Theoretical modeling of laser fragmentation of nanoparticles in liquid media

A physical model of metal nanoparticle fragmentation in liquid media upon exposure to femtosecond laser pulses is proposed by the example of gold nanoparticle fragmentation in water. The model is based on electrolyzing the metal nanoparticles heated by laser pulses and their fragmentation during development of the instability of charged liquid metal drops. The nanoparticle charge gained upon exposure to laser radiation in water is estimated and the nanoparticle fragmentation parameter is determined.     

Silicon nanoparticles formation by means of laser ablation in liquid media

The possibility to produce silicon nanoparticles by the method of the pulse laser ablation of monocrystaline silicon targets in the water, glycerol and liquid nitrogen have been shown. Studies by the atomic-force microscopy and Raman scattering methods revealed the nanoparticles have a crystalline structure and their mean size depends on the buffer liquid composition.     

Investigation of different liquid media and ablation times on pulsed laser ablation synthesis of aluminum nanoparticles

Aluminum nanoparticles were synthesized by pulsed laser ablation of Al targets in ethanol, acetone, and ethylene glycol. Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM) images, Particle size distribution diagram from Laser Particle Size Analyzer (LPSA), UV-visible absorption spectra, and weight changes of targets were used for the characterization and comparison of products. The experiments demonstrated that ablation efficiency in ethylene glycol is too low, in ethanol is higher, and in acetone is highest. Comparison between ethanol and acetone clarified that acetone medium leads to finer nanoparticles (mean diameter of 30 nm) with narrower size distribution (from 10 to 100 nm). However, thin carbon layer coats some of them, which was not observed in ethanol medium. It was also revealed that higher ablation time resulted in higher ablated mass, but lower ablation rate. Finer nanoparticles, moreover, were synthesized in higher ablation times.     

Action of femtosecond laser pulses on metal nanoparticles in a liquid

A theoretical model describing the influence that ultrashort laser pulses have on spherical metal nanoparticles placed in a liquid is proposed. The kinetics of excitation and damping of acoustic vibrations in gold nanoparticles is studied, and the propagation processes of acoustic disturbances in their environment are discussed.     

Modeling of breakdown of a gas by electrons of the boundary layer during irradiation of metal targets by picosecond laser pulses

Results are presented from a numerical simulation of the breakdown of a dense inert gas by boundary-layer electrons upon irradiation of a metal target by high-power picosecond laser pulses. It is shown that taking the electric field of the boundary layer into account leads to a substantial increase in the concentration of the seed electrons near the target surface, which accelerates the ionization process. The dependence of the breakdown time on the electric field intensity of the incident wave and the concentration of the gas atoms is derived. Zh. Tekh. Fiz. 68, 48–53 (August 1998)     

Effects of particle size and laser wavelength on heating of silver nanoparticles under laser irradiation in liquid

Laser energy absorption results in significant heating of metallic nanoparticles and controlling the heating of nanoparticles is one of the essential stages of selective cell targeting. It is necessary to note that the laser action should be done by laser pulses with a wavelength that is strongly absorbed by the particles and it is important to select wavelengths that are not absorbed by the medium. Laser pulse duration must be chosen sufficiently short to minimize heat flow emitted from absorbing particles. Numerical calculations based on Mie theory were used to obtain the effect of laser wavelength and particle size on absorption factor for colloidal silver nanoparticles with radii between 5 and 50 nm. Calculations for acquiring temperatures under irradiations of pulsed KrF laser and pulsed Nd:YAG laser were performed. We showed that for low wavelengths of the laser, smaller nanoparticles have larger absorption efficiency compared to larger nanoparticles and in high wavelengths, temperature of all particles increased in the same way.     

Excitation of electric signals during laser irradiation of liquid surface

Mechanisms for excitation of electric signals (ESs) during laser irradiation of an open water surface are analyzed. An explanation is proposed for the time dependence of the ESs, and the maximum possible ES amplitudes are evaluated.     

Dynamic processes in metal nanoparticles under irradiation

Dynamic processes occurring in a nanoscale metal object during the implantation of high-energy ions are studied in detail. It is shown that the elastic and thermoelastic lattice responses to irradiation form force factors considerably affecting the evolution of the defect-impurity system, which, in turn, leads to a decrease in the number of structural defects. Quantitative estimates of the spatial redistribution of defects resulting in their emergence on the surface are obtained. Such self-organization of nanoparticles under ionizing radiation forms a basis for the production of nanostructured radiation-resistant materials capable of sustaining a long-term intense radiation load.     

Development of systems for the laser synthesis of nanoparticles starting from liquid precursors

The laser synthesis of nanoparticles starting from liquid precursors is particularly suitable as synthesis technique for obtaining nanoparticles. In the present work the laser pyrolysis is performed in a novel setup where the liquid precursor is brought with the aid of an original evaporator system to temperatures in excess of the boiling point and is finally fed into the reactor under the form of heated vapors.The process occurs in the gas phase and ensures the avoidance of the condensation. The temperature control system allows for the maintaining of the overall system temperature below the decomposition temperature and above the boiling. Temperatures up to 500 °C are assured for the mixed precursors. The control of the amount of the active substances is performed upstream, in the liquid phase. The set-up is able to offer safety conditions at the synthesis of substances with high toxicity. This experimental set-up was proposed in order to synthesize TiO₂ nanoparticles from TTIP because its boiling temperature is relatively high (239 °C grades). Different analytical techniques such as EDX, TEM, XRD and HRTEM were used in order to evaluate the structural characteristics of the produced nanopowders.     

