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Low dimensionality and influence of nanostructuration

Low dimensionality and influence of nanostructuration

Graphene nanoribbons : ballistic transport and gap opening

Ballistic transport in graphene nanoribbons

GeorgiaTech has developped an original method to synthesize graphene nanoribbons. A first lithographic step allows to place ribbons on a SiC surface. A subsequent annealing leads to the ribbon growth at certain surface facets. Our photoemission measurements show the ideal electronic structure for graphene (linear bands and a nearly undoped graphene) on the main body of these ribbons. The ribbons exhibit ballistic transport up to 16 μm at room temperature, as determined at GeorgiaTech and at Hannover University.

On the left, electronic structure on graphene nanoribbons obtained by photoemission. On the right, transport measurements as a function of the spacement between the contacts for different annealing temperatures of ribbons. All the curves converge towards h/e2. Figures from J. Baringhaus et al., Nature 506, 349 (2014).

The ballistic transport could be favored by ribbon edges with a low defect density, as shown by our STM images. In other kinds of ribbons, defects open a transport gap that is absent here, according to our tunnel spectroscopy measurements.

In any case, the existence of ballistic transport at room temperature is the keystone to promote devices with low heat dissipation, necessary requirement for high density integration.

Scanning tunneling microscopy and spectroscopy on the ribbons. STM images show different regions on the nanostructures (top plateau, ribbon and bottom trench). On the plateau and the trench, the structure is different from that of graphene and an electronic gap is present. On the facet, the honeycomb of graphene as well as its characteristic spectrum are appreaciated. Image from J. Baringhaus et al., Nature 506, 349 (2014).

Other references

Highlight in Nature Physics News & Views 10, 182 (2014).

CNRS press release : http://www2.cnrs.fr/presse/communique/3419.htm.

Cover page of the CNRS annual report 2013.

Band gap opening in miniribbons

We have studied the last-layer topography and beyond by combining scanning tunneling microscopy and transmission electron microscopy. The main body of the ribbons is bordered by regions where the SiC substrate has minifacets. It is possible to grow a single graphene layer over the minifacets as well as over the main facet. We have observed by photoemission that the electronic properties of the continuous graphene layer change depending on the region. On top of the minifacets there are graphene mini-ribbons. These mini-ribbons are attached to the SiC substrate on their two edges, in a semiconducting region. The graphene between the attached parts is decoupled from the substrate and it exhibits a band gap of more than 500 meV because of the electronic confinement in these small mini-ribbons of 1-2 nm width.

Schematics and details of the structure adapted with permission from I. Palacio et al., Nano Lett. 15, 182 (2015). Copyright 2015 American Chemical Society. Photoemission data from J. Hicks et al., Nature Phys 9, 49 (2012).

Electronic confinement in self-organized networks

The growth of self-organized systems can be used to tailor the electronic properties of the substrate. This is possible because surface states are localized in the last atomic layers and are sensitive to modifications in these layers. It is possible to create a self-organized network of islands that induces a periodic modification of the surface potential. In this way, a network of periodic quantum boxes can be settled to study their electronic properties. Co islands can be indeed self-organized on top of a stepped (vicinal) Au surface. The Au surface state is confined by the Co islands and the surface steps. The periodicity of the system allows to use photoemission to measure the band structure and obtain the surface potential.

On the left, differential conductance image of self-organized Co islands on a stepped substrate of Au. The intensity in between each four islands shows that the surface state is confined there. On the right, simulation of the photoemission measurements in order to determine the surface potential induced by Co islands.

"Interacting quantum box superlattice by self-organized Co nanodots on Au(788)". C. Didiot, A. Tejeda, Y. Fagot-Revurat, V. Repain, B. Kierren, S. Rousset, and D. Malterre, Phys. Rev. B 76, 081404(R) (2007).

First direct observation of the ideal dispersion on graphene

Graphene growing on the carbon face of SiC has a growth rate for a given temperature very different from that of Si face. Graphene layers on the C-face are stacked with angles different from 60°, the characteristic angle of graphite and of the Si-face. As the stacking is different from that of graphite, the electronic properties of ideal graphene are preserved, as proved by our photoemission measurements. Bands are linear for the different stacked layers and the doping of each layer is almost zero, as the Dirac point is located at the Fermi level.

Figure from M. Sprinkle et al., Phys. Rev. Lett. 103, 226803 (2009). Copyright 2009 by the American Physical Society." http://dx.doi.org/10.1103/PhysRevLett.103.226803