Nanoparticle formation by laser ablation in air and by spark discharges at atmospheric pressure

Recent promising methods of nanoparticle fabrication include laser ablation and spark discharge. Despite different experimental conditions, a striking similarity is often observed in the sizes of the obtained particles. To explain this result, we elucidate physical mechanisms involved in the formation of metallic nanoparticles. In particular, we compare supersaturation degree and sizes of critical nucleus obtained under laser ablation conditions with that obtained for spark discharge in air. For this, the dynamics of the expansion of either ablated or eroded products is described by using a three-dimensional blast wave model. Firstly, we consider nanosecond laser ablation in air. In the presence of a background gas, the plume expansion is limited by the gas pressure. Nanoparticles are mostly formed by nucleation and condensation taking place in the supersaturated vapor. Secondly, we investigate nanoparticles formation by spark discharge at atmospheric pressure. After efficient photoionization and streamer expansion, the cathode material suffers erosion and NPs appear. The calculation results allow us to examine the sizes of critical nuclei as function of the experimental parameters and to reveal the conditions favorable for the size reduction and for the increase in the nanoparticle yield.     

Angular distributions of plume components in ultrafast laser ablation of metal targets

In view of its fundamental interest and relevance to nanoparticle film production, we have characterised the nanoparticle component of the ablation plume generated in femtosecond laser irradiation of metals. The results are compared to those of the ion plume, which is considered as representative of the atomic component. At moderate laser fluences, the angular distributions of both nanoparticle and ionic components were studied by measuring the spatial distribution of deposition on a transparent substrate and with a planar Langmuir probe, respectively. Our results show that both angular profiles of the plume components can be described by Anisimov model of isentropic expansion. As the laser fluence is increased above a value of several times the ablation threshold, the shape of the nanoparticle angular distribution progressively differs from the Anisimov prediction, contrary to what is observed for the ion component. This effect is interpreted in terms of the influence of the pressure exerted by the nascent atomic plasma plume on the initial hydrodynamic evolution of nanoparticle material.     

Nanoparticle plume dynamics in femtosecond laser ablation of gold

This paper describes some recent results on femtosecond laser ablation of gold. We have studied both the fast vapour/plasma and slow nanoparticle plumes using Langmuir probe, time-resolved ICCD imaging and time-resolved optical absorption measurements. The nanoparticle plume dynamics was analysed by comparing the optical emission absorption measurements with an adiabatic isentropic model of ablation plume expansion, leading to an estimate of the amount of material in the nanoparticle plume.     

Dynamical study of femtosecond-laser-ablated liquid-aluminum nanoparticles using spatiotemporally resolved x-ray-absorption fine-structure spectroscopy

We study the temperature evolution of aluminum nanoparticles generated by femtosecond laser ablation with spatiotemporally resolved x-ray-absorption fine-structure spectroscopy. We successfully identify the nanoparticles based on the L-edge absorption fine structure of the ablation plume in combination with the dependence of the edge structure on the irradiation intensity and the expansion velocity of the plume. In particular, we show that the lattice temperature of the nanoparticles is estimated from the L-edge slope, and that its spatial dependence reflects the cooling of the nanoparticles during plume expansion. The results reveal that the emitted nanoparticles travel in a vacuum as a condensed liquid phase with a lattice temperature of about 2500 to 4200 K in the early stage of plume expansion.     

An analytical continuum-based model of time-of-flight distributions for pulsed laser ablation

An analytical model for time-of-flight (TOF) distributions of particles produced by pulsed laser ablation in vacuum has been proposed. The model takes into account the hydrodynamic expansion stage of the ablation plume and is based on a ‘sudden freeze’ model developed previously for steady-state supersonic jets. It is assumed that a continuum-like expansion of the plume takes place until a freezing time moment t _free (or, alternatively, until a ‘freezing distance’ x _free) whereupon the collisionless expansion begins. The proposed model is applied for analysis of experimental data on graphite ablation with nanosecond laser pulses. For verification, the analytical distributions are compared with calculated results obtained using a hybrid model combining a thermal model of laser-induced material heating with calculations of the plume dynamics by the direct simulation Monte Carlo (DSMC) method. It is shown that the proposed model can accurately estimate the surface temperature for conditions when the common approach fails.     

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.     

Pulsed laser ablation of metals in vacuum: DSMC study versus experiment

We propose a hybrid model describing the processes of heating and ablation of a metal target with expansion of vaporized material into a vacuum under the action of nanosecond laser pulses of moderate intensity. The model is based on the integration of the thermal problem of laser-induced material heating and the direct Monte Carlo simulation method to describe the hydrodynamics of the ablation products. Modeling has been performed for niobium as an example and the results obtained are in good agreement with experimental data on mass removal and time-of-flight (TOF) measurements. Application of the commonly used fitting procedures for characterization of the laser plume parameters by the TOF signal is discussed. PACS 52.38.Mf; 79.20.Ds     

Characterization of aerosol plumes in nanosecond laser ablation of molecular solids at atmospheric pressure

