Electronic structure of titanylphthalocyanine layers on Ag(111)

We have investigated the electronic structures of axially oxo functionalized titanylphthalocyanine (TiOPc) on Ag(111) by X-ray and ultraviolet photoelectron spectroscopies, two-photon photoemission, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. Furthermore, we use complementary data of TiOPc on graphite and planar copper phthalocyanine (CuPc) on Ag(111) for a comparative analysis. Both molecules adsorb on Ag(111) in a parallel orientation to the surface, for TiOPc with an oxygen-up configuration. The interaction of nitrogen and carbon atoms with the substrate is similar for both molecules, while the bonding of the titanium atom to Ag(111) in the monolayer is found to be slightly more pronounced than in the CuPc case. Ultraviolet photoemission spectroscopy reveals an occupation of the lowest unoccupied molecular orbital (LUMO) level in monolayer thick TiOPc on Ag(111) related to the interaction of the molecules and the silver substrate. This molecule-metal interaction also causes an upward shift of the Ag(111) Shockley state that is transformed into an unoccupied interface state with energies of 0.23 and 0.33 eV for the TiOPc monolayer and bilayer, respectively, at the Brillouin zone center.The authors acknowledge financial support from the Deutsche Forschungsgemeinschaft through SFB 1083 “Structure and Dynamics of Internal Interfaces”, the Spanish CSIC I-Link programm, the Spanish Ministry of Economy and Competitiveness, MINECO (under Contract No. MAT2016-78293-C6-2-R, and Severo Ochoa No. SEV-2013-0295.), and by the Secretariat for Universities and Research, Knowledge Department of the Generalitat de Catalunya (2014 SGR 715). M. Paradinas thanks the Spanish Government for financial support through PTA2014-09788-I fellowships. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya.Peer Reviewe

Tuning the Magnetic Anisotropy of Lanthanides on a Metal Substrate by Metal–Organic Coordination

Taming the magnetic anisotropy of lanthanides through coordination environments is crucial to take advantage of the lanthanides properties in thermally robust nanomaterials. In this work, the electronic and magnetic properties of Dy-carboxylate metal–organic networks on Cu(111) based on an eightfold coordination between Dy and ditopic linkers are inspected. This surface science study based on scanning probe microscopy and X-ray magnetic circular dichroism, complemented with density functional theory and multiplet calculations, reveals that the magnetic anisotropy landscape of the system is complex. Surface-supported metal–organic coordination is able to induce a change in the orientation of the easy magnetization axis of the Dy coordinative centers as compared to isolated Dy atoms and Dy clusters, and significantly increases the magnetic anisotropy. Surprisingly, Dy atoms coordinated in the metallosupramolecular networks display a nearly in-plane easy magnetization axis despite the out-of-plane symmetry axis of the coordinative molecular lattice. Multiplet calculations highlight the decisive role of the metal–organic coordination, revealing that the tilted orientation is the result of a very delicate balance between the interaction of Dy with O atoms and the precise geometry of the crystal field. This study opens new avenues to tailor the magnetic anisotropy and magnetic moments of lanthanide elements on surfaces.The ALBA synchrotron is acknowledged for providing beam time at BOREAS beamline (proposal number 2015091454). This project has received funding from the European Research Council (ERC, grant 766555) and Marie Sklodowska-Curie Actions (MSCA, project 894924) under the European Union's Horizon 2020 research and innovation programme. This work has been financed by the Spanish Ministerio de Economía, Industria y Competitividad (projects FIS2016-78591-C3-1-R, RTI2018-097895-B-C42, MAT2016-78293-C6-2-R, MAT2017-85089-C2-1-R, and PID2019-107338RB-C65); the Comunidad de Madrid (Projects S2013/MIT-2850, P2018/NMT4321, and S2018/NMT-4367); the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI); and “Severo Ochoa” Programme for Centres of Excellence in R&D (grants SEV-2016-0686, and SEV-2017-0706)

Heterogeneous nanotribological response of polymorphic self-assembled monolayers arising from domain and phase dependent friction

