8 research outputs found
Ordered Superstructures of a Molecular Electron Donor on Au(111)
The
molecular donor tetraphenyldibenzoperiflanthene (DBP) forms
coverage-dependent superstructures on Au(111). At submonolayer coverage,
the molecules align parallel to each other. They arrange in row-like
structures, which exhibit a nearly rectangular primitive unit cell.
By contrast, the molecular monolayer is characterized by a herringbone-type
DBP arrangement spanned by an almost square unit cell containing two
molecules. Both superstructures occur simultaneously in a narrow coverage
range close to completion of the molecular monolayer. The adsorbate–substrate
interaction is similar to other physisorbed molecular films on Au(111),
but differs for the two adsorption phases as inferred from the different
modification of the Au(111) surface reconstruction. Structural properties
were consistently probed in real and reciprocal space by scanning
tunneling microscopy and low-energy electron diffraction, respectively
Formation and Structure of Calcium Carbonate Thin Films and Nanofibers Precipitated in the Presence of Poly(Allylamine Hydrochloride) and Magnesium Ions
That the cationic
polyelectrolyte poly(allylamine hydrochloride)
(PAH) exerts a significant influence on CaCO<sub>3</sub> precipitation
challenges the idea that only anionic additives have this effect.
Here, we show that in common with anionic polyelectrolytes such as
poly(aspartic acid), PAH supports the growth of calcite thin films
and abundant nanofibers. While investigating the formation of these
structures, we also perform the first detailed structural analysis
of the nanofibers by transmission electron microscopy (TEM) and selected
area electron diffraction. The nanofibers are shown to be principally
single crystal, with isolated domains of polycrystallinity, and the
single crystal structure is even preserved in regions where the nanofibers
dramatically change direction. The formation mechanism of the fibers,
which are often hundreds of micrometers long, has been the subject
of intense speculation. Our results suggest that they form by aggregation
of amorphous particles, which are incorporated into the fibers uniquely
at their tips, before crystallizing. Extrusion of polymer during crystallization
may inhibit particle addition at the fiber walls and result in local
variations in the fiber nanostructure. Finally, we investigate the
influence of Mg<sup>2+</sup> on CaCO<sub>3</sub> precipitation in
the presence of PAH, which gives thinner and smoother films, together
with fibers with more polycrystalline, granular structures