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Intrinsic Nature of the Excess Electron Distribution at the TiO2 (110) Surface


The deposition of Na on the TiO2(110) surface injects
charge into Ti atoms originating a new electronic state in
the band gap, similar to the well-known Defect State due
to oxygen vacancies. In order to spatially localize the injected
charge, we have performed RESPED (Resonant Photoelectron
Diffraction) measurements of the valence band. The RESPED
from the stoichiometric, albeit Na-doped, surface is identical to
that obtained from the reduced surface. This evidence points to
a general property of TiO2(110), that is the redistribution of the
excess of charge is independent of the injection mechanism.
P. Kruger, et al. prl.aps.org/pdf/PRL/v108/i12/e126803

The gap state that appears upon reduction of TiO2 plays a key role in many of titania’s interesting properties but its origin and spatial localization have remained unclear. In the present work, the TiO2(110) surface is reduced in a chemically controlled way by sodium adsorption. By means of resonant photoelectron diffraction, excess electrons are shown to be distributed mainly on subsurface Ti sites strikingly similar to the defective TiO2
(110) surface, while any significant contribution from interstitial Ti ions is discarded.
In agreement with first principles calculations, these
findings demonstrate that the distribution of the band gap charge is an intrinsic property of TiO2(110), independent of the way excess electrons areproduced.

Retrieve article
Intrinsic Nature of the Excess Electron Distribution at the TiO2 (110) Surface, P. Kruger, J. Jupille, S. Bourgeois, B. Domenichini, A. Verdini, L. Floreano, and A. Morgante PRL 108, 126803 (2012)

Donor–Acceptor Shape Matching Drives Performance in Photovoltaics

Shape-complementarity of donor and acceptor molecules drives self-assembly into an extended interface with a ball-and-socket structural motif, which increases both the active volume and exciton dissociation rates to improve the efficiency of organic solar cells.
T. Schiros, et al. http://onlinelibrary.wiley.com/doi/10.1002/aenm.201201125/pdf

While the demonstrated power conversion efficiency of organic photovoltaics (OPVs) now exceeds 10%, new design rules are required to tailor interfaces at the molecular level for optimal exciton dissociation and charge transport in higher efficiency devices. We show that molecular shape-complementarity between donors and acceptors can drive performance in OPV devices. Using core hole clock (CHC) X-ray spectroscopy and density functional theory (DFT), we compare the electronic coupling, assembly, and charge transfer rates at the interface between C60 acceptors and flat- or contorted-hexabenzocorone (HBC) donors. The HBC donors have similar optoelectronic properties but differ in molecular contortion and shape matching to the fullerene acceptors. We show that shape-complementarity drives self-assembly of an intermixed morphology with a donor/acceptor (D/A) ball-and-socket interface, which enables faster electron transfer from HBC to C60. The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved efficiency of organic photovoltaic devices that incorporate these concave/convex D/A materials.

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Donor–Acceptor Shape Matching Drives Performance in Photovoltaics,
Theanne Schiros, Gregor Kladnik, Deborah Prezzi, Andrea Ferretti, Giorgia Olivieri,     Albano Cossaro, Luca Floreano, Alberto Verdini, Christine Schenck, Marshall Cox, Alon A. Gorodetsky, Kyle Plunkett, Dean Delongchamp, Colin Nuckolls, Alberto Morgante, Dean Cvetko, Ioannis Kymissis

Tuning the catalytic activity of Ag(110)-supported Fe phthalocyanine in the oxygen reduction reaction

A careful choice of the surface coverage of iron phthalocyanine (FePc) on Ag (110) around the single monolayer allows us to drive with high precision both the long-range supramolecular arrangement and the local adsorption geometry of FePc molecules on the given surface. We show that this opens up the possibility of sharply switching the catalytic activity of FePc in the oxygen reduction reaction and contextual surface oxidation in a reproducible way. A comprehensive and detailed picture built on diverse experimental evidence from scanning tunnelling microscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, coupled with density functional theory calculations, sheds new light on the nature of the catalytically active molecule–surface coordination and on the boundary conditions for its occurrence. The results are of relevance for the improvement of the catalytic efficiency of metallo-macrocycles as viable substitutes for platinum in the cathodic compartment of low-temperature fuel cells.
F. Sedona, et al. http://www.nature.com/nmat/journal/v11/n11/pdf/nmat3453.pdf

A combination of different techniques such as Scanning Tunneling Microscope, synchrotron radiation X-ray photoelectron and absorption spectroscopy combined with density functional theory (DFT) calculations shows that the molecular local chemisorption site and the long-range supramolecular arrangement of Metallo-Phthalocyanine molecules in the monolayer coverage range on a metal surface can be controlled by fine tuning of the overlayer coverage. This in turn opens the possibility of reliably mastering adsorption-site-selective properties such as the molecular catalytic activity, as will be shown here with respect to the FePc-catalysed oxygen reduction reaction, which can be reproducibly switched on or quenched by controlling the Fe-Phthalocyanine local adsorption site in the single-ML range. Our claims are substantiated by characterizing the molecular coordination in the catalytically active phase in the presence of oxygen and by checking that the Fe-Phthalocyanine molecules remain intact throughout the catalytic cycle. Finally, we show that the catalyst pushes the Ag support oxidation to levels unattainable in ultra high vacuum by dosing oxygen on clean silver single-crystal surfaces, and that reduced oxygen thus formed can be fully removed by providing hydrogen ions to the interface. The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved efficiency of organic photovoltaic devices that incorporate these concave/convex D/A materials.

