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ANDREA FERRETTI

DIPENDENTE ALTRO ENTE DI RICERCA
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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Pubblicazioni

2015 - Surface-Assisted Reactions toward Formation of Graphene Nanoribbons on Au(110) Surface [Articolo su rivista]
Massimi, Lorenzo; Ourdjini, Oualid; Lafferentz, Leif; Koch, Matthias; Grill, Leonhard; Cavaliere, Emanuele; Gavioli, Luca; Cardoso, Claudia; Prezzi, Deborah; Molinari, Elisa; Ferretti, Andrea; Mariani, Carlo; Betti, Maria Grazia
abstract

Scanning tunneling microscopy and X-ray spectroscopy measurements are combined to first-principles simulations to investigate the formation of graphene nanoribbons (GNRs) on Au(110), as based on the surface-mediated reaction of 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules. At variance with Au(111), two different pathways are identified for the GNR self-assembly on Au(110), as controlled by both the adsorption temperature and the surface coverage of the DBBA molecular precursors. Room-temperature DBBA deposition on Au(110) leads to the same reaction steps obtained on Au(111), even though with lower activation temperatures. For DBBA deposition at 470 K, the cyclodehydrogenation of the precursors preceds their polymerization, and the GNR formation is fostered by increasing the surface coverage. While the initial stages of the reaction are found to crucially determine the final configuration and orientation of the GNRs, the molecular diffusion is found to limit in both cases the formation of high-density long-range ordered GNRs. Overall, the direct comparison between the Au(110) and Au(111) surfaces unveils the delicate interplay among the different factors driving the growth of GNRs.

2014 - Exciton-dominated optical response of ultra-narrow graphene nanoribbons [Articolo su rivista]
Denk, Richard; Hohage, Michael; Zeppenfeld, Peter; Cai, Jinming; Pignedoli, Carlo A; Söde, Hajo; Fasel, Roman; Feng, Xinliang; Müllen, Klaus; Wang, Shudong; Prezzi, Deborah; Ferretti, Andrea; Ruini, Alice; Molinari, Elisa; Ruffieux, Pascal
abstract

Narrow graphene nanoribbons exhibit substantial electronic bandgaps and optical properties fundamentally different from those of graphene. Unlike graphene--which shows a wavelength-independent absorbance for visible light--the electronic bandgap, and therefore the optical response, of graphene nanoribbons changes with ribbon width. Here we report on the optical properties of armchair graphene nanoribbons of width N=7 grown on metal surfaces. Reflectance difference spectroscopy in combination with ab initio calculations show that ultranarrow graphene nanoribbons have fully anisotropic optical properties dominated by excitonic effects that sensitively depend on the exact atomic structure. For N=7 armchair graphene nanoribbons, the optical response is dominated by absorption features at 2.1, 2.3 and 4.2 eV, in excellent agreement with ab initio calculations, which also reveal an absorbance of more than twice the one of graphene for linearly polarized light in the visible range of wavelengths.

2014 - Probing the mechanism for graphene nanoribbon formation on gold surfaces through X-ray spectroscopy [Articolo su rivista]
Batra, Arunabh; Cvetko, Dean; Kladnik, Gregor; Adak, Olgun; Cardoso, Claudia; Ferretti, Andrea; Prezzi, Deborah; Molinari, Elisa; Morgante, Alberto; Venkataraman, Latha
abstract

We studied the formation of graphene nanoribbons (GNRs) via the self-assembly of 10,10'-dibromo-9,9'-bianthryl precursor molecules on gold surfaces with different synchrotron spectroscopies. Through X-ray photoemission spectroscopy core-level shifts, we followed each step of the synthetic process, and could show that the Br-C bonds of the precursors cleave at temperatures as low as 100 degrees C on both Au(111) and Au(110). We established that the resulting radicals bind to Au, forming Au-C and Au-Br bonds. We show that the polymerization of the precursors follows Br desorption from Au, suggesting that the presence of halogens is the limiting factor in this step. Finally, with angle-resolved ultraviolet photoemission spectroscopy and density functional theory we show that the GNR/Au interaction results in an upshift of the Shockley surface state of Au(111) by similar to 0.14 eV, together with an increased electron effective mass.