Ablation of molecular solids with pulsed ultraviolet lasers at atmospheric pressure is an important process in (bio-)organic mass spectrometry. Of practical importance for analytical sampling and analysis are the plume formation and expansion. Plumes formed by atmospheric-pressure laser ablation of anthracene and 2,5-dihydroxybenzoic acid (2,5-DHB) were studied by light scattering imaging, which showed significant material release in the form of aerosols. The monitored plume expansion dynamics could be fitted to the drag-force model, yielding initial plume velocities of 150 m/s for anthracene and 43 m/s for DHB. While the angle of incidence does not affect the plume direction and propagation, a large dependence of the plume-expansion velocity on the laser pulse energy could be found, which is limited at atmospheric pressure by the onset of plasma shielding. With respect to analytical applications, the efficiency of sampling of the laser ablation products by a capillary could be experimentally visualized.     

Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c     

Dynamics of femtosecond-laser-ablated liquid-aluminum nanoparticles probed by means of spatiotemporally resolved X-ray absorption spectroscopy

We investigated spatiotemporal evolution of expanding ablation plume of aluminum created by a 100-fs, 10¹⁴–10¹⁵-W/cm² laser pulse. For diagnosing dynamic behavior of ablation plume, we employed the spatiotemporally resolved X-ray absorption spectroscopy (XAS) system that consists of a femtosecond-laser-plasma soft X-ray source and a Kirkpatrick–Baez (K–B) microscope. We successfully assigned the ejected particles by analyzing structure of absorption spectra near the L _II,III absorption edge of Al, and we clarified the spatial distribution of Al⁺ ions, Al atoms, and liquid droplets of Al in the plume. We found that the ejected particles strongly depend the irradiated laser intensity. The spatial distribution of atomic density and the expansion velocity of each type of particle were estimated from the spatiotemporal evolution of ablation particles. We also investigated a temperature of the aluminum fine particles in liquid phase during the plume expansion by analyzing the slope of the L _II,III absorption edge in case of 10¹⁴-W/cm² laser irradiation where the nanoparticles are most efficiently produced. The result suggests that the ejected particles travel in a vacuum as a liquid phase with a temperature of about 2500 to 4200 K in the early stage of plume expansion.     

Laser ablation of silicon and copper targets. Experimental and finite elements studies

The ablation process induced by excimer lasers is a collective phenomenon that basically involves two phenomena: the laser radiation–matter interaction and the dynamic of the ablation plume. The laser parameters, the thermal and optical properties of the material, and the surface morphology are critical factors in the ablation mechanisms affecting the direction of the ablation plume expansion. In this study, the role of the surface roughness and the evolution of its morphology under the laser irradiation were investigated. Assuming a thermal ablation model, a theoretical study of the initial steps of the laser ablation process by a finite element method using ANSYS (6.1) was performed. Different ablation experiments were carried out on silicon and copper targets using a XeCl laser. The target surface morphology changes were observed by SEM and the plume deflection was recorded by a digital camera. An acceptable agreement between the experimental and simulated results was found. This study contributes to a better understanding of the physical processes involved in the laser ablation and the relations between the plume deflection angle and the surface roughness. PACS 79.20.Ds; 81.40.Gh; 44.05.+e     

Features of plasma plume evolution and material removal efficiency during femtosecond laser ablation of nickel in high vacuum

We present an experimental characterization describing the characteristics features of the plasma plume dynamics and material removal efficiency during ultrashort, visible (527 nm, ≈300 fs) laser ablation of nickel in high vacuum. The spatio-temporal structure and expansion dynamics of the laser ablation plasma plume are investigated by using both time-gated fast imaging and optical emission spectroscopy. The spatio-temporal evolution of the ablation plume exhibits a layered structure which changes with the laser pulse fluence F. At low laser fluences (F<0.5 J/cm²) the plume consists of two main populations: fast Ni atoms and slower Ni nanoparticles, with average velocities of ≈10⁴ m/s for the atomic state and ≈10² m/s for the condensed state. At larger fluences (F>0.5 J/cm²), a third component of much faster atoms is observed to precede the main atomic plume component. These atoms can be ascribed to the recombination of faster ions with electrons in the early stages of the plume evolution. A particularly interesting feature of our analysis is that the study of the ablation efficiency as a function of the laser fluence indicates the existence of an optimal fluence range (a maximum) for nanoparticles generation, and an increase of atomization at larger fluences. PACS 52.50.-b; 52.38.Mf; 79.20.Ds; 81.07.-b     

Correlation between ablation efficiency and nanoparticle generation during the short-pulse laser ablation of metals

The mechanisms of material ablation and nanoparticle generation from metal samples exposed to intense short laser pulses are experimentally investigated. We performed measurements of the ablated volume using optical microscopy and the analysis of the ablation plume by fast imaging. The results confirm the existence of two distinguished ablation regimes as a function of the laser fluence, and give a deeper insight in the involved physical mechanisms. Thus, both regimes are found to be related to the relative amount of atoms and nanoparticles within the plume. Comparing the results obtained for copper and gold, it is possible to determine the influence of electron-lattice coupling on the sample heat regime and the resulting plume properties.     

