Optical Extinction Spectra Research Articles

Optical properties of plasmonic silver nanoparticles exposed to organic solvents

The influence of ethanol and acetic acid on the structure and optical properties of silver granular films formed by physical vapor deposition in a high vacuum on sapphire substrates was studied via optical spectroscopy and scanning electron microscopy. It was found that irregularly shaped oblate silver grains transforms into almost spherical nanoparticles. Simultaneously, optical extinction spectra shift in the short wavelength range and become narrower. The same effect was observed when liquid crystal was poured on such film. It was noticed, that nanoparticle shapes change under the action of these fluids in the same way as in the process of thermal annealing. This analogy suggests that the observed effects could be explained by acceleration of atomic diffusion over the islet surfaces. It was noticed also that the resistance of thick granular films changes abruptly several minutes after pouring such film with ethanol. This jump of resistance is also very similar to the resistance jump observed previously in the case of annealing.

Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol.

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS2 and WS2 shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

Systematic control of edge length, tip sharpness, thickness, and localized surface plasmon resonance of triangular Au nanoprisms

Triangular gold (Au) nanoprisms of various sizes were synthesized in a controlled way using a modified three-step seed-mediated method with different volumes of starting seed solution and subsequent first step’s growth solution. The structures and optical properties of the triangular Au nanoprisms were investigated using transmission electron microscopy (TEM), atomic force microscopy, and UV–Vis–NIR spectrophotometry. The Au nanoprisms synthesized also varied in optical response frequency of localized surface plasmon resonance (LSPR) owing to electric dipole polarizations of the Au nanoprisms. This variation depended nonlinearly on the volume of the seed solution. From optical extinction spectra and careful TEM observations, the dipole LSPR peak frequency was found to be linearly proportional to the edge length of the Au nanoprisms. Consequently, it was experimentally shown that the LSPR optical response frequency of their colloidal solutions could be controlled in the near-infrared region (700–1200 nm), corresponding to an edge length of 40–180 nm of the Au nanoprisms. It was also demonstrated that the tip sharpness of triangular Au nanoprisms was improved by using fine Au seeds instead of coarse Au seeds, and the resulting Au nanoprisms were smaller and thinner. A formation mechanism of triangular Au nanoprisms shall also be discussed with a prospect of synthesizing very tiny Au nanoprisms.

Fröhlich resonance in carbon nanospiroids and the 2175 Å interstellar absorption feature

This paper demonstrates that a free electron gas model accurately simulates the spectral dependence of optical extinction spectra for carbon spiroids under the assumption that free electrons are confined in an homogeneous spherical particle owing to the delocalisation of π electrons that occurs in the actual spectral range. This effect can occur in the spiroid, rather than a spheroid (onion) due to the variable radii of the spiral turns as a function of distance from the centre, which are smaller than typical values for the spheroid.

Effects of hydriding and ageing of Pd nanoparticles to contact between nanoparticles and quartz and contacts among nanoparticles investigated by the pump-probe technique

Closely contacted Pd nanoparticles with average size of 8 nm are uniformly deposited on quartz. The ageing, hydriding, hydriding and then ageing of the nanoparticles are characterized by the transmission electron microscopy, optical extinction spectrum, and X-ray photoelectron spectroscopy, which show little chemical and crystallographic change. The contact between the nanoparticles and quartz and contacts among the nanoparticles are investigated by the pump-probe technique. The contact between nanoparticles and quartz looses with the ageing, hydriding, hydriding and then ageing of the nanoparticles, and the contacts among nanoparticles loose with the ageing whereas compresses with the hydriding.

In-Plane Optical Anisotropy of Layered Gallium Telluride.

Layered gallium telluride (GaTe) has attracted much attention recently, due to its extremely high photoresponsivity, short response time, and promising thermoelectric performance. Different from most commonly studied two-dimensional (2D) materials, GaTe has in-plane anisotropy and a low symmetry with the C2h(3) space group. Investigating the in-plane optical anisotropy, including the electron-photon and electron-phonon interactions of GaTe is essential in realizing its applications in optoelectronics and thermoelectrics. In this work, the anisotropic light-matter interactions in the low-symmetry material GaTe are studied using anisotropic optical extinction and Raman spectroscopies as probes. Our polarized optical extinction spectroscopy reveals the weak anisotropy in optical extinction spectra for visible light of multilayer GaTe. Polarized Raman spectroscopy proves to be sensitive to the crystalline orientation of GaTe, and shows the intricate dependences of Raman anisotropy on flake thickness, photon and phonon energies. Such intricate dependences can be explained by theoretical analyses employing first-principles calculations and group theory. These studies are a crucial step toward the applications of GaTe especially in optoelectronics and thermoelectrics, and provide a general methodology for the study of the anisotropy of light-matter interactions in 2D layered materials with in-plane anisotropy.

