New reference materials for organic depth profiling

With the commercial availability of new tools for organic depth profiling (e.g. Ar1000 gas cluster ion beams) it is now possible to analyse organic multilayer systems while retaining the molecular information. To get valid information on the chemical in-depth distribution of organic compounds appropriate organic reference materials with sharp interfaces and interlayers at known depths are necessary for the development, optimization and validation of analytical methods. Self Assembled Monolayers (SAM) and Metal Organic Frameworks (MOF), due to their controlled layer formation, are expected to meet these requirements. R&D and quality control in recent applications of organic layer stacks in molecular electronics, organic photovoltaics and biosensing devices relies on valid methods of organic depth profiling. It is the aim of this project to characterise the surface and interface of layered organic samples by surface analysis techniques such as Time of Flight Secondary Ion Mass Spectrommetry (ToF-SIMS), X-ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS). Further the suitability of SAMs and MOFs as potential new organic reference materials for the establishment of valid methods of organic depth profiling is explored.

The aim of this research project was to contribute to the development of organic reference materials for classical surface chemical analysis, such as time-of-flight secondary ion mass spectrometry (ToF- SIMS), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) as a method of analytical electron microscopy (SEM). Driven by the increasing scientific and industrial interest on thin organic (multi)-layer structures used e.g. in organic electronics or organic sensor devices has led to the development of nano-structured devices in a bottom-up materials-by-design approach. Thereby, the surface and interface of such devices play a critical role: on the one hand they determine many properties and on the other hand affect their performance. Therefore, there is a need for reliable, valid and accurate analysis methodologies to obtain in-depth knowledge of the chemical composition of such (multi)-layers and their interfaces. To further ensure the highest quality of measurement, reference materials (RMs) are essential. Unfortunately, the number of certified reference materials (CRMs) is rather limited and available CRMs are mostly based on pure inorganic substances provided as thin layer on suitable substrates. Within this project it was possible to develop ionic liquids (organic-inorganic salts, that are molten at room temperature) as a multi-elemental RM for (i) the calibration and regular performance check for energy dispersive spectrometers (EDS) following ISO 15632:2012, specifically addressing light elements in the delicate range below 1 keV and (ii) the determination of the transmission function for X-ray photoelectron spectrometers following ISO 15470:2004 to considerably improve the quantification accuracy for photo peaks measured in the binding energy range of 150 to 750 eV, specifically addressing R&D of (bio)organic surfaces. The use of ionic liquids as RM impresses most notably by their simple handling and their time and cost efficiency. A more fundamental aspect of the research resulted from the aim to design a thin organic multilayer RM based on self-assembly processes. In this course it was possible to develop an analytical methodology for the process and quality control for the bottom-up approach of nano-scaled devices using layer-by-layer self-assembly processes. We were able to follow and visualise the formation mechanism of self-assembled monolayers and define minimum reaction times for the formation of a complete monolayer leading to high quality thin organic films. The research activities within this project has an impact on the huge communities covered by ISO/TC 201 (microbeam analysis) and ISO/TC 202 (surface chemical analysis) such as e.g. nanotechnology (global market by 2010: ~$700 billion), semiconductor, steel industry, implant materials, ceramics and industries dealing with thin organic films, such as e.g. organic electronics in general, (medical) sensor devices and coatings. The benefits expected from ISO/TC 201 & 202 are in principal innovations in industry by providing key analytical standards as well as trade enhancements by providing reliable standards for quality management.