Lithuanian Journal of Physics, Vol. 51. No. 3, pp. 248–259 (2011) © lietuvos mokslų akademija, 2011 GENERATION OF METAL NANOPARTICLEs BY LAsER ABLATION V. Dudoitis, V. Ulevičius, G. Račiukaitis, N. Špirkauskaitė, and K. Plauškaitė State Research Institute Center for Physical Sciences and Technology, Savanorių 231, LT-02300 Vilnius, Lithuania E-mail: ulevicv@ktl.mii.lt Received 2 June 2011; revised 11 July 2011; accepted 21 September 2011 he process of nanoparticle generation during nanosecond and picosecond laser ablation of various metals (Ni, Al, W and stainless steel) in ambient air and argon gas was investigated. he number concentration of nanoparticles generated by laser ablation in argon gas was up to 100 times higher compared to the concentration in ambient air. hree stable separate size peaks of nucleation, Aitken and accumulation modes of nanoparticles were observed in case of argon gas, while in ambient air particles of a wide size spectrum (8–200 nm) were generated. he natural precursors in ambient air can have an efect on size spectrum of the particles composed of various chemical compounds with target material during the ablation process. he inluence of laser process parameters and properties of investigated metals on the number concentration and size distribution of nanoparticles generated during ns- and ps-laser ablation was observed. Keywords: nanoparticles, nanosecond and picosecond laser ablation, metals, size distribution, number concentration, ambient air, argon gas PACs: 52.38.Mf, 78.70.-g, 81.10.St 1. Introduction Comprehension of the aerosol formation as inevitable process in laser micromachining technology is established and the application of laser ablation in production of nanoparticles is widely spread. However, characteristics of particles generated by laser ablation are not fully known. herefore, the studies of particle concentration and their size distribution are hot topics in current laser ablation investigations recently [1–9]. Nanoparticle generation by laser ablation is used for a variety of applications such as biotechnology, electronic industry, etc. he laser ablation is also an intensive source of submicron particle generation and their possible leakage into ambient air therefore investigations of the particle formation kinetics in ambient air, during laser processing are essential for the evaluation of potential generation of particle emission and their impact on human health [7]. For example, during laser processing particles formed of metals (e. g. iron, aluminum), especially toxic (e. g. manganese, zinc) or carcinogenic substances (e. g. chromium (VI) compounds, nickel), might penetrate into a human organism and cause health concerns [8, 9]. he physical process of particle generation begins with the absorption of laser irradiation in the beam/material interaction zone. Depending on the wavelength of the laser beam and the material properties, kinetic energy is transformed into thermal (in metals 10–13 s) [10], which cuts of chemical bonds in material. he phase transition occurs if the energy threshold for that material is reached. As a result, material can be melted, vaporized or sublimated and the particle generation process begins. Both the particle concentration and size distribution depend on laser operating parameters such as wavelength, pulse duration, energy and repetition rate, beam scanning speed. [3, 7, 10, 11]. Studies show that experimental conditions during laser ablation could be established in order to control the particle size and distribution [1, 3]. Besides, the medium 249 V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) (ambient air, argon, water, etc.) in the ablation chamber also plays an important part in the particle formation process. It was experimentally estimated that the mass of generated nanoparticles in ambient air was up to 100 times higher than in water [3]. he aim of the study was to investigate the peculiarities of nanoparticle formation during laser ablation of diferent metals (nickel, aluminum, stainless steel, tungsten) and to characterize the diference of size distribution of the generated particles in respect of the laser irradiation type (nanosecond, picosecond) and media in the ablation chamber (partially particle-free ambient air, argon gas). 