Optics of a single ultrasharp groove in metal.

Optical properties of a single ultrasharp groove of subwavelength width cut in an otherwise flat metal surface are examined theoretically. We calculate optical extinction, scattering, and absorption cross-section spectra for a wide range of groove profiles, establishing several fundamental trends. As grooves are made sharper, amplitudes of oscillations in cross-section spectra and their period decrease, while the absorption level increases, leading eventually to efficient broadband (nonresonant) absorption. Oscillations in scattering spectra are generally more pronounced than those in corresponding absorption spectra. For ultrasharp grooves, oscillations in all spectra can be suppressed by increasing the groove depth. Finally, the level of absorption relative to that of scattering increases as the top groove width decreases, a trend that is analogous to that found when decreasing the size of metal nanoparticles.

Size-Modulation of Plasmonic Nanorings Obtained by the Self-Assembly of Gold Nanoparticles and Block Copolymers

Metal nanoparticles exhibit interesting optical properties due to the collective excitation of conduction electrons called the plasmon. Within appropriate metal nanostructures, cooperative plasmon modes appear and the resonance plasmon frequency is modified. This article reports a simple method for the formation of such structures, in the form of self-assembled nanorings. Rings of alkanethiol-capped gold nanoparticles are obtained by the Langmuir–Blodgett technique and a block copolymer (PS-b-P2VP) template. With this approach, organized nanoparticle arrangements covering a large surface area are obtained. Furthermore, geometric parameters such as ring diameter, ring-to-ring separation, and ring width can be systematically varied by the addition of homopolymer or in situ nanoparticle regrowth. Optical extinction spectra recorded for the nanoparticle rings depend both on ring diameter and particle size. In particular, after in situ particle regrowth, the plasmon extinction spectrum exhibits a red-shift tha...

Contrast- and intensity-enhancement of sensor signals based on Rayleigh anomaly in metal-coated gratings

We investigate the spectroscopic response of Rayleigh anomaly in a dielectric grating before and after it is coated with a layer of aluminum. The metallic coating enabled effective prohibition of the diffractions into the substrate and suppressed the background of the optical extinction spectrum of Rayleigh anomaly. This enhanced significantly both the contrast and the amplitude of the sensor signal based on the spectral shift of Rayleigh-anomaly. Sensor measurements were performed on the glucose/water solutions with different concentrations, which show improved performance due to the enhancement of the spectral contrast.

Role of Evaporated Silver Nanoparticles in Organic Field-Effect Transistor: Electrical Effects and Dependence on the Size

The electrical effects of metal nanoparticles are determined by the nature of a single nanoparticle (shape, size, surface) and their correlation with the nanoscale electronic structure. In this work, we report that electrical properties of evaporated silver nanoparticles can be controlled by different thicknesses and thermal annealing times. The particle size and size distribution were first fully characterized by the AFM and optical extinction spectra, and then their electrical properties such as current trapping and threshold voltage were studied by the organic field-effect transistor with a device structure of Si/SiO2/Ag-NPs/PMMA/PTB7/Ag. The results show that the thickness decrease and thermal annealing are effective ways for a lower charge trapping, which corresponds to smaller particle size and homogeneous particle distribution without particle aggregates. These results would be helpful for the optoelectronic applications of metal nanoparticles.

Resonant surface plasmon-exciton interaction in hybrid MoSe2@Au nanostructures.

In this work we investigate the interaction between plasmonic and excitonic resonances in hybrid MoSe2@Au nanostructures. The latter were fabricated by combining chemical vapor deposition of MoSe2 atomic layers, Au disk processing by nanosphere lithography and a soft lift-off/transfer technique. The samples were characterized by scanning electron and atomic force microscopy. Their optical properties were investigated experimentally using optical absorption, Raman scattering and photoluminescence spectroscopy. The work is focused on a resonant situation where the surface plasmon resonance is tuned to the excitonic transition. In that case, the near-field interaction between the surface plasmons and the confined excitons leads to interference between the plasmonic and excitonic resonances that manifests in the optical spectra as a transparency dip. The plasmonic-excitonic interaction regime is determined using quantitative analysis of the optical extinction spectra based on an analytical model supported by numerical simulations. We found that the plasmonic-excitonic resonances do interfere thus leading to a typical Fano lineshape of the optical extinction. The near-field nature of the plasmonic-excitonic interaction is pointed out experimentally from the dependence of the optical absorption on the number of monolayer stacks on the Au nanodisks. The results presented in this work contribute to the development of new concepts in the field of hybrid plasmonics.

