Visible Extinction Spectra Research Articles

Optical Simulations of Self-assembly Relevant Gold Aggregates: A Comparative Study

In this study, visible light extinction spectra of different gold nanoparticle assemblies were simulated using boundary element method (BEM) in order to investigate the optical properties of structures which determine the experimentally measured spectra during the self-assembly of the particles. Numerous different optically dominant particle arrangements can evolve depending on the underlying physicochemical mechanism of the clustering process itself: one-dimensional chains, two-dimensional arrays or three-dimensional clusters can be formed in the solutions or at interfaces. Experimentally the aggregation or clustering of gold nanoparticles can be conveniently followed by spectroscopic techniques due to the plasmon coupling related profound effect of particle aggregation on the visible extinction spectrum. However, the measured spectra usually contain the optical response of various aggregate structures. Additionally, small changes of the interparticle distance can have a significant impact on the frequency of the coupled mode. In order to assess the contribution of the different structures to the experimentally measurable extinction spectra during particle clustering, different model structures (chains, array and 3D-cluster) have been simulated, where the distance between the particles was varied as well.

Trace-Level Detection of Secondary Explosives Using Hybrid Silver–Gold Nanoparticles and Nanostructures Achieved with Femtosecond Laser Ablation

Hybrid silver–gold targets were achieved by effortless mixing of pure silver (Ag) and gold (Au) metals at different ratios (Ag0.65Au0.35, Ag0.5Au0.5, and Ag0.35Au0.65) and embracing a manual melting process. The obtained targets were ablated by ultrafast (∼40 fs) laser pulses in acetone ensuing the fabrication of Ag–Au bimetallic nanoparticles (NPs) and nanostructures (NSs) in a single experiment. UV–visible extinction spectra of Ag–Au colloids demonstrated the tuning of localized surface plasmon resonance (LSPR) in the spectral range of 406–524 nm. The morphologies of NSs were investigated by the field emission scanning electron microscopy (FESEM) technique. Ag–Au NPs and NSs were utilized as surface enhanced Raman scattering (SERS) platforms to detect secondary explosive molecules such as 1,1-diamino-2,2-dinitroethene (FOX-7, 5 μM concentration) and 1-nitro pyrazole (1NPZ, 20 nM concentration). Our experimental observations clearly demonstrated that the increment in gold percentage reduced the surface a...

Kinetics of reaction of gold nanoparticles following partial removal of stabilizers

Citrate-stabilized gold nanoparticles (Au NPs) of 17-nm diameter were allowed to react following partial depletion of the stabilizer using dialysis. Kinetics of the reaction was investigated by following time-dependent changes in the visible extinction spectrum. Thus, surface plasmon resonance peak (SPR) of isolated Au NPs (reactant) at 522 nm decreased, while SPR peak due to product—which was agglomerated Au NPs—occurring at 600 nm increased with time. The reaction followed first-order kinetics with respect to concentration of reactant (Au NP) with a rate constant on the order of (2.10 ± 0.34) × 10−3 min−1. Further, product concentration correspondingly increased with time. Transmission electron microscopy investigation indicated the presence of individual NPs, along with agglomerated structures in the beginning of reaction—the extent of which increased with time, rather than the formation of smaller agglomerates. A model has been proposed based on reaction of individual NPs with agglomerated structures which accounted for the observed kinetics.

Synthesis and Growth Mechanism of Lead Sulfide Quantum Platelets in Lamellar Mesophase Templates

Thin (1 nm) PbS quantum platelets are prepared from lead acetate and thiourea in n-octylamine solvent at and near room temperature. Mechanistic studies establish that the quantum platelets are formed within lamellar amine-bilayer mesophase templates. UV–visible extinction spectra contain discrete features assigned to the n = 1 and n = 2 quantum-well transitions in the quantum platelets. The observed shifting of the spectral features to lower energies during growth of the quantum platelets is ascribed to increasing multi-quantum-well coupling between adjacent quantum platelets in the template stacks. Re-examination of a previously published higher-temperature synthesis of PbS nanosheets provides strong evidence for an analogous oleate-bilayer templated pathway. The lamellar mesophases investigated here undergo elastic expansion in the high-vacuum environment of the TEM, which appears to be a general phenomenon. Thus, lamellar spacings measured by TEM are larger than the actual spacings at ambient pressure.

