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Kohlenstoff Nanomaterialien
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Carbon-based Nanomaterials

 

Defects in carbon-based nanomaterials

In carbon nanomaterials such as graphene or graphene nanoribbons the honeycomb lattice of carbon atoms can be locally disturbed by vacancies, adatoms, substitutional defects, edge structures or topological defects such as Stone-Wales bond rotations. Very often, irregularities or defects in a material are associated with imperfection and have the connotation of a detriment. This view is, however, too narrow minded as the controlled introduction of defects can be a pathway for the specific modification and control of a variety of electronic, magnetic, mechanical and structural properties.

In our research we are, on the one hand, interested in how defects develop in the on-surface chemical synthesis process, and how they can be avoided in order to achieve higher structural perfection. On the other hand we explore how specific structural motifs influence the electronic or magnetic properties of carbon-based nanostructures. Examples of our research range from understanding the importance of precursor surface diffusion and coupling efficiency with respect to the formation of structurally perfect porous graphene, the stability of the anti-ferromagnetic zigzag edge states upon edge defect introduction, or the pseudo-spin and momentum dependence in electron scattering at vacancy defects of single walled carbon nanotubes (SWNTs). We apply experimental tools such as scanning tunneling microscopy (STM) and spectroscopy (STS) as well as high-resolution noncontact AFM in combination with computational approaches at different levels of theory.

Atomically precise graphene nanoribbons

Not just since the 2010 Nobel prize, graphene is the rising star on the horizon of novel electronic materials. It combines all the outstanding electronic properties already unveiled in SWNT with CMOS compatible processability. Whereas properties such as high charge carrier mobility and ballistic transport relate to graphene as well as to carbon nanotubes, graphene has a distinctive new feature: The edges!

When graphene is “cut” into narrow ribbons the confinement leads to the opening of a band gap and the semimetal becomes a true semiconductor. However, in order to obtain a band gap comparable to the one of silicon, the width of the ribbon needs to be 2 nm or less. To produce such narrow ribbons by lithography and etching is at the moment out of reach. Furthermore it is obvious that the precision of the edge structure will be crucial to determine the electronic quality. In view of these challenges we have pioneered, in collaboration with the group of Prof. Klaus Müllen in Mainz (Germany), the bottom-up synthesis of atomically precise graphene nanoribbons by surface assisted polymerization and cyclodehydrogenation of molecular precursors. Our current focus in the field of nanoribbon research is on the one hand the extension of our abilities to synthesize ribbons with different shapes, widths and edge structures and hence electronic properties. We do this by exploring the polymerization of different molecular precursors and searching for new coupling schemes. The produced ribbons are investigated in detail with respect to their physical properties, with a strong emphasis on electronic properties. On the other hand, we target the integration of graphene nanoribbons in prototypical electronic devices, which involves the development and optimization of technologies for efficient transfer and contacting of the ribbons.

Electron field emission of carbon nanotubes

Among the many extraordinary properties of CNTs one of the most apparent is their very high aspect ratio. Combined with their small dimension and with the very high current densities, that can be sustained by CNTs, this qualifies them as excellent field emission electron sources.

When a conductive tip like structure is exposed to an electric field, this field will be amplified at its apex. Depending on the tip geometry this amplification can reach several orders of magnitude. In this way it is possible to attain field strengths exceeding 30 million Volts/Meter, which are required to eject electrons from the tip by field emission. The investigation and development of field emission cathodes, not only based on CNT, has a long tradition in our laboratory. We have developed 3 generations of Scanning Anode Field Emission Microscopes (SAFEM) for the microscopic investigation of the properties of planar field emission cathodes. The SAFEM allows the measurement of the statistical distribution of all relevant emission parameters from a large set of emission sites. This information in combination with extensive modeling allows a targeted identification of the limiting factors regarding the overall cathode performance. This information is invaluable in the process of cathode development and optimization. This competency of our laboratory is expressed by numerous collaborations with industrial partners such as Motorola, SONY, Thales and Philips. We restrict these activities however not only to technology transfer with industry, but are open for collaborations with academic partners in the search of new electron emission materials.

ADDRESS

nanotech@surfaces Laboratory

Empa, Swiss Federal Laboratories for

Materials Science & Technology

Ueberlandstrasse 129

8600 Duebendorf

Switzerland

How to find us :

printable map, interactive map

 

CONTACT

Prof. Dr. Roman Fasel, Head of Laboratory

Dr. Oliver Gröning, Deputy head

Ms. Christine Tran, Head assistant

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