Surface Plasmon Resonance of an Individual Nano‐Object on an Absorbing Substrate: Quantitative Effects of Distance and 3D Orientation

Modification of the plasmonic response of a metal nano‐object due to interaction with a substrate is experimentally investigated measuring the quantitative optical extinction spectra of individual nano‐objects with various elongated shapes (bipyramids and rods) deposited on a dielectric (silica) or absorbing (carbon) membrane. Apart from the expected dependence of the nanoparticle surface plasmon resonance (SPR) frequency on the nature of the substrate, large substrate and particle shape dependent modifications of its SPR width are demonstrated. These dependencies are ascribed to strong localization of the electromagnetic field associated with the longitudinal SPR of an elongated nano‐object around its tips, leading to different interaction with the substrate depending on the particle shape and 3D orientation relative to the substrate. Both parameters have been precisely determined by electron tomography, permitting excellent reproduction of the experimental data. Experiments performed on silver‐encapsulated bipyramids, whose shape evolves from a pyramidal one towards a cylindrical one, further confirm this effect.

Higher Order Plasmonic Modes Excited in Ag Triangular Nanoplates by an Electron Beam

Ag triangular nanoplates are known to generate strong plasmonic resonances when excited by both light and electron beams. Experimental electron energy-loss spectra (EELS) and maps were acquired using an aberration-corrected JEOL-ARM microscope. The corner, edge and centre modes that are often observed in such structures were also observed in these measurements. In addition, novel higher order internal modes were observed and were found to be well reproduced by theoretical calculations using boundary element method (BEM). These modes are “dark modes” so are not observed in the optical extinction spectra. They are confined surface propagating modes and are analogous to laser cavity modes.

Plasma-induced brightening and coarsening of tarnished Ag nanoparticles

The recovery of the optical properties of tarnished silver nanoparticles (Ag NPs) has been demonstrated using exposure to plasma. As-prepared Ag NPs on a SiO2 substrate exhibited an intense localized surface plasmon resonance (LSPR) in the UV–vis optical extinction spectrum. The LSPR band weakened gradually when an Ag NP sample was stored in ambient air forming tarnished Ag. X-ray photoelectron spectroscopy (XPS) and ion-beam analysis revealed the presence of impurity elements, such as N and S. In addition, the shape of the valence-band XPS spectra changed significantly with exposure to ambient air. Ar plasma treatments recovered the intense LSPR and the XPS spectral shape. We conclude that both the reduction of Ag2S to Ag and the morphological change toward isolated Ag NPs most likely lead to their recovery. A simulation based on the Mie theory, as well as direct observation with an atomic force microscope, supports our findings.

Waveguide‐Plasmon Polariton Enhanced Photochemistry

One key process in plasmon‐enhanced photocatalysis is the transfer of hot charge–carriers from metal nanostructures into photocatalytically active materials. This process is secondary to the initial step of light absorption by the metal nanostructures. Light absorption in these structures can be controlled by designing complex geometries with tailored optical cross‐sections. Here, a study on one of the simplest nanostructures exhibiting plasmon resonances is presented: 1D gratings of metal wires. Results on the effect of the periodicity of these arrays are presented on the resonant absorption of light and on the hot charge–carrier transfer to a supporting TiO2 thin film. This charge transfer process is monitored by following the photocatalytic decomposition of small organic molecules in solution. Strong enhancements in the rate of decomposition are found that exhibit a correlation with the measured optical extinction spectra of the plasmonic nanostructures.

Linear length-dependent light-harvesting ability of silicon nanowire

Silicon nanowire (SiNW) is of great promising for photovoltaic applications due to its excellent performance in light-harvesting. Some experimental and theoretical results indicate its light-harvesting is dramatically length dependent, while there is still no investigation on this dependency. Through reliable simulations on the optical extinction and absorption spectra of SiNWs with varying lengths, we find that the light-harvesting ability of SiNW is linear with its length. For the SiNWs of the optimal diameter, 80nm, the linearity between the light-concentration (light-absorption) multiples and length is about 133μm−1 (50μm−1). This linear relationship can be explained reasonably by the leaky modes theory.

