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The following resources related to this article are available online at www. Copyright by the American Association for the Advancement of Science; all rights reserved. Downloaded from www. Bonn, Nat. Ilani, A. Yacoby, D. Mahalu, H. Shtrikman, Science , Feist et al.
Imry and A. Klug for their continued support. Supported by the Fonds zur F? Supporting Online Material www. S1 to S3 References 1 November ; accepted 7 January 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 solutionphase—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—nanometer GNRs produced were semiconductors and afforded graphene field effect transistors with on-off ratios of about at room temperature.
G raphene single-layer graphite has emerged as a material with interesting low-dimensional physics and potential applications in electronics 1—6. Chemical approaches 14—17 and self-assembly processes may produce graphene structures with desired shape and dimensions for fundamental and practical applications. We report that, by using a widely available and abundant graphite material, we can develop simple chemical methods to produce GNRs.
The resulting exfoliated graphite was dispersed in a 1,2-dichloroethane DCE solution of poly m-phenylenevinylene-co- 2,5-dioctoxy-p-phenylenevinylene PmPV by sonication for 30 min to form a homogeneous suspension.
Centrifugation then removed large pieces of materials from the supernatant Fig. S1 To whom correspondence should be addressed. E-mail: hdai stanford.
Chemically derived graphene nanoribbons down to subnm width. Right Schematic drawing of a graphene nanoribbon with two units of a PmPV polymer chain adsorbed on top of the graphene via p stacking. Some of the GNRs narrow down to a sharp point near the ends.
In C , the three GNRs are two to three layers thick. In D , ribbons are one right image to three layers. Two GNRs crossing in the left image are observed. In E , ribbons are two- to three-layered. In the middle image, a single ribbon exhibits varying width along its length with mechanical bends bright regions between segments. All scale bars indicate nm. Transmission electron microscopy TEM, fig. S5 18 , electron diffraction fig.
S5 18 , and Raman spectroscopy fig. S6 18 graphene G-band were used to characterize the GNRs. Because of their topographical resemblance to SWNTs, we carried out extensive control experiments to ensure that the subnm GNRs in our samples were not SWNTs present from contamination or other causes. For example, we performed surface-enhanced Raman measurements on many GNR samples deposited on Au substrates and never observed any radial breathing modes intrinsic to SWNTs fig.
S6 The formation of our GNRs constitutes several key steps. The PmPV conjugated polymers Fig. S1 18, 22, We were not able to form homogeneous suspension by the same process without using PmPV. We suggest that sonication is responsible for chemomechanical breaking of the stably suspended graphene sheets into smaller pieces, including nanoribbons.
Sonochemistry and ultrahot gas bubbles involved in sonication cause graphene to break into various structures, with an appreciable yield of GNRs. The supernatant after centrifugation contains micrometer-sized graphene sheets and GNRs albeit at lower yield than sheets in various sizes, shapes, and morphologies. Besides regularly shaped ribbons, we observed graphene structures that were shaped irregularly, such as wedges Fig.
S5, TEM data 18 ]. These results suggest that GNRs could be formed by breaking off narrow pieces of graphene from larger sheets during sonication. However, we found that continuous sonication does not lead to higher yield of GNRs and that the degree of sonication needs to be controlled for optimal yield of GNRs. Imaging with AFM indicated that almost no subnm ribbons were obtained if sonication were excessive for hours because of continued cutting and breaking of ribbons into small particle-like structures.
The observed graphene nanoribbons narrowing down to diminishing width and to a point Fig. GNRs comprised of segments of varying widths Fig. Single-layered GNRs displayed remarkably mechanical flexibility and resilience, with mechanical bending and folding without obvious breakage Fig.
This suggests that the GNRs are semiconducting and have substantial band gaps. The band gaps extracted this way were fit into an empirical form :8 of Eg? Graphene nanoribbons with interesting morphologies and graphene-junctions.
C and D AFM images of knifelike graphene ribbons with width changing narrowing down from tens of nanometers to ultra sharp points. Inset of C A schematic drawing to illustrate the sharp tip. Inset of E A schematic drawing of the ribbon. All scale bars are nm. Possible errors in our Eg versus w analysis include uncertainties in w based on AFM and in the assumption of negligible SB for holes in ultranarrow e. In armchair-edged GNRs, band gaps arise from quantum confinement and increased hopping integral of the p orbitals of the edge atoms caused by slight changes in atomic bonding lengths.
In zigzagedged GNRs, band gaps result from a staggered sublattice potential from magnetic ordering 7. The all-semiconductor nature found for our subnm GNRs is consistent with the band gap opening in GNRs with various edge structures suggested theoretically 7, 8. Band gap values extracted from our experimental data fall in between the limits of theoretical calculations 7 for zigzag- and armchair-edged GNRs with various widths Fig.
More quantitative comparisons of our extracted band gaps with theory are difficult at the present time because the precise edge structures of the GNRs in our FETs are unknown and are likely to vary between ribbons in various devices with either zigzag, armchair, or mixed edges. Assessment of carrier mobility requires more accurate gate capacitances 26, 27 than are currently available. This high mobility suggests that the GNRs are of high quality, nearly pristine, and free of excessive covalent functionalization, which is consistent with spectroscopic data [fig.
S2 18 ]. More precise mobility analysis will require accurate data on gate capacitances, GNR width and edge structures 28 , and ensuring ohmic contacts to the ribbons. At the present time, the experimentally observed all-semiconducting nature of narrow GNRs appears to be a key advantage over SWNTs as candidates for future nanoelectronics. References and Notes 1. Geim, K. Novoselov, Nat. Novoselov et al.
Zhang, Y. Tan, H. Stormer, P. Kim, Nature , Berger et al. B , Son, M. Cohen, S. Louie, Phys. Barone, O. Hod, G. Scuseria, Nano Lett. Areshkin, D. Gunlycke, C. White, Nano Lett. Liang, N. Neophytou, D. Nikonov, M.
Chemically derived, ultrasmooth graphene nanoribbon semiconductors.
Bonn, Nat. Ilani, A. Yacoby, D. Mahalu, H. Shtrikman, Science Shtrikman, Science ,
Chemically Derived, Ultrasmooth Graphene Nanoribbon Semiconductors