Observation of intrinsic size effects in the optical response of individual gold nanoparticles

Boosting Gas-Phase TiO2 Photocatalysis with Weak Electric Field Strengths of Volt/Centimeter

Among semiconductor nanomaterials, titanium dioxide is at the forefront of heterogeneous photocatalysis, but its catalytic activity greatly suffers from the loss of photoexcited charge carriers through deleterious recombination processes. Here, we investigate the impact of an external electric field (EEF) applied to conventional P25 TiO2 nanopowder with or without Au nanoparticles (NPs) to circumvent this issue. The study of two redox reactions in the gas phase, water splitting and toluene degradation, reveals an enhancement of the photocatalytic activity with rather modest electric fields of a few volt/centimeters only. Such an improvement arises from the electric-field-induced quenching of the green emission in anatase, allowing the photoexcited charge carriers to be transferred to the adsorbed reactants instead of pointless radiative recombinations. Applying an EEF across a trap-rich metal oxide material, such as TiO2, which, when impregnated with Au NPs, leads, respectively, to 12- and 6-fold enhancements in the production of hydrogen and the oxidation of toluene for an electric field of 8 V/cm, without any electrolysis, is a simple and elegant strategy to meet higher photocatalytic efficiencies.

Bandwidth of quantized surface plasmons: competition between radiative and nonradiative damping effects

We investigate the damping effects of coherent electron oscillations on the bandwidth of a quantized nanoparticle plasmon resonance. The nanoparticle (NP) is treated as a two-level quantum system, and the total relaxation time involves both the population relaxation time associated with radiative processes and the collisional relaxation time associated with nonradiative processes that result in dephasing/decoherence of electron oscillations. We describe the optical response of NPs to an external electromagnetic field by the optical Bloch equations employing the density matrix formalism to capture the quantum description nature of dipolar plasmon resonance and suggest a generalized criterion for the validity of dipole approximation. Then we explore the competition between the radiative and nonradiative damping in determining the plasmon bandwidth of two typical NP models; metallic nanospheres and dielectric core-metal shell NPs (nanoshells). We show that the frequency of plasmon resonance, in addition to the NP size, plays an important role in the competition between the damping mechanisms. Consequently, the damping processes are significantly influenced by the factors that determine the resonance frequency, such as the core size, the dielectric constant of the medium, and the shell thickness (for nanoshells). For both models of NPs, we identify the optimum parameters that achieve a narrower plasmon bandwidth (minimal damping), which is a prerequisite for advanced sensing and medical applications. We demonstrate excellent agreement of the simulated spectral features of the plasmon resonance with previously reported experimental results for a single NP where the inhomogeneous broadening of the plasmon line is excluded. For NP ensembles where inhomogeneous broadenings and interface chemical effects are significant, our theoretical approach successfully predicts the overall trend of size-dependent damping rates.

Gold nanoshells with magnetic cores and a urea-based receptor for SERS sensing of fluoride anions: experimental and computational study

The study demonstrates that a combination of plasmonic nanostructures and artificial receptors can be applied for sensing small molecular species. Gold nanoshells containing magnetic cores are used as the SERS-active substrates, which opens the way for the development of multimodal contrast agents with applicability extended to sensing or for the separation of analytes by magnetic solid-phase extraction. Disubstituted ureas forming hydrogen-bonded complexes with certain anions can be employed as molecular sensors. In this case study, gold nanoshells with silica-coated Mn-Zn ferrite cores were prepared by a multistep procedure. The nanoshells were co-functionalized with an N-(4-mercaptophenyl)-N'-(4-nitrophenyl)urea sensor synthesized directly on the gold surface, and with 4-nitrothiophenol, which is adopted as an internal standard. SERS measurements were carried out with acetonitrile solutions of tetrabutylammonium fluoride (Bu4NF) over a concentration range of 10-10-10-1 mol L-1. The spectral response of the sensor is dependent on the fluoride concentration in the range of 10-5-10-1 mol L-1. To investigate further the SERS mechanism, a model sensor, N-(4-bromophenyl)-N'-(4-nitrophenyl)urea, was synthesized and used in Raman spectroscopy with solutions of Bu4NF, up to a molar ratio of 1 : 20. The spectra and the interactions between the sensors and fluoride anions were also studied by extensive DFT computations.

Preparation, Structure, and Properties of PVA-AgNPs Nanocomposites

The aim of the work was to prepare a polymer matrix composite doped by silver nanoparticles and analyze the influence of silver nanoparticles (AgNPs) on polymers' optical and toxic properties. Two different colloids of AgNPs were prepared by chemical reduction. The first colloid, a blue one, contains stable triangular nanoparticles (the mean size of the nanoparticles was ~75 nm). UV-vis spectrophotometry showed that the second colloid, a yellow colloid, was very unstable. Originally formed spherical particles (~11 nm in diameter) after 25 days changed into a mix of differently shaped nanoparticles (irregular, triangular, rod-like, spherical, decahedrons, etc.), and the dichroic effect was observed. Pre-prepared AgNPs were added into the PVA (poly(vinyl alcohol)) polymer matrix and PVA-AgNPs composites (poly(vinyl alcohol) doped by Ag nanoparticles) were prepared. PVA-AgNPs thin layers (by a spin-coating technique) and fibers (by electrospinning and dip-coating techniques) were prepared. TEM and SEM techniques were used to analyze the prepared composites. It was found that the addition of AgNPs caused a change in the optical and antibiofilm properties of the non-toxic and colorless polymer. The PVA-AgNPs composites not only showed a change in color but a dichroic effect was also observed on the thin layer, and a good antibiofilm effect was also observed.

