John W. Keto

Metal nanoparticles generated by laser ablation

Abstract We study a new method for producing ultrafine metal particles (nanopartides) that employs Laser Ablation of Microparticles (LAM). Pulsed excimer laser radiation at 248 nm wavelength was used to ablate ~2 μm feedstock of silver, gold, andpermalloy (Ni 81 %:Fe 19% ) under both normal atmospheric conditions and in other gases and pressures. A model for nanoparticle formation by LAM is proposed that includes plasma breakdown and shock-wave propagation through the initial microparticle. Behind the shock a large fraction of the original microparticle mass is converted to nanoparticles that diffuse to silicon substrates and TEM grids for collection and analysis. Nanoparticle morphologies are spherical except for gold nanoparticles >100 nm that are generally cubes. Electron micrographs of the samples were analyzed by computer-aided image processing to determine the effect of irradiation conditions on the nanoparticle size distribution. The results showed that mean particle diameters were normally in the range from 10 to 100 nm and that the particle size distributions were generally log-normal, with dispersion (diameter/standard deviation) ranging from 0.2 to 0.5. For metallic microparticle feedstock, the mean size of the produced nanoparticles generally increased with increasing laser fluence and were smallest for fluences not too far above the breakdown threshold.

Large-scale production of nanocrystals by laser ablation of microparticles in a flowing aerosol

We experimentally demonstrate the production of nanoparticles by laser ablation of microparticles entrained at high density in a flowing aerosol. The currently measured production rate of 20 grams per hour could be scaled to industrially useful rates. We have characterized the size distribution of particles and found nearly monodisperse distributions where mean sizes were smaller and varied less with laser fluence than was observed for ablation of microparticles held on flat plates. Mean size was controlled from 4–20 nm by varying the type and pressure of carrier gas. We found Ag and CdSe nanoparticles were crystalline having few dislocations. Materials tested included metals (Ag, Au, and W), semiconductors (Si, CdSe, GaN, and ZnO), ceramics (WC, SiC, and YBa2Cu3O7), and a ferroelectric. Two types of collection processes are described that preserve the nonagglomerated nature of the particles, even at high mass densities.

Collisional deactivation of two‐photon laser excited xenon 5p5 6p. II. Lifetimes and total quench rates

Lifetimes and total binary quench rate constants of Xe 5p5 6p were measured using two‐photon laser excitation. For Xe 2p5, state‐changing three‐body quenching is observed, as well. The deactivation rates are discussed on the basis of the molecular potentials involved. Strong coupling of Xe 2p10 with states of Xe 6s’ is observed and analyzed using a kinetic model which gives consistent results with data of other workers. The dominant path for deactivation of the Xe 6s’ levels is found to involve molecular states of Xe 2p10. The total cross sections obtained are compared to measurements of the quenching of Xe 6p by other rare gases.

Radiative lifetimes and collisional deactivation of two‐photon excited xenon in argon and xenon

Radiative lifetimes and bimolecular rates have been determined for two‐photon laser excited states of Xe* (5p56p, 5p56p’, 5p57p) in argon and xenon buffer gases. The collisional deactivation rates are found to be very large for the Xe 6p’ and 7p states [∼(2–5)×10−10 cm3/s] while the rates for the Xe 6p states are comparatively smaller [∼(0.06–1.2)×10−10 cm3/s]. In general, the quenching rates in argon are about a factor of 2 smaller than the xenon quenching rates for the same excited state with the notable exception of Xe 6p[1/2]0. For Xe 6p[1/2]0, a multicomponent decay has been observed in argon buffer gases. The second component is attributed to collisional coupling to Xe 5d[1/2]1 which lies 132.3 cm−1 below Xe 6p[1/2]0. Quench rates determined from the collisionally induced VUV fluorescence from Xe 5d[1/2]1 at 125 nm are in excellent agreement with this assignment. Furthermore, these experiments have unambiguously identified the product channel involved in the curve crossings observed in studies of op...

Collisional deactivation of two‐photon laser excited xenon 5p5 6p. I. State‐to‐state reaction rates

Laser‐induced fluorescence of two‐photon excited Xe 5p5 6p is analyzed to obtain collisional branching fractions and state‐to‐state reaction rates. The intramultiplet quenching is found to be described surprisingly well by a simple statistical model. Based on the molecular potentials involved, curve crossing mechanisms are discussed.

Deactivation of two‐photon excited Xe(5p56p,6p’,7p) and Kr(4p55p) in xenon and krypton

Lifetimes and bimolecular quenching rate constants have been determined for two‐photon laser excited states of Xe*(5p56p,5p56p’,5p57p) and Kr*(4p55p) in krypton and xenon buffer gases. Collisional mixing between Kr*5p[5/2]2 and Kr*5p[5/2]3 in krypton is observed and analyzed using a coupled two‐state model to obtain the rate of mixing. The measured rate constants for quenching of Xe*(6p’,7p) by krypton are 15%–20% smaller than those measured previously in xenon while bimolecular rates for the Kr*(5p) states are an order of magnitude larger in xenon than those in a krypton buffer. Measurements of state‐to‐state rate constants for deactivation and excitation transfer are also reported for these states in krypton and xenon buffer gases.

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.

Synthesis of nanometer glass particles by pulsed‐laser ablation of microspheres

We describe a new method for producing ultrafine glass particles that employs pulsed‐laser particle ablation. Pulsed‐laser radiation with wavelength of 249 nm was used to ablate 8‐μm‐diam glass microspheres under normal atmospheric conditions. The ejected particles were collected on silicon substrates for further analysis. Scanning electron micrographs of the samples were analyzed by computer aided image processing to determine the effect of laser fluence on the particle size distribution. Results showed that mean particle diameter, in the range from 60 to 80 nm, was generally inversely proportional to laser fluence. The particle size distributions were closely log‐normal with small geometric standard deviations.

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.

Dynamics of laser ablation of microparticles prior to nanoparticle generation

To better understand the process of nanoparticle formation when microspheres are ablated by a high-energy laser pulse, we investigated the Nd:YAG laser-induced breakdown of 20 μm glass microspheres using time-resolved optical shadow images and Schlieren images. Time-resolved imaging showed the location of the initial breakdown and the shockwave motion over its first 300 μm of expansion. From these measurements, we determined the shockwave velocity dependence on laser fluence. Measured shockwave velocities were in the range of 1–10 km/s. We also developed a numerical model that simulated breakdown in the glass microsphere and the propagation of this disturbance through the edge of the sphere where it could launch an air shock. Our objective was to simulate the shockwave velocity dependence on laser fluence and to generate glass density, temperature, and mass velocity profiles after breakdown. The simulation and experimental data compared favorably.