Optical Extinction Spectra Research Articles

Quantitative Determination of the Size Dependence of Surface Plasmon Resonance Damping in Single Ag@SiO2 Nanoparticles

The optical extinction spectra of single silver nanoparticles coated with a silica shell were investigated in the size range 10-50 nm. Measurements were performed using the spatial modulation spectroscopy technique which permits independent determination of both the size of the metal nanoparticle under study and the width of its localized surface plasmon resonance (LSPR). These parameters can thus be directly correlated at a single particle level for the first time. The results show a linear increase of the width of the LSPR with the inverse diameter in the small size regime (less than 25 nm). For these nanoparticles of well-controlled environment, this can be ascribed to quantum confinement of electrons or, classically, to increase of the electron surface scattering processes. The impact of this effect was measured quantitatively and compared to the predictions by theoretical models.

Application of a conproportionation reaction to a synthesis of shape-controlled gold nanoparticles

Synthesis of gold nanoparticles with multiple shapes by a modified seeding growth method is reported. The optical extinction spectra of a mixture of the seed and growth solutions indicate that the seed particle size decreases during stirring the mixture by a conproportionation reaction of Au metal with Au(III) ions. This conproportionation reaction is used to investigate the effects of variation in seed particle size on the resultant gold nanoparticle shape. When single crystalline and multiply twinned particles are used as the seed particles, they grow into gold nanorods and nanobipyramids, respectively. By letting the seed particles experience the conproportionation reaction before the particle growth to decrease the size, the surface of resultant nanoparticles roughens. Increasing the conproportionation reaction time up to 5 min, multi-branched gold nanoparticles with many lattice defects grow from both of the seed particles. This indicates that the conproportionation reaction is useful to generate lattice defects during the particle growth, which lead to the formation of gold nanoparticles with complex shapes.

Probing formally forbidden optical transitions in PbSe nanocrystals by time- and energy-resolved transient absorption spectroscopy

The first two peaks of the optical extinction spectrum of PbSe nanocrystals in solution have been assigned in the literature to the 1Sh1Se and 1Ph1Pe transitions. In the present work we assign the transitions causing extinction in the energy region between these two lowest-energy peaks. Our femtosecond transient absorption data indicate that optical extinction in this region is neither due to Rayleigh scattering nor due to local-field effects but due to the formally forbidden 1Ph1Se and 1Sh1Pe transitions.

Luminescence properties of Tb implanted ZnO

ZnO [0 0 0 1] crystals were irradiated at room temperature with Tb+ ions of 400 keV with fluences from 1×1016 to 2×1017 cm−2. The implanted layer was examined by several methods, including radioluminescence (RL), Rutherford backscattering spectrometry (RBS) and optical spectroscopy. The optical extinction spectra were simulated using Mie scattering theory. Absorption spectra predicted by Mie theory for particles of decreasing diameter were compared with those obtained experimentally. Some qualitative agreement between theoretical and experimental data was achieved. It was also shown that the intensities of the characteristic green emission bands associated with Tb produced by 5D4→7Fj=5,45D4→7Fj=5,4 transitions have increased about 8 times after annealing. Optical spectroscopy and radioluminescence data have revealed that the ion implantation is a promising tool for synthesizing Tb nanoparticles in the ZnO surface. The Tb nanoparticles exhibit a rather weak plasma resonance.

Magnetic and optical properties of cobalt nanowires fabricated in polycarbonate ion-track templates

Cobalt nanowires with diameter 75 nm were synthesized in ion track-etched membranes by electrochemical deposition. Scanning electron microscopy displays cylindrical wires with smooth and homogeneous contours. X-ray diffraction studies indicate that the wires possess a face centered cubic structure and a preferred growth orientation along the [1 1 0] direction. The wires exhibit magnetic anisotropy, which is observed and is ascribed to shape anisotropy. The optical extinction spectrum shows a band which probably originates from a surface plasmon resonance.