Selecting High-Performance Gold Nanorods for Photothermal Conversion

In this work, we establish a new paradigm on identifying optimal arbitrarily shaped metallic nanostructures for photothermal applications. Crucial thermo-optical parameters that rule plasmonic heating are appraised, exploring a nanoparticle size-dependence approach. Our results indicate two distinct figures of merit for the optimization of metallic nanoheaters, under both non-cumulative femtosecond and continuum laser excitation. As a case study, gold nanorods are evaluated for infrared photothermal conversion in water, and the influence of the particle length and diameter are depicted. For non-cumulative femtosecond pulses, efficient photothermal conversion is observed for gold nanorods of small volumes. For continuous wave (CW) excitation at 800 nm and 1064 nm, the optimal gold nanorod dimensions (in water) are, respectively, 90 × 25nm and 150 × 30 nm. Figure of Merit (FoM) variations up to 700% were found considering structures with the same peak wavelength. The effect of collective heating is also appraised. The designing of high-performance plasmonic nanoparticles, based on quantifying FoM, allows a rational use of nanoheaters for localized photothermal applications.

Formation of kinetically trapped small clusters of PEGylated gold nanoparticles revealed by the combination of small-angle X-ray scattering and visible light spectroscopy

Gold nanoparticles coated with polyethylene glycol (PEG) are able to form clusters due to the collapse of the surface-grafted polymer chains when the temperature and ion concentration of the aqueous medium are increased. The chain collapse reduces the steric repulsion, leading to particle aggregation. In this work, we combine small angle X-ray scattering (SAXS) and visible light spectroscopy to elucidate the structure of the developing clusters. The structure derived from the SAXS measurements reveals a decrease in interparticle distance and drastic narrowing of its distribution in the cluster, indicating restricted particle mobility and displacement within the cluster. Surprisingly, instead of forming a large crystalline phase, the evolving clusters are composed of about a dozen particles. The experimental optical extinction spectra measured during cluster formation can be very well reproduced by optical simulations based on the SAXS-derived structural data.

Multidimensional characterization of noble metal alloy nanoparticles by multiwavelength analytical ultracentrifugation

In this study, we introduce a method for the simultaneous retrieval of two-dimensional size-composition distributions of noble metal Ag-Au alloy nanoparticles utilizing an analytical ultracentrifuge equipped with a multiwavelength extinction detector (MWL-AUC). MWL-AUC is used to measure coupled optical and sedimentation properties of the particles. The optical response of the nanoparticles is calculated using Mie's theory, where the particles' complex refractive index is corrected due to the effect of reduced mean free path of electrons. Using a combined analysis of the hydrodynamic and spectral data captured by MWL-AUC, the size and composition of the alloy particles is retrieved. Our method is validated through the analysis of synthetic data and by the very good agreement between experimental scanning transmission electron microscopy and our AUC data. The presented comprehensive characterization approach contributes to improved synthesis, scale-up and production of particulate systems as it provides a simple, fast and direct method to determine noble metal alloy nanoparticle size and composition distributions simultaneously.

Quantitatively linking morphology and optical response of individual silver nanohedra

The optical response of metal nanoparticles is governed by plasmonic resonances, which are dictated by the particle morphology. A thorough understanding of the link between morphology and optical response requires quantitatively measuring optical and structural properties of the same particle. Here we present such a study, correlating electron tomography and optical micro-spectroscopy. The optical measurements determine the scattering and absorption cross-section spectra in absolute units, and electron tomography determines the 3D morphology. Numerical simulations of the spectra for the individual particle geometry, and the specific optical set-up used, allow for a quantitative comparison including the cross-section magnitude. Silver nanoparticles produced by photochemically driven colloidal synthesis, including decahedra, tetrahedra and bi-tetrahedra are investigated. A mismatch of measured and simulated spectra is found in some cases when assuming pure silver particles, which is explained by the presence of a few atomic layers of tarnish on the surface, not evident in electron tomography. The presented method tightens the link between particle morphology and optical response, supporting the predictive design of plasmonic nanomaterials.

Electrochemical aspects of coinage metal nanoparticles for catalysis and spectroscopy

Down scaling bulk materials can cause colloidal systems to evolve into microscopically dispersed insoluble particles. Herein, we describe the interesting applications of coinage metal nanoparticles (MNPs) as colloid dispersions especially gold and silver. The rich plasmon bands of gold and silver in the visible range are elaborated using the plasmon resonance and redox potential values of grown metal microelectrode (GME). The gradation of their standard reduction potential values (E 0), as evaluated from the Gibbs free energy change for bulk metal, is ascribed to the variation in their size. Also, the effect of nucleophiles in the electrolytic cell with metal nanoparticles (MNPs) is described. The nucleophile-guided reduction potential value is considered, which is applicable even for bulk noble metals. Typically, a low value (as low as E 0 = +0.40 V) causes the oxidation of metals at the O2 (air)/H2O interface. Under this condition, the oxidation of noble metal particles and dissolution of the noble metal in water are demonstrated. Thus, metal dissolution as a function of the size of metal nanoparticles becomes eventful and demonstrable with the addition of a surfactant to the solution. Interestingly, the reversal of the nobility of gold (Au) and silver (Ag) microelectrodes at the water/electrode interface is confirmed from the evolution of normal and inverted 'core-shell' structures, exploiting visible spectrophotometry and surface-enhanced Raman scattering (SERS) analysis. Subsequently, the effect of the size, shape, and facet- and support-selective catalysis of gold nanoparticles (NPs) and the effect of incident photons on current conversion without an applied potential are briefly discussed. Finally, the synergistic effect of the emissive behaviour of gold and silver clusters is productively exploited.

Nanosecond-resolution photothermal dynamic imaging via MHZ digitization and match filtering

Photothermal microscopy has enabled highly sensitive label-free imaging of absorbers, from metallic nanoparticles to chemical bonds. Photothermal signals are conventionally detected via modulation of excitation beam and demodulation of probe beam using lock-in amplifier. While convenient, the wealth of thermal dynamics is not revealed. Here, we present a lock-in free, mid-infrared photothermal dynamic imaging (PDI) system by MHz digitization and match filtering at harmonics of modulation frequency. Thermal-dynamic information is acquired at nanosecond resolution within single pulse excitation. Our method not only increases the imaging speed by two orders of magnitude but also obtains four-fold enhancement of signal-to-noise ratio over lock-in counterpart, enabling high-throughput metabolism analysis at single-cell level. Moreover, by harnessing the thermal decay difference between water and biomolecules, water background is effectively separated in mid-infrared PDI of living cells. This ability to nondestructively probe chemically specific photothermal dynamics offers a valuable tool to characterize biological and material specimens.