2. heoretical background During During the interaction of the laser beam and material, the light absorption is a general physical process [11–13]. he energy coupled into material causes thermal heating, melting and vaporization of material, plasma formation and particle emission. In metals, free electrons interact with the intense electromagnetic irradiation. he energy is instantly absorbed by electrons and further distributed to the lattice. he laser pulse duration τL is important for understanding what happens with the energy in the beam / material interaction zone. is the lattice heating time, is the electron cooling time, where Ce and Ci are electron and lattice thermal capacities, respectively, and γ is the electron– lattice coupling parameter speciic to every material. Based on the two-temperature difusion model by Chichkov et al. [13] the laser beam/material interaction can be described by electron and lattice subsystem temperatures Te and Ti, respectively: , , (1) (2) where Q(z) is the heat lux, S is the heat source (laser pulse). For picosecond pulses, condition τe << τL << τi is fulilled. he laser pulse duration is shorter than the lattice heating time. Particles from the material are removed partly by solid state – vapor transition and direct breaking of chemical bonds. For picosecond pulses (Eq. (1)) the electron temperature Te becomes quasi-stationary: , (3) where ke is the electron thermal conductivity, α is the absorption coeicient, Ia is the laser intensity transmitted to the material, z is the depth of laser beam penetration into material perpendicular to the surface. In this case, lattice temperature Ti is , (4) where T0 is the initial electron temperature. Taking this into account Eq. (4) can be simpliied due to quasi-stationary condition of electron temperature: , (5) where t is the time ater the laser exposure. It means that electron cooling temperature Te stays longer than lattice heating temperature Ti ater exposure to picosecond laser pulses. Electron cooling temperature Te at the end of the pulse is . (6) Lattice heating temperature Ti at the end of the laser pulse is , (7) where Fa is the absorbed laser luence, which is Fa = IaτL. By using the condition of strong evaporation it can be described as Fa ≥ Fth exp (αz), (8) where Fth is the threshold laser luence for evaporation. Equations (6)–(8) describe how laser intensity, material transmissivity and thermal capacities inluence the temperature of the electron and lattice. In the second case for nanosecond pulses, condition τL >>τi >> τe is fulilled. Equations (1), (2) for nanosecond pulses are: , (9) where k0 is the conventional equilibrium thermal conductivity of a metal and T represents general 250 V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) temperature of the subsystem, because in case of ns-laser ablation Te = Ti = T. he energy absorbed by electrons is fully transferred to the lattice during the laser pulse duration. Material is heated due to the absorbed energy of the lattice. In this case, solid material melts, the beam/interaction zone spreads and particles are evaporated from liquid state. Metals have good thermal conductivity properties, thus increasing energy loss due to lattice heating for nanosecond pulses. 3. Experimental set-up he experimental set-up is schematically shown in Fig. 1. he main parts are: the laser, the galvoscanner for laser beam control and an ablation chamber, a diferential mobility particle sizer (DMPS) (ELAS-05MC, working range: 10–200 nm) and the condensation particle counter (CPC) (UF-02, cutsize at 4.5 nm). Both devices are developed at the Institute of Physics (Lithuania) [14, 15]. Laser ablation was carried out using two diferent Nd : YAG lasers: nanosecond (ns) (Ekspla Ltd., NL640, λ = 1064 nm, 15 ns) and picosecond (ps) (Ekspla Ltd., PL10100, λ = nm, 10 ps). Materials chosen for the investigation were the following metals: stainless steel, nickel, aluminum and tungsten. he experiments were performed in the ablation chamber at atmospheric pressure with a constant low of partially particle-free ambient air which was processed through a HEPA ilter. It cleaned gas low from particles up to 5 nm. In case of argon, the gas was delivered into the chamber from a pressurized tank at a constant low rate of 3 000 cm3/min. he exhaust low was divided for the CPC and DMPS measurements. In case of argon, only DMPS was used. he aerosol low rate for DMPS was 2 l/min and 0.3 l/min and 0.3l/min [15] for CPC. he ambient air temperature at the inlet was 25 ± 2 °C (the argon temperature was – 18 ± 1 °C, relative humidity 25–30%. he movement of the laser beam was controlled by a galvoscanner. During the experiments, a laser beam was moving in the form of zigzag, illing a square (5 × 5 mm2). It means that the faster moved the beam, the less time it took to ill a square and to start over ablating the same area again. he laser beam focus diameter D and area S in the case of nslaser were D = 40±5 µm, S = 1.26·10–5 cm2; in the case of the ps-laser D = 20±5 µm, S = 3.14 · 10–6 cm2. he nanosecond laser process parameters were: the average power 0.1–3 W, the pulse repetition rate 1–40 kHz and the beam scanning speed 10– 1000 mm/s. hose of the picosecond laser: the average power 0.1–2 W, the pulse repetition rate 50 or 100 kHz and the beam scanning speed 10–1000 mm/s. For data analysis of the size distribution of generated Fig. 1. Experimental set-up for generated particle measurement. V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) 251 particles by laser ablation in ambient air the Aitken classiication of atmospheric aerosol particle modes was used [16]. 