Improvements in the characterisation of complex metal oxide thin films using online-laser ablation of solids in liquid (online-LASIL)

Abstract

The number of applications for complex metal oxides (CMO) is steadily increasing at the beginning of the 21st century. Regardless, whether these materials are used for energy conversion in solar cells, solid oxide fuel cells, as chemical sensors or in touch screens of electronic devices, their high flexibility and outstanding properties make them to so called "functional materials". Without such materials some of the applications would not be possible. An ongoing research progress of these functional materials extends and creates new fields of applications. Properties and functionalities of the mentioned CMOs are closely linked to the composition of the materials. Tailoring the properties of such advanced materials is often related to the determination of their precise elemental composition (stoichiometry), the monitoring of present contaminants or the control of added dopant concentrations and their distribution within the samples. Therefore, powerful analytical tools, which keep up with the developments in material research, are necessary. For material analysis, several analytical techniques exist, which all come with certain advantages and disadvantages. One of the common lab based techniques is inductively coupled plasma-mass spectrometry (ICP-MS) due to its high sensitivity for most elements and large linear range. For the analysis of solid samples either a direct solid sampling approach or a solution based approach is performed. The latter requires the transformation of the solid sample into a liquid. Both methods have certain drawbacks: (i) The digestion of chemically inert solid material can be quite challenging and additional structural information possibly present in the sample is lost during the digestion process. (ii) Signal quantification, in the other case, is usually hard to achieve for a direct solid sampling approach if no suitable solid reference material is available. In this work, a novel analytical method, called online-laser ablation of solids in liquid (LASIL), circumventing these drawbacks of conventional ICP-MS measurements is presented. A new sampling approach in which the solid target is submerged in a liquid during the laser ablation process is adopted and further developed. The batch-wise approach proposed in literature is improved to an online hyphenation set-up, where the ablation cell is directly coupled to the detection device. This set-up strongly resembles the conventional laser ablation with the major difference that the sample is submerged into a liquid in the LASIL process. The liquid particle transportation medium provides the possibility to use liquid reference solutions for signal quantification. This is a big advantage, because solid samples do not need to be elaborately digested any more, and though commercially available liquid reference material can be used for signal quantification. The beneficial combination of the advantages in online-LASIL are shown in this thesis. A user-friendly and improved online-ablation cell is designed and manufactured. The capability for an easy quantification is demonstrated with the analysis of homogeneous complex metal oxide thin films. The stoichiometry of these thin films in the range of 220 to 14 nm thickness was determined with an outstanding sensitivity and precision even for the lowest sample thickness which was not possible with an energy dispersive X-ray measurement in conventional secondary electron microscopy (SEM-EDX). The possibility to obtain laterally resolved measurements is an important feature of any analytical technique, because nowadays many samples show structures in 2D or 3D. Therefore, a main goal is to develop and demonstrate these imaging capability for online-LASIL as well. In the course of this project, some aspects of the online-LASIL set-up are improved, especially concerning the particle transportation process and the data acquisition. The adjustment of the cell geometry and transport tubing results in an improved washout behaviour, which is a prerequisite for imaging applications. Additionally, the operating conditions of the ICP-MS (e.g., dwell times) are refined for an improved precision of the measurements. Such an increased precision is important for an accurate assessment of the stoichiometry of thin films. For signal quantification, an adapted standard addition concept is developed, which allows a very time-effective calibration process without any manual or automated changes between different types of solutions and thus, signal changes due to instrumental drifts can be easily compensated. As a result, laterally resolved measurements with a spot size of 20 μm were recorded and the reconstructed images give a correct representation of the geometrical structures and chemical composition of the thin films. These are the first images ever obtained with online-LASIL enabling the spatially resolved assessment of inhomogeneities in sample composition. Minor constituents and trace elements can significantly change the materials properties, especially those of CMOs. Sulfur (S) is involved in a technological relevant degradation process of solid oxide fuel cells and therefore the measurement of traces of S is of analytical interest. However, ICP-MS analysis of S is challenging due to severe polyatomic interferences in ICP-MS instrumentation equipped with a quadrupole mass filter. Thus, a measurement mode to overcome the isobaric interferences by converting S+ to SO+ is optimised. A new design of the online-LASIL cell enables the adjustment of the carrier solution to stabilize the analytes and to reduce unwanted memory effects. These measures allow the detection of traces of S in Gd doped ceria thin films. The measurements demonstrate that online-LASIL is suitable for the detection of non-metals of low masses. Due to the small liquid volumes present in the ablation process, the analyte concentration is possibly higher in suspensions obtained in the LASIL process compared to solutions obtained in a conventional digestion approach. This work highlights the potential, the development and the already achieved improvementsof online-LASIL. Application examples are designed and prepared to demonstrate the capabilities of this novel technique for a direct analysis of solid samples to obtain bulk measurements but also spatially resolved investigations. Stoichiometry determination and quantification of contaminants is realised without solid reference materials, because online-LASIL provides an extraordinary combination of advantages for elemental analysis. It can be seen as a valuable contribution to the existing variety of techniques.