Suchergebnisse

Communication.-- et al. ; A 2D array of electronically coupled quantum boxes is fabricated by means of on-surface self-assembly assuring ultimate precision of each box. The quantum states embedded in the boxes are configured by adsorbates, whose occupancy is controlled with atomic precision. The electronic interbox coupling can be maintained or significantly reduced by proper arrangement of empty and filled boxes. ; The authors would like to acknowledge financial support from the Swiss Nanoscience Institute (SNI), Swiss National Science Foundation (Grants Nos. 200020-149713 and 206021-121461), the Spanish Ministry of Economy (Grant No. MAT2013-46593-C6-4-P), the Basque Government (Grant No. IT621-13), the São Paulo Research Foundation (Grant No. 2013/04855-0), Swiss Government Excellence Scholarship Program, Netherlands Organization for Scientific Research NWO (Chemical Sciences, VIDI-Grant No. 700.10.424), the European Research Council (ERC-2012-StG 307760-SURFPRO), University of Basel, University of Heidelberg, Linköping University, University of Groningen, Paul Scherrer Institute, and the Japan Science and Technology Agency (JST) "Precursory Research for Embryonic Science and Technology (PRESTO)" for a project of "Molecular technology and creation of new function." ; Peer Reviewed

Two-dimensional organic and metal-organic nanoporous networks can scatter surface electrons, leading to their partial localization. Such quantum states are related to intrinsic surface states of the substrate material. We further corroborate this relation by studying the thermally induced energy shifts of the electronic band stemming from coupled quantum states hosted in a metal-organic array formed by a perylene derivative on Cu(111). We observe by angle-resolved photoemission spectroscopy (ARPES), that both, the Shockley and the partially localized states, shift by the same amount to higher binding energies upon decreasing the sample temperature, providing evidence of their common origin. Our experimental approach and results further support the use of surface states for modelling these systems, which are expected to provide new insight into the physics concerning partially confined electronic states: scattering processes, potential barrier strengths, excited state lifetimes or the influence of guest molecules. ; We acknowledge financial support from the Spanish Ministry of Economy (grant MAT2013-46593-C6-4-P), the Basque Government (grant IT621-13), the Spanish Research Council (CSIC-201560I022), the Swiss Nanoscience Institute (SNI), Swiss National Science Foundation (grants Nos. 200020-149713, 206021-121461), the Netherlands Organization for Scientific Research (NWO Vidi grant No. 700.10.424), the European Research Council (ERC-2012-StG 307760-SURFPRO) and the Paul Scherrer Institute. ; Peer Reviewed

We present electronic structure calculations based on a single-parameter plane wave expansion method for basic graphene building blocks, namely n-oligophenylenes and n-oligoacenes, revealing excellent agreement with density-functional theory. When oligophenylene molecules are joined through meta (zigzag) or ortho (chevron) junctions, the resulting molecular dimers and polymers exhibit a semiconducting character. While zigzag dimers of oligoacenes also exhibit gapped electronic structures, their chevron-phase features a sharp metallic band at the Fermi energy. This zero-point-energy state, which transforms into Dirac-like band in chevron polymers, survives at the outer elbows of the dimer irrespective of the molecular length, and has the same origin as reported for the polyacetylene and topologically induced edge states at edge-decorated graphene nanoribbons. These findings assist the engineering of topological electronic states at the molecular level and complement the toolbox of quantum phases in carbon-based nanostructures. ; JEO acknowledges financial support from the Spanish Ministry of Science and Innovation (Grants PID2019-107338RB-C63) and the Basque Government (Grant IT-1255-19). ; Peer reviewed

Theoretical progress in graphene physics has largely relied on the application of a simple nearest-neighbor tight-binding model capable of predicting many of the electronic properties of this material. However, important features that include electron-hole asymmetry and the detailed electronic bands of basic graphene nanostructures (e.g., nanoribbons with different edge terminations) are beyond the capability of such a simple model. Here we show that a similarly simple plane-wave solution for the one-electron states of an atom-based two-dimensional potential landscape, defined by a single fitting parameter (the scattering potential), performs better than the standard tight-binding model, and levels to density-functional theory in correctly reproducing the detailed π-band structure of a variety of graphene nanostructures. In particular, our approach identifies the three hierarchies of nonmetallic armchair nanoribbons, as well as the doubly-degenerate flat bands of free-standing zigzag nanoribbons with their energy splitting produced by symmetry breaking. The present simple plane-wave approach holds great potential for gaining insight into the electronic states and electro-optical properties of graphene nanostructures and other two-dimensional materials with intact or gapped Dirac-like dispersions. ; This work has been supported in part by the Spanish MINECO (Grants No. MAT2014-59096-P, No. MAT2016-78293-C6, No. MAT-2017-88374-P, No. MAT2017-88492-R, and No. SEV2015-0522), the Basque Government (Grant No. IT-1255-19), the European Research Council (Advanced Grant No. 789104-eNANO), the European Commission (Graphene Flagship 696656), the Catalan CERCA Program, Fundació Privada Cellex, and AGAUR (Grant No. 2017 SGR 1651).