4. Results Samples were placed into the chamber and it was illed with the operating gas. he laser beam was scanned over the sample surface and both techniques were applied to measure the particles in the exhaust gasses. During the experiments the particle size distribution was averaged from 3 to 8 measurements. he geometric mean of the particle size diameter dp and the standard deviation σg were calculated. Solid curves in Figs. 2, 3, 5 were obtained by itting the data using DistFit program [17]. Fig. 2. Particle size (Dp) distribution for the ns-laser ablation of nickel in ambient air at 0.4 W, 10 kHz, 200 mm/s and 12.73 J/cm2. Nickel. Particle generation data by ps- and nslaser ablation of nickel in ambient air are shown in Figs. 2 and 3. As seen, the processes involved in particle formation were partially diferent. Smallest particles of 8 nm size (nucleation mode) with the concentration up to 5·104 cm-3 were observed in case of the ps-laser. Particles generated in the nucleation mode with the size of 18 nm and with three times higher number concentration were observed during ns-laser ablation as well (see Table 1). However, in case of the ns-laser ablation, a wide size spectrum of particles from 10 to 180 nm with a sharp peak only in the nucleation mode was observed (Fig. 2). CPC data showed a growth of the particle number concentration in time during the laser ablation (Fig. 4(a)). Fig. 3. Particle size Dp distribution for the ps-laser ablation in ambient air: (a) nickel at 0.25 W, 100 kHz, 10 mm/s and 0.80 J/cm2; (b) aluminum at 0.6 W, 50 kHz, 10 mm/s and 3.82 J/cm2; (c) tungsten at 0.25 W, 100 kHz, 1 000 mm/s and 0.80 J/cm2. Sharp peaks of the nucleation, Aitken and accumulation modes at 8, 57 and 164 nm, respectively, were observed in the particle size 252 V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) Table 1. Maximum number concentration of generated particles during nanosecond (ns) and picosecond (ps) laser ablation in diferent media, cm-3. Aerosol particle modes in the atmosphere Laser Metals Accumulation (130– type Nucleation (1–30 nm) Aitken (30–130 nm) 200 nm) In ambient air (0.90 ± 0.17) × 105 (1.2 ± 0.2) × 105 Nickel (Ni) ns (peak at 61 nm) (peak at 18 nm) Nickel (Ni) ps (5.0 ± 0.2) × 104 (peak at 8 nm) (1.60 ± 0.15) × 104 (peak at 57 nm) Aluminum (Al) ps (1.6 ± 0.5) × 103 (peak at 20 nm) (0.7 ± 0.3) × 103 (peak at 43 nm) Tungsten (W) ps Nickel (Ni) ns Aluminum (Al) ns (1.13 ± 0.03) × 106 (peak at 21 nm) Stainless steel (302) ns (1.86 ± 0.06) × 106 (peak at 17 nm) Nickel (Ni) ps (1.8 ± 0.3) × 105 (peak at 8 nm) Aluminum (Al) Tungsten (W) (1.8 ± 0.2) × 104 (peak at 31 nm) In argon gas (2.0 ± 0.6) \ × 105 (1.40 ± 0.03) × 106 (peak at 107 nm) (peak at 20 nm) (6.20 ± 0.15) × 104 (peak at 164 nm) (1.1 ± 0.2) × 104 (peak at 186 nm) (3.4 ± 0.3) × 105 (peak at 99 nm) (5.8 ± 0.15) × 105 (peak at 182 nm) (5.9 ± 0.3) × 105 (peak at 46 nm) (2.9 ± 0.2) × 105 (peak at 166 nm) ps (1.7 ± 0.3) × 105 (peak at 79 nm) (2.5 ± 0.7) × 105 (peak at 170 nm) ps (3.9 ± 0.4) × 105 (peak at 36 nm) (2.5 ± 0.9) × 105 (peak at 131 nm) distribution during the ps-laser ablation (Fig. 3(a)). Particle number concentrations of these modes were (5.0±0.2)·104, (1.6±0.15)·104 and (6.2±0.15)·104 cm-3, respectively. he CPC data show an intensive particle generation process in the irst minutes and a slow declining tendency in time using a diferent laser beam scanning speed (Fig. 4(b)). he highest nickel particle concentration (up to 1.2·105 cm-3) was generated in ambient air using the ns-laser (Table 1). Generation of particles was also performed in the argon gas low. It was observed that generated nickel particles using both types of lasers had a narrow size distribution peaked at 20 nm with the high number concentration (1.4·106 cm-3) and concentration lower by order of magnitude at 107 nm (2.0·105 cm-3) using the ns-laser and at 8 nm (1.8·105 cm-3), at 46 nm (5.9·105 cm-3) and at 166 nm (2.9·105 cm-3) using the ps-laser ablation (Table 1). Generation