arXiv subjects

Jorge Lobo-Checa

Publications and source records attributed to Jorge Lobo-Checa.

At least 19 recordsLinked to original sources

arXiv2026

Prebiotic Chemistry Assemblies of L-Cysteine on Defect-Free Pyrite Terraces

W\"achtersh\"auser's theory proposes iron-sulfur minerals as key platforms for molecular synthesis and supramolecular organization in prebiotic environments. However, defects have been traditionally considered at the center of such assemblies, thereby underestimating the contributions of regular and pristine interfaces. Here, we combine scanning tunneling microscopy and spectroscopy (STM/STS) with density functional theory (DFT) to investigate the fundamental prebiotic chemistry system of L-Cysteine (L-Cys) on defectless FeS$_2$(100) terraces. To do so, we first achieved atomically ordered, defect-free terraces that act as support of two distinct supramolecular phases of L-Cys: one compact, highly ordered supramolecular network and another less packed, labile supramolecular network. We unveil trimer-based intermolecular interactions to be at the origin of these pattern formations. These results demonstrate that L-Cys self-assemblies can be hosted on flawless FeS$_2$ terraces due to the cooperative interplay between substrate electronic structure and intermolecular interactions, without the participation of dominant defects. Therefore, the autocatalytic activity of pyrite could have triggered the on-surface polymerization process of these non-static self-assembled structures under primordial conditions, thereby endorsing W\"achtersh\"auser's postulates on the origin of life.

cond-mat.mtrl-sci↗

arXiv2026

Iron-mediated on-surface synthesis of substrate-decoupled graphdiyne monolayers

Graphdiynes are emerging two-dimensional sp-sp$^2$ carbon materials with electronic structures complementing those of graphene, yet their on-surface synthesis is limited by the persistence of metalated intermediates or yields disordered covalent networks. Here, we report an iron-assisted route to covalent hydrogenated graphdiyne monolayers on Au(111) from 1,3,5-tris(bromoethynyl)benzene. Low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy and density functional theory show that Fe scavenges chemisorbed Br byproducts forming FeBr$_2$, in turn promoting the removal of Au adatoms from the organometallic network, thus enabling its metalated-to-covalent conversion under mild thermal treatment. Subsequent annealing removes FeBr$_2$ and yields covalent, ordered domains weakly coupled to the substrate. Scanning tunnelling spectroscopy, combined with density functional theory, reveals a semiconducting gap of about 1.6 eV associated with carbon p$_z$ frontier orbitals. This Fe-mediated on-surface synthesis strategy provides a route to atomically precise, weakly substrate-coupled graphdiyne networks and offers a design principle for two-dimensional carbon semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

Atomic scale demonstration of ferromagnetism in a single layer FeCl2 on Au(111)

FeCl2 is a promising single-layer material with sizeable magnetic susceptibility and insulating character that can be easily grown by molecular beam epitaxy on various surfaces. In order to include it into the select palette of van der Waals materials used to engineer functional heterostructures, it is necessary to confirm its magnetic and electronic ground states, and understand the influence of the supporting substrate. In this work, we unambiguously demonstrate ferromagnetic ordering in a single-layer FeCl2 on Au(111) by means of spin-polarized scanning tunnelling microscopy. The material features a relatively wide insulating gap of 3.3 eV and a strongly spin-polarized conduction band that emerges at 1.5 eV above the Fermi level. Atomic scale defects with triangular shape play a primary role in the electronic gap and spin density distribution. Specifically, in a region of 1.6 nm around each defect, the conduction band is locally suppressed and the tunnelling magneto-conductance is reduced a factor of four. By tracking the spin-dependent tunnelling conductance as a function of the applied magnetic field, we record atomically resolved hysteresis loops, revealing a soft ferromagnetic ground state with pronounced out-of-plane anisotropy and coercive fields in the range of 15-50 mT.

cond-mat.mtrl-sci↗

arXiv2024

Unravelling the Band Structure and Orbital Character of a $π$-Conjugated 2D Graphdiyne-Based Organometallic Network

