(333d) How Graphene Nanoribbons Grow during Chemical Vapor Deposition on Ge(001)

Chemical vapor deposition (CVD) of carbon containing precursors is a convenient way to synthesize graphene on different substrates. Interestingly Ge(001) is one of the few substrates, where – at slow growth rates - elongated graphene shapes with smooth armchair edges, so called graphene nanoribbons (GNRs), can be synthesized [1,2]. Due to their finite bandgap at width below 10 nm, the controlled synthesis of GNRs is highly desirable, but the underlying reasons for the favorable growth behavior on Ge(001) are poorly understood.

In this contribution we study the growth of graphene on Ge(001) during chemical vapor deposition using an empirical kinetic Monte Carlo approach. We find that two different growth regimes exist, which are determined by the competition between the formation of new rows and their growth. Our results indicate that the smoothness of the edges is influenced by row growth rate and the mobility of the graphene precursor species being attached to the graphene edges. Lastly, we find that anisotropy in the formation of this precursor species at different edges, its thermodynamic stability, and the formation of new graphene rows can lead to high aspect-ratio graphene shapes. Comparing our results to experimentally observed growth properties of graphene on Ge(001), we identify the anisotropic thermodynamic stabilization of graphene precursors as the most likely reason for the formation of narrow graphene nanoribbons on this surface. We also demonstrate that kinetic effects impose limits on the obtained aspect ratios.

[1] R.M. Jacobberger, B. Kiraly, M. Fortin-Deschenes, P.L. Levesque, K.M. McElhunny, G.J. Brady, R. Rojas Delgado, S.S. Roy, A. Mannix, M.G. Lagally, P.G. Evans, P. Dejardins, R. Martel, M.C. Hersam, N.P. Guisinger, Nat. Commun. 6, 8006, 2015

[2] R.M. Jacobberger,E.A. Murray, M. Fortin-Deschenes, F. Göltl, W.A. Behn, Z.J. Krebs, P.L. Levesque, F.E. Savage, C. Smoot, M.G. Lagally, P. Desjardins, R. Martel, B. Brar, O. Moutanabbir. M. Mavrikakis, M.S. Arnold, Nanoscale 11, 4864, 2019