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.     

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     

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     

Phosphorus cluster production by laser ablation

Neutral and charged phosphorus clusters of a wide size range have been produced by pulsed laser ablation (PLA) in vacuum at 532, 337, and 193 nm ablating wavelengths and investigated by time-of-flight mass spectrometry. The neutral P_n clusters are even-numbered with local abundance maxima at n=10 and 14, while the cationic and anionic clusters are preferentially odd-numbered with P₇⁺, P₂₁⁺, and P₁₇^- being the most abundant ions. The dominance of the magic clusters is more pronounced at 337-nm ablation that is explained by efficient direct ejection of their building blocks under these conditions. Nanocrystalline phosphorus films have been produced by PLA in ambient helium gas. PACS 52.38.MF; 61.46.+w; 79.20.Ds; 81.07.B; 81.16.Mk     

Stability, size and optical properties of silver nanoparticles prepared by laser ablation in different carrier media

We studied the effects of the surrounding liquid environment on the size and optical properties of silver nanoparticles prepared by laser ablation by a pulsed Nd:YAG laser operated at 1064 nm. The silver targets used were kept in acetone, water and ethanol. TEM observations and optical extinction were employed for characterization of particle size, shape and optical properties, respectively. Nano silver in acetone showed a narrow size distribution with a mean size of 5 nm and the colloidal solution was stable. In deionised water a rather narrow size distribution with a mean size of 13 nm was observed and nanoparticles were precipitated slowly after about two weeks. In ethanol, a broadening in size distribution and optical extinction spectra was observed. Silver nanoparticles in ethanol with a mean size of 22 nm were completely precipitated after 48 h. In acetone, deionised water and ethanol, the wavelengths of maximum optical extinction are 399, 405 and 411 nm respectively, which is attributed to increasing the size of the nanoparticles. Growth, aggregation and precipitation mechanisms were related to the dipole moment of the surrounding molecules in order to clarify the difference in size, optical properties and stability of the nanoparticles. PACS 79.20.Ds; 81.07.-b; 61.46.+w     

Size,composition and optical properties of copper nanoparticles prepared by laser ablation in liquids

Colloidal copper nanoparticles were prepared by pulsed Nd:YAG laser ablation in water and acetone. Size and optical properties of the nanoparticles were characterized by transmission electron microscopy and UV–visible spectrophotometry, respectively. The copper particles were rather spherical and their mean diameter in water was 30 nm, whereas in acetone much smaller particles were produced with an average diameter of 3 nm. Optical extinction immediately after the ablation showed surface plasmon resonance peaks at 626 and 575 nm for the colloidal copper in water and acetone, respectively. Time evaluation showed a blue shift of the optical extinction maximum, which is related to the change of the particle size distribution. Copper nanoparticles in acetone are yellowish and stable even after 10 months. In water, the color of the blue-green solution was changed to brown-black and the nanoparticles precipitated completely after two weeks, which is assigned to oxidation of copper nanoparticles into copper oxide (II) as was confirmed by the electron diffraction pattern and optical absorption measurements. We conclude that the ablation of bulk copper in water and acetone is a physical and flexible method for synthesis of stable colloidal copper and oxidized copper nanoparticles. PACS 42.62.-b; 81.07.-b; 61.46.+w     

Effect of oxygen injection on the size and compositional evolution of ZnO/Zn(OH)<Subscript>2</Subscript> nanocomposite synthesized by pulsed laser ablation in distilled water

Zinc oxide/hydroxide nanocomposite materials are synthesized by pulsed laser ablation of zinc in double distilled water. Effect of simultaneous flow of oxygen in the closed vicinity of laser ablated plasma plume on the size, morphology, crystallinity, and composition of synthesized oxide/hydroxide nanocomposite structures is investigated. As synthesized nanocomposite materials are characterized using UV–visible absorption, Scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), Differential thermal analysis (DTA), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and photoluminescence spectroscopic methods. It is observed that injection of oxygen induces a new mechanism in the particle synthesis, which causes decrease in particle size, distribution as well as Zn(OH)₂/ZnO ratio and increase of order of crystallinity of product. There are some novel findings in the direction of development of pulsed laser ablation in aqueous media (PLAAM) for the synthesis of nanostructured materials.     