Discrete dipole approximation simulations of gold nanorod optical properties: Choice of input parameters and comparison with experiment

Gold nanorods have interesting optical properties due to surface plasmon resonance effects. A variety of biomedical applications of these particles have been envisaged and feasibilities demonstrated in imaging, sensing, and therapy based on the interactions of light with these particles. In order to correctly interpret experimental data and tailor the nanorods and their environments for optimal use in these applications, simulations of the optical properties of the particles under various conditions are essential. Of various numerical methods available, the discrete dipole approximation (DDA) approach implemented in the publicly available DDSCAT code is a powerful method that had proved popular for studying gold nanorods. However, there is as yet no universal agreement on the shape used to represent the nanorods and on the dielectric function of gold required for the simulations. We systematically study the influence of these parameters on simulated results. We find large variations in the position of plasmon resonance peaks, their amplitudes, and shapes of the spectra depending on the choice of the parameters. We discuss these in the light of experimental optical extinction spectra of gold nanorods synthesized in our laboratory. We show that much care should be taken and prudence applied before DDA results be used to interpret experimental data and to help characterize nanoparticles synthesized.

Improved Localized Surface Plasmon Resonance Immunoassay with Gold Bipyramid Substrates

Gold nanoparticles bound to substrates exhibit localized surface plasmon resonance (LSPR) in their optical extinction spectra at visible and near-infrared wavelengths. The LSPR wavelength is sensitive to the surrounding refractive index, enabling a simple, label-free immunoassay when capture antibodies are bound to the nanoparticles. Gold bipyramids are nanoparticles with a penta-twinned crystal structure, which have a sharp LSPR because of their high monodispersity. Bipyramid substrates were found to have a refractive index sensitivity ranging from 288 to 381 nm/RIU (-0.62 to -0.68 eV/RIU), increasing with the nanoparticle size and aspect ratio. In an immunoassay, the bipyramid substrates yielded higher sensitivity than nanorods and nanospheres. An immunoassay sensitivity constant which depends on both the optical properties of the nanoparticle and conjugation chemistry was found to be K(LSPR) = 0.01 nm x microm(2) for gold bipyramids.

Tuning Optical Properties of Rhombic Hybrid Au-Ag Nanoparticles: A Discrete Dipole Approximation Calculation

Optimization of metallic nanoparticles was presented in this paper by aid of computational numerical calculation. The optical extinction spectra of rhombic hybrid Au-Ag nanoparticles have been calculated by the discrete dipole approximation (DDA) aided design method. Both material and the thickness of the particles can be used to effectively tune localized surface plasmon resonance. On the basis of the calculated extinction spectra, the crucial parameters of the nanostructure arrays such as thickness can be determined. Using this DDA aided approach, a hybrid Au-Ag nanoparticles array is put forth and designed with the optimized parameter of thickness of metal thin films (h(Au) = 5 nm, and h(Ag) = 25 nm). This study shows that the material of the particles have significant effect on the optical properties. The DDA aided design method can provide the optimized structure parameters for the hybrid nanostructures.

Formation of Au−Ag Core−Shell Nanostructures in Silica Matrix by Sequential Ion Implantation

Formation of Au core-Ag shell bimetallic nanoparticles in silica matrix is demonstrated through sequential implantation of Ag and Au ions and subsequent thermal annealing. Formation of core-shell structures is verified through optical absorption spectroscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy, and simulated optical extinction spectra. A mechanism for the formation of such unusual structures in ion-implanted silica is proposed. By controlling the implantation energy of the two ions properly and keeping the implantation sequence Ag first and then Au, it is possible to create Au core-Ag shell nanoparticles in the silica matrix with homogeneous distribution.

Optical extinction for determining the size distribution of gold nanoparticles fabricated by ultra-short pulsed laser ablation

This work presents a method, based on measurements of the optical extinction spectra, to determine the size of spherical gold nanoparticles produced using the femtosecond laser ablation process in deionized water. By using an improved theoretical model that modifies the contribution of the free electrons to the dielectric function introducing a size-dependent term, it is possible to fit the full experimental extinction spectrum considering a certain size distribution. Additionally, in order to obtain complementary measurements of the size distribution, TEM analysis was performed. The results obtained showed that the predominant nanoparticle size distribution ranges from 1 to 11 nm in terms of radii. An optical extinction measurement together with an appropriate theoretical model based on Mie’s theory represents a simple, low-cost, fast and easy method to describe a multimodal size distribution of spherical gold nanoparticles.

Quantitative Analysis of Dipole and Quadrupole Excitation in the Surface Plasmon Resonance of Metal Nanoparticles

Metal nanoparticles have received increasing attention for their peculiar capability to control local surface plasmon resonance (SPR) when interacting with incident light waves. In this article, we calculate the optical extinction spectra of silver nanocubes with the edge length ranging from 15 to 200 nm by using the discrete dipole approximation method. An increasing number of SPR peaks appear in the optical spectra, and their positions change when the nanocube size increases. We have developed a method to quantitatively separate the contributions of the individual dipole component and quadrupole component of the optical extinction cross sections. This allows us to specify unambiguously the physical origin of each SPR peak in the spectra. We have also extensively analyzed the distribution patterns of electric fields and electric charges within and around the silver nanoparticle. These patterns clearly show the dipole and quadrupole excitation features at the SPR peaks. The near-field analyses are consistent with the far-field extinction spectra analyses. This suggests that the combination of far-field spectra and near-field pattern analysis can greatly help to uncover the intrinsic physics behind light interaction with metal nanoparticles and excitation dynamics of local surface plasmonic waves.

Core and shell sizing of small silver-coated nanospheres by optical extinctionspectroscopy

Silver metal nanoparticles (Nps) are extensively used in different areas of research andtechnology due to their interesting optical, thermal and electric properties, especially forbare core and core–shell nanostructures with sizes smaller than 10 nm. Since theseproperties are core–shell size-dependent, size measurement is important in manipulatingtheir potential functionalization and applications. Bare and coated small silver Npsfabricated by physical and chemical methods present specific characteristics in theirextinction spectra that are potentially useful for sizing purposes. This work presents anovel procedure to size mean core radius smaller than 10 nm and mean shell thicknessof silver core–shell Nps based on a comparative study of the characteristics intheir optical extinction spectra in different media as a function of core radii, shellthickness and coating refractive index. From the regularities derived from theserelationships, it can be concluded that plasmon full width at half-maximum (FWHM)is sensitive to core size but not to coating thickness, while plasmon resonancewavelength (PRW) is related to shell thickness and mostly independent of core radius.These facts, which allow sizing simultaneously both mean core radius and shellthickness, can also be used to size bare silver Nps as a special case of core–shell Npswith zero shell thickness. The proposed method was applied to size experimentalsamples and the results show good agreement with conventional TEM microscopy.

Optical extinction spectrum of a single metal nanoparticle: Quantitative characterization of a particle and of its local environment

Optical absorption spectroscopy of a single metal nanoparticle is used to characterize its properties and to obtain quantitative information on its local environment. Experiments were performed using the spatial modulation spectroscopy (SMS) technique on 16 nm mean diameter gold nanoparticles embedded in different medium (i.e., deposited on glass or embedded in a polymer layer). Extraction of the nanoparticle characteristics and determination of the dielectric constant of its environment are discussed, focusing on the impact of the particle shape assumption. The refractive index of the local environment deduced from these measurements shows large particle-to-particle variation, yielding information about fluctuations of the dielectric properties of the surrounding medium on a nanometric scale, inaccessible in ensemble measurements. The influence of the environment of a nanoparticle on its optically extracted geometry and its surface plasmon resonance broadening by surface effect are also studied at a single-particle level.

Optical properties of helical and multiring Ag nanostructures: The effect of pitch height

The pitch height–dependent optical extinction spectra and the electric field enhancement of helical and multiring nanostructures are studied by the discrete dipole approximation method. For light incident along the axis of the helix, with the increase of the pitch height, the plasmon peak wavelengths redshift monotonically and the electric coupling between two adjacent pitches weakens. The plasmon peak can also be tuned by changing the polarization of the incident light, particularly by circularly polarized light. In most cases, the maximum electric field is distributed near the surface of the helix. As a comparison, since multiring structures do not have helicity, the extinction spectra are polarization independent and the maximum electrical fields always distribute on the top ring. These results show that the optical properties of helical structures are different when irradiated by left or right circular polarized light. Understanding and manipulating pitch height allow one to tune the plasmon peak and rearrange the electric field enhancement distribution of helical nanostructures.

