In the last decade, there have been significant advances in the research and development of heterogeneous and surface guided crystallization of organic semiconductors and their application as functional materials in optoelectronic devices. This works presents bioinspired synthetic organic macrocrystalline systems. Highly uniform single crystal films of xanthine derivatives are grown on top of quartz single crystal with an area reaching several square millimeters and thickness of hundreds of nanometers. The crystal sheets are characterized with a well-defined orientation both in and out of the substrate plane, giving rise to high optical anisotropy in the plane parallel to the quartz surface.

a) Schematic description of the in-plane and out-of-plane XRD techniques used.

b) 2theta-theta (out-of-plane) scan showing only {100} planes of theophylline arising from surface guided growth on a (11̄20) single crystal quartz surface.

c) Rocking curve (omega scan) analysis showing the out-of-plane orientation distribution of the organic phase with an orientation distribution of 0.195°.

d) In-plane scan (2theta/phi) 2𝜃/𝜑, also showing very high texture; At such crystal orientation we observe a preferred orientation of theophylline in the [002] direction.

e) 𝜑-rotation at 2𝜃 = 84°, showing the overlap of peaks corresponding to (017) of theophylline and (2204) of quartz. This epitaxial growth can be rationalized also from a molecular insight, where the distance between the dangling oxygen (O) atoms on the surface of the quartz is 7.599 Å, which matches well with twice the distance between the (O) atoms of theophylline in the (017) plane (7.526 Å). The lattice mismatch is less than 1% which allows efficient hydrogen bonding between the organic layer and the quartz substrate.

f) 𝜑-rotation at 2𝜃 = 20.9°, showing the angle between (002) of theophylline and (1100) of quartz, namely that the slow axis of theophylline is in 45° to the slow optical axis of quartz as expected.

In-plane and out-of-plane measurements employed in this study are demonstrated in the video below:

To learn more about the work done by Prof D. Oron's group please see:

“Surface-Guided Crystallization of Xanthine Derivatives for Optical Metamaterial Applications”

A. Niazov-Elkan, M. Shepelenko, L. Alus, M. Kazes, L. Houben, K. Rechav, G. Leitus, A. Kossoy, Y. Feldman, L. Kronik, P. G. Vekilov, D. Oron. Advanced Materials (2024) 36, 2306996