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.     

Nanoparticles by Laser Ablation

This review concerns nanoparticles collected in the form of nanopowder or a colloidal solution by laser ablating a solid target that lies in a gaseous or a liquid environment. The paper discusses the advantages of the method as compared with other methods for nanoparticle synthesis, outlines the factors on which the properties of the produced nanoparticles depend, explains the mechanisms and models involved in the generation of nanoparticles by laser ablation, clarifies the differences between nanoparticle generation in gaseous and liquid environments, presents some experimental desigins and equipment used by the several groups for nanoparticle generation by laser ablation, describes the techniques used for “tuning” the width of the nanoparticles size distribution, and finally presents a few interesting examples of nanoparticles generated by laser ablation.     

Nanoparticle Formation by Laser Ablation

The properties of nanoparticle aerosols of size ranging from 4.9nm to 13nm, generated by laser ablation of solid surfaces are described. The experimental system consisted of a pulsed excimer laser, which irradiated a rotating target mounted in a cylindrical chamber 4cm in diameter and 18-cm long. Aerosols of oxides of aluminum, titanium, iron, niobium, tungsten and silicon were generated in an oxygen carrier gas as a result of a reactive laser ablation process. Gold and carbon aerosols were generated in nitrogen by non-reactive laser ablation. The aerosols were produced in the form of aggregates of primary particles in the nanometer size range. The aggregates were characterized using a differential mobility analyzer and electron microscopy. Aggregate mass and number concentration, electrical mobility size distribution, primary particle size distribution and fractal dimension were measured. System operating parameters including laser power (100mJ/pulse) and frequency (2Hz), and carrier gas flow rate (1l/min) were held constant.A striking result was the similarity in the properties of the aerosols. Primary particle size ranged between 4.9 and 13nm for the eight substances studied. The previous studies with flame reactors produced a wider spread in primary particle size, but the order of increasing primary particle size follows the same trend. While the solid-state diffusion coefficient probably influences the size of the aerosol in flame reactors, its effect is reduced for aerosols generated by laser ablation. It is hypothesized that the reduced effect can be explained by the collision-coalescence mechanism and the very fast quenching of the laser generated aerosol.     

氧化铜纳米粒子的制备及其SERS特性

采用激光刻蚀法在水溶液中制备了氧化铜纳米粒子,刻蚀完成后将氧化铜胶体沉积在铝片上形成一层氧化铜岛膜.所得纳米粒子的紫外—可见吸收峰在280 nm处,表明所得为氧化铜.原子力显微镜观察表明所制得的纳米粒子具有椭圆状结构,长轴约为30～50 nm,厚度约为8 nm.扫描电镜图片显示氧化铜粒子沉积在铝基底后形成了大小在0.5...     

飞秒激光烧蚀合成Y_2O_3∶Pr~(3+),Yb~(3+)上转换发光材料

利用飞秒激光烧蚀方法合成了Y2O3∶Pr3+,Yb3+纳米颗粒并对其上转换发光性质进行了研究。对合成产物的形貌分析显示,在一定的激光烧蚀功率密度下,即可制备出小尺寸的纳米颗粒,提高激光功率密度可以得到更高的纳米颗粒产量,所获得的纳米颗粒尺寸小于50 nm。荧光测试结果表明,Y2O3∶Pr3+,Yb3+纳米颗粒具有510 nm为中心的上转换发光带,上转换发光随纳米颗粒数量的增加而增强。     

Size Control of Porous Silicon Nanoparticles by Electrochemical Perforation Etching

Size‐controlled porous silicon‐based nanoparticles are prepared by pulsed electrochemical etching of single crystal silicon wafers, followed by ultrasonic fracture of the freestanding porous layer. When high‐current density pulses are applied periodically during the porous layer etching process, a porous multilayer results in which porous layers are separated by thin layers of much higher porosity. Ultrasonic fracture selectively cleaves the porous film along these high‐porosity perforations, providing greater size control and improved yields (by 5x) of the resulting porous nanoparticles. The effect of pulse width and repetition rate is systematically studied: tunability of the average nanoparticle size in the range 160–350 nm is demonstrated.     

