Templating Effect of Different Low-Miller-Index Gold Surfaces on the Bottom-Up Growth of Graphene Nanoribbons

This work is focused on the structural control of graphene nanoribbons (GNRs) and intermediate polymeric wires [poly(p-phenylene), PPP] during their thermoactivated bottom-up synthesis from room temperature to 480 °C. The first step of the synthesis relies on the Ullmann coupling between 4,4″-dibromo-p-terphenyl (DBTP) molecular precursor units that lead to PPP wires in the temperature range between 170 and 200 °C. Thermal annealing at higher temperatures (360–480 °C) triggers the PPP lateral fusion to yield GNRs. We systematically studied the deposition of DBTP on the three main low-Miller-index gold surfaces, i.e., Au(100), Au(110), and Au(111), to elucidate the templating effects of such surfaces due to the pronounced anisotropy of their reconstructions via a multitechnique approach (scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction). The best results are obtained on Au(100) in terms of (i) GNR length (up to 80 nm), (ii) narrow width distribution (only 6- and 9-GNRs), (iii) long-range order, and (iv) excellent alignment along the reconstruction. Au(111) produces longer GNRs than Au(100) but poorer molecular ordering. Concerning PPP wires, they are stable within a wide temperature range and exhibit an interesting improvement of the long-range order with increasing temperature on Au(100), but the best overall organization and unidirectionality have been achieved on Au(110).