Atomic Resolution Transmission Electron Microscopy: On the way toward the ultimate spatial resolution and accuracy
The capability to see where the atoms are in the matter, distinguishing from atom to atom, has been an old dream since the time of Democritus. This dream has contributed along the centuries to milestone stories in the Science Fiction. Today we are close to make this dream true by atomic resolution methods in electron microscopy. Nevertheless, despite the recent strong advances, still significant efforts are necessary to achieve the ultimate resolution and accuracy, overcoming the intrinsic limitations related to the physics of the electron-matter interaction and to the electron optics. In this lecture I shall discuss, with the help of many experimental examples, the features of atomic resolution imaging methods in a Transmission Electron Microscope (TEM) and Scanning TEM (STEM). The intention is to make evident how the ultimate result is a puzzle that, in order to be solved, needs equipment capable to achieve the spatial resolution down to the diffraction limit, but this is definitely not enough. Definitely it needs knowledge and relevant methods capable to maximize and quantify the information that can be extracted from the experimental data. In particular, electron coherent diffraction imaging in a TEM has demonstrated promising results that will be discussed in this lecture. The strong electron-matter interaction makes TEM/STEM approaches very effective in achieving information from nano-volumes of the matter but at the same time it complicates the quantitative interpretation of results, posing also the issue of the damage of the specimen structure. This is of particular relevance for materials containing weakly-bound low atomic number atoms, like polymers or proteins, with a paramount relevance for basic and applicative points of view. It is widely recognized that, in many cases, the spatial resolution in a TEM experiment on radiation sensitive specimens is not related only to the electron-optic limitations of a TEM but mainly on the damage threshold of the specimen. Hence, TEM methods capable to handle the radiation damage in a specimen are of paramount importance as testified by the Nobel prize in chemistry in 2017 to Henderson, Dubochet and Frank for the development of cryo-microscopy in TEM to image the structure of biomolecules at spatial resolution otherwise not achievable. The development of TEM/STEM methodologies capable to tackle these issues will be discussed underlining how the concurrence of theoretical, experimental and computational efforts are definitively necessary to awake and make the dream true.