激光刻蚀法制备Pd纳米颗粒及其光谱特性初探

用Nd:YAG脉冲激光器1 064 nm激发光在二次去离子水中制备了Pd胶体.此体系中Pd颗粒表面不存在杂质离子,为"化学纯净".刻蚀后将Pd胶体沉积在铝基底上形成一层Pd岛膜.紫外-可见吸收光谱表明Pd胶体在200～800 nm测量范围内没有出现特征吸收峰,说明Pd胶体的吸收光谱并不能反映Pd胶体中Pd颗粒的粒径分布等信息.SEM显示Pd岛膜由Pd纳米颗粒组成,其平均粒径在200 nm左右.以101-3mo/·L-1的4-巯基吡啶(4MPY)为探针分子对Pd胶体和Pd岛膜的表面增强拉曼散射(SERS)进行了研究.结果显示,在Pd胶体中没有获得4MPY的SERS信号;Pd岛膜是一种非常高效的活性增强基底,从实验数据估算得出其增强因子约为8.7×104.SERS光谱分析显示4MPY分子通过S原子以倾斜的方式吸附于Pd岛膜表面;与Pd岛膜表面相比,4MPY分子在Ag岛膜上更趋于采用垂直于表面的吸附取向.     

Dependence of Optical Absorption in Silicon Nanostructures on Size of Silicon Nanoparticles

The amorphous silicon nanoparticles (Si NPs) embedded in silicon nitride (SiN_x) films prepared by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique are studied. From Raman scattering investigation, we determine that the deposited film has the structure of silicon nanocrystals embedded in silicon nitride (nc-Si/SiNx) thin film at a certain hydrogen dilution amount. The analysis of optical absorption spectra implies that the Si NPs is affected by quantum size effects and has the nature of an indirect-band-gap semiconductor. Further, considering the effects of the mean Si NP size and their dispersion on oscillator strength, and quantum-confinement, we obtain an analytical expression for the spectral absorbance of ensemble samples. Gaussian as well as lognormal size-distributions of the Si NPs are considered for optical absorption coefficient calculations. The influence of the particlesize-distribution on the optical absorption spectra was systematically studied. We present the fitting of the optical absorption experimental data with our model and discuss the results.     

Two‐Photon Imaging of a Cellular Line Using Organic Fluorescent Nanoparticles Synthesized by Laser Ablation

A comparative study of the optical properties of organic fluorescent nanoparticles fabricated by laser ablation (NPs‐LA), reprecipitation (NPs‐RP), and microemulsion (NPs‐ME) methods is presented. These nanoparticles contain a fluorene‐based p‐conjugated molecule (BT2). Distinctive electronic transitions are observed in samples due to the specific way in which the molecule BT2 is assembled in each type of nanoparticles; for instance, transitions involved in absorption and emission spectra of NPs‐LA result in blueshifting with respect to the molecular solution of BT2, whereas redshifting is observed in NPs‐RP and NPs‐ME. Further, the results show that under infrared excitation, the aqueous suspensions of NPs‐LA exhibit the highest fluorescence induced by two‐photon absorption (≈790 GM at 740 nm), as well as the best photostability, compared with aqueous suspensions of NPs‐RP and NPs‐ME. The nanoparticles synthetized by the three aforementioned methods are employed as exogenous agents for the visualization of human cervical cancer cell line (HeLa) using confocal and two‐photon microscopy. Under similar experimental conditions, it is found that microscopy images of the best quality are obtained with NPs‐LA. These results show that laser ablation is a suitable technique for the fabrication of organic fluorescent nanoparticles used as contrast agents for in vitro fluorescence microscopy.     

