Surfaces 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).

Spectroscopy of adsorbates and the role of interfacial interactions / G. Fratesi. ((Intervento presentato al convegno Congress of the Department of Physics tenutosi a Milano nel 2017.

Spectroscopy of adsorbates and the role of interfacial interactions

G. Fratesi
Primo
2017

Abstract

Surfaces 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).
29-giu-2017
Settore FIS/03 - Fisica della Materia
https://sites.google.com/view/cdip2017
Spectroscopy of adsorbates and the role of interfacial interactions / G. Fratesi. ((Intervento presentato al convegno Congress of the Department of Physics tenutosi a Milano nel 2017.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/511935
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