Organic materials with fascinating/intriguing electronic properties have been the driving force for many research activities in the past, and in particular for important progress in materials science covering both new functional materials as well as theoretical developments. In addition, charge transfer, i. e., the addition or removal of charges to or from molecules in organic solids is one route to modify and control their electronic properties. Recently, the discovery of superconductivity in several alkali metal intercalated hydrocarbon systems (picene, phenanthrene, coronene and 1,2;8,9-dibenzopentacene) with rather high transition temperatures has opened a new chapter in organic material science as well as solid-state physics.
The search for a microscopic understanding of the mechanism that drives materials superconducting always has initiated a large number of scientific activities, and there are numerous examples where these activities have provided major advancement. A basic foundation of this understanding is the knowledge of the electronic properties of the material under investigation.
In this context, this thesis reports first, very detailed insight into the electronic structure of both undoped as well as potassium doped picene, coronene and 1,2;8,9-dibenzopentacene using electron energy-loss spectroscopy (EELS) as main experimental method. Additionally, also photoemission spectroscopy experiments have been performed to investigate the occupied electronic density of states close to the chemical potential. In order to learn more about the electronic structure we have compared the results we obtained from EELS and photoemission spectroscopy with theoretical calculations based on Density functional theory (DFT) using the local-density approximation (LDA).
We identify the peculiar case of very close lying conduction bands that upon doping harbour the electrons that form the Cooper-pairs in the superconducting state. Moreover, the presented data display substantial changes in the electronic excitation spectrum upon doping, whereas in the doped case the appearance of one new peak (for picene) and several new peaks (for coronene and 1,2;8,9-dibenzopentacene) in the former optical gap is reported. By using a Kramers–Kronig analysis (KKA) it is possible to gain information about the nature of this doping introduced excitations. In particular, in case of picene, the new low energy feature can be assigned to a charge carrier plasmon. Interestingly, this plasmon disperses negatively upon increasing momentum transfer, which deviates significantly from the traditional picture of metals based on the homogeneous electron gas. The comparison with calculations of the loss function of potassium intercalated picene shows how this finding is the result of the competition between metallicity and electronic localization on the molecular units.
Furthermore, core level excitation measurements show the reduction of the lowest lying C 1s excitation feature, which clearly demonstrates that potassium intercalation leads to a filling of the conduction bands with electrons. Additionally, the measurements of potassium intercalated 1,2;8,9-dibenzopentacene clearly indicate the formation of particular doped phases with compositions K_xdibenzopentacene (x = 1, 2, 3), whereas the data suggest that K_1dibenzopentacene has an insulating ground state with an energy gap of about 0.9 eV, while K_2dibenzopentacene and K_3dibenzopentacene might well be metallic, because of the absent of an energy gap in the electronic excitation spectra.
Interestingly, a comparison of the photoemission as well as EELS spectra of undoped 1,2;8,9-dibenzopentacene and pentacene reveal that the electronic states close to the Fermi level and the electronic excitation spectra of the two materials are extremely similar, which is due to the fact, that the additional two benzene rings in 1,2;8,9-dibenzopentacene virtually do not contribute to the delocalized pi molecular orbitals close to the Fermi level. This close electronic similarity is in contrast to the behavior upon potassium doping, where evidence for a Mott state has been reported in the case of pentacene.
A comparison of the low energy excitation spectra of chrysene with picene (phenacenes) as well as tetracene with pentacene (acenes) crystals reveal a significant difference between the former and the latter two materials. While for the phenacenes (zigzag arrangement) the excitation onset is characterized by up to five weak excitation features with only small anisotropy and without visible Davydov splitting within the a*, b*-planes, the acene (linear arrangement) spectra are dominated by a large excitation close to the onset and a sizable Davydov splitting. The presented data show further that the spectral shape of the pentacene excitation spectrum provides clear evidence for a large admixture of molecular Frenkel-type excitons with charge-transfer excitations resulting in excited states with a significantly mixed character. This conclusion is in good agreement with recent advanced calculations which predicted a charge-transfer admixture to the lowest singlet excitation which is significantly dependent upon the length of the acene molecules. Moreover, also for picene and chrysene we observe differences which point towards an increased charge-transfer contribution to the singlet excitation spectrum in the former.
Finally, investigations of the electronic properties of undoped and potassium doped chrysene, a close relative of picene, show that the doping introduced changes are in a similar range such as observed in case of picene. Interestingly, due to the analogy between the observed changes in the electronic structure upon potassium doping between chrysene and picene and further similarity in the crystal structure we speculate that chrysene is a promising candidate for another aromatic hydrocabon superconductor.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:26700 |
| Date | 22 February 2013 |
| Creators | Roth, Friedrich |
| Contributors | Knupfer, Martin, Büchner, Bernd, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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