S. Petrović

Femtosecond laser-induced periodic surface structure on the Ti-based nanolayered thin films

Laser-induced periodic surface structures (LIPSSs) and chemical composition changes of Ti-based nanolayered thin films (Al/Ti, Ni/Ti) after femtosecond (fs) laser pulses action were studied. Irradiation is performed using linearly polarized Ti:Sapphire fs laser pulses of 40 fs pulse duration and 800 nm wavelength. The low spatial frequency LIPSS (LSFL), oriented perpendicular to the laser polarization with periods slightly lower than the irradiation wavelength, was typically formed at elevated laser fluences. On the contrary, high spatial frequency LIPSS (HSFL) with uniform period of 155 nm, parallel to the laser light polarization, appeared at low laser fluences, as well as in the wings of the Gaussian laser beam distribution for higher used fluence. LSFL formation was associated with the material ablation process and accompanied by the intense formation of nanoparticles, especially in the Ni/Ti system. The composition changes at the surface of both multilayer systems in the LSFL area indicated the intermixing between layers and the substrate. Concentration and distribution of all constitutive elements in the irradiated area with formed HSFLs were almost unchanged.

Laser-induced formation of intermetallics in multilayered Al/Ti nano-structures

In this study, multilayered structures consisting of eight (Al/Ti) bilayers deposited by d.c. ion sputtering onto Si (100) wafers, to a total thickness of ~300xa0nm were treated in air with a picosecond-pulsed Nd:YAG laser in a defocused regime. Irradiation was done with 200 successive pulses, at 1,064-nm wavelength, energy per pulse 15xa0mJ, the incidence angle of ~45°, covering an area of 2-mm in diameter. The samples were analyzed by scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. It was found that laser irradiation-induced melting and a progressed Al–Ti nanoalloying within the top four bilayers (up to a depth of ~150xa0nm), while the underlying part of the multilayered structure remained intact. Another interesting result is that of a regularly rippled surface topography (~1.4-μm period and 100-nm amplitude) developed throughout the zone of the melted and reacted material.

Angular distributions of ions channeled in the Si crystals

In this study we analyze the angular distributions of Ne 10þ ions channeled in the h 100 i Si crystals. The ion energy is 60 MeV and the crystal thickness is varied from 286 to 3435 nm. This thickness range corresponds to the reduced crystal thickness range from 0.5 to 6, i.e. from the second to the twelfth rainbow cycle. The angular distributions were obtained via the numerical solution of the ion equations of motion and the computer simulation method. The analysis shows that the angular distribution has a periodic behavior. We also analyze the transmission patterns corresponding to the angular distributions. These patterns should be compared to the experimental patterns obtainable by a two-dimensional position sensitive detector. We demonstrate that, when the ion beam divergence is sufficiently large, i.e. much larger than the critical angle for channeling, the channeling star effect occurs in the transmission patterns. 2002 Elsevier Science B.V. All rights reserved.

Laser irradiation of nano-metric Al/Ti multilayers

Multilayered 5×(Al/Ti) structures, deposited on a Si substrate to a total thickness of 130xa0nm, were treated by unfocused Nd:YAG laser pulses (150xa0ps) with energies of 85 and 65xa0mJ. Irradiations were performed in air with 10, 50 and 100 successive laser pulses. Characterizations were done by using Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The results obtained show that laser irradiation, at either energy, induced almost full intermixing of deposited layers and formation of intermetallic compounds, but this was more pronounced for the applied laser pulses with a higher energy. The intermixed layer–silicon interface remains intact for all numbers of applied laser pulses and for both energies. The formation of an oxide layer on the sample surfaces was also observed, the thickness of which is greater for the higher laser beam energy.

Ion Beam Modification of Al/Ti Multilayers

Aluminum/titanium multilayers were sputter deposited on (100) Si wafers and irradiated with 200 keV Ar+ and N ions. Irradiation fluences were in the range from 5 · 1015 to 4 · 1016 ions cm−2, for argon ions, and 1 · 1017 to 2 · 1017 ions cm−2 for nitrogen. The samples were analyzed by Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Nano-hardness measurements were performed by Vickers method. Irradiation with argon ions induces mixing between Ti and Al layers, which is most pronounced in the vicinity of the projected range of implanted species. TEM cross-section analysis shows significant growth of grain size with the increase of Ar ion fluence. Implantation with nitrogen ions, up to these high fluences, causes a full intermixing of Al/Ti layers, resulting in multilayered structures with different content of Al, Ti, and N. In the latter case, nitrogen ion irradiation, probably, induces the formation of nitrides of titanium, aluminum, and AlTi. Nano-hardness measurements show significant increase of hardness for all applied ion fluences.

