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.     

电场中不同能量密度激光烧蚀制备纳米硅晶粒分布特性

在室温和10 Pa氩气环境中,引入平行于靶面方向的直流电场,通过改变脉冲激光能量密度烧蚀单晶硅靶,在与羽辉轴线呈不同角度的衬底上沉积纳米硅晶薄膜。利用扫描电子显微镜和拉曼散射谱对沉积样品进行分析,结果表明：随着激光能量密度的增加,位于相同角度衬底上的晶粒尺寸和面密度逐渐变大;在同一激光能量密度下,零度角处衬底上的晶粒尺寸和面密度最大,且靠近接地极板处的值比与之对称角度处略大。通过朗缪尔探针对不同能量密度下烧蚀羽辉中硅离子密度变化的诊断、结合成核区内晶粒成核生长动力学过程,对晶粒分布特性进行了分析。     

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.     

电场中不同能量密度激光烧蚀制备纳米硅晶粒分布特性

在室温和10 Pa氩气环境中,引入平行于靶面方向的直流电场,通过改变脉冲激光能量密度烧蚀单晶硅靶,在与羽辉轴线呈不同角度的衬底上沉积纳米硅晶薄膜。利用扫描电子显微镜和拉曼散射谱对沉积样品进行分析,结果表明:随着激光能量密度的增加,位于相同角度衬底上的晶粒尺寸和面密度逐渐变大;在同一激光能量密度下,零度角处衬底上的晶粒尺寸和面密度最大,且靠近接地极板处的值比与之对称角度处略大。通过朗缪尔探针对不同能量密度下烧蚀羽辉中硅离子密度变化的诊断、结合成核区内晶粒成核生长动力学过程,对晶粒分布特性进行了分析。     

Gold nanoparticles produced by laser ablation in water and in graphene oxide suspension

The comparison between two different approaches based on the use of the laser ablation in medium to synthetise gold nanoparticles is presented and discussed. Deionised water as well as a graphene oxide (GO) suspension in deionised water have been employed as solution to produce gold nanoparticles by laser ablation. In the former case, the nanoparticles assembly has been stabilised by using surfactants, but in the latter case to avoid undesired effects the use of chemicals was not necessary and Au reduced graphene oxide (Au-rGO) nanocomposites have been obtained. The structure, size and composition of the gold nanoparticles and of the Au–rGO nanocomposites have been monitored by UV–Vis–NIR absorption spectroscopy and Raman spectroscopy, the transmission and scanning electron microscopies and the X-ray energy-dispersive spectroscopy. The presented methodology of Au rGO nanocomposites preparation could represent a green alternative on the production of metallic nanoparticles in biocompatible environment.     

Experimental study on reflection of high-intensity nanosecond Nd:YAG laser pulses in ablation of metals

The total reflectivity of tin and magnesium in ablation by nanosecond Nd:YAG laser pulses in air is studied. It was found that the high initial reflectivity of the studied metals undergoes a significant drop to values of 0.11 for Sn and 0.16 for Mg within a laser fluence range between about 0.8 and 11 J/cm². These reduced reflectivity values remain virtually unchanged with further increasing laser fluence. This study shows that a significant reflectivity decrease of the studied metals is caused by plasma formation in front of the irradiated surface. Below the plasma formation threshold, the reflectivity of the studied metals is observed to be virtually independent of laser fluence, indicating a small role of Drude׳s temperature effect on the reflectivity of the studied samples.     

