102,212 research outputs found
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
Femto-magnetism and electron transport at core-excited organic molecule/graphene interfaces
No full textWe investigate by ab initio theoretical methods phenomena occurring on the femtosecond time scale as a result of core level excitations in organic molecules adsorbed on graphene. Molecular chemisorption induces a magnetic ground state in graphene, that relaxes towards a non-spin polarized configuration upon the excitation of a molecular core state due to the coupling of the adsorbate energy levels with the graphene mid-gap (defect) ones. Conversely, physisorbed molecules shift from a non-magnetic to a magnetic state [1].
Femtosecond electron transfer times at interfaces can be measured by resonant core-level spectroscopies, where backward transfer (substrate-to-molecule) was also observed following the excitation at the molecule. We describe this phenomenon within a theoretical framework based on density-functional theory (DFT) and a molecular break junction setup [2,3]. The ultrafast transfer (τ=4fs) induced by N 1s excitation for bipyridine molecules on epitaxial graphene/Ni(111) [4] is significantly slowed down by the addition of a second layer of graphene [5]. This is rationalized by the transition from the strong hybridization between C and metal states in epitaxial graphene, to a decoupled interface for bilayer graphene where the C layer in contact with the molecule is less hybridized with Ni underneath. The absence of transfer in principle expected by the current approach for molecules on free-standing graphene is a stimulus for further developments.
References
1. A. Ravikumar, A. Baby, H. Lin, G. P. Brivio, and G. Fratesi, Scientific Reports 6, 24603 (2016), doi:10.1038/srep24603.
2. G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118, 8775 (2014), doi:10.1021/jp500520k.
3. D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, Phys. Chem. Chem. Phys. 18, 22140 (2016), doi:10.1039/c6cp04099c.
4. O. Adak, G. Kladnik, G. Bavdek, A. Cossaro, A. Morgante, D. Cvetko, and L. Venkataraman, Nano Lett. 15, 8316, (2015), doi:10.1021/acs.nanolett.5b03962.
5. 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, submitted
Electronic and spectral properties of clean and C60-covered atom-thick Chromium oxide at the Fe(001) surface
No full textChemisorption of a single atomic layer of oxygen on the Fe(001) surface yields a highly ordered and reproducible benchmark substrate [1] for theoretical and experimental studies, and for the epitaxial growth of metal oxides, including atom-thick CrxOy layers, and hybrid interfaces with foreseen applications e.g. in organic spintronics.
This talk initially presents ab initio investigations that have supplemented microscopy and spectroscopy experiments of the electronic and magnetic properties of two-dimensional Chromium oxides of Cr3O4 and Cr4O5 stoichiometry grown on Fe(001), featuring antiferromagnetic magnetic configurations with underlying Fe(001) [2,3]. Despite Cr / CrO systems are notoriously difficult for mean field approaches, generalized-gradient results are found to explain most experimental findings, with a rigid shift of oxygen bands accounting for electronic correlation effects.
We eventually consider the effect of inserted Cr4O5 layers at the interface between the prototypical C60 organic semiconductor and Fe(001), which is shown to enhance the magnetic hybridization between the molecule and the surface through x-ray magnetic circular dichroism (XMCD) [4,5]. By means of ab initio calculation we characterize the local interface morphology, the magnetic configuration of the surface and the induced spin dependent electronic properties of the molecule, the latter reflecting the magnetic electronic properties of the surface at the relevant energy range. As seen from the substrate, adsorbates can influence the magnitude and even orientation of surface Cr magnetic moments. The interest in this interface is then twofold: on one side the thin magnetic oxide allows tailoring the magnetic properties of the organic layer, on the other side the adsorption of C60 can be envisioned as a tool to control the magnetic ordering of Cr atoms at the interface.
[1] A. Picone, M. Riva, A. Brambilla, A. Calloni, G. Bussetti, M. Finazzi, F. Ciccacci, L. Duò, Surface Science Reports 71, 32 (2016).
[2] A. Picone, G. Fratesi, M. Riva, G. Bussetti, A. Calloni, A. Brambilla, M. I. Trioni, L. Duò, F. Ciccacci, and M. Finazzi, Phys. Rev. B 87, 085403 (2013).
[3] A. Calloni, G. Fratesi, S. Achilli, G. Berti, G. Bussetti, A. Picone, A. Brambilla, P. Folegati, F. Ciccacci, and L. Duò, Phys. Rev. B 96, 085427 (2017).
