Coverage Dependence of the Level Alignment for Methanol on TiO2_2(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 $G_0W_0$ calculations to determine the coverage dependence of the level alignment for a prototypical photocatalytic interface: 1/2 and 1 monolayer (ML) intact and dissociated CH$_3$OH on rutile TiO$_2$(110). We find changes in the wavefunction's spatial distribution, and a consequent renormalization of the quasiparticle energy levels, as a function of CH$_3$OH 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 CH$_3$O 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 G0W0G_0W_0 Level Alignment to Identify Catechol's Structure on TiO2_2(110)

We perform state-of-the-art calculations for a prototypical dye sensitized solar cell: catechol on rutile TiO$_2$(110). Catechol is often used as an anchoring group for larger more complex organic and inorganic dyes on TiO$_2$ and forms a type II heterojunctions on TiO$_2$(110). In particular, we compare quasiparticle (QP) $G_0W_0$ with hybrid exchange correlation functional (HSE) density functional theory (DFT) calculations for the catechol-rutile TiO$_2$(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 OH$-$O bonds, and interfacial hydrogen bonds to the surface bridging O atoms (O$_{br}$H$-$C and O$_{br}$H$-$O) 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 catechol$-$TiO$_2$(110) interfaces.Comment: 7 pages, 4 figures, corrected table

Comparing quasiparticle H2_2O level alignment on anatase and rutile TiO2_2

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 H$_2$O photooxidation activities on two prototypical photocatalytic TiO$_2$ surfaces: anatase (A)-TiO$_2$(101) and rutile (R)-TiO$_2$(110). Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) $G_0W_0$ calculations, we assess the relative photocatalytic activity of intact and dissociated H$_2$O on coordinately unsaturated (Ti$_{\textit{cus}}$) sites of idealized stoichiometric A-TiO$_2$(101)/R-TiO$_2$(110) and bridging O vacancies (O$_{\textit{br}}^{\textit{vac}}$) of defective A-TiO$_{2-x}$(101)/R-TiO$_{2-x}$(110) surfaces ($x=\frac{1}{4},\frac{1}{8}$) 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$_2$(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$_2$(101) than R-TiO$_2$(110) and (2) HO@Ti$_{\textit{cus}}$ is more photocatalytically active than intact H$_2$O@Ti$_{\textit{cus}}$

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