778 research outputs found
Using surface plasmonics to turn on fullerene's dark excitons
Using our recently proposed Bethe-Salpeter formulation, we explore
the optical absorption spectra of fullerene (C) near coinage metal
surfaces (Cu, Ag, and Au). We pay special attention to how the surface plasmon
influences the optical activity of fullerene. We find the lower
energy fullerene excitons at 3.77 and 4.8 eV only weakly interact with the
surface plasmon. However, we find the surface plasmon strongly interacts with
the most intense fullerene exciton, i.e.\ the dipolar mode at
6.5 eV, and the quadrupolar mode at
6.8 eV. When fullerene is close to a copper surface
( 5.3 \AA) the dipolar mode and "localized" surface
plasmons in the molecule/surface interface hybridize to form two coupled modes
which both absorb light. As a result, the molecule gains an additional
optically active mode. Moreover, in resonance, when
, the strong interaction with the surface plasmon
destroys the quadrupolar character and it becomes an optically
active mode. In this case the molecule gains two additional very intense
optically active modes. Further, we find this resonance condition, , is satisfied by silver and gold metal surfaces.Comment: 10 pages, 8 figure
Influence of O2 and N2 on the conductivity of carbon nanotube networks
We have performed experiments on single-wall carbon nanotube (SWNT) networks
and compared with density-functional theory (DFT) calculations to identify the
microscopic origin of the observed sensitivity of the network conductivity to
physisorbed O2 and N2. Previous DFT calculations of the transmission function
for isolated pristine SWNTs have found physisorbed molecules have little
influence on their conductivity. However, by calculating the four-terminal
transmission function of crossed SWNT junctions, we show that physisorbed O2
and N2 do affect the junction's conductance. This may be understood as an
increase in tunneling probability due to hopping via molecular orbitals. We
find the effect is substantially larger for O2 than for N2, and for
semiconducting rather than metallic SWNTs junctions, in agreement with
experiment.Comment: 6 pages, 5 figures, 1 tabl
Influence of Functional Groups on Charge Transport in Molecular Junctions
Using density functional theory (DFT), we analyze the influence of five
classes of functional groups, as exemplified by NO2, OCH3, CH3, CCl3, and I, on
the transport properties of a 1,4-benzenedithiolate (BDT) and
1,4-benzenediamine (BDA) molecular junction with gold electrodes. Our analysis
demonstrates how ideas from functional group chemistry may be used to engineer
a molecule's transport properties, as was shown experimentally and using a
semiempirical model for BDA [Nano Lett. 7, 502 (2007)]. In particular, we show
that the qualitative change in conductance due to a given functional group can
be predicted from its known electronic effect (whether it is pi/sigma
donating/withdrawing). However, the influence of functional groups on a
molecule's conductance is very weak, as was also found in the BDA experiments.
The calculated DFT conductances for the BDA species are five times larger than
the experimental values, but good agreement is obtained after correcting for
self-interaction and image charge effects.Comment: 6 pages, 3 figures, J. Chem. Phys (in press
Quasiparticle spectra and excitons of organic molecules deposited on substrates: G0W0-BSE approach applied to benzene on graphene and metallic substrates
We present an alternative methodology for calculating the quasi-particle
energy, energy loss, and optical spectra of a molecule deposited on graphene or
a metallic substrate. To test the accuracy of the method it is first applied to
the isolated benzene (C6H6) molecule. The quasiparticle energy levels and
especially the energies of the benzene excitons (triplet, singlet, optically
active and inactive) are in very good agreement with available experimental
results. It is shown that the vicinity of the various substrates
(pristine/doped graphene or (jellium) metal surface) reduces the quasiparticle
HOMO-LUMO gap by an amount that slightly depends on the substrate type. This is
consistent with the simple image theory predictions. It is even shown that the
substrate does not change the energy of the excitons in the isolated molecule.
