Graphene nanoribbons

Graphene nanoribbons (also called nano-graphene ribbons or nano-graphite ribbons), often abbreviated GNRs, are strips of graphene with ultra-thin width (<50 nm). Graphene ribbons were originally introduced as a theoretical model by Mitsutaka Fujita and co-authors to examine the edge and nanoscale size effect in graphene.^[1][2][3]

Production

Large quantities of width controlled GNRs can be produced via graphite nanotomy ^[4] shown by Berry group, where sharp diamond knife application on graphite produces graphite nanoblocks, which are exfoliated to produce GNRs. GNRs can also be produced by unzipping or cutting open nanotubes.^[5] In one such method by Tour group multi-walled carbon nanotubes were unzipped in solution by action of potassium permanganate and sulfuric acid.^[6] In another method by Dai group GNRs were produced by plasma etching of nanotubes partly embedded in a polymer film.^[7] More recently, graphene nanoribbons have been grown onto silicon carbide (SiC) substrates using ion implantation followed by vacuum or laser annealing.^[8][9][10] The latter technique allows one to write any pattern onto SiC substrates with 5 nm precision.^[11]

Electronic structure

The electronic states of GNRs largely depend on the edge structures (armchair or zigzag). Zigzag edges provide the edge localized state with non-bonding molecular orbitals near the Fermi energy. They are expected to have large changes in optical and electronic properties from quantization. Calculations based on tight binding theory predict that zigzag GNRs are always metallic while armchairs can be either metallic or semiconducting, depending on their width. However, DFT calculations show that armchair nanoribbons are semiconducting with an energy gap scaling with the inverse of the GNR width.^[12] Indeed, experimental results show that the energy gaps do increase with decreasing GNR width.^[13] Graphene nanoribbons with controlled edge orientation have been fabricated by scanning tunneling microscope (STM) lithography.^[14] Opening of energy gaps up to 0.5 eV in a 2.5 nm wide armchair ribbon was reported. Zigzag nanoribbons are also semiconducting and present spin polarized edges. Their gap opens thanks to an unusual antiferromagnetic coupling between the magnetic moments at opposite edge carbon atoms. This gap size is inversely proportional to the ribbon width^[15][16] and its behavior can be traced back to the spatial distribution properties of edge-state wave functions, and the mostly local character of the exchange interaction that originates the spin polarization. Therefore, the quantum confinement, inter-edge superexchange, and intra-edge direct exchange interactions in Zigzag GNR are important for its magnetism and band gap. The edge magnetic moment and band gap of Zigzag GNR are reversely proportional to the electron/hole concentration, and they also can be memorably controlled by alkaline adatoms.^[17]

Tight-binding numerical simulation ^[18] obtained by means of the open-source code NanoTCAD ViDES have demonstrated that field effect transistors exploiting GNR as channel material can comply with ITRS requirements for next-generation devices.

Their 2D structure, high electrical and thermal conductivity, and low noise also make GNRs a possible alternative to copper for integrated circuit interconnects. Some research is also being done to create quantum dots by changing the width of GNRs at select points along the ribbon, creating quantum confinement.^[19]

Graphene nanoribbons possess semiconductive properties and may be a technological alternative to silicon semiconductors.^[20] and may be capable of sustaining microprocessor clock speeds in the vicinity of 1 THz^[21] field-effect transistors less than 10 nm wide have been created with GNR – "GNRFETs" – with an I_on/I_off ratio >10⁶ at room temperature.^[22][23]

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  GNR band structure for armchair type. Tight binding calculations show that armchair type can be semiconducting or metallic depending on width (chirality).

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  GNR band structure for zigzag type. Tight binding calculations predict that zigzag type is always metallic.

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  TEM micrographs of GNRs of (a) w=15, (b) w=30, (c) w=40 (exfoliating), and (d) w=60 nm deposited on 400 mesh lacey carbon grids and (e) FESEM micrograph of 600 nm ribbon. (f) Electron microscope images of a 120-nm graphene ribbons (FESEM), (g) 50 nm square GQDs (FESEM), (h,i) 25×100 nm2 rectangular GQDs (FESEM), and (j) 8°-angled tapered GNR (or triangular GQD) (FESEM)). The large densities of square and rectangular GQDs (g) showed extensive folding (white arrows). Bar sizes=(a) 250 nm, (b,g,i) 50 nm, (c,d) 500 nm, and (h) 1 μm.^[1]

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Exciton Properties

First-principle calculations with quasiparticle corrections and many-body effects are performed to study the electronic and optical properties of graphene-based materials.^[24] With GW calculation, the properties of graphene-based materials are accurately investigated, including graphene nanoribbons,^[25] edge and surface functionalized armchair graphene nanoribbons,^[26] hydrogen saturated armchair graphene nanoribbons,^[27] and scaling properties in armchair graphene nanoribbons.^[28]

Polymeric Nanocomposites

Graphene nanoribbons and their oxidized counterparts called graphene oxide nanoribbons have been investigated as nano-fillers to improve the mechanical properties of polymeric nanocomposites. Raifee, Koratkar, and Tour et al. report increases in the mechanical properties of epoxy composites on loading of graphene nanoribbons.^[29] In a recent study, Lalwani, Sitharaman and co-workers report an increase in the mechanical properties of biodegradable polymeric nanocomposites of poly(propylene fumarate) at low weight% loading of oxidized graphene nanoribbons, fabricated for bone tissue engineering applications.^[30]

See also

References

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External links

• Graphene nanoribbons on arxiv.org