Characterization of Ag and Au nanoparticles created by nanosecond pulsed laser ablation in double distilled water

Pulsed laser ablation of Ag and Au targets, immersed in double-distilled water is used to synthesize metallic nanoparticles (NPs). The targets are irradiated for 20 min by laser pulses at different wavelengths—the fundamental and the second harmonic (SHG) (λ = 1064 and 532 nm, respectively) of a Nd:YAG laser system. The ablation process is performed at a repetition rate of 10 Hz and with pulse duration of 15 ns. Two boundary values of the laser fluence for each wavelength under the experimental conditions chosen were used—it varied from several J/cm² to tens of J/cm². Only as-prepared samples were measured not later than two hours after fabrication. The NPs shape and size distribution were evaluated from transmission electron microscopy (TEM) images. The suspensions obtained were investigated by optical transmission spectroscopy in the near UV and in the visible region in order to get information about these parameters. Spherical shape of the NPs at the low laser fluence and appearance of aggregation and building of nanowires at the SHG and high laser fluence was seen. Dependence of the mean particle size at the SHG on the laser fluence was established. Comments on the results obtained have been also presented.     

Formation of periodic structures by laser ablation of metals in liquids

Experimental results are presented on ablation of metals (W, Cu, brass and bronze) in a liquid environment (e.g., ethanol or water) by irradiation with either a pulsed copper vapor laser (0.51 μm) or a pulsed Nd:YAG laser (1.06 μm). The target material is ejected into surrounding liquid in the form of nanoparticles. In a certain range of laser parameters (fluence and number of laser shots) the surface of the solid target is composed of micro-cones having a regular structure. The distance between neighboring micro-cones in the structure depends on the laser spot size. The structures allow the observation of up-conversion of the laser frequency due to generation of the second harmonics in the eye retina.     

The production and oxidation of uranium nanoparticles produced via pulsed laser ablation

Depleted uranium samples were ablated using five nanosecond pulses from a Nd:YAG laser and produced films of ∼1600 Å thickness that were deposited with an angular distribution typical of a completely thermal ablation (cos¹ θ). The films remained contiguous for many months in vacuum but blistered due to tensile stress induced in the films several days after being brought into air. While under vacuum (2 × 10⁻¹⁰ Torr base pressure) the films were allowed to oxidize from the residual gases, of which water vapor was found to be the primary oxidizer. During the oxidation, the samples were monitored with both X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) and were found to oxidize following Langmuir kinetics. That a 2D-surface growth model described the oxidation indicates that, even at these low pressures, oxygen accumulation on the surface is a much faster process than diffusion into the bulk. While bulk diffusion did occur, the oxygen present at the surface saturated the measurements taken using photoemission and diffusion was difficult to observe. A method for determining oxide concentration via photoemission from the valence level, as opposed to the more conventional core levels, is also presented.     

Submicron foaming in gelatine by nanosecond and femtosecond pulsed laser irradiation

We compare the foaming characteristics induced by irradiation with single ns and fs laser pulses of UV, VIS and IR wavelengths on gelatines differing in gel strength (bloom values 75 and 225) and in crosslinking degree. We have observed that while laser irradiation with nanoseconds leads to the formation of a microfoam layer at 266 nm, and melting and crater formation at longer wavelengths (532 and 1064 nm), fs pulse irradiation leads to submicron foaming at all wavelengths studied (266, 400 and 800 nm). These results show the possibility of controlling the submicrometric foam structure in this biomaterial and can shed light into the working mechanisms of fs laser nanoprocessing in biomaterials, where increase of temperature, thermoelastic stress generation, and stress-induced bubble formation are mediated by the generated plasma.     

Characterization of optical and nonlinear optical properties of silver nanoparticles prepared by laser ablation in various liquids

The optical, structural, and nonlinear optical properties of silver nanoparticles prepared by laser ablation in various liquids were investigated at 397.5, 532, and 795 nm. The TEM and spectral measurements have shown temporal dynamics of size distribution of Ag nanoparticles in solutions. The thermal-induced self-defocusing dominated in the case of high pulse repetition rate as well as in the case of nanosecond pulses. In the case of low pulse repetition rate, the self-focusing (γ = 3 × 10⁻¹³ cm² W⁻¹) and saturated absorption (β = −1.5 × 10⁻⁹ cm W⁻¹) of picosecond and femtosecond radiation were observed in these colloidal solutions. The nonlinear susceptibility of Ag nanoparticles ablated in water was measured to be 5 × 10⁻⁸ esu (at λ = 397.5 nm).     

