Mimicking the Real World and Exploring New Ones With In-Situ Electron Microscopy
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The influence of the electron beam is probably significant in many of these in-situ experiments and negligible or well understood in only a few of them. Radiation damage observations, particularly of the knock-on damage produced by the electron beam itself in high momentum transfer, displacement collisions with atomic nuclei, have long been recognised as a topic of special importance. Though still of considerable and wide interest [14,15], they attracted peak attention over a decade ago because of their relevance to the damage process in nuclear reactors. The development of visible knock-on damage, usually appearing as vacancy or interstitial dislocation loops, tetrahedra or voids, can be followed on a greatly accelerated time scale compared with the same processes in the reactor thanks to the much higher dose rate intensities prevailing in the electron microscope. Quantitative translation of the results from the EM to the reactor situation is far from straightforward however, not simply because of the differences between neutron and electron collisions, but also because the visible damage is only a late stage in a complex chain of processes involving diffusion, aggregation or annihilation of point defects [16]. Most of the vacancies or interstitials, which are created in pairs in the initial collision, rapidly recombine and only a very small fraction survive to agglomerate in visible damage. Even before any damage has become visible, it is possible for every atom in the sample to have suffered a considerable number of displacement events. This picture was confirmed by measurements of the electron beam induced conductivity (EBIC) signal near a p-n junction in silicon during irradiation with high energy electrons [17]. Long before any point defect clusters are visible, the EBIC signal is drastically reduced by the presence of isolated vacancies and interstitials which act as recombination centres. The elevated point defect concentrations characteristic of high voltage EM observation also have profound effects on dislocation motion which have been long recognised in high voltage in-situ deformation work. The processes of ionisation damage are more complex than knock-on damage and are as yet not completely understood. Nevertheless it is increasingly clear that in many insulators at least a parallel situation probably applies. Although there may be not always be signs of visible damage, the concentration of electron-hole pairs may be sufficient to have a strong influence on various other processes of interest for in-situ observations. We conclude by reviewing some of the evidence on this topic. DEVELOPMENTS IN INSTRUMENTATION Transmission Imaging and Spectroscopy The steady improvement over the past two decades in transmission instrumentation has led to atomic resolution microscopy and more recently, thanks to PEELS and associated imaging and detection equipment, to spatially resolved spectroscopy at the same level. Even without specialist modifications, these achievements have generally been accompanied with improved
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