Graphene Nanoribbons - Electronic Structure

Electronic Structure

The electronic states of GNRs largely depend on the edge structures (armchair or zigzag, the first being the upper side of the picture on the left, and the later being the right side). 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 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. Indeed, experimental results show that the energy gaps do increase with decreasing GNR width. Graphene nanoribbons with controlled edge orientation have been fabricated by scanning tunneling microscope (STM) lithography. 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 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.

Tight-binding numerical simulation 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.

Graphene nanoribbons possess semiconductive properties and may be a technological alternative to silicon semiconductors. and may be capable of sustaining microprocessor clock speeds in the vicinity of 1 THz 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.

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

• GNR band structure for zig-zag type. Tight binding calculations predict that zigzag type is always metallic.

• 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.