Silver nanoparticles generated by pulsed laser ablation in supercritical CO2 medium

Pulsed laser ablation (PLA) has been widely employed in industrial and biological applications and in other fields. The environmental conditions in which PLA is conducted are important parameters that affect both the solid particle cloud and the deposition produced by the plume. In this work, the generation of nanoparticles (NPs) has been developed by performing PLA of silver (Ag) plates in a supercritical CO₂ medium. Ag NPs were successfully generated by allowing the selective generation of clusters. Laser ablation was performed with an excitation wavelength of 532 nm under various pressures and temperatures of CO₂ medium. On the basis of the experimental result, both surface of the irradiated Ag plate and structure of Ag NPs were significantly affected by the changes in supercritical CO₂ pressure and temperature. With increasing irradiation pressure, plume deposited in the surrounding crater created by the ablation was clearly observed. In Field Emission Scanning Electron Microscopy (FE-SEM) the image of the generated Ag NPs on the silicon wafer and the morphology of Ag particles were basically a sphere-like structure. Ag particles contain NPs with large-varied diameter ranging from 5 nm to 1.2 μm. The bigger Ag NPs melted during the ablation process and then ejected smaller spherical Ag NPs, which formed nanoclusters attached on the molten Ag NPs. The smaller Ag NPs were also formed around the bigger Ag NPs. Based on the results, this new method can also be used to obtain advanced nano-structured materials.     

Effects of laser field and electric field on impurity states in zinc-blende GaN/AlGaN quantum well

Based on the effective-mass approximation, the competition effects between the laser field and applied electric field on impurity states have been investigated variationally in the ZB GaN/AlGaN quantum well (QW). Numerical results show that for any laser field, the electric field makes the donor binding energy present asymmetric distribution with respect to the center of the QW. Moreover, when the laser field is weak, the electric field effects are obvious on the donor binding energy; however, the electric field effects are insensitive to the variation of donor binding energy in the ZB GaN/AlGaN QW with strong laser field.     

Ag nanoparticles obtained by pulsed laser ablation in water: surface properties and SERS activity

We studied the surface properties and reactivity of silver nanoparticles obtained by picosecond or nanosecond pulsed laser ablation in water and with 1064‐nm wavelength. Ultraviolet–visible spectroscopy results and subsequent modelling by Mie theory indicated the presence of an oxide layer on the nanoparticle surface, which favours the colloidal stability, but reduces the interaction with the environment. The oxide layer is also responsible for the reduced surface enhanced Raman spectroscopy (SERS) activity of these colloids with respect to those obtained by chemical reduction. However, SERS activation can be efficiently obtained by addition of chloride ions to the colloids, leading to SERS enhancement factors that are comparable with those of the chemically prepared counterparts. Copyright © 2015 John Wiley & Sons, Ltd.     

Preparation of silver nanoparticles by laser ablation in polyvinylpyrrolidone solutions

We performed laser ablation of a silver plate in polyvinylpyrrolidone (PVP) aqueous solutions to prepare silver nanoparticles. Secondary laser irradiation onto the prepared colloidal solutions was also carried out. It was revealed that the formation efficiency was increased by addition of PVP as well as the stability of nanoparticles. The result of shadowgraph measurements suggested that the increased ablation efficiency by PVP is attributable to increased secondary etching efficiency by the solvent-confined plasma toward the silver plate. On the other hand, the size decrease of the nanoparticles by addition of PVP was more remarkable during the secondary irradiation process than in the laser ablation (nanoparticle preparation) process. This result indicates that emitted materials interact less sufficiently with PVP molecules in the laser ablation process than in the secondary laser irradiation process.     

