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Fullerenes are nanomaterials that have properties intermediate between those of large molecules and those of bulk materials. They are becoming increasingly important as electron-acceptor constituents of organic solar cells and doped fullerene crystals show the highest critical temperatures of any “organic” superconductors. In spite of their considerable interest as new organic electronic materials, surprisingly little is known about the fundamental properties of the excited electronic states of the molecules and how these develop into band structure as aggregates or crystals are formed. Evidence has recently been found, using scanning tunnel microscopy, for the presence of diffuse hydrogenic orbitals associated with fullerenes deposited on a metal substrate. These socalled “superatom” states (SAMO) are distinct from the molecular s- and p-orbitals that form through hybridization of the s and p orbitals on the carbon atoms. Instead of being bound to individual carbon atoms the SAMOs assume the radial and angular distributions of spherical harmonic functions that are defined by the central potential of the hollow C60
core and thus look like large, relatively simple atomic orbitals. When the fullerene molecules self-assemble into chains, the diffuse orbitals are seen to readily combine into delocalized bands and are predicted to play an important role in defining the electronic properties of fullerene-based materials.
We have recently found evidence for the presence of these SAMOs in gas phase photoelectron spectroscopy of fullerenes with fs laser pulses. Gas phase studies have the potential to provide more detailed information about these unusual molecular states and will provide a stringent test of theoretical predictions.
This project will expand on the initial investigations to study the properties of SAMOs for a range of hollow fullerene-based molecular systems (functionalized fullerenes, endohedral fullerenes, small carbon nanotubes). The aim will be to understand how the properties of the orbitals can be tuned by modifying the fullerene cage, ultimately leading to the development of materials with specific electronic properties. The project will combine experiment and theory. Advanced experimental techniques such as velocity map imaging photoelectron spectroscopy using amplified, wavelength-tunable fs laser pulses will be used to probe the properties of the SAMO excited states. The experimental work will be complemented by theoretical calculations of photoelectron angular distributions using time-dependent density functional theory and will involve spending some time in the group of our theoretical collaborators in Belgium.
Successful applicants should have, or be expecting to achieve, a first or upper second class Honours degree, or equivalent, in chemistry, physics or chemical physics.
Funding Notes:
Funded by the Leverhulme Trust
References:
“Angular-resolved Photoelectron Spectroscopy of Superatom Orbitals of Fullerenes”, J.O. Johansson, G.G. Henderson, F. Remacle, E.E.B. Campbell, Phys. Rev. Lett. 108 (2012) 173401
“Anisotropic hot electron emission from fullerenes”, J.O. Johansson, J. Fedor, M. Goto, M. Kjellberg, J. Stenfalk, G.G. Henderson, E.E.B. Campbell, K. Hansen, J. Chem. Phys. 136 (2012) 164301
Research Assessment Exercise (RAE) 2008 Results