Abstract.A molecular dynamics simulation of quartz at different temperatures both in the e and in the/~ phase has been conducted. The e-/~ phase transition could be observed. A phonon analysis of the/%phase confirms and rounds out in a quantitative way the origin of the incommensurate (ic) modulated phase. In particular it traces the optic soft mode at Y becoming (to a good approximation) a so-called rigid unit mode (RUM) at q ~ 0, and elucidates its coupling to the transverse acoustic mode which precipitates the incommensurate transition. This success underpins and illuminates the concept of RUMs and their role in structural phase transitions.
Hexacene, composed
of six linearly fused benzene rings, is an organic
semiconductor material with superior electronic properties. The fundamental
understanding of the electronic and chemical properties is prerequisite
to any possible application in devices. We investigate the orientation
and interface properties of highly ordered hexacene monolayers on
Ag(110) and Cu(110) with X-ray photoemission spectroscopy (XPS), photoemission
orbital tomography (POT), X-ray absorption spectroscopy (XAS), low-energy
electron diffraction (LEED), scanning tunneling microscopy (STM),
and density functional theory (DFT). We find pronounced differences
in the structural arrangement of the molecules and the electronic
properties at the metal/organic interfaces for the two substrates.
While on Cu(110) the molecules adsorb with their long molecular axis
parallel to the high symmetry substrate direction, on Ag(110), hexacene
adsorbs in an azimuthally slightly rotated geometry with respect to
the metal rows of the substrate. In both cases, molecular planes are
oriented parallel to the substrate. A pronounced charge transfer from
both substrates to different molecular states affects the effective
charge of different C atoms of the molecule. Through analysis of experimental
and theoretical data, we found out that on Ag(110) the LUMO of the
molecule is occupied through charge transfer from the metal, whereas
on Cu(110) even the LUMO+1 receives a charge. Interface dipoles are
determined to a large extent by the push-back effect, which are also
found to differ significantly between
6A
/Ag(110) and
6A
/Cu(110).
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