Real-space investigation of surface reactions of organic adsorbates on silicon surfaces using a combination of fast laser heating and scanning tunneling microscopy

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The adsorption of organic molecules on semiconductor surfaces proceeds in most cases via an intermediate state which controls the reaction and the final product. For molecules containing a heteroatom such as nitrogen or oxygen, the intermediate is typically formed by a dative bond between the heteroatom and the unoccupied dangling bond state of the surface. The reaction further proceeds via the so-called conversion barrier into the covalently bound final state on the surface. Conventional experiments typical address the pathway with the lowest barrier for conversion between intermediate and final state only; higher-activated reaction channels contribute significantly to the reaction rate only at temperatures at which the overall rate is too high compared to the typical time scale of STM or spectroscopy experiments. In the proposed study, the potential energy surface of model-type organic molecules, such as alcohols, amines, and ethers, on Si(001) will be investigated with focus on the higher-activated reaction channels. In order to do so, we will heat the surface by means of laser-induced heating pulses on the nanosecond timescale; between these fast heating cycles, rearrangement on the surface will be followed by means of STM. The fast heating cycles are thus decoupled from the slow measurement cycle and rates as high as 10^9 1/s can be observed with atomic precision. In consequence, a wider range of temperature is accessible by the experiments; from the distribution of final states as a function of temperature the activation energies for the respective reaction channels are then deduced. Thus not only the lowest-activated reaction channel but a larger part of the potential energy surface can be accessed. We choose the molecules in such a way that the different variables a separately addressed. E.g., in the case of the adsorption of ether molecules, the influence of the local surface electronic structure on the final configurations will be most important since the reaction will predominantly take place via O-C cleavage. On the other hand, in the case of the adsorption of alcohols, the competition between O-H and O-C cleavage will be in the focus of our studies.The molecules to be investigated can be seen as model systems for a wide range of adsorbate systems on Si(001) and the results should thus lead to a better understanding of surface reactions on semiconductor surfaces in general, especially with respect to the underlying driving forces. Furthermore, the results are important with respect to the functionalization of semiconductor surfaces with multi-functional organic molecules: In that case, control of the reaction, e.g., by the adsorption temperature, requires the knowledge on the higher-activated reaction channels.