2012 - Electronic Structure of Atomically Precise Graphene NanoribbonsACS Nano, 2012, 6 (8), pp 6930–6935Publication Date (Web): August 1, 2012 (Article)DOI: [Articolo su rivista]
P., Ruffieux; J., Cai; N. C., Plumb; L., Patthey; Prezzi, Deborah; Ferretti, Andrea; Molinari, Elisa; X., Feng; K., Müllen; C. A., Pignedoli; R., Fasel
abstract

Some of the most intriguing properties of graphene are predicted for specifically designed nanostructures such as nanoribbons. Functionalities far beyond those known from extended graphene systems include electronic band gap variations related to quantum confinement and edge effects, as well as localized spin-polarized edge states for specific edge geometries. The inability to produce graphene nanostructures with the needed precision, however, has so far hampered the verification of the predicted electronic properties. Here, we report on the electronic band gap anddispersion of the occupied electronic bands of atomically precise graphene nanoribbons fabricated via on-surface synthesis. Angle-resolved photoelectron spectroscopy and scanning tunnelingspectroscopy data from armchair graphene nanoribbons of width N = 7 supported on Au(111) reveal a band gap of 2.3 eV, an effective mass of 0.21 m0 at the top of the valence band, and anenergy-dependent charge carrier velocity reaching 8.2 105 m/s in the linear part of the valence band. These results are in quantitative agreement with theoretical predictions that include image charge corrections accounting for screening by the metal substrate and confirm the importance of electron-electron interactions in graphene nanoribbons

2009 - Atomic and electronic structure of the nonpolar GaN (1 1̄ 00) surface [Articolo su rivista]
Bertelli, Matteo; Löptien, P.; Wenderoth, M.; Rizzi, Angela; Ulbrich, R. G.; Righi, Maria Clelia; Ferretti, Andrea; Martin Samos, L.; Bertoni, Carlo Maria; Catellani, A.
abstract

We present a cross-section scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and ab initio density-functional theory simulations study of the cleaved nonpolar (1[overline 1]00) surface (m-plane) of n-type HVPE GaN free-standing quasisubstrates. Atomically resolved empty and filled states STM topographies show that no reconstruction occurs upon cleavage, as predicted by theory. STS measurements on clean and atomically flat cleaved surfaces (defect concentration sigmad<=2×10**12 cm−2) show that the Fermi energy is not pinned and the tunneling current flows through Ga-like electronic states lying outside the fundamental band gap. On surface areas with defect concentration sigmad>=3×10**13 cm−2, the Fermi energy is pinned inside the band gap in defect-derived surface states and tunneling through filled (empty) N-like (Ga-like) states takes place.

2007 - Mixing of electronic states in pentacene adsorption on copper [Articolo su rivista]
Ferretti, Andrea; C., Baldacchini; Calzolari, Arrigo; DI FELICE, Rosa; Ruini, Alice; Molinari, Elisa; M. G., Betti
abstract

By combining experimental and theoretical approaches, we study the adsorption of pentacene on copper as a model for the coupling between aromatic molecules and metal surfaces. Our results for the interface electronic structure are not compatible with a purely physisorption picture, which is conventionally employed for such systems. Nay, we demonstrate electronic mixing between molecular orbitals and metal electronic states

2007 - Polarization properties of (1-100) and (11-20) SiC surfaces from first principles [Articolo su rivista]
Brandino, Gp; Cicero, G; Bonferroni, Benedetta; Ferretti, Andrea; Calzolari, Arrigo; Bertoni, Carlo Maria; Catellani, A.
abstract

We report on first-principles density functional calculations of nonpolar low-index surfaces of hexagonal silicon carbide. We provide an accurate analysis of the macroscopic bulk spontaneous polarization as a function of the hexagonality of the compound, and we describe in detail the electronic and structural properties of the relaxed surfaces. We revise the methodology to achieve a detailed description of the surface polarization effects. Our results on low-index surfaces reveal a strong in-plane polar contribution, opposing the spontaneous polarization field present in hexagonal polytypes. This in-plane surface polarization component has not been considered before, although it is of significant impact in adsorption experiments, affecting functionalization and growth processes, as well as the electronic properties of confined, low-dimensional systems.

2007 - Symmetry lowering of pentacene molecular states interacting with a Cu surface [Articolo su rivista]
Baldacchini, C.; Mariani, C.; Betti, M. G.; Vobornik, I.; Fujii, J.; Annese, E.; Rossi, G.; Ferretti, A.; Calzolari, A.; Di Felice, R.; Ruini, A.; Molinari, E.
abstract

Pentacene adsorbed on the Cu(119) vicinal surface forms long-range ordered chain structures. Photoemission spectroscopy measurements and ab initio density functional theory simulations provide consistent evidences that pentacene molecular orbitals mix with the copper bands, giving rise to interaction states localized at the interface. Angular-resolved and polarization dependent photoemission spectroscopy shows that most of the pentacene derived intensity is strongly dichroic. The symmetry of the molecular states of the free pentacene molecules is reduced upon adsorption on Cu(119), as a consequence of the molecule-metal interaction. Theoretical results show a redistribution of the charge density in π molecular states close to the Fermi level, consistent with the photoemission intensities (density of states) and polarization dependence (orbital symmetry).

2006 - First-principles investigation of functionalization-defects on silicon surfaces [Articolo su rivista]
C. S., Cucinotta; B., Bonferroni; Ferretti, Andrea; Ruini, Alice; M. J., Caldas; Molinari, Elisa
abstract

We present a theoretical study of chemisorption of CH C-CH2-COOH molecules on the H:Si(100) surface. We perform simulations for different chemisorbed configurations, attained by reactions through the alkyne tail. We use the periodic slab approximation for the extended surface, within ab initio density functional theory, and analyse results from several different approaches. We conclude that structures composed of single Si-C bridges are very stable.. while a previously proposed structure, with a double Si-C-Si bridge, should be metastable on the flat surface, and introduce electron and hole traps in the Si band gap. (c) 2006 Elsevier B.V. All rights reserved.

2005 - First-principle theory of correlated transport through nano-junctions [Articolo su rivista]
Ferretti, Andrea; Calzolari, Arrigo; DI FELICE, Rosa; Manghi, Franca; M. J., Caldas; M., BUONGIORNO NARDELLI; Molinari, Elisa
abstract

We report the inclusion of electron-electron correlation in the calculation of transport properties within an ab initio scheme. A key step is the reformulation of Landauer's approach in terms of an effective transmittance for the interacting electron system. We apply this framework to analyze the effect of shortrange interactions on Pt atomic wires and discuss the coherent and incoherent correction to the mean-field approach.

2003 - Charge transport and radiative recombination in polythiophene crystals: a first-principles study [Articolo su rivista]
Ruini, A.; Bussi, G.; Ferretti, A.; Caldas, M. J.; Molinari, E.
abstract

We investigate two phases of polythiophene crystals by means of first-principles calculations, focusing on the effect of the different structure on charge transport parameters and luminescence quantum yield. The resulting microscopic interpretation highlights the impact of solid-state interchain coupling on both transport and emissive properties of semiconducting polymer crystals.

2003 - Electronic properties of polymer crystals: The effect of interchain interactions [Articolo su rivista]
Ruini, Alice; Molinari, Elisa; Caldas, Marilia J.; Ferretti, Andrea
abstract

We present a theoretical study of the transport parameters in a prototype conjugated-polymer, poly-para-phenylenevinylene, in two different possible crystalline packings. Our analysis is performed through density-functional electronic structure calculations, and allows one to obtain the fundamental parameters describing charge transport. The transfer integrals are found to be a crucial quantity to appreciate the effects of crystalline aggregation on conduction properties: our results indicate that interchain interactions can be viewed as a tunable parameter for the design of efficient electronic devices based on organic materials.