Atomic Structure

2009

 

The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons

 

Nature Materials 8, 235 - 242 (2009)

 

Kyle A. Ritter & Joseph W. Lyding

 

 

Graphene shows promise as a future material for nanoelectronics owing to its compatibility with industry-standard lithographic processing, electron mobilities up to 150 times greater than Si and a thermal conductivity twice that of diamond. The electronic structure of graphene nanoribbons (GNRs) and quantum dots (GQDs) has been predicted to depend sensitively on the crystallographic orientation of their edges; however, the influence of edge structure has not been verified experimentally. Here, we use tunnelling spectroscopy to show that the electronic structure of GNRs and GQDs with 2–20 nm lateral dimensions varies on the basis of the graphene edge lattice symmetry. Predominantly zigzag-edge GQDs with 7–8 nm average dimensions are metallic owing to the presence of zigzag edge states. GNRs with a higher fraction of zigzag edges exhibit a smaller energy gap than a predominantly armchair-edge ribbon of similar width, and the magnitudes of the measured GNR energy gaps agree with recent theoretical calculations.

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Growth of Semiconducting Graphene on Palladium

 

Nano Lett., 2009, 9 (12), pp 3985–3990

 

Soon-Yong Kwon, Cristian V. Ciobanu, Vania Petrova, Vivek B. Shenoy, Javier Bareo, Vincent Gambin, Ivan Petrov and Suneel Kodambaka

 

 

We report in situ scanning tunneling microscopy studies of graphene growth on Pd(111) during ethylene deposition at temperatures between 723 and 1023 K. We observe the formation of monolayer graphene islands, 200-2000 Å in size, bounded by Pd surface steps. Surprisingly, the topographic image contrast from graphene islands reverses with tunneling bias, suggesting a semiconducting behavior. Scanning tunneling spectroscopy measurements confirm that the graphene islands are semiconducting, with a band gap of 0.3 ± 0.1 eV. On the basis of density functional theory calculations, we suggest that the opening of a band gap is due to the strong interaction between graphene and the Pd substrate. Our findings point to the possibility of preparing semiconducting graphene layers for future carbon-based nanoelectronic devices via direct deposition onto strongly interacting substrates.

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Structure and Electronic Properties of Graphene Nanoislands on Co(0001)

 

Nano Lett., 2009, 9 (8), pp 2844–2848

 

Daejin Eom, Deborah Prezzi, Kwang Taeg Rim, Hui Zhou, Michael Lefenfeld, Shengxiong Xiao, Colin Nuckolls, Mark S. Hybertsen, Tony F. Heinz and George W. Flynn

 

 

 

We have grown well-ordered graphene adlayers on the lattice-matched Co(0001) surface. Low-temperature scanning tunneling microscopy measurements demonstrate an on-top registry of the carbon atoms with respect to the Co(0001) surface. The tunneling conductance spectrum shows that the electronic structure is substantially altered from that of isolated graphene, implying a strong coupling between graphene and cobalt states. Calculations using density functional theory confirm that structures with on-top registry have the lowest energy and provide clear evidence for strong electronic coupling between the graphene ?-states and Co d-states at the interface.

 

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2008

Free-standing graphene at atomic resolution

 

Nature Nanotechnology 3, 676-681 (2008)

 

Mhairi H. Gass, Ursel Bangert, Andrew L. Bleloch, Peng Wang, Rahul R. Nair & A. K. Geim

 

Research interest in graphene, a two-dimensional crystal consisting of a single atomic plane of carbon atoms, has been driven by its extraordinary properties, including charge carriers that mimic ultra-relativistic elementary particles. Moreover, graphene exhibits ballistic electron transport on the submicrometre scale, even at room temperature, which has allowed the demonstration of graphene-based field-effect transistors and the observation of a room-temperature quantum Hall effect. Here we confirm the presence of free-standing, single-layer graphene with directly interpretable atomic-resolution imaging combined with the spatially resolved study of both the p -p* transition and the p 1s plasmon. We also present atomic-scale observations of the morphology of free-standing graphene and explore the role of microstructural peculiarities that affect the stability of the sheets. We also follow the evolution and interaction of point defects and suggest a mechanism by which they form ring defects.

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2007

Atomic Structure of Graphene on SiO2

 

NANO LETTERS 2007 Vol. 7, No. 6 1643-1648

 

Masa Ishigami, J. H. Chen, W. G. Cullen, M. S. Fuhrer, and E. D. Williams

 

We employ scanning probe microscopy to reveal atomic structures and nanoscale morphology of graphene-based electronic devices (i.e., a graphene sheet supported by an insulating silicon dioxide substrate) for the first time. Atomic resolution scanning tunneling microscopy images reveal the presence of a strong spatially dependent perturbation, which breaks the hexagonal lattice symmetry of the graphitic lattice. Structural corrugations of the graphene sheet partially conform to the underlying silicon oxide substrate. These effects are obscured or modified on graphene devices processed with normal lithographic methods, as they are covered with a layer of photoresist residue. We enable our experiments by a novel cleaning process to produce atomically clean graphene sheets.

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