Formation, structure and nonlinear optical properties of carbon nanoparticles synthesized by pulsed laser ablation

Carbon nanoparticles were prepared by pulsed laser ablation in argon gas for studying their formation, properties and applications. The nanoparticles were produced at different background pressures and different substrate temperatures, respectively. Micro-Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the samples. Nonlinear optical limiting properties of the carbon nanoparticles were characterized using nanosecond laser pulses. Uniform carbon nanoparticles can be synthesized in a background gas for nonlinear optical applications. PACS 61.46.+w; 81.07.-b; 78.20.-e     

Generation of titanium-oxide nanoparticles in liquid using a high-power, high-brightness continuous-wave fiber laser

Previous studies on laser-assisted nanomaterial formation in liquids have focused on using pulsed laser ablation of metals. We report, for the first time to our knowledge, the fabrication of nanoparticles via high-power high-brightness continuous-wave fiber laser ablation of titanium in liquids. Analysis revealed the generation of spherical nanoparticles of titanium-oxide ranging mainly between 5 nm and 60 nm in diameter. A mechanism of formation for crystallized nanoparticles, based on the self-organized pulsations of the evaporated metal, is proposed. This may account for the observed substantial efficiency gain owing to the high average power and brightness of the source. PACS 42.62.-b; 81.07.-b; 52.50.-b; 47.20.-k; 45.55.-t     

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.     

γ-Fe2O3 nanoparticles prepared by laser ablation of a tiny wire

A new method of preparing nanoparticles by pulsed laser ablation of a tiny wire is reported. A Nd:YAG pulsed laser with a wavelength of 1064 nm was used to ablate a 0.5-mm-diameter iron wire in a sealed chamber in a flowing mixed gas of N₂, O₂, and air to generate -Fe₂O₃ nanoparticles. In the meantime, a bulk Fe sample was ablated in the same chamber with the same laser processing parameters in order to compare the effect of the bulk sizes on the production rates and the sizes of the nanoparticles. The experimental results demonstrated that the production rate of nanoparticles prepared by laser ablation of tiny wires was about eight times that of laser ablation of bulk targets with the same composition, while the sizes of the nanoparticles were basically the same. With a higher power density and/or smaller diameters of the metal wires, it is possible to obtain smaller sizes of the nanoparticles with higher production rates. PACS 81.07.Wx     

Correlation between surface oxide and photoluminescence properties of Si nanoparticles prepared by pulsed laser ablation

Hydrogenated silicon nanoparticles were prepared by pulsed laser ablation (PLA) of Si target in hydrogen gas. We observed native oxidation process for 250 days by infrared (IR) absorption measurement and investigated correlation between native oxidation and photoluminescence (PL) properties. We found three PL peak regions, around 800 nm, 600–700 nm and 400–500 nm. These PL peak wavelengths depended on the Si-O bond density and remarkable correlation with composition of the oxide layer was not observed. The native oxidation is a passive method to modify the surface. We propose plasma surface treatment as an active method. The PL wavelength varied by the surface treatments due to suppression of native oxidation. The surface modification is an important technique to control PL peak wavelength of silicon nanoparticles. PACS 81.15.Fg; 81.07.Bc; 78.67.-n     

Bimodal Nanoparticle Size Distributions Produced by Laser Ablation of Microparticles in Aerosols

Silver nanoparticles were produced by laser ablation of a continuously flowing aerosol of microparticles in nitrogen at varying laser fluences. Transmission electron micrographs were analyzed to determine the effect of laser fluence on the nanoparticle size distribution. These distributions exhibited bimodality with a large number of particles in a mode at small sizes (3–6-nm) and a second, less populated mode at larger sizes (11–16-nm). Both modes shifted to larger sizes with increasing laser fluence, with the small size mode shifting by 35% and the larger size mode by 25% over a fluence range of 0.3–4.2-J/cm². Size histograms for each mode were found to be well represented by log-normal distributions. The distribution of mass displayed a striking shift from the large to the small size mode with increasing laser fluence. These results are discussed in terms of a model of nanoparticle formation from two distinct laser–solid interactions. Initially, laser vaporization of material from the surface leads to condensation of nanoparticles in the ambient gas. Material evaporation occurs until the plasma breakdown threshold of the microparticles is reached, generating a shock wave that propagates through the remaining material. Rapid condensation of the vapor in the low-pressure region occurs behind the traveling shock wave. Measurement of particle size distributions versus gas pressure in the ablation region, as well as, versus microparticle feedstock size confirmed the assignment of the larger size mode to surface-vaporization and the smaller size mode to shock-formed nanoparticles.     