Nanostructured and nanopatterned gold surfaces: application to the surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) has enormous potential for a range of applications where high sensitivity needs to be combined with good discrimination between molecular targets. However, the SERS technique has trouble finding its industrial development, as was the case with the surface plasmon resonance technology. The main reason is the difficulty to produce stable, reproducible, and highly efficient substrates for quantitative measurements. In this paper, we report a method to obtain two-dimensional regular nanopatterns of gold nanoparticles (AuNPs). The resulting patterns were evaluated by SERS. Our bottom-up strategy was divided into two steps: (a) nanopatterning of the substrate by e-beam lithography and (b) electrostatic adsorption of AuNPs on functionalized substrates. This approach enabled us to highlight the optimal conditions to obtain monolayer, rows, or ring of AuNPs, with homogeneous distribution and high density (800 AuNPs/μm2). The nanostructure distributions on the substrates were displayed by scanning electron microscopy and atomic force microscopy images. Optical properties of our nanostructures were characterized by visible extinction spectra and by the measured enhancements of Raman scattering. Finally, we tried to demonstrate experimentally that, to observe a significant enhancement of SERS, the gold diffusers must be extremely closer. If electron beam lithography is a very attractive technique to perform reproducible SERS substrates, the realization of pattern needs a very high resolution, with distances between nanostructures probably of less than 20 nm.

Localized Surface Plasmon Resonance of Cu@Cu2O core–shell nanoparticles: Absorption, Scattering and Luminescence

By co-deposition via RF-Sputtering and RF-PECVD methods and using Cu target and acetylene gas, we prepared Cu@Cu2O core–shell nanoparticles on the a-C:H thin film at room temperature. Mie absorption of Cu cores, scattering from Cu2O shell and luminescence that rises from carrier transfer in Cu@Cu2O interface were employed to fit the whole range of visible extinction spectrum of these core–shells. From simulation it was found that scattering and luminescence have an important effect on the energy, width and shape of LSPR absorption peak. Shift of LSPR peak is more affected by the dielectric coefficient of shell than Cu core size particularly for Cu core diameter above 4nm. Also, the LSPR absorption peak is damped by decreasing Cu core size and dielectric coefficient of shell. The energy of LSPR absorption peak is independent of shell thickness and host dielectric coefficient. The LSPR peak is damped by increasing shell thickness and host dielectric coefficient too. The scattering contribution in extinction spectra was affected more by shell size than dielectric coefficient. These points are important for detection techniques based on LSPR peak.

Multilayer Substrate‐Mediated Tuning Resonance of Plasmon and SERS EF of Nanostructured Silver

A thin-film of dielectric on a reflecting surface constituting a multilayer substrate modulates light intensity due to the interference effect. A nanostructure consisting of randomly oriented silver particles of different shapes, sizes, and interparticle spacings supports multiple plasmon resonances and is observed to have a broad extinction spectrum that spans the entire visible region. Combining the two systems by fabricating the nanostructure on the thin-dielectric film of the multilayer substrate yields a new composite structure which is observed to modulate both the extinction spectrum and the SERS EF (surface enhanced Raman scattering enhancement factor) of the nanostructure as the thickness of the thin-film dielectric is varied. The frequency and intensity of the visible extinction spectrum vary dramatically with the dielectric thickness and in the intermediate thickness range the spectrum has no visible band. The SERS EF determined for the composite structure as a function of the thin-film dielectric thickness varies by several orders of magnitude. Strong correlation between the magnitude of the SERS EF and the extinction intensity is observed over the entire dielectric thickness range indicating that the extinction spectrum corresponds to the excitation of the plasmon resonances of the nanostructure. A significant finding which has potential applications is that the composite structure has synergic effect to boost SERS EF of the nanostructure by an order of magnitude or more compared to the same nanostructure on an unlayered substrate.

SERRS and visible extinction spectroscopy of copper chlorophyllin on silver colloids as a function of pH

A study of sodium copper chlorophyllin adsorbed on silver colloids (CuChl/Ag) is conducted using surface-enhanced resonance Raman scattering (SERRS) and visible extinction spectroscopy to examine how the system changes as a function of pH. Initially at basic pH, SERRS signal is not detected even though CuChl is adsorbed onto the silver surface and deprotonated. Upon decreasing the solution pH slightly, colloidal aggregation is induced, evidenced by the broadening of the visible extinction spectra. The larger aggregates possess a surface plasmon that is in resonance with the laser excitation frequency (633-nm) and SERRS signal is detected. As the acid protonates CuChl, the overall negatively-charged surface approaches neutrality which induces more aggregation. Complete protonation of CuChl by pH 4.6 results in colloidal precipitation. However, when aggregation is halted about pH 5, adding NaOH( aq) to the system maintains the extent of aggregation and an intense SERRS signal is detected at basic pH.

