Scanning Microscopy


The electron microscope has brought over the last fifty years a wealth of information about the structure of solids, from surface topography to the details of atomic arrangements. Like any probe, however, the electron beam is subject to the epistomological constraint that the investigator inevitably perturbs what is being investigated, and from its beginnings questions were raised about the integrity of the ·images generated in the microscope from a specimen which was subjected to such an aggressive probe. A fast electron is about as likely to be scattered inelastically as elastically from a collection of atoms, and the density of energy transfer to these atoms, under conditions where their positions or identities can be established, approaches that in a modest nuclear explosion. It is, indeed, a tribute to the redundancy of atomic bonding in solids that atomic organization is largely maintained during investigation. It was early recognized that biological solids were substantially affected; only recently has it been realized that the integrity of atomic-scale information from inorganic solids as well can be seriously compromised by the investigating electron. This contribution reviews the interaction modes which are relevant to the deterioration of specimens in a fast electron beam, outlines the mechanisms by which these interactions lead to irreversible alterations in structure, and assesses the rates at which these alterations proceed in the several instrumental configurations of the electron microscope represented by the scanning electron microscope (SEM), conventional transmission electron microscope (CTEM), and scanning transmission electron microscope (STEM). Incidences of degradation are illustrated for investigation of several structural classes, and several palliative measures are described.

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