X-Ray Probes for In Situ Studies of Interfaces
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In Situ Studies of Interfaces D.D. Fong, C.A. Lucas, M.-I. Richard, and M.F. Toney
Abstract Surfaces and buried interfaces play critical roles in many environmental, catalytic, and tribological processes and in a wide variety of applications, including microelectronics and optoelectronics. Interfacial structure and composition are closely coupled to their surroundings, and probes that yield information about materials in situ are essential to obtain a thorough understanding of interface functions and properties. The highly brilliant, hard x-rays available from synchrotron light sources can easily penetrate through gas or liquid environments, or even solid thin-film overlayers, and enable real-time monitoring of the evolving chemistry and structure of the interface with atomic-scale resolution. Here we review the in situ study of interfaces by a variety of synchrotron x-ray scattering techniques and provide several examples of their application in electrochemical processes and thin-film island growth. We also discuss recent advances in analytical techniques and x-ray optics that are facilitating the in situ study of surfaces and buried interfaces with direct imaging.
Introduction X-rays interact weakly with matter, making them nearly an ideal probe for in situ studies (i.e., studies of materials in their “natural” place). While ex situ studies suffice in many instances, there is mounting evidence that they often fail to capture the physics and chemistry governing many interfacial processes, giving rise to the so-called “pressure gap” in gas-phase catalysis or the “emersion gap” in electrochemistry. For in situ studies, however, attenuation of the x-ray beam in the host environment often means that a sufficient signal-to-noise ratio can only be obtained with synchrotron radiation. With the opening of third-generation hard x-ray facilities in the 1990s, along with the many initiatives in nanoscale materials, there has been a surge of activity in the interfacial sciences, and many beamlines across the globe are dedicated to their study in situ. Researchers are not only taking advantage of source brightness and coherence but also energy tunability and polarization, which enable a wide variety of electronic, chemical, and magnetic studies in addition to structural investigations. Furthermore, with the ongoing
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advances in in situ instrumentation, x-ray optics, detectors, and analytical techniques, there is an ever-expanding variety of tools with which one can probe material response to external stimuli. The purpose of this article is to describe a few of the most important synchrotron x-ray techniques employed in interfacial studies through their application to particular cases involving electrochemical reactions and thin-film island growth. We conclude with a discussion of recent developments in phase retrieval and x-ray microscopy that will significantly impact the direction of this field in the years to come.
X-Ray Techniques Three examples of x-ray techniques applicable to in situ interface studies are shown
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