Graphdiyne-based carbon systems generate intriguing layered sp-sp$^2$ organometallic lattices, characterized by flexible acetylenic groups connecting planar carbon units through metal centers. At their thinnest limit, they can result in two-dimensional (2D) organometallic networks exhibiting unique quantum properties and even confining the surface states of the substrate, which is of great importance for fundamental studies. In this work, we present the on-surface synthesis of a highly crystalline 2D organometallic network grown on Ag(111). The electronic structure of this mixed honeycomb-kagome arrangement - investigated by angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy - reveals a strong electronic conjugation within the network, leading to the formation of two intense electronic band-manifolds. In comparison to theoretical density functional theory calculations, we observe that these bands exhibit a well-defined orbital character that can be associated with distinct regions of the sp-sp$^2$ monomers. Moreover, we find that the halogen by-products resulting from the network formation locally affect the pore-confined states, causing a significant energy shift. This work contributes to the understanding of the growth and electronic structure of graphdiyne-like 2D networks, providing insights into the development of novel carbon materials beyond graphene with tailored properties.

cond-mat.mes-hall↗

arXiv2023

Optimizing the Ullmann coupling reaction efficiency on an oxide surface by metal atom addition

The bottom-up synthesis of carbon based nanomaterials directly on semiconductor surfaces allows to decouple their electronic and magnetic properties from the substrates. However, the lack of reactivity on these non-metallic surfaces hinders or reduces significantly the yield of these reactions. Such hurdles practically precludes transferring bottom-up synthesis strategies onto semiconducting and insulating surfaces. Here, we achieve a high polymerization yield of terphenyl molecules on the semiconductor TiO$_2$(110) surface by incorporating cobalt atoms as catalysts in the Ullmann coupling reaction. Cobalt atoms trigger the debromination of 4,4-dibromo-p-terphenyl (DBTP) molecules on TiO$_2$(110) and the formation of an intermediate organometallic phase already at room-temperature (RT). As the debromination temperature is drastically reduced, the homo-coupling temperature is also significantly lowered, preventing the desorption of DBTP molecules from the TiO$_2$(110) surface and leading to a radical improvement on the poly-para-phenylene (PPP) polymerization yield. The universality of this mechanism is demonstrated with an iodinated terphenyl derivative (DITP), which shows analogous dehalogenation and polymerization temperatures with a very similar reaction yield. Consequently, we propose to use minute amounts of metal catalyst to drive forward generic bottom-up synthesis strategies on non-metallic surfaces.

cond-mat.mtrl-sci↗

arXiv2023

Ferromagnetism on an atom-thick and extended 2D-metal-organic framework

Ferromagnetism (FM) is the cornerstone of permanent magnets, data storage and other technologies that directly impact our everyday life by their implementation in standard applications and devices. When downscaling bulk materials into their two-dimensional (2D) magnetic isotropic form, the Mermin-Wagner theorem precludes this collective state mediated by short-range exchange interactions at finite temperatures. Interestingly, this prediction fails when significant magnetic anisotropy is present in the material, as recently demonstrated in single layered van der Waals crystals. Before the latter, single layer metal-organic frameworks (MOFs) grown on metallic supports were one of the earliest candidates for achieving 2D-FM. Such high expectations were based on the chemical and spacing control of the 2D-MOF magnetic centers, the tunability of the organic linkers and the rich self-assembled architectures displayed. However, despite many attempts, extended FM in 2D-MOFs has been experimentally elusive. In this work, we demonstrate that extended, cooperative FM takes place in an atom thick 2D-MOF consisting of 9,10-dicyanoanthracene (DCA) molecules and Fe adatoms grown on Au(111). We show this by means of an experimental multitechnique approach that is endorsed by state-of-the art first-principles calculations. Particularly, this 2D ferromagnet follows a first order transition with TC ~ 35 K, which is driven by exchange interactions mainly through the molecular linkers (J=2 meV) and exhibits an out-of-plane square-like hysteresis loop. The strict periodicity of our 2D-MOF allows us to envision the fabrication of ultra-dense single atom magnetic memories and opens the way to explore periodic magnetic 2D-models that could considerably increase the fundamental superparamagnetic limit.

cond-mat.mtrl-sci↗

arXiv2023

Detecting the spin-polarization of edge states in graphene nanoribbons

Low dimensional carbon-based materials are interesting because they can show intrinsic $π$-magnetism associated to p-electrons residing in specific open-shell configurations. Consequently, during the last years there have been impressive advances in the field combining indirect experimental fingerprints of localized magnetic moments with theoretical models. In spite of that, a characterization of their spatial- and energy-resolved spin-moment has so far remained elusive. To obtain this information, we present an approach based on the stabilization of the magnetization of $π$-orbitals by virtue of a supporting substrate with ferromagnetic ground state. Remarkably, we go beyond localized magnetic moments in radical or faulty carbon sites: In our study, energy-dependent spin-moment distributions have been extracted from spatially extended one-dimensional edge states of chiral graphene nanoribbons. This method can be generalized to other nanographene structures, representing an essential validation of these materials for their use in spintronics and quantum technologies.

