1,721,012 research outputs found
Electron transfer with core-level excitations at hybrid interfaces
No full textElectron core-level spectroscopies have emerged as effective tools to investigate several aspects of the hybrid interface between organic molecules and a substrate. In particular, resonant photoemission spectroscopy can measure interfacial electron transfer times down to the femtosecond timescale. Furthermore, the strong perturbation induced by the core hole opens up the several questions on how the properties of the interface are modified, calling for a theoretical description of the core-excited system.
We adopt a theoretical framework based on density-functional theory (DFT), where the excitation is introduced explicitly in the core-level occupation of an atom in a molecule, to investigate the electronic structure and electron transfer from/to organic molecules adsorbed on metal, semimetal, and semiconducting substrates.
The perturbing potential lowers the energy of the molecular orbitals. Focusing on the lowest-unoccupied (LUMO), a filling of the core-excited LUMO* by substrate electrons may occur within the core-hole lifetime, as found for molecules on metals where the adsorption angle is also shown to influence the electron transfer rate [1,2]. In the case of a semimetal graphene substrate, a spin-polarized LUMO* pinned at the Fermi level can be determined for physisorbed molecules. In that case electron transfer would be suppressed given the low density of states of unsupported graphene at that energy, but still possible for graphene supported on a metal [3]. For molecules adsorbed on a semiconductor, the LUMO* may form a bound exciton in the gap [4]. Here, we found especially interesting to consider the influence of thermal motion on the energy-level alignment and the absorption coefficient [5,6].
References
[1] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, submitted.
[2] A. Baby, G. Fratesi, S.R. Vaidya, L.L. Patera, C. Africh, L. Floreano, G.P. Brivio, J. Phys. Chem. C 119 (2015) 3624.
[3] A. Ravikumar, A. Baby, H. Lin, G.P. Brivio, and G. Fratesi, Scientific Reports 6 (2016) 24603.
[4] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118 (2014) 8775
[5] H. Lin, G. Fratesi, S. Selçuk, G.P. Brivio, and A. Selloni, J. Phys. Chem. C, 120 (2016) 3899.
[6] M. Muller, D. Sànchez-Portal, H. Lin, G. Fratesi, G.P. Brivio, and A. Selloni, in preparation
New feature in the Auger peak of adsorbed oxygen
No full textRecent joint theoretical and experimental investigations of Auger core-core-valence spectra of alkali adatoms on simple metals have revealed that such technique is capable to ascertain contributions from different adsorption environments in the signal [M.I. Trioni, S. Caravati, G.P. Brivio, L. Floreano, F. Bruno, A. Morgante, Phys. Rev. Lett. 93 (2004) 206802]. Consequently, to verify if such an effect is present also for other chemical species, we study theoretically the KLV transition of oxygen either as a bulk impurity or as an adsorbate in/on Al and Ag (jellium-like). We make use of the Fermi golden rule in which the matrix elements of the interaction are calculated within DFT. We verify that the relevant physical quantity of this phenomenon is the excited local density of states (LDOS), calculated within a region centered on the core ionized atom. The Auger rate for oxygen in Ag bulk displays a single asymmetric peak, while for adsorbed oxygen a second smaller feature at lower energies, and very close to the first one, appears. This unexpected result follows from the removal of the degeneracy of the m quantum number of the 2p states of oxygen at the surface. It is only displayed on the electronically less dense metal (Ag), but not on Al
Quantum and Classical Image Charges at Metal Surfaces
No full textPoint charges at a metal surface are balanced by image charges canceling their electrostatic potential below the surface. This well-known phenomenon is at the basis of important observations also at the nanometer length scale, where a quantum description proves essential, but may escape common first-principle theoretical approaches.
By reviewing two different examples, I will discuss cases where standard description by Density Functional Theory with the independent-particle Kohn-Sham formalism (KS-DFT) can / cannot grasp the true findings. (i) The potential for an electron at a jellium surface in common uses of KS-DFT dramatically misses the correct decay at large distances, which is due to coupling with dynamical fluctuations in the surface charge density, and is restored by higher level of theory explicitly including electron-electron many-body interactions [1-3]. (ii) An alkali atom adsorbed on a metal typically charges positively, resulting in a strong dipole whose electric field below the surface is balanced by image dipole. The occurrence of the image dipole affects the interaction between the adsorbates, the charge transfer and magnitude of the dipole itself, aspects that we show to be described by KS-DFT calculations [4-9] well validated by the quantitative agreement to a variety of experimental findings [10,6,8].
