158 research outputs found
Recent progress in the consistent interpretation of complementary spectroscopic results obtained on molecular systems
Abstract Research on organic thin films is largely driven by potential (optoâ)electronic applications and turns out to be no less intriguing from a fundamental point of view. Numerous studies make it clear that the understanding of deviceârelevant molecular thin film architectures is quite challengingâoften hampered by insufficient spectroscopic data and the lack of a consistent interpretation of the available datasets. Consequently, speculative aspects prevail in the discussion of energy levels in conjunction with the optical properties of organic thin films. Adequate spectroscopic techniques applicable to thin films of organic molecules (typical thicknesses required for devices are in the nanometer range) with the necessary sensitivities are rather demanding. Some of those methods were developed or significantly improved in the recent past. Here, the now available complementary spectroscopies are briefly surveyed with particular emphasis on some techniques that have not yet become widespread standards, and a nonâexhaustive set of examples of acquired experimental results are provided. For a consistent interpretation of the latter, the concepts brought forward in the literature considering the role of initial and final states of spectroscopic processes are outlined, with important consequences for quantitatively correct energy diagrams
Identification of vibrational excitations and optical transitions of the organic electron donor tetraphenyldibenzoperiflanthene (DBP)
Tetraphenyldibenzoperiflanthene (DBP) attracts interest as an organic electron donor for photovoltaic applications. In order to assist in the analysis of vibrational and optical spectra measured during the formation of thin films of DBP, we have studied the vibrational modes and the electronic states of this molecule. Information on the vibrational modes of the electronic ground state has been obtained by IR absorption spectroscopy of DBP grains embedded in polyethylene and CsI pellets and by calculations using density functional theory (DFT). Electronic transitions have been measured by UV/vis absorption spectroscopy applied to DBP molecules isolated in rare-gas matrices. These measurements are compared with the results of ab initio and semi-empirical calculations. Particularly, the vibrational pattern observed in the S1 <- S0 transition is interpreted using a theoretical vibronic spectrum computed with an ab initio model. The results of the previous experiments and calculations are employed to analyze the data obtained by high-resolution electron energy loss spectroscopy (HREELS) applied to DBP molecules deposited on a Au(111) surface. They are also used to examine the measurements performed by differential reflectance spectroscopy (DRS) on DBP molecules deposited on a muscovite mica(0001) surface. It is concluded that the DBP molecules in the first monolayer do not show any obvious degree of chemisorption on mica(0001). Regarding the first monolayer of DBP on Au(111), the HREELS data are consistent with a face-on anchoring and the absence of strong electronic coupling
Suicidal Behavior and Alcohol Abuse
Suicide is an escalating public health problem, and alcohol use has consistently been implicated in the precipitation of suicidal behavior. Alcohol abuse may lead to suicidality through disinhibition, impulsiveness and impaired judgment, but it may also be used as a means to ease the distress associated with committing an act of suicide. We reviewed evidence of the relationship between alcohol use and suicide through a search of MedLine and PsychInfo electronic databases. Multiple genetically-related intermediate phenotypes might influence the relationship between alcohol and suicide. Psychiatric disorders, including psychosis, mood disorders and anxiety disorders, as well as susceptibility to stress, might increase the risk of suicidal behavior, but may also have reciprocal influences with alcohol drinking patterns. Increased suicide risk may be heralded by social withdrawal, breakdown of social bonds, and social marginalization, which are common outcomes of untreated alcohol abuse and dependence. People with alcohol dependence or depression should be screened for other psychiatric symptoms and for suicidality. Programs for suicide prevention must take into account drinking habits and should reinforce healthy behavioral patterns
The Evolution of Intermolecular Energy Bands of Occupied and Unoccupied Molecular States in Organic Thin Films
Complex Stoichiometry-Dependent Reordering of 3,4,9,10-Perylenetetracarboxylic Dianhydride on Ag(111) upon K Intercalation
The Evolution of Intermolecular Energy Bands of Occupied and Unoccupied Molecular States in Organic Thin Films
In
organic semiconductors, the hole and electron transport occurs
through the intermolecular overlaps of highest occupied molecular
orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), respectively.