cond-mat.mes-hall↗

arXiv2021

Order from a mess: the growth of 5-armchair graphene nanoribbons

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon-nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of 5-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this novel reaction path is responsible for the straight GNR growth, something remarkable considering the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants, but also serve as a guide for the design of new precursor molecules.

cond-mat.mtrl-sci↗

arXiv2021

Engineering interfacial quantum states and electronic landscapes by molecular nanoarchitectures

Surfaces are at the frontier of every known solid. They provide versatile supports for functional nanostructures and mediate essential physicochemical processes. Being intimately related with 2D materials, interfaces and atomically thin films often feature distinct electronic states with respect to the bulk, which are key for many relevant properties, such as catalytic activity, interfacial charge-transfer, or crystal growth mechanisms. Of particular interest is reducing the surface electrons' dimensionality and spread with atomic precision, to induce novel quantum properties via lateral scattering and confinement. Both atomic manipulation and supramolecular principles provide access to custom-designed molecular superlattices, which tailor the surface electronic landscape and influence fundamental chemical and physical properties at the nanoscale. Herein, we review the confinement of surface state electrons focusing on their interaction with molecule-based scaffolds created by molecular manipulation and self-assembly protocols under ultrahigh vacuum conditions. Starting from the quasi-free 2D electron gas present at the (111)-terminated surface planes of noble metals, we illustrate the enhanced molecule-based structural complexity and versatility compared to simple atoms. We survey low-dimensional confining structures in the form of artificial lattices, molecular nanogratings or quantum dot arrays, which are constructed upon appropriate choice of their building constituents. Whenever the realized (metal-)organic networks exhibit long-range order, modified surface band structures with characteristic features emerge, revealing intriguing physical properties, such as discretization, quantum coupling or energy and effective mass renormalization. Such collective electronic states can be additionally modified by positioning guest species at the voids of open nanoarchitectures [...].

cond-mat.mes-hall↗

arXiv2020

Hierarchy in the halogen activation during surface-promoted Ullmann coupling

Within the collection of surface-supported reactions currently accessible for the production of extended molecular nanostructures under ultra-high vacuum, Ullmann coupling has been the most successful in the controlled formation of covalent single C-C bonds. Particularly advanced control of this synthetic tool has been obtained by means of hierarchical reactivity, commonly achieved by the use of different halogen atoms that consequently display distinct activation temperatures. Here we report on the site-selective reactivity of certain carbon-halogen bonds. We use precursor molecules halogenated with bromine atoms at two non-equivalent carbon atoms and found that the Ullmann coupling occurs on Au(111) with a remarkable predilection for one of the positions. Experimental evidence is provided by means of scanning tunneling microscopy and a rationalized understanding of the observed preference is obtained from density functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2020

Switching From Reactant to Substrate Engineering in the Selective Synthesis of Graphene Nanoribbons

The challenge of synthesizing graphene nanoribbons (GNRs) with atomic precision is currently being pursued along a one-way road, based on the synthesis of adequate molecular precursors that react in predefined ways through self-assembly processes. The synthetic options for GNR generation would multiply by adding a new direction to this readily successful approach, especially if both of them can be combined. We show here how GNR synthesis can be guided by an adequately nanotemplated substrate instead of by the traditionally designed reactants. The structural atomic precision, unachievable to date through top-down methods, is preserved by the self-assembly process. This new strategy s proof-of-concept compares experiments using 4,4 -dibromo-para-terphenyl as molecular precursor on flat Au(111) and stepped Au(322) substrates. As opposed to the former, the periodic steps of the latter drive the selective synthesis of 6 atom-wide armchair GNRs, whose electronic properties have been further characterized in detail by scanning tunneling spectroscopy, angle resolved photoemission and density functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2020

Electronic structure tunability by periodic meta-ligand spacing in one-dimensional organic semiconductors

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 unravelled 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 bandgap 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.

cond-mat.mtrl-sci↗

arXiv2019

Tunable energy and mass renormalization from homothetic Quantum dot arrays

Quantum dot arrays in the form of molecular nanoporous networks are renown for modifying the electronic surface properties through quantum confinement. Here we show that, compared to the pristine surface state, the fundamental energy 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. The absence to date of the experimental band structure resulting from single adatom metal-coordinated nanoporous networks has precluded the observation of the significant surface state renormalization reported here, which we infer are general of low interacting and well-defined adatom arrays.

