X-ray Structural Studies of Bi2Sr2CaCu2O8$+\delta $ Exfoliated Nanocrystals

Structural studies of nanocrystals produced via mechanical exfoliation are not only essential for examining structure quality or structural changes at reduced-dimensionality, but also for understanding the role of substrates in the exfoliation process. Highly focused, tunable synchrotron X-ray beams enable the use of non-destructive characterization tools to study exfoliated samples on a variety of substrates. We demonstrate that structural and spectroscopic information can be obtained on nanocrystals as thin as 6 nm, by using a combination of micro X-ray fluorescence ($\mu$ XRF), micro X-ray absorption near-edge spectroscopy ($\mu$ XANES), and X-ray microdiffraction ($\mu$ XRD) techniques. $\mu$ XRF is used to locate the sample of desired thickness; $\mu$ XANES and $\mu$ XRD are used to obtain electronic and structural information, respectively. We report a substantial substrate effect for Bi2Sr2CaCu2O8$+\delta $ nanocrystals exfoliated on Si/SiO2 and mica substrates. The ``4.7 b'' structural modulation, characteristic of bulk crystals, vanishes below a thickness of 60 nm on mica, and is drastically suppressed below 60 nm for the Si/SiO2 substrate.