1,721,083 research outputs found
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
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)
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)
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
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
Theoretical core-level spectroscopy from adsorbed organic molecules
No full textHybrid interfaces between organic molecules and inorganic substrates are key systems in many technological applications, ranging from photovoltaics to organic field effect transistors and light emitting devices. Several properties of such interfaces can be effectively studied by electron core-level spectroscopies: X-ray photoemission (XPS) addresses changes in the chemical state of the atoms; near-edge X-ray absorption fine structure (NEXAFS) accesses molecular orientations and provides information on the unoccupied electronic levels; resonant photoemission spectroscopies (RESPES) can measure interfacial electron transfer times down to the femtosecond timescale.
The potential of these experimental techniques is fully obtained by complementing them with theoretical investigations and reliable numerical modeling. The perturbation by charged atomic cores is efficiently taken into account within a density-functional theory (DFT) based approach, as will be exemplified here. Numerical simulation of the dependence of NEXAFS molecular spectra on the photon electric field direction for pentacene and perylene derivatives [1,2] demonstrates how one can completely determine the orientation (polar and azimuthal) of molecules at interfaces. Disentangling the contribution of specific atomic cores (initial states) and molecular orbitals (final states) to NEXAFS [2], is further used to demonstrate filling of the lowest-unoccupied molecular orbital of pentacene in the peculiar V-bent conformation it assumes on Al(001) [3]. Excitonic effects due to the presence of core-excited atoms in the molecule can be included in the modeling to understand the electron-transfer times as measured by RESPES, which are evaluated numerically for molecules on a semi-infinite TiO2(110) substrate whose continuum of conduction states is described by Green’s function techniques [4].
[1] J. Phys. Chem. C, 2013, 117 (13), pp 6632–6638
[2] Phys. Chem. Chem. Phys., 2014, 16 (28), pp 14834-14844
[3] J. Phys. Chem. C, 2015, 119 (7), pp 3624–3633
[4] J. Phys. Chem. C, 2014, 118 (17), pp 8775–878
Theoretical core-level spectroscopy from adsorbed organic molecules
No full textHybrid interfaces between organic molecules and inorganic substrates are key systems in many technological applications, ranging from photovoltaics to organic field effect transistors and light emitting devices. Several properties of such interfaces can be effectively studied by electron core-level spectroscopies: X-ray photoemission (XPS) addresses changes in the chemical state of the atoms; near-edge X-ray absorption fine structure (NEXAFS) accesses molecular orientations and provides information on the unoccupied electronic levels; resonant photoemission spectroscopies (RESPES) can measure interfacial electron transfer times down to the femtosecond timescale.
The potential of these experimental techniques is fully obtained by complementing them with theoretical investigations and reliable numerical modeling. The perturbation by charged atomic cores is efficiently taken into account within a density-functional theory (DFT) based approach, as will be exemplified here. Numerical simulation of the dependence of NEXAFS molecular spectra on the photon electric field direction for pentacene and perylene derivatives [1,2] demonstrates how one can completely determine the orientation (polar and azimuthal) of molecules at interfaces. Disentangling the contribution of specific atomic cores (initial states) and molecular orbitals (final states) to NEXAFS [2], is further used to demonstrate filling of the lowest-unoccupied molecular orbital of pentacene in the peculiar V-bent conformation it assumes on Al(001) [3]. Excitonic effects due to the presence of core-excited atoms in the molecule can be included in the modeling to understand the electron-transfer times as measured by RESPES, which are evaluated numerically for molecules on a semi-infinite TiO2(110) substrate whose continuum of conduction states is described by Green’s function techniques [4].
[1] J. Phys. Chem. C, 2013, 117 (13), pp 6632–6638
[2] Phys. Chem. Chem. Phys., 2014, 16 (28), pp 14834-14844
[3] J. Phys. Chem. C, 2015, 119 (7), pp 3624–3633
[4] J. Phys. Chem. C, 2014, 118 (17), pp 8775–878
Depolarization and bonding in quasi-one-dimensional Na structures on Cu(001)
No full textThe formation of quasi-one-dimensional (Q1D) -Na/Cu(001) structures is addressed by density-functional theory investigations for adsorbate coverage from low to the saturation one. A general dependence of the dipole moment on the given configuration is deduced by extending that for uniform distributions, and is greatly affecting the energetics of the Na overlayer. Larger stability for Q1D arrangements aligned along and holds at coverage larger than ~ML, in agreement with low-temperature He scattering experiments, and can be explained by a reduced dipole-dipole repulsion for the with respect to hex-like distributions. Interatomic bonding charge displacements along zig-zag rows of Na atoms further support the Q1D structure and contribute significantly to the surface corrugation as seen by the He probe
Potential energy surface of alkali atoms adsorbed on Cu(001)
Full text linkWe report a comprehensive density-functional theory (DFT) calculation of the lateral adiabatic potential-energy surface (PES) of individual alkali atoms adsorbed on the Cu(001) surface. The corrugation of the PES monotonically decreases by one order of magnitude from Li to Cs, the latter being mobile on the surface even at low temperatures. Substrate relaxations in metastable configurations are consistent with a harmonic response to the adsorbate and account for a significant fraction of the PES corrugation. A simple nonseparable cosine expression provides a good description of values computed between high-symmetry configurations. Results are in good agreement with published He atom scattering measures of surface diffusion, especially at the saddle point, and with structural characterization, pointing out the accuracy of DFT calculations for this class of systems
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