Photothermal microscopy is limited for imaging of thermal dynamics. Here, the authors introduce a lock-in free, mid-infrared photothermal dynamic imaging system, which significantly increases SNR and imaging speed, and demonstrate metabolism analysis at single-cell level and background removal.

Near-infrared photothermal performance of a metal-organic framework-based composite

The construction of heterostructures is a universal method to hinder the radiative recombination of hot electrons and hot holes, which can effectively enhance the photothermal effect of semiconductors. In this work, a one-pot method was employed to prepare a composite named Bi₂Se₃@ZIF-8 NPs, which incredibly increased the photothermal conversion efficiency of Bi₂Se₃ NPs. The temperature elevation of Bi₂Se₃@ZIF-8 NPs was almost double that of the Bi₂Se₃ NPs; specifically, the temperature of the irradiated Bi₂Se₃@ZIF-8 NPs was strikingly increased to 130 °C within 6 seconds, and finally stabilized at 165 °C. Furthermore, the photothermal conversion ability was maintained over multiple irradiation cycles, which endows this composite with great potential to be an excellent photothermal agent.

Programmable Metal Nanoclusters with Atomic Precision

With the recent establishment of atomically precise nanochemistry, capabilities toward programmable control over the nanoparticle size and structure are being developed. Advances in the synthesis of atomically precise nanoclusters (NCs, 1-3 nm) have been made in recent years, and more importantly, their total structures (core plus ligands) have been mapped out by X-ray crystallography. These ultrasmall Au nanoparticles exhibit strong quantum-confinement effect, manifested in their optical absorption properties. With the advantage of atomic precision, gold-thiolate nanoclusters (Au_n (SR)_m ) are revealed to contain an inner kernel, Au-S interface (motifs), and surface ligand (-R) shell. Programming the atomic packing into various crystallographic structures of the metal kernel can be achieved, which plays a significant role in determining the optical properties and the energy gap (E_g ) of NCs. When the size increases, a general trend is observed for NCs with fcc or decahedral kernels, whereas those NCs with icosahedral kernels deviate from the general trend by showing comparably smaller E_g . Comparisons are also made to further demonstrate the more decisive role of the kernel structure over surface motifs based on isomeric Au NCs and NC series with evolving kernel or motif structures. Finally, future perspectives are discussed.

Spectral and photophysical modifications of porphyrins attached to core-shell nanoparticles. Theory and experiment

Plasmonic nanostructures, of which gold nanoparticles are the most elementary example, owe their unique properties to localized surface plasmons (LSP), the modes of free electron oscillation. LSP alter significantly electromagnetic field in the nanostructure neighborhood (i.e., near-field), which can modify the electric dipole transition rates in organic emitters. This study aims at investigating the influence of Au@SiO₂core-shell nanoparticles on the photophysics of porphyrins covalently attached to the nanoparticles surface. Guided by theoretical predictions, three sets of gold nanoparticles of different sizes were coated with a silica layer of similar thickness. The outer silica surface was functionalized with either free-basemeso-tetraphenylporphyrin or its zinc complex. Absorption and emission bands of porphyrin overlap in energy with a gold nanoparticle LSP resonance that provides the field enhancement. Silica separates the emitters from the gold surface, while the gold core size tunes the energy of the LSP resonance. The signatures of weak-coupling regime have been observed. Apart from modified emission profiles and shortened S₁lifetimes, Q band part intensity of the excitation spectra significantly increased with respect to the Soret band. The results were explained using classical transfer matrix simulations and electronic states kinetics, taking into account the photophysical properties of each chromophore. The calculations could reasonably well predict and explain the experimental outcomes. The discrepancies between the two were discussed.

Size effect of bifunctional gold in hierarchical titanium oxide-gold-cadmium sulfide with slow photon effect for unprecedented visible-light hydrogen production

Gold nanoparticles (Au NPs) with surface plasmonic resonance (SPR) effect and excellent internal electron transfer ability have widely been combined with semiconductors for photocatalysis. However, the in-depth effects of Au NPs in multicomponent photocatalysts have not been completely understood. Herein, ternary titanium oxide-gold-cadmium sulfide (TiO₂-Au-CdS, TAC) photocatalysts, based on hierarchical TiO₂ inverse opal photonic crystal structure with different Au NPs sizes have been designed to reveal the SPR effect and internal electron transfer of Au NPs in the presence of slow photon effect. It appears that the SPR effect and internal electron transfer ability of Au NPs, depending on their sizes, play a synergistic effect on the photocatalytic enhancement. The ternary TAC-10 photocatalyst with ~ 10 nm Au NPs demonstrates an unprecedented hydrogen evolution rate of 47.6 mmolh^-1g^-1 under visible-light, demonstrating ~ 48% enhancement comparing to the sample without slow photon effect. In particular, a 9.83% apparent quantum yield under 450 nm monochromatic light is achieved for TAC-10. A model is proposed and finite-difference time-domain (FDTD) simulations reveal the size influence of Au NPs in ternary TAC photocatalysts. This work suggests that the rational design of bifunctional Au NPs coupling with slow photon effect could largely promote hydrogen production from visible-light driven water splitting.

Highly stable measurement for nanoparticle extinction cross section by analyzing aperture-edge blurriness

In order to stabilize the extinction cross section measurement of a single nanoparticle, we propose to analyze the blurriness parameter of aperture edge images in real time, which provides a feedback to lock the sample position. Unlike the conventional spatial modulation spectroscopy (SMS) technique, a probe beam experiences both the spatial modulation by a piezo stage and the temporal modulation by a chopper. We experimentally demonstrate that the measurement uncertainty is one order magnitude less than that in the previous report. The proposed method can be readily implemented in conventional SMS systems and can help to achieve high stability for sensing based on light extinction by a single nanoparticle, which alleviate the impact from laboratory environment and increase the experimental sensitivity.