of nickel particles by ps-laser ablation, as can be seen in Figs. 5(a,d), was weaker compared to the ns-laser ablation and caused a shit of the size distribution peak from 20 to 46 nm and to a coarse particle range (166 nm). Aluminum. Generation of aluminum particles by the ps-laser ablation in ambient air was ineficient. As seen in Fig. 3(b), generated particles were of nucleation and Aitken modes. he highest number concentration of generated particles was in the nucleation mode at 20 nm (1.6·103 cm-3) (Table 1). CPC data showed a rapid sink trend of the particle number concentrations in time (Fig. 4(e)). he opposite situation was observed in case of the aluminum particle generation in argon gas using the ns-laser compared to the ps-laser. he hughest number concentration of particles was measured during the ns-laser ablation. he size spectrum of generated aluminum particles using the ns-laser was similar to that of nickel particles, i. e. two peaks were observed at 21 nm and V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) 253 Fig. 4. Dynamics of generated particle number concentration during laser ablation of various metals in ambient air: (a) nickel by ns-laser at 0.4 W, 20 kHz, 1.59 J/cm2 and (b) by ps-laser at 0.49 W, 100 kHz, 1.56 J/cm2; (c) tungsten by ns-laser at 0.85 W, 22 kHz, 3.07 J/cm2 and (d) by ps-laser at 0.25 W, 100 kHz, 1.80 J/cm2; (e) aluminum by ns-laser at 0.6 W, 50 kHz, 3.82 J/cm2. 99 nm (Fig. 5(b)). he maximum particle number concentration reached 1.13·106 cm-3 for the smallest size of 21 nm (Table 1). he measurement data showed that the maximum number concentration of generated alumi- num particles by the ps-laser was (1.7–2.5)·105 cm-3 and it was higher than in case of the ns-laser, i. e. the particle size ranged widely from 50 to 200 nm with two maximums: at 79 and 170 nm (Fig. 5(e), Table 1). 254 V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) Fig. 5. Particle size dp distribution for the ns-laser ablation of various metals in argon gas: (a) nickel at 2.7 W, 30 kHz, 10 mm/s, 7.16 J/cm2; (b) aluminum at 1.36 W, 20 kHz, 100 mm/s, 5.41 J/cm2; (c) stainless steel at 1.75 W, 5 kHz, 10 mm/s, 27.85 J/cm2 and the ps-laser ablation in argon gas: (d) nickel at 1.5 W, 50 kHz, 100 mm/s, 9.55 J/cm2; (e) aluminum at 0.6 W, 50 kHz, 100 mm/s, 3.82 J/cm2; (f) tungsten at 1.2 W, 100 kHz, 100 mm/s, 3.82 J/cm2. Tungsten. Generation of tungsten particles by laser ablation was investigated using only the pslaser in ambient air and argon gas medium. he size distribution of generated particles was completely diferent in both cases and evidently showed that the particle size distribution strongly depended on the experimental conditions. he highest number concentration (3.9·105 cm-3) was found during the ps-laser ablation in argon gas and the particle size distribution in this case was observed in a wide V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) range from 18 to 200 nm (Table 1). In both media, particles of a narrow size mode were not formed, but a wide size spectrum was observed with a shit to the smaller particle size in ambient air, while in argon gas it was shited to coarse particle size (Figs. 3(c) and 5(f)). Measurement in argon gas showed two number concentration peaks at 36 nm and 131 nm. CPC data of tungsten particle formation process are shown in Figs. 4(c,d) during the ns- and the ps-laser ablation. As seen, the dynamics of the generated particle number concentration shows that during ablation in ambient air and argon gas the production of particles was diferent. In case of tungsten CPC data showed a slight growth of the particle number concentration for the ns- laser ablation when a small declining tendency in the irst six minutes for the ps-laser ablation and a further slow growth of the particle number concentration were observed (Fig. 4(d)). Stainless steel. he experimental data on generation of particles in stainless steel by the nanosecond laser ablation in argon gas are shown in Fig. 5(c). It should be noted that the particle number concentration reached 1.86·106 cm-3 (the distinct peak at 17 nm), but the size spectrum of generated particles was ranging to 200 nm. he second small peak at 190 nm in the size distribution of generated particles was observed. 