Electronic Transport of Recrystallized Freestanding Graphene Nanoribbons

Abstract

The use of graphene and other two-dimensional materials in next-generation electronics is hampered by the significant damage caused by conventional lithographic processing techniques employed in device fabrication. To reduce the density of defects and increase mobility, Joule heating is often used since it facilitates lattice reconstruction and promotes self-repair. Despite its importance, an atomistic understanding of the structural and electronic enhancements in graphene devices enabled by current annealing is still lacking. To provide a deeper understanding of these mechanisms, atomic recrystallization and electronic transport in graphene nanoribbon (GNR) devices are investigated using a combination of experimental and theoretical methods. GNR devices with widths below 10 nm are defined and electrically measured in situ within the sample chamber of an aberration-corrected transmission electron microscope. Immediately after patterning, we observe few-layer polycrystalline GNRs with irregular sp(2)-bonded edges. Continued structural recrystallization toward a sharp, faceted edge is promoted by increasing application of Joule heat. Monte Carlo-based annealing simulations reveal that this is a result of concentrated local currents at lattice defects, which in turn promotes restructuring of unfavorable edge structures toward an atomically sharp state. We establish that intrinsic conductance doubles to 2.7 e(2)/h during the recrystallization process following an almost 3-fold reduction in device width, which is attributed to improved device crystallinity. In addition to the observation of consistent edge bonding in patterned GNRs, we further motivate the use of bonded bilayer GNRs for future nanoelectronic components by demonstrating how electronic structure can be tailored by an appropriate modification of the relative twist angle of the bonded bilayer.

Nanocrystalline graphene at high temperatures: insight into nanoscale processes

“…These close caged structures also migrate and are pinned by defects, but merging was not observed for these kinds of structures. Furthermore, it has been shown in a number of studies that few layer graphene can contain back-folded edges or bilayers, 47,48 which are presumably also present in the nanocrystalline graphene investigated here. These structures are known to be more stable and less reactive and we suggest that the shape and size stability can be attributed to closed cage and folded structures.…”

Graphene nanodevices for DNA sequencing

“…Patterning graphene at elevated temperatures (>600 • C) provides a way to minimize defects to preserve graphene's crystallinity [84]. Narrow ribbons with crystalline edges were also produced through Joule heating [83,90], where a voltage of ∼2-3 V applied across a ribbon resulted in a local heating of 2000K, leading to recrystallization of the edges [90]. Alternatively, narrow bottom-up graphene nanoribbons that are chemically synthesized with perfect zigzag or armchair edges may represent the ultimate approach for ultra-sensitive graphene devices [105].…”

“…Crystalline ribbons were also obtained using Joule heating, where a large voltage (∼3 V) is applied across the ribbon, leading to local heating (> 2000 K) due to the high current densities. This heating recrystallizes the edges of the nanoribbon that rearrange along either a zigzag or armchair profile [90]. With that approach, armchair ribbons down to 0.7nm in width were made, which were highly conducting and could sustain microampere currents at low voltages [82,83].…”

Electronic properties of graphene-based bilayer systems

“…However, the fabrication of nanoribbons with smooth edges is a very difficult task, and experimental attempts in this direction are ongoing (see, for example, Ref. [156]). …”