Doping as a Strategy for Modulating Crystallinity and Optical Properties of Large Area Films of Semiconducting...

Two-dimensional covalent organic frameworks (2D-COFs) represent a promising class of 2D materials with semiconducting properties, making them ideal candidates for optoelectronic applications. The key to their applicability lies in the ability to fine-tune their electronic properties and the large crystalline order in a film-like morphology. In this article, we demonstrate that the use of a dopant molecule, melamine (Me), can drastically enhance the crystallinity and modulate the optical band gap of large self-standing thin films of 2D-COF, synthesized through interfacial Schiff base condensation. 2D-COFs are synthesized using a variable ratio of p-phenylenediamine (PDA), benzene-1,3,5-tricarboxaldehyde (TCA), and melamine (Me) dopant, leading to significant alterations in both crystallinity and optical properties. Notably, the optical band gap shows a strong blue shift from ≈2.2 eV for the undoped 2D-COF to ≈2.6 eV for the optimally Me-doped 2D-COF. In addition, for the optimum ratio of melamine to PDA and TCA, we observed large, highly crystalline domains within the film. The effect of melamine doping on the structural and electronic modifications of the films was characterized using various spectroscopic and microscopic techniques and density functional theory calculations.