82 research outputs found
Coverage Dependence of the Level Alignment for Methanol on TiO(110)
Electronic level alignment at the interface between an adsorbed molecular
layer and a semiconducting substrate determines the activity and efficiency of
many photocatalytic materials. We perform calculations to determine
the coverage dependence of the level alignment for a prototypical
photocatalytic interface: 1/2 and 1 monolayer (ML) intact and dissociated
CHOH on rutile TiO(110). We find changes in the wavefunction's spatial
distribution, and a consequent renormalization of the quasiparticle energy
levels, as a function of CHOH coverage and dissociation. Our results
suggest that the occupied molecular levels responsible for hole trapping are
not those observed in the ultraviolet photoemission spectroscopy (UPS)
spectrum. Rather, they are those of isolated CHO on the surface. We find
the unoccupied molecular levels have either 2D character with weight above the
surface at 1 ML coverage, or significant hybridization with the surface at 1/2
ML coverage. These results suggest the resonance observed in the two photon
phooemission (2PP) spectrum arises from excitations to unoccupied "Wet
electron" levels with 2D character.Comment: 8 pages, 5 figures, 1 tabl
Using Level Alignment to Identify Catechol's Structure on TiO(110)
We perform state-of-the-art calculations for a prototypical dye sensitized
solar cell: catechol on rutile TiO(110). Catechol is often used as an
anchoring group for larger more complex organic and inorganic dyes on TiO
and forms a type II heterojunctions on TiO(110). In particular, we compare
quasiparticle (QP) with hybrid exchange correlation functional (HSE)
density functional theory (DFT) calculations for the catechol-rutile
TiO(110) interface. In so doing, we provide a theoretical interpretation of
ultraviolet photoemission spectroscopy (UPS) and inverse photoemission
spectroscopy (IPES) experiments for this prototypical system. Specifically, we
demonstrate that the position, presence, and intensity of peaks associated with
catechol's HOMO, intermolecular OHO bonds, and interfacial hydrogen bonds to
the surface bridging O atoms (OHC and OHO) may be used to
fingerprint deprotonation of catechol's OH anchoring groups. Furthermore, our
results suggest deprotonation of these groups, while being nearly isoenergetic
at high coverages, may significantly increase the photovoltaic efficiency of
catecholTiO(110) interfaces.Comment: 7 pages, 4 figures, corrected table
Comparing quasiparticle HO level alignment on anatase and rutile TiO
Knowledge of the molecular frontier levels' alignment in the ground state can
be used to predict the photocatalytic activity of an interface. The position of
the adsorbate's highest occupied molecular orbital (HOMO) levels relative to
the substrate's valence band maximum (VBM) in the interface describes the
favorability of photogenerated hole transfer from the VBM to the adsorbed
molecule. This is a key quantity for assessing and comparing HO
photooxidation activities on two prototypical photocatalytic TiO surfaces:
anatase (A)-TiO(101) and rutile (R)-TiO(110). Using the projected
density of states (DOS) from state-of-the-art quasiparticle (QP)
calculations, we assess the relative photocatalytic activity of intact and
dissociated HO on coordinately unsaturated (Ti) sites of
idealized stoichiometric A-TiO(101)/R-TiO(110) and bridging O vacancies
(O) of defective
A-TiO(101)/R-TiO(110) surfaces ()
for various coverages. Such a many-body treatment is necessary to correctly
describe the anisotropic screening of electron-electron interactions at a
photocatalytic interface, and hence obtain accurate interfacial level
alignments. The more favorable ground state HOMO level alignment for
A-TiO(101) may explain why the anatase polymorph shows higher
photocatalytic activities than the rutile polymorph. Our results indicate that
(1) hole trapping is more favored on A-TiO(101) than R-TiO(110) and (2)
HO@Ti is more photocatalytically active than intact
HO@Ti
Level alignment of a prototypical photocatalytic system: Methanol on TiO2(110)
Photocatalytic and photovoltaic activity depends on the optimal alignment of
electronic levels at the molecule/semiconductor interface. Establishing level
alignment experimentally is complicated by the uncertain chemical identity of
the surface species. We address the assignment of the occupied and empty
electronic levels for the prototypical photocatalytic system of methanol on a
rutile TiO2 (110) surface. Using many-body quasiparticle (QP) techniques we
show that the frontier levels measured in ultraviolet photoelectron and two
photon photoemission spectroscopy experiments can be assigned with confidence
to the molecularly chemisorbed methanol, rather than its decomposition product,
the methoxy species. We find the highest occupied molecular orbital (HOMO) of
the methoxy species is much closer to the valence band maximum, suggesting why
it is more photocatalytically active than the methanol molecule. We develop a
general semi-quantitative model for predicting many-body QP energies based on
the appropriate description of electronic screening within the bulk, molecular
or vacuum regions of the wavefunctions at molecule/semiconductor interfaces.Comment: 5 pages, 5 figure
PFO-BPy solubilizers for SWNTs: Modelling of polymers from oligomers
arXiv:1411.3275v1Due to their exeptional physical properties, single walled carbon nanotubes (SWNTs) embedded in organic polymers (polymer-SWNT hybrid systems) are promising materials for organic photovoltaic devices. Already at the SWNT sorting and debundling step, polymers such as the copolymer of 9,9-dioctylfluorenyl-2,7-diyl and bipyridine (PFO-BPy) are used as solubilizers. However, to model polymer-SWNT hybrid systems, we must first determine the smallest oligomer needed to sufficiently describe the electronic and optical absorption properties of the polymer. To do so, we use time dependent density functional theory (TDDFT) to model the PFO-BPy polymer using the monomers, dimers and trimers of the PFO-BPy and Py-PFO-Py building blocks, which are also compared to the infinitely long polymer. We find the Py-PFO-Py monomer, with shortened side chains, already describes the PFO-BPy polymer within the expected accuracies of TDDFT.We acknowledge funding from the European Projects DYNamo (ERC-2010-AdG-267374), CRONOS (280879-2CRONOS CP-FP7) and POCAONTAS (FP7-PEOPLE-2012-ITN-316633); Spanish Grants (FIS2012-37549-C05-02, FIS2010-
21282-C02-01, PIB2010US-00652, JCI-2010-08156); and Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07).Peer Reviewe
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.D.J.M. acknowledges financial support from the Spanish “Juan de la Cierva” program (JCI-2010-08156), MICINN (FIS2010-21282-C02-01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (IT-319-07), and ACI-Promociona (ACI2009-1036).Peer Reviewe
Quantum-ionic features in the absorption spectra of homonuclear diatomic molecules
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We show that additional features can emerge in the linear absorption spectra of homonuclear diatomic molecules when the ions are described quantum mechanically. In particular, the widths and energies of the peaks in the optical spectra change with the initial configuration, mass, and charge of the molecule. We introduce a model that can describe these features and we provide a quantitative analysis of the resulting peak energy shifts and width broadenings as a function of the mass.We acknowledge financial support from the European Research Council Advanced Grant DYNamo (Grant No. ERC-2010-AdG-267374), Spanish Grants No. FIS2013-46159-C3-1-P and No. PIB2010US-00652, and Grupo Consolidado UPV/EHU del Gobierno Vasco (Grant No. IT578-13). A.C.-U. acknowledges financial
support from the Departamento de Educacion, Universidades e Investigacion del Gobierno Vasco (Reference No. BFI-2011-26).Peer Reviewe
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