Cluster growth in an ablation plume propagating through a buffer gas

A phenomenological mixed-propagation model that describes the expansion of an ablation plume through a buffer gas is introduced. Selected experiments including LaMnO₃ and tin ablation in oxygen, as well as tungsten ablation in argon, are analysed. For given ablation conditions the expansion parameters required to model the growth of clusters in the expanding plasma plume are deduced and the average asymptotic size of the clusters is calculated and compared (for tungsten) with the size of clusters measured by transmission electron microscopy.     

Magnetic studies on mass-selected iron particles

We have investigated the magnetic properties of mass-selected iron clusters using the Magneto-Optical Kerr effect (MOKE) in longitudinal geometry. For the production of these clusters, a newly developed continuous arc cluster ion source (ACIS) was applied. The source is based on cathodic arc erosion in inert gas environment and subsequent expansion into high vacuum. Its intensity and stability allows mass selection within a wide size range. The source efficiency is demonstrated in deposition experiments and transmission electron microscopy. For mass-selected iron particles deposited into a silver matrix we could observe a change in the magnetic behaviour from ferromagnetism to superparamagnetism around a size of 10 nm at room temperature. Received 1st December 2000     

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.     

Substrate heating effects on the propagation dynamics of laser produced plume during pulsed laser deposition of oxides

We report on the expansion dynamics of laser-produced plasma plumes of complex oxides in an oxygen atmosphere. In particular, we have studied the combined effects of background gas pressure and substrate heating on the plume propagation in typical pressure and temperature regimes of oxides thin film deposition by pulsed laser deposition. Our results evidence a reduced resistance of the background gas to the plume propagation as the substrate temperature increases. The experimental data are analyzed in the frame of a model describing the plume propagation into the background gas. Our experimental findings clearly indicate that the deposition temperature might influence film growth, not only through its direct thermal effect on the surface kinetics of adatoms, but also by affecting the energetic properties of the precursors in the gas phase.     

Fluorescence spectroscopy of silver clusters formed in rare gas droplets

The Pick Up technique allows continuous formation of high density metal cluster beams in the otherwise hardly accessible size range Me₂ to about Me₅₀. With an apparatus based on the use of cryo-condensation pumps, Ar as inert gas for production of the host clusters by adiabatic expansion and silver as metal we demonstrate how the various source parameters influence the composition and mean size of the resulting cluster distributions. The analysis in the gas phase by means fluorescence spectroscopy allows to conclude that an Ar shell encapsulates the metal clusters. Received 30 November 2000     

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster-size distributions can be relatively well described by a power law Y(N)∼N^-τ with exponents different for small, up to ∼15 molecules, and large clusters. The decay is much slower in the high-mass region of the distribution. Received: 13 October 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-434/982-5660, E-mail: lz2n@virginia.edu     

Plume splitting in pico-second laser-material interaction under the influence of shock wave

In this work, molecular dynamics simulations are conducted to study the physics of plume splitting in pico-second laser material interaction in background gas. The velocity distribution shows a clear split into two distinctive components. Detailed atom trajectory track reveals the behavior of atoms within the peaks and uncovers the mechanisms of peak formation. The observed plume velocity splitting emerges from two distinguished parts of the plume. The front peak of the plume is from the faster moving atoms and smaller particles during laser-material ablation. This region experiences strong constraint from the ambient gas and has substantial velocity attenuation. The second (rear) peak of the plume velocity originates from the larger and slower clusters in laser-material ablation. These larger clusters/particles experience very little constraint from the background, but are affected by the relaxation dynamics of plume and appear almost as a standing wave during the evolution. Density splitting only appears at the beginning of laser-material ablation and quickly disappears due to spread-out of the slower moving clusters. It is found that higher ambient pressure and stronger laser fluence favor earlier plume splitting.     

Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution by UV/VIS spectroscopy and STM/SEM microscopy

Silver colloids in aqueous solution were studied by different scanning microscopy techniques and UV/VIS spectroscopy. The silver colloids were produced either by chemical reduction or by nanosecond laser ablation from a solid silver foil in water. Variation of laser power and ablation time leads to solutions of metal clusters of different sizes in water. We characterized the electronic absorption of the clusters by UV/VIS spectroscopy. STM (scanning tunneling microscope) imaging of the metal colloids shows atomic resolution of rod- or tenon-like silver clusters up to 10-nm length formed by laser ablation. Our scanning electron microscope measurements, however, show that much larger silver colloids up to 5-μm length are also formed, which are not visible in the STM due to their roughness. We correlate them with the long-wavelength tail of the multimodal UV/VIS spectrum. The silver colloids obtained by chemical reduction are generally larger and their electronic spectra are red-shifted compared to the laser-ablated clusters. Irradiation of the colloid solution with nanosecond laser pulses of appropriate fluence at 532 nm and 355 nm initially reduced the colloid size. Longer irradiation at 355 nm, however, leads to the formation of larger colloids again. There seems to be a critical lower particle size, where silver clusters in aqueous solution become unstable and start to coagulate. Received: 24 June 2002 / Revised version: 25 July 2002 / Published online: 25 October 2002 RID="*" ID="*"This work is part of the thesis of H. M?ltgen RID="**" ID="**"Corresponding author. Fax: +49-211/811-5195, E-mail: kleinermanns@uni-duesseldorf.de     

Optical properties of mixed clusters: comparative study of Ni/Ag and Pt/Ag clusters

The optical properties of mixed (Ni_0.5Ag_0.5)_n and Pt/Ag clusters are investigated in the size range 2-5 nm. Low Energy Ion Spectroscopy (LEIS) experiments show that the cluster surface is entirely covered by silver atoms for the two systems. The optical spectra of Ni/Ag clusters exhibit a large Surface Plasmon Resonance (SPR), damped and widened when the cluster size decreases, in agreement with a classical model assuming a core-shell geometry and including the reduction of the conduction electron mean-free path in the silver shell. For Pt/Ag clusters, no SPR emerges in the size range 2-5 nm, although it is predicted within a classical model, a pronounced SPR appearing only for clusters larger than 10 nm in diameter.     

Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

Influence of ambient gas ionization on the deposition of clusters formed in an ablation plume

The effect of inert gas ionization on the dynamics of a laser ablation plume expanding through a background inert gas is studied. Charge transfer reactions between ablated ions and neutral background gas atoms yield to the formation of a charged layer on the plume expansion front. The energy lost by ablated ions when the plume is slowed down is calculated. The observed microstructure differences between carbon films prepared by pulsed laser deposition in helium, where the ionization mechanism is absent and respectively in argon, where it is present, are well correlated to model predictions.     

Propagation of LaMnO3 laser ablation plume in oxygen gas

The propagation of LaMnO₃ laser ablation plume in oxygen background has been investigated using fast photography of overall visible plume emission and time-resolved optical emission spectroscopy. The plume expansion was studied with ambient oxygen pressures ranging from vacuum level to 100 Pa. Free-expansion, splitting, sharpening and stopping of the plume were observed at different pressures and time delays after the laser pulse. Time-resolved optical emission spectroscopy showed that oxides are mainly formed through reaction of the atomic species ablated from the target with oxygen in the gas-phase. These reactions mainly affect the content of lanthanum oxide in the plume, while emission of manganese oxide is barely observed in all the range of pressure investigated.     

Nanostructured silver thin films deposited by pulsed laser ablation

Nanostructured thin films were deposited by excimer laser ablation of silver targets in controlled atmospheres of He and Ar. The film structural properties were investigated by means of scanning electron microscope and transmission electron microscope imaging. The film growth mechanism was identified as the result of coalescence of nanometric clusters formed during plume flight. Cluster formation involves plume confinement as a consequence of the increased collisional rate among plasma species. Fast photography imaging of the laser-generated silver plasma allowed to identify plasma confinement, shock wave formation and plasma stopping.     