et al. ; A vicinal rutile TiO(110) crystal with a smooth variation of atomic steps parallel to the [1-10] direction was analyzed locally with STM and ARPES. The step edge morphology changes across the samples, from [1-11] zigzag faceting to straight [1-10] steps. A step-bunching phase is attributed to an optimal (110) terrace width, where all bridge-bonded O atom vacancies (O vacs) vanish. The [1-10] steps terminate with a pair of 2-fold coordinated O atoms, which give rise to bright, triangular protrusions (S) in STM. The intensity of the Ti 3d-derived gap state correlates with the sum of O vacs plus S protrusions at steps, suggesting that both O vacs and steps contribute a similar effective charge to sample doping. The binding energy of the gap state shifts when going from the flat (110) surface toward densely stepped planes, pointing to differences in the Ti polaron near steps and at terraces. ; We acknowledge financial support from the Spanish Ministry of Economy (Grants MAT2013-46593-C6-4-P and MAT2013-46593-C6-2-P) and the Basque Government (Grant IT621-13 and IT756-13). M.S. and U.D. acknowledge support from the ERC Advanced Grant "OxideSurfaces". D.S.P. and M.M. acknowledge support from the Marie Curie ITN "THINFACE" and financial support by the Deutsche Forschungsgemeinschaft. through SFB 1083 "Structure and Dynamics of Internal Interfaces". ; Peer Reviewed

Recently, mixed honeycomb–kagome lattices featuring metal–organic networks have been theoretically proposed as topological insulator materials capable of hosting nontrivial edge states. This new family of so-called "organic topological insulators" are purely two-dimensional and combine polyaromatic-flat molecules with metal adatoms. However, their experimental validation is still pending given the generalized absence of edge states. Here, we generate one such proposed network on a Cu(111) substrate and study its morphology and electronic structure with the purpose of confirming its topological properties. The structural techniques reveal a practically flawless network that results in a kagome network multi-band observed by angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. However, at the network island borders we notice the absence of edge states. Bond-resolved imaging of the network exhibits an unexpected structural symmetry alteration that explains such disappearance. This collective lifting of the network symmetry could be more general than initially expected and provide a simple explanation for the recurrent experimental absence of edge states in predicted organic topological insulators. ; We gratefully acknowledge financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6-6-R, MAT2016-78293-C6-4-R and MAT2017-83468-R) and the Ministry of Science and Innovation (MICINN, Grant No. PID2019-107338RB-C6-3 and PID2019-107338RB-C6-4), from the regional Government of Aragon (E12-20R RASMIA project and Formación de Personal Investigador, 2018 grant) and the regional Government of the Basque Country (IT-1255-19), from the "Juan de la Cierva" program, and from the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA194/16 TNSI). C. D. is supported by the Royal Society University Research Fellowship (URF/R1/201158). ; We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI). ; Peer reviewed

A combined scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory study of graphene on a Fe-Ir(111) alloy with variable Ir concentration is presented. Starting from an intercalated Fe layer between the graphene and Ir(111) surface we find that graphene-substrate interaction can be fine-tuned by Fe-Ir alloying at the interface. When a critical Ir-concentration close to 0.25 is reached in the Fe layer, the Dirac cone of graphene is largely restored and can thereafter be tuned across the Fermi level by further increasing the Ir content. Indeed, our study reveals an abrupt transition between a chemisorbed phase at small Ir concentrations and a physisorbed phase above the critical concentration. The latter phase is highly reminiscent of the graphene on the clean Ir(111) surface. Furthermore, the transition is accompanied by an inversion of the graphene's induced magnetization due to the coupling with the Fe atoms from antiferromagnetic when chemisorbed to weakly ferromagnetic in the physisorption regime, with spin polarizations whose magnitude may be tuned with the amount of Fe content. ; This work has been funded by the Spanish MINECO under contract Nos. FIS2013-48286-C2-1-P, MAT2013-47878-C2-R, MAT2015-66888-C3-1R, and MAT2013-46593-C6-4-P as well as the Basque Government Grants IT621-13 and IT756-13. ; Peer Reviewed

