Surface modifications and functionalization of highly aligned multi-walled carbon nanotubes

Highly aligned multi-wall carbon nanotubes (MWCNTs) prepared as vertical arrays onto a silicon substrate, were functionalized and ion-bombarded in clean controlled conditions, to modify their electronic properties. We studied their morphology and electronic states, before and after induced modifications, by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Scanning Electron Microscopy (SEM). Ion-beam induced modifications were obtained by medium energy (3 keV) noble gas ions of different mass (He, Ne, Ar), so to generate controlled defects without causing chemical interaction with the carbon matrix. Ion irradiation leads to change in morphology, deformation of the carbon (C) honeycomb lattice and different structural defects in MWCNTs. One of the major effects is the production of bond distortions, as determined by micro-Raman and micro-X-ray photoelectron spectroscopy. We observe an increase of sp3 distorted bonds at higher binding energy with respect to the expected sp2 associated signal of the C 1s core level, and increase of dangling bonds. Furthermore, the surface damage as determined by the XPS C 1s core level is equivalent upon bombarding with ions of different mass, while the impact and density of defects in the lattice of the MWCNTs, as determined by micro-Raman and SEM, are dependent on the bombarding ion mass, huge damage for lighter helium ions, smaller damage for heavier argon ions. CNT functionalization has been achieved by exposing the nanotubes to atomic deuterium in clean and controlled ultra-high-vacuum conditions. Bonding of deuterium (D) atoms on the C mesh of the nanotubes has been established, so to modify the electronic response, without changing the average morphology of the CNT arrays. X-ray photoelectron spectroscopy of the C 1s core level provides clear evidence of deuterium and carbon chemical interaction, by evidencing the establishment of sp3 bonds with suppression of the π plasmon excitation. We demonstrate ∼70 at. % D:C percentage of deuterated carbon atoms in the MWCNTs. The electronic structure modification induced by D chemisorption also affects the energy loss spectrum extrinsically excited by the outgoing photoelectrons, showing quenching of the π-plasmon. Ultraviolet photoelectron spectroscopy showed the opening of an energy gap in the valence band of the D-MWCNTs, with the valence band maximum at about 3.2 eV below the Fermi level. The bond distortion is also evidenced by the modification of the Raman response at the deuterated nanotubes. These results on controlled increase of sp3 distorted bonds via ion bombardment and increasing D:C bonds via atomic deuterium irradiation on the MWCNTs, open new functionalization perspective. This work represents that the molecular cracking of D2 in an ultra-high vacuum is an efficient way to obtain stable, homogeneous and high uptake of deuterium atoms with minimal induction of defects. Ion bombardment opens up the ways to improve and increase atomic deuterium uptake on ion-bombarded MWCNTs, towards potential applications in hydrogen storage into solid state.