59 research outputs found
Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications
Based on the arrangement of two-dimensional 'melon', we construct a unit cell
for polymeric carbon nitride (PCN) synthesized via thermal polycondensation,
whose theoretical diffraction powder pattern includes all major features
measured in x-ray diffraction. With the help of this unit cell, we describe
the process-temperature-induced crystallographic changes in PCN that occur
within a temperature interval between 510 and 610 °C. We also discuss further
potential modifications of the unit cell for PCN. It is found that both
triazine- and heptazine-based g-C3N4 can only account for minor phases within
the investigated synthesis products
An optical quasimonomer
A comprehensive investigation of the luminescent properties of carbon nitride
polymers, based on tri-s-triazine units, has been conducted. Steady-state
temperature- and excitation-power-dependent as well as time-resolved
measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong
photoluminescence, covering the visible spectrum. The spectral, thermal, and
temporal features of the photoluminescence can be satisfactorily described by
the excitation and radiative recombination of molecular excitons, localized at
single tri-s-triazine units. The discussed model is in accordance with the
recently reported absorption features of carbon nitride polymers. Thus, from
the point of view of optical spectroscopy, the material effectively behaves as
a monomer
A complementary neutron and anomalous x-ray diffraction study
Distinguishing the scattering contributions of isoelectronic atomic species by
means of conventional x-ray- and/or electron diffraction techniques is a
difficult task. Such a problem occurs when determining the crystal structure
of compounds containing different types of atoms with equal number of
electrons. We propose a new structural model of Cu(InxGa1−x)3Se5 which is
valid for the entire compositional range of the CuIn3Se5–CuGa3Se5 solid
solution. Our model is based on neutron and anomalous x-ray diffraction
experiments. These complementary techniques allow the separation of scattering
contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly
identically in monoenergetic x-ray diffraction experiments. We have found that
CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification
exhibits a modified stannite structure (space group I4¯2m). Different
occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies
have been found at three different cationic positions of the structure
(Wyckoff sites 2a, 2b, and 4d) depending on the composition of the compound.
Significantly, Cu+ does not occupy the 2b site for the In-free compound, but
does for the In-containing case. Structural parameters, including lattice
constants, tetragonal distortions, and occupation factors are given for
samples covering the entire range of the CuIn3Se5–CuGa3Se5 solid solution. At
the light of the result, the denotation of Cu-poor 1:3:5 compounds as
chalcopyrite-related materials is only valid in reference to their
composition
Transport properties of CuGaSe(2)-based thin-film solar cells as a function of absorber composition
The transport properties of thin-film solar cells based on wide-gap CuGaSe(2) absorbers have been investigated as a function of the bulk [Ga]/[Cu] ratio ranging from 1.01 to 1.33. We find that (i) the recombination processes in devices prepared from absorbers with a composition close to stoichiometry ([Ga]/[Cu] = 1.01) are strongly tunnelling assisted resulting in low recombination activation energies (E(a)) of approx. 0.95 eV in the dark and 1.36 eV under illumination. (ii) With an increasing [Ga]/[Cu] ratio, the transport mechanism changes to be dominated by thermally activated Shockley-Read-Hall recombination with similar E(a) values of approx. 1.52-1.57 eV for bulk [Ga]/[Cu] ratios of 1.12-1.33. The dominant recombination processes take place at the interface between CdS buffer and CuGaSe(2) absorber independently from the absorber composition. The increase of E(a) with the [Ga]/[Cu] ratio correlates with the open circuit voltage and explains the better performance of corresponding solar cells
Structure and properties of a novel fulleride Sm6C60
A novel fulleride Sm6C60 has been synthesized using high temperature solid
state reaction. The Rietveld refinement on high resolution synchrotron X-ray
powder diffraction data shows that Sm6C60 is isostructural with body-centered
cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60,
and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of
Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules
and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak
T-linear behavior above 180 K, the transport at low temperature is mainly
dominated by granular-metal theory.Comment: 9 pages, 3 figures, submitted to Phys. Rev. B (March 12, 1999
Orbital character of O 2p unoccupied states near the Fermi level in CrO2
The orbital character, orientation, and magnetic polarization of the O 2
unoccupied states near the Fermi level () in CrO was determined using
polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic
circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak
observed just above is excited only by the electric field vector () normal to the tetragonal -axis, characteristic of a narrow band
( 0.7 eV bandwidth) constituted from O 2 orbitals perpendicular to
(O 2) hybridized with Cr 3 states. By comparison
with band-structure and configuration-interaction (CI) cluster calculations our
results support a model of CrO as a half-metallic ferromagnet with large
exchange-splitting energy ( 3.0 eV) and
substantial correlation effects.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. B Rapid
Com
Sub-surface Oxygen and Surface Oxide Formation at Ag(111): A Density-functional Theory Investigation
To help provide insight into the remarkable catalytic behavior of the
oxygen/silver system for heterogeneous oxidation reactions, purely sub-surface
oxygen, and structures involving both on-surface and sub-surface oxygen, as
well as oxide-like structures at the Ag(111) surface have been studied for a
wide range of coverages and adsorption sites using density-functional theory.
Adsorption on the surface in fcc sites is energetically favorable for low
coverages, while for higher coverage a thin surface-oxide structure is
energetically favorable. This structure has been proposed to correspond to the
experimentally observed (4x4) phase. With increasing O concentrations, thicker
oxide-like structures resembling compressed Ag2O(111) surfaces are
energetically favored. Due to the relatively low thermal stability of these
structures, and the very low sticking probability of O2 at Ag(111), their
formation and observation may require the use of atomic oxygen (or ozone, O3)
and low temperatures. We also investigate diffusion of O into the sub-surface
region at low coverage (0.11 ML), and the effect of surface Ag vacancies in the
adsorption of atomic oxygen and ozone-like species. The present studies,
together with our earlier investigations of on-surface and
surface-substitutional adsorption, provide a comprehensive picture of the
behavior and chemical nature of the interaction of oxygen and Ag(111), as well
as of the initial stages of oxide formation.Comment: 17 pages including 14 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Insights into the function of silver as an oxidation catalyst by ab initio, atomistic thermodynamics
To help understand the high activity of silver as an oxidation catalyst,
e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of
methanol to formaldehyde, the interaction and stability of oxygen species at
the Ag(111) surface has been studied for a wide range of coverages. Through
calculation of the free energy, as obtained from density-functional theory and
taking into account the temperature and pressure via the oxygen chemical
potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a
thin surface-oxide structure is most stable for the temperature and pressure
range of ethylene epoxidation and we propose it (and possibly other similar
structures) contains the species actuating the catalysis. For higher
temperatures, low coverages of chemisorbed oxygen are most stable, which could
also play a role in oxidation reactions. For temperatures greater than about
775 K there are no stable oxygen species, except for the possibility of O atoms
adsorbed at under-coordinated surface sites Our calculations rule out thicker
oxide-like structures, as well as bulk dissolved oxygen and molecular
ozone-like species, as playing a role in the oxidation reactions.Comment: 15 pages including 9 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Defect band transport in p type CuGaSe2
Hall effect and electrical resistivity is measured on slightly Cu rich epitaxial CuGaSe2 films in the temperature range of 15 300 K. The temperature dependence of the Hall coefficient is described by the two band model with holes in both the valence and defect bands, as manifested by a maximum in the Hall coefficient. At the temperature of the maximum, the mobility in the defect band is about 250 times lower than that in the valence band. The model can be used to separate holes in the valence band and the defect band, allowing the determination of the activation energies and concentrations of the acceptors, and the concentration of the compensating donor
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