In-situ study of Ag nanoparticle hydrosol optical spectra evolution during laser ablation/fragmentation

The results of in-situ monitoring of a laser fragmentation process of a largely polydisperse and morphologically heterogeneous citrate-reduced Ag hydrosol containing a fraction of Ag nanowires are presented. The laser fragmentation was performed using several wavelengths of the incident laser pulses (1064, 532 and 355 nm). Surface plasmon extinction spectra monitoring the nanoparticle fragmentation process were acquired pulse by pulse and related to transmission electron microscopy (TEM) images and statistical TEM image analysis of Ag nanoparticles collected in selected stages of the fragmentation. It was found that, due to different interactions of the laser pulses of various wavelengths with a specific fraction of the Ag nanoparticles in the hydrosol, the course of the fragmentation process depends on the wavelength, leading to different size distributions of the nanoparticles in the resulting hydrosol. The laser pulses of 532 nm wavelength are the most effective for the fragmentation process of the citrate-reduced Ag hydrosol, yielding the narrowest size distribution and the smallest mean radius of the Ag nanoparticles. PACS 81.07.-b; 73.21.-b; 81.16.-c     

Modeling photothermal and acoustical induced microbubble generation and growth

Previous experimental studies showed that powerful heating of nanoparticles by a laser pulse using energy density greater than 100 mJ/cm², could induce vaporization and generate microbubbles. When ultrasound is introduced at the same time as the laser pulse, much less laser power is required. For therapeutic applications, generation of microbubbles on demand at target locations, e.g. cells or bacteria can be used to induce hyperthermia or to facilitate drug delivery. The objective of this work is to develop a method capable of predicting photothermal and acoustic parameters in terms of laser power and acoustic pressure amplitude that are needed to produce stable microbubbles; and investigate the influence of bubble coalescence on the thresholds when the microbubbles are generated around nanoparticles that appear in clusters.

We develop and solve here a combined problem of momentum, heat and mass transfer which is associated with generation and growth of a microbubble, filled with a mixture of non-vaporized gas (air) and water vapor. The microbubble’s size and gas content vary as a result of three mechanisms: gas expansion or compression, evaporation or condensation on the bubble boundary, and diffusion of dissolved air in the surrounding water. The simulations predict that when ultrasound is applied relatively low threshold values of laser and ultrasound power are required to obtain a stable microbubble from a single nanoparticle. Even lower power is required when microbubbles are formed by coalescence around a cluster of 10 nanoparticles. Laser pulse energy density of 21 mJ/cm² is predicted for instance together with acoustic pressure of 0.1 MPa for a cluster of 10 or 62 mJ/cm² for a single nanoparticle. Those values are well within the safety limits, and as such are most appealing for targeted therapeutic purposes.     

Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c     

Synthesis and investigation of stable copper nanoparticle colloids

The synthesis of stable nanoparticle colloids by laser ablation of the copper target in water and fragmentation of Cu + CuO nanopowder with pulsed fiber laser irradiation with a wavelength of 1064 nm and pulse duration of 100 ns has been investigated experimentally. The influence of the technological parameters on the nanoparticle size and stability of the colloid has been studied. It has been shown that the laser ablation creates the CuO spherical nanoparticles. Subsequent fragmentation makes it possible to reduce the nanoparticle size in a colloid and to produce a stable colloidal solution from an aqueous suspension of Cu + CuO nanopowder.     

Synthesis of nanoparticles and suspensions by pulsed laser ablation of microparticles in liquid

Recent studies demonstrated that the process to produce metal and oxide nanoparticles by laser ablation of consolidated microparticles is a convenient and energy-efficient way to prepare nanoparticles. In this work, the novel process is applied to nanoparticle synthesis in the liquid environment and the results are compared with those by the gas-phase process. Metal and oxide nanoparticles are synthesized by pulsed laser ablation of the compacted metal microparticles using a Q-switched Nd:YAG laser in water. It is shown that the process is effective for preparing nanoparticle suspensions having relatively uniform size distributions. While the laser fluence and the degree of compaction strongly influence the size of the produced nanoparticle in air, the sedimentation time is shown to be the most critical factor to determine the mean size of the suspended particles.     

