Using surface plasmonics to turn on fullerene's dark excitons

Using our recently proposed Bethe-Salpeter $G_0W_0$ formulation, we explore the optical absorption spectra of fullerene (C$_{60}$) near coinage metal surfaces (Cu, Ag, and Au). We pay special attention to how the surface plasmon $\omega_S$ 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 $\pi$ exciton, i.e.\ the dipolar mode at $\hbar\omega_+\approx$ 6.5 eV, and the quadrupolar mode at $\hbar\omega_-\approx$ 6.8 eV. When fullerene is close to a copper surface ($z_0\approx$ 5.3 \AA) the dipolar mode $\omega_+$ 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 $\omega_S\approx\omega_\pm$, the strong interaction with the surface plasmon destroys the $\omega_-$ 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, $\omega_S \approx \omega_\pm$, is satisfied by silver and gold metal surfaces.Comment: 10 pages, 8 figure

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

Analytical expression for stopping force acting on a slow charged particle moving parallel to a thick graphene-sapphire-graphene structure

We derive an analytical expression for the stopping force acting on an external charged particle moving parallel to a sandwich-like structure consisting of two undoped graphene sheets separated by a layer of Al2O3 (sapphire)

Interactions of ions with graphene-sapphire-graphene composite system: Stopping force and image force

We derive general expressions for the stopping and image forces on an external charged particle moving parallel to a sandwich-like structure consisting of two doped graphene sheets separated by a layer of Al2O3 (sapphire

Quasiparticle interfacial level alignment of highly hybridized frontier levels: H2_2O on TiO2_2(110)

Knowledge of the frontier levels' alignment prior to photo-irradiation is necessary to achieve a complete quantitative description of H$_2$O photocatalysis on TiO$_2$(110). Although H$_2$O on rutile TiO$_2$(110) has been thoroughly studied both experimentally and theoretically, a quantitative value for the energy of the highest H$_2$O occupied levels is still lacking. For experiment, this is due to the H$_2$O levels being obscured by hybridization with TiO$_2$(110) levels in the difference spectra obtained via ultraviolet photoemission spectroscopy (UPS). For theory, this is due to inherent difficulties in properly describing many-body effects at the H$_2$O-TiO$_2$(110) interface. Using the projected density of states (DOS) from state-of-the-art quasiparticle (QP) $G_0W_0$, we disentangle the adsorbate and surface contributions to the complex UPS spectra of H$_2$O on TiO$_2$(110). We perform this separation as a function of H$_2$O coverage and dissociation on stoichiometric and reduced surfaces. Due to hybridization with the TiO$_2$(110) surface, the H$_2$O 3a$_1$ and 1b$_1$ levels are broadened into several peaks between 5 and 1 eV below the TiO$_2$(110) valence band maximum (VBM). These peaks have both intermolecular and interfacial bonding and antibonding character. We find the highest occupied levels of H$_2$O adsorbed intact and dissociated on stoichiometric TiO$_2$(110) are 1.1 and 0.9 eV below the VBM. We also find a similar energy of 1.1 eV for the highest occupied levels of H$_2$O when adsorbed dissociatively on a bridging O vacancy of the reduced surface. In both cases, these energies are significantly higher (by 0.6 to 2.6 eV) than those estimated from UPS difference spectra, which are inconclusive in this energy region. Finally, we apply self-consistent QP$GW$ (scQP$GW$1) to obtain the ionization potential of the H$_2$O-TiO$_2$(110) interface.Comment: 12 pages, 12 figures, 1 tabl

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

The dissociation of molecules, even the most simple hydrogen molecule, cannot be described accurately within density functional theory because none of the currently available functionals accounts for strong on-site correlation. This problem led to a discussion of properties that the local Kohn-Sham potential has to satisfy in order to correctly describe strongly correlated systems. We derive an analytic expression for the nontrivial form of the Kohn-Sham potential in between the two fragments for the dissociation of a single bond. We show that the numerical calculations for a one-dimensional two-electron model system indeed approach and reach this limit. It is shown that the functional form of the potential is universal, i.e., independent of the details of the two fragments.We acknowledge funding by the Spanish MEC (Grant No. FIS2007-65702-C02-01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (Grant No. IT-319-07), and the European Community through e-I3 ETSF project (Grant Agreement No. 211956).Peer reviewe

Low-energy plasmons in quantum-well and surface states of metallic thin films

We studied low-energy plasmons in ultrathin films of silver in the thickness regimes where the surface states as well as quantum-well states must play significant roles. Realistic band structure was adopted for assessing the quantum-mechanical effect on the low-energy charge dynamics. In addition to the expected quasi-two-dimensional plasmon mode, we find the modes that resemble an acoustic surface plasmon on semi-infinite metal surfaces and an additional plasmon mode related to the interband transitions between the two slab-split surface states. It is found that the dispersion of the latter mode is almost identical to the acoustic surface plasmon except the energy offset at small momenta values. The peaks in the surface response function related to interband transitions between the surfacelike states and bulklike states are identified as well. The present work elucidates the role of quantized electronic states and surface states on the plasmonic excitations in the ultimately thin films potentially used in the future nano-optics devices. © 2011 American Physical Society.Peer Reviewe