Multiple scattering in grazing-incidence X-ray diffraction: impact on lattice-constant determination in thin films

Abstract

Dynamical scattering effects are observed in grazing-incidence X-ray diffraction experiments using an organic thin film of 2,2 0 :6 0 ,2 00 -ternaphthalene grown on oxidized silicon as substrate. Here, a splitting of all Bragg peaks in the out-ofplane direction (z-direction) has been observed, the magnitude of which depends both on the incidence angle of the primary beam and the out-of-plane angle of the scattered beam. The incident angle was varied between 0.09 and 0.25 for synchrotron radiation of 10.5 keV. This study reveals comparable intensities of the split peaks with a maximum for incidence angles close to the critical angle of total external reflection of the substrate. This observation is rationalized by two different scattering pathways resulting in diffraction peaks at different positions at the detector. In order to minimize the splitting, the data suggest either using incident angles well below the critical angle of total reflection or angles well above, which sufficiently attenuates the contributions from the second scattering path. This study highlights that the refraction of X-rays in (organic) thin films has to be corrected accordingly to allow for the determination of peak positions with sufficient accuracy. Based thereon, a reliable determination of the lattice constants becomes feasible, which is required for crystallographic structure solutions from thin films.

GIWAXS-SIIRkit: scattering intensity, indexing and refraction calculation toolkit for grazing-incidence wide-angle X-ray scattering of organic materials

“…We also evaluate the position of an additional peak which arises due to reflection off the substrate. This effect was previously examined for a thin film of 2,2 0 :6 0 ,2 00 -ternaphthalene by Resel et al (2016); we compare our results with those of Resel et al (2016) in Appendix B. In cases where two or more similarly positioned peaks are visible, our tool can help distinguish whether these arise from reflection effects or from scattering of multiple polymorphs.…”

“…The intensity of the reflection peak falls off at incident angles above 0.13 , and by an incident angle of 0.15 this peak is barely visible. Another example showing the movement of a reflected peak with incident angle is reported by Resel et al (2016).…”

“…B6. Comparison of the results of our simulation with those of Resel et al (2016) We modified the procedure somewhat in order to compare the results of our equations with the scattering pattern observed by Resel et al (2016). This work assumed a direct conversion between the out-of-plane scattering angle and Q z .…”

“…In addition to refractive shift, GIWAXS data of well ordered organic materials may display a 'double-peak' effect, especially in the out-of-plane direction, as shown by Resel et al (2016) and in Fig. 9(a).…”

“…9(b), are consistent with the experimentally observed peak positions. We also used a similar procedure to predict the peak positions observed by Resel et al (2016) Fig. 10(c)], refraction changes the measured outof-plane Q position of the directly scattered peak by 0.006 Å À1 relative to the Bragg's law position.…”