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.     

Laser ablation of microparticles for nanostructure generation

The process of laser ablation of microparticles has been shown to generate nanoparticles from microparticles; but the generation of nanoparticle networks from microparticles has never been reported before. We report a unique approach for the generation of nanoparticle networks through ablation of microparticles. Using this approach, two samples containing microparticles of lead oxide (Pb₃O₄) and nickel oxide (NiO), respectively, were ablated under ambient conditions using a femtosecond laser operating in the MHz repetition rate regime. Nanoparticle networks with particle diameter ranging from 60 to 90 nm were obtained by ablation of microparticles without use of any specialized equipment, catalysts or external stimulants. The formation of finer nanoparticle networks has been explained by considering the low pressure region created by the shockwave, causing rapid condensation of microparticles into finer nanoparticles. A comparison between the nanostructures generated by ablating microparticle and those by ablating bulk substrate was carried out; and a considerable reduction in size and narrowed size distribution was observed. Our nanostructure fabrication technique will be a unique process for nanoparticle network generation from a vast array of materials.     

Synthesis of silver nanoparticles by laser ablation in pure water

We have successfully produced silver nanoparticles by irradiating an Ag target with a 532-nm laser beam in pure water. By working with high laser power and small spot sizes, we were able to synthesize very small spherical particles with a typical size of 2–5 nm. The influence of the beam spot size, the laser power, and the ablation time were studied, and the possible mechanisms of particle formation are discussed. PACS 79.20.Ds; 81.07.-b     

Synthesis of nanoparticles by pulsed laser ablation of consolidated metal microparticles

This paper describes a novel process of nanoparticle synthesis by pulsed laser ablation of consolidated microparticles. Metal microparticles, including Cu, Al and Ag, are consolidated by a cold isobaric press with pressures up to a few hundred MPa before laser irradiation. Nanoparticles are then synthesized in air by high-power pulsed laser ablation of the microparticles using a Q-switched Nd:YAG laser. It is shown that the degree of compaction plays a significant role in determining the size of the produced nanoparticles. The effect of laser fluence and collector position on the distribution of particle size is examined. Photoacoustic deflection probing and nanosecond time-resolved visualization indicate that the novel process attains increased efficiency of laser-energy coupling with the target. PACS 81.07.-b; 81.07.Wx; 42.62.Cf     

Gas and Pressure Dependence for the Mean Size of Nanoparticles Produced by Laser Ablation of Flowing Aerosols

Silver nanoparticles were produced by laser ablation of a continuously flowing aerosol of microparticles entrained in argon, nitrogen and helium at a variety of gas pressures. Nanoparticles produced in this new, high-volume nanoparticle production technique are compared with our earlier experiments using laser ablation of static microparticles. Transmission electron micrographs of the samples show the nanoparticles to be spherical and highly non-agglomerated under all conditions tested. These micrographs were analyzed to determine the effect of carrier gas type and pressure on size distributions. We conclude that mean diameters can be controlled from 4 to 20 nm by the choice of gas type and pressure. The smallest nanoparticles were produced in helium, with mean sizes increasing with increasing molecular weight of the carrier gas. These results are discussed in terms of a model based on cooling via collisional interaction of the nanoparticles, produced in the laser exploded microparticle, with the ambient gas.     

Visualization of microparticle explosion and flow field in nanoparticle synthesis by pulsed laser ablation

Laser ablation of microparticles is one of the promising methods for efficient nanoparticle synthesis but the physical mechanisms of nanoparticle formation are not yet clearly understood, particularly in the case of metallic or ceramic particles. In this work, we report, for the first time, the results of in situ visualization of the metal microparticle explosion. Ablation of a Cu microparticle in CuO nanoparticle synthesis by a Q-switched Nd:YAG laser is visualized using laser-flash shadowgraphy. The dynamics of the ablation plume and shock wave is also analyzed by monitoring the photoacoustic probe-beam deflection. Strong shock-wave emission from a single particle has been observed in the velocity range 1000–4000 m/s even at near-threshold fluences. The threshold laser fluence for particle ablation has been found to be substantially lower than that required for bulk metal ablation. Consequently, the results confirm that the photomechanical mechanism associated with the shock-wave generation plays a significant role in the ablation process. PACS 81.07.-b; 81.07.Wx; 42.62.Cf     

Nanoparticle formation by laser ablation in air and by spark discharges at atmospheric pressure