[4] A. Brambilla, A. Picone, D. Giannotti, A. Calloni, G. Berti, G. Bussetti, S. Achilli, G. Fratesi, M. I. Trioni, G. Vinai, P. Torelli, G. Panaccione, L. Duò, M. Finazzi, and F. Ciccacci, Nano Lett. 17, 7440 (2017).
[5] A. Brambilla, A. Picone, S. Achilli, G. Fratesi, A. Lodesani, A. Calloni, G. Bussetti, M. Zani, M. Finazzi, L. Duò, and F. Ciccacci, Journal of Applied Physics 125, 142907 (2019)
Spectroscopy of adsorbates and the role of interfacial interactions
No full textSurfaces of solids forming interfaces with nanometer-sized adsorbed layers can be used for the growth and/or the study of the adsorbed species. For example, large area samples of graphene can be grown on copper and nickel surfaces; organic molecules can be immobilized at a surface for further characterization, which is especially effective if ordered samples are obtained. The strength of the adsorbate-substrate interaction is crucial since a compromise is necessary between a weak and ineffective one and one where the adsorbate properties are spoiled by too strong electronic coupling at the interface. Electronic and optical spectroscopy techniques, and their theoretical understanding at the nanometer length scale, are especially useful to address these issues, as we exemplify by two test cases from our recent work.
Two-dimensional silicon sheets (“silicene”) can be grown on the surface of silver in various forms, which differ for out-of-plane atomic buckling and registry to the substrate but retain an honeycomb structure analogous to graphene. The peculiar electronic structure of the perfect free-standing film are however disrupted by the strong hybridization between Si and Ag states. By combining our first-principles calculations with angle-resolved photoemission experiments we show that all allotropes display similar electronic bands despite the structural differences, missing the massless Dirac fermions [1]. Optical spectra present the fingerprint of silicene-induced transitions, although now with major participation by silver states and photoinduced charge carriers dynamics consequently approaching typical metal timescales [2].
We investigated silicon surfaces covered by uracil-like nucleobases by simulating the reflectance anisotropy spectra (RAS), that can be used to monitor non-destructively the interface. A characteristic RAS lineshape weakly dependent on the adsorbed species provides the mark of uracile-like adsorption. Differences between nucleobases for the molecular transitions in the visible range are however overwhelmed by modifications in the substrate response. The sign and position of the RAS peaks at higher energy can be fully rationalized in terms of the molecular orbitals involved. Our theoretical results call for a RAS experimental study in the near-UV region [3].
[1] P.M. Sheverdyaeva, S.Kr. Mahatha, P. Moras, L. Petaccia, G. Fratesi, G. Onida, and C. Carbone, “Electronic States of Silicene Allotropes on Ag(111)”, ACS Nano 11, 975 (2017).
[2] E. Cinquanta, G. Fratesi, S. dal Conte, C. Grazianetti, F. Scotognella, S. Stagira, C. Vozzi, G. Onida, and A. Molle, “Optical response and ultrafast carrier dynamics of the silicene-silver interface”, Phys. Rev. B 92, 165427 (2015).
[3] E. Molteni, G. Cappellini, G. Onida, and G. Fratesi, “Optical properties of organically functionalized silicon surfaces: Uracil-like nucleobases on Si(001)”, Phys. Rev. B Just Accepted (9 Feb. 2017)
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
Azimuthal dichroism in near-edge X - ray absorption fine structure spectra of planar molecules
No full textAzimuthal Dichroism in Near-Edge X - ray Absorption Fine Structure Spectra of Planar Molecules:
The dependence of the near-edge x-ray absorption fine structure (NEXAFS) spectrum of molecules on the photon electric field direction is investigated by means of first principles simulations based on density-functional theory with the transition-potential approach. In addition to the well-known dependence of the NEXAFS resonances on the orientation of the electric field with respect to the molecular plane, we demonstrate that for planar molecules with sufficient in-plane anisotropy such as pentacene, a dichroic effect is computed with a splitting of the σ* resonance as a function of the azimuthal orientation of the photon electric field in the molecular plane. The σ* splitting is investigated as a function of the length of acenes and closely related molecules. A proper assignment of such spectral features guided by theory together with variable polarization experiments, may allow one to completely determine the orientation of molecules at interfaces.
[1] G. Fratesi, V. Lanzilotto, L. Floreano, and G. P. Brivio, J. Phys. Chem. C (2013), vol. 117, pages 6632-663
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
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
Territorio, istituzioni, crescita: il quadro teorico e il contesto italiano
No full textThe paper analyzes the situation in Italy and its regions in the international context and in the light of the economic crisis, showing that that the various Italian regions are affected in very different ways
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)
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