5. Discussion In this research, as in many cases of such type of investigations [3, 5, 10, 11, 18–20], it has been shown that concentration of generated particles and their size distribution depend on the experimental medium, target material properties and laser process parameters. At irst, the formation of nanoparticles their size were afected by parameters of laser irradiation – the intensity, pulse length and radiation wavelength. Our experimental data showed that the smallest (~8 nm) particles of nucleation mode were formed by the picosecond laser ablation (Figs. 3 and 5). Larger (17–21 nm) particles of nucleation mode were generated during the nanosecond laser ablation. Diferent physical mechanisms of laser beammatter interaction for ns- and ps-lasers have an effect on the diameter of generated particles. he primary thermal nature of nanosecond laser ablation can lead to formation of a molten layer and ejection 255 of melted particles at evaporation, in addition to particles formed from condensation [1]. he metal surface impacted by picosecond laser radiation can be attributed to electrons and mass emitted from the target surface, while particle generation ater several 10 ns pulses proceeds due to emission of particles and droplets ater a thermal boiling process. Vaporized material, in contact with metal, is ionized and over the target surface a formation of plasma cloud from electrons, ions and particles of metals starts [5, 11, 19, 21]. Our registered smallest nickel particles of nucleation mode (~8 nm) by the ps-laser ablation can be produced during this process (Figs. 3 and 5). Experimental results have shown that the generated particles (nickel, aluminum) during the pslaser ablation are characterized by the maximum in the particle size range of 160–170 nm, while during the ns-laser ablation particles of size range of 90–110 nm were produced (Table 1). he efects of various laser process parameters on the nickel particle size and number concentration during the nanosecond laser ablation in argon gas at four diferent laser intensities were observed, and two almost identical narrow peaks of size distribution of particles for all energies were obtained (Fig. 6(a)). Two narrow peaks of nickel particles in the range of 14–18 nm and 100– 150 nm at pulse energies from 90 to 207 µJ were observed. It should be noted that by changing the laser beam scanning speed (10 or 100 mm/s) the diference in the particle concentration and size distribution was very small. he same results were observed in experiments with aluminum (Fig. 6(b)). Two narrow peaks of particles in the range of 20–24 nm and 80 nm at laser pulse energies from 47 to 89 µJ with stable particle number concentration by changing laser beam scanning speed (100 mm/s or 1000 mm/s) were obtained. he production of the maximum number concentration of metal particles by laser ablation in argon was reached: for nickel at 93 µJ, stainless steel at 350 µJ and aluminum at 89 µJ. As seen in Fig. 6(c), the largest efect on stainless steel particle generation at various (150–385 µJ) nslaser pulse energies was observed at 350 µJ pulse energy and the beam scanning speed of 10 mm/s. he variations of the nickel particle number concentration only of the smallest size particles (10–12 nm) during the ps-laser ablation in argon 256 V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) Fig. 6. Particle size dp distribution for ns-laser ablation of metals in argon gas at diferent beam intensities: (a) nickel at 10–30 kHz, 10–100 mm/s (16.47, 11.70, 7.40 and 7.16 J/cm2 respectively to the laser pulse energy); (b) aluminum at 10–20 kHz, 100–500 mm/s (7.08, 5.41 and 3.74 J/cm2 respectively to the laser pulse energy) and (c) stainless steel at 1–20 kHz, 10–100 mm/s (27.85, 30.64, 13.13 and 11.94 J/cm2 respectively to the laser pulse energy). gas by changing the laser beam scanning speed (100 or 1000 mm/s) were observed. In case of stainless steel during the ns-laser ablation by changing the beam scanning speed from 10 to 100 mm/s, the particle number concentration varied about two times in the range of 14–18 nm. he optimal regime for particle generation (nickel and stainless steel) was observed at following laser parameters: the pulse frequency 10 kHz and the beam scanning speed 100 mm/s. As seen in Figs. 2, 3 and 5, the size distribution and number concentration of generated metal particles during laser ablation in ambient air difered from those in argon gas. First of all, we consider that formation of particle size and their number concentration was inluenced by the complex of precursors (oxygen, water vapor, suspended matter <5 nm, etc.) present in ambient air, which was passing through the ablation chamber. Precursors themselves are actively forming various chemical compounds with generated metal particles (NiO, Al2O3, FeO) during the ablation. Studies of particle formation by Gonzalez et al. [1] show that the particle size strongly depends on the precursor concentration (atoms and clusters), which will affect