Electron Microscopy on Tailored CNT / 3d Transition Metal Interfaces

In the field of carbon nanotube (CNT) research, the trend goes from investigations of fundamental properties towards more application related projects. In this context, some shortcomings in the understanding of their behaviour become more apparent. For the implementation of CNTs in electronic devices the observed contact resistance towards metals is one of the limiting obstacles. To overcome this contact resistance the understanding of the junctions needs to be improved and new interface formation techniques need to be developed. In the frame of this project these problems have been addressed with electron microscopy analysis and tailoring of the interface of 3d transition metals towards CNTs. The interfaces of Cu, Ni, Cr and V towards multi-walled carbon nanotubes (MWCNTs) are analysed by electron microscopy techniques. A high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) analysis showed a dependence of the wetting behaviour of the metals from the number of free states in their valence d-orbitals. Where the more free electron states are in the valence d-orbitals the better the observed wetting behaviour. Between Cr and V no difference could be observed in the wetting behaviour, which is also expected, as both metals have all d-orbital states free that are concerned in the interaction with the CNTs π-system. A more detailed analysis was done of the interface of Cu and Ni towards MWCNTs. For those two metals a new mild chemical one-step deposition technique has been adapted. A 1 μm thick metal layer could be deposited on forests of vertically aligned MWCNTs (VAMWCNTs), simultaneously the MWCNTs are covered with homogeneously distributed metal nanoparticles over a length of about 200 μm. For Cu, but not for Ni, filled MWCNTs (that have not previously been opened deliberately) are observed, leading to 5 nm thick Cu-rods reaching lengths of up to 250 nm. The deposition technique does not destroy the sp2 system of the graphitic CNT-walls, as it does not require any aggressive chemicals, but only heating to 140 ◦C for 48 h in benzyl alcohol. This mild deposition technique made it possible to experimentally show the stronger interaction of Ni nanoparticles with the CNTwalls, compared to Cu. With STEM electron energy-loss spectroscopy (EELS) C K-edge maps, rehybridisation of the outermost graphitic wall of the MWCNTs could be shown at the interface towards Ni nanoparticles, while no such rehybridisation could be observed towards Cu. These findings are confirmed with HAADF STEM images showing a better coverage of the MWCNTs with Ni, compared to Cu, when the MWCNTs were dispersed during the reaction. A better electron transparency of the chemically formed contacts, compared to sputtered ones, could be demonstrated with field emission measurements. The reduced contact resistance makes this joining method interesting for CNT applications in electronic devices. The homogeneous metal nanoparticle functionalization makes the forests of VAMWCNTs very promising for applications in pseudocapacitor electrodes. The advantages of the new chemical technique, compared to sputtering deposition, could also be demonstrated in high resolution transmis-