Recent promising methods of nanoparticle fabrication include laser ablation and spark discharge. Despite different experimental conditions, a striking similarity is often observed in the sizes of the obtained particles. To explain this result, we elucidate physical mechanisms involved in the formation of metallic nanoparticles. In particular, we compare supersaturation degree and sizes of critical nucleus obtained under laser ablation conditions with that obtained for spark discharge in air. For this, the dynamics of the expansion of either ablated or eroded products is described by using a three-dimensional blast wave model. Firstly, we consider nanosecond laser ablation in air. In the presence of a background gas, the plume expansion is limited by the gas pressure. Nanoparticles are mostly formed by nucleation and condensation taking place in the supersaturated vapor. Secondly, we investigate nanoparticles formation by spark discharge at atmospheric pressure. After efficient photoionization and streamer expansion, the cathode material suffers erosion and NPs appear. The calculation results allow us to examine the sizes of critical nuclei as function of the experimental parameters and to reveal the conditions favorable for the size reduction and for the increase in the nanoparticle yield.     

Effect of laser-ablated copper nanoparticles on polymerization of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane

We have studied the effect of copper nanoparticles produced by laser ablation on the ring-opening polymerization of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane (TPDC). The particle size and distribution of laser-ablated, copper nanoparticles were controlled by laser-ablation conditions such as ambient pressure, laser fluence, and distance between target and substrate. Laser-ablated, copper nanoparticles induced polymerization of TPDC effectively, resulting in formation of poly(diphenylsilylenemethylene) (PDPhSM) thin films. The polymerization efficiency depended mainly on the particle size and the surface concentration of copper nanoparticles deposited on the TPDC films by laser ablation. No clear tendency was observed between oxidation of Cu nanoparticles and polymerization efficiency. This technique enables us to easily fabricate PDPhSM thin films and may become a new method for fabricating polymer–metal nanocomposites. PACS 81.05.Lg; 82.35.Np; 82.50.Hp     

Properties of nanoparticles generated during femtosecond laser machining in air and water

Femtosecond laser ablation is used to generate nanoparticle aerosols and colloids from solid targets of various materials (Ti, Ag, Au, Co, etc.) in air and water ambience. We determine the influence of different laser parameters (pulse energy, pulse overlap) and properties of media (air, airbrush, water) on the rate of production and size distribution of the laser-generated nanoparticles. It is shown that the pulse overlap and laser fluence are the parameters determining the nanoparticle size. At optimum conditions the nanoparticle productivity can be increased by 150–300%. The generation of multimaterial nanoparticle dispersions is demonstrated. Being free of toxic impurities, the laser-produced nanoparticles may be promising for biomedical applications. PACS 79.20.Ds; 81.16.Mk; 81.16.-c; 52.38.Ph; 06.60.Jn     

Synthesis of CaWO<Subscript>4</Subscript> nanocolloidal suspension via pulsed laser ablation and its optical properties

Pulsed laser ablation (PLA) in the liquid phase was successfully employed to synthesize calcium tungstate (CaWO₄) nanocolloidal suspension. The crystalline phase, particle morphology and laser ablation mechanism for the colloidal nanoparticles were investigated using XRD, TEM and SEM. The obtained colloidal suspension consisted of well-dispersed CaWO₄ nanoparticles which showed a spherical shape with sizes ranging from 5 to 30 nm. The laser ablation and the nanoparticle forming process were discussed under consideration of the photo-ablation mechanism, where the nanoparticles were generated by rapid condensation of the plume in high pressured ethanol vapor. The optical properties of the prepared CaWO₄ colloidal nanoparticles were analyzed in detail using XPS, Raman spectroscopy, UV-Vis spectroscopy and PL spectrophotometry. The optical band gap was estimated by Tauc and Menths law. PACS 42.62.-b; 82.70.Dd; 78.55.Hx; 81.07.Wx     

