Structural,optical, and nonlinear optical properties of indium nanoparticles prepared by laser ablation

A study of indium nanoparticles prepared by two laser ablation techniques is reported. The suspensions of indium nanoparticles were prepared using the laser ablation of bulk indium in liquids. The prepared suspensions of indium nanoparticles were analyzed by the X-ray fluorescence spectroscopy and absorption spectroscopy. The position of the surface plasmon resonance of In-containing suspensions (350 nm) was consistent with the estimations taking into account the average size of In nanoparticles (43 nm) measured using the X-ray fluorescence spectroscopy. The nonlinear optical parameters of indium nanoparticles-containing liquids were studied by the z-scan technique using a picosecond Nd:YAG laser. We compare the laser ablation in liquids with the laser ablation of indium in vacuum at the tight and weak focusing conditions of a Ti:sapphire laser and analyze the 60 nm indium nanoparticles synthesized in the latter case. PACS 42.65.An; 42.65.Hw; 42.65.Jx; 61.46.Df; 78.67.Bf     

Pulsed laser deposition and characterization of NiTi-based MEMS prototypes

We report the fabrication and characterization of microelectromechanical systems (MEMS) prototypes based on a microcantilever coated with a NiTi thin film. The NiTi film was deposited by laser ablation and deposition in ultra-high vacuum. The samples were characterized from the mechanical point of view by measuring the resonance frequency of the cantilever as a function of the temperature. Data show a clear phase transition of the NiTi film, with relevant transition temperatures close to those of the bulk, which demonstrates the congruency of the deposition method. We have also developed a technique to control the deflection of the NiTi-coated cantilever by all-optical means, both continuous and pulsed, the latter with typical time scales in the hundreds of s range. PACS 81.15.Fg; 85.85.+j; 81.30.Kf     

Thin films of silver nanoparticles deposited in vacuum by pulsed laser ablation using a YAG:Nd laser

We report the deposition of thin films of silver (Ag) nanoparticles by pulsed laser ablation in vacuum using the third line (355 nm) of a YAG:Nd laser. The nanostructure and/or morphology of the films was investigated as a function of the number of ablation pulses, by means of transmission electron microscopy and atomic force microscopy. Our results show that films deposited with a small number of ablation pulses (500 or less), are not continuous, but formed of isolated nearly spherical Ag nanoparticles with diameters in the range from 1 nm to 8 nm. The effect of increasing the number of pulses by one order of magnitude (5000) is to increase the mean diameter of the globular nanoparticles and also the Ag areal density. Further increase of the number of pulses, up to 10,000, produces the formation of larger and anisotropic nanoparticles, and for 15,000 pulses, quasi-percolated Ag films are obtained. The presence of Ag nanoparticles in the films was also evidenced from the appearance of a strong optical absorption band associated with surface plasmon resonance. This band was widened and its peak shifted from 425 nm to 700 nm as the number of laser pulses was increased from 500 to 15,000.     

Pulsed laser deposition and in situ diagnostics of the process applied to shape-memory alloys

NiTi films deposited by pulsed laser ablation on Si/SiO₂ are shown to exhibit structural and functional properties related to the shape-memory effect. Film characterization suggests that relevant temperatures for the solid-to-solid transformation responsible for the shape memory are in substantial agreement with those of the bulk target material, demonstrating a good congruency of the deposition process. Besides the technological interest for this class of thin films, our findings point out the suitability of laser ablation for metal alloy deposition. An investigation based on in situ ion-mass spectroscopy and covariance mapping analysis allows us to determine the main vapor-phase processes leading to the formation of stoichiometric clusters expected to play a relevant role in assisting the growth of NiTi thin films. Received: 6 August 2001 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-50/2214-333, E-mail: fuso@df.unipi.it     

Simple synthetic route for hydroxyapatite colloidal nanoparticles via a Nd:YAG laser ablation in liquid medium

Pulsed laser ablation (PLA) in liquid medium was successfully employed to synthesize hydroxyapatite (HAp) colloidal nanoparticles. The crystalline phase, particle morphology, size distribution and microstructure of the HAp nanoparticles were investigated in detail. The obtained HAp nanoparticles had spherical shape with sizes ranging from 5 to 20 nm. The laser ablation and the nanoparticle forming process were studied in terms of the explosive ejection mechanism by investigating the change of the surface morphology on target. The stoichiometry and bonding properties were studied by using XPS, FT-IR and Raman spectroscopy. A molar ratio of Ca/P of the prepared HAp nanoparticles was more stoichiometric than the value reported in the case of ablation in vacuum.     

