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Graphene is a one-atom thick, two-dimensional layer of carbon atoms in a honeycomb structure. Its intriguing electrical, optical, and mechanical qualities have sparked monumental interest from various scientific fields.
Both carbon nanotubes and graphene nanoribbons can essentially be formed from graphene. Nanoribbons can be formed from the ‘unzipping’ of carbon nanotubes, although the process is still very much in the research phase.
Carbon nanotubes are essentially rolled up sheets of graphene that can be as long as millimetres, but still retain nanometre dimensions. Nanotubes can be single-walled or have multiple walls, being essentially cylinders within cylinders.
Like sheets of graphene, carbon nanotubes have a very wide range of unique electronic, thermal, and structural properties, which can vary based diameter, length and chirality. They can have metallic properties or exhibit semiconducting behaviour.
Graphene nanoribbons are thin strips of graphene. The quasi one-dimensional character of nanoribbons results in this unique form of carbon having more functionality than graphene, the two-dimensional counterpart to nanoribbons.
The structure and physical qualities of nanoribbons vary considerably and are based on the method used to synthesize them. Currently, there are three main processes for the production of nanoribbons: slicing ribbons from graphene via lithography; bottom-up production from polycyclic molecules; and unzipping of carbon nanotubes.
The lithography method has been thoroughly-researched and produces single-layer nanoribbons on a substrate surface. The use of lithographically-generated nanoribbons is restricted to applications where they sit flat on a surface. Bulk volumes cannot be generated using lithographic techniques. As a result of built-in limits for lithography-based processes, lithographically-generated nanoribbons have irregular edges. While lithography generates highly-precise, narrow nanoribbons, the resulting jagged edges make it challenging to control the electronic qualities of the resulting ribbons.
The bottom-up production of nanoribbons entails a multi-step organic synthesis founded on cyclization of pre-synthesized polymer chains. This technique grows very narrow ribbons with atomically precise edges. Until recently, these nanoribbons could only be made on the surface of a substrate, restricting the possibility for bulk creation. The most recent efforts have led to nanoribbons being synthesized on a scale of hundreds of milligrams.
Producing Graphene Nanoribbons from Carbon Nanotubes
The third method of producing of nanoribbons is by using longitudinal opening, or 'unzipping', of multi-walled carbon nanotubes. Published methods vary, but nearly all of them are solution-based operations that involves the intercalation of the potassium-sodium alloy found between nanotube walls in a 1,2-dimethoxyethane (DME) solvent. This lattice enlargement triggers enough stress to longitudinally break the nanotube walls. The carbon atoms at the newly-established edges are reduced to their highly-reactive carboanionic form, making them highly prone to electrophilic infiltration. If the intermediate product is quenched with methanol, followed by aqueous washing, the metal cations on the ends are substituted with protons, yielding hydrogen-terminated nanoribbons.
These nanoribbons, however, are not totally flat as a result of van der Waals interaction between the nanotube walls. The nanoribbons can be compressed and exfoliated to some degree by bath sonication in chlorosulfonic acid. The electrical conductivity of the compressed 3.5 to 5 nm thick nanoribbon stacks, composed of 10 to 14 layers, is in the range of 70,000 to 95,000 S/m, which is similar to reported values for other graphitic structures.
To create alkylated nanoribbons, the intermediate product is subjected to halo-alkanes. Intercalated potassium is effectively replaced by haloalkanes that functionalize the edges and act as intercalents in the resulting alkylated-nanoribbons. The alkylated-nanoribbons with long alkyl chains are well-dispersible in organic solvents like alcohols, alkanes, ethers and ketones. Highly-stable dispersions can be generated in chloroform or chlorobenzene.
The main benefit of this ‘unzipping’ approach is its potential for production on the kilogram scale. Considerably lower cost is another advantage. Also, recent publications have indicated nanoribbons derived from unzipped nanotubes will be the first to see widespread application.