Gold nanomaterials for optical biosensing and bioimaging

Gold nanoparticles (AuNPs) are highly compelling nanomaterials for biomedical studies due to their unique optical properties. By leveraging the versatile optical properties of different gold nanostructures, the performance of biosensing and biomedical imaging can be dramatically improved in terms of their sensitivity, specificity, speed, contrast, resolution and penetration depth. Here we review recent advances of optical biosensing and bioimaging techniques based on three major optical properties of AuNPs: surface plasmon resonance, surface enhanced Raman scattering and luminescence. We summarize the fabrication methods and optical properties of different types of AuNPs, highlight the emerging applications of these AuNPs for novel optical biosensors and biomedical imaging innovations, and discuss the future trends of AuNP-based optical biosensors and bioimaging as well as the challenges of implementing these techniques in preclinical and clinical investigations.

Novel semi-analytical optoelectronic modeling based on homogenization theory for realistic plasmonic polymer solar cells

Numerical-based simulations of plasmonic polymer solar cells (PSCs) incorporating a disordered array of non-uniform sized plasmonic nanoparticles (NPs) impose a prohibitively long-time and complex computational demand. To surmount this limitation, we present a novel semi-analytical modeling, which dramatically reduces computational time and resource consumption and yet is acceptably accurate. For this purpose, the optical modeling of active layer-incorporated plasmonic metal NPs, which is described by a homogenization theory based on a modified Maxwell-Garnett-Mie theory, is inputted in the electrical modeling based on the coupled equations of Poisson, continuity, and drift-diffusion. Besides, our modeling considers the effects of absorption in the non-active layers, interference induced by electrodes, and scattered light escaping from the PSC. The modeling results satisfactorily reproduce a series of experimental data for photovoltaic parameters of plasmonic PSCs, demonstrating the validity of our modeling approach. According to this, we implement the semi-analytical modeling to propose a new high-efficiency plasmonic PSC based on the PM6:Y6 PSC, having the highest reported power conversion efficiency (PCE) to date. The results show that the incorporation of plasmonic NPs into PM6:Y6 active layer leads to the PCE over 18%.

Between plasmonics and surface-enhanced resonant Raman spectroscopy: toward single-molecule strong coupling at a hotspot

The purpose of this minireview is to build a bridge between two research fields: surface-enhanced resonant Raman spectroscopy (SERRS) under near-single-molecule conditions and the branch of plasmonics treating strong coupling between plasmons and molecular excitons. SERRS enables single-molecule spectroscopy owing to its significant enhancement at SERRS hotspots (HSs), localized at gaps or junctions between plasmonic nanoparticle aggregates. SERRS is SERS (surface enhanced Raman spectroscopy) under a resonant Raman excitation condition. The origin of the Raman enhancement in SERRS is electromagnetic coupling between plasmons and molecular excitons at HSs. It has been reported that the coupling energy at HSs reaches the strong coupling region, meaning that they are potential platforms for applications of single molecular excitons modified by strong coupling. In this review, we discuss recent progress related to electronic strong coupling in near-single-molecule SERRS: collective (e.g., vibrational) strong coupling is out of the scope of this minireview. First, we explain the relationship between the electromagnetic enhancement factor and coupling energy. Second, we introduce three theoretical methods for obtaining evidence of strong coupling at HSs. Third, we discuss a method for reproducing enhanced and modified molecular Raman and fluorescence spectra at HSs using the coupling energy. Finally, we propose the use of two experimental methods of absorption spectroscopy at HSs for modifying molecular electronic dynamics by strong coupling and comment on future applications of SERRS HSs to photophysics and photochemistry.

Simulation of the optical properties of gold nanoparticles on sodium alginate

In this contribution, we report on the simulation of optical reflectance and transmittance (R&T) taken on a set of gold nanoparticles thin film, deposited on sodium alginate by magnetron sputtering. The gold layer is very thin, so that the films are not continuous and the material is arranged in nanostructured layers. R&T spectra are simulated using the Generalized Transfer Matrix method applied to the film-on-substrate model. The gold NP films are simulated using the Drude-Lorentz model, by taking into account that the optical function of nanostructured gold exhibits increased collision frequency and reduced relaxation time. Moreover, the signal of localized surface plasmon, evident in the spectra, is simulated by introducing a dedicated modified Lorentz oscillator. The experimental results are well reproduced by the applied model. All trends (amplitude and energy position of the plasmon oscillator, film thickness, relaxation time) are correlated with the deposition parameters. The procedure represents a useful tool in the characterisation of such nanoparticles thin films.

Accelerated Development of Colloidal Nanomaterials Enabled by Modular Microfluidic Reactors: Toward Autonomous Robotic Experimentation

In recent years, microfluidic technologies have emerged as a powerful approach for the advanced synthesis and rapid optimization of various solution-processed nanomaterials, including semiconductor quantum dots and nanoplatelets, and metal plasmonic and reticular framework nanoparticles. These fluidic systems offer access to previously unattainable measurements and synthesis conditions at unparalleled efficiencies and sampling rates. Despite these advantages, microfluidic systems have yet to be extensively adopted by the colloidal nanomaterial community. To help bridge the gap, this progress report details the basic principles of microfluidic reactor design and performance, as well as the current state of online diagnostics and autonomous robotic experimentation strategies, toward the size, shape, and composition-controlled synthesis of various colloidal nanomaterials. By discussing the application of fluidic platforms in recent high-priority colloidal nanomaterial studies and their potential for integration with rapidly emerging artificial intelligence-based decision-making strategies, this report seeks to encourage interdisciplinary collaborations between microfluidic reactor engineers and colloidal nanomaterial chemists. Full convergence of these two research efforts offers significantly expedited and enhanced nanomaterial discovery, optimization, and manufacturing.