Ultrafast laser ablative generation of gold nanoparticles: the influence of pulse energy, repetition frequency and spot size

Previous studies investigating the role of the operating parameters on ultrafast laser ablative generation of gold nanoparticles have reported a wide range of nanoparticle size distribution and plasmon resonant properties. In some cases the reported role of fluence and other processing parameters is contradictory. In this systematic investigation, we deconstruct and examine the role of the component parts of fluence, namely pulse energy and ablation spot size, on nanoparticle generation. Other parameters such as exposure time and scan speed are also studied. We show that the nanoparticle average size and distribution is related to different contributions from pulse energy, pulse repetition frequency and spot size. We also correlate the average particle size and distribution with the wavelength and width of the plasmon resonance peak, and apply Mie theory in order to develop clearer physical insights into the mechanisms dominating nanoparticle generation.     

ZnO nano-rods synthesized by nano-particle-assisted pulsed-laser deposition

We succeeded in synthesizing ZnO nanorods by nanoparticle assisted pulsed-laser deposition (PLD) without using any catalyst where nanoparticles formed by condensation of ablated particles play an important role. The nanorods have an average size of about 120 nm. Stimulated emission was observed from ZnO nanorods at 388 nm by optical pumping. The size-controlling of nanorods can be achieved by controlling the size and the density of these nanoparticles. PACS 61.46.+w; 81.07.Bc; 78.66.Hf; 78.67.Bf; 81.16.Mk.     

The effect of target size on α-Fe nanoparticle preparation by?pulsed laser ablation

Two methods of preparing Fe nanoparticles at atmospheric pressure were conducted using pulsed laser ablation of a 0.5-mm-diameter Fe wire and a bulk Fe target. Passivated α-Fe nanoparticles covered with a shell of γ-Fe₂O₃ were prepared at different process parameters. The influences of average laser power, repetition rate, pulse duration and carrier-gas pressure on the mean particle size for two laser ablation methods were investigated, respectively. The results show that the target size has a large effect on the nanoparticle preparation though we have the same range of laser process parameters. Except the carrier-gas pressure, the influence of the laser parameters on the mean particle size is almost opposite for the two laser ablation methods. Besides, the ablation mechanisms were discussed to understand the variation of mean particle sizes with target size.     

Pulsed-laser generation of gold nanoparticles with on-line surface plasmon resonance detection

Size of nanoparticles is an important parameter for their applications. The real-time monitoring is required for reliable and reproducible production of nanoparticles with controllable size. We present results of our research on development of the system for the online nanoparticle characterization during their production by a laser. The laser ablation chamber which allows measurements of surface plasmon resonance spectra during the nanoparticle generation process has been designed and fabricated. The online characterization system was tested by producing and modification of gold nanoparticles. Nanoparticles were generated by nanosecond-laser (wavelength 1064 nm) ablation of gold target in deionized water, and optimal conditions for the highest nanoparticle productivity were estimated. The mean diameter of nanoparticles was determined using their absorption spectra measured in the real-time during the ablation experiments and from the TEM images analysis, and it varied from 20 to 45 nm. The mismatch between nanoparticle diameters, estimated using these two methods, is due to the polydispersity of the generated nanoparticles. The further experiments of laser-induced modification of colloidal gold nanoparticles were carried out using second harmonic (wavelength 532 nm) of nanosecond Nd:YAG laser and alteration in nanoparticle size were acquired by the online measurement system.     

Size distribution control of metal nanoparticles using femtosecond laser pulse train: a molecular dynamics simulation

Microscopic mechanisms and optimization of metal nanoparticle size distribution control using femtosecond laser pulse trains are studied by molecular dynamics simulations combined with the two-temperature model. Various pulse train designs, including subpulse numbers, separations, and energy distributions are compared, which demonstrate that the minimal mean nanoparticle sizes are achieved at the maximal subpulse numbers with uniform energy distributions. Femtosecond laser pulse trains significantly alter the film thermodynamical properties, adjust the film phase change mechanisms, and hence control the nanoparticle size distributions. As subpulse numbers and separations increase, alternation of film thermodynamical properties suppresses phase explosion, favors critical point phase separation, and significantly reduces mean nanoparticle size distributions. Correspondingly, the relative ratio of two phase change mechanisms causes two distinct nanoparticle size control regimes, where phase explosion leads to strong nanoparticle size control, and increasing ratio of critical point phase separation leads to gentle nanoparticles size control.     

