Prof.dr. Paola Ayala: Understand the governing absorption mechanisms vs. doping in ultra-clean carbon nanotubes
|Wanneer:||vr 28-02-2014 14:00 - 15:00|
Establishing the prerequisites for studying the rich low-dimensional physics of pristine vs. doped Single-Walled Carbon Nanotubes (SWCNTs) using photoemission and X-ray absorption was not possible for a long time due to the material quality available until recent years. We will focus this discussion on our recent progress towards the identification of the changes in the site selective electronic structure within various types of SWCNTs metallicity pure SWCNTs and substitutionally doped SWCNTs (with B and N). N is the most studied experimentally as substitutional dopant. However, it can also be part of molecules like NO2 or NO, which can present distinctive adsorption kinetics and alter the functionalization of a tube. In this context, the mechanisms governing NOx-adsorption had always been thought to be related to chemisorption but we have compellingly proved that this mechanism must be rethought. Strikingly, Franck Condon satellites, never detected before in nanotube-NOx systems were resolved in the N1s X-ray absorption signal revealing a weak chemisorption intrinsically related to NO dimer molecules.
I shall then focus on substitutional doping of B, contemplating the possibility of the formation of an acceptor state in the SWCNT´s electronic structure even at very low B concentrations. Quantifying the amount and bonding environments of B in the SWCNT lattice has been challenging, particularly when the doping concentration is below 1 at%. In B doped samples, EELS studies carried out in a TEM failed to detect the substituted B in the C network. For this reason, B induced changes in the intensity of the RBM and shifts in G' band have been used as indirect proof of substituted B in the SWCNT lattice so far. The case of ultra-low doping (comparable to the doping levels in Si semiconductor technology) will be discussed based on X-ray absorption spectroscopy findings.