Non-Thermal Activation of Reactions of Organic Molecules on Si(001)- Molecular Beam and Scanning Tunneling Microscopy Experiments
In the presented cumulative thesis, the reaction of different organic molecules on Si(001) surface are studied by using molecular beam techniques and scanning tunneling microscopy. The obtained results are discussed with a special focus on the dynamics of the reactions, the underlying potential ener...
Scanning tunneling microscopy Molecular beam Semiconductor Surface Science Adsorption Dynamics Tip electric field Potential energy curve
Rastertunnelmikroskopie Molekularstrahl Halbleiter Oberflächenphysik Adsorptionsdynamik STM-Spitze Elektrisches Feld Potentielle Energiekurve
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|Summary:||In the presented cumulative thesis, the reaction of different organic molecules on Si(001) surface are studied by using molecular beam techniques and scanning tunneling microscopy. The obtained results are discussed with a special focus on the dynamics of the reactions, the underlying potential energy curve, and the resulting possibility to control these reactions.
In the first part, we used molecular beam experiments to investigate the adsorption pathway of methanol, water, and acetylene on Si(001). We found that the adsorption of these three molecules proceeds via an intermediate state. The binding energy of the intermediate state e_d depends on the configuration of the molecule and the resulting type of the intermediate. The difference between e_d and the conversion barrier from the intermediate into the final state e_a, is found to be e_d − e_a = 0.37, 0.36, 0.16 eV for methanol, water, and acetylene respectively. By comparing our results to well-established systems such as diethyl ether and ethylene on Si(001), we conclude on a low activated and fast conversion process from the intermediate into the final state for the three molecules. The conversion in the case of methanol and water proceeds via a proton-transfer reaction, which is known to be more facile than the O-C cleavage in the case of ether adsorption. The fast conversion for acetylene is attributed to the weak binding energy of the molecule in the intermediate, which involves the weaker three center bond between the Pi electrons of the molecule and the positively charged D_down state of the silicon dimer in combination with a generally high reactivity of the triple bond. Based on these results on the energetics with substantial differences for these systems, a comprehensive study of the adsorption dynamics of these molecules was carried out by determining the dependence of the initial sticking coefficient s0 on
the kinetic energy of the adsorbing molecules, E_kin. We found that the main factor governing the adsorption dynamics is the type of the intermediate state, regardless the details of the adsorbed molecules or their possible further reaction, i.e., the conversion to the final state. The different dependence of the s_0 on the E_kin allows to control the reaction with respect to adsorption into the intermediate state.
In the second part of the thesis, a further manipulation of the adsorbates after the conversion from the intermediate into the final state is discussed. The system of
diethyl ether on Si(001) is taken as an example as it shows two different configurations in the final state. Mutual conversions between the two configurations is observed by using scanning tunneling microscopy. This conversion between the two sub-states is attributed to a field-assisted thermally activated hopping of the alkyl fragment of the cleaved diethyl ether on top of one Si dimer. The relative hopping rate shows clear dependence on the bias voltage, but remains constant when varying the tunneling current. This observation is attributed to a reduction of the energy barrier by depolarizing the C-Si bond by the electric field of the negative STM tip. The energetic contribution of the field was found to be at least 0.3 eV, which correlates well with the value of the electric field at the respective distance between tip and sample.|
|Physical Description:||53 Pages|