Silicon nanoparticles generated by femtosecond laser ablation in a liquid environment

Silicon nanoparticles were generated by femtosecond laser [387 nm, 180 fs, 1 kHz, pulse energy = 3.5 μJ (fluence = 0.8 J/cm²)] ablation of silicon in deionized water. Nanoparticles with diameters from ~5 up to ~200 nm were observed to be formed in the colloidal solution. Their size distribution follows log-normal function with statistical median diameter of ≈20 nm. Longer ablation time leads to a narrowing of the nanoparticle size distribution due to the interaction of the ablating laser beam with the produced nanoparticles. Raman spectroscopy measurements confirm that the nanoparticles exhibit phonon quantum confinement effects and indicate that under the present conditions of ablation they are partially amorphous.     

Evolution of terahertz conductivity in ZnSe nanocrystal investigated with optical-pump terahertz-probe spectroscopy

The unique optical properties of nanoparticles are highly sensitive in respect to particle shapes, sizes, and localization on a sample. This demands for a fully controlled fabrication process. The use of femtosecond laser pulses to generate and transfer nanoparticles from a bulk target towards a collector substrate is a promising approach. This process allows a controlled fabrication of spherical nanoparticles with a very smooth surface. Several process parameters can be varied to achieve the desired nanoparticle characteristics. In this paper, the influence of two of these parameters, i.e. the applied pulse energy and the laser beam shape, on the generation of Si nanoparticles from a bulk Si target are studied in detail. By changing the laser intensity distribution on the target surface one can influence the dynamics of molten material inducing its flow to the edges or to the center of the focal spot. Due to this dynamics of molten material, a single femtosecond laser pulse with a Gaussian beam shape generates multiple spherical nanoparticles from a bulk Si target. The statistical properties of this process, with respect to number of generated nanoparticles and laser pulse energy are investigated. We demonstrate for the first time that a ring-shaped intensity distribution on the target surface results in the generation of a single silicon nanoparticle with a controllable size. Furthermore, the generated silicon nanoparticles presented in this paper show strong electric and magnetic dipole resonances in the visible and near-infrared spectral range. Theoretical simulations as well as optical scattering measurements of single silicon nanoparticles are discussed and compared.     

Analysis of thermal radiation from laser-heated nanoparticles formed by laser-induced decomposition of ferrocene

Thermal radiation, originating from laser-heated gas-phase nanoparticles, was detected in the 400–700 nm wavelength range by means of optical emission spectroscopy. The particles were formed upon laser-induced photolytic decomposition of ferrocene (Fe(C₅H₅)₂) and consisted of an iron core surrounded by a carbon shell. The laser-induced excitation was performed as the particles were still within the reactor zone, and the temperature of the particles could be determined from thermal emission. Both the temperature of the nanoparticles and the relative intensity changes of the emission were monitored as a function of time (with respect to the laser pulse), laser fluence and Ar ambient pressure. At high laser fluences, the particles reached high temperatures, and evidence was found for boiling of iron. Modeling of possible energy-releasing mechanisms such as black-body radiation, thermionic electron emission, evaporation and heat transfer by the ambient gas was also performed. The dominant cooling mechanisms at different ranges of temperature were clarified, together with a determination of the accommodation factor for the Ar–nanoparticle collisions. The strong evaporation at elevated temperatures also led to significant iron loss from the produced particles. PACS 61.46.+w; 81.16.Mk; 65.80.+n     

Noble metal nanoparticles produced by nanosecond laser ablation

Silver and gold thin films were deposited by pulsed laser ablation in a controlled Ar atmosphere at pressures between 10 and 100 Pa. Different morphologies, ranging from isolated nanoparticle arrays up to nanostructured thin films were observed. Fast imaging of the plasma allowed deducing the expansion dynamics of the ablated plume. Plasma velocity and volume were used together with the measured average ablated mass per pulse as input parameters in a model to estimate the average size of nanoparticles grown in the plume. The nanoparticle size is expected to decrease from 4 nm down to 1 nm with decreasing Ar pressure between 100 and 10 Pa: this was confirmed by transmission electron micrographs which indicate a reduced dispersion of particle size over narrow size ranges. The production of substrates for surface enhanced Raman scattering whose performances critically depend on nanoparticle size, shape, and structure is discussed.     

