68 research outputs found
Imaging isodensity contours of molecular states with STM
We present an improved way for imaging the local density of states with a
scanning tunneling microscope, which consists in mapping the surface topography
while keeping the differential conductance (d/d) constant. When
archetypical C molecules on Cu(111) are imaged with this method, these
so-called iso-d/d maps are in excellent agreement with theoretical
simulations of the isodensity contours of the molecular orbitals. A direct
visualization and unambiguous identification of superatomic C orbitals
and their hybridization is then possible
Electroluminescence from a polythiophene molecular wire suspended in a plasmonic scanning tunneling microscope junction
The electroluminescence of a polythiophene wire suspended between two
metallic electrodes is probed using a scanning tunneling microscope. Under
positive sample voltage, the spectral and voltage dependencies of the emitted
light are consistent with the fluorescence of the wire junction mediated by
localized plasmons. This emission is strongly attenuated for the opposite
polarity. Both emission mechanism and polarity dependence are similar to what
occurs in organic light emitting diodes (OLED) but at the level of a single
molecular wire.Comment: to be published in Physical Review Letter
Optical Probe of Quantum Shot Noise Reduction at a Single-Atom Contact
Visible and infra-red light emitted at a Ag-Ag(111) junction has been
investigated from tunneling to single atom contact conditions with a scanning
tunneling microscope. The light intensity varies in a highly nonlinear fashion
with the conductance of the junction and exhibits a minimum at conductances
close to the conductance quantum. The data are interpreted in terms of current
noise at optical frequencies, which is characteristic of partially open
transport channels
Topologically localized excitons in single graphene nanoribbons
Excitonic emission from atomically precise graphene nanoribbons (GNRs)
synthesised on a metal surface is probed with atomic-scale spatial resolution
using a scanning tunneling microscopy (STM) approach. A STM-based strategy to
transfer the GNRs to a partially insulating surface is used to prevent light
emission quenching of the ribbons by the metal substrate. Sub-nanometer
resolved STM-induced fluorescence spectra reveal emission from localized dark
excitons build upon the topological end states of the GNRs. A low frequency
vibronic emission comb whose characteristics change with the GNR length is
attributed to longitudinal acoustic modes confined to a finite box. Overall,
our study provides a novel path to investigate the interplay between excitons,
vibrons and topology in atomically precise graphene nanostructures
Pulling and Stretching a Molecular Wire to Tune its Conductance
A scanning tunnelling microscope is used to pull a polythiophene wire from a
Au(111) surface while measuring the current traversing the junction. Abrupt
current increases measured during the lifting procedure are associated to the
detachment of molecular sub-units, in apparent contradiction with the expected
exponential decrease of the conductance with wire length. \textit{Ab initio}
simulations reproduce the experimental data and demonstrate that this
unexpected behavior is due to release of mechanical stress in the wire, paving
the way to mechanically gated single-molecule electronic devices
Demokratie auf dem Rückzug? Zur Konfrontation zwischen amerikanischem Präsidenten und Kongress über einen Truppenabzug aus dem Irak
Im US-Präsidentschaftswahlkampf 2008 ist der Irakkrieg eines der wichtigsten Themen. Die Amerikaner sind allmählich kriegsmüde und die Mehrheit befürwortet einen Truppenrückzug aus dem Irak. Das brachten bereits der Ausgang der Kongresswahlen im Jahr 2006 sowie regelmäßige Meinungsumfragen zum Ausdruck.
Es ist nun anzunehmen, dass in der ältesten Demokratie der Welt Volkes Wille direkten Einfluss auf die Politik haben müsste. Doch dies ist nur bedingt der Fall. Besonders Präsident George W. Bush hat wiederholt klargestellt, dass die Stimmung in der Bevölkerung für ihn in der Irakfrage nicht maßgebend sei. Für ihn zähle vielmehr, wie die Generäle vor Ort die aktuelle Sicherheitslage und die Fortschritte einschätzen. Dagegen versucht der seit Ende 2006 von Demokraten dominierte Kongress immer wieder, Rückzugspläne mit festen Fristen zu etablieren. Bislang ist sein Erfolg jedoch bescheiden.
Vor allem die fehlende Unterstützung der Republikaner im Kongress und die Blockadepolitik des Präsidenten waren dafür verantwortlich. Hinter der Konfrontation steht eine tiefergehende Auseinandersetzung über die Kompetenzverteilung zwischen Präsident und Kongress bei Kriegsentscheidungen, die in der US-Verfassung in einer Grauzone bleibt.
Stephanie Sohnius verknüpft vor diesem verfassungsrechtlichen Hintergrund die Aufarbeitung der aktuell wichtigsten USaußenpolitischen Debatte mit der Frage, ob sich die Demokratie in den USA in der Ära des „Krieges gegen den Terrorismus“ (weiterhin) auf dem Rückzug befindet
Many-body description of STM-induced fluorescence of charged molecules
A scanning tunneling microscope is used to study the fluorescence of a model
charged molecule (quinacridone) adsorbed on a sodium chloride (NaCl)-covered
metallic sample. Fluorescence from the neutral and positively charged species
is reported and imaged using hyper-resolved fluorescence microscopy. A
many-body excitation model is established based on a detailed analysis of
voltage, current and spatial dependencies of the fluorescence and electron
transport features. This model reveals that quinacridone adopts a large palette
of charge states, transient or not, depending on the voltage used and the
nature of the underlying substrate. This model has a universal character and
explains the electronic and fluorescence properties of many other molecules
adsorbed on thin insulators
Single azopyridine-substituted porphyrin molecules for configurational and electronic switching
A new azopyridine functionalized Ni–porphyrin was synthesized
as a model compound for deposition and switch on surfaces. Two
geometrically and electronically different states of single molecules
on Au(111) were found by scanning tunneling microscopy/
spectroscopy and analyzed with density functional calculations
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