Configuring electronic states in an atomically precise array of quantum boxes

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 Reviewe

Interplay between steps and oxygen vacancies on curved TiO2(110)

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 Reviewe

Formation of the BiAg2 surface alloy on lattice-mismatched interfaces

We report on the growth of a monolayer-thick BiAg2 surface alloy on thin Ag films grown on Pt(111) and Cu(111). Using low energy electron diffraction (LEED), angle resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) we show that the surface structure of the 13 ML Bi/x-ML Ag/Pt(111) system (x≥2) is strongly affected by the annealing temperature required to form the alloy. As judged from the characteristic (3×3)R30 LEED pattern, the BiAg2 alloy is partially formed at room temperature. A gentle, gradual increase in the annealing temperatures successively results in the formation of a pure BiAg2 phase, a combination of that phase with a (2×2) superstructure, and finally the pure (2×2) phase, which persists at higher annealing temperatures. These results complement recent work reporting the (2×2) as a predominant phase, and attributing the absence of BiAg2 alloy to the strained Ag/Pt interface. Likewise, we show that the growth of the BiAg2 alloy on similarly lattice-mismatched 1 and 2 ML Ag-Cu(111) interfaces also requires a low annealing temperature, whilst higher temperatures result in BiAg2 clustering and the formation of a BiCu2 alloy. The demonstration that the BiAg2 alloy can be formed on thin Ag films on different substrates presenting a strained interface has the prospect of serving as bases for technologically relevant systems, such as Rashba alloys interfaced with magnetic and semiconductor substrates.This work was supported by the Spanish Gouvernment (Grant No. MAT2013-46593-C6-4-P), the Basque Gouvernment (Grant No. IT621-13), and the Spanish Research Council (Grant No. CSIC-201560I022). Z.M.A. would like to acknowledge funding from DAAD and DIPC. P.L. would also like to acknowledge funding from the Deutsche Forschungsgemeinschaft via Project No.RE 1469/8-1.Peer Reviewe

Giant confinement of excited surface electrons in a two-dimensional metal-organic porous network

Two-dimensional metal-organic porous networks (2D-MOPNs) are highly ordered quantum boxes for exploring surface confinements. In this context, the electron confinement from occupied Shockley-type surface states (SS) has been vigorously studied in 2D-MOPNs. In contrast, the confinement of excited surface states, such as image potential states (IPSs), remains elusive. In this work, we apply two-photon photoemission to investigate the confinement exemplarily for the first image state in a Cu-coordinated T4PT porous network (Cu-T4PT). Due to the lateral potential confinement in the Cu-T4PT, periodic replicas of the IPS as well as the SS are present in a momentum map. Surprisingly, the first IPS transforms into a nearly flat band with a substantial increase of the effective mass (> 150 %), while the band dispersion of the SS is almost unchanged. The giant confinement effect of the excited electrons can be attributed to the wavefunction location of the first IPS perpendicular to the surface, where the majority probability density mainly resides at the same height as repulsive potentials formed by the Cu-T4PT network. This coincidence leads to a more effective scattering barrier to the IPS electrons, which is not observed in the SS. Our findings demonstrate that the vertical potential landscape in a porous architecture also plays a crucial role in surface electron confinement

Temperature-driven confinements of surface electrons and adatoms in a weakly interacting 2D organic porous network

Two-dimensional organic porous networks (2DOPNs) have opened new vistas for tailoring the physicochemical characteristics of metallic surfaces. These typically chemically bound nanoporous structures act as periodical quantum wells leading to the 2D confinements of surface electron gases, adatoms and molecular guests. Here we propose a new type of porous network with weakly interacting 2,4,6-triphenyl-1,3,5-triazine (TPT) molecules on a Cu(111) surface, in which a temperature-driven (T-driven) phase transition can reversibly alter the supramolecular structures from a close-packed (CP-TPT) phase to a porous-network (PN-TPT) phase. Crucially, only the low-temperature PN-TPT exhibits subnano-scale cavities that can confine the surface state electrons and metal adatoms. The confined surface electrons undergo a significant electronic band renormalization. To activate the spin degree of freedom, the T-driven PN-TPT structure can additionally trap Co atoms within the cavities, forming highly ordered quantum dots. Our theoretical simulation reveals a complex spin carrier transfer from the confined Co cluster to the neighbouring TPT molecules via the underlying substrate. Our results demonstrate that weakly interacting 2DOPN offers a unique quantum switch capable of steering and controlling electrons and spin at surfaces via tailored quantum confinements

Precise engineering of quantum dot array coupling through their barrier widths

Quantum dots are known to confine electrons within their structure. Whenever they periodically aggregate into arrays and cooperative interactions arise, novel quantum properties suitable for technological applications show up. Control over the potential barriers existing between neighboring quantum dots is therefore essential to alter their mutual crosstalk. Here we show that precise engineering of the barrier width can be experimentally achieved on surfaces by a single atom substitution in a haloaromatic compound, which in turn tunes the confinement properties through the degree of quantum dot intercoupling. We achieved this by generating self-assembled molecular nanoporous networks that confine the twodimensional electron gas present at the surface. Indeed, these extended arrays form up on bulk surface and thin silver films alike, maintaining their overall interdot coupling. These findings pave the way to reach full control over two-dimensional electron gases by means of self-assembled molecular networks

