Dynamic in situ microscopy relating structure and function

Contemporaneous and complementary progress in addressing technical challenges with dynamic in situ structure and function characterization are being rapidly developed in the physical and life sciences. As they are advanced, the science requires and supports a new emphasis on identifying strategic questions in the present understanding of chemical and biological processes. They primarily take place at the atomic and molecular levels, with active structures forming under dynamic conditions. They require corresponding local analysis of diversity; now becoming possible under complex controlled dynamic reaction conditions. Articles in this theme issue bring together complementary strands of state-of-the-art atomic-scale understanding and review recent developments in new ways of probing it.

An underlying theme of this issue is the relation of atomic-scale structure with the functions of physical, chemical and biological activity. Most recently the work has been extended by considering the dynamic changes which take place in the structure and consequential function under controlled reaction conditions. Characterizing dynamic structures is key to understanding and developing the outputs of many practical chemical and biological processes. Previous misconceptions are corrected and new avenues of development are suggested. It is necessary to have more informed framing of questions as a route to breakthrough answers. The newly developed ability to gain unparalleled insight into materials function within broadly defined ‘devices' has led researchers to push the boundaries of in situ and operando methods. In the former, change conditions, such as chemical reactions, are experimented to understand atomic-scale reaction mechanisms, and replicated on larger samples ex situ for product data. In the latter, there can be direct in situ analysis of products; albeit often with some concomitant diminution in the flexibility, quality or sensitivity of analysis which can be employed. Crystal structures, their surfaces and the structures of nanoparticles can be predicted with modelling, as described in this issue (by [1]) with examples taken from metallic, inorganic and organic systems.

Sequencing the dynamic chemistry at the core of heterogeneous gas–solid catalysis is vital to inform and develop the scientific basis of key technologies at the heart of the industrial economy, for producing fuels, chemicals, food, pharmaceuticals and many other products. Of ever-increasing societal importance are environmental controls, waste management and pollution remediation measures on both input and output ‘sides' of reactions. Catalysis plays a vital role in improving reaction possibilities and to make reactions more efficient, economical, energy efficient and environmentally friendly. In catalytic reactions, nanoparticles, less structured atom cluster entities and increasingly single atoms, with low loadings, are understood to be active species in many catalysts containing precious metals on high surface area ceramic or carbon supports. The aim is to generate and keep active species highly dispersed to present the maximum percentage (for product quantity) of atoms at the outside reaction surface in nanostructures with low coordination (for product quality), and therefore integrated reactivity effectiveness. However, the more finely divided and hence reactive species are also often inherently less structurally stable, especially under active reaction conditions at high temperatures. Atomic-level sintering by Ostwald Ripening plays a critical role in deactivation and the consequent loss of catalyst performance. In the research presented in this issue, dynamic in situ atom-by-atom analyses of sintering dynamics and stability of supported precious metal nanoparticles in chemical reactions reveal the complex nature of sintering and deactivation at the single-atom level [2].

The use of conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) methods can be highly informative; however, when conducted ex situ regular analyses may be subject to change after reaction by removal of the temperature and gas reaction environment conditions. A second set of distortions may well occur during transfer through air, of the often by definition chemically highly active surfaces, for ex situ analysis under equally alien high vacuum conditions. The developing science now supports the realization and study of intermediate phases which may be local, transient, active and metastable with respect to operating reaction conditions. The goal is, therefore, to follow cycles to prepare, activate, react, deactivate, reactivate or recover precious materials—all under controlled conditions of a continuous gas atmosphere at operating temperature—combined with single-atom imaging and the highest quality analysis. Thus, a novel atomic resolution-environmental scanning transmission electron microscope (ESTEM), with a differentially pumped gas-in-microscope open aperture system, has been created [3] to access and analyse key processes in situ, with single-atom resolution under controlled dynamic reaction conditions. The aim is to provide a more accurate continuous characterization in situ during a dynamic reaction sequence and with it to supply a better-informed basis for the rational development of materials, process design and operation.

Currently, the complex requirements of combining UHV conditions and representative gas atmospheres are handled in differentially pumped open aperture systems (gas-in-microscope) or in cells (gas-in-holder) with electron semi-transparent windows. It is also necessary to moderate the deleterious, radiolytic and displacement impact of the analysing radiation: its energy, sample cross-sections, dose and dose rate; as pioneered in biology and polymer science with their higher sensitivities and now highly developed condition management. The whole field is being helped by, serendipitous as well as synergistic, breakthroughs in detection efficiency (especially recent camera developments), data collection rates, result processing algorithms and ever-increasing computing power and accessibility.

The use of TEM/STEM and associated techniques for the analysis of beam sensitive materials and complex multiphase systems in situ, or close to their native state, are reviewed by the LEMAS group at Leeds [4]. This requires the determination of a dose budget for radiolytic and displacement damage to achieve an optimum dose-limited resolution for a given choice of microscope operating parameters, imaging modes and detectors. The authors show how this understanding can be extended for the in situ analysis of samples interacting with liquids and gases. They also discuss how cryo-TEM of samples snap frozen in vitreous ice can play a significant role in benchmarking dynamic materials processes, as well as for the well-established biology applications. The paper neatly combines many key elements of the development of in situ methodology for analysis of systems inherently unstable, especially under the electron beam and/or high vacuum; for now limiting what can be achieved with dynamic studies. It is also a report of some of the inspiring progress which has already been made and helps to define ways developments can and must be pushed forward expeditiously. However, the descriptive methods are not an end in themselves. They must be directed and exploited to inform and test understanding of the science to deliver societal needs.

