Fabrication and electrical engineering of graphene nanoribbons

Graphene, as a typical representative of advanced materials, exhibits excellent electronical properties due to its unique and unusual crystal structure. The valence band and conduction band of pristine graphene meet at the corners of the Brillouin zone, leading to a half-metal material with zero bandgap. However, although the extraordinary electronical properties make graphene possess excellent electrical conductivity, it also restricts its applications in electronic devices, which usually needs an appropriate bandgap. Therefore, opening and tuning the bandgap of graphene has aroused great scientific interest. To date, many efforts have been made to open the bandgap of graphene, including defects, strain, doping, surface adsorptions, structure tunning, etc. Among these methods, graphene nanoribbon, the quasi-one-dimensional strips of graphene with finite width (< 10 nm) and high aspect ratios, possesses a band gap opening at the Dirac point due to the quantum confinement effects. Thus, graphene nanoribbon has been considered as one of the most promising candidates for the future electronic devices due to its unique electronic and magnetic properties. Specifically, the band gap of graphene nanoribbons is strongly dependent on the lateral size and the edge geometry, which has attracted tremendous attention. Furthermore, it has been reported that armchair graphene nanoribbons possess gaps inversely proportional to their width, and numerous efforts have been devoted to fabricating the graphene nanoribbons with different widths by top-down or bottom-up approaches. Moreover, based on the on-surface reaction, the bottom-up approach shows the capability of controlling the width and edge structures, and it is almost contamination-free processing, which is suitable to performing further characterizations. Ultra-high-vacuum scanning tunneling microscope is a valid tool to fabricate and characterize the graphene nanorribons, and it can also obtain the band structure information when combined with the scanning tunneling spectroscopy. Taking the advantage of the bottom-up synthetic technique, the nearly perfect graphene nanoribbons can be fabricated based on the organic molecule reaction on surface, which is a promising strategy to study the original electronic properties. To precisely tuning the band engineering of graphene nanoribbons, the researchers have adopted various effective methods, such as changing the widths and topological morphologies of graphene nanoribbons, doping the graphene nanoribbons with heteroatoms, fabricating the heterojunctions under a controlable condition. The precise control of graphene synthesis is therefore crucial for probing their fundamental physical properties. Here we highlight the methods of fabricating the graphene nanoribbons and the precise tuning of graphene bandgap structure in order to provide a feasible way to put them into application.