Interplay between steps and oxygen vacancies on curved TiO2(110)

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 reviewe

Electronic bands of nanoporous networks and one-dimensional covalent polymers assembled on metal surfaces

A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy in Physics of Nanostructures and Advanced Materials in the Nanophysics Lab, Centro de Física de Materiales (CFM-CSIC).Complex molecular layers self-assembled on surfaces with engineered architectures and tailored properties, are expected to play an important role in the development of future devices at the nanoscale. The reversibility of non-covalent interactions such as hydrogen bonds or metal ligand interactions, allows error correction processes in the formed structures. Such elimination of defective structures can give rise to almost defect-free, long-range ordered formations. Metal-organic networks grown on metallic surfaces fall into such self-healing structures and show novel magnetic properties, catalytic effects, oxidation states, exotic tesellation patterns and even bear the prospect of exhibiting topological band structures. Nanoporous networks featuring long-range order belong to error corrected noncovalent structures. The recent finding of electron confinement of the two-dimensional electron gas (2DEG) within the nanopores of self-assembled supramolecular nanoporous networks, is an experimental demonstration of a quantum box effect. This is an effect which may play a crucial role in engineering future molecular devices. By using scanning tunneling microscopy and spectroscopy (STM/STS), in a similar fashion to quantum corrals, it is possible to probe such localized electronic states at the single pore or quantum dot (QD) level. However, studies on the long-range ordered and robust 3deh-DPDI metal-organic network on Cu(111), revealed that nanopores are rather imperfect or leaky confining entities, leading to significant coupling to neighboring nanopores. The periodicity of the highly-ordered supramolecular network induces the formation of Bloch-wave states that result into new electronic bands that can be observed by spatially averaging angle-resolved photoemission spectroscopy (ARPES). The well-established control of the structures of porous networks, together with its characteristic degree of coupling between ad-molecules and the surface state, is our starting point for the fabrication and investigation of coupled electronic systems with tailored band structures. Based on the concepts of Supramolecular Chemistry on surfaces, by choosing suitable molecular constituents (functional groups and/or carbon backbone size) and guided by reversible, non-covalent bonding mechanisms, we are able to generate six different long-range ordered nanoporous networks on (111)-terminated coinage metal surfaces in ultra-high vacuum (UHV). Such nanoporous structures are analogous to QD arrays on surfaces, bearing distinct sizes, barrier separations and scattering strengths. As a result, with each particular nanoporous system grown, we not only engineer the local confinement properties at each QD, but also modulate the coherent electronic band structure steming from the overall array. We observe changes in its fundamental energy, band dispersion, effective mass, zone boundary gaps and Fermi surface contour. Our experimental findings are supported by the electron boundary elements method in combination with the electron plane wave expansion (EBEM/EPWE) modelling, density functional theory (DFT) calculations, and the phase accumulation model (PAM). In this way, we disentangle the repulsive scattering potential landscape of each nanoporous network and delve into subtle surface-organic overlayer interactions, such as hybridization and geometry induced effects, which are altogether responsible for the confinement effects and distinct electronic band modulations. Our findings envision the engineering of 2D electronic metamaterials, in analogy to the well-established optical metamaterials. The studied electronic structure from nanoporous networks correspond to the modified substrate’s surface state, which isindependent of the molecular states. However, low-dimensional organic electronic states, such as the one obtained in graphene nanoribbons (GNRs) and oligophenylene chains are currently very attractive to the Scientific Community based on their industrial prospects. These one-dimensional polymeric structures have been extensively studied as simple, appealing nanostructures leading to distinct electronic features, such as gap opening and peculiar edge states. Their quantum confinement origin can be readily tuned through their width, shape, and edge terminations. The rapidly progressing on-surface chemistry is a highly versatile bottom-up tool for the controlled-synthesis of such atomically precise, graphene-based nanostructures. This achievement has paved the way towards the precise mapping of their intriguing electronic structures with ARPES and STS, making them promising candidates for the realization of exotic graphene-based nanodevices. In this thesis, we engineer the electronic band structure of the well-known poly-(para-phenylene) (PPP), namely the Nα = 3 armchair GNR, by introducing periodically spaced meta-junctions into its conductive path. We synthesize and macroscopically align a saturated film of cross-conjugated oligophenylene zigzag chains on a vicinal Ag(111) surface. We find that these atomically precise chains, hosting periodically spaced meta-junctions, remain sufficiently decoupled from each other and from the substrate. ARPES reveals weakly dispersing one-dimensional electronic bands along the chain direction, which is reproduced by DFT and EPWE. In addition, STS shows a significantly larger frontier orbital bandgap than PPP chains and that straight segments are able to confine electrons. These weakly interacting QDs confirm that periodically spaced meta-junctions constitute strong scattering centers for the electrons. These findings corroborate the important effects that the conductive path topology of a molecular wire has on its electronic states, which are responsible for defining its chemical, optical and electronic properties. Such arrays of semiconducting QDs hold potential for designing future oligophenylene-based quantum devices such as electrically driven, telecom-wavelength, room-temperature singlephoton sources.Peer reviewe