function of the number of dipoles is relatively good for solid spheres and rounded cubes whose size is of the same order as the wavelength of the localized surface plasmon resonance in silver. However, we find that for solid particles much smaller than the resonance wavelength, and for silver–silica core-shell particles in particular, DDA does not converge to the correct limit even for 106 dipoles. We also find that the slow convergence tends to be accompanied by strong, discretization dependent oscillations in the particle׳s internal electric field. We demonstrate that the main factor behind the slow convergence of the DDA is due to inaccuracies in the plasmonic resonances of the dipoles at the surface of the particles.