Effects of laser intensity and ambient conditions on the laser-induced plume

Laser ablation presents a promising technique for material processing. The quality of products is strongly influenced by the properties of the laser-induced plume. In compressible flow, the ambient conditions can be transmitted upstream. Therefore, the laser ablation process is strongly affected by the ambient conditions. In this paper, the effects of laser intensity, back pressure and temperature on the laser-induced plume were studied using a numerical model, which calculates the density, pressure and temperature of the laser-induced plume at different laser intensity and ambient conditions. The results are in agreement with experimental results available in the literature and can be used for the optimization of the pulsed laser deposition process.     

Laser ablation and deposition of wide bandgap semiconductors: plasma and nanostructure of deposits diagnosis

Nanostructured CdS and ZnS films on Si (100) substrates were obtained by nanosecond pulsed laser deposition at the wavelengths of 266 and 532 nm. The effect of laser irradiation wavelength on the surface structure and crystallinity of deposits was characterized, together with the composition, expansion dynamics and thermodynamic parameters of the ablation plume. Deposits were analyzed by environmental scanning electron microscopy, atomic force microscopy and X-ray diffraction, while in situ monitoring of the plume was carried out with spectral, temporal and spatial resolution by optical emission spectroscopy. The deposits consist of 25–50 nm nanoparticle assembled films but ablation in the visible results in larger aggregates (150 nm) over imposed on the film surface. The aggregate free films grown at 266 nm on heated substrates are thicker than those grown at room temperature and in the former case they reveal a crystalline structure congruent with that of the initial target material. The observed trends are discussed in reference to the light absorption step, the plasma composition and the nucleation processes occurring on the substrate.     

Fast imaging of the plume expansion produced by laser ablation of LiNbO3

Fast photography of the plume produced by laser ablation of LiNbO₃ in vacuum has been performed using an image intensified CCD (ICCD) camera in a time interval up to 2 7s after the laser pulse. Two differently oriented single crystalline LiNbO₃ targets were used. The results show that although the emission intensity of the laser-generated plume initially depends on the crystalline orientation of the target, it reaches a stationary state after several minutes which is the same for both targets orientations. Under these stationary conditions, the angular distribution of the Li atoms is found to be broader than that of Nb atoms. The observed less forward directed expansion of the Li species may explain the poor Li content normally observed in films grown by laser ablation of LiNbO₃ in vacuum.     

Study of the plume generated by Nd:YAG laser ablation of a hydroxyapatite target

The plume generated by Nd:YAG laser ablation of a hydroxyapatite target has been investigated in vacuum and at 0.1 and 0.2 mbar of water vapor. The investigation has been carried out by means of fast intensified CCD imaging with the aid of bandpass interferential filters that allow the following single species to be isolated: neutral calcium, calcium oxide radicals and neutral oxygen. Results obtained in vacuum reveal that expansion takes place at a constant velocity of about 2᎒⁴ m/s for the atomic species and about 3᎒³ m/s for the molecular ones and that emission is completely dominated by emissive neutral calcium. When ablation is carried out in a water atmosphere, the background gas confines the species in the leading edge of the plume, which results in the formation of a planar shock wave at 0.1 mbar and a spherical shock wave at 0.2 mbar. Comparison of the images with those obtained at 0.1 mbar of Ne has revealed the existence of chemical reactions between the plume and the water atmosphere, leading to the formation of calcium oxide radicals. In that case, plume emission is dominated by these molecular species.     

Subpicosecond laser ablation of copper and fused silica: Initiation threshold and plasma expansion

We investigated the subpicosecond laser ablation of copper and fused silica under 100 fs laser irradiation at 800 nm in vacuum by means of fast plume imaging and time- and space-resolved optical emission spectroscopy. We found that, to the difference of copper ablation, the laser-generated plasma from a fused silica target exhibited one “main” component only. The “slow” plasma component, observed during copper ablation and usually assigned to optical emission from nanoparticles was not detected by either plasma fast imaging or optical emission spectroscopy even when fused silica targets were submitted to the highest incident fluences used in our experiments. The characteristic expansion velocity of this unique component was about three times larger than the velocity of the fast plume component observed during copper ablation. The dependence of laser fluence on both plasma expansion and ablation rate was investigated and discussed in terms of ablation efficiency and initiation mechanisms.     

Growth of ZnO nanoparticles and nanorods with ultrafast pulsed laser deposition

In this work, we study the application of ultrafast pulsed laser deposition (PLD) in ZnO nanomaterial synthesis, including nanoparticles and nanorods. PLD using long pulse (nanosecond) lasers has been widely used as a method for growing prototype materials. The recently-emerged ultrafast PLD is expected to be able to overcome the problem of large liquid droplet formation. Using near infrared and femtosecond laser pulses in ablation, we first characterize the ablation plume using a Langmuir probe and plasma optical emission spectroscopy. We then examine the structural properties of the nanoparticles generated during low-fluence ablation. Finally, we demonstrate that using nanoparticle aggregates as templates, assisted by plume-excited nitrogen radicals at a high fluence, high quality ZnO nanorods can be grown free of metal catalysts.