Graphene nanoribbons

Scanning tunneling microscopy images of graphene nanoribbons having periodic width and boron doping pattern. The polymerization reaction used for their synthesis is shown on top.^[1]

Graphene nanoribbons (GNRs, also called nano-graphene ribbons or nano-graphite ribbons), are strips of graphene with ultra-thin width (<50 nm). Graphene ribbons were introduced as a theoretical model by Mitsutaka Fujita and coauthors to examine the edge and nanoscale size effect in graphene.^[2][3][4]

Production

Nanotomy

Large quantities of width controlled GNRs can be produced via graphite nanotomy,^[5] where applying a sharp diamond knife on graphite produces graphite nanoblocks, which can then be exfoliated to produce GNRs. GNRs can also be produced by "unzipping" or cutting open nanotubes.^[6] In one such method multi-walled carbon nanotubes were unzipped in solution by action of potassium permanganate and sulfuric acid.^[7] In another method GNRs were produced by plasma etching of nanotubes partly embedded in a polymer film.^[8] More recently, graphene nanoribbons have been grown onto silicon carbide (SiC) substrates using ion implantation followed by vacuum or laser annealing.^[9][10][11] The latter technique allows any pattern to be written on SiC substrates with 5 nm precision.^[12]

Epitaxy

GNRs were grown on the edges of three-dimensional structures etched into silicon carbide wafers. When the wafers are heated to approximately 1,000 °C (1,270 K; 1,830 °F), silicon is preferentially driven off along the edges, forming nanoribbons whose structure is determined by the pattern of the three-dimensional surface. The ribbons had perfectly smooth edges, annealed by the fabrication process. Electron mobility measurements surpassing one million correspond to a sheet resistance of one ohm per square— two orders of magnitude lower than in two-dimensional graphene.^[13]

Chemical vapor deposition

Nanoribbons narrower than 10 nanometers grown on a germanium wafer act like semiconductors, exhibiting a band gap. Inside a reaction chamber, using chemical vapor deposition, methane is used to deposit hydrocarbons on the wafer surface, where they react with each other to produce long, smooth-edged ribbons. The ribbons were used to create prototype transistors.^[14] At a very slow growth rate, the graphene crystals naturally grow into long nanoribbons on a specific germanium crystal facet. By controlling the growth rate and growth time, the researchers can tune the nanoribbon width.^[15]

Electronic structure

The electronic states of GNRs largely depend on the edge structures (armchair or zigzag). In zigzag edges each successive edge segment is at the opposite angle to the previous. In armchair edges, each pair of segments is a 120/-120 degree rotation of the prior pair. 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, density functional theory (DFT) calculations show that armchair nanoribbons are semiconducting with an energy gap scaling with the inverse of the GNR width.^[16] Experiments verified that energy gaps increase with decreasing GNR width.^[17] Graphene nanoribbons with controlled edge orientation have been fabricated by scanning tunneling microscope (STM) lithography.^[18] Energy gaps up to 0.5 eV in a 2.5 nm wide armchair ribbon were reported.

Zigzag nanoribbons are 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^[19][20] 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 can be controlled by alkaline adatoms.^[21]

Their 2D structure, high electrical and thermal conductivity and low noise also make GNRs a possible alternative to copper for integrated circuit interconnects. Research is exploring the creation of quantum dots by changing the width of GNRs at select points along the ribbon, creating quantum confinement.^[22]

Graphene nanoribbons possess semiconductive properties and may be a technological alternative to silicon semiconductors^[23] capable of sustaining microprocessor clock speeds in the vicinity of 1 THz^[24] 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.^[25][26]

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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]

1. ^ Cite error: The named reference GQD was invoked but never defined (see the help page).

Exciton Properties

First-principle calculations with quasiparticle corrections and many-body effects explored the electronic and optical properties of graphene-based materials.^[27] With GW calculation, the properties of graphene-based materials are accurately investigated, including graphene nanoribbons,^[28] edge and surface functionalized armchair graphene nanoribbons^[29] and scaling properties in armchair graphene nanoribbons.^[30]

Applications

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. Increases in the mechanical properties of epoxy composites on loading of graphene nanoribbons were observed.^[31] An increase in the mechanical properties of biodegradable polymeric nanocomposites of poly(propylene fumarate) at low weight% was achieved by loading of oxidized graphene nanoribbons, fabricated for bone tissue engineering applications.^[32]

Contrast agent for bioimaging

Hybrid imaging modalities, such as photoacoustic (PA) tomography (PAT) and thermoacoustic (TA) tomography (TAT) have been developed for bioimaging applications. PAT/TAT combines advantages of pure ultrasound and pure optical imaging/radio frequency (RF), providing good spatial resolution, great penetration depth and high soft-tissue contrast. GNR synthesized by unzipping single- and multi-walled carbon nanotubes have been reported as contrast agents for photoacoustic and thermoacoustic imaging and tomography.^[33]

See also

References

External links

• Graphene nanoribbons on arxiv.org