Temporal and Spectral Characterization of Breakdown Plasma Induced by Laser Radiation in Colloidal Solutions of Gold Nanoparticles

Temporal and spectral characteristics of laser-induced breakdown plasma in colloidal solutions of gold nanoparticles were experimentally studied. Near-infrared laser sources of nanosecond pulses were used. It was shown that under certain experimental conditions nanosized plasma around nanoparticles might change to laser-induced breakdown plasma in liquid. The dependencies of the plasma temporal and spectral characteristics on laser pulse duration as well as resulting nanoparticles properties were studied. Laser-induced breakdown plasma lifetime was shown to be comparable with laser pulse duration. The efficiency of gold nanoparticles fragmentation was shown to depend on laser pulse duration. Similar experiments were carried out under reduced external pressure. It turned out to affect the properties of both plasma plume and nanoparticles. Transmission electron microscopy and disc measuring centrifuge were used for nanoparticle morphology and size analysis. Extinction spectra of colloidal solutions and emission spectra of plasma were studied by means of optical spectroscopy.     

Hydrodynamic size distribution of gold nanoparticles controlled by repetition rate during pulsed laser ablation in water

Most investigations on the laser generation and fragmentation of nanoparticles focus on Feret particle size, although the hydrodynamic size of nanoparticles is of great importance, for example in biotechnology for diffusion in living cells, or in engineering, for a tuned rheology of suspensions. In this sense, the formation and fragmentation of gold colloidal nanoparticles using femtosecond laser ablation at variable pulse repetition rates (100-5000 Hz) in deionized water were investigated through their plasmon resonance and hydrodynamic diameter, measured by Dynamic Light Scattering. The increment of the repetition rate does not influence the ablation efficiency, but produces a decrease of the hydrodynamic diameter and blue-shift of the plasmon resonance of the generated gold nanoparticles. Fragmentation, induced by inter-pulse irradiation of the colloids was measured online, showing to be more effective low repetition rates. The pulse repetition rate is shown to be an appropriate laser parameter for hydrodynamic size control of nanoparticles without further influence on the production efficiency.     

Generation of web-like structures and nanoparticles by femtosecond laser ablation of silicon target in ambient air

Silicon nanoparticles were produced by femtosecond laser ablation in ambient air. Obtained samples were studied using dark-field optical microscopy, scanning electron microscopy and Raman-scattering spectroscopy. Two groups of structures can be found: (1) branched amorphous structures with a minimum element size of about 10 nm and incorporations of nanocrystals (0.6–6.6 nm from Raman scattering analysis); (2) larger crystal particles with smooth surface and a typical size of 50–200 nm that provide directional visible light scattering (at dark-field optical microscopy observations). An influence of environment on resulting phase composition of silicon nanoparticles was investigated by numerical evaluation of nanoparticle’s cooling rate. The calculation shows that cooling in ambient air ensures cooling rate sufficient for crystallization.     

Effect of picosecond laser induced cavitation bubbles generated on Au targets in a nanoparticle production set-up

This work is aimed at an analysis of the influence on the efficiency of nanoparticle production of a cavitation bubble (CB), which forms during the laser ablation process in high-fluence regime. The CB is produced on an Au metal target immersed in water by 1064 nm ps Nd:YAG laser pulses at different fluences. Its time–space evolution is monitored by a shadowgraphic set-up, while the Au nanoparticles production rate is tagged by the growth of the plasmon resonance, which is detected by measuring shot-by-shot the UV-Vis absorbance. We analyze the dependence of bubble size on the experimental parameters. Our results appear of interest to enhance the nanoparticle production efficiency in a liquid medium.     

Excitation of high energy levels under laser exposure of suspensions of nanoparticles in liquids

Laser exposure of suspensions of nanoparticles in liquids leads to excitation of high energy levels in both liquid and nanoparticle material. The emission spectrum of the colloidal solution under exposure of a suspension metallic nanoparticles in water to radiation of a Nd:YAG laser of a picosecond range of pulse duration is discussed. Excitation of nuclear energy levels and neutron release is experimentally studied on the model system of transmutation of Hg into Au that occurs under exposure of Hg nanodrops suspended in D₂O. The proposed mechanism involves: (i) emission of X-ray photons by Hg nanoparticles upon laser exposure, leading to neutron release from D₂O, (ii) initiation of Hg → Au transmutation by the capture of neutrons. The effect of transmutation is more pronounced using ¹⁹⁶Hg isotope instead of Hg of natural isotope composition. The influence of laser pulse duration on the degree of transmutation (from fs through ns range) is discussed.