A new environmental high-voltage transmission electron microscope (E-HVEM) operating at 1000 kV has been developed by Nagoya University [5] in collaboration with JEOL Ltd for dynamic studies of thicker samples in catalytic reactions, as well as for mechanical deformation studies in higher pressure gaseous conditions; all with the flexibility of greater experimental space made possible with a 1000 kV system.

Conical metallic tapers represent an intriguing sub-class of metallic nanostructures [6], as their plasmonic properties show characteristics in strong correlation with their geometrical properties. Favourable plasmonic resonances can be tailored by optimizing structural parameters like surface roughness or face opening angles. Through TEM study of tapers, the underlying resonance mechanisms can be accessed, assessed and exploited.

The femtosecond photoexcitation of semiconducting materials [7] leads to the generation of coherent acoustic phonons (CAPs) linked to intrinsic and engineered electronic, optical and structural properties. The influence of nanoscale structure and morphology on CAP dynamics can be challenging to resolve with all-optical pump-probe spectroscopic techniques. Ultrafast electron microscopy (UEM) is reported to resolve variations in CAP dynamics caused by differences in the degree of crystallinity in as-prepared and annealed GaAs lamellae. Following in situ femtosecond photoexcitation the generation and propagation of the dynamics of hypersonic CAPs can be followed; extending the method to elucidate correlated electronic, optical and structural behaviours in semiconducting materials.

Biological physics may be defined as the study of biological systems from which physical insights stand independently of the original system under study [8]. Nanotechnology has provided a new scale of observation. It is at this scale that biological physics can observe emergent physical phenomena, as competing forms of energy (including mechanical, electrostatic, thermal) are roughly of the same magnitude and induce the emergence of self-organization. The development of high-speed atomic force microscopy (HS-AFM) is providing new capability to obtain nanometre-scale structural information and dynamics with sub-second resolution. HS-AFM provides also a tool to measure minute forces within microseconds, giving access to the exploration of intermediate states in protein unfolding or receptor/ligand interactions. This timescale also allows direct comparison with all-atom molecular dynamics simulations.

An under-exploited complementary tool over longer length scales in dynamic in situ studies is the scanning electron microscope (SEM). In this issue, the potential is exploited to quantify the diffusionless transformation of austenite to martensite in steels and to follow diffusional oxidation of copper through cyclic in situ heating and cooling, together with dynamic in situ electron backscatter analysis of crystallography [9].

Collectively the contributions highlight the importance of dynamic in situ microscopy in improved understanding of atomic-scale reaction mechanisms, the role of otherwise ‘hidden' metastable intermediate states and the relationships between the diversity of local structure and function in a wide range of dynamic physical, chemical and biological processes. The new dynamic in situ methods open up new areas of research in the development of materials and processes. The scope for improvement in the contribution each can make to well-being is large; with many processes much less efficient than they could be with better scientific understanding, and others yet to be invented, to meet ever-increasing societal needs and desires.

Editors' biographies

Dame Pratibha Gai, FREng, FRS, is Prof. and Chair of Electron Microscopy in the Departments of Chemistry and Physics and founding Co-Director of the York JEOL Nanocentre at the University of York, UK. Previously she held senior positions as DuPont research fellow and concurrently as adjunct Prof. at the University of Delaware, USA and led in situ electron microscopy and catalysis Group at the University of Oxford, after her PhD in physics from Cavendish Laboratory, University of Cambridge. She co-invented the in situ atomic resolution-environmental transmission electron microscope (atomic resolution-ETEM) with E. D. Boyes, to visualize and analyse fundamental catalysis and chemical reactions at the atomic level. The ETEM invention is used globally.

Ed Boyes is Research Prof. of Physics at the University of York. His PhD from Cambridge was followed by research fellowships at Materials Dept and Wolfson College, Oxford. Twenty year move to DuPont CR&D in Wilmington, DE, USA, produced the original in situ catalyst applications-oriented atomic lattice resolution environmental TEM (ETEM, with P. L. Gai) and the ultrahigh resolution (1 nm at 1 kV) low-voltage SEM/LVEDX. Member technical advisory group for NNI review, (US) President's Council of Advisors on Science and Technology (2003–2008). Returned to UK as Prof. of Physics and Electronics and founding co-director of the York JEOL Nanocentre at York. Design and catalysis applications of the novel aberration-corrected single-atom resolution analytical environmental STEM (ESTEM).

Rik Brydson holds a Chair in Nanostructural Materials Characterization in the School of Chemical and Process Engineering and newly formed Bragg Centre for Materials Research at the University of Leeds. He possesses greater than 35 years' research experience in the analytical science of materials. He is Honorary Secretary Physical Sciences and Council member of the Royal Microscopical Society, and former member of the European Microscopy Management Board (2004–2016). He was co-chair of the 2014 EPSRC-funded working group report into the coordination of UK electron microscopy capabilities in the Physical and Engineering Sciences and leads the Advisory Board of the EPSRC National Facility for Advanced Electron Microscopy (SuperSTEM) at Daresbury Laboratories.

Richard Catlow, FRS, is a Prof. at University College London and Cardiff University. Previously, he was Director of the Davy-Faraday Research Laboratory, and Wolfson Prof. of Natural Philosophy at the Royal Institution. His scientific programme develops and applies computer models to solid state and materials chemistry—areas of chemistry that investigate the synthesis, structure and properties of functional materials. He has worked extensively on collaborative projects with the developing world, especially in Africa, and was elected Foreign Secretary of the Royal Society.

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