Templated synthesis of gold–iron alloy nanoparticles using pulsed laser deposition

A means for synthesizing paramagnetic nanoparticles composed of anAu–Fe alloy is described using pulsed laser deposition (PLD) of thealloy into a mesoporous alumina membrane template. Nanoparticles46 ± 13 nm in diameter and composed of a 17% Fe alloy have been created bydepositing a 35% Fe alloy into a template with 65 nm diameter pores.These paramagnetic nanoparticles had a saturation magnetization of11.5 emu g−1 at2000 G, and their UV–visible extinction spectrum was dominated by strong absorption similar to thatof Fe3O4 nanoparticles. The surfaces of these nanoparticles were readilyfunctionalized with a dense monolayer of DNA oligonucleotides that had a5′ thiol group. The Au–Fe nanoparticles appear to be well suited for biotechnologicalapplications and single molecule measurements as they can be synthesized in a specific sizerange, are strongly paramagnetic, and may be easily functionalized with biologicalmacromolecules.

Coherent Light Scattering from Semicontinuous Silver Nanoshells near the Percolation Threshold

We report on measurements of visible extinction spectra of semicontinuous silver nanoshells grown on colloidal silica spheres. We find that thin, fractal shells below the percolation threshold exhibit geometrically tunable plasmon resonances. A modified scaling theory approach is used to model the dielectric response of such shells, which is then utilized to obtain the extinction cross section in a retarded Mie scattering formalism. We show that such spherical resonators support unique plasmon dynamics: in the visible there is a new regime of coherently driven cluster-localized plasmons, while crossover to homogeneous response in the infrared predicts a delocalized shell plasmon.

Preparation of spindle-shape silver core-shell particles

Silver core-shell nanoparticles with spindle shape were successfully synthesized by the seed-mediated growth technique. Small amount of silver nanoparticles (4±1.5 nm) were immobilized on polyethylene amine (PEI) coated spindle-shape β-FeOOH particles and served as ‘seeds’ for further growth of silver shell. The seed catalyzed growing process was repeated for 1 to 4 times, the surface coverage of the silver core-shell particles was increased until a complete silver layer was formed finally. UV–visible extinction spectra showed that two absorption peaks red-shifted with the increasing surface coverage and shell completeness. Our research showed that these spindle-shape silver core-shell particles were promisingly to be used as surface enhanced Raman scattering (SERS) nanosensors.

Linear aggregation of gold nanoparticles in ethanol

Gold colloidal particles were found to spontaneously organize into linear aggregates in ethanol. The morphology of these aggregates was characterized with transmission electron microscope (TEM) and atomic force microscope (AFM), which revealed the quasi one-dimensional (1D) chain-like shape. Immobilization of these chains from solution onto substrate surface as well as the measurement of UV–visible extinction spectrum was performed to investigate the aggregation mechanism. The results presented the evidence for the existence of the nanoparticle chains in solution. Moreover, the kinetic aspects of the chain-like aggregation were detailed studied using UV–visible spectroscopy. It is believed that dipole–dipole interaction is the driving force of nanoparticle linear aggregation. This finding provides a novel strategy for the construction of quasi 1D structures, which are suitable for the fabrication of solid-state devices based on single-electron tunneling.

Morphology of Silver and Gold “SERS Active” Substrates from Optical Spectroscopy Experiments and Numerical Simulations

In order to understand more deeply the surface enhanced Raman scattering (SERS) effect, we develop different models allowing the determination (i) of the morphology of silver or gold clusters in colloidal sols from their ultraviolet–visible extinction spectra and (ii) of the roughness features of silver islands chemically deposited on various metals from differential reflectivity measurements. For ultraviolet–visible absorption spectra, the model leads to calculated spectra of aggregated colloids in good agreement for both profiles and intensities with experimental ones and allows to approach the histogram of the clusters appearing in the aggregated colloid. This histogram reveals that the aggregated sols mainly consist of small spheres, reflecting unaggregated particles, and of elongated spheroids formed by the coalescence of several spherical particles. In the last part of this paper, we present preliminary results showing how to obtain “SERS active substrates” by chemically depositing silver islands onto various metals (Zn, Al, Cu). The observed SERS spectra are qualitatively related to the differential reflectivity spectra obtained on the same substrates.

