Carrying out electron holography (EH) in gaseous and liquid environment offers a whole new range of opportunities to study in-situ physical and chemical reactions of materials, liquid-solid interfaces, plasma effects, and beam effects in general. Inelastic interactions between beam electrons and the molecules of the gas or liquid produce ions, radicals, molecular fragments, free electrons and protons, and many other reactive species that promote local chemical/physical transformations of the sample, whose effects can be directly observed with the phase-sensitive eye of EH.

To establish gas-EH in an environmental TEM (ETEM), inspire by the work of Duchamp et al.[1] we have designed and fabricated an exchangeable electron biprism, which is resistant to corrosive gasses and placed it within the ETEM column. We have assessed its performance with various gasses, H2, N2, O2, by measuring the change in overlap width, visibility, electron count and phase sensitivity as a function of pressure. The analysis of the data shed light into fundamental aspects of gas-electron interactions, especially regarding coherence coherence loss. Our findings reveal that gas-EH is a viable setup that opens the door to a whole new class of experiments, within fields such as magnetic nanoparticles, catalysis and doped semiconductors. We have then used gas-EH to follow the progressive oxidation of FeO nanoparticles from magnetite to maghemite to hematite, and the results of these experiments will be presented at the conference.