Fully inorganic, colloidal gold nanoclusters (NCs) constitute a new class of nanomaterials that are clearly distinguishable from their commonly studied metal-organic ligand-capped counterparts. As their synthesis by chemical methods is challenging, details about their optical properties remain widely unknown. In this work, laser fragmentation in liquids is performed to produce fully inorganic and size-controlled colloidal gold NCs with monomodal particle size distributions and an fcc-like structure. Results reveal that these NCs exhibit highly pronounced photoluminescence with quantum yields of 2%. The emission behavior of small (2-2.5 nm) and ultrasmall (<1 nm) NCs is significantly different and dominated by either core- or surface-based emission states. It is further verified that emission intensities are a function of the surface charge density, which is easily controllable by the pH of the surrounding medium. This experimentally observed correlation between surface charge and photoluminescence emission intensity is confirmed by density functional theoretical simulations, demonstrating that fully inorganic NCs provide an appropriate material to bridge the gap between experimental and computational studies of NCs. The presented study deepens the understanding of electronic structures in fully inorganic colloidal gold NCs and how to systematically tune their optical properties via surface charge density and particle size.
Pulsed laser ablation in liquid (PLAL) is a powerful method for producing nanoparticle colloids with a long-term stability despite the absence of stabilizing organic agents. The colloid stability involves different reactivities and chemical equilibria with complex ionic-specific effects at the nanoparticle/solvent interface which must be strongly influenced by their chemical composition. In this work, the surface composition of PLAL-produced gold nanoparticles in alkaline and saline (NaBr) water is investigated by X-ray photoelectron spectroscopy on free-flying nanoparticles, exempt from any substrate or radiation damage artifact. The Au 4f photoelectron spectra with a depth profiling investigation are used to evaluate the degree of nanoparticle surface oxidation. In alkaline water, the results preclude any surface oxidation contrary to the case of nanoparticles produced in NaBr solution. In addition, the analysis of Br 3d core-level photoelectron spectra agrees with a clear signature of Br on the nanoparticle surface, which is confirmed by a specific valence band feature. This experimental study is supported by DFT calculations, evaluating the energy balance of halide adsorption on different configurations of gold surfaces including oxidation or adsorbed salts.
Although oxide nanoparticles are ubiquitous in science and technology, a multitude of compositions, phases, structures, and doping levels exist, each one requiring a variety of conditions for their synthesis and modification. Besides, experimental procedures are frequently dominated by high temperatures or pressures and by chemical contaminants or waste. In recent years, laser synthesis of colloids emerged as a versatile approach to access a library of clean oxide nanoparticles relying on only four main strategies running at room temperature and ambient pressure: laser ablation in liquid, laser fragmentation in liquid, laser melting in liquid and laser defect-engineering in liquid. Here, established laser-based methodologies are reviewed through the presentation of a panorama of oxide nanoparticles which include pure oxidic phases, as well as unconventional structures like defective or doped oxides, non-equilibrium compounds, metal-oxide core-shells and other anisotropic morphologies. So far, these materials showed several useful properties that are discussed with special emphasis on catalytic, biomedical and optical application. Yet, given the endless number of mixed compounds accessible by the laser-assisted methodologies, there is still a lot of room to expand the library of nano-crystals and to refine the control over products as well as to improve the understanding of the whole process of nanoparticle formation. To that end, this review aims to identify the perspectives and unique opportunities of laser-based synthesis and processing of colloids for future studies of oxide nanomaterial-oriented sciences.
Rational design and precise engineering are needed to optimize the structural and chemical parameters of functional materials. In this work, we demonstrate how pre-formed binary metal selenides can be an excellent synthetic choice for the synthesis of ternary coinage metal selenide nanoparticles (NPs) with controlled composition. The mild conditions required to obtain these ternary coinage metal selenide NPs offered an easy synthesis of n% CuAgSe-TiO2 (n = 0.01, 0.1, 0.3 and 1.0 mol%) nanocomposites for photocatalytic applications without compromising the structural and morphological characteristics of TiO2 and without having any organic ligands around the NPs. The use of ternary metal selenide nanocomposites CuAgSe-TiO2 results in a clear improvement in their photocatalytic activity for the photodegradation of formic acid as compared to the well-known benchmark for photocatalysis, TiO2 (P25), and its binary metal selenide nanocomposites Cu2-xSe-TiO2. DFT calculations establish semi-metallic behavior of CuAgSe NPs and show that CuAgSe-TiO2 forms a semimetallic-semiconductor heterojunction allowing a better charge separation to enhance its photocatalytic activity.
Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.
The design of high-performance energy-converting materials is an essential step for the development of sensors, but the production of the bulk materials currently used remains costly and difficult. Therefore, a different approach based on the self-assembly of nanoparticles has been explored. We report on the preparation by solvothermal synthesis of highly crystalline CeF3 nanodiscs. Their surface modification by bisphosphonate ligands led to stable, highly concentrated, colloidal suspensions in water. Despite the low aspect ratio of the nanodiscs (â¼6), a liquid-crystalline nematic phase spontaneously appeared in these colloidal suspensions. Thanks to the paramagnetic character of the nanodiscs, the nematic phase was easily aligned by a weak (0.5 T) magnetic field, which provides a simple and convenient way of orienting all of the nanodiscs in suspension in the same direction. Moreover, the more dilute, isotropic, suspensions displayed strong (electric and magnetic) field-induced orientation of the nanodiscs (Kerr and Cotton-Mouton effects), with fast enough response times to make them suitable for use in electro-optic devices. Furthermore, an emission study showed a direct relation between the luminescence intensity and magnetic-field-induced orientation of the colloids. Finally, with their fast radiative recombination decay rates, the nanodiscs show luminescence properties that compare quite favorably with those of bulk CeF3. Therefore, these CeF3 nanodiscs are very promising building blocks for the development and processing of photosensitive materials for sensor applications.
The surface chemistry of gold nanoparticles produced by the pulsed laser ablation in liquids method is investigated by X-ray photoelectron spectroscopy (XPS). The presence of surface oxide expected on these systems is investigated using synchrotron radiation in conditions close to their original state in solvent but free from substrate or solvent effects which could affect the interpretation of spectroscopic observations. For that purpose we performed the experiment on a controlled free-standing nanoparticle beam produced by combination of an atomizer and an aerodynamic lens system. These results are compared with those obtained by the standard situation of deposited nanoparticles on silicon substrate. An accurate analysis based on Bayesian statistics concludes that the existence of oxide in the free-standing conditions cannot be solely confirmed by the recorded core-level 4f spectra. If present, our data indicate an upper limit of 2.15 ± 0.68% of oxide. However, a higher credence to the hypothesis of its existence is brought by the structureless valence profile of the free-standing beam. Moreover, the cross-comparison with the deposited nanoparticles case clearly evidences an important misleading substrate effect. Experiment with free-standing nanoparticles is then demonstrated to be the right way to further investigate oxidation states on Au nanoparticles.
Repeated attacks using organophosphorus compounds, in military conflicts or terrorist acts, necessitate developing inexpensive and readily available decontamination systems. Nanosized cerium oxide is a suitable candidate, acting as a heterogeneous catalyst for the degradation of organophosphorus compounds such as VX agent or sarin. However, the reaction mechanism of the phosphatase mimetic activity of CeO2 nanoparticles is not fully described. Adsorption, surface-promoted hydrolysis, and desorption cycles strongly depend on the physico-chemical characteristics of the facets. In this study, CeO2 nanoparticles with different shapes were elaborated by hydrothermal synthesis. Nano-octahedra, nanocubes, or nanorods were selectively obtained under different conditions (temperature, concentration and nature of the precursors). The degradation activity according to the crystal faces was evaluated in vitro by measuring the degradation kinetics of paraoxon organophosphate in the presence of CeO2 nanoparticles. The results show an influence of both specific surface area and crystal faces of the nanoparticles, with higher activity for {111} facets compared to {100} facets at 32 °C. The relative activity between the facets is ascribed to the adsorption probability, assuming coordination between the phosphoryl oxygen and cerium atoms, but also to the surface density of the Ce doublets with relevant spacing for phosphatase mimetic activity.
While doping of semiconductors or oxides is crucial for numerous technological applications, its control remains difficult especially when the material is reduced down to the nanometric scale. In this paper, we show that pulsed laser ablation of an undoped solid target in an aqueous solution containing activator ions offers a new way to synthesise doped-nanoparticles. The doping efficiency is evaluated for laser ablation of an undoped Gd2O3 target in aqueous solutions of EuCl3 with molar concentration from 10-5 mol L-1 to 10-3 mol L-1. Thanks to luminescence experiments, we show that the europium ions penetrate the core of the synthesised monoclinic Gd2O3 nanoparticles. We also show that the concentration of the activators in the nanoparticles is proportional to the initial concentration in europium ions in the aqueous solution, and a doping of about 1% ([Eu]/[Gd] atomic ratio) is reached. On the one hand, this work could open new ways for the synthesis of doped nanomaterials. On the other hand, it also raises the question of undesired penetration of impurities in laser-generated nanoparticles in liquids.