the size of the primary particles. he oxidation process of nickel particles during laser ablation in ambient air is shown in works of Liu et al. [5]. For example, in Fig. 2 it can be seen that the nickel particle size spectrum is wide and the generation of particles by the ns-laser in narrow ranges was not observed. he standard deviation of the generated particle number concentration at 61 nm was σg = 3.21 and only a sharp peak of smallest nickel particles at 18 nm (σg = 1.15) was observed. Investigation of nickel particle formation by the ns- and ps-laser ablation in ambient air showed that the inluence of precursors on the particle size and concentration was diferent as well. he size distribution of the generated nickel particles showed that during the ps-laser ablation in argon gas the smallest particles in the nucleation mode (dp = 8 nm) and coarser particles in Aitken (dp = 46 nm) and accumulation (dp = 166 nm) modes were produced (Fig. 5(d)). he standard deviations of particle concentration for these sizes were σg = 1.35, σg = 1.13 and σg = 1.56, respectively. In addition to the particle size in the range from 10 to 100 nm for nickel during ps-laser ablation in ambient air the inluence of precursors was observed, except for particles less than 10 nm with σg = 1.35 (Fig. 3(a)). Comparison of the aluminum particle size with the size of nickel and tungsten particles under the same ablation conditions (ambient air, 257 V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) ps-laser) showed a wide variety range of all particles (Fig. 3). Aluminum particles in a wide range with peaks at 20 nm (the standard deviation σg = 1.31) and with a negligible peak at 43 nm (σg = 2.32) were formed (Fig. 3(b)). Based on aluminum properties, it is possible to explain that the ablated particles can be originated from Al2O3. A constant ilm of aluminum oxide is formed on the surface of this metal in contact with air [10]. Besides, the inluence of precursors during the picosecond laser ablation in ambient air was observed in case of generated tungsten particles. As seen in Fig. 3(c) Aitken mode particles dominated (dp = 31 nm, σg = 2.46) in a wide size spectrum. he accumulation mode was clearly separated in the particle size distribution (dp = 186 nm, σg = 1.13). In general, the particle generation of all investigated metals in ambient air was very small. Experimental data of particle formation by the ns- and ps-laser ablation in argon gas have shown that the particle size distribution and concentration depended on the processed metal characteristics, on the laser process parameters as well as on the thermodynamic conditions in the processed zone. For example, during the ns-laser ablation in argon gas smaller particles were produced (17–21 nm) than during the ps-laser ablation, except for nickel particles (dp = 8 nm, σg = 1.35) (Table 1, Fig. 5(a, d). Furthermore, the dependence of the nanoparticle size distribution on material properties of various metals during the ns- or ps-laser ablation was observed. he laser pulse energy inluencing the size distribution of nanoparticles generated during laser ablation of various materials depends on their relectivity [10]. he smallest particles are generated at higher pulse energies, but a size shit decreases for materials with lower relectivity. Our measurement data showed that during the ns-laser ablation of stainless steel, nickel and aluminum the shit to a smaller size range was observed at higher pulse energy of metals with lower relectivity. We observed the smallest generated particles (dp = 17 nm) of stainless steel (34% relectivity) at 350 μJ pulse energy, whereas aluminum (relectivity – 91%) generated particles were of 21 nm magnitude at 89 µJ pulse energy (Fig. 6, Table 2). he explanation for separate number concentrations of nanoparticles and their size spectrum of investigated metals is more complicated due to the diferent physical properties of each material. As can be seen in Fig. 5, a major variety of discrepancy of modes of produced particles among metals is observed during the picosecond laser ablation. Our results for tungsten are in agreement with [22] where an increase in the formation of larger particles of elements with a high melting point was determined. As seen in Table 2, the melting point of tungsten is highest among investigated metals and a wide size spectrum of produced tungsten particles with a shit to the coarse particle size compared to other metals was observed (Fig. 5). According to the investigations by Gonzalez et al. [1], Brikas [10] and Hola et al. [22], the inluence of material properties on the size and concentration of particles generated by the nanosecond laser ablation is important. he higher number concentration of stainless steel and nickel particles produced by the ns-laser (Table 1) could be relatted to the lowest thermal conductivity of these metals compared with other investigated metals (Table 2). Generally, the generation of stainless steel, nickel and aluminum particles by the ns-laser ablation in argon gas was the most intensive. 