Mechanisms of small clusters production by short and ultra-short laser ablation

The mechanisms involved into the formation of clusters by pulsed laser ablation are studied both numerically and experimentally. To facilitate the model validation by comparison with experimental results, the time and length scales of the simulation are considerably increased. This increase is achieved by using a combination of molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) methods. The combined MD-DSMC model is then used to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking and aggregation in the ablated gas flow. Calculation results demonstrate that both of these mechanisms play a role. The initial cluster ejection provides cluster precursors thus eliminating the three-body collision bottleneck in the cluster growth process. The presence of clusters thus facilitates the following collisional condensation and evaporation processes. The rates of these processes become considerable, leading to the modification of not only the plume cluster composition, but also the dynamics of the plume expansion. Calculation results explain several recent experimental findings.     

Nanostructured high valence silver oxide produced by pulsed laser deposition

Among silver oxides, Ag₄O₄, i.e. high valence Ag(I)Ag(III) oxide, is interesting for applications in high energy batteries and for the development of antimicrobial coatings. We here show that ns UV pulsed laser deposition (PLD) in an oxygen containing atmosphere allows the synthesis of pure Ag₄O₄ nanocrystalline thin films, permitting at the same time to control the morphology of the material at the sub-micrometer scale. Ag₄O₄ films with a crystalline domain size of the order of tens of nm can be deposited provided the deposition pressure is above a threshold (roughly 4 Pa pure O₂ or 20 Pa synthetic air). The formation of this particular high valence silver oxide is explained in terms of the reactions occurring during the expansion of the ablated species in the reactive atmosphere. In particular, expansion of the PLD plasma plume is accompanied by formation of low stability Ag-O dimers and atomic oxygen, providing reactive species at the substrate where the film grows. Evidence of reactive collisions in the expanding ablation plume is obtained by analysis of the plume visible shape in inert and reactive atmospheres. In addition, we show how the dimensionless deposition parameter L, relating the target-to-substrate distance to the ablation plume maximum expansion length, can be used to classify different growth regimes. It is thus possible to vary the stoichiometry and the morphology of the films, from compact and columnar to foam-like, by controlling both the gas pressure and the target-to-substrate distance.     

Expansion of a laser-produced silver plume in light background gases

The expansion of a silver ablation plume in a helium and an argon background gas has been studied over the pressure range 10^-6 to 1 mbar. The angular distribution of silver atoms deposited on an array of quartz-crystal microbalances as well as time-of-flight signals of the plume ions in both gases were measured. There is a distinct sharpening of the angular distribution of the ablated silver atoms in the helium gas within a narrow range from 0.2 to 0.4 mbar. In contrast, the width of the angular distribution in the argon gas increases throughout the interval. The time-of-flight signals show a peak of fast ions which gradually merges into a more complex structure. PACS 79.20.Ds; 81.15.Fg; 52.38.Mf     

Computational and experimental study of the cluster size distribution in MAPLE

A combined experimental and computational study is performed to investigate the origin and characteristics of the surface features observed in SEM images of thin polymer films deposited in matrix-assisted pulsed laser evaporation (MAPLE). Analysis of high-resolution SEM images of surface morphologies of the films deposited at different fluences reveals that the mass distributions of the surface features can be well described by a power-law, Y(N) ∝ N^−t, with exponent −t ≈ −1.6. Molecular dynamic simulations of the MAPLE process predict a similar size distribution for large clusters observed in the ablation plume. A weak dependence of the cluster size distributions on fluence and target composition suggests that the power-law cluster size distribution may be a general characteristic of the ablation plume generated as a result of an explosive decomposition of a target region overheated above the limit of its thermodynamic stability. Based on the simulation results, we suggest that the ejection of large matrix-polymer clusters, followed by evaporation of the volatile matrix, is responsible for the formation of the surface features observed in the polymer films deposited in MAPLE experiments.