93 research outputs found
Adsorption of 5-Fluorouracil on Au(111) and Cu(111) surfaces
The adsorption of 5-Fluorouracil (5FU) on Au(111) and Cu(111) surfaces as a function of molecular coverage and temperature has been studied, using x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The nature of 5-Fluorouracil bonding with the two substrates is remarkably different. The Cu substrate forms a chemisorbed complex with 5-FU while the Au substrate shows only physisorption. NEXAFS data at the C, N and O K-edge show a strong angular dependence, indicating that 5-FU lies nearly parallel on the inert Au(111) surface, and at a steep angle on the Cu(111) surface. 5-FU is a biomolecule used for cancer treatment and the results are relevant for those using metal surfaces to prepare 5-FU for applications such as drug delivery.The adsorption of 5-Fluorouracil (5FU) on Au(111) and Cu(111) surfaces as a function of molecular coverage and temperature has been studied, using x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The nature of 5-Fluorouracil bonding with the two substrates is remarkably different. The Cu substrate forms a chemisorbed complex with 5-FU while the Au substrate shows only physisorption. NEXAFS data at the C, N and O K-edge show a strong angular dependence, indicating that 5-FU lies nearly parallel on the inert Au(111) surface, and at a steep angle on the Cu(111) surface. 5-FU is a biomolecule used for cancer treatment and the results are relevant for those using metal surfaces to prepare 5-FU for applications such as drug delivery
Complete determination of molecular orbitals by measurement of phase symmetry and electron density.
Several experimental methods allow measuring the spatial probability density of electrons in atoms, molecules and solids, that is, the absolute square of the respective single-particle wave function. But it is an intrinsic problem of the measurement process that the information about the phase is generally lost during the experiment. The symmetry of this phase, however, is a crucial parameter for the knowledge of the full orbital information in real space. Here, we report on a key experiment that demonstrates that the phase symmetry can be derived from a strictly experimental approach from the circular dichroism in the angular distribution of photoelectrons. In combination with the electron density derived from the same experiment, the full quantum mechanical wave function can thus be determined experimentally
Direct observation of the band gap transition in atomically thin ReS
ReS is considered as a promising candidate for novel electronic and
sensor applications. The low crystal symmetry of the van der Waals compound
ReS leads to a highly anisotropic optical, vibrational, and transport
behavior. However, the details of the electronic band structure of this
fascinating material are still largely unexplored. We present a
momentum-resolved study of the electronic structure of monolayer, bilayer, and
bulk ReS using k-space photoemission microscopy in combination with
first-principles calculations. We demonstrate that the valence electrons in
bulk ReS are - contrary to assumptions in recent literature - significantly
delocalized across the van der Waals gap. Furthermore, we directly observe the
evolution of the valence band dispersion as a function of the number of layers,
revealing a significantly increased effective electron mass in single-layer
crystals. We also find that only bilayer ReS has a direct band gap. Our
results establish bilayer ReS as a advantageous building block for
two-dimensional devices and van der Waals heterostructures
Signatures of an Atomic Crystal in the Band Structure of a Molecular Thin Film
Transport phenomena in molecular materials are intrinsically linked to the
orbital character and the degree of localization of the valence states. Here,
we combine angle-resolved photoemission with photoemission tomography to
determine the spatial distribution of all molecular states of the valence band
structure of a C thin film. While the two most frontier valence states
exhibit a strong band dispersion, the states at larger binding energies are
characterized by distinct emission patterns in energy and momentum space. Our
findings demonstrate the formation of an atomic crystal-like band structure in
a molecular solid with delocalized -like valence states and strongly
localized -states at larger binding energies
Enhancing electron correlation at a 3D ferromagnetic surface
Spin-resolved momentum microscopy and theoretical calculations are combined beyond the one-electron approximation to unveil the spin-dependent electronic structure of the interface formed between iron (Fe) and an ordered oxygen (O) atomic layer, and an adsorbate-induced enhancement of electronic correlations is found. It is demonstrated that this enhancement is responsible for a drastic narrowing of the Fe d-bands close to the Fermi energy (EF) and a reduction of the exchange splitting, which is not accounted for in the Stoner picture of ferromagnetism. In addition, correlation leads to a significant spin-dependent broadening of the electronic bands at higher binding energies and their merging with satellite features, which are manifestations of a pure many-electron behavior. Overall, adatom adsorption can be used to vary the material parameters of transition metal surfaces to access different intermediate electronic correlated regimes, which will otherwise not be accessible. The results show that the concepts developed to understand the physics and chemistry of adsorbate–metal interfaces, relevant for a variety of research areas, from spintronics to catalysis, need to be reconsidered with many-particle effects being of utmost importance. These may affect chemisorption energy, spin transport, magnetic order, and even play a key role in the emergence of ferromagnetism at interfaces between non-magnetic systems
Valence structures of aromatic bioactive compounds: a combined theoretical and experimental study.
