By exposing single walled carbon nanotubes (SWNT) to ion bombardment or atomic radicals we deliberately create local modifications of their structure by creating vacancies, chemisorption sites or combinations of both. These defects can change the local electronic properties of the SWNT considerably. The most prominent effects that can be expected are enhanced electron scattering, introduction of impurity states, local or global doping and creation of magnetic centers. We investigate these electronic modifications mainly by scanning tunneling spectroscopy (STS). Here we are especially interested in the occurrence of new electronic states localized spatially as well as in energy and in the details of the electron scattering at these defects e.g. the pseudo-spin and momentum dependence. The tool which gives us the required experimental access to these effects is Fourier transformed STS at low temperatures. Whereas currently this research is still very much focused on SWNT, in a next phase we want to extend these studies to graphene nanoribbons.

When graphene is “cut” into narrow ribbons the confinement leads to the opening of a gap and the semi-metal can become 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 smaller. 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 of molecular precursors. Our current focus in the field of nanoribbon research is on one hand the expansion of our abilities to synthesize ribbons with different structures. We do this by exploring the polymerization of different molecular precursors and searching for new coupling schemes. The second line of study is the investigation of the physical properties of the produced ribbons with strong emphasis on the electronic properties.

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.