REPOSITORY > RESULTS

Abstract

Laser ablation (LA) is a versatile technique that can be used for the elemental analysis of diverse materials as well as for synthesis purposes, i.e. for pulsed laser deposition (PLD) of a wide range of materials. Analysis and synthesis will only be successful if the ablated material can be stoichiometrically transferred to a detector or a substrate, respectively. Although LA and PLD are conceptually simple techniques, the ablation and deposition processes involved are very complex and not yet completely understood.
The main goal of this thesis is to implement a fundamental study of ablation and deposition processes, i.e. to understand the ejection of the target material in the plume and eventual deposition of the ablated species onto the substrate using two different PLD setups: a commercial laser (213 nm solid state Nd:YAG laser) and a 157 nm molecular F2 laser. A Fe-Sm-Ta alloy (Sm13.8Fe82.2Ta4.0) was used as a target material to study PLD; this material was obtained through induction melting of Fe (13.8 at. %), Sm (82.2 at. %) and Ta (4.0 at. %) via a number of phases (SmFe2, SmFe3, Sm2Fe17, and TaFe2), each with clearly defined boundaries.
In this study, a 213 nm Nd-YAG commercial laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS) intended for microanalysis work was used for the first time for PLD under atmospheric pressure and in and ex situ ICPMS analysis for diagnosing the nanoparticle fabrication process. An actual PLD instrument, comprising a 157 nm molecular F2 laser and a micro-controlled X-Y-Z-Θ translation stage in a 316 stainless-steel chamber was used parallel to this work for fabricating nanoparticles from the same targets. The products obtained at 213 nm were compared with the ones obtained at 157 nm using a suite of characterization techniques (scanning electron microscopy-energy dispersive X-ray spectroscopy [SEM-EDXS], a superconducting quantum interference device [SQUID], X-ray diffractometry [XRD], transmission electron microscopy [TEM] and atomic force microscopy [AFM]).
The results obtained indicate that the PLD diagnostics process indeed led to the fabrication of Sm13.8Fe82.2Ta4.0 particles using the commercial LA-ICPMS instrument. In a helium atmosphere, under suitable PLD conditions, more than 95 % [w/w] of the particles generated had a diameter smaller than 0.75 µm (independently checked via multi-stage cascade impaction of particles leaving the laser ablation chamber) and an (average) stoichiometry indistinguishable from that of the target. Furthermore, element maps of the deposited particles showed that optimal target-substrate geometry (target and substrate positioned at an angle of 45° with respect to the incident beam direction and parallel to each other) can be achieved in a short time (< 1 day) for the appropriate layer density and spatial stoichiometry on a Si/Ta substrate. Detailed SEM-EDXS analysis revealed a deposition pattern which upon close inspection (zooming in) revealed random agglomerates of single spherical particles. In addition to very fine “dust”, differently-sized spherical particles (10−50 nm) could also be seen. EDXS analysis proved that oxidation of the Fe-Sm-Ta particles was low and that the average composition of the particles (N = 10) reflected that of the target (Fe, 82.6 at. %; Sm, 12.6 at. %; Ta, 4.8 at. %). The Fe/Sm ratio measured in larger particles was, in general, somewhat higher than in the target. This is in accordance with cascade impactor findings for the fractions between 0.41 and 2.8 µm or higher and may be explained by ejection from the melted liquid due to plasma-initiated splashing leading to a melted residue that is rich in lower volatility Fe. The SQUID measurements of the magnetic properties showed that the deposited material, after annealing and thermal treatment in nitrogen, was ferromagnetic.
Experiments at 157 nm were used to unravel the mechanisms related to formation of magnetic nanoparticles from the Sm13.8Fe82.2Ta4.0 target. The deposition parameters such as laser fluence and distance between the target and the substrate were optimized to ensure that the particles had the desired crystal structure, composition and ferromagnetic properties. The samples were thoroughly characterized by morphological, structural and magnetic measurements. More reliable information concerning the structural properties of the particles was obtained through high-resolution transmission electron microscopy. TEM images show bi-phase spherical nanodroplets (50−100 nm) consisting of a 5−10 nm crystalline nucleus surrounded by an external amorphous phase. These nanodroplets exhibit a ferromagnetic response with coercivity of 2.5 kOe, which could be further increased to 5.0 kOe through annealing and thermal treatment in nitrogen. The surrounding amorphous shell prevents the post-ablation oxidization of the crystalline magnetic nucleus of the nanodroplet due to oxidization being confined to the surface.
It appears that the commercial 213 nm Nd:YAG LA-ICPMS intended for microanalysis work can be used for PLD under atmospheric pressure and in and ex situ ICPMS analysis for diagnostics of the PLD process. In this way, the size distribution and elemental composition of particles in the plume may be probed in addition to fabricating the elemental density, homogeneity and stoichiometry of the deposited material.
On the other hand, a 157 nm PLD can contribute positively toward improving the properties of magnetic core-shell Sm-Fe-Ta-N nanodroplets. The results presented in this work form a good basis for further studies such as optimization of the PLD process (under atmospheric conditions) for all kinds of novel materials.