GIWAXS Characterization of Metal–Organic Framework Thin Films and Heterostructures : Quantifying Structure and Orientation
For optoelectronic applications of metal–organic framework (MOF) thin films, it is important to be able to fabricate films and heterostructures that are highly oriented relative to the substrate’s surface normal. However, process optimization to achieve this is difficult without sufficiently detailed structural characterization of the deposited films. It is demonstrated that 2D grazingincidence wide-angle X-ray scattering (GIWAXS) data from a laboratory
system go a long way to providing such characterization and can 1) better test structural models than 1D scans, 2) provide a quantitative estimate—useful for process optimization—of the fraction of the deposited film that has the desired surface-oriented texture (2D powder), and 3) deliver such information as a function of depth into the film—useful for heterostructure characterization. Herein, GIWAXS data collection and analysis are introduced in the context of understanding MOF thin films, then it is shown how the desired oriented fraction (2D powder fraction) of UiO-66 fabricated by vapor-assisted conversion can be increased from 4% to over 95% by minimizing nucleation in solution. Finally, it is demonstrated that heterostructures of UiO-66 and UiO-67 can be grown wherein both layers are highly ordered (UiO-66 83%, UiO-67 >94%) once synthetic protocols are optimized.