Micro-/nanoelectromechanical systems demand robust ultrathin films for lubrication. As they can drastically modify the frictional properties of surfaces, few nanometers thick self-assembled monolayers (SAMs) constitute accepted candidates as boundary lubricants. Their high stability and easy preparation make them attractive also for low cost applications. Given their high order, organosulfur SAMs have been archetypal systems for structural investigations, but few efforts have been devoted to analyze the influence of lateral inhomogeneities on their surface properties. The impact on the frictional response of the surface due to the existence of crystalline domains with lateral dimension in the sub-micrometer range is considered here. To this end, two polymorphic structures of self-assembled monolayers of ω-(4′-methylbiphenyl-4-yl) butane-1-thiol coexisting on Au(111) are investigated by scanning tunneling and force microscopy. Described by rectangular 5(raiz de 5) x3 (α-phase) and oblique 6 (raiz de 3) x 2 (raiz de 3) (β-phase) unit cells, they exhibit pronouncedly different frictional responses. The lateral nano-tribological heterogeneity of the surface is further influenced by the azimuthal orientation dependence of friction for each phase. In particular, this phenomenon is exploited in the less densely packed β-phase for which the separate analysis of forward and backward lateral force scans is used to differentiate domains rotated 180°. The results demonstrate the level of structural control required in the design of SAMs for nano-tribology applications.This work has been supported by the Spanish Government through grants MAT2010-20020 and NANOSELECT CSD2007-00041. M. Paradinas thanks financial support through the Spanish BES-2008-003588 FPI fellowship and C. Munuera from the ‘‘Juan de la Cierva’’ postdoctoral program JCI-2011-08815. Partial support by EPSRC (EP/E061303/1, EP/D036828/1) is also gratefully acknowledged.Peer Reviewe

Giant reversible nanoscale piezoresistance at room temperature in Sr2IrO4 thin films

This article has been highlighted as "Hot Paper" in Nanoscale.-- Domingo, Neus et al.Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin–orbit coupling, which opens up a narrow bandgap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, with the giant and fully reversible perpendicular piezoresistance rendering iridates as promising materials for room temperature piezotronic devices.N.D wants to acknowledge the Spanish Ministerio de Ciencia e Innovación for a Ramon y Cajal research grant RYC-2010-06365. X.M. acknowledges the Grant Agency of the Czech Republic No. P204/11/P339. G.C. acknowledges ERC Starting Grant 308023. T.J. acknowledges ERC Advanced Grant 268066 and Praemium Academiae of the Academy of Sciences of the Czech Republic. Di Yi was sponsored by the National Science Foundation through the Penn State MRSEC. J.L. is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 through the Quantum Material program in the Materials Sciences Division of Lawrence Berkeley National Laboratory. Financial support has been obtained under projects from the Spanish Ministerio de Economía y Competitividad under projects MAT2010-17771, MAT2010-20020, MAT2013-47869-C4-1-P, FIS2013-48668-C2-1-P and NANOSELECT CSD2007-00041 and Severo Ochoa Excellence Programme 2013-0295, and the Generalitat de Catalunya under projects 2014 SGR 733 and 2014 SGR 1216.Peer reviewe

Real Space Demonstration of Induced Crystalline 3D Nanostructuration of Organic Layers

The controlled 3D nanostructuration of molecular layers of the semiconducting molecules C22H14 (pentacene) and N,N′-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) is addressed. A tip-assisted method using atomic force microscopy (AFM) is developed for removing part of the organic material and relocating it in up to six layer thick nanostructures. Moreover, unconventional molecular scale imaging combining diverse friction force microscopy modes reveals the stacking sequence of the piled layers. In particular, we unambiguously achieve epitaxial growth, an issue of fundamental importance in thin film strategies for the nanostructuration of more efficient organic nanodevices.We thank M. Aghamohammadi for her help with the film growth. This work has been supported by the Spanish Government under projects MAT2013-47869-C4-1-P and MAT2016-77852-C2-1-R (AEI/FEDER, UE) and the Generalitat de Catalunya 2014 SGR501. We acknowledge the MINEICO project MAT2015-68994-REDC and the “Severo Ochoa” Program for Centers of Excellence in R&D (SEV-2015- 0496). M.P. thanks the Spanish Government for financial support through BES-2008-003588 FPI and PTA2014-09788-I fellowships.Peer reviewe

Heterogeneous nanotribological response of polymorphic self-assembled monolayers arising from domain and phase dependent friction

Micro-/nanoelectromechanical systems demand robust ultrathin films for lubrication. As they can drastically modify the frictional properties of surfaces, few nanometers thick self-assembled monolayers (SAMs) constitute accepted candidates as boundary lubricants. Their high stability and easy preparation make them attractive also for low cost applications. Given their high order, organosulfur SAMs have been archetypal systems for structural investigations, but few efforts have been devoted to analyze the influence of lateral inhomogeneities on their surface properties. The impact on the frictional response of the surface due to the existence of crystalline domains with lateral dimension in the sub-micrometer range is considered here. To this end, two polymorphic structures of self-assembled monolayers of omega-(4'-methylbiphenyl-4-yl) butane-1-thiol coexisting on Au(111) are investigated by scanning tunneling and force microscopy. Described by rectangular 5 root 5 x 3 (alpha-phase) and oblique 6 root 3 x 2 root 3 (beta-phase) unit cells, they exhibit pronouncedly different frictional responses. The lateral nano-tribological heterogeneity of the surface is further influenced by the azimuthal orientation dependence of friction for each phase. In particular, this phenomenon is exploited in the less densely packed beta-phase for which the separate analysis of forward and backward lateral force scans is used to differentiate domains rotated 180 degrees. The results demonstrate the level of structural control required in the design of SAMs for nano-tribology applications.</p