A measure of such intermolecular electronic coupling is the transfer
integral, which can experimentally be observed as energy level splittings
or the width of the respective energy bands. Quantum chemistry textbooks
describe how an energy level splits into two levels in molecular dimers,
into three levels in trimers and evolves into an energy band in infinite
systems, a process that has never been observed for the LUMO or beyond
dimers for the HOMO. In this work, our new technique, low-energy inverse
photoelectron spectroscopy, was applied to observe the subtle change
of the spectral line shape of a LUMO-derived feature while we used
ultraviolet photoelectron spectroscopy to investigate the occupied
states. We show at first that tin-phthalocyanine molecules grow layer-by-layer
in quasi-one-dimensional stacks on graphite, and then discuss a characteristic
and systematic broadening of the spectral line shapes of both HOMO
and LUMO. The results are interpreted as energy-level splittings due
to the intermolecular electronic couplings. On the basis of the HuÌckel
approximation, we determined the transfer integrals for HOMOâ1,
HOMO, and LUMO to be â€15 meV, (100 ± 10) meV, and (128
± 10) meV, respectively
Self-Assembly of Tetraphenyldibenzoperiflanthene (DBP) Films on Ag(111) in the Monolayer Regime
Tetraphenyldibenzoperiflanthene (DBP)
is a promising candidate
as a component of highly efficient organic photovoltaic cells and
organic light-emitting diodes. The structural properties of thin films
of this particular lander-type molecule on Ag(111) were investigated
by complementary techniques. Highly ordered structures were obtained,
and their mutual alignment was characterized by means of low-energy
electron diffraction (LEED). Scanning tunneling microscopy (STM) images
reveal two slightly different arrangements within the first monolayer
(ML), both describable as specific herringbone patterns with two molecules
per unit cell whose dibenzoperiflanthene framework is parallel to
the surface. In contrast, single DBP molecules in the second ML were
imaged with much higher intramolecular resolution, resembling the
shape of the frontier orbitals in the gas phase as calculated by means
of density functional theory (DFT). Further deposition leads to the
growth of highly ordered bilayer islands on top of the first ML with
identical unit cell dimensions and orientation but slightly inclined
molecules. This suggests that the first ML acts as a template for
the epitaxial growth of further layers. Simultaneously, a significant
number of second-layer molecules mainly located at step edges or scattered
over narrow terraces do not form highly ordered aggregates
Complex Stoichiometry-Dependent Reordering of 3,4,9,10-Perylenetetracarboxylic Dianhydride on Ag(111) upon K Intercalation
Alkali metal atoms are frequently
used for simple yet efficient
n-type doping of organic semiconductors and as an ingredient of the
recently discovered polycyclic aromatic hydrocarbon superconductors.
However, the incorporation of dopants from the gas phase into molecular
crystal structures needs to be controlled and well understood in order
to optimize the electronic properties (charge carrier density and
mobility) of the target material. Here, we report that potassium intercalation
into the pristine 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)
monolayer domains on a Ag(111) substrate induces distinct stoichiometry-dependent
structural reordering processes, resulting in highly ordered and large
K<sub><i>x</i></sub>PTCDA domains. The emerging structures
are analyzed by low-temperature scanning tunneling microscopy, scanning
tunneling hydrogen microscopy (STÂ[H]ÂM), and low-energy electron diffraction
as a function of the stoichiometry. The analysis of the measurements
is corroborated by density functional theory calculations. These turn
out to be essential for a correct interpretation of the experimental
STÂ[H]ÂM data. The epitaxy types for all intercalated stages are determined
as point-on-line. The K atoms adsorb in the vicinity of the oxygen
atoms of the PTCDA molecules, and their positions are determined with
sub-Ă
ngstroÌm precision. This is a crucial prerequisite
for the prospective assessment of the electronic properties of such
composite films, as they depend rather sensitively on the mutual alignment
between donor atoms and acceptor molecules. Our results demonstrate
that only the combination of experimental and theoretical approaches
allows for an unambiguous explanation of the pronounced reordering
of K<sub><i>x</i></sub>PTCDA/AgÂ(111) upon changing the K
content
Growth and characterization of novel Ir1âxCrxO2 thin films
Novel Ir1âxCrxO2 thin films have been prepared by reactive coâsputtering deposition. Composition, structure and electric and magnetic behavior have been analyzed by different techniques including EDX, XRR, XRD, SQUID magnetometry, electrical resistivity and XANES and XMCD spectroscopies. Despite the difficulty in growing CrO2 by physical deposition techniques, an Ir1âxCrxO2 solid solution phase could be achieved for 0 = x = 0.8, where the oxidation state of Cr is found to remain as 4+. Both the electrical and the magnetic behavior are shown to starkly depart from those of the parent IrO2 (paramagnetic metal) and CrO2 (halfâmetal ferromagnet) compounds. In particular, they show a semiconducting behavior, dÂż/dT < 0 and giant magnetic coercivity at low temperatures. XMCD reveals a significant contribution of Ir to the magnetic response of the Ir1âxCrxO2 films. In addition, the nature of the magnetic moment of the Ir4+ ion (/ = 0.09) is completely different from the large orbital moment that is a hallmark of insulating Ir4+ oxides. This suggests a Crâinduced magnetic moment, which is a remarkably surprising result for an oxide
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