Tightly Pitched Sub-10 Nm Nanoribbons Grown Via Seeded Anisotropic Synthesis on Ge(001)

Abstract

Graphene nanoribbons with sub-10 nm widths and smooth armchair edges possess exceptional electrical and thermal properties making them a material of interest for next-generation electronics. However, the difficulty of synthesizing ribbons with the qualities required for device applications has remained a challenge. Ribbons must have sub-10 nm widths, lengths greater than 100 nm, controlled orientation and placement, and a smooth, armchair edge structure. Top-down lithographic patterning of monolayer graphene into ribbons can address the issues of placement, orientation, and length. However, patterning resolution limits ribbons to greater than 10 nm widths and they suffer from poor edge structure, which degrades electrical and thermal properties. Bottom-up techniques such as the unzipping of graphite and nanotubes can result in narrow ribbons with smooth edges but do not provide control over placement, orientation, and edge chirality. Another method is polymerization, which results in a smooth, armchair edge structure and sub-10 nm widths but it lacks control over orientation and placement and the ribbons possess short lengths (~20 nm). Our group has shown the CVD growth of graphene on Ge(001) directly yields long, smooth armchair edged, semiconducting graphene nanoribbons [1-2]. The ribbons evolve from the bottom-up from CH4 due to highly anisotropic growth kinetics. The ribbons possess exceptional charge transport properties, as compared to others in literature, demonstrating an on/off conduction of 2x104 and an on-state conductance of 5 µS [3]. However, this fabrication method causes nanoribbons (with two orientations possible) to stochastically nucleate at random locations and times giving rise to length, width, and bandgap polydispersity. We have recently shown that seed-mediated growth of graphene nanoribbons on Ge(001) enables control over ribbon position and orientation, reduces length and width polydispersity, and enables the fabrication of nanoribbon arrays [4]. However, the diameter of our graphene seeds before growth was unknown, the effect of pitch (ribbon to ribbon distance) on ribbon growth was not studied, and sub-10 nm ribbon arrays were not achieved. Here, we present on the synthesis of sub-10 nm graphene nanoribbons from sub-10 nm seeds, at pitches varying from 50 to 500 nm, for the first time. We use lithography to create graphene seeds that vary from 20 to 55 nm in diameter and then etch the seeds in an Ar/H2 environment before nanoribbon synthesis to create sub-10 nm seeds. We find that the evolution of nanoribbons from the seeds proceeds similarly, regardless of pitch – indicating that growth is dictated by attachment limited processes that do not depend on inter-nanoribbon separation. These findings are used to fabricate tightly pitched arrays of registered, unidirectionally aligned nanoribbons, with sub-10 nm widths and lengths exceeding 200 nm, for the first time. These results show that seed-mediated nanoribbon growth is a viable route for creating dense arrays of nanoribbons for semiconductor electronics. [1] Jacobberger, R. M. et al. Nat. Commun. 2015, 6, 8006. [2] Kiraly, B. et al. Appl. Phys. Lett. 2016, 108, 213101. [3] Jacobberger, R. M. et al. ACS Nano 2017, 11, 8924−8929. [4] Way, A. J. et al. Nano Letters 2018, 18, 898.