End-to-End Assembly of CTAB-Stabilized Gold Nanorods by Citrate Anions

We report studies on the end-to-end assembly of CTAB-stabilized gold nanorods (GNRs) dispersing in an aqueous solution. The addition of sodium citrate to the solution of GNRs with an aspect ratio (AR) of 3 causes the end-to-end assembly accompanied with a red-shift of longitudinal surface plasmon resonance (LSPR) band of the GNRs in the optical extinction spectrum. The red-shift in the LSPR band continues until the solution becomes colorless and transparent, whereas adding CTAB brings the color change to a stop, resulting in the long term-stable LSPR of the end-to-end connected GNRs. Added citrate anions are found to be adsorbed on only the end faces of GNRs so that they are considered to neutralize the surface charge of the GNR ends, resulting in the end-to-end assembly of the GNRs. On the other hand, GNRs with an AR of 16 are end-to-end assembled by decreasing CTAB concentration in the solution. The assembled GNRs are welded at their connecting points by the further decrease of CTAB concentration in the...

Formation and properties of metal colloids and colloidal nano-alloys by laser ablation and irradiation of metals in liquids

We have successfully synthesized noble metal colloids by laser ablation of metal targets in liquids and Au/Ag colloidal nano-alloys with a wide range of compositions by a re-irradiation of mixed colloidal suspensions. Optical extinction spectra as well as electron microscopy data can give a clear picture of the structure and properties of the obtained particles, while an X-ray photoelectron spectroscopy is used for an estimation of the clusters’ composition.

Optical response of a single gold nanoparticle

Optical detection and spectroscopy of single gold nanoparticles using the spatial-modulation spectroscopy technique is described, focusing on the connection between the nanoparticle and environment parameters with the measured linear optical extinction spectrum. Characterisation of the size, shape and orientation on the substrate of a nanoparticle through quantitative determination of its optical extinction cross-section spectrum is illustrated in the case of gold nanospheres and nanorods. Extension of this technique to ultrafast nonlinear spectroscopy of a single gold nanoparticle is also discussed.

Optical properties of cluster-based titanium and titanium oxide films

The optical properties of nanostructured titanium and titanium oxide films grown on transparent substrates have been investigated. Titanium films were produced either by cluster deposition or by atom deposition, and subsequently the films were oxidised by controlled exposure to oxygen. Optical extinction spectra in the ultraviolet and visible were recorded during film growth and oxidation. Before oxidation the extinction showed the characteristics of metallic titanium, with the spectrum of the cluster-based film indicating a structure of small spherical particles. After oxidation a band gap opened up and the films became transparent in the visible, and peaks appeared in the spectrum of the sample grown by atom deposition.

Extinction spectra and electrical field enhancement of Ag nanorods with different topologic shapes

We use the discrete dipole approximation method to investigate the length-dependent optical extinction spectra and the E-field enhancement of Ag nanorods with different topologic shapes, such as cylindrical, needlelike, periodic, L, and Y shapes. Comparing to nanorods with a simple shape (cylinder and needle), irregular nanorods show many distinct plasmon resonances over a large spectral range. The main plasmon peaks are redshifted with the increase of the lengths and enhanced E-field distribution at those wavelengths depends strongly on the shapes of the nanorods. More hot spots are observed for the nanorods with more defects (i.e., periodic, L, and Y shapes) and the maximum E fields are in the same order of magnitude for cylindrical nanorods and depends on the polarization and propagation directions of the excitation light. The polarization dependent extinction spectra of nanorods with different shapes have also been calculated and are shown to be dependent on the symmetry of the nanorod structure. With specific incident direction, by changing the incident polarization angle, the maximum E field of L- and Y-shaped nanorods can vary in location and magnitude. These results show that defects or imperfections in Ag nanorod structures could drastically change the optical properties, generate extra hot spots for surface enhanced spectroscopy, and have different enhanced field distribution for future plasmonic applications.

Precious Metal Core−Shell Spindles

A simplified method to produce spindle-shaped particles with a hematite core and a silica shell is described. The silica shell can, in turn, serve as the substrate for an outer coating of Ag or Au nanoparticles. The resulting multilayer core−shell particles display a flexible optical extinction spectrum, due primarily to the sensitivity of their plasmon resonance to the morphology of the precious metal outer coating.