The thermodynamic stability and simulated STM images of graphene nanoribbons

In this study, we investigated the edge formation energy of hydrogen-terminated graphene nanoribbons as a function of the chemical potential of the environment as well as their electronic and magnetic properties by means of density functional theory. The single-hydro.-en-terminated zigzag ribbons, well known for their antiferromagnetic edge states, were found to be stable only under extremely low vacuum pressures. At more standard conditions, the most stable structures are the single- and double-hydrogenated armchair edges and a zigzag edge configuration with one double- and two single-hydrogenated sites. At high hydrogen pressure, the edge formation energy becomes negative, meaning that graphene spontaneously breaks into ribbons. Clar type bond formulas proved to be an excellent tool for the rationale behind the stability, existence of edge states, and appearance of magnetism, and they are in perfect agreement with simulated scanning tunneling microscope (STM) images of the ribbons. [GRAPHICS] Bond formulas and simulated STM images show an identical pattern of Clar sextets. Left: single-hydrogen-terminated armchair ribbon. Right: double-hydrogen-terminated armchair ribbon. U= -0.5 V, sample-tip distance fixed at 3 angstrom. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim