3,674 research outputs found
Energy Level Alignment in Organic-Organic Heterojunctions: The TTF-TCNQ Interface
The energy level alignment of the two organic materials forming the TTF-TCNQ
interface is analyzed by means of a local orbital DFT calculation, including an
appropriate correction for the transport energy gaps associated with both
materials. These energy gaps are determined by a combination of some
experimental data and the results of our calculations for the difference
between the TTF_{HOMO} and the TCNQ_{LUMO} levels. We find that the interface
is metallic, as predicted by recent experiments, due to the overlap (and charge
transfer) between the Density of States corresponding to these two levels,
indicating that the main mechanism controlling the TTF-TCNQ energy level
alignment is the charge transfer between the two materials. We find an induced
interface dipole of 0.7 eV in good agreement with the experimental evidence. We
have also analyzed the electronic properties of the TTF-TCNQ interface as a
function of an external bias voltage \Delta, between the TCNQ and TTF crystals,
finding a transition between metallic and insulator behavior for \Delta~0.5 eV
Effect of van der Waals forces on the stacking of coronenes encapsulated in a single-wall carbon nanotube and many-body excitation spectrum
We investigate the geometry, stability, electronic structure and optical
properties of C24H12 coronenes encapsulated in a single-wall (19,0) carbon
nanotube. By an adequate combination of advanced electronic-structure
techniques, involving weak and van derWaals interaction, as well as many-body
effects for establishing electronic properties and excitations, we have
accurately characterized this hybrid carbon nanostructure, which arises as a
promising candidate for opto-electronic nanodevices. In particular, we show
that the structure of the stacked coronenes inside the nanotube is
characterized by a rotation of every coronene with respect to its neighbors
through van derWaals interaction, which is of paramount importance in these
systems. We also suggest a tentative modification of the system in order this
particular rotation to be observed experimentally. A comparison between the
calculated many-body excitation spectrum of the systems involved reveals a
pronounced optical red-shift with respect to the coronene-stacking gas-phase.
The origin of this red-shift is explained in terms of the confinement of the
coronene molecules inside the nanotube, showing an excellent agreement with the
available experimental evidence
Three-Dimensional Wave Packet Approach for the Quantum Transport of Atoms through Nanoporous Membranes
Quantum phenomena are relevant to the transport of light atoms and molecules
through nanoporous two-dimensional (2D) membranes. Indeed, confinement provided
by (sub-)nanometer pores enhances quantum effects such as tunneling and zero
point energy (ZPE), even leading to quantum sieving of different isotopes of a
given element. However, these features are not always taken into account in
approaches where classical theories or approximate quantum models are
preferred. In this work we present an exact three-dimensional wave packet
propagation treatment for simulating the passage of atoms through periodic 2D
membranes. Calculations are reported for the transmission of He and He
through graphdiyne as well as through a holey graphene model. For
He-graphdiyne, estimations based on tunneling-corrected transition state theory
are correct: both tunneling and ZPE effects are very important but competition
between each other leads to a moderately small He/He selectivity. Thus,
formulations that neglect one or another quantum effect are inappropriate. For
the transport of He isotopes through leaky graphene, the computed transmission
probabilities are highly structured suggesting widespread selective adsorption
resonances and the resulting rate coefficients and selectivity ratios are not
in agreement with predictions from transition state theory. Present approach
serves as a benchmark for studies of the range of validity of more approximate
methods.Comment: 4 figure
Graphdiyne based membranes: exceptional performances for helium separation applications
Graphdiyne is a novel two-dimensional material deriving from graphene that
has been recently synthesized and featuring uniformly distributed sub-nanometer
pores. We report accurate calculations showing that graphdiyne pores permit an
almost unimpeded helium transport which can be used for its chemical and
isotopic separation. Exceptionally high He/CH_4 selectivities are found which
largely exceed the performance of the best membranes used to date for
extraction from natural gas. Moreover, by exploiting slight differences in the
tunneling probabilities of ^3He and ^4He, we also find promising results for
the separation of the Fermionic isotope at low temperature
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