Xinran Wang

We developed a chemical route to produce graphene nanoribbons (GNR) with width below 10 nanometers, as well as single ribbons with varying widths along their lengths or containing lattice-defined graphene junctions for potential molecular electronics. The GNRs were solution-phase-derived, stably suspended in solvents with noncovalent polymer functionalization, and exhibited ultrasmooth edges with possibly well-defined zigzag or armchair-edge structures. Electrical transport experiments showed that, unlike single-walled carbon nanotubes, all of the sub-10-nanometer GNRs produced were semiconductors and afforded graphene field effect transistors with on-off ratios of about 10(7) at room temperature.

Graphene is readily p-doped by adsorbates, but for device applications, it would be useful to access the n-doped material. Individual graphene nanoribbons were covalently functionalized by nitrogen species through high-power electrical joule heating in ammonia gas, leading to n-type electronic doping consistent with theory. The formation of the carbon-nitrogen bond should occur mostly at the edges of graphene where chemical reactivity is high. X-ray photoelectron spectroscopy and nanometer-scale secondary ion mass spectroscopy confirm the carbon-nitrogen species in graphene thermally annealed in ammonia. We fabricated an n-type graphene field-effect transistor that operates at room temperature.

Sub-10 nm wide graphene nanoribbon field-effect transistors (GNRFETs) are studied systematically. All sub-10 nm GNRs afforded semiconducting FETs without exception, with ${I}_{\mathrm{on}}/{I}_{\mathrm{off}}$ ratio up to ${10}^{6}$ and on-state current density as high as $\ensuremath{\sim}2000\text{ }\text{ }\ensuremath{\mu}\mathrm{A}/\ensuremath{\mu}\mathrm{m}$. We estimated carrier mobility $\ensuremath{\sim}200\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{V}\text{ }\mathrm{s}$ and scattering mean free path $\ensuremath{\sim}10\text{ }\text{ }\mathrm{nm}$ in sub-10 nm GNRs. Scattering mechanisms by edges, acoustic phonon, and defects are discussed. The sub-10 nm GNRFETs are comparable to small diameter ($d\ensuremath{\le}\ensuremath{\sim}1.2\text{ }\text{ }\mathrm{nm}$) carbon nanotube FETs with Pd contacts in on-state current density and ${I}_{\mathrm{on}}/{I}_{\mathrm{off}}$ ratio, but have the advantage of producing all-semiconducting devices.