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Interfacial topological states are a key element of interest for topological insulator thin films, and their properties can depend sensitively on the atomic bonding configuration. We employ in situ nonresonant and resonant surface x-ray scattering to study the interfacial and internal structure of a prototypical topological film system: Bi2Te3 grown on Si(111). The results reveal a Te-dominated buffer layer, a large interfacial spacing, and a slightly relaxed and partially strained bottom quintuple layer of an otherwise properly stacked bulklike Bi2Te3 film. The presence of the buffer layer indicates a nontrivial process of interface formation and a mechanism for electronic decoupling between the topological film and the Si(111) substrate.

The contrast mechanism for imaging molecular-scale features on solid surfaces is described for X-ray reflection interface microscopy (XRIM) through comparison of experimental images with model calculations and simulated measurements. Images of elementary steps show that image contrast is controlled by changes in the incident angle of the X-ray beam with respect to the sample surface. Systematic changes in the magnitude and sign of image contrast are asymmetric for angular deviations of the sample from the specular reflection condition. No changes in image contrast are observed when defocusing the condenser or objective lenses. These data are explained with model structure-factor calculations that reproduce all of the qualitative features observed in the experimental data. These results provide new insights into the image contrast mechanism, including contrast reversal as a function of incident angle, the sensitivity of image contrast to step direction (i.e. up versus down), and the ability to maximize image contrast at almost any scattering condition defined by the vertical momentum transfer, Q(z). The full surface topography can then, in principle, be recovered by a series of images as a function of incident angle at fixed momentum transfer. Inclusion of relevant experimental details shows that the image contrast magnitude is controlled by the intersection of the reciprocal-space resolution function (i.e. controlled by numerical aperture of the condenser and objective lenses) and the spatially resolved interfacial structure factor of the object being imaged. Together these factors reduce the nominal contrast for a step near the specular reflection condition to a value similar to that observed experimentally. This formalism demonstrates that the XRIM images derive from limited aperture contrast, and explains how non-zero image contrast can be obtained when imaging a pure phase object corresponding to the interfacial topography.

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The use of x-ray reflection interface microscopy (XRIM) to image molecular-scale topography at the aqueous–solid interface, in situ, is described. Specifically, we image interfacial topography of the orthoclase-(001)–aqueous solution interface at room temperature and describe the challenges associated with in situ XRIM imaging. The measurements show that the reflectivity signal for in situ XRIM measurements is substantially smaller than that for ex situ measurements, because of both intrinsic and extrinsic factors. There is also a systematic temporal reduction in the image intensity with increasing x-ray dose, revealing that interaction of the focused x-ray beam with the orthoclase interfaces leads to interfacial perturbations, presumably in the form of surface roughening. This image fading is localized to the x-ray beam footprint, suggesting that the primary damage mechanism is initiated by photoelectrons produced by x-ray beam absorption near the substrate–electrolyte interface. Finally, the role of aqueous solution composition in controlling the sensitivity of the orthoclase surface to x-ray beam-induced effects is explored. A substantial increase in the orthoclase (001) surface stability was observed in solutions having elevated ionic strength, apparently as a result of the reduced lifetime of radiation chemistry products at these conditions.