Probing the electronic and geometric structure of adsorbed porphyrins and fullerenes

Taylor, Simon (2016) Probing the electronic and geometric structure of adsorbed porphyrins and fullerenes. PhD thesis, University of Nottingham.

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In this thesis, the geometric and electronic structure of tetra(4-bromophenyl) porphyrin (Br4TPP) and H2O@C60/C60 molecules are studied on Cu, Au and Ag surfaces. Developing an understanding of the physics at the single molecule level is not only important to the nanoscience field, but also in future progression towards nanoscale devices.

Adsorption of Br4TPP molecules on Au(111) and Cu(111) substrates reveal two conformers. It was previously understood that chemisorption drove conformational changes of the molecules with respect to the substrate, but work resulting from studies in this thesis using STM for single molecule manipulation, combined with DFT and MD simulations concludes that large conformational changes are dominated by van der Waals interactions. The mechanochemical response of Br4TPP was also investigated, and by using NC-AFM it was found that there was a large difference in requirements for lateral force manipulation between the two conformers. Dispersion corrected DFT calculations emphasise significant differences in diffusion barrier for each conformer. STM was also used to reproducibly de-brominate Br4TPP in a controlled sequence, and the electronic mapping of molecular orbitals is also presented.

In the second half of this thesis, H2O@C60/C60 has been investigated on Cu(111) and H2O@C60 on Ag(111) surfaces using STM/NC-AFM and XPS/NIXSW respectively. Both STM and NC-AFM are unable to distinguish between fillled and empty C60 cages. Remarkably, H2O@C60 behaves similarly to C60 during molecule-induced surface reconstruction on Cu(111). A careful NC-AFM study shows apparent intermolecular bonding between neighbouring H2O@C60/C60 molecules. This highlights the fact that intermolecular artifacts are common across all SPM studies. Due to the shortfall of STM and NC-AFM in this particular case, XPS was used to further probe the C60 cages to see if they were encapsulated with H2O, and was confirmed by the O1s peak at 532eV. Unsurprisingly, valence band spectra reiterated the similarity between filled and empty C60 molecules. NIXSW was used to attempt to triangulate the O1s signal within the C60 cage. Initial calculations suggest the encapsulated water molecule is located 0.56nm relative to the Ag(111) substrate, which is approximately central in inside the C60 cage.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Moriarty, P.J.
Beton, P.H.
Keywords: Scanning Tunnelling Microscopy, Atomic Force Microscopy, single molecule manipulation, X-ray Photoelectron Spectroscopy, X-ray Standing Wavefield, Porphyrin, Fullerene, Endohedral Fullerene, Low Energy Electron Diffraction
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 35487
Depositing User: Taylor, Simon
Date Deposited: 02 Mar 2017 13:25
Last Modified: 15 Oct 2017 14:58

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