Ether-Moleküle und bifunktionale Cyclooctine auf Si(001) - vom mikroskopischen Verständnis der Adsorption zur chemoselektiven Anbindung
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Date
2016-09-29
Authors
Publisher
Philipps-Universität Marburg
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Abstract
In this thesis, the functionalization of semiconductor surfaces with
organic molecules was investigated. By analyzing the adsorption
configurations, kinetics, and dynamics of ether molecules on
Si(001), a understanding of the adsorption process on the
microscopic level could be obtained. Based on these results, further
studies of our group, and methods adopted from biological chemistry,
a concept for the chemoselective adsorption of bifunctional organic
molecules on Si(001) was developed.
For tetrahydrofuran (THF) and diethyl ether (Et2O), it was
shown that these ether molecules adsorb in a nonactivated process
via an intermediate state on Si(001). In the datively bonded
intermediate state, electron density of the oxygen lone pairs is
donated into an empty Ddown-state of a zwitterionic silicon
dimer. At 80 K, the conversion rate into the final state is
negligible, the intermediate state could thus be characterized in
STM-, XPS- and UPS-experiments. Thermal activation leads to the
cleavage of the O-C bond of the ether group and thus the formation
of covalent Si-O and Si-C bonds on two neighboring dimer rows. As both, the ringlike and the linear
ether molecule adsorb over two neighboring dimer rows, arguments
based on a geometrically more favorable final state can be excluded
for this unusual final state. Instead, this final state might be
preferred by the nature of the surface mediated reaction mechanism,
e.g., due to an interaction with the Dup-state of the
neighboring dimer row. The fragmentation of diethyl ether into two
parts further leads to the observation of a tip induced
hopping-process. The key parameters of
the potential energy curve of Et2O, the barrier Ea
for the conversion into the final state and the binding energy
Ed of the intermediate state, were measured by a
combination of optical second-harmonic generation and molecular beam
techniques: Using optical second-harmonic generation, the conversion
rates from the intermediate state into the final state were measured
as a function of surface temperature; the barrier
Ea=0.38+-0.05 eV was
determined. By measuring the initial sticking coefficient as a
function of surface temperature, the energy difference between
desorption and conversion barrier,
Ed-Ea=0.24+-0.03 eV, and thus the binding
energy of the datively bonded intermediate state
Ed=0.62+-0.08 eV
was determined.
The insights obtained for these model systems were applied to the
functionalization of semiconductor surfaces with bifunctional
organic molecules. The major hinderance for the chemoselective
adsorption of such bifunctional molecules is the high reactivity of
the Si(001) surface. In STM- and XPS-experiments, a chemoselective
adsorption process of cyclooctyne ether and cyclooctyne ester with
the strained triple bond was observed; the second functionality
(ether, ester) remained unreacted. This chemoselectivity can be
explained with the qualitatively different adsorption dynamics of
both functional groups: The strained triple bond of cyclooctyne
adsorbs directly into the final state of the underlying potential
energy curve. If the second functionality reaches the surface first,
the molecule is trapped in an intermediate state with a finite
lifetime. In the intermediate state, the molecule can sample the
surface with the strained triple bond, thus enabling a conversion
into the direct pathway of the strained triple bond with the
concomitant cleavage of the weak bonding of the second
functionality. The qualitatively different potential energy curves
thus enable the chemoselective adsorption of bifunctional molecules
Si(001). In analogy to bio-orthogonal chemistry, we term this type
of selectivity surface-orthogonal chemistry. Such
chemoselective adsorption of bifunctional molecules is the first
step for building molecular architectures on semiconductor surfaces.
Review
Metadata
Contributors
Supervisor:
Dates
Created: 2015Issued: 2016-09-29
Faculty
Fachbereich Physik
Language
ger
Data types
DoctoralThesis
Keywords
SemiconductorPhotoelectron SpectroscopyNonlinear OpticsMolekularstrahltechnik ,OberflächenphysikScanning Tunneling MicroscopySurface Science
DFG-subjects
Nichtlineare OptikHalbleiterPhotoelektronenspektroskopieRastertunnelmikroskopie
DDC-Numbers
530
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Reutzel, Marcel: Ether-Moleküle und bifunktionale Cyclooctine auf Si(001) - vom mikroskopischen Verständnis der Adsorption zur chemoselektiven Anbindung. : Philipps-Universität Marburg 2016-09-29. DOI: https://doi.org/10.17192/z2016.0093.
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This item has been published with the following license: In Copyright