6. Conclusions he peculiarities of nanoparticle formation during the ns- and ps-lasers ablation of nickel, aluminum, stainless steel and tungsten in ambient air and argon gas were investigated experimentally. Table 2. Physical properties of investigated metals. Metals hermal conductivity K, W / (cm K) Density ρ, g/cm3 Melting point TM, K Boiling point, TB, K Relectivity coeicient R Aluminum (Al) Tungsten (W) Stainless steel (302) Nickel (Ni) 2.37 1.74 0.16 0.91 2.7 19.3 7.8–8 8.9 933 3673–3683 1644–1672 1 726 2467 5930 3000 3005 0.91 0.49 0.34 0.70 258 V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) It was found that the size distribution and number concentration of generated metal particles during the laser ablation in ambient air difered from those in argon gas medium. he number concentrations of generated nanoparticles during the laser ablation in argon gas, compared to the produced nanoparticle concentrations in ambient air, were up to 100 times higher and characterized by stable separate size peaks (nucleation, Aitken and accumulation modes) of nanoparticles, while a wide size spectrum (from 8 to 200 nm) of generated nanoparticles without clear separation of the size modes in ambient air was observed. he natural precursors in ambient air can have an efect on the on size spectrum of particles composed of various chemical compounds with target material during ablation process. Primary particles of 8 nm size during the pslaser ablation and of 17–21 nm in nucleation mode by the ns-laser ablation were produced. It was evidently caused by diferent physical processes in the laser beam material interaction zone. he inluence of laser process parameters on the number concentration and a shit in dynamics of size distribution of nanoparticles generated during the ns- and ps-laser ablation in argon gas were observed. he maximum number concentration of generated metal particles by laser ablation in argon was reached:: for nickel at 93 µJ, stainless steel at 350 µJ and aluminum at 89 µJ. Moreover, the dependence of the investigated metal properties (relectivity, melting point and thermal conductivity) on the size distribution and number concentration of generated nanoparticles was determined. he study showed that according to particular laser process parameters during the nanosecond and picosecond laser ablation in argon gas the generation of stable separate modes of nickel and aluminum particle size distribution can be controlled. Further investigations will focus on the development of technology of generation of monodisperse metal nanoparticles (the standard deviation σg = 1.1) based on primary and secondary particle formation physical processes. References [1] J.J. Gonzalez, C. Liu, S.-B. Wen, X. Mao, and R.E. Russo, Metal particles produced by laser ablation for ICP–MS measurements, Talanta 73(3), 567–576 (2007). [2] S.-B. Wen, X. Mao, R. Greif, and R.E. Russo, Experimental and theoretical studies of particle generation ater laser ablation of copper with a background gas at atmospheric pressure, J. Appl. Phys. 101, 123105 (2007). [3] S. Barcikowski, A. Hahn, A.V. Kabashin, and B.N. Chichkov, Properties of nanoparticles generated during femtosecond laser machining in air and water, Appl. Phys. A 87, 47–55 (2007). [4] D. Breitling, A. Ruf, and F. Dausinger, Fundamental aspects in machining of metals with short and ultrashort laser pulses, Proc. SPIE 5339, 49 (2004). [5] B. Liu, Z. Hu, Y. Che, Y. Chen, and X. Pan, Nanoparticle generation in ultrafast pulsed laser ablation of nickel, Appl. Phys. Lett. 90, 044103 (2007). [6] M.-H. Tsai, S.-Y. Chen, and P. Shen, Laser ablation condensation of TiO2 particles: Efects of laser energy, oxygen low rate and phase transformation, J. Aerosol Sci. 36, 13–25 (2005). [7] D.-W. Lee and M.