Photothermal Microscopy: Imaging the Optical Absorption of Single Nanoparticles and Single Molecules

The photothermal (PT) signal arises from slight changes of the index of refraction in a sample due to absorption of a heating light beam. Refractive index changes are measured with a second probing beam, usually of a different color. In the past two decades, this all-optical detection method has reached the sensitivity of single particles and single molecules, which gave birth to original applications in material science and biology. PT microscopy enables shot-noise-limited detection of individual nanoabsorbers among strong scatterers and circumvents many of the limitations of fluorescence-based detection. This review describes the theoretical basis of PT microscopy, the methodological developments that improved its sensitivity toward single-nanoparticle and single-molecule imaging, and a vast number of applications to single-nanoparticle imaging and tracking in material science and in cellular biology.

Plasmonic evolution of atomically size-selected Au clusters by electron energy loss spectrum

The plasmonic response of gold clusters with atom number (N) = 100–70 000 was investigated using scanning transmission electron microscopy-electron energy loss spectroscopy. For decreasing N, the bulk plasmon remains unchanged above N = 887 but then disappears, while the surface plasmon firstly redshifts from 2.4 to 2.3 eV above N = 887 before blueshifting towards 2.6 eV down to N = 300, and finally splitting into three fine features. The surface plasmon's excitation ratio is found to follow N^0.669, which is essentially R². An atomically precise evolution picture of plasmon physics is thus demonstrated according to three regimes: classical plasmon (N = 887–70 000), quantum confinement corrected plasmon (N = 300–887) and molecule related plasmon (N < 300).

A novel concept of two-component dielectric function for gold nanostars: theoretical modelling and experimental verification

Rational design of AuNST morphology requires adequate computational models. The bulk dielectric function is not applicable to sharp nanostar spikes. We suggest a two-component dielectric function in which the nanostar core is treated as a bulk material, whereas the size-corrected dielectric function of the spikes is treated by a modified Coronado-Schatz model. In addition to the strong broadening of plasmonic peaks, the simulated absorption and scattering spectra show unusual properties, which are not observed with bulk dielectric functions. The effect of NIR water absorption on nanostar spectra is small, and the absorption peak demonstrates the expected small decrease in the absorbing media. Surprisingly, however, water absorption increases the scattering peak by 30%. For the common surfactant-free Vo-Dinh AuNSTs, we report, for the first time, very intense SWIR plasmonic peaks around 1900 nm, in addition to the common strong peak in the UV-vis-NIR band (here, at 1100 nm). For bilayers of AuNSTs in air, we recorded two similarly intense peaks near 800 and 1500 nm. To simulate the experimental extinction spectra of colloids and bilayers on glass in air, we develop a statistical model that includes the major fraction of typical Vo-Dinh AuNSTs and two minor fractions of sea urchins and particles with protrusions. In contrast to the general belief, we show that the common UV-vis-NIR plasmonic peak of surfactant-free AuNSTs is related to short spikes on a spherical core, whereas long spikes produce an intense SWIR plasmonic mode. Such a structural assignment of vis-NIR and SWIR peaks does not seem to have been reported previously for surfactant-free nanostars. With our model, we demonstrate good agreement between simulated and measured spectra of colloids and bilayers on glass in air.

Controlled Synthesis of Gold Nanoparticles in Copolymers Nanomolds by X-ray Radiolysis

We show by X-ray and neutron small-angle scattering that gold nanoparticles with controlled sizes and morphologies can be obtained by the metallic reduction of AuCl₄^- ions trapped in 3D organic molds by X-ray radiolysis. The molds are spherical frozen micelles of polystyrene-b-poly(dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) block copolymer in acidic aqueous solution with a PS spherical core surrounded by a corona of PDMAEMA chains in good solvent. The behavior of micelles is controlled by the [AuCl₄^-]/[DMAEMA] ratio R_{AuCl4^-/DMAEMA}. At low gold concentration, AuCl₄^- ions condense on the positively charged DMAEMA moieties without changing the behavior of the PDMAEMA chains. At intermediate gold concentration, the ions induce a progressive contraction of the corona's chains and dehydration of micelles. At large gold concentration, the corona becomes a fully dry phase loaded with gold ions, which induces micelle aggregation. Radiolysis of the solution by an intense X-ray beam produces different types of gold nanoparticles with respect to R_{AuCl4^-/DMAEMA} and irradiation time. At R_{AuCl4^-/DMAEMA} = 0.033, irradiation produces in the first step gold clusters in the micelle corona which in the second step merge to form nanoparticles of a similar size to that of the micelle. Conversely, at R_{AuCl4^-/DMAEMA} = 0.33, micelles do not operate as templates but only as nucleation zones and large nanoparticles grow outside the micelles.

Device for real-time monitoring of oil-in-water and suspended solids based on thermal lens spectrometry and light scattering

A novel system suitable for simultaneous monitoring of both oil-in-water and suspended solids based on thermal lens spectroscopy and forward light scattering is presented. The technique measures the concentration of dissolved hydrocarbons and simultaneously detects single oil droplets and suspended particles separately. The device was tested with injection water samples from an on-field water treatment plant, and hydrocarbon concentrations were measured with a precision better than 5% in the range of up to 100 ppm, reaching resolutions as low as 0.03 ppm. Particle detection was tested with model samples of dyed and undyed polystyrene spheres acting as absorption and scattering centers, which simulated oil droplets and suspended solids, respectively. We show that particles of different sizes are distinguished by the magnitude of the perturbations introduced in the signals, and their concentrations can be measured independently of dissolved components.