Vaporization and Plasma Shielding during High Power Nanosecond Laser Ablation of Silicon and Nickel

A thermal model to describe the high-power nanosecond pulsed laser ablation is presented. It involves the vaporization and the following plasma shielding effect on the whole ablation process. As an example of Si target, we obtainthe time evolution of the calculated surface temperature, ablation rate and ablation depth. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the ablation depth with laser fluence based on different models is shown. Moreover, we simulate the pulsed laser irradiation Ni target. The evolution of the transmitted intensity and the variation of ablation depth per pulse with laser fluence are performed. Under the same experimental conditions, the numerical results calculated with our thermal model are more in agreement with the experimental data.     

Effect of low-threshold air breakdown on material ablation by short laser pulses

Ablative formation of channels in steel by picosecond and nanosecond pulses of Nd lasers was studied. It was found that significant screening of the incident energy (up to 80–90%) in this pulse duration range is caused by breakdown of air contaminated with ablated microparticles. The breakdown threshold, size of particles, and time of their settling down were estimated. It was shown that this kind of plasma screening results in a decrease in the ablation rate and significant channel widening. Practical approaches to eliminate the low-threshold breakdown induced by microparticles were proposed and implemented. These approaches are based on experimental results of the study of the dependences of laser ablation on the pressure and repetition rate. It was shown that a moderate decrease in the pressure below 300–400 mbar makes it possible to avoid screening. In high-repetition-rate ablation, it was found that values above several kilohertz correspond to quasi-vacuum conditions in the ablation spot.     

外加气流对脉冲激光烧蚀制备纳米Si晶粒尺寸分布的影响

提出一种控制脉冲激光烧蚀制备纳米Si晶粒尺寸分布的新方法。在10Pa的Ar环境中,采用脉冲激光烧蚀高阻抗单晶硅靶沉积制备了纳米Si晶薄膜。在羽辉正上方2.0cm,距靶0.3～3.0cm范围内的不同位置引入氩气流,在烧蚀点正下方2.0cm处水平放置单晶Si(111)衬底来收集制备的纳米Si晶粒。利用扫描电子显微镜观察样品表面形貌,并对衬底不同位置上纳米Si晶粒进行统计。结果表明:在不引入气流时,晶粒的尺寸随靶衬间距的增加先增大后减小,晶粒尺寸峰值出现在距靶1.7cm处;引入气流后,晶粒尺寸分布发生变化,在距靶1.7cm引入气流时晶粒尺寸峰值最大,在距靶3.0cm引入气流时晶粒尺寸峰值最小,且出现晶粒尺寸峰值的位置随着引入气流位置的增加而增大。     

Nanosecond pulsed laser ablation of silicon in liquids

Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at 532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond pulsed laser irradiation at 532 nm. Silicon surface’s ablated diameter growth was measured at different applied laser fluences and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the water interfaces. Multiple-shot laser ablation threshold of silicon is determined. Pulsed energy accumulation model is used to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the incubation effect is also found to be absent.     

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.     

Analysis of the sizes and number of microparticles in the ablation plume,which attenuates laser radiation during ablation of a constructive material

The effect of laser radiation with a wavelength of 1.3 μm, power of 25 kW, pulse width of 1 s, and irradiated spot area of 9 cm² on carbon silicon carbide composite material (CSCCM) is analyzed. The formation of an ablation plume (which consists of vapor of irradiated material, burning products, drops and microparticles of various chemical composition and size) above the irradiated surface leads to a significant loss of laser energy. The fractions of the scattered and absorbed laser radiation in the plume are determined, the size and mass distributions of microparticles in the plume are estimated, their concentration is calculated, and the microparticle escape velocities from the irradiated CSCCM surface are evaluated.