Studies2

Graduate

Studies2

Seeking the Ultimate Physical Properties
- Formation of graphene nanostructures and related electronic structures -

ABSTRACT:
The Nobel Prize in Physics 2010 was awarded to researchers "for groundbreaking experiments regarding the two-dimensional material graphene." Graphene has a simple honeycomb structure comprising carbon atoms, and is an ultimate two-dimensional material system because it has only one atomic layer of thickness. Electrons confined in this planar world behave differently than they do in ordinal semiconductors and three-dimensional metals. For example, electrons in graphene can move extremely fast with photon-like behavior, and are expected to be used for ultra-fast electronic devices. It is furthermore theoretically predicted to exhibit very unique physical properties by lowering structural dimensions to one (ribbon) and zero (dot), or by making a corrugated structure. We are working to obtain experimental evidence of the unique characteristics of these graphene nanostructures.


Graphene has a very simple structure, a two-dimensional network of carbon atoms (Fig. 1). Stacking of graphene creates graphite, which is a common material used in many fields, but this changes its physical characteristics, including its electronic, optical and magnetic properties. The unique electronic structure in graphene, described by the Dirac equation for relativistic particles, provides linear energy dispersion at K-points (Figs. 2 and 3), resulting in an ultra-high mobility environment where electrons can behave like photons.

It has long been a dream of physicists to experimentally observe these characteristics, as it had been extremely difficulty to obtain two-dimensionally isolated materials. However, in 2004, Prof. Geim's group in the UK finally opened up the graphene world by using a surprisingly simple exfoliating method that employs Scotch tape. They and others who followed performed successful experiments that demonstrated unique graphene properties, which were even beyond the results theorists had predicted.

Other than the "Scotch tape method," several ways to obtain high quality graphene have been proposed. SiC surface decomposition, which we are pursuing, is one of the more promising ways for device processes in particular, because of recent SiC wafer technologies. Graphene can be grown on SiC wafers (Fig. 4), which are now commercially available and demanded for next-generation high power electronic circuits. SiC surface decomposition is a simple process, and advantageous in obtaining epitaxial graphene in a very wide area, a process that is essential to both device applications and experimental research.

We are especially interested in the physical properties of graphene, such as its electronic structures, correlated to nano-scale morphologies and dimensions. Graphene nanoribbons (GNRs) are one-dimensional and thus expected to reveal band-gap openings due to quantum effects. (Graphene is a zero gap semiconductor.) For electronic applications such as high speed FETs, it is essential to open a gap for gate-controlled operation. GNRs are difficult to fabricate via standard lithographic techniques, but can be obtained by our approach using SiC surface decomposition. We have found that SiC nano-surfaces, periodically ordered nanofacets in nano-scale, are a good template for GNRs. Moreover, graphene on such SiC nano-surfaces consists of unique morphologies, including curved and periodic ripples indicating modification of electronic structures, which have never been previously observed. We are planning to investigate the correlations of graphene nanostructures with electronic/optical/phonon characteristics.

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[Fig. 1] Structure of graphite and graphene
Graphene is a single sheet of graphite.

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[Fig. 2] Electronic structure of graphene (energy dispersion)
Graphene is characteristic in a linear dispersion at K-points.


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[Fig. 3] A linear dispersion observed in one layer of graphene by angle-resolved photoemission spectroscope.

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[Fig. 4] Cross-sectional TEM image of three monolayer graphene epitaxially grown on SiC surfaces.


Department of Applied Quantum Physics and Nuclear Engineering, Faculty of Engineering, Kyushu University
Professor Satoru Tanaka
Assistant Professor Anton Visikovskiy
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