Laser-produced plasma ion characteristics in laser ablation of lithium manganate

Laser ablation is widely used to assist in the fabrication of prototype lithium manganate (LiMn₂O₄) thin film structures for Li-ion battery electrodes via the pulsed laser deposition technique. However, films can be considerably Li and/or O deficient, depending the deposition conditions used. Here we present data on the ionic component of laser-produced plasma in laser ablation of lithium manganate with ns excimer laser. Plasma was monitored using an electrical Langmuir ion probe, in time-of-flight mode in conjunction with mass spectrometry to identify the dominant ionic species. Ablation in vacuum at ∼2.5 J cm⁻² revealed the plasma's ionic component was composed primarily of singly charged Li and Mn ions. The time-of-flight data indicates significant deceleration of the plasma when ablation is carried out in an oxygen background gas pressure of the order of 10 Pa. The implications for thin film growth are considered in terms of the possible gas phase interactions and/or thin film re-sputtering yield.     

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.     

Nanoparticle preparation of quinacridone and β-carotene using near-infrared laser ablation of their crystals

Quinacridone nanoparticles with a mean size of about 200 nm are successfully prepared using nanosecond near-infrared (NIR) laser ablation of its microcrystalline powders in heavy water. The absorption spectra of the formed colloidal solutions depend on the excitation wavelengths, which is eventually ascribed to number and energy of absorbed photons. β-carotene has low photostability and is easily decomposed upon UV/VIS laser ablation of its solid, while its nanoparticles are prepared utilizing this NIR laser ablation technique. The advantage of nanoparticle preparation by NIR laser ablation is discussed.     

Micromachining NiTi tubes for use in medical devices by using a femtosecond laser

Recent growth in medical device technology has been substantially driven by developments in laser micromachining, which is a powerful fabrication technique in which nickel–titanium (Nitinol, NiTi) alloy materials that exhibit superelastic and shape memory properties are formed (e.g., self-expanding stents). In this study a NiTi tube curve surface process is proposed, involving a femtosecond laser process and a galvano-mirror scanner. The diameter of the NiTi tube was 5.116 mm, its thickness was 0.234 mm, and its length was 100 mm. The results indicated that during the machine process the ablation mechanism of the NiTi tubes was changed by altering the machining path. The path alteration enhanced the laser ablation rate from 12.3 to 26.7 μm/J. Thus the path alteration contributed to a wide kerf line, enabling the assisted air to efficiently remove the debris deposited at the bottom of the kerf during the laser ablation process. The results indicated that the NiTi tube curve process enhanced the laser ablation rate by two times and reduced the amount of energy accumulated within the materials by 50% or more. By altering the machining path using the scanning system, this process can decrease the production of heat affected zones (the accumulation of thermal energy) in medical device applications.     

Structure and size control of ZnO nanoparticles by applying high pressure to ambient liquid in liquid-phase laser ablation

We synthesized ZnO nanoparticles by laser ablation of a Zn target in water at pressures up to 30 MPa. We observed the enhancement of the crystallinity of synthesized ZnO nanoparticles when high pressure was applied to ambient water. In addition, we found that ZnO nanoparticles with smaller sizes were synthesized by pressurizing ambient water. Considering our previous understanding on the effect of high pressure applied to ambient liquid, the controls of the structure and the size of nanoparticles were considered to be obtained via the controls of the dynamics of laser ablation plasma and ablation-induced cavitation bubble.     

Nanoparticles size modifications during femtosecond laser ablation of nickel in vacuum

Nanoparticles were synthesized by irradiating a nickel target with femtosecond laser pulses in high vacuum, and subsequently analyzed. The proof-of-principle experiments aim to modify the size characteristics of the produced nanoparticles. For nickel it is found that: (i) ultraviolet laser pulses lead to a remarkable change in the nanoparticles size distribution with respect to visible laser pulses; (ii) irradiation of the femtosecond pulses induced ablation plume with a second, delayed ultraviolet laser pulse can change the size characteristics of the produced nanoparticles.     

