What are Armchair Graphene Nanoribbons?Gary Kardys | May 04, 2017
Graphene is a supermaterial because the material has superlative properties such as:
- Strongest material
- Thinnest material (0.345 nanometers thick)
- Best thermal conductor
- Best transparent, electrical conductor
- Very high specific surface area
- Unique electronic properties from Dirac cones
What characteristics cause the outstanding properties of graphene?
Intrinsically, graphene does not have a band gap like conventional insulators or semiconductors, or a partially filled band like metals rather properties like a tunable conductivity. Graphene has unique electronic properties resulting from the bonding states between carbon atoms. The carbon atoms in graphene are sp2 hybridized, resulting in covalent or sigma bonds between atoms within the graphene plane. Carbon atoms with sp3 hybridization occur in diamond, another supermaterial (super for superhardness and thermal conductivity). The atoms in the graphene plane are arranged in a hexagonal pattern, meaning each carbon atom forms three bonds to other carbon atoms. The strong carbon bonds within the graphene nanoribbons or nanosheets give rise to graphene’s 130 GPa ultimate tensile strength, which is orders of magnitude higher than the strength of A36 structural steel (0.4 GPa).
Carbon has a valence of four, so the fourth bond or remaining p or Pi (π) orbital becomes a delocalized electron cloud on the surface. In some ways, this is not unlike metals, where a sea of electrons are shared by all of the metal atoms. Graphene is often described as a zero-overlap semi-metal. The shared sea of electrons gives rise to high conductivity in metals. Electrons in metal are still slowed down by localized fields from impurities and defects when traveling through a metallic crystal. The electrons on the surface of the graphene are not encumbered by crystal lattice variations. The Pi orbital electrons are tied to the bonding and anti-bonding (the valance and conduction bands), which gives rise to the “topological insulator” or “Dirac cone” electronic nature of graphene. At the Dirac point, the electrons act like they have zero mass akin to photons. The cones help give graphene massless fermions or charge carriers, which lead to various quantum Hall effects and enormous carrier mobility.
What forms of graphene are available?
In conventional semiconductors, conductivity is controlled by doping to narrow the gap and by an applied voltage, which allow electrons to tunnel through or bridge the gap. With a Dirac cone, electrons behave more like light in a relativistic fashion and like light moving at high speeds. An applied voltage can be used to control the number and nature of the graphene charge carriers, electrons or holes. Conductivity can be adjusted by doping to alter the Fermi level.
Conventional materials are available in a variety of forms such as plate, sheet, foil, strip, wire, block, square bar or rod, which are defined by the three dimensions of the stock shape. Graphene is a two dimensional material and the thinnest known material. The two dimensional nature limits the form available to monolayer sheets, bilayer sheets, superlattices, quantum dots, fibers, nanocoils, honeycombs, aerogel foams and nanoribbons. Doping, nanotube reinforcement, oxidation and other modifications are used to tailor graphene’s properties for specific applications. When graphene is in the form of narrow sheet less than 50 nanometers wide, it's called a graphene strip, a nanoribbon or nanostripe.
How do zig-zag and armchair graphene nanoribbons differ?
Graphene nanoribbons are available in two forms or orientations: “zig-zag” and “armchair” edge shapes. Zig-zag graphene nanostripes form at low temperatures and have spin-polarized metallic edge currents. Graphene zig-zag nanoribbons hold promise for spintronics applications. Graphene "armchair" nanoribbon edges are particularly interesting because they behave like semiconductors. Research has shown nanomaterials provide continued miniaturization and performance enhancement in microelectronics device and manufacturing. Some believe the limits of conventional semiconductor materials will end Moore’s Law. Armchair graphene nanoribbons might provide the next leap in semiconductor technology. The chemical nature or bonding energies between zig-zag and armchair graphene nanoribbons differs, which can impact functionalization or forming chemical modifications such as fluorographene, chlorographene, graphene oxide and other functionalized graphene nanoribbon hybrids.
Graphene nanoribbons also have interesting physical and chemical properties for other applications. The carbon atoms in graphene are bonded to only three other atoms, so the fourth atom on the surface is available for bonding to other chemicals, coupling agents or binder resins in composites. Forming a good interface in a composite is a critical in fully utilizing a high strength reinforcement and transferring the structural load from the resin matrix to nanoribbon. Composite applications look promising for graphene nanoribbon when the capability for bonding is combined with ultrahigh tensile strength and the high specific surface area.