The advent of short-pulse electron and x-ray sources has enabled pump-probe approaches for elucidating ultrafast materials dynamics. From such studies, a comprehensive picture of the time-dependent evolution of the initial steps of energy deposition, propagation, relaxation, and conversion in a wide range of materials can be generated. In this article, we provide an overview of the capabilities of femtosecond electron and x-ray scattering for resolving structural dynamics of materials. With such approaches, time resolutions are ultimately limited by the durations of the electron and x-ray pulses, and dynamics can be studied at length scales spanning atomic to mesoscale dimensions. The articles in this issue represent a cross section of the vigorous activity occurring in the study of light-induced ultrafast materials dynamics as it relates to charge carriers, surfaces and interfaces, lattice-coupling mechanisms, coherent structural motions, and next-generation instrument development. The approaches highlighted here are leading to new physical insights, new possibilities for engineering the properties of matter, and ultimately, a new understanding of materials functionality on ultrasmall and ultrashort spatiotemporal scales.

Conventional electron microscopy during the last three decades has experienced tremendous developments, especially in equipment design and engineering, to become one of the most widely recognized and powerful tools for key research areas in materials science and nanotechnology. In this article, we discuss scanning ultrafast electron microscopy (S-UEM) as a new methodology for four-dimensional electron imaging of material surfaces. We also illustrate a few unique applications. By monitoring secondary electrons emitted from surfaces of photoactive materials, photo- and electron-impact-induced electrons and holes near surfaces, interfaces, and heterojunctions can be imaged with adequate spatial and temporal resolution. Charge separation, transport, and anisotropic motions as well as their dependence on carrier energies can be resolved. S-UEM is poised to directly image and visualize relevant interfacial dynamics in real space and time for emerging optoelectronic devices and help push their performance.

In the quest for dynamic multimodal probing of a material’s structure and functionality, it is critical to be able to quantify the chemical state on the atomic-/nanoscale using element-specific electronic and structurally sensitive tools such as electron energy-loss spectroscopy (EELS). Ultrafast EELS, with combined energy, time, and spatial resolution in a transmission electron microscope, has recently enabled transformative studies of photoexcited nanostructure evolution and mapping of evanescent electromagnetic fields. This article aims to describe state-of-the-art experimental techniques in this emerging field and its major uses and future applications.

Ultrafast measurement technology provides essential contributions to our understanding of the properties and functions of solids and nanostructures. Atomic-scale vistas with ever-growing spatial and temporal resolution are offered by methods based on short pulses of x-rays and electrons. Time-resolved electron diffraction and microscopy are among the most powerful approaches to investigate nonequilibrium structural dynamics. In this article, we discuss recent advances in ultrafast electron imaging enabled by significant improvements in the coherence of pulsed electron beams. Specifically, we review the development and first application of ultrafast low-energy electron diffraction for the study of structural dynamics at surfaces, and discuss novel opportunities for ultrafast transmission electron microscopy facilitated by laser-triggered field-emission sources. These and further developments will render coherent electron beams an essential component in future ultrafast nanoscale imaging.

Ultrafast electron diffraction has been employed for the study of structural dynamics at surfaces in the time domain. Experiments were performed in a pump-probe setup with femtosecond-laser excitation and subsequent probing through diffraction of a femtosecond electron pulse at a temporal resolution of 350 fs. The system of interest is one atomic layer of indium atoms on a Si(111) surface. Through self-assembly, indium atomic wires form and exhibit a Peierls-like, insulator-to-metal phase transition that can be driven nonthermally through a femtosecond laser pulse. The transient intensity of the diffraction spots indicates the lifting of the Peierls transition and melting of a charge-density wave in only 700 fs, heating of the surface in 6 ps, and formation of a metastable and supercooled phase, which exists for nanoseconds.

Over the past few years, x-ray free-electron lasers (FELs) have demonstrated the possibility for probing materials with femtosecond time resolution and Angstrom spatial sensitivity. Here, we review a novel development of Fourier transform inelastic x-ray scattering (FT-IXS), which exploits the ultrafast pulses from an FEL to capture frozen snapshots of the lattice vibrations at multiple length scales simultaneously, as they oscillate when excited by a short laser pulse. This article includes an overview of the principle behind this method and a review of recent work that uses this technique to access microscopic, wave vector-dependent information on how electrons couple to the lattice and to capture phonon–phonon scattering events in real time.

Since the first report in 2012 of a solid-state perovskite solar cell (PSC) with a power-conversion efficiency (PCE) of 9.7% and 500 h stability, research on perovskite photovoltaics has unprecedentedly and exponentially increased. Currently, certified PCE for perovskite solar cells tops 22.7%, which surpasses the PCEs of conventional thin-film solar cells. Perovskite solar cells are thus a disruptive technology in photovoltaics due to their low cost and superb performance. In this article, the emergence of PSCs is introduced, and an overview of progress in our laboratory is presented. In addition, future research directions that could lead to higher efficiencies are described. Beyond photovoltaic applications of halide perovskites, results for light-emitting diodes, resistive memories, and x-ray imaging are described.

A hallmark of life is plasticity, which enables reproduction, evolution, and environmental adaptivity. It is natural to wonder if these remarkable features in nature and biology can be realized in the materials world and implemented in the emerging fields of autonomous systems, artificial intelligence, and animal–machine interfaces. First, we describe fundamental features of neurons and synapses in the brain that are responsible for information processing. Then we discuss mechanisms governing electronic plasticity in correlated electronic quantum materials that mimic organismic behavior. We give examples of learning networks and circuits designed using quantum materials that can be implemented for machine intelligence. We conclude with suggestions for future interdisciplinary research wherein synergistic interactions between orbital filling, defects, and strain could give rise to new functionality of relevance to sensory interfaces (e.g., haptics), neural information processing, and neuroscience.