Elaboration and characterization of bimetallic nanoparticles obtained by laser ablation of Ni75Pd25 and Au75Ag25 targets in water

A YAG laser operating at the second harmonic wavelength (532 nm, 10 Hz, 8 ns and 40 mJ) was used to elaborate bimetallic nanoparticles by laser ablation of Ni₇₅Pd₂₅ and Au₇₅Ag₂₅ targets in water. TEM–EDX, UV–Vis spectroscopy and PIXE measurements were performed to obtain information on their mean sizes, size distributions and chemical composition as a function of the time of laser ablation. The surface of the laser impacted regions of the targets were characterized by RBS in order to check their composition after the laser ablation. The so-obtained bimetallic nanoparticles always show a homogeneous composition. However, while the composition of Au–Ag nanoparticles was found to be very similar to the one of the alloy target, the composition of the Ni–Pd nanoparticles can be different from the nominal composition of the alloy target. Segregation phenomena can be invoked to explain the difference between the Ni–Pd nanoparticles and the Au–Ag nanoparticles compositions obtained in the same conditions. However, an influence of chemical reactions occurring in the high pressure plasma created locally at liquid–solid interface (called ‘reactive quenching’) cannot be completely ruled out.     

Drilling enhancement by nanosecond-nanosecond collinear dual-pulse laser ablation of titanium in vacuum

Laser ablation of titanium in vacuum was performed using single- and dual-pulse regime in order to study crater formation. Crater profiles were analyzed by optical microscopy. It was found that the repetition-rate plays an important role in a process of laser ablation. The drilling is most effective for the highest repetition-rate. For the same total number of laser pulses clear drilling enhancement was achieved by dual-pulse regime of ablation in comparison to single-pulse regime. The strongest ablation rate in dual-pulse regime was achieved for the delay time between the pulses τ = 370 ns. Results are discussed in terms of decreased ablation threshold due to continuous heating of the target during the experiment.     

Emission properties of laser ablation of SnO2: Sb transparent conducting film and KTiOPO4 crystal

Optical emission spectra of Nd:YAG laser ablation of KTiOPO₄ (KTP) crystal and SnO₂:Sb transparent conducting thin film were recorded and analyzed in vacuum and in air. The integral intensities of spectral lines from laser-ablated KTP crystal were obtained as functions of distance from the target surface and laser power density in vacuum and in air. The ambient gas effects on pulsed laser ablation of target were discussed. We also performed laser ablation of SnO₂:Sb transparent conducting thin film in air and the electron temperature and full-width at half-maximum (FWHM) of atomic and ionic spectral lines in the plasma were quantified using Boltzmann plot method and Lorentzian fit, respectively. Integral intensities of atomic and ionic Sn spectral lines were also obtained as functions of distance from the target surface and laser irradiance. The intensity ratio of ionic and atomic Sn spectral lines as a function of laser power density was got which gives some information about the variation of ionization ratio with laser irradiance in the plasma produced by high-power laser.     

Size,composition and optical properties of copper nanoparticles prepared by laser ablation in liquids

Colloidal copper nanoparticles were prepared by pulsed Nd:YAG laser ablation in water and acetone. Size and optical properties of the nanoparticles were characterized by transmission electron microscopy and UV–visible spectrophotometry, respectively. The copper particles were rather spherical and their mean diameter in water was 30 nm, whereas in acetone much smaller particles were produced with an average diameter of 3 nm. Optical extinction immediately after the ablation showed surface plasmon resonance peaks at 626 and 575 nm for the colloidal copper in water and acetone, respectively. Time evaluation showed a blue shift of the optical extinction maximum, which is related to the change of the particle size distribution. Copper nanoparticles in acetone are yellowish and stable even after 10 months. In water, the color of the blue-green solution was changed to brown-black and the nanoparticles precipitated completely after two weeks, which is assigned to oxidation of copper nanoparticles into copper oxide (II) as was confirmed by the electron diffraction pattern and optical absorption measurements. We conclude that the ablation of bulk copper in water and acetone is a physical and flexible method for synthesis of stable colloidal copper and oxidized copper nanoparticles. PACS 42.62.-b; 81.07.-b; 61.46.+w     