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.     

C60薄膜的准分子激光刻蚀及形貌分析

报道了准分子ＸｅＣｌ（３０８ｎｍ）紫外激光刻蚀Ｃ６０膜的实验研究及对刻蚀薄膜的形貌分析。基于形貌分析的结果提出其刻蚀机理。     

Crystal Phases of TiO2 Ultrafine Particles Prepared by Laser Ablation of Solid Rods

Titanium dioxide ultrafine particles (UFPs) are produced by pulsed laser ablation of titanium or titanium dioxide (anatase and rutile) rods in an atmosphere of He or O₂/He mixture. The collected UFPs on cellulose membrane filters at the exit of the ablation chamber are analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The TiO₂ particles produced are composed of very small particles (diameter: 10–50nm) that are completely anatase, irrespective of the rod material, and relatively large particles (diameter: 100nm–1m) that are a mixture of anatase and rutile. The large particles consist of the direct strip-off fragments coming from the rod surface. The particles obtained from the laser ablation on TiO₂ rods in an atmosphere of He contains gray particles that are supposed to be amorphous TiO₂ (x < 2). In the presence of O₂ in the ablation chamber, these oxygen defects in amorphous TiO₂ are stabilized and anatase UFPs are formed. These results suggest that the crystal phase of the products can be controlled by adjusting the rod material and the gases used in the ablation process.     

Effect of Particle Size and Phase Composition of Titanium Dioxide Nanoparticles on the Photocatalytic Properties

Titanium dioxide (TiO₂) nanoparticles were prepared by the oxidation of titanium tetrachloride (TiCl₄) in a diffusion flame reactor. The average diameter of particles was 15–30 nm and mass fraction of anatase ranged from 40% to 80%. Effects of particle size and phase composition of those TiO₂ nanoparticles on photocatalytic properties such as decomposition of methylene blue, bacteria and ammonia gas were investigated. The degree of decomposition of methylene blue by the TiO₂ nanoparticles under the illumination of the black light was directly proportional to the anatase mass fraction, but inversely to the particle size. The decomposition of bacteria and ammonia gas by the TiO₂ nanoparticles under the illumination of the fluorescent light showed the same trend as in the case of the methylene blue.     

脉冲激光烧蚀过程中靶材表面熔融前温度分布与激光功率密度分布的研究

用较为符合实际的高斯分布表示了脉冲激光输出功率密度分布，讨论了脉冲激光功率密度分布函数形状变化对烧蚀过程中靶材表面熔融前温度分布的影响。建立了考虑热源项的热传导方程，并给出了相应的边界条件。以Si为例，用有限差分方法模拟了温度随时间、位置的变化规律，模拟过程中强调了对边界条件的处理，使整体截断误差保持最小。通过改变脉冲激光功率密度分布函数的形状，分析了温度分布的变化。结果表明，相比恒定脉冲功率密度输出，功率密度高斯分布的激光束与靶材作用时高温阶段的温度变化率变大，靶材表面熔融时刻热扩散距离增加；当激光器上升沿变陡时，在有效作用时间内温度上升得更快，对加工区域周围热效应的影响明显减弱，而热扩散距离变小。     

聚四氟乙烯材料表面激光改性与刻蚀

利用波长为248 nm的准分子激光束在不同激光能量密度下照射聚四氟乙烯(PTFE)材料的表面,并用扫描电镜(SEM)、X射线光电子能谱(XPS)、拉曼(Raman)光谱等手段对激光处理前后样品的表面形貌、化学成分和结构进行测量和分析,进而对激光与聚四氟乙烯相互作用的机理进行了研究。实验结果表明,激光辐照使聚四氟乙烯表面产生去氟效应,导致表面碳化、分子链的交联以及含氧基团的产生,随着激光能量密度的增加,C=C双键逐渐形成。这些结构的变化可以导致表面硬度和粘结性增强。激光能量密度的大小对照射后样品表面的物理性质和化学结构有着重要的影响,它是聚合物表面激光改性和烧蚀的关键因素。     