References:
[1] A.G. Eguiluz, M. Heinrichsmeier, A. Fleszar, and W. Hanke, Phys. Rev. Lett. 68, 1359 (1992), DOI: http://dx.doi.org/10.1103/PhysRevLett.68.1359
[2] G. Fratesi, G.P. Brivio, P. Rinke, and R.W. Godby, Phys. Rev. B 68, 195404 (2003), DOI: http://dx.doi.org/10.1103/PhysRevB.68.195404
[3] G. Fratesi, G.P. Brivio, and L.G. Molinari, Phys. Rev. B 69, 245113 (2004), DOI: http://dx.doi.org/10.1103/PhysRevB.69.245113
[4] G. Fratesi, G. Alexandrowicz, M.I. Trioni, G.P. Brivio, and W. Allison, Phys. Rev. B 77, 235444 (2008), DOI: http://dx.doi.org/10.1103/PhysRevB.77.235444
[5] G. Fratesi, Phys. Rev. B 80, 045422 (2009), DOI: http://dx.doi.org/10.1103/PhysRevB.80.045422
[6] H. Hedgeland, P.R. Kole, H.R. Davies, A.P. Jardine, G. Alexandrowicz, W. Allison, J. Ellis, G. Fratesi, and G.P. Brivio, Phys. Rev. B 80, 125426 (2009), DOI: http://dx.doi.org/10.1103/PhysRevB.80.125426
[7] G. Fratesi, A. Pace, and G.P. Brivio, J. Phys.-Condens. Matter 22, 304005 (2010), DOI: http://dx.doi.org/10.1088/0953-8984/22/30/304005
[8] C. Huang, G. Fratesi, D.A. MacLaren, W. Luo, G.P. Brivio, and W. Allison, Phys. Rev. B 82, 081413(R) (2010), DOI: http://dx.doi.org/10.1103/PhysRevB.82.081413
[9] G. Fratesi, Phys. Rev. B 84, 155424 (2011), DOI: http://dx.doi.org/10.1103/PhysRevB.84.155424
[10] G. Alexandrowicz, A. P. Jardine, H. Hedgeland, W. Allison, and J. Ellis, Phys. Rev. Lett. 97, 156103 (2006), DOI: http://dx.doi.org/10.1103/PhysRevLett.97.15610
Femtosecond electron transfer at core-excited adsorbed molecules
No full textCharge transfer phenomena at metal/organic interfaces are a crucial step affecting the efficiencies of devices for organic based electronics and photovoltaics. A quantitative study of electron transfer rates, which take place on the femtosecond timescale, is often difficult, especially since in most systems the molecular adsorption geometry is unknown. Electron core-level spectroscopies have emerged as effective tools to investigate several aspects of the hybrid interface between organic molecules and a substrate. In particular, X-ray resonant photoemission spectroscopy can measure interfacial electron transfer times down to the femtosecond timescale. Furthermore, the strong perturbation induced by the core hole opens up the several questions on how the properties of the interface are modified, calling for a theoretical description of the core-excited system.
Here, we use X-ray resonant photoemission spectroscopy to measure ultrafast charge transfer rates across pyridine/Au(111) interfaces while also controlling the molecular orientation on the metal [1]. We demonstrate that a bi-directional charge transfer across the molecule/metal interface is enabled upon creation of a core-exciton on the molecule with a rate that has a strong dependence on the molecular adsorption angle.
We adopt a theoretical framework based on density-functional theory (DFT), where the excitation is introduced explicitly in the core-level occupation of an atom in a molecule, to investigate the electronic structure and electron transfer from/to the molecules adsorbed on a semi-infinite metal, whose continuum of states is described by a Green's function method [2]. We show that the alignment of molecular levels relative to the metal Fermi level is dramatically altered when a core-hole is created on the molecule, allowing the lowest unoccupied molecular orbital to fall partially below the metal Fermi level opening to substrate-to-molecule electron transfer in X-ray photoemission experiments. We also calculate charge transfer rates as a function of molecular adsorption geometry and find a trend in semiquantitative agreement with the experiment [1].
References:
[1] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, Phys. Chem. Chem. Phys. 18 (2016) 22140
[2] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118 (2014) 877
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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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