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     

Effect of laser-ablated copper nanoparticles on polymerization of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane

We have studied the effect of copper nanoparticles produced by laser ablation on the ring-opening polymerization of 1,1,3,3-tetraphenyl-1,3-disilacyclobutane (TPDC). The particle size and distribution of laser-ablated, copper nanoparticles were controlled by laser-ablation conditions such as ambient pressure, laser fluence, and distance between target and substrate. Laser-ablated, copper nanoparticles induced polymerization of TPDC effectively, resulting in formation of poly(diphenylsilylenemethylene) (PDPhSM) thin films. The polymerization efficiency depended mainly on the particle size and the surface concentration of copper nanoparticles deposited on the TPDC films by laser ablation. No clear tendency was observed between oxidation of Cu nanoparticles and polymerization efficiency. This technique enables us to easily fabricate PDPhSM thin films and may become a new method for fabricating polymer–metal nanocomposites. PACS 81.05.Lg; 82.35.Np; 82.50.Hp     

Stability, size and optical properties of silver nanoparticles prepared by laser ablation in different carrier media

We studied the effects of the surrounding liquid environment on the size and optical properties of silver nanoparticles prepared by laser ablation by a pulsed Nd:YAG laser operated at 1064 nm. The silver targets used were kept in acetone, water and ethanol. TEM observations and optical extinction were employed for characterization of particle size, shape and optical properties, respectively. Nano silver in acetone showed a narrow size distribution with a mean size of 5 nm and the colloidal solution was stable. In deionised water a rather narrow size distribution with a mean size of 13 nm was observed and nanoparticles were precipitated slowly after about two weeks. In ethanol, a broadening in size distribution and optical extinction spectra was observed. Silver nanoparticles in ethanol with a mean size of 22 nm were completely precipitated after 48 h. In acetone, deionised water and ethanol, the wavelengths of maximum optical extinction are 399, 405 and 411 nm respectively, which is attributed to increasing the size of the nanoparticles. Growth, aggregation and precipitation mechanisms were related to the dipole moment of the surrounding molecules in order to clarify the difference in size, optical properties and stability of the nanoparticles. PACS 79.20.Ds; 81.07.-b; 61.46.+w     

Investigation of plumes produced by material ablation with two time-delayed femtosecond laser pulses

We experimentally investigated and herewith reported the results of laser ablation of copper and gold with two time-delayed femtosecond laser pulses at 800 nm in vacuum. The ablation plume dynamic was monitored by fast plume imaging and time- and space-resolved optical emission spectroscopy. Optical microscopy was used to follow the ablation depth as a function of the delay between the two laser pulses. Nanoparticles deposition on mica substrates was analysed by atomic force microscopy.We estimate roughly the plume's atomization degree - that is the mass fraction of atomized material over the total ablated mass - from the relative intensities of radiation emitted from atoms and nanoparticles. It is shown that the atomization degree depends critically on the time delay between both laser pulses and on the characteristic time of electron-lattice relaxation. The increase of the atomization degree is accompanied by the decrease of the ablation depth. Atomic force microscopy measurements confirm the partial atomization of nanoparticles, as the analyses of particle deposition on mica substrates show a large decrease of the number of nanoparticles for large delay between the two pulses.     

Neutrals' temperature in laser-generated plasma at LNS

A Nd–YAG laser operating at 532 nm with a maximum intensity of 10¹⁰ W/cm² was used to ablate aluminium and tantalum targets placed in vacuum.

A mass quadrupole spectrometer (MQS) at high sensitivity, operating in the range of 1–300 amu, with a resolution better than 1 amu, was used to analyse the atomic emission produced by the laser ablation. The neutrals' emission produced by laser-generated plasma at INFN-LNS was investigated in terms of temperature, ablation yield, angular distribution and velocity.

The neutrals' detection through the MQS permitted to measure the mass energy distribution. Results demonstrate that the maximum temperatures of the neutral species are of the order of 100 eV. The angular emission of neutrals is peaked along the normal to the target surface, as it was observed for the ions; the ablation yield increases suddenly at low laser intensity and decreases at high laser intensity, owing to the higher ionization processes; the flow velocity follows the adiabatic expansion of the plasma in vacuum and it is of the order of 10⁴ m/s.

Measurements will be presented and discussed, according to the available models.