Modified Drude model for small gold nanoparticles surface plasmon resonance based on the role of classical confinement

In this paper, we study the effect of restoration force caused by the limited size of a small metallic nanoparticle (MNP) on its linear response to the electric field of incident light. In a semi-classical phenomenological Drude-like model for small MNP, we consider restoration force caused by the displacement of conduction electrons with respect to the ionic positive background taking into account a free coefficient as a function of diameter of nanoparticle (NP) in the force term obtained by the idealistic Thomson model in order to adjust the classical approach. All important mechanisms of the energy dissipation such as electron-electron, electron-phonon and electron-NP surface scatterings and radiation are included in the model. In addition a correction term added to the damping factor of mentioned mechanisms in order to rectify the deficiencies of theoretical approaches. For determining the free parameters of model, the experimental data of extinction cross section of gold NPs with different sizes doped in the glass host medium are used and a good agreement between experimental data and results of our model is observed. It is shown that by decreasing the diameter of NP, the restoration force becomes larger and classical confinement effect becomes more dominant in the interaction. According to experimental data, the best fitted parameter for the coefficient of restoration force is a third order negative powers function of diameter. The fitted function for the correction damping factor is proportional to the inverse squared wavelength and third order power series of NP diameter. Based on the model parameters, the real and imaginary parts of permittivity for different sizes of gold NPs are presented and it is seen that the imaginary part is more sensitive to the diameter variations. Increase in the NP diameter causes increase in the real part of permittivity (which is negative) and decrease in the imaginary part.

A novel method for in situ visualization of the growth kinetics, structures and behaviours of gas-phase fabricated metallic alloy nanoparticles

Modulation of gas-phase nanoparticles is unmethodical as there is a lack of information on the growth kinetics and its determinants. Here, we developed a novel in situ evaporation-and-deposition (EAD) method inside a transmission electron microscope which enables direct visualization of the nucleation, growth, coalescence and shape/phase evolution of gas-phase fabricated nanoparticles. Using a Bi₄₉Pb₁₈Sn₁₂In₂₁ alloy as a sample, the critical factors that determine the feasibility of this EAD method are revealed. By direct observation, it is unambiguously evidenced that pristine nanoparticles with ultra-clean surfaces are extremely energetic during growth. Coalescence between EAD-fabricated nanoparticles takes place in a manner beyond conventional understanding acquired by postmortem analyses. Moreover, the EAD-fabricated diverse nanoparticles show distinct size distributions and sandwich-type or Janus-type phase segregations. These features offer an effective tool to identify atomic surface steps of thin films and can provide an ideal case for exploring the phase diagrams of nanoalloys in the future.

In situ visualizing the growth kinetics and behaviours of alloy nanoparticles by a novel EAD method.

Toward Real-Time Monitoring and Control of Single Nanoparticle Properties with a Microbubble Resonator Spectrometer

Optical microresonators have widespread application at the frontiers of nanophotonic technology, driven by their ability to confine light to the nanoscale and enhance light-matter interactions. Microresonators form the heart of a recently developed method for single-particle photothermal absorption spectroscopy, whereby the microresonators act as microscale thermometers to detect the heat dissipated by optically pumped, nonluminescent nanoscopic targets. However, translation of this technology to chemically dynamic systems requires a platform that is mechanically stable, solution compatible, and visibly transparent. We report microbubble absorption spectrometers as a versatile platform that meets these requirements. Microbubbles integrate a two-port microfluidic device within a whispering gallery mode microresonator, allowing for the facile exchange of chemical reagents within the resonator's interior while maintaining a solution-free environment on its exterior. We first leverage these qualities to investigate the photoactivated etching of single gold nanorods by ferric chloride, providing a method for rapid acquisition of spatial and morphological information about nanoparticles as they undergo chemical reactions. We then demonstrate the ability to control nanorod orientation within a microbubble through optically exerted torque, a promising route toward the construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator spectrometer technology by permitting room-temperature, aqueous experimental conditions, which may be used for time-resolved single-particle experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.

Photothermal microspectroscopy with Bessel-Gauss beams and reflective objectives

Here, we investigate scanning photothermal microspectroscopic imaging of metal nanoparticles with reflective objectives. We show that correction-less collection of spectra from single spherical nanoparticles embedded in a polymer is possible over a wide spectral band, with large depth of focus, long working distance, and high lateral spatial resolution. We posit that these beneficial characteristics are inherent of the Bessel-Gauss character of the focused beam. When compared with other types of optical microscopy, the combination of these characteristics give photothermal imaging with reflective objectives unique appeal for material characterization applications.

Making waves: Radiation damping in metallic nanostructures

Metal nanostructures display several types of resonances. In the visible and near-IR spectral regions, there are localized surface plasmon resonances (LSPRs) that involve the coherent oscillation of the conduction electrons. Extended metal nanostructures, such as nanowires or nanoplates, also exhibit propagating surface plasmon polaritons (PSPPs), which are motions of the electrons at the surface of the structure that have a well-defined momentum. In addition, the vibrational normal modes of metal nanostructures give rise to low frequency resonances in the gigahertz to terahertz range. These different types of motions/resonances suffer energy losses from internal effects and from interactions with the environment. The goal of this perspective is to describe the part of the energy relaxation process due to the environment. Even though the plasmon resonances and acoustic vibrational modes arise from very different physics, it turns out that environmental damping is dominated by radiation of waves. The way the rates for radiation damping depend on the size of the nanostructure and the properties of the environment will be discussed for the different processes. For example, it is well known that for LSPRs, the rate of radiation damping increases with particle size. However, the radiation damping rate decreases with increasing dimensions for PSPPs and for the acoustic vibrational modes.

