What Are Nanoribbons?

Nanoribbons are quasi-one-dimensional nanostructures in which one lateral dimension is on the order of nanometers while the longitudinal dimension is comparatively large, giving a ribbon-like geometry. The term is most commonly applied to graphene nanoribbons (GNRs), strips cut or grown from a single layer of graphene, though the concept extends to nanoribbons of molybdenum disulfide, boron nitride, silicon, and other two-dimensional or quasi-two-dimensional materials. The electronic, magnetic, and optical properties of nanoribbons differ substantially from those of the parent two-dimensional sheet because quantum confinement in the width direction opens a bandgap and because the edge atomic configuration introduces localized states that have no analog in the bulk. The field draws on condensed matter physics, surface chemistry, and materials engineering.

Graphene itself is a semimetal with no bandgap, which limits its utility in digital logic. Forming it into nanoribbons with widths below approximately 10 nm induces a bandgap through lateral quantum confinement, making the material useful as a transistor channel. The magnitude of this bandgap is inversely proportional to ribbon width, so precise width control at the atomic scale is essential for device reproducibility. As reviewed in Graphene Nanoribbons: Current Status, Challenges, and Opportunities in Quantum Frontiers, GNRs are considered a candidate channel material for post-silicon transistors because of their high carrier mobility and compatibility with planar fabrication.

Electronic Structure and Edge Effects

The electronic structure of a GNR depends critically on the orientation of its edges relative to the graphene lattice. Ribbons terminated along the armchair direction exhibit a semiconducting bandgap that scales as approximately 0.9 eV·nm divided by ribbon width in nanometers, making a 5 nm wide armchair GNR comparable to a medium-bandgap semiconductor. Zigzag-terminated ribbons possess a flat band at the Fermi energy associated with localized edge states, which gives rise to edge magnetism: ab initio calculations predict that opposite edges of a zigzag GNR carry antiparallel spin polarization, a property that could be exploited in spin-based logic devices. The PMC review of electronic properties of edge-decorated graphene nanoribbons explores how these edge states are modified by adsorption of atoms or functional groups, opening routes to chemical tuning of bandgap and magnetic order.

Synthesis and Fabrication

Two broad synthesis strategies have been developed for GNRs: top-down patterning of graphene sheets and bottom-up on-surface synthesis from molecular precursors. Top-down approaches include electron beam lithography and plasma etching, but these methods produce rough edges that scatter carriers, degrading transport properties. Unzipping carbon nanotubes by plasma etching or chemical oxidation creates nanoribbons from a cylindrical template, with edge quality that depends on the nanotube diameter and the unzipping chemistry. Bottom-up synthesis, in which designed polyaromatic hydrocarbon precursor molecules are deposited on a metal surface and allowed to undergo thermally induced polymerization and cyclodehydrogenation, produces GNRs with atomically precise edge termination and well-defined widths. The bottom-up approach, developed on gold and silver surfaces, currently offers the best edge quality for fundamental studies and device prototyping, though transfer to insulating substrates remains a processing challenge.

Properties and Functionalization

Chemical functionalization of nanoribbon edges modifies the bandgap and surface chemistry without changing ribbon width. Hydrogen termination is the standard reference state; substitution of edge hydrogens with nitrogen, oxygen, or halogens shifts the bandgap and can introduce additional localized states or magnetic moments. Boron nitride nanoribbons, isostructural with GNRs but fully insulating, are studied as dielectric encapsulants for GNR transistors or as components in lateral heterostructures where the interface between a semiconducting GNR and an insulating BN nanoribbon defines a one-dimensional quantum wire. Nanoribbons of transition-metal dichalcogenides such as MoS2 combine a direct bandgap with strong spin-orbit coupling, properties relevant for valley-polarized transistors and spintronic applications. Research published in Scientific Reports on the geometric and electronic properties of edge-decorated GNRs covers the range of edge-chemistry effects systematically.

Applications

Nanoribbons have applications in a wide range of fields, including:

• Nanoscale transistor channels in post-silicon CMOS architectures, where GNR bandgaps enable high on-off ratios at gate lengths below 10 nm
• Chemical and biological sensors, where edge functionalization creates selective adsorption sites that shift electrical resistance in proportion to analyte concentration
• Spintronic devices, exploiting the edge magnetism of zigzag GNRs for spin-polarized current sources or logic gates
• Thermal management in nanoscale electronics, using high-conductivity GNR interconnects to transport heat away from hotspots
• Quantum information, where GNR quantum dots defined by width modulation serve as spin or charge qubit hosts