(538i) Understanding Armchair Graphene Nanoribbon Growth on Mis-Cut Ge(001) Surfaces through Experiments and Density Functional Theory Calculations

Graphene nanoribbons containing smooth, armchair edges have unique electronic properties that allow them to act as semiconductors while maintaining a high charge carrier mobility when their widths are confined to below 10 nm¹. Using chemical vapor deposition of methane and hydrogen (CVD) at low partial pressures, epitaxial armchair graphene nanoribbons can be grown on the Ge(001) surface². However, ribbons produced with this method self-align evenly in both Ge<110> directions, making it impossible to form ordered arrays of nanoribbons that are required for fabricating transistor devices from the ribbons.

Here, we explore the impact of step edges on growth behavior of graphene nanoribbons. Experimental measurements reveal that on mis-cut Ge(001) surface, where additional step edges are created in the <110> direction, the length of the graphene nanoribbons preferentially align along step edges. Additionally, we observed that when the ribbon growth rate was increased, graphene crystals grew into larger ribbons on the flat surface and into semicircular shapes on mis-cut surfaces. To understand this behavior, we examined the interaction of graphene nanoribbons with flat and mis-cut Ge surfaces using self-consistent density functional theory (DFT) calculations. Our calculations show that a carbon-terminated edges of the graphene nanoribbons and step edges of the surface form strong bonds, which are thermodynamically unfavorable on the flat surface. We hypothesize that the strong bond between graphene and step-edge suppresses growth at this edge, and graphene crystals grow outward from this pinned edge, which explains the straight termination in one direction.

Combining experimental and theoretical methods, we offer insights into the mechanism of graphene nanoribbon alignment on Ge(001) during CVD growth. In the future, these insights carry the potential to optimize the production of graphene nanoribbon arrays for device applications.

References
[1] L. Chen, Y. Hernandez, X. Feng, and K. Müllen, “From Nanographene and Graphene Nanoribbons to Graphene Sheets: Chemical Synthesis,” Angew. Chemie Int. Ed., vol. 51, no. 31, pp. 7640–7654, Jul. 2012.
[2] R. M. Jacobberger, B. Kiraly, M. Fortin-Deschenes, P. L. Levesque, K. M. McElhinny, G. J. Brady, R. Rojas Delgado, S. Singha Roy, A. Mannix, M. G. Lagally, P. G. Evans, P. Desjardins, R. Martel, M. C. Hersam, N. P. Guisinger, and M. S. Arnold, “Direct oriented growth of armchair graphene nanoribbons on germanium,” Nat. Commun., vol. 6, p. 8006, Aug. 2015