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Understanding and designing sophisticated new materials require measurements of not only their average structural properties but also their dynamic behavior. X-ray photon correlation spectroscopy (XPCS) provides this information by characterizing fluctuations in condensed matter across a broad range of length scales and timescales. Over the past two decades, XPCS has provided a wide variety of results in the study of materials properties. In this review, we provide an overview of coherence, photon correlation spectroscopy, and the dynamic structure factor as well as information on the mechanics of XPCS experiments. We highlight the impact that this infrastructure has had on materials research and the bright future that is forthcoming with the anticipated upgrade of many third-generation synchrotron sources to fourth-generation multibend achromat sources.

The properties of artificially grown thin films are often strongly affected by the dynamic relationships between surface growth processes and subsurface structure. Coherent mixing of X-ray signals promises to provide an approach to better understand such processes. Here, we demonstrate the continuously variable mixing of surface and bulk scattering signals during real-time studies of sputter deposition of a-Si and a-WSi2 films by controlling the X-ray penetration and escape depths in coherent grazing-incidence small-angle X-ray scattering. Under conditions where the X-ray signal comes from both the growth surface and the thin film bulk, oscillations in temporal correlations arise from coherent interference between scattering from stationary bulk features and from the advancing surface. We also observe evidence that elongated bulk features propagate upwards at the same velocity as the surface. Furthermore, a highly surface-sensitive mode is demonstrated that can access the surface dynamics independently of the subsurface structure.

X-ray photon correlation spectroscopy experiments were conducted on bicontinuous microemulsion and lamellar phases in polystyrene/polyisoprene/poly(styrene-b-isoprene) blends. Regardless of the microstructure, the scattering intensity autocorrelation function at low temperatures in the vicinity of the primary X-ray scattering peak, g2(q,t) (qis the scattering vector), was found to be a condensed exponential, i.e., g2(q,t) ~ exp-t/(q)with > 1. This is in direct contrast to previous observations of exponential and stretched exponential correlation functions ( = 1) in microstructured materials comprising polymers as well as small molecules. At higher temperatures, more conventional stretched exponential correlation functions are recovered. We suggest that condensed exponential relaxation functions may be related to the spontaneous breakup of microsctructure.