Quantitative Measurement of the Optical Cross Sections of Single Nano-objects by Correlative Transmission and Scattering Microspectroscopy

The scattering and absorption of light by nano-objects is a key physical property exploited in many applications, including biosensing and photovoltaics. Yet, its quantification at the single object level is challenging and often requires expensive and complicated techniques. We report a method based on a commercial transmission microscope to measure the optical scattering and absorption cross sections of individual nano-objects. The method applies to microspectroscopy and wide-field image analysis, offering fine spectral information and high throughput sample characterization. Accurate cross-section determination requires detailed modeling of the measurement, which we develop, accounting for the geometry of the illumination and detection as well as for the presence of a sample substrate. We demonstrate the method on three model systems (gold spheres, gold rods, and polystyrene spheres), which include metallic and dielectric particles, spherical and elongated, placed in a homogeneous medium or on a dielectric substrate. Furthermore, by comparing the measured cross sections with numerical simulations, we are able to determine structural parameters of the studied system, such as the particle diameter and aspect ratio. Our method therefore holds the potential to complement electron microscopy as a simpler and cost-effective tool for structural characterization of single nano-objects.

Plasmonic performance of AuxAgyCu1-x-y alloys from many-body perturbation theory

We present a detailed appraisal of the optical and plasmonic properties of ordered alloys of the form Au_xAg_yCu_1-x-y, as predicted by means of first-principles many-body perturbation theory augmented by a semi-empirical Drude-Lorentz model. In benchmark simulations on elemental Au, Ag, and Cu, we find that the random-phase approximation (RPA) fails to accurately describe inter-band transitions when it is built upon semi-local approximate Kohn-Sham density-functional theory band-structures. We show that non-local electronic exchange-correlation interactions sufficient to correct this, particularly for the fully-filled, relatively narrow d-bands which contribute strongly throughout the low-energy spectral range (0-6 eV), may be modeled very expediently using band-stretching operators that imitate the effect of a perturbative [Formula: see text] self-energy correction incorporating quasiparticle (QP) mass renormalization. We thereby establish a convenient work-flow for carrying out approximated [Formula: see text] spectroscopic calculations on alloys and, in particular here, we have considered alloy concentrations down to 12.5% in [Formula: see text], including all possible crystallographic orderings of face-centred cubic type. We develop a pragmatic procedure for calculating the Drude plasmon frequency from first principles, including self-energy effects, as well as a semi-empirical scheme for interpolating the plasmon inverse lifetimes between stoichiometries. A distinctive M-shaped profile is observed in both quantities for binary alloys, in qualitative agreement with previous experimental findings. A range of optical and plasmonic figures of merit are discussed, and plotted for ordered [Formula: see text] at three representative solid-state laser wavelengths. On this basis, we predict that certain compositions may offer improved performance over elemental Au for particular application types. We predict that while the loss functions for both bulk and surface plasmons are typically diminished in strength through binary alloying, certain stoichiometric ratios may exhibit higher-quality (longer-lived) localized surface-plasmons and surface-plasmon polaritons, at technologically-relevant wavelengths, than those in elemental Au.

Nonclassical Optical Properties of Mesoscopic Gold

Gold nanostructures have important applications in nanoelectronics, nano-optics, and in precision metrology due to their intriguing optoelectronic properties. These properties are governed by the bulk band structure but to some extent are tunable via geometrical resonances. Here we show that the band structure of gold itself exhibits significant size-dependent changes already for mesoscopic critical dimensions below 30 nm. To suppress the effects of geometrical resonances and grain boundaries, we prepared atomically flat ultrathin films of various thicknesses by utilizing large chemically grown single-crystalline gold platelets. We experimentally probe thickness-dependent changes of the band structure by means of two-photon photoluminescence and observe a surprising 100-fold increase of the nonlinear signal when the gold film thickness is reduced below 30 nm allowing us to optically resolve single-unit-cell steps. The effect is well explained by density functional calculations of the thickness-dependent 2D band structure of gold.

Plasmon-induced hot-carrier generation differences in gold and silver nanoclusters

In the last thirty years, the study of plasmonic properties of noble metal nanostructures has become a very dynamic research area. The design and manipulation of matter in the nanometric scale demands a deep understanding of the underlying physico-chemical processes that operate in this size regimen. Here, a fully atomistic study of the spectroscopic and photodynamic properties of different icosahedral silver and gold nanoclusters has been carried out by using a Time-Dependent Density Functional Tight-Binding (TD-DFTB) model. The optical absorption spectra of different icosahedral silver and gold nanoclusters of diameters between 1 and 4 nanometers have been simulated. Furthermore, the energy absorption process has been quantified by means of calculating a fully quantum absorption cross-section using the information contained in the reduced single-electron density matrix. This approach allows us take into account the quantum confinement effects dominating in this size regime. Likewise, the plasmon-induced hot-carrier generation process under laser illumination has been explored from a fully dynamical perspective. We have found noticeable differences in the energy absorption mechanisms and the plasmon-induced hot-carrier generation process in both metals which can be explained by their respective electronic structures. These differences can be attributed to the existence of ultra-fast electronic dissipation channels in gold nanoclusters that are absent in silver nanoclusters. To the best of our knowledge, this is the first report that addresses this topic from a real time fully atomistic time-dependent approach.

Spectrophotometric Characterization of Thin Copper and Gold Films Prepared by Electron Beam Evaporation: Thickness Dependence of the Drude Damping Parameter

Copper and gold films with thicknesses between approximately 10 and 60 nm have been prepared by electron beam evaporation and characterized by spectrophotometry from the near infrared up to the near ultraviolet spectral regions. From near normal incidence transmission and reflection spectra, dispersion of optical constants have been determined by means of spectra fits utilizing a merger of the Drude model and the beta-distributed oscillator model. All spectra could be fitted in the full spectral region with a total of seven dispersion parameters. The obtained Drude damping parameters shows a clear trend to increase with decreasing film thickness. This behavior is discussed in the context of additional non-optical characterization results and turned out to be consistent with a simple mean-free path theory.

