Polyaromatic molecules : structure and reactivity on copper single crystals

Abstract

In recent years there has been a significant interest in the use of polyaromatic molecules in (opto-) electronic devices. This family of molecules tends to be vibrantly coloured and have interesting optical and semiconductor properties hence a specific interest in display devices. In this thesis, thin films of organic polyaromatic molecules 3,4,9,10- perylenetetracarboxylic dianhydride (PTCDA), perylene and tetracene on copper single crystals were investigated. The molecules were deposited in Ultra High Vacuum (UHV) onto clean copper crystals under a number of different substrate conditions where they formed ordered thin films. The films were characterised using scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), thermal desorption, X-ray photoelectron spectroscopy (XPS) and vibrational spectroscopies, electron energy loss spectroscopy (EELS) and reflection/absorption infra-red spectroscopy (RAIRS). PTCDA, the most widely investigated of the molecules, was analysed on Cu(211) and (110) surfaces and was shown to have strong interactions with the copper rows of the substrates strongly influencing the structures formed. XPS results indicate a strong bond to the surface through loss of the anhydride oxygen atoms with subsequent layers physisorbed. On Cu(211) PTCDA formed a unit cell of a = 16.1 Å, b = 24. 1Å, β = 85°. On Cu(110), PTCDA forms a unit cell of a= 18.9 Å, b = 18.9 Å, β = 75°. Perylene proved to be the most complex molecule with annealing of the initial monolayer vital in determining the structure of the subsequent layers. A highly ordered structure with molecular rows far larger than the terraces of the underlying copper could be formed after heating as opposed to a less ordered structure. The three thin film structures of perylene recorded were: α -phase: a = 11.0 Å, b = 11.0 Å, β = 77°, β -phase: a = 10.84Å, b = 10.84Å, β = 65° and γ-phase: a = 20.7 Å, b = 19.3 Å, β = 90° all of which are significantly different from the bulk crystal structures. Photoluminescence experiments show that the different multilayer structures lead to different wavelengths of emission due to different intermolecular interactions within the films. The recorded photon emission results from a excimer due to overlapping of π-orbitals in the novel crystal structure. Below two monolayers, no emission is recorded due to quenching by the substrate. Tetracene forms only monolayers at room temperature. If cooled, multilayers can be formed. Two tetracene monolayer structures were observed, the first driven by interactions with the copper rows produced a centred 12 x 2 structure, and the second, formed by annealing the first, involved molecules which were close packed and interdigitated, giving a compressed primitive 6 x 1.83 structure.