Formation of Au–Ag nanoalloy through Au core/Ag shell intermediate phase by laser ablation

We report on the formation of Au–Ag nanoalloy by laser ablation of Ag plate in the Au nanocolloidal solution. Development of an intermediate phase of Au core/Ag shell nanostructures is noticed at the initial stage of alloying which is evident from the optical extinction spectra and is further confirmed from Transmission Electron Microscopic image. The intermediate phase is found to be stable and continued ablation of Ag target resulted in the formation of nanoalloys with plasmon band tunable depending on the time of ablation.

Ag nanoparticles formed by femtosecond pulse laser ablation in water: self-assembled fractal structures

We report for the first time on the formation of self-assembled fractals of spherical Ag nanoparticles (Nps) fabricated by femtosecond pulse laser ablation of a solid silver target in water. Fractal structures grew both in two and three Euclidean dimensions (d). Ramified-fractal assemblies of 2 nm height and 5–14 μm large, decorated with Ag Nps of 3 nm size, were obtained in a 2d geometry when highly diluted drops of colloidal suspension were dried at a fast heating rate over a mica substrate. When less-diluted drops were dried at slow heating rate, isolated single Nps or rosette-like structures were formed. Fractal aggregates about 31 nm size in 3d geometry were observed in the as-prepared colloidal suspension. Electron diffraction and optical extinction spectroscopy (OES) analyses performed on the samples confirmed the presence of Ag and Ag2O. The analysis of the optical extinction spectrum, using the electrostatic approximation of Mie theory for small spheres, showed the existence of Ag bare core, Ag–Ag2O and air–Ag core–shell Nps, Ag–Ag2O being the most frequent type [69 % relative abundance (r.a.)]. Core-size and shell-thickness distribution was derived from OES. In situ scattering measurements of the Ag colloidal suspension, carried out by small-angle X-ray scattering, indicate a mass fractal composed of packaged 〈D SAXS〉 = (5 ± 1) nm particles and fractal dimension d f = 2.5. Ex situ atomic force microscopy imaging displayed well-ramified structures, which, analyzed with box-counting method, yield a fractal dimension d f = 1.67. The growing behavior of these 2d and 3d self-assembled fractals is consistent with the diffusion-limited aggregation model.

Fano coupling between Rayleigh anomaly and localized surface plasmon resonance for sensor applications

Fano coupling between Rayleigh anomaly and localized surface plasmon resonance has been observed in diffractive grating structures consisting of aluminum nanolines deposited on the top surface of photoresist with each nanoline composed of tightly aggregated aluminum nanoparticles. Localized surface plasmon resonance is excited both in the nanoparticles and in the nanolines by differently polarized light. The surface propagation mode excited by the first- and second-order Rayleigh diffraction anomaly is strongly scattered and diffracted by the plasmonic aluminum grating structures, producing light rays in the same direction as the reflected light beam with the same spectral feature as the Rayleigh anomaly. The narrow-band diffracted and scattered light appears as sharp dips in the broad-band reflective optical extinction spectrum of plasmon resonance, which is recognized as a kind of Fano coupling. This kind of coupled mode is utilized successfully in refractive-index-sensor devices with excellent sensitivity.

Nanoparticle assembly following Langmuir-Hinshelwood kinetics on a Langmuir film and chain networks captured in LB films.

The Langmuir-Blodgett (LB) technique is an elegant protocol for the steered assembly of metal nanoparticles, the deposition pressure serving as a convenient parameter to tune the assembly. Adsorption of nanoparticles from the subphase to the air-water interface can provide further control of the process. Citrate-stabilized gold nanoparticles in the aqueous subphase are shown to assemble into extended 2-dimensional chain networks following adsorption on a cationic amphiphile Langmuir film at the air-water interface. Kinetic investigations show that the process can be visualized as a surface-catalyzed reaction and explained in terms of the Langmuir-Hinshelwood mechanism. The LB deposition proves to be a unique route to capture the reaction product together with the amphiphile film. The deposition pressure is used to tune the density of nanoparticle chain networks in the LB film, and their optical extinction spectrum. The unusual blue shift of the extinction observed with increasing deposition pressure is attributed to the impact of the amphiphile monolayer environment. The extent of formation of the chain network is analyzed in terms of the pathways in the corresponding graph representation, and shown to scale with the deposition pressure. The current investigation highlights the use of a charged monolayer as a heterogeneous catalyst surface, provides fundamental insight into the kinetics of nanoparticle assembly at interfaces, and demonstrates the utility of the LB technique in tuning the formation of 2-dimensional nanoparticle chain networks.