Femtosecond laser surface patterning of steel and titanium alloy

In this work, we present the results obtained by femtosecond laser processing of AISI D2 steel and the widely applicable titanium-based alloy, Ti6Al4V. Patterning the materials surfaces was done by a Ti:sapphire system at 775 nm wavelength and 200 fs pulse duration, while varying the output pulse energies and the scanning speed. The formation of laser-induced periodical surface structures were found for both materials.

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm2 were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.

Modification of multilayered TiAlN/TiN coating by nanosecond and picosecond laser pulses

A multilayered TiAlN/TiN coating deposited on H11 work-steel was irradiated by a TEA CO2 laser (ns pulses) and a Nd:YAG laser (ps pulses), and the effects compared. The coating was 2.17 µm thick and consisted of 45 layers. The laser-induced modifications showed dependence on laser pulse duration, pulse count and laser wavelength. The conditions for coating ablation in both cases were determined. The experiment has revealed laser-induced periodic surface structures (LPSS) on nanometre and micrometre scales, depending on the laser wavelength used. Sample surfaces were characterized before and after laser irradiation by an optical microscope, scanning electron microscope (SEM), focused ion beam (FIB) microscope and profilometry.

Partial ablation of Ti/Al nano-layer thin film by single femtosecond laser pulse

The effects of ultra-short laser pulses on reactive Ti/Al nano-layered thin film were investigated. The thin film composed of alternated titanium and aluminium nano-layers, was deposited by ion-sputtering. Single pulse irradiation was conducted in the air with focused and linearly polarized femtosecond laser beam - of 1026 nm wavelength and pulse duration of 170 fs. Laser induced composition and morphological changes, using different microscopy techniques and energy dispersive X-ray spectroscopy, were investigated. Following results were obtained: (i) one step partial/selective ablation of upper Ti layer from nano-layer Ti/Al at low laser fluence and (ii) two step ablation or entire ablation of nano-layer Ti/Al at higher laser fluence. Single pulse selective ablation of the upper Ti layer was confirmed based on profiling (AFM) along the ablation steps and reduction of Ti concentration (EDX) in the ablated areas. Ablation threshold was estimated using well known procedure for ultra-short laser pulses - spot diameter square versus logarithm of pulse energy. To interpret the experimental observations, simulations have been performed to explore the thermal response of the multiple layered structure (Ti(5x(Al/Ti))) after irradiation with a characteristic value of single laser pulse of fluence F = 320 mJ/cm2. The results are in agreement with the calculations.

Agglomeration in core-shell structure of CuAg nanoparticles synthesized by the laser ablation of Cu target in aqueous solutions

Metallic copper Cu and bimetallic copper?silver CuAg nanoparticles (NPs) are generated by the ablation of copper bulk target in water and aqueous Ag colloidal solution, respectively. The experiments were performed using nanosecond Nd:YAG laser operating at 1064 nm. The generated NPs are characterized by UV?vis absorption spectroscopy, laser-induced breakdown spectroscopy, dynamic light scattering and scanning electron microscopy. The conducted investigations can be summarized as follows: (i) CuAg NPs colloidal solution possess the absorption in UV?vis spectral region, which can be attributed to the Cu-component; (ii) the primary bimetallic CuAg NPs have near uniform dimensions with diameter of about 15 nm, and as a rule, they are grouped into larger agglomerates without defined morphology; (iii) the obtained Cu NPs have mainly spherical form with average diameters up to 20 nm. Both types of NPs show a tendency towards the formation of large agglomerates with different morphology. Bimetallic NPs show the plasmon resonance in the vicinity of 640 nm with a good coincidence with formation of the colloidal solution of pure Cu NPs. The results also demonstrate that the core?shell structure (Ag-rich core/Cu-rich shell) is important for the formation of the bimetallic NPs, also agreeing very well with theory.