Early stage of the material removal during ArF laser ablation of graphite

Material removal during ArF excimer laser ablation of graphite at atmospheric pressure was investigated by two independent methods; 1) by observation of the propagating properties of the shock wave generated by the carbonaceous ejecta and 2) by in situ measurement of the size distribution of carbon nanoparticles condensing in the ablation plume. This latter was carried out by a scanning mobility particle sizer system based on a differential mobility analyser. The performed measurements indicate that the material removal during ArF laser ablation consists of two steps at fluences above the threshold fluence. First, a thin layer of carbon (of the order of 1 nm) is removed by a quick desorption process, leading to shockwave formation. This process takes place in a ns time scale, and desorption rate estimations reveal that this can not be explained by thermal surface evaporation. Since to our knowledge there is no thermal process that could account for the estimated desorption rate, it is argued that this is a fast photochemical (i.e. non-thermal) process. The size distribution of the condensed nanoparticles related to this step shows a rising edge at diameters below 10 nm. At fluences above the ablation threshold, the majority of the material is ejected in the second phase, resulting in condensation of carbon nanoparticles, peaking at 50 nm diameters in the size spectrum. Both shockwave formation and material removal are also detected well below the ablation threshold fluence, which is attributed to the photochemical process. PACS 61.46.+w; 81.16.Mk     

Measurements of nanoparticle size distribution produced by laser ablation of tungsten and boron-carbide in N2 ambient

Nanoparticles (NPs) were produced by ablating tungsten and boron-carbide (B₄C) target materials in atmospheric pressure nitrogen ambient using ArF excimer laser pulses. The size distributions of the NPs formed during the ablation were monitored—within a 7-133 nm size window—by a condensation particle counter connected to a differential mobility analyzer. The laser repetition rate was varied between 1-50 Hz, and the fluence was systematically changed in the range of 0.5-15 J/cm², for both materials, allowing a comparative study in an extended laser parameter regime. The multishot ablation threshold (Φ_th) of B₄C was determined to be ∼1.9 J/cm² for the laser used (ArF excimer, λ = 193 nm). Similarly to earlier studies, it was shown that the size distributions consist of mainly small nanoparticles (<∼20 nm) attributed to a non-thermal ablation mechanism below Φ_th. An additional broad peak appears (between 20 and 40 nm) above Φ_th as a consequence of the thermally induced macroscopic ablation. Chemical composition of deposited polydisperse nanoparticles was studied by X-ray photoelectron spectroscopy showing nitrogen incorporation into the boron-carbide.     

Differential mobility analysis of nanoparticles generated by laser vaporization and controlled condensation (LVCC)

Silicon and iron aluminide (FeAl) nanoparticles were synthesized by a laser vaporization controlled condensation (LVCC) method. The particles generated by the laser ablation of solid targets were transported and deposited in the presence of well-defined thermal and electric field in a newly designed flow-type LVCC chamber. The deposition process of nanoparticles was controlled by the balance of the external forces; i.e., gas flow, thermophoretic and electrostatic forces. The size distributions of generated nanoparticles were analyzed using a low-pressure differential mobility analyzer (LP-DMA). The effect of synthesis condition on the size distribution was analyzed by changing the pressure of the carrier gas (20–200 Torr), the temperature gradient in the LVCC chamber (ΔT=0–190°C) and the electric field applied between the LVCC chamber plates (E=0–3000 V/m). It was found that electrostatic field was effective to selectively deposit small size nanoparticles (about 10 nm) with expelling large droplet-like particles.     

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.     

CdSe &; ZnS core/shell nanoparticles generated by laser ablation of microparticles

Laser Ablation of Microparticles (LAM) is a process of nanoparticle formation in which microparticles in a flowing aerosol are continuously ablated by high-power laser pulses. For the first time, we have produced CdSe/ZnS core/shell nanoparticles using a double ablation apparatus, designed to undergo a two-step LAM process. This process can be inverted to produce ZnS/CdSe core/shell nanoparticles. The present work focuses on the range around ∼15 nm radius heterostructures and uses high-resolution transmission electron microscopy (HRTEM) to image core and shells. For smaller particles, core shell structures have been detected with energy dispersive spectroscopy (EDS) 5 nm spot size beam and fast Fourier transform (FFT) spectra. Differences in the ablation behavior were measured between the two IIB–VIA type semiconductors.     

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.     

Nanoparticle preparation of quinacridone and β-carotene using near-infrared laser ablation of their crystals

Quinacridone nanoparticles with a mean size of about 200 nm are successfully prepared using nanosecond near-infrared (NIR) laser ablation of its microcrystalline powders in heavy water. The absorption spectra of the formed colloidal solutions depend on the excitation wavelengths, which is eventually ascribed to number and energy of absorbed photons. β-carotene has low photostability and is easily decomposed upon UV/VIS laser ablation of its solid, while its nanoparticles are prepared utilizing this NIR laser ablation technique. The advantage of nanoparticle preparation by NIR laser ablation is discussed.     

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.     

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.