<i>Ab initio</i> investigation of the cyclodehydrogenation process for polyanthrylene transformation to graphene nanoribbons

Atomically precise synthesis of graphene nanoribbons (GNRs) may enable GNR-based nanoelectronics. We investigated GNR synthesis from DBBA molecular precursors on an Au(111) surface [1]. The growth process consists of dehalogenation/polymerization followed by cyclodehydrogenation. We investigated the latter using the nudged elastic band method within DFT. Our studies found that the metal substrate: (i) enhances the reaction energetics because adsorption of the product (GNR) on Au (111) is stronger than the adsorption of the reactant (polyanthrylene), and (ii) acts as a catalyst to lower the energy barriers for cyclodehydrogenation. In comparison, an underlying adsorbed GNR screens the metal substrate and hinders on-top GNR growth by lowering the adsorption energy and increasing the energy barriers. We also investigated electronic levels of various intermediate structures and found that molecular orbitals play an important role in directing the reaction, e.g., the injected electrons (or holes) lower the energy barrier through arenium ion effect. These findings provide new insight into GNR growth and offer guidance for the design of new graphitic structures.

[1] Z. Xiao, C. Ma, W. Lu, J. Huang, L. Liang, K. Hong, A.-P. Li, B. G. Sumpter, J. Bernholc, npj Comp. Mat. 5, 91 (2019).