The role of a plasmonic substrate on the enhancement and spatial resolution of tip-enhanced Raman scattering

Since the first report in the early 2000s, there have been several experimental configurations that have demonstrated enhancement and spatial resolution of tip-enhanced Raman spectroscopy (TERS). The combination of a plasmonic substrate and a metallic tip is one suitable approach to achieve even higher enhancement and lateral resolution. In this contribution, we demonstrate TERS on a monolayer of MoS₂ on an array of Au nanodisks. The Au nanodisks were prepared by electron beam writing. Thereafter, MoS₂ was transferred onto the plasmonic substrate via the exfoliation technique. We witness an unprecedented enhancement and spatial resolution in the experiments. In the TERS image a ring-like shape is observed that matches the edges of the nanodisks. TERS enhancement at the edges is about 170 times stronger than at the center of the nanodisks. For a better understanding of the experimental results, finite element method (FEM) simulations were employed to simulate the TERS image of the MoS₂/plasmonic heterostructure. Our calculations show a higher electric field concentration at the edges that exponentially decays to the center. Therefore, it reproduces the ring-like shape of the experimental image. Moreover, the calculations suggest a TERS enhancement of 135 at the edges compared to the center, which is in very good agreement with the experimental data. According to our calculations, the spatial resolution is also increased at the edges. For comparison, FEM simulations of a tip-flat metal substrate system (conventional gap-mode TERS) were carried out. The calculations confirmed a 110 times stronger enhancement at the edges of the nanodisks than that of conventional gap-mode TERS and explained the experimental maps. Our results provide not only a deeper understanding of the TERS mechanism of this heterostructure, but can also help in realizing highly efficient TERS experiments using similar systems.

Direct optical excitation of dark plasmons for hot electron generation

We demonstrate the excitation of dark modes and creation of hot electrons using linearly polarized light and scalable, cost-effective plasmonic surfaces.

White-Light Photosensors Based on Ag Nanoparticle-Reduced Graphene Oxide Hybrid Materials

The unique and outstanding electrical and optical properties of graphene make it a potential material to be used in the construction of high-performance photosensors. However, the fabrication process of a graphene photosensor is usually complicated and the size of the device also is restricted to micrometer scale. In this work, we report large-area photosensors based on reduced graphene oxide (rGO) implemented with Ag nanoparticles (AgNPs) via a simple and cost-effective method. To further optimize the performance of photosensors, the absorbance and distribution of the electrical field intensity of graphene with AgNPs was simulated using the finite-difference time-domain (FDTD) method through use of the surface plasmon resonance effect. Based on the simulated results, we constructed photosensors using rGO with 60–80 nm AgNPs and analyzed the characteristics at room temperature under white-light illumination for outdoor environment applications. The on/off ratio of the photosensor with AgNPs was improved from 1.166 to 9.699 at the bias voltage of −1.5 V, which was compared as a sample without AgNPs. The proposed photosensor affords a new strategy to construct cost-effective and large-area graphene films which raises opportunities in the field of next-generation optoelectronic devices operated in an outdoor environment.

A comparative study of stability, antioxidant, DNA cleavage and antibacterial activities of green and chemically synthesized silver nanoparticles

Silver nanoparticles have a wide range of research, industrial and biomedical applications that make it essential to develop a low cost and eco-friendly approach with scaling up potential. Green synthesis of nanoparticles through bio-reactions leads to a reduction of silver ions to particles could be an acceptable selection using no additional reducing chemicals. Moreover, the simplicity of scale-up processes of the method makes it more efficient than chemical and physical synthesis methods. In this study, Datura stramonium leaf extract and sodium citrate were used as biological and chemical reducing and stabilizing agents to make silver nanoparticles. The main goal is to comprise properties and evaluate antibacterial activity of nanoparticles synthesized through two approaches. Size and morphology compared between the two types of the synthesized nanoparticle by UV-Visible spectroscopy, DLS, AFM, TEM and their antibacterial effects were evaluated through growth inhibition MIC and MBC methods. The results showed narrow size range, spherical shape, high anti-oxidant, antibacterial and DNA cleavage activities of green synthesized silver nanoparticles comparing to less average size, wider range of nanoparticle size, no anti-oxidant activity and less antibacterial and DNA cleavage activities of chemically synthesized nanoparticles. The green synthesized silver nanoparticles had more desirable characteristics and biological activities compared to chemically synthesized nanoparticles. For instance, the green nanoparticles showed narrow size range, spherical shape, high anti-oxidant, antibacterial and DNA cleavage activities versus the chemically synthesized which had less average size, higher range of nanoparticles size, no anti-oxidant activity and less antibacterial and DNA cleavage activities.

Strategies to synthesize various nanostructures of silver and their applications - a review

Due to their various beneficial application-based properties, such as behavior, structure, and size, the synthesis of silver nanoparticles (Ag-NPs) with different structures has become an interesting yet common task for researchers to produce nanostructures for applications in various fields. This is because silver nanoparticles have interesting and unique properties, such as optical and catalytic, resulting from their different structures and sizes. These properties extend the use of nanostructures in various fields of research, especially in medicine, pharmacy, electronics, etc. Also, variations in their parameters affect the structures and sizes of Ag-NPs. This review provides an overview/brief presentation of various methodologies used to synthesize different application-based silver nanoparticles and lists areas where these nanoparticles are suitable for use according to their specific structures and sizes.

Imaging and Spectroscopy of Single Metal Nanostructure Absorption

The highly tunable optical properties of metal nanoparticles make them an ideal building block in any application that requires control over light, heat, or electrons on the nanoscale. Because of their size, metal nanoparticles both absorb and scatter light efficiently. Consequently, improving their performance often involves shifting the balance between absorption and scattering to promote desirable features of their optical properties. Scattering by single metal nanoparticles is commonly characterized using dark-field scattering spectroscopy, but routine methods to characterize pure absorption over a broad wavelength range are much more complex. This article reviews work from our lab using photothermal imaging in combination with dark-field scattering and electron microscopy to separate radiative and nonradiative properties of single nanoparticles and their assemblies. We present both initial work using different laser wavelengths to explore pure absorption free from scattering contributions based on the heat released into the environment as well as the development of photothermal spectroscopy over a broad wavelength range, making it possible to resolve details that are otherwise hidden in ensemble measurements that most of the time also do not separate radiative and nonradiative properties.