This article belongs to the Special Issue On-Surface Synthesis of Low-Dimensional Organic Nanostructures. ; Chiral graphene nanoribbons are extremely interesting structures due to their narrow band gaps and potential development of spin-polarized edge states. Here, we study their band structure on low work function silver surfaces. The use of a curved Ag single crystal provides, within the same sample, regions of disparate step structure and step density. Whereas the former leads to distinct azimuthal growth orientations of the graphene nanoribbons atop, the latter modulates the substrate's work function and thereby the interface energy level alignment. In turn, we disclose the associated charge transfer from the substrate to the ribbon and assess its effect on the nanoribbon's properties and the edge state magnetization. ; We acknowledge financial support from MCIN/AEI/10.13039/501100011033 (grant nos. PID2019-107338RB-C62, PID2019-107338RB-C63, PID2019-107338RB-C66, and PCI2019-111933-2 and MAT-2017-88374-P), the European Union's Horizon 2020 research and innovation program (FET Open project SPRING, grant no. 863098), the Xunta de Galicia (Centro Singular de Investigación de Galicia, 2019–2022, grant no. ED431G2019/03), the European Regional Development Fund, the Basque Government (Grant IT-1255-19), Gobierno Vasco-UPV/EHU (Project No. IT1246-19) and the Dpto. Educación Gobierno Vasco (Grant No. PIBA-2020-1-0014). ; Peer reviewed

Trabajo presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 1 al 7 de marzo de 2020. ; Conventional spin-degenerated surface electrons have been effectively manipulated by using organic and inorganic self-assembled nano-arrays as resonators. Step superlattices naturally assembled in vicinal surfaces are a particularly interesting case since they represent simple one-dimensional (1D) models for fundamental studies, and can imprint strong anisotropies in surface electron transport in real devices. Here we present the first realization of periodic resonator arrays on the BiAg2 atom-thick surface alloy with unprecedented atomic precision, and demonstrate their potential ability for tuning helical Rashba states. We employ a Ag cylindrical crystal curved around the Ag(645) vicinal surface [c-Ag(645) sample] to be able of selecting local vicinal planes with kinked steps. After sublimation and post-annealing of a 0.3 ML Bi layer, we achieve a homogenous BiAg2 surface alloy, featuring sharp arrays of extraordinarily straight Bi-terminated, monoatomic steps, with variable (tunable) density across the curved surface. Scanning the ultraviolet light beam on such Bi/c-Ag(645) interface in angle-resolved photoemission (ARPES) experiments allowed us to observe the characteristic signatures of strong and coherent step-lattice scattering in the spin-textured bands of the BiAg2 alloy, that is, step-umklapps, and a large, step-density-dependent Rashba band shift. First-principle DFT calculations reveal a complex terrace/step spin/orbital composition, which in turn explains the large spectral broadening and strong photon-energy dependence observed in ARPES. DFT also predicts the split-off of 1D electron-like step and terrace bands dispersing parallel to surface steps. ; This research was supported by the CERCA Programme/Generalitat de Catalunya, and funded by the Spanish Ministry of Science, Innovation and Universities (Grants MAT2016-78293-C6 (2-R, 4-R and 6-R), MAT-2017-88374-P and Severo Ochoa No. SEV-2017-0706), the Basque Government (Grant IT-1255-19), the regional Government of Aragón (RASMIA project) and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI). ; Peer reviewed

Vicinal surfaces exhibiting arrays of atomic steps are frequently investigated due to their diverse physical-chemical properties and their use as growth templates. However, surfaces featuring steps with a large number of low-coordinated kink-atoms have been widely ignored, despite their higher potential for chemistry and catalysis. Here, the equilibrium structure and the electronic states of vicinal Ag(111) surfaces with densely kinked steps are investigated in a systematic way using a curved crystal. With scanning tunneling microscopy we observe an exceptional structural homogeneity of this class of vicinals, reflected in the smooth probability distribution of terrace sizes at all vicinal angles. This allows us to observe, first, a subtle evolution of the terrace-size distribution as a function of the terrace-width that challenges statistical models of step lattices, and second, lattice fluctuations around resonant modes of surface states. As shown in angle resolved photoemission experiments, surface states undergo stronger scattering by fully-kinked step-edges, which triggers the full depletion of the two-dimensional band at surfaces with relatively small vicinal angles. ; We acknowledge the financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6 and Severo Ochoa No. SEV-2013-0295), the Basque Government (Grant No. IT-621-13), the regional Government of Aragon (RASMIA project), the CERCA Programme/Generalitat de Catalunya, and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA194/16 TNSI). ; Peer reviewed