Synthesis of Nano-Cadmium Sulfide by Pulsed Laser Ablation in Liquid Environment

A pulsed laser-assisted in liquid environment method has been developed successfully to synthesize size-tunable (5–12 nm) and different shapes (sphere, rod, rope) of nano II–VI semiconductor (cadmium sulfide). This method can be carried out in two ways; the first one is the top-down technique, which has been discussed in publications in the last few decades, and the other one is the bottom-up technique, which appears for the first time in this paper. X-ray diffraction, ultraviolet-visible spectroscopy, and transmission electron microscopy confirm that the nanoparticles are crystalline. The methods lead to the production of nanomaterials, which are important for photonics and biosensing applications. Both synthesized methods can be applied in all materials because of their ability to ablate almost all kinds of materials due to the ultrahigh energy density and control over the growth process by manipulating the process parameters such as intensity, wavelength, and so on.     

发射光谱法研究纳秒激光烧蚀硅等离子体特性

利用调Q Nd3+∶YAG激光器三倍频355 nm激光脉冲烧蚀空气环境的硅样品,观测不同脉冲激光能量下产生的等离子体在380～420 nm范围内的时间-空间分辨等离子体发射光谱,观测到在等离子体羽膨胀初期存在N+发射光谱。在局域热力学平衡近似条件下,根据时间-空间分辨等离子体发射光谱计算得到等离子羽体电子温度和电子密度随时间延时存在二次指数衰减变化,等离子体羽体电子温度和电子密度的空间分布近似呈Lorentz分布,发现在确定激光脉冲能量下电子密度空间分布最大值偏离光谱强度最大空间位置并对产生原因进行分析,探讨了等离子体羽参数与激光脉冲能量的关系。     

Silicon micro-hemispheres with periodic nanoscale rings produced by the laser ablation of single crystalline silicon

We describe the fabrication of silicon micro-hemispheres by adopting the conventional laser ablation of single crystalline silicon in the vacuum condition without using any catalysts or additives. The highly oriented structures of silicon micro-hemispheres exhibit many periodic nanoscale rings along their outer surfaces. We consider that the self-organized growth of silicon micro-structures is highly dependent on the laser intensity and background air medium. The difference between these surface modifications is attributed to the amount of laser energy deposited in the silicon material and the consequent cooling velocity.     

Nd：YAG激光器切割金刚石膜的特性研究

采用Ｎｄ：ＹＡＧ激光器对金刚石膜进行切割，研究了激光器在不同输出功率下切割金刚石膜的速率和切割深度与时间的关系，以及Ｎｄ：ＹＡＧ激光器切割的金刚石膜在成份和形状上的变化特点，给出了比较理想的切割工艺条件。     

Atomic and Electronic Structures of Traps in Silicon Oxide and Silicon Oxynitride

Silicon oxide (SiO₂) and silicon oxynitride (SiO_xN_y) are two key dielectrics used in silicon devices. The excellent interface properties of these dielectrics with silicon have enabled the tremendous advancement of metal-oxide-semiconductor (MOS) technology. However, these dielectrics are still found to have pronounced amount of localized states which act as electron or hole traps and lead to the performance and reliability degradations of the MOS integrated circuits. A better understanding of the nature of these states will help to understand the constraints and lifetime performance of the MOS devices. Recently, due to the available of ab initio quantum-mechanical calculations and some synchrotron radiation experiments, substantial progress has been achieved in understanding the atomic and electronic nature of the defects in these dielectrics. In this review, the properties, formation and removal mechanisms of various defects in silicon oxide and silicon oxynitride films will be critically discussed. Some remarks on the thermal ionization energies in connection with the optical ionization energies of electron and hole traps, as well as some of the unsolved issues in these materials will be highlighted.