Discrete dipole approximation for ultraviolet–visible extinction spectra simulation of silver and gold colloids

In order to understand more deeply the surface enhanced Raman scattering (SERS) effect, this article develops a model, based upon the simulation of the UV–visible extinction spectra which allows the determination of the morphology of metallic particles in silver and gold colloids either unaggregated or in any aggregation state. The main assumptions of this model are (i) light scattering by independent particles or clusters (objects) which enables us to express the total extinction cross section as a suitably weighted sum of cross sections of individual objects (targets); (ii) these targets are supposed to be compact and their individual cross sections are determined either from the Mie theory for spheres or from the discrete dipole approximation (DDA) for objects of any shape and size; (iii) the weight of each individual cross section is determined using a minimization process (simplex method) which looks for the best possible agreement between the experimental and calculated spectrum; (iv) lastly a simple calculation, based on the assumption of compact objects, provides the absorbance (optical density). In the case of unaggregated silver and gold colloids, this model gives a very good agreement between experimental and simulated extinction spectra thus leading to a particle size histogram which is consistent to that determined from transmission electronic microscopy (TEM) measurements. For aggregated colloids, an excellent agreement is still obtained between experimental and simulated band profiles; a slight discrepancy is observed between experimental and calculated intensities which might result from the tendency of DDA to underestimate the individual cross sections and/or from a lack of validity of the compact approximation. The clusters histogram deduced from the simulation process reveals small spheres (unaggregated particles) and elongated objects small compared to the visible light wavelength. This latter result is different from that obtained by TEM data in which the likely occurrence of a further aggregation leads to the observation of large aggregates.

Characterization and Surface-Enhanced Raman Spectroscopy of Alkali Metal Sols

Colloidal suspensions of Li, Na, K, Rb and Cs were prepared in diethyl ether. The effective sizes of particle aggregates were estimated by comparison of experimental visible extinction spectra with calculated spectra using the full equations of Mie scattering. The extinction spectra of Cs sols demonstrate unusual blue shifting as particle aggregation proceeds. The blue-shifted band is assigned to the transverse resonance of a linear string of Cs particles. Surface-enhanced Raman (SER) spectra are reported for Na, K and Rb sols. Observation of an OH stretching band (3585 cm−1) on the Na sol and growth of the SER bands as moisture is introduced to the Na and K sols strongly suggest that the bands derive from alkali metal hydroxides on the metal surfaces. When a trace of D2O is present in the Na sol a new SER band at 2640 cm−1 is observed, and is assigned to the OD stretch of surface NaOD. The Raman excitation spectrum for the 290 cm−1 SER band on Na is compared with the extinction band of the Na sol. The result is found to be in good qualitative agreement with the electromagnetic theory of surface-enhanced Raman scattering.

Stabilization of Silver Colloids by Various Types of Anions and Their Effect on the Surface-Enhanced Raman Spectra of Organic Dyes

Using a recently published procedure, silver colloids with uniform particle size were prepared and stabilized by nine different anions. Although the UV–visible extinction spectra of the colloids showed only small variations with the nature of the stabilizing ions, dramatic differences were observed in the SERS spectra of seven dyes studied. It was demonstrated that each family of dyes experiences a large enhancement factor only with certain stabilizing ions. The adsorption geometry of oxazine dyes changes with the nature of the stabilizing anion and with the concentration of the dye. By comparison of observed and calculated normal modes, conclusions about the adsorption geometry are drawn. The results are discussed in the framework of the ‘adatom model’.

Characterization of alkali-metal sols in diethyl ether. Visible extinction and surface-enhanced Raman spectra

Colloidal suspensions of Na, K, Rb, and Cs have been prepared in diethyl ether. The visible extinction spectra of each are compared with results of theoretical Mie-scattering spectra, calculated using a variety of optical constant data sets. Comparison of the experimental extinction spectra to the theoretical spectra demonstrates that fresh sols of Na, K and Rb contain small metal particles with diameters suitable for surface-enhanced Raman (SER) spectroscopic studies. Fresh Cs colloids, reported for the first time, show signs of particle aggregation in long strings. Raman spectra of Na, K and Rb colloids are presented in the region between 150 and 700 cm −1. We find that Na colloids display two broad SER features at 290 and 530 cm −1, K colloids display two SER features at 220 and 480 cm −1, and Rb colloids display one band at 215 cm −1. The lower-frequency bands for Na and K sols and the 215 cm −1 band of Rb sols may be due to lattice modes of amorphous alkali-metal hydroxides on the metal surfaces. The two higher-frequency bands at 480 and 530 cm −1 remain unassigned.