Trabajo presentado al Symposium on Surface Science (3S), celebrado en St. Christoph am Arlberg (Austria) del 21 al 27 de febrero de 2016. ; We acknowledge financial support from the Spanish Ministry of Economy (grant MAT2013-46593-C6-4-P and MAT2013-46593-C6-2-P) and the Basque Government (grant IT621-13 and IT756-13), the ERC Advanced Grant "OxideSurfaces"., and the Marie Curie ITN "THINFACE". ; Peer reviewed

Trabajo presentado al 32nd Symposium on Surface Science (3S'19), celebrado en Baqueira Beret, Lleida (España) del 10 al 16 de marzo de 2019. ; Support by the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6-6-R) from the regional Government of Aragón (RASMIA project) and from the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI) is thankfully acknowledged.

Quantum dot arrays in the form of molecular nanoporous networks are renowned for modifying the electronic surface properties through quantum confinement. Here we show that, compared to the pristine surface state, the band bottom of the confined states can exhibit downward shifts accompanied by a lowering of the effective masses simultaneous to the appearance of tiny gaps at the Brillouin zone boundaries. We observed these effects by angle resolved photoemission for two self-Assembled homothetic (scalable) Co-coordinated metal-organic networks. Complementary scanning tunneling spectroscopy measurements confirmed these findings. Electron plane wave expansion simulations and density functional theory calculations provide insight into the nature of this phenomenon, which we assign to metal-organic overlayer-substrate interactions in the form of adatom-substrate hybridization. To date, the absence of the experimental band structure resulting from single metal adatom coordinated nanoporous networks has precluded the observation of the significant surface state renormalization reported here, which we infer to be general for low interacting and well-defined adatom arrays. ; We acknowledge Prof. J. García de Abajo for providing the EPWE code and the financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6 and FIS2016-75862-P), from the Basque Government (Grant No. IT-1255-19 and IT-756-13), from the Regional Government of Aragon (RASMIA project), from the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI) and from the European Research Council (ERC-2012-StG 307760-SURFPRO).

On-surface metal-organic nanoporous networks generally refer to adatom coordinated molecular arrays, which are characterized by the presence of well-defined and regular nanopores. These periodic structures constructed using two types of components confine the surface electrons of the substrate within their nanocavities. However, the confining (or scattering) strength that individual building units exhibit is a priori unknown. Here, we study the modification of the substrate's surface electrons by the interaction with a Cu-coordinated TPyB metal-organic network formed on Cu(111) and disentangle the scattering potentials and confinement properties. By means of STM and angle-resolved photoemission spectroscopy we find almost unperturbed free-electron-like states stemming from the rather weak electron confinement that yields significant coupling between adjacent pores. Electron plane wave expansion simulations match the superlattice induced experimental electronic structure, which features replicating bands and energy renormalization effects. Notably, the electrostatic potential landscape obtained from our ab initio calculations suggests that the molecules are the dominant scattering entities while the coordination metal atoms sandwiched between them act as leaky channels. These metal atom transmission conduits facilitate and enhance the coupling among quantum dots, which are prone to be exploited to engineer the electronic structure of surface electron gases. ; Financial support is acknowledged from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grants No. MAT2016-78293-C6 and No. FIS2016-75862-P), from the regional Government of Aragon (Grant No. E12-17R), from the Basque Government (Grant No. IT-1255-19), from the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI) and from the Hong Kong RGC 16304016. ; Peer reviewed

Designing molecular organic semiconductors with distinct frontier orbitals is key for the development of devices with desirable properties. Generating defined organic nanostructures with atomic precision can be accomplished by on-surface synthesis. We use this "dry" chemistry to introduce topological variations in a conjugated poly(para-phenylene) chain in the form of meta-junctions. As evidenced by STM and LEED, we produce a macroscopically ordered, monolayer thin zigzag chain film on a vicinal silver crystal. These cross-conjugated nanostructures are expected to display altered electronic properties, which are now unraveled by highly complementary experimental techniques (ARPES and STS) and theoretical calculations (DFT and EPWE). We find that meta-junctions dominate the weakly dispersive band structure, while the band gap is tunable by altering the linear segment's length. These periodic topology effects induce significant loss of the electronic coupling between neighboring linear segments leading to partial electron confinement in the form of weakly coupled quantum dots. Such periodic quantum interference effects determine the overall semiconducting character and functionality of the chains. ; We acknowledge the financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6), from the Basque Government (Grant No. IT-621-13), from the regional Government of Aragon (RASMIA project), from the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI), from the German Science Foundation (DFG) through the CRC 1083 and Grant No. GO 1812/2 and funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 635919). ; Peer reviewed