We prove (in terms of simple image theory) that energies of the excitons are
indeed influenced by two mechanisms which cancel each other. We demonstrate
that the benzene singlet optically active (E1u) exciton couples to real
electronic excitations in the substrate. This causes it substantial decay, such
as {\Gamma} = 174 meV for pristine graphene and {\Gamma} = 362 meV for metal
surfaces as the substrate. However, we find that doping graphene does not
influence the E1u exciton decay rate.Comment: 16 pages, 14 figure
Tailoring electronic and optical properties of TiO2: nanostructuring, doping and molecular-oxide interactions
Titanium dioxide is one of the most widely investigated oxides. This is due
to its broad range of applications, from catalysis to photocatalysis to
photovoltaics. Despite this large interest, many of its bulk properties have
been sparsely investigated using either experimental techniques or ab initio
theory. Further, some of TiO2's most important properties, such as its
electronic band gap, the localized character of excitons, and the localized
nature of states induced by oxygen vacancies, are still under debate. We
present a unified description of the properties of rutile and anatase phases,
obtained from ab initio state of the art methods, ranging from density
functional theory (DFT) to many body perturbation theory (MBPT) derived
techniques. In so doing, we show how advanced computational techniques can be
used to quantitatively describe the structural, electronic, and optical
properties of TiO2 nanostructures, an area of fundamental importance in applied
research. Indeed, we address one of the main challenges to TiO2-photocatalysis,
namely band gap narrowing, by showing how to combine nanostructural changes
with doping. With this aim we compare TiO2's electronic properties for 0D
clusters, 1D nanorods, 2D layers, and 3D bulks using different approximations
within DFT and MBPT calculations. While quantum confinement effects lead to a
widening of the energy gap, it has been shown that substitutional doping with
boron or nitrogen gives rise to (meta-)stable structures and the introduction
of dopant and mid-gap states which effectively reduce the band gap. Finally, we
report how ab initio methods can be applied to understand the important role of
TiO2 as electron-acceptor in dye-sensitized solar cells. This task is made more
difficult by the hybrid organic-oxide structure of the involved systems.Comment: 32 pages, 8 figure
TDDFT study of time-dependent and static screening in graphene
Time-dependent density functional theory (TDDFT) within the random phase approximation (RPA) is used to obtain the time evolution of the induced potential produce by the sudden formation of a C 1s core hole inside a graphene monolayer, and to show how the system reaches the equilibrium potential. The characteristic oscillations in the time-dependent screening potential are related to the excitations of π and σ+π plasmons as well as the low energy 2D plasmons in doped graphene. The equilibrium RPA screened potential is compared with the DFT effective potential, yielding good qualitative agreement. The self energy of a point charge near a graphene monolayer is shown to demonstrate an image potential type behavior, Ze/(z−z0), down to very short distances (4 a.u.) above the graphene layer. Both results are found to agree near quantitatively with the DFT ground state energy shift of a Li+ ion placed near a graphene monolayer
Magneto-optical study of thermally annealed InAs-InGaAs-GaAs self-assembled quantum dots
We report a magneto-optical study of InAs-InGaAs-GaAs self-assembled quantum dots (QDs) subjected to post-growth thermal annealing at different temperatures. At low temperatures annealing strongly affects the bimodal distribution of QDs; at higher temperatures a strong blueshift of the emission occurs. Magnetophotoluminescence reveals that the annealing increases the QD size, with a larger effect occurring along the growth axis, and decreases the carrier effective masses. The main contribution to the blueshift is deduced to be an increase in the average Ga composition of the QDs. The inadvertent annealing which occurs during the growth of the upper AlGaAs cladding layer in laser structures is also studied
Donut and dynamic polarization effects in proton channeling through carbon nanotubes
We investigate the angular and spatial distributions of protons of the energy
of 0.223 MeV after channeling through an (11,~9) single-wall carbon nanotube of
the length of 0.2 m. The proton incident angle is varied between 0 and 10
mrad, being close to the critical angle for channeling. We show that, as the
proton incident angle increases and approaches the critical angle for
channeling, a ring-like structure is developed in the angular distribution -
donut effect. We demonstrate that it is the rainbow effect. When the proton
incident angle is between zero and a half of the critical angle for channeling,
the image force affects considerably the number and positions of the maxima of
the angular and spatial distributions. However, when the proton incident angle
is close to the critical angle for channeling, its influence on the angular and
spatial distributions is reduced strongly. We demonstrate that the increase of
the proton incident angle can lead to a significant rearrangement of the
propagating protons within the nanotube. This effect may be used to locate
atomic impurities in nanotubes as well as for creating nanosized proton beams
to be used in materials science, biology and medicine.Comment: 17 pages, 14 figure
Understanding Charge Transfer in Donor-Acceptor/Metal Systems: A Combined Theoretical and Experimental Study
We develop an effective potential approach for assessing the flow of charge
within a two-dimensional donor-acceptor/metal network based on core-level
shifts. To do so, we perform both density functional theory (DFT) calculations
and x-ray photoemission spectroscopy (XPS) measurements of the core-level
shifts for three different monolayers adsorbed on a Ag substrate. Specifically,
we consider perfluorinated pentacene (PFP), copper phthalocyanine (CuPc) and
their 1:1 mixture (PFP+CuPc) adsorbed on Ag(111).Comment: 12 pages, 10 figure
- …