-D. Cheng, Particle generation by laser ablation during surface decontamination, J. Aerosol Sci. 35, 1527–1540 (2004). [8] S. Barcikowski, A. Hahn, A.V. Kabashin, and B.N. Chichkov, Nanoparticles as potential risk during femtosecond laser ablation, J. Laser Appl., 19(2), 65–73 (2007). [9] N. Morris, he ine art of micromachining, ElectroOptics.com (2006), http://electrooptics. com/features/aprmay06/aprmay06micromachines.html. [10] M. Brikas, Micromachining of silicon and metals with high repetition rate picosecond laser, PhD thesis, Vilnius University and Institute of Physics, Center for Physical Sciences and Technology, Vilnius (2011) [in Lithuanian]. [11] T.V. Kononenko, V.I. Konov, S.V. Garnov, R. Danielius, A. Piskarskas, G. Tamoshauskas, and F. Dausinger, Comparative state of the ablation of materials by femto second and pico- or nanosecond laser pulses, Quantum Electron. 29(8), 724– 728 (1999). [12] N.B. Dahotre and S.P. Harimkar, in: Laser Fabrication and Machining of Materials (Springer Science, 2008) pp. 34–65. [13] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, and A. Tünnermann, Femtosecond, picosecond and nanosecond laser ablation of solids, Appl. Phys. A 63(2), 109–115 (1996). [14] K. Plauškaitė, V. Ulevičius, N. Špirkauskaitė, S. Byčenkienė, T. Zieliński, T. Petelski, and A. Ponczkowska, Observations of new particle formation events in the south-eastern Baltic Sea, Oceanologia, 52(1), 53–75 (2010). V. Dudoitis et al. / Lithuanian J. Phys. 51, 248–259 (2011) [15] G. Mordas, M. Kulmala, T. Petäjä, P.P. Aalto, V. Matulevicius, V. Grigoraitis, V. Ulevicius, V. Grauslys, A. Ukkonen, and K. Hämeri, Design and performance characteristics of a condensation particle counter UF-02proto, Boreal Env. Res. 10, 543–552 (2005). [16] W. Haaf and R. Jaenicke, Results of improved size distribution measurements in the Aitken range of atmospheric aerosols, J. Aerosol Sci. 11(3), 321–330 (1980). [17] www.distit.com/Front/DistFit/DistFit_Brochure. pdf. [18] E.G. Gamaly, N.R. Madsen, M. Duering, A.V. Rode, V.Z. Kolev, and B. Luther-Davies, Ablation of metals with picosecond laser pulses: Evidence of longlived nonequilibrium conditions at the surface, Phys. Rev. B 71, 174405 (2005). 259 [19] J. Konig, S. Nolte, and A. Tünnermann, Plasma evolution during metal ablation with ultrashot laser pulses, Opt. Express 13(26), 10597 (2005). [20] A. Hahn, S. Barcikowski, and B.N. Chichkov, Inluences on nanoparticle production during pulsed laser ablation, J. Laser Micro Nanoeng. 3(2), 73–77 (2008). [21] M.S. Tillack, D.W. Blair, and S.S. Harilal, he efect of ionization on cluster formation in laser ablation plumes, Nanotechnol. 15, 390–403 (2004). [22] M. Holá, V. Konečná, P. Mikuška, J. Kaiser, and V. Kanický, Inluence of physical properties and chemical composition of sample on formation of aerosol particles generated by nanosecond laser ablation at 213 nm, Spectrochim. Acta, Part B 65 (1), 51–60 (2010). METALŲ NANODALELIŲ GENERAVIMAs LAZERINE ABLIACIJA V. Dudoitis, V. Ulevičius, G. Račiukaitis, N. Špirkauskaitė, K. Plauškaitė Valstybinis mokslinių tyrimų institutas Fizinių ir technologijos mokslų centras, Vilnius, Lietuva santrauka Lazerinė abliacija, kaip efektyvus ir spartus įvairių medžiagų smulkių struktūrų formavimo būdas, pastaruoju metu vis sėkmingiau įsitvirtina nanodarinių gamyboje. Metalai yra vieni tinkamiausių medžiagų mikrosistemoms gaminti. Norint formuoti tikslias mikrosistemas turi būti atliekami generuojamų nanodalelių dydžių spektro bei koncentracijos eksperimentiniai tyrimai. Darbe nagrinėti nikelio, aliuminio, nerūdijančio plieno ir volframo nanodalelių generavimo procesai, taikant nanosekundinės ir pikosekundinės trukmės impulsų lazerius. Nustatyta, kad generuojamų visų tirtų metalų nanodalelių dydžių spektras ir skaitinė koncentracija, vykstant lazerinei abliacijai oro ar argono dujų aplinkoje, ženkliai skiriasi. Vykstant lazerinei abliacijai ore generuojamos nanodalelių (1–12).104 cm-3 koncentracijos bei 8–200 nm dydžių platus spektras su nukleacinės ir akumuliacinės modų nedidelėmis smailėmis. Manome, kad atskirų dydžių generuojamų nanodalelių spektrui įtakos turėjo oro sudėties komponentės, kurios abliacijos proceso metu dalyvavo susidarant cheminiams junginiams (pvz., FeO, NiO, Al2O3). Lazerinės abliacijos ore atveju argono dujų aplinkoje generuojamos metalų nanodalelių (0,1–1,9).106 cm-3 koncentracijos su gerai besiformuojančiu nukleacinės, Aitkeno ir akumuliacinės modų spektru. Stebėta lazerinės abliacijos parametrų ir metalų savybių įtaka generuojamų nanodalelių skaitinei koncentracijai bei dalelių dydžių pasiskirstymo spektre poslinkiui. Mažiausios nanodalelės (~8 nm) generuotos abliuojant nikelį pikosekundinės trukmės impulso lazeriu argono dujų aplinkoje. Manome, kad registruotos pirminės nikelio dalelės galėjo būti generuojamos susidarius plazmai lazerinės spinduliuotės ir metalo sąveikos zonoje. Nerūdijančio plieno, nikelio ir aliuminio nanodalelių (10–200 nm) generavimo procesas intensyviausiai vyko abliuojant metalus nanosekundinės trukmės impulso lazeriu argono dujose.