Surface-confined dehalogenation reactions
are versatile bottom-up
approaches for the synthesis of carbon-based nanostructures with predefined
chemical properties. However, for devices generally requiring low-conductivity
substrates, potential applications are so far severely hampered by
the necessity of a metallic surface to catalyze the reactions. In
this work we report the synthesis of ordered arrays of poly(p-phenylene) chains on the surface of semiconducting TiO2(110) via a dehalogenative homocoupling of 4,4″-dibromoterphenyl
precursors. The supramolecular phase is clearly distinguished from
the polymeric one using low-energy electron diffraction and scanning
tunneling microscopy as the substrate temperature used for deposition
is varied. X-ray photoelectron spectroscopy of C 1s and Br 3d core
levels traces the temperature of the onset of dehalogenation to around
475 K. Moreover, angle-resolved photoemission spectroscopy and tight-binding
calculations identify a highly dispersive band characteristic of a
substantial overlap between the precursor’s π states
along the polymer, considered as the fingerprint of a successful polymerization.
Thus, these results establish the first spectroscopic evidence that
atomically precise carbon-based nanostructures can readily be synthesized
on top of a transition-metal oxide surface, opening the prospect for
the bottom-up production of novel molecule–semiconductor devices.
Understanding
how specific atom sites on metal surfaces lower the
energy barrier for chemical reactions is vital in catalysis. Studies
on simplified model systems have shown that atoms arranged as steps
on the surface play an important role in catalytic reactions, but
a direct comparison of how the light-off temperature is affected by
the atom orientation on the step has not yet been possible due to
methodological constraints. Here we report in situ spatially resolved
measurements of the CO2 production over a cylindrical-shaped
Pd catalyst and show that the light-off temperature at different parts
of the crystal depends on the step orientation of the two types of
steps (named A and B). Our finding is supported by density functional
theory calculations, revealing that the steps, in contrast to what
has been previously reported in the literature, are not directly involved
in the reaction onset but have the role of releasing stress.
We present a detailed study on the ionic transport properties of polyethylene oxide (PEO) thin films prepared under different conditions. Using a state-of-the-art Atomic Force Microscopy (AFM) methodology, we simultaneously acquired the nanostructured topography of these semicrystalline polymer films as well as the corresponding dielectric function; in the latter case by probing the frequency-dependent tip-sample electrical interactions. By means of this AFM protocol, we studied the ionic conductivity in the PEO amorphous phase and its dependence on film preparation conditions. In general, for any preparation method, we found a distribution of conductivities ranging from 10 to 10 S cm. Specifically, PEO thin films crystallized from the melt presented relatively high conductivity values, which decreased in the PEO films prepared from solutions at room temperature depending on solvent polarity. We discuss our results by considering the molecular arrangement of the polymer segments in the complex amorphous phase, which is strongly influenced by the PEO crystallization route.
A vicinal rutile TiO2(110) crystal with a smooth variation of atomic steps parallel to the [1-10] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1-11] zigzag faceting to straight [1-10] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (Obr vacs) vanish. The [1-10] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (St) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of Obr vacs plus St protrusions at steps, suggesting that both Obr vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti(3+) polaron near steps and at terraces.
The use of an atomic force microscope for studying molecular dynamics through dielectric spectroscopy with spatial resolution in the nanometer scale is a recently developed approach. However, difficulties in the quantitative connection of the obtained data and the material dielectric properties, namely, frequency dependent dielectric permittivity, have limited its application. In this work, we develop a simple electrical model based on physically meaningful parameters to connect the atomic force microscopy (AFM) based dielectric spectroscopy experimental results with the material dielectric properties. We have tested the accuracy of the model and analyzed the relevance of the forces arising from the electrical interaction with the AFM probe cantilever. In this way, by using this model, it is now possible to obtain quantitative information of the local dielectric material properties in a broad frequency range. Furthermore, it is also possible to determine the experimental setup providing the best sensitivity in the detected signal. V
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.