Carbon-based materials are of great technological and scientific interest in materials science. Pulsed laser ablation in liquids (PLAL) is extensively used as a method to produce nanoparticles including nanodiamond and related materials. In this feature article, we will review the use of PLAL to tackle the challenges of synthesizing carbon-based nanostructures. Surprisingly, reported results have shown very poor reproducibility despite the use of similar experimental conditions. We use plasma spectroscopy and shadowgraph imaging to provide a picture of the thermodynamic properties, and then to better understand this apparent contradiction. Our study was carried out under traditional conditions which involve nanosecond laser, and radiant exposures from tens to thousands J/cm2. Prompted by these results, the different scenarios reported in the literature are discussed including shockwave induced phase transition, growth in high temperatures-high pressures like conditions, and vapor phase chemistry.
A predictive model for nanoparticle nucleation has not yet been successfully achieved. Classical nucleation theory fails because the atomistic nature of the seed has to be considered. Indeed, geometrical structure as well as stoichiometry do not always match the bulk values. We present a fully microscopic approach based on a first-principle study of aluminum oxide clusters. We calculated stable structures of AlxOy and their associated thermodynamic properties. From these data, the chemical composition of a gas composed of aluminum and oxygen atoms can be calculated as a function of temperature, pressure, and aluminum to oxygen ratio. We demonstrate the accuracy of this approach in reproducing experimental results obtained with time-resolved spectroscopy of a laser-induced plasma from an Al2O3 target. We thus extended the calculation to lower temperatures, i.e., longer time scales, to propose a scenario of composition gas evolution leading to the first alumina seeds.
Scintillating nanoparticles (NPs) in combination with X-ray or γ-radiation have a great potential for deep-tissue cancer therapy because they can be used to locally activate photosensitizers and generate singlet oxygen in tumours by means of the photodynamic effect. To understand the complex spatial distribution of energy deposition in a macroscopic volume of water loaded with nanoscintillators, we have developed a GEANT4-based Monte Carlo program. We thus obtain estimates of the maximum expected efficiency of singlet oxygen production for various materials coupled to PS, X-ray energies, NP concentrations and NP sizes. A new parameter, ηnano, is introduced to quantify the fraction of energy that is deposited in the NPs themselves, which is crucial for the efficiency of singlet oxygen production but has not been taken into account adequately so far. We furthermore emphasise the substantial contribution of primary interactions taking place in water, particularly under irradiation with high energy photons. The interplay of all these contributions to the photodynamic effect has to be taken into account in order to optimize nanoscintillators for therapeutic applications.
Energy Transfer/radiation effects , Models, Statistical , Nanoparticles/chemistry , Nanoparticles/radiation effects , Photochemotherapy/methods , X-Rays , Computer Simulation , Monte Carlo Method , Radiation Dosage , Singlet Oxygen/chemistry
We present an unbiased and robust analysis method for power-law blinking statistics in the photoluminescence of single nanoemitters, allowing us to extract both the bright- and dark-state power-law exponents from the emitters' intensity autocorrelation functions. As opposed to the widely used threshold method, our technique therefore does not require discriminating the emission levels of bright and dark states in the experimental intensity timetraces. We rely on the simultaneous recording of 450 emission timetraces of single CdSe/CdS core/shell quantum dots at a frame rate of 250 Hz with single photon sensitivity. Under these conditions, our approach can determine ON and OFF power-law exponents with a precision of 3% from a comparison to numerical simulations, even for shot-noise-dominated emission signals with an average intensity below 1 photon per frame and per quantum dot. These capabilities pave the way for the unbiased, threshold-free determination of blinking power-law exponents at the microsecond time scale.
Pulsed laser ablation has proved its reliability for the synthesis of nano-particles and nano-structured materials, including metastable phases and complex stoichiometries. The possible nucleation of the nanoparticles in the gas phase and their growth has been little investigated, due to the difficulty of following the gas composition as well as the thermodynamic parameters. We show that such information can be obtained from the optically active plasma during its short lifetime, only a few microseconds for each laser pulse, as a result of a quick quenching due to the liquid environment. For this purpose, we follow the laser ablation of an α-Al2O3 target (corindon) in water, which leads to the synthesis of nanoparticles of γ-Al2O3. The AlO blue-green emission and the Al(I) (2)P(0)-(2)S doublet emission provide the electron density, the density ratio between the Al atoms and AlO molecules, and the rotational and vibrational temperatures of the AlO molecules. These diagnostic considerations are discussed in the framework of theoretical studies from the literature (density functional theory). We have found that starting from a hot atomized gas, the nucleation cannot occur in the first microseconds. We also raise the question of the influence of water on the control of the stoichiometry.