Valence electronic structures of three recently isolated aryl bioactive compounds, namely 2-phenylethanol (2PE), p-hydroxyphenylethanol (HPE) and 4-hydroxybenzaldehyde (HBA), are studied using a combined theoretical and experimental method. Density functional theory-based calculations indicate that the side chains cause electron charge redistribution and therefore influence the aromaticity of the benzene derivatives. The simulated IR spectra further reveal features induced by the side chains. Solvent effects on the IR spectra are simulated using the polarizable continuum model, which exhibits enhancement of the O-H stretch vibrations with significant red-shift of 464 cm(-1) in 2PE. A significant spectral peak splitting of 94 cm(-1) between O(4)-H and O(8)-H of HPE is revealed in an aqueous environment. Experimental measurements for valence binding energy spectra for 2PE, HPE and HBA are presented and analyzed using outer-valence Green function calculations. The experimental (predicted) first ionization energies are measured as 9.19 (8.79), 8.47 (8.27) and 8.97 (8.82) eV for 2PE, HPE and HBA, respectively. The frontier orbitals (highest occupied molecular orbitals, HOMOs, and lowest unoccupied molecular orbitals, LUMOs) have similar atomic orbital characters although the HOMO-LUMO energy gaps are quite different
Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe/MoSe heterobilayers
We identify an optical excitation mechanism that transfers a valley
polarization from photo-excited electron-hole pairs to free charge carriers in
twisted WSe/MoSe heterobilayers. For small twist angles, the valley
lifetimes of the charge carriers are surprisingly short, despite the occurrence
of interlayer excitons with their presumably long recombination and
polarization lifetimes. For large twist angles, we measure an increase in both
the valley polarization and its respective lifetime by more than two orders of
magnitude. Interestingly, in such heterobilayers we observe an interlayer
transfer of valley polarization from the WSe layer into the MoSe layer.
This mechanism enables the creation of a photo-induced valley polarization of
free charge carriers in MoSe, which amplitude scales with the gate-induced
charge carrier density. This is in contrast to monolayer MoSe, where such a
gate-tunable valley polarization cannot be achieved. By combining time-resolved
Kerr rotation, photoluminesence and angle-resolved photoemission spectroscopy
measurements with first principles calculations, we show that these findings
can be explained by twist angle dependent interlayer scattering mechanisms
involving the Q- and -valleys.Comment: 21 pages, 10 figure
VUV photon induced fluorescence study of SF5CF3
The interaction of SFCF with vacuum-UV radiation has been investigated by photon induced fluorescence spectroscopy. Total fluorescence yield and dispersed fluorescence spectra of SFCF were recorded in the 200-1000 nm fluorescence window. In all cases, the fluorescence spectra resemble those of CFX (X=H, F, Cl, and Br) molecules. At photon energies below 20 eV, the emission is attributed to the excited CF and CF fragments. The threshold for the CF emission is 10.2 ± 0.2 eV, giving an upper-limit estimate for the SF-CF bond dissociation energy of 3.9 ± 0.3 eV. The excitation functions of the CF3 and CF2 emissions were measured in the photon energy range 13.6 – 27.0 eV. The resonant structures observed in SFCF are attributed to electronic transitions from valence to Rydberg orbitals, following similar assignments in CFX molecules. The photoabsorption spectrum of SFCF shows features at the same energies, indicating a strong contribution from Rydberg excitations
Electronic band structure changes across the antiferromagnetic phase transition of exfoliated MnPS probed by -ARPES
Exfoliated magnetic 2D materials enable versatile tuning of magnetization,
e.g., by gating or providing proximity-induced exchange interaction. However,
their electronic band structure after exfoliation has not been probed, most
likely due to their photochemical sensitivity. Here, we provide micron-scale
angle-resolved photoelectron spectroscopy of the exfoliated intralayer
antiferromagnet MnPS above and below the N\'{e}el temperature down to one
monolayer. The favorable comparison with density functional theory calculations
enables to identify the orbital character of the observed bands. Consistently,
we find pronounced changes across the N\'{e}el temperature for bands that
consist of Mn 3d and 3p levels of adjacent S atoms. The deduced orbital mixture
indicates that the superexchange is relevant for the magnetic interaction.
There are only minor changes between monolayer and thicker films demonstrating
the predominant 2D character of MnPS. The novel access is transferable to
other MPX materials (M: transition metal, P: phosphorus, X: chalcogenide)
providing a multitude of antiferromagnetic arrangements.Comment: 26 pages, 17 figure
Metalloporphyrins on Oxygen-Passivated Iron: Conformation and Order Beyond the First Layer
On-surface metal porphyrins can undergo electronic and conformational changes
that play a crucial role in determining the chemical reactivity of the
molecular layer. Therefore, understanding those properties is pivotal for the
design and implementation of organic-based devices. Here, by means of
photoemission orbital tomography supported by density functional theory
calculations, we investigate the electronic and geometrical structure of two
metallated tetraphenyl porphyrins (MTPPs), namely ZnTPP and NiTPP, adsorbed on
the oxygen-passivated Fe(100)-p(1x1)O surface. Both molecules weakly interact
with the surface as no charge transfer is observed. In the case of ZnTPP our
data correspond to those of moderately distorted molecules, while NiTPP
exhibits a severe saddle-shape deformation. From additional experiments on
NiTPP multilayer films, we conclude that this distortion is a consequence of
the interaction with the substrate, as the NiTPP macrocycle of the second layer
turns out to be flat. We further find that distortions in the MTPP macrocycle
are accompanied by an increasing energy gap between the highest occupied
molecular orbitals (HOMO and HOMO-1). Our results demonstrate that
photoemission orbital tomography can simultaneously probe the energy level
alignment, the azimuthal orientation, and the adsorption geometry of complex
aromatic molecules even in the multilayer regime
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