Decoding crystallographic domains of molecular systems by cantilever torsion imaging

Resumen del póster presentado a la 13th European Conference on Surface Crystallography and Dynamics, celebrada en Donostia-San Sebastián (España) del 19 al 21 de junio de 2017.From the physical point of view, the tribological response, in particular frictional properties, of organic surfaces is an obvious subject of interest by itself. As a powerful tool to understand different dissipation mechanisms at surfaces, we commonly employ the scanning force microscope by measuring the lateral force perpendicular to the scan direction (also known as friction force microscopy, FFM). As for inorganic materials, FFM helps visualizing regions of different chemical nature. However, more subtle surface characteristics influence the frictional properties of molecular systems and well-designed and sensitive FFM measurements permit obtaining detailed structural details as molecular order (stick-slip) as well as molecular tilt angles and/or tilt angle azimuths. This is the case of observing friction anisotropy or friction asymmetry. The term anisotropy refers to the variation of friction with the relative orientation angle between sliding surfaces and is commonly correlated with surface crystallographic orientations, while asymmetry refers to a change in friction when the sliding direction is changed by 180°. FFM has been successfully employed in organic self-assembled monolayers (SAMs) not only to discriminate between ordered configurations presenting different friction coefficients or different packing, but also to decipher between equivalent structural domains as well as to identify highly dissipative transient molecular configurations during phase transitions. Outstandingly, crystallographic aspects including "stick-slip" can also be extracted from the torsion of the cantilever during scanning along its axis in the so-called transverse shear microscopy (TSM).Peer reviewe

Surface-guided graphene nanoribbon superlattices

Resumen del trabajo presentado al International workshop On-Surface Synthesis (OSS), celebrado en Sant Feliu de Guíxol (España) del 23 al 28 de septiembre de 2018.The on-surface synthesis of graphene nanoribbons (GNRs) has been subject of intense research in recent years. Focused mainly on the realization of GNRs with different shape, edge structure and chemical composition, few studies have paid attention to the control of their distribution throughout the surface, which is critical for integration in devices. The most representative advance in this direction has been the use of stepped surfaces as templates to align the GNRs along one direction at the cm scale, a distribution that can be then replicated after transfer to insulating substrates. Here we show how the herringbone reconstruction can be used as template for inducing different GNR superlattice patterns. Hydrogenated and fluorinated triphenylene derivatives lead to GNRs that interact strongly with the surface reconstruction, which constrains their growth along the fcc domains. This results in a 2D superlattice that replicates the herringbone pattern, and limits the GNR size to the straight domains of the zig-zag. In contrast, GNRs obtained with diphenyl–DBBA undergo a more subtle interaction, where the interaction with the reconstruction is only exploited for the parallel alignment of GNRs without limiting their size. As a consequence we obtain parallel arrays of ultra-long GNRs (>100nm) of different periodicity depending on the coverage (Fig .1). The parallel distribution is optimum for the realization of multichannel electronic devices and optical superlattices, and additionally provides an ideal configuration for the subsequent lateral coupling of GNRs that give rise to nanoporous graphene.Peer reviewe

Electronic properties and magnetic interaction of metal organic molecules on a monolayer thick GdAu2 substrate

Trabajo presentado en el X Iberian Conference on Tribology y XI Iberian Vacuum Conference (IBERTRIVA) celebrada en Sevilla (España), del 26 al 28 de junio de 2019Fundamental research on new states of the matter and technological applications in fields like spintronics (1) relays on the interfaces between magnetic materials and other states of the matter like superconductors, semiconductors or topological insulators. In the case of twodimensional (2D) ferromagnetic materials, it is necessary to keep the long-range magnetic order. To maintain this 2D magnetism the chemical and structural stability is mandatory. A molecular overlayer could influence these properties due to charge transfer. 2D rareearth/Noble-metal monolayer alloys show great potential as nanostructured magnetic patterns. Many of the lanthanides have shown already its potential by forming highly ordered corrugated structures when deposited on Au(111) and Ag(111) (2) nevertheless their magnetic and electronic structure after molecular deposition remains unknown. Here, we show evidence of the interaction between a GdAu2 monolayer alloy and magnetic molecules (metal centered porphyrin and phthalocyanines) that mutually influence their magnetic behavior. For example in the case of a one monolayer (ML) Co-tetraphenylporphyrin we observe a ferromagnetic interaction of the Co atom to the substrate, while the underlying material keeps basically its electronic and magnetic properties (Fig. 1). On the other hand a ML thick Co-phthalocyanine molecules film does not reveal any magnetic interaction. Only the second layer show a small magnetic signal