The historical backdrop for the role of microscopy in the development of human knowledge is reviewed. Atomic-scale investigations are a logical step in a natural progression of increasingly more powerful microscopies. A brief outline of the concept of atomic-scale analytical tomography (ASAT) is given, and its implications for science and technology are anticipated. The intersection of ASAT with advanced computational materials engineering is explored. The chapter concludes with a look toward a future where ASAT will become common.

We discuss how ASAT has the potential to make important advances on critical frontiers in crystallography. These key frontiers include unequivocal quantification of the nearest-neighbour relationships in materials, compositional information, and details of the degree of both short-range order and long-range order. Interfaces represent a particular opportunity. We discuss the present challenges in experimental microscopy-based methods to incorporate both the structural crystallographic information at crystal interfaces with the local chemical compositional information. We anticipate that ASAT will drive forward the field of interface science and interface engineering.

We conclude our contribution with a prospective and optimistic look to the art of what might be possible with the advent of ASAT. We see a convergence between the digital or computational world and the experimental, and envisage ASAT as an enabler for the design and development of new materials. This potential arises because real-world 3D atomic-scale information will bring direct insights into thermodynamic, kinetic, and engineering properties. Moreover, when coupled with machine learning and other computational techniques, it may be envisaged that discovery-based procedures could follow that adjust the observed real-world atomistic configurations toward configurations that exhibit the desired engineering properties. This will fundamentally change the role of microscopy from a tool that provides inference to a materials behaviour to one that provides a quantitative assessment. This opens the pathway to atomic-scale materials informatics.

A complete, albeit brief review of the history of atoms and atomic-scale microscopy is offered. From the concept of the atom developed by Greek philosophers to the ultimate microscopy, the path of development is examined. Atomic-Scale Analytical Tomography (ASAT) is cited as the ultimate microscopy in the sense that the objects, atoms, are the smallest building blocks of nature. The concept of atoms developed as the scientific method grew in application and sophistication beginning in the Middle Ages. The first images of atoms were finally obtained in the mid-twentieth century. Early field ion microscopy evolved eventually into three-dimensional atom probe tomography. The crucial role of the electron microscope in atomic-scale microscopy is examined. Recently, combining atom probe tomography and electron microscopy has emerged as a path toward ASAT. The chapter concludes with the point that ASAT can be expected in the next decade.

Based on the discussion in Chapters 4 and 5, combining information from both electron microscopy, presumably (Scanning) Transmission Electron Microscopy ((S)TEM), and Atom Probe Tomography (APT) is a likely path toward ASAT. Experimentally, concurrent (S)TEM and APT may appear to be a straightforward experiment, but the instrumentation required can be complex and require significant capital investment. In this chapter, we consider what instrumentation is necessary for each technique and what could be done to both simplify and improve the ASAT technique in a combined instrument that solves many of the complexities in experimentation. Experimental conditions such as vacuum pressure, cryogenic temperatures, electron imaging and diffraction, laser wavelength and positioning, and specimen holder designs must all be taken into account.

A burgeoning number of research studies are emerging where scientific questions are being successfully addressed because of the combination of information revealed from atom probe microscopy and density functional theory (DFT). Situations where high-quality experimental data alone would not wholly answer the question at hand and, equally, situations where atomistic simulations would have no obvious starting place were it not for the atom probe. Atomic-scale analytical tomography holds great potential to expand the realm of mediation between experimentation and computer simulation of materials properties. Any model framework is applicable, but we have delved into detail for the case of DFT because it is a self-consistent theory that has arguably the most immediate and exciting intersection with ASAT data.

We proposed in Chapter 5 that a combination of STEM and APT is the most likely method through which ASAT could be achieved, using either APT-centric atomic positioning or STEM-centric atom positioning. The approaches laid out are expected to achieve ASAT at some level, but we cannot expect absolute perfection since all experimental techniques have limitations. Limitations may be due either to the underlying physics (physical) or due to the technology available (technical). It is important to consider these limitations to understand where improvements might be made if limited primarily by the technology (technical). This chapter explores the most significant of these potential limitations using both APT- and STEM-centric atom positioning methods. Changes to experimental best practices as well as forward-looking advances in hardware and software are needed in order to achieve ASAT.

This chapter looks, not at the big picture, but at the details of an operational ASAT instrument. Can specimens withstand repeated STEM/APT cycling? Is an integrated STEM/APT instrument needed or can they be coupled by a vacuum transport? Will specimen evolution models suffice to deliver a realistic model of the specimen shape throughout an ASAT experiment? In an integrated instrument, can APT and STEM be operated simultaneously? Concerns about radiation damage in ASAT experiments and means for mitigating these effects are explored. The role of electron diffraction in ASAT is considered, and it is seen as an important adjunct to atom probe crystallography. The importance of complementary analytical information such as EDS and especially EELS is illustrated. Since atom probe tomography is a compositional mapping tool, EELS as a chemical mapping tool takes on added import. The interplay among the many elements of ASAT and its intrinsic correlative microscopy opportunities serve as an internal check on results. A synergistic ecosystem of AST information with chemical information correlated with physical properties and image simulations defines the opportunity inherent in Atomic-Scale Analytical Tomography.

This chapter begins with a formal definition of Atomic-Scale Analytical Tomography (ASAT) and the origins of the concept. The progression of experimental atomic-scale microscopies that led to ASAT concepts is reviewed, and the people and projects are highlighted. Once ASAT is established as a concept, its implications for structure-properties microscopy, coupled through Integrated Computational Materials Engineering (ICME), become obvious. A forward-looking roadmap for ASAT considers what length scales and atom counts in ASAT images are needed to address important microstructural questions. The chapter concludes with the notion that microscopy is at an inflection point: having reached the ultimate building blocks, the drive to see smaller and smaller must now evolve to a drive to see more and more of a structure.

Based on our preceding discussions of atomic-resolution characterization techniques in Chapter 4, no technique has yet achieved ASAT. Combining information from FIM or (S)TEM along with APT has demonstrated some very promising results, and each combination seems to be a likely path toward ASAT. In this chapter, we propose how ASAT might be achieved using correlative and/or combined techniques such as (S)TEM + APT. Such a combination would allow several routes for determination of the ion transfer function, or how imaging occurs during an APT experiment. If we can determine the transfer function with high-enough fidelity, we make the argument that it should be possible to achieve ASAT using a combination of (S)TEM and APT with inputs from simulations.