Ar环境气压对脉冲激光烧蚀制备纳米Si晶粒平均尺寸的影响

采用XeCl脉冲准分子激光器，烧蚀高阻抗单晶Si靶，在1—500 Pa的Ar气环境下沉积制备了纳米Si薄膜. x射线衍射谱测量证实，纳米Si晶粒已经形成.利用扫描电子显微镜观测了所形成纳米Si薄膜的表面形貌，结果表明，随着环境气压的增加，所形成的纳米Si晶粒的平均尺寸增大，气压为100 Pa时达到最大值20 nm，而后开始减小. 从晶粒形成动力学角度，对实验结果进行了定性分析. 关键词： 纳米Si晶粒 脉冲激光烧蚀 表面形貌     

Small size TiO2 nanoparticles prepared by laser ablation in water

Titanium dioxide nanoparticles in distilled H₂O solvent were prepared by laser ablation. The experiments were performed irradiating a Ti target with a second harmonic (532 nm) output of a Nd:YAG laser varying the operative fluence between 1 and 10 J cm⁻² and for an ablation time ranging from 10 to 30 min. Electron microscopy measurements have evidenced the predominant presence of nanoparticles with diameter smaller than 10 nm together with agglomerations of 100-200 nm whose content increases with the laser fluence. At low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while at mid and high fluences the presence of 5-7 nm nanoparticles is predominant. XPS analysis has revealed the presence of different titanium suboxide phases with the prevalence of Ti-O bonds from TiO₂ species. The optical bandgap values, determined by UV-vis absorption measurements, are compatible with the anatase phase.     

Theoretical and experimental study of laser induced damage on GaAs by nanosecond pulsed irradiation

The surface damage experiments of gallium arsenide (GaAs) single crystal irradiated by 1.06 and 0.53 μm nanosecond irradiations are carried out with fundamental and frequency-doubled Nd:YAG laser, respectively. The surface damage thresholds for both wavelengths are experimentally determined and the damaged morphologies and elementary component are analyzed with electron probe microanalyzer (EPM). It is found that the components of Ga and As almost keep constant in our experiments when the irradiated fluence is just around the surface damage threshold and no oxygen is found at all. The theoretical calculations on temperature rise for both wavelengths are carried out using the purely thermal model. It is shown that for irradiation with photon energy above the corresponding band gap the theoretical calculation is in good agreement with the experimental results; however, for that with photon energy just below the band gap, the experimental results cannot be effectively explained by the purely thermal heating mechanism. Combining with the experiment of multi-shot damage from references we finally conclude that the damage by laser irradiation with photon energy below the band gap should be explained by the micro-defect accumulation and consequently enhanced absorption heating mechanism.     

Investigation of metal nanoparticles produced by laser ablation and their catalytic activity

Polydiphenylsilylenemethylene (PDPhSM) thin films, which are difficult to fabricate by conventional methods because of their insolubility and high melting point, have been synthesized by using laser-ablated metal nanoparticles for the thermal ring-opening polymerization of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane (TPDC) in this paper. TPDC was first evaporated on silicon substrates and then exposed to metal (Pt, Cu and Ag) nanoparticles deposition by laser ablation prior to heat treatment. The catalytic activity of Pt, Cu and Ag nanoparticles has been studied. The results showed that the mean diameter of Pt nanoparticles was the smallest, Cu nanoparticles the moderate and Ag nanoparticles the biggest, while the polymerization efficiency for Pt nanoparticles was the highest, Cu nanoparticles the moderate and Ag nanoparticles the lowest. In addition, the penetration behaviours of Pt, Cu and Ag nanoparticles into the TPDC monomer films during laser ablation were different due to the particle size or the chemical interaction between metal nanoparticles and TPDC molecules.     

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10 Pa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2.0 cm,距靶0.3～3.0 cm范围内的不同位置引入氩气流,在烧蚀点正下方2.0 cm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1.7 cm处;引入气流后,晶粒尺寸分布发生变化,在距靶1.7 cm引入气流时晶粒尺寸峰值最大,在距靶3.0 cm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。