The adsorption of organic molecules on metal and semiconductor surfaces.
PhD thesis, University of Glasgow.
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The first part of this project focused on the effects of coadsorbates on the adsorption of organic molecules bonded to Cu(111). First, the influence of coadsorbed CO on the structure and bonding of thiophene was investigated using a combination of LEED, AES, TPD and synchrotron-based NIXSW and NEXAFS techniques. It was found that the coadsorption of CO does not induce an ordering of the disordered chemisorbed thiophene layer, in contrast to the behaviour of related benzene coadsorption systems where CO induces ordering of benzene disordered layers. Detailed analysis of our NIXSW and NEXAFS data showed that CO and thiophene both adopt stop adsorption sites within the coadsorbed overlayer, the same site adopted by both molecules in pure layers, and the orientation of the thiophene molecules within the coadsorbed layer is also similar to that adopted in the pure layer. The results of our NIXSW measurements, however, showed that CO molecules within the coadsorbed layer are tilted, which contrasts with a linear geometry observed in pure CO layers of a similar coverage. We propose that the lack of any significant coopertive effects between the CO and thiophene within the coadsorbed overlayers is due to the relatively weak adsorbate - substrate interactions.
The second coadsorption study concerned the influence of sulfur precovered Cu(111) surfaces on the adsorption of thiophene, benzene, cyclohexene and cyclohexene molecules using TPD, AES, LEED, XPS, and UPS. The characterisation experiments established that all four molecules are reversibly adsorbed on all the surfaces studied, and more importantly our TPD and UPS data clearly showed that the co-adsorption of sulfur influences the bonding of each of the probe molecules in particular ways. At a pre-coverage of 0.12 ML of sulfur, the desorption of thiophene and benzene in our TPD experiments is shifted to higher temperatures, clearly showing that co-adsorbed sulfur at this precise coverage stabilises the adsorption of the aromatic molecules. With increasing S pre-coverage, the stabilising effects of similar on these two molecules diminish and by ca. 0.33 ML of sulfur destabilisation takes place. The stabilisation of cyclohexene is also effective but occurred at higher sulfur coverages (up to qS = 0.33 ML). We believe that steric blocking by sulfur adatoms is responsible for the destabilisation of thiophene, benzene and cyclohexene. For cyclohexene, however, stabilisation does not occur and the appearance of a new desorption peak indicates the formation of a less stable adsorption state at all sulfur coverages studied. We believe that the stabilisation of thiophene, benzene and cyclohexene on Cu(111) in presence of sulfur can be explained in terms of a simple electrostatic model. The formation of induced anti-parallel dipoles, which are caused by the charge transfer from the unsaturated molecules to the substrate and from the substrate to sulfur adatoms, provoke an increase of charge donation from the p-levels of the unsaturated molecules into unoccupied levels of the substrate. This model illustrates the enhancement of the bond strength of the p-bonded species to Cu(111) experimentally observed. A similar electrostatic model can also be used to describe the destabilisation of the cyclohexene molecules. The electrostatic field set up by sulfur results in the formation of induced parallel dipoles which reduce the charge transfer from the substrate to the saturated molecule and destabilise and saturated molecule.
In the second part of this project, the surface reactivity of thiophene, benzene and benzonitrile with Si(100)-(2x1), Si(111)-(7x7) and Ge(100)-(2x1) was investigated using synchrotron-based valence band photoemission. To the best of our knowledge, the adsorption of thiophene and benzonitrile on the Ge(100)-(2x1) surface has not been reported in the literature. For the three molecules studied, our experimental results show that the relative reaction rates for the (2x1) semiconductor surfaces studied give Si(100) > Ge(100), with Si(100) being more reactive than the Ge(100) surface as a result of the higher degree of polarisation within the Si dimers than the Ge dimers. The detailed analysis of the collected valence band data reveals that the adsorption of thiophene, benzene and benzonitrile on all three semiconductor surfaces leads to the formation of 2,5-dihydrothiophene- 1,4-cyclohexadiene- and benzoimine-like moieties, respectively.
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