Initiation of nuclear reactions under laser irradiation of metal nanoparticles in the presence of thorium aqua ions

Laser irradiation of tungsten and gold nanoparticles in aqueous solutions of Th(NO₃)₄ was experimentally studied. Picosecond Nd:YAG lasers with a wavelength of 1.06 μm and a peak power from 10¹¹ to 10¹³ W cm⁻² were used. The composition of colloidal solutions before and after laser irradiation was analyzed using atomic absorption and gamma spectrometry. It was found that laser irradiation initiates nuclear reactions involving thorium nuclei, occurring via two different channels. Radioactive decay of thorium nuclei within its radioactive series is enhanced under laser irradiation in D₂O; one of the fission fragments is ¹³⁷Cs. Possible mechanisms of the process are discussed.     

液相法制备金属纳米粒子

液相法是在均相溶液中，利用各种途径引发化学反应，通过均相或异相成核及随后的扩散生长而制备出粒径分布窄且表面功能化的纳米尺度材料．介绍了液-液两相法、反相胶束、高温液相法等制备单分散金属纳米粒子的方法和高温液相法制备金属纳米粒子的影响因素，以及近年来在金属纳米粒子的制备和性能研究上的进展，尤其是Co等多种磁性纳米粒子的制备、磁性研究．     

Synthesis of silver nanoparticles and antimony oxide nanocrystals by pulsed laser ablation in liquid media

Pulsed laser ablation in liquid media (PLALM) is a prominent technique for the controlled fabrication of nanomaterials via rapid reactive quenching of ablated species at the interface between the plasma and liquid. Results on nanoparticles and nanocrystals formed by PLALM of silver (Ag) and antimony (Sb) solid targets in different liquid environments (Sodium Dodecyl Sulfate, distilled water) are presented. These experiments were done by irradiating solid targets of Ag and Sb with a nanosecond pulsed Nd:YAG laser output of wavelength 532 nm. Nanoparticles of silver and nanocrystals of antimony oxide (Sb₂O₃) obtained were characterized using UV-Vis spectrometry, Scanning Electron Microscopy (SEM), transmission electron microscopy (TEM), X-ray Energy Dispersion Analysis (EDAX) and X-ray diffractometry (XRD). The morphology of nanomaterials formed is studied as a function of surfactant environment. The silver nanoparticles obtained were spherical of size in the order of 10–35 nm in solution of SDS having different concentrations. In case of the Sb target, ablation was performed in two different molarities of SDS solution and distilled water. Nanocrystals of Sb₂O₃ in powder form having cubic and orthorhombic phases were formed in SDS solution and as fibers of nanocrystals of cubic Sb₂O₃ in distilled water.     

Study of the plasma plume generated during near IR femtosecond laser irradiation of silicon targets

Ti:sapphire femtosecond laser ablation of silicon has been investigated by Langmuir probe and time-gated optical emission spectroscopy. The measured spectra show the presence of a fast ion population preceding the main plume core of slow ions and neutrals produced by a thermal ablation mechanism. By analyzing the fluence thresholds for the emission of the two ion populations, we provide clear experimental evidence that fast ions are ejected non-thermally from the sample surface as a result of the Si surface supercritical state induced by the intense ultrashort laser pulse irradiation. PACS 52.50.Jm; 52.38.Mf; 79.20.Ds; 61.82.Fh     

Hot electrons and laser optoacoustics in metal nanoparticles

The theory of sound generation and heat transfer in matrix-embedded metal nanoparticles under ultra-short laser irradiation has been developed. The shape and time dependence of acoustic waves generated by a sharp change in the pressure of electrons have been investigated for a single nanoparticle and for the group of nanoparticles located on a 2-D flat matrix plane. The dependence of the electronic temperature and the temperature of the interface between the dielectric matrix and nanoparticle as a function of time has been derived.     

Promoting the yield of nanoparticles from laser ablation in liquid

We have systematically studied the relationship between the preparation of Ge nanoparticles and the thickness of the water layer upon the pulsed-laser ablation of a Ge target in water. It was found that the average size of the prepared nanoparticles decreases with the thickness of water, and there is an optimum the thickness of water for the largest yield of the produced nanopartciles, which means the thickness of water can be regarded as an important parameter to control the size and yield of the prepared nanoparticles from laser ablation in liquid. The relevant physical mechanism was discussed.     

General method of synthesis for metal nanoparticles

Sugar-assisted stable monometallic nanoparticles were synthesized by wet chemical method following a general scheme. Judicious manipulation of the reducing capabilities of different sugars has shown to have a bearing on the particle size that corroborates the shift of the absorbance peak positions and TEM analysis. Evaporation of the precursor solutions on the solid surface (strong metal--support interaction), led to the formation of smaller particles. Under the experimental condition, spherical nanoparticles of approximately 1, 3, 10 and 20 nm sizes were prepared reproducibly for gold, platinum, silver and palladium, respectively. Fructose has been found to be the best suited sugar for the synthesis of smaller particles and remained stable for months together.     

Pulsed laser induced synthesis of scheelite-type colloidal nanoparticles in liquid and the size distribution by nanoparticle tracking analysis

Pulsed laser ablation (PLA) of ceramic target in liquid phase was successfully employed to prepare calcium tungstate (CaWO₄) and calcium molybdate (CaMoO₄) colloidal nanoparticles. The crystalline phase, particle morphology and optical property of the colloidal nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. The produced stable colloidal suspensions consisted of the well-dispersed nanoparticles showing a spherical shape. The mechanism for the laser ablation and nanoparticle forming was discussed under consideration of photo-ablation process. Nanoparticle tracking analysis using optical microscope combined with image analysis was proposed to determine the size distribution function of the prepared colloidal nanoparticles. The mean size of the CaWO₄ and CaMoO₄ colloidal nanoparticles were 16 and 29 nm, with a standard deviations of 2.1 and 5.2 nm, respectively.