39 research outputs found
Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces
No full textThe properties of novel and prospective 2D materials are dramatically influenced by the interaction with a substrate. For example, the electronic hybridization of silicene states on Ag(111) or graphene ones on Ni(111) disrupts the Dirac fermions of the freestanding layers. This calls for efficient approaches to tune the interaction strength at the interface. Here we focus on the case of graphene functionalized by organic molecules and grown on Ni(111) and on the interfacial charge transfer dynamics. This is investigated by X-ray resonant photoemission spectroscopy, that is able to measure electron transfer rates occurring within few femtoseconds, and by a theoretical framework based on density-functional theory [1,2].
We use 4,4’-bipyridine as the prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection (τ=4fs) of electrons from the substrate to the molecule adsorbed on epitaxial graphene/Ni(111), which is characterized by a strong hybridization between C and metal states. We demonstrate that this interface can be decoupled by the addition of a second layer of graphene, where the one in contact with the metal acts as a buffer layer and the one in contact with the molecule is less hybridized with Ni underneath. As a result, the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.
[1] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118, 8775 (2014)
[2] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, Phys. Chem. Chem. Phys. 18, 22140 (2016)
[3] A. Ravikumar, G. Kladnik, M. Müller, A. Cossaro, G. Bavdek, L. Patera, D. Sánchez-Portal, L. Venkataraman, A. Morgante, G. P. Brivio, D. Cvetko, and G. Fratesi, Nanoscale 10, 8014 (2018)
Phase diagram of pentacene growth on Au(110)"
No full textWe studied the growth of pentacene (C22H14) on the Au(110) surface by means of He atom scattering and Synchrotron X-ray photoemission. We found that two-dimensional commensurate growth only occurs in the monolayer range for a substrate temperature, T-s, higher than similar to 370 K. Larger amounts of deposited molecules forms three-dimensional uncorrelated clusters on the wetting layer. The desorption of second layer molecules occurs at T-s >= 420 K. The highest coverage ordered phase displays a (6 x 8) symmetry and corresponds to the saturation coverage at T, = 420 K. The (3 x 6) symmetry phase, recently reported for a multilayer planar film [Ph. Guaino, et al. Appl. Phys. Lett. 2004, 85, 2777], is only found at a coverage slightly lower than the (6 x 8) one. The (3 x 6) phase corresponds to the saturation coverage of the first layer at T-s = 470 K
TiO2(110) charge donation to an extended π-Conjugated molecule
No full textThe surface reduction of rutile TiO2(110) generates a state in the band gap whose excess electrons are spread among multiple sites, making the surface conductive and reactive. The charge extraction, hence the surface catalytic properties, depends critically on the spatial extent of the charge redistribution, which has been hitherto probed by small molecules that recombine at oxygen vacancy (Ovac) sites. We demonstrate by valence band resonant photoemission (RESPES) a very general charge extraction mechanism from a reduced TiO2(110) surface to an extended electron-acceptor organic molecule. Perylene-tetra-carboxylic-diimide (PTCDI) is not trapped at Ovac sites and forms a closely packed, planar layer on TiO2(110). In this configuration, the perylene core spills out the substrate excess electrons, filling the lowest unoccupied molecular orbital (LUMO). The charge transfer from the reduced surface to an extended pi-conjugated system demonstrates the universality of the injection/extraction mechanism, opening new perspectives for the coupling of reducible oxides to organic semiconductors and supported catalysts
Substrate induced ultrafast electron injection dynamics at organic-graphene interfaces
No full textElectron core-level spectroscopies can effectively be used to investigate electron transfer rates at organic/inorganic interfaces occurring within few femtoseconds. The core-level excitation at an adsorbed molecule strongly perturbs the system and calls for a proper theoretical description. On the other hand it induces novel phenomena such as backward electron transfer (substrate-to-molecule) as we measure by X-ray resonant photoemission and calculate by a theoretical framework based on density-functional theory (DFT) [1]. The rates can be controlled by varying molecular properties like the adsorption angle [2], as well as by tailoring the substrate like we show here for molecules on graphene.
N1s core excitation induces ultrafast electron transfer (τ=4fs) for bipyridine molecules on epitaxial graphene/Ni(111), which is characterized by a strong hybridization between C and metal states. We demonstrate that this interface can be decoupled by the addition of a second layer of graphene, so that the one in contact with the molecule is less hybridized with Ni underneath. In that case, transfer rates decrease by about one order of magnitude in the experiments and in the simulations, whereas no transfer is in principle expected for molecules on freestanding graphene within the current description.