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Tuning the catalytic activity of Ag(110) supported Fe phthalocyanine in the oxygen reduction reaction, F. Sedona,    M. Di Marino, D. Forrer, A. Vittadini, M. Casarin, A. Cossaro, L. Floreano, A. Verdini and M. Sambi

Quantifying Through-Space Charge Transfer Dynamics in π-Coupled Molecular Systems

This work probes the relation between the rate of charge delocalization and the strength of through-space π-π coupling in stacked aromatic systems. With resonant photoemission we determine charge transfer (CT) dynamics in two molecular bi-layer systems with different inter-ring separation, [2,2]paracyclophane (22PCP) and [4,4]paracyclophane (44PCP) adsorbed on Au(111), which allows us to quantitatively probe the carrier transport as a function of inter-ring coupling strength.
A.Batra et al.  http://www.nature.com/ncomms/journal/v3/n9/full/ncomms2083.html

We use the core-hole clock implementation of resonant photoemission spectroscopy to study the femtosecond charge-transfer dynamics in cyclophanes, which consist of two precisely stacked π-rings held together by aliphatic chains. We study two systems, [2,2]paracyclophane (22PCP) and [4,4]paracyclophane (44PCP), with inter-ring separations of 3.0 Å and 4.0 Å respectively. We find that charge transport across the π-coupled system of 22PCP occurs in 2fs whereas it is 20 times slower in the 44PCP. We attribute this difference to the reduced inter-ring electronic coupling in 44PCP, and illustrate the use of core-hole clock spectroscopy as a general tool for quantifying through-space coupling in p-stacked systems Retrieve article
Quantifying Through-Space Charge Transfer Dynamics in π-Coupled Molecular Systems, Arunabh Batra, Gregor Kladnik, Héctor Vázquez, Jeffrey S. Meisner, Luca Floreano, Colin Nuckolls, Dean Cvetko, Alberto Morgante, Latha Venkataraman

Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization

Surface polymerization is of great interest as it enables the realization of graphene-like layers with tunable properties by simply modifying the architecture of the molecular building blocks used as precursors. We focus on some open points concerning the fundamentals of surface-catalyzed dehalogenative polymerization based on Ullmann coupling, widely used over the past decade to obtain 1D and 2D polymers on surfaces. M. Di Giovannantonio et al. ACS Nano 7 (9), 8190 (2013)

1,4-dibromobenzene molecules were used as precursors, forming poly(para-phenylene) polymers by Ullmann coupling on Cu(110). Chemically sensitive techniques such as x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy allow to unequivocally identify the existence of an organometallic intermediate product of reaction and the of a final extended conjugated structure. Scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and first-principles calculations provide a deeper insight into the intermediate organometallic phase and on the fundamental role of the halogen in stabilizing specific structures. Fast-XPS analysis of the system during the transformation from organometallic chains to polymers unveils the exact transition temperature for this process. Retrieve article
Insight into Organometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymerization, Marco Di Giovannantonio, Mohamed El Garah, Josh Lipton-Duffin, Vincent Meunier, Luis Cardenas, Yannick Fagot Revurat, Albano Cossaro, Alberto Verdini, Dmitrii F. Perepichka, Federico Rosei and Giorgio Contini. ACS Nano 2013, 7 (9), pp 8190-8198 DOI:10.1021/nn4035684

Trimethyltin-Mediated Covalent Gold–Carbon Bond Formation

Spectroscopic evidence of C-Au bond formation, responsible for  
the “electron gateway” state, is shown in the process of
TrimethylTin break-up on gold.

A.Batra et al., J. Am. Chem. Soc., 2014, 136 (36), pp 12556–12559

The formation of covalent gold-carbon bonds is studied in benzyltrimethylstannane (C10H16Sn) deposited on Au in ultra high vacuum. Through X-ray Photoemission Spectroscopy and X-ray absorption measurements, we find that the molecule fragments at the Sn-Benzyl bond when exposed to gold surfaces at temperatures as low as 380K. We show that the resulting benzyl species is stabilized by the presence of Au(111), but only forms covalent Au-C bonds on more reactive Au surfaces like Au(110). In addition, we present spectroscopic proof for the existence of an electronic ‘gateway state’ localized on the Au-C bond that is responsible for its unique electronic properties. Finally, we use density functional theory based nudged elastic band calculations to elucidate the crucial role played by the undercoordinated Au surface in the formation of Au-C bonds.

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Trimethyltin-Mediated Covalent Gold–Carbon Bond Formation, Arunabh Batra, Gregor Kladnik, Narjes Gorjizadeh, Jeffrey Meisner, Michael Steigerwald, Colin Nuckolls, Su Ying Quek, Dean Cvetko, Alberto Morgante, and Latha Venkataraman

Last Updated on Friday, 19 December 2014 17:40