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Sources, Interaction with Matter, Detection and Analysis of Low Energy Electrons 2

Final Report Summary - SIMDALEE2 (Sources, Interaction with Matter, Detection and Analysis ofLow Energy Electrons 2)

Electron microscopy (EM) has played and still is playing a paramount role in making the invisible visible. For technological developments in industry as well as in fundamental research, EM-techniques presently are indispensable tools, e.g. for characterization of semiconductor devices, steels, hybrid or composite materials, nanomaterials, etc. The availability of low-cost, tabletop design electron microscopes operating with low energy electrons constitutes an important factor in progress in technological developments, fundamental research and improvement of high-definition commercial products.
Within the SIMDALEE2 ITN network a European multi-site network of 10 Early Stage Researchers (ESRs) and 4 Experienced Researchers (ERs) was established at 13 host institutions. The ITN-fellows were provided with a well structured and extensive training programme which not only involved the scientific research for their theses supervised at their host institutions, but also encompassed enhanced technical knowledge as well as multidisciplinary and business skills that will enable them to contribute to strengthening the role of the EU in areas of science and technology where low energy electron (LEE) technology constitutes an indispensable tool.
SCIENTIFIC OBJECTIVES of the ITN comprised i.a.:
• (O1) Put the theory of field-emission on a modern basis taking into account the 3-dimensional shape of the emitter and relate it to the lateral resolution of microscopy using field emission sources.
• (O2) Certified model for the interaction of low energy electrons with solid surfaces.
• (O3) Optimisation of lensless imaging and electron detection in a strong field ambiance.
• (O4) Establish a common theoretical basis for the interpretation of various contrast mechanisms observed with low energy
• (O5) Development of a prototype Scanning Field Emission Electron Microscope (SFEM).
• (O6) Investigate the Economic Impact of Low Energy Electron Beam Techniques.
Each fellow adopted the maximum level of training for career development. All ESRs were enrolled in the doctorate programme of their host institutions. Network training included summer schools, dedicated courses offered by experts during network meetings on a variety of subjects spanning from vacuum technology to marketing, secondments to other network hosts, participation in international conferences, participation in the organisation as well as the scientific programme of the SIMDALEE2 conference in 2017 Pula, Sardinia. Transferable skills were also offered to fellows on the local level, at their respective hosts. The large majority of fellows has written at least one scientific publication in a peer-reviewed journal as first author. At the time of writing, a few ESRs are in the course of finishing the writing up of their thesis, preparing for their defense in 2019. All fellows are expected to successfully submit/defend their doctoral thesis in the course of 2019/2020. The ERs have well integrated in their respective work environment. Some of the ESRs as well as the ERs have already received and accepted offers for further employment (e.g. a postdoctoral position) at institutions associated with the network.
• A 3D-theory of field emission taking into account the shape of the emitter has been developed and compared with experimental results and provides an explanation for the observed lateral resolution. Ab initio DFT calculations for carbon nanotube field emission sources provide the 3D potential distribution, exchange-correlation energy, work function and individual molecular orbitals
• A model for the signal generation process in the SFEM has been developed, combining stochastic motion of electron scattering and emission in the solid with Newtonian motion in vacuum. This model may lead to a paradigm shift in electron spectroscopy since it predicts a 2-D electron cascade along the sample surface, in stark contrast to the 3-D pear-shaped electron cascade that can be found in any textbook on electron microscopy.
• Magnetic as well as chemical imaging in the SFEM has been successfully demonstrated on an optimised SFEM prototype that was built.
• An AFM-based SFEM was designed and the building blocks for prototype were tested. The assembly is in progress.
• Field emission from W5O14 nanowires was found to follow the Fowler-Nordheim law with huge field enhancement factors.
• All required components for quasi in-situ electron holography of field emission tips have been designed and constructed.
• Concerning the mechanism of secondary electron (SE) emission it was found that (1) SEs are generated in single electron scattering events in which the full energy and momentum lost by the incident electron is transferred to a bound electron of the target; (2) For the SE emission to be possible, energy and momentum transferred in the collision must be such to couple an initially occupied bound state of the target with an empty state above the vacuum level; and (3) The SE emission process is assisted by collective excitations, such as plasmons, and by coherent diffractions from the crystal lattice.
• Values for the low energy electron inelastic mean free paths for non-crystalline solids have been extracted from experimental data for the secondary electron yield
• The limits of the validity of a Boltzmann type description of electron transport in crystalline substances have been exposed. The concept of the inner potential to describe the k-perpendicular problem (as in photoemission) has shown to be questionable and needs to be revisited.
• Spin-polarised low energy electron (LEE) loss spectra of iron on silver (001) and on tungsten (110) were taken in order to clarify magnetic interactions of LEE with matter
• Retrieval of the projected charge distribution for a given specimen has been achieved using a model-based iterative reconstruction algorithm of electron holography measurements in the TEM,
• An optimised method for charge reduction has been developed, demonstrated and implemented on a commercial SEM instrument.
• Crystallographic contrast via local density of states measurement in the SEM was achieved.
• A low-voltage SEM / time-of-flight (LVSEM / ToF) system that allows angle and energy measurements of transmitted electrons has been designed, built and successfully tested
• A miniature Bessel-box electron energy analyser has been developed and tested and is being incorporated in a system in use within the network.
• Nine products for economic exploitation have been identified as documented in a report on economic impact. These comprise scientific software and microscopy accessories, as well as the complete SFEM setup.
SIMDALEE2-fellows presented their research at international conferences, workshops, seminars and meetings. The SIMDALEE2-fellows have also organised and participated in outreach activities such as open days etc. Approximately 30 articles have been written by the researchers which have been published or are about to be published in peer-reviewed journals. Editing of a special issue of ”Journal of Electron Spectroscopy and Related Phenomena” is in progress, featuring invited contributions of the SIMDALEE2-conference. Most importantly, this issue also contains gold open access publications authored by (the majority of) fellows of the network. In view of the future career of the researchers, it was decided that this is the best way to publicise the importance of the subject investigated by the network and is to be preferred over the publication of a SIMDALEE2-book, as originally planned.

CONTACT: Prof. Dr. Wolfgang WERNER - -