Thermosensitive fluorescent nanoparticles seeded in deionized water combined with confocal microscopy enables thermal mapping over three dimensions of the liquid phase flowing through a microchannel interrupted by a microdiaphragm. This experiment reveals the presence of a strong thermal gradient up to ~10(5) K/m only when hydrodynamic cavitation is present. Here hydrodynamic cavitation is the consequence of high shear rates downstream in the diaphragm. This temperature gradient is located in vortical structures associated with eddies in the shear layers. We attribute such overheating to the dissipation involved by the cavitating flow regime. Accordingly, we demonstrate that the microsizes of the device enhance the intensity of the thermal gap.
A series of anhydrous cerium(III) trifluoroacetate complexes with neutral O-donor ligands, namely Ce2(OAc)(TFA)5(DMF)3 (1), Ce(TFA)3(L)x [x = 2, L = THF (2), DMF (3), DMSO (4); x = 1, L = diglyme (5)] and Ce2(TFA)6(DMSO)x(DMF)y [x = 6, y = 0 (6); x = 4, y = 2 (7)] (where OAc = acetate, TFA = trifluoroacetate, THF = tetrahydrofuran, DMF = dimethylformamide, DMSO = dimethylsulphoxide, and diglyme = MeO(C2H4O)2Me] were synthesized and completely characterized by elemental analysis, FT-IR spectroscopy and TG-DTA-MS studies. A partially hydrated complex [Ce(TFA)3(diglyme)(H2O)] (8) was obtained by slow evaporation of the THF solution of anhydrous 5 in the air. The single crystal X-ray diffraction analysis of 1, 3, 4, and 68 showed the versatile bonding mode of the TFA ligand (terminal, chelating and bridging). These complexes, on decomposition in 1-octadecene in inert atmosphere, gave CeF3 nanoparticles of 811 nm size. The complex 5 proved to be the best precursor in the series because of the ability of the diglyme ligand to act as capping reagent during decomposition to render the CeF3 particles monodisperse in organic solvents. The obtained CeF3 nanoparticles were characterized by FT-IR, EDX analysis and TEM studies and their luminescence and scintillation responses under UV and X-ray excitation were studied and compared with that of CeF3 single crystal.
We present a novel approach for convenient tuning of the local refractive index around nanostructures. We apply this technique to study the influence of the local refractive index on the radiative decay time of CdSe/ZnS quantum dots with three distinct emission wavelengths. The dependence of the luminescence decay time on the environment is well described by an effective medium approach. A critical distance of about 80 nm is found for the determination of the effective local index of refraction. An estimation for the emitting-state quantum efficiency can be extracted.
Algorithms , Luminescent Measurements/methods , Materials Testing/methods , Quantum Dots , Refractometry/methods , Semiconductors , Spectrometry, Fluorescence/methods
The evanescent coupling of a 1.5 mum radius silicon microdisk with one or two Silicon-On-Insulator waveguides is studied. Thanks to the high refractive index contrast between Silica and Silicon materials, this very-small-diameter microdisk exhibits the highest quality factor measured in wavelength range from 1500 nm to 1600 nm. Coupled to a single monomode waveguide, the optical resonator behaves as a stop-band filter. Even if the microdisk is a largely multimode resonator, only its fundamental modes are efficiently excited. The filter's transmission is measured for different gap between the waveguide and the resonator. The critical coupling is clearly observed and gives access to 1.63 nm linewidth. A 20 dB decrease of the transmission signal is also observed. Coupled to two waveguides, the resonator becomes a compact symmetric wavelength-demultiplexer. In this case, the optimal response comes from a compromise between the gap and the desired linewidth dropped in the second waveguide. Finally, our measurements are also compared to analytic models showing a good agreement especially for the critical gap prediction.
We study the higher order harmonics of scalar modulational instability in the regime where it arises spontaneously through amplification of vacuum fluctuations. We obtain detailed predictions concerning the detunings, intensities, growth rates, and spectral widths of the harmonics. These predictions are well verified by experimental results obtained by propagating high intensity light pulses through optical fibers.