[1] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118 (2014) 8775
[2] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, Phys. Chem. Chem. Phys. 18 (2016) 2214
Order-disorder transition of the (3X3) Sn/Ge(111) phase
No full textWe have measured the long-range order of the alpha phase of Sn on the Ge(111) surface throughout the (3 x 3) --> (root3 x root3)R30 degrees phase transition. The transition has been found of the order-disorder type with a critical temperature T-c similar to 220 K. The expected three-state Potts critical exponents are shown to be consistent with the observed power-law dependence of the (3 x 3) order parameter and its correlation length close to T-c, thus excluding a charge-density wave driven phase transition
Quantum size effects in the low temperature layer-by-layer growth of Pb on Ge(001)
No full textThe electronic properties of thin metallic films deviate from the corresponding bulk ones when the film thickness is comparable with the wavelength of the electrons at the Fermi level. This phenomenon, referred to as quantum size effect (QSE), is also expected to affect the film morphology and structure leading to the "electronic growth" of metals on semiconductors. Such effect may be observed when metals are grown on substrates held at low temperature and are manifested through the occurrence of "magical" thickness islands or critical thickness for layer-by-layer growth. In particular, layer-by-layer growth of Pb(111) films has been reported for deposition on Ge(001) below similar to130 K. An extremely flat morphology is preserved throughout deposition from four up to a dozen of monolayers. These flat films are shown to be metastable and to reorganize into large clusters uncovering the first Pb layer pseudomorphic to the underlying Ge(001) substrate already at room temperature. Indications of QSE induced structural variations of the growing films have been reported for Pb growth on both Si(111) and Ge(001). In the latter case, the apparent height of the Pb(111) monatomic step was shown to change in an oscillatory fashion by He atom scattering (HAS) during layer-by-layer growth at low temperature. The extent of the structural QSE has been obtained by a comparison of the HAS data with X-ray diffraction (XRD) and reflectivity experiments. Whereas step height variations as large as 20% have been measured by HAS reflectivity, the displacement of the atomic planes from their bulk position, as measured by XRD, has been found to mainly affect the topmost Pb layer, but with a lower extent, i.e. the QSE observed by HAS are mainly due to a perpendicular displacement of the topmost layer charge density. The effect of the variable surface relaxation on the surface vibration has been studied from the acoustic dispersion of the low energy phonons, as measured by inelastic HAS
Using A Buffer Layer To Tune Electron Injection Dynamics At The Organic-graphene/metal Interface
No full textThe properties of novel and prospective 2D materials are dramatically influenced by the interaction with a substrate. For example, the electronic hybridization of silicene states on Ag(111) or graphene ones on Ni(111) disrupts the Dirac fermions of the freestanding layers. This calls for efficient approaches to tune the interaction strength at the interface. Here we focus on the case of graphene functionalized by organic molecules and grown on Ni(111) and on the interfacial charge transfer dynamics. This is investigated by X-ray resonant photoemission spectroscopy, that is able to measure electron transfer rates occurring within few femtoseconds, and by a theoretical framework based on density-functional theory [1,2].
We use 4,4’-bipyridine as the prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection (τ=4fs) of electrons from the substrate to the molecule adsorbed on epitaxial graphene/Ni(111), which is characterized by a strong hybridization between C and metal states. We demonstrate that this interface can be decoupled by the addition of a second layer of graphene, where the one in contact with the metal acts as a buffer layer and the one in contact with the molecule is less hybridized with Ni underneath. This decreases the charge transfer rates by about one order of magnitude and is seen in both theory and experiments.
[1] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118 (2014) 8775
[2] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, Phys. Chem. Chem. Phys. 18 (2016) 2214
Characterization of early growth stages of Pb/Ge(001)
No full textEarly stages of thin Pb film growth on Ge(001) substrate, exhibiting quantum size effects (QSE), are characterized by means of Photoelectron Diffraction and Helium Atom Scattering. Pb is found to form a commensurate first monolayer, while an ordered layer-by-layer growth only sets in after deposition of 4 monolayers. In the intermediate coverage range no long range order of the overlayer is established and we find that uncorrelated islands of preferred four-layer thickness are formed. Continuous Pb film with long range order emerges through islands coalescence close to a coverage of 4 monolayers, upon which a more regular layer-by-layer growth mode sets in
Copper-phthalocyanine induced reconstruction of Au(110)
No full textThe structure of ultrathin Cu-phthalocyanine (Cu-Pc) film on Au(110) has been studied by means of several diffraction tecniques: helium atom scattering (HAS), low energy electron diffraction (LEED) and grazing incidence X-ray diffraction (GIXD). HAS has been used to measure the long range order of the organic overlayer, whereas LEED at 200 eV has been used to probe the corresponding substrate reconstruction. At the monolayer coverage, the Au(110) substrate displays a reconstruction with a 3-fold periodicity along the [001] direction, whose structure has been studied by out of plane GIXD and variable polarization X-ray absorption near edge spectroscopy (XANES). We found the structure of the substrate unit cell to be an asymmetric shallow (1 x 3) reconstruction with the Cu-Pc molecules tilted by an angle of similar to32degrees from the (110) surface plane
Pentacene nanorails on Au(110)
No full textWe studied the molecular orientation of pentacene monolayer phases on the Au(110) surface by means of near-edge X-ray absorption spectroscopy at the carbon K-shell and scanning tunneling microscopy. The highest coverage phase, displaying a (6 x 8) symmetry, is found to be formed by two types of differently oriented molecules mimicking regular arrays of nanorails. Flat-lying molecules, aligned side-by-side with the long molecular axis along the [001] direction, form long crosstie chains extending in the [1 (1) over bar0] direction. In between the adjacent flat chains, additional molecules, tilted by 90 degrees around their molecular axis, line up head-to-tail into rails extending along [1<(1)over bar0]. These molecules are very weakly hybridized with the substrate, as indicated by their lowest unoccupied molecular orbitals, which closely resemble those of the free molecule. The nanorail structure is found to be stable up to 420 K in vacuum and to also remain in place after exposure to air, thus being a template well suited for further self-assembly of organic heterostructures. The tilted quasi-free molecules open the possibility for an optimal lateral pi-coupling to other molecules or molecular assemblies