physics.chem-ph↗

arXiv2019

Atomically precise step grids for the engineering of helical states

Conventional spin-degenerated surface electrons have been effectively manipulated by using organic and inorganic self-assembled nanoarrays 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. By employing curved crystals to select local vicinal planes we achieve tunable arrays of monoatomic steps with different morphology and orientation. Scanning the ultraviolet light beam on the curved surface during angle-resolved photoemission experiments allows one to unveil the scattering behavior of spin-textured helical states. In this way, we find coherent scattering of helical Rashba states from the step arrays, as well as step-density-dependent Rashba band shifts and spin-orbit splitting compared to the extended BiAg2 plane.

cond-mat.mtrl-sci↗

arXiv2018

Electronic Properties of Substitutionally Boron-doped Graphene Nanoribbons on a Au(111) Surface

High quality graphene nanoribbons (GNRs) grown by on-surface synthesis strategies with atomic precision can be controllably doped by inserting heteroatoms or chemical groups in the molecular precursors. Here, we study the electronic structure of armchair GNRs substitutionally doped with di-boron moieties at the center, through a combination of scanning tunneling spectroscopy, angle-resolved photoemission, and density functional theory simulations. Boron atoms appear with a small displacement towards the surface signaling their stronger interaction with the metal. We find two boron-rich flat bands emerging as impurity states inside the GNR band gap, one of them particularly broadened after its hybridization with the gold surface states. In addition, the boron atoms shift the conduction and valence bands of the pristine GNR away from the gap edge, and leave unaffected the bands above and below, which become the new frontier bands and have negligible boron character. This is due to the selective mixing of boron states with GNR bands according to their symmetry. Our results depict that the GNRs band structure can be tuned by modifying the separation between di-boron moieties.

cond-mat.mes-hall↗

arXiv2018

Symmetry, Shape and Energy Variations in Frontier Molecular Orbitals at Organic/Metal Interfaces: the Case of F$_4$TCNQ

Near Edge X-ray Absorption, Valence and Core-level Photoemission and Density Functional Theory calculations are used to study molecular levels of tetracyano-2,3,5,6-tetrafluoroquinodimethane (F$_4$TCNQ) deposited on Ag(111) and BiAg$_2$/Ag(111). The high electron affinity of F$_4$TCNQ triggers a large static charge transfer from the substrate, and, more interestingly, hybridization with the substrate leads to a radical change of symmetry, shape and energy of frontier molecular orbitals. The Lowest Unoccupied Molecular Orbital (LUMO) shifts below the Fermi energy, becoming the new Highest Occupied Molecular Orbital ($n$-HOMO), whereas the $n$-LUMO is defined by a hybrid band with mixed $π^*$ and $σ^*$ symmetries, localized at quinone rings and cyano groups, respectively. The presence of Bi influences the way the molecule contacts the substrate with the cyano group. The molecule/surface distance is closer and the bond more extended over substrate atoms in F$_4$TCNQ/Ag(111), whereas in F$_4$TCNQ/BiAg$_2$/Ag(111) the distance is larger and the contact more localized on top of Bi. This does not significantly alter molecular levels, but it causes the respective absence or presence of optical excitations in F$_4$TCNQ core-level spectra.

cond-mat.mtrl-sci↗

arXiv2016

Tunable Band Alignment with Unperturbed Carrier Mobility of On-Surface Synthesized Organic Semiconducting Wires

The tunable properties of molecular materials place them among the favorites for a variety of future generation devices. In addition, to maintain the current trend of miniaturization of those devices, a departure from the present top-down production methods may soon be required and self-assembly appears among the most promising alternatives. On-surface synthesis unites the promises of molecular materials and of self-assembly, with the sturdiness of covalently bonded structures: an ideal scenario for future applications. Following this idea, we report the synthesis of functional extended nanowires by self-assembly. In particular, the products correspond to one-dimensional organic semiconductors. The uniaxial alignment provided by our substrate templates allows us to access with exquisite detail their electronic properties, including the full valence band dispersion, by combining local probes with spatial averaging techniques. We show how, by selectively doping the molecular precursors, the product's energy level alignment can be tuned without compromising the charge carrier's mobility.

cond-mat.mtrl-sci↗

arXiv2016

Π Band Dispersion along Conjugated Organic Nanowires Synthesized on a Metal Oxide Semiconductor

Surface confined dehalogenation reactions are versatile bottom-up approaches for the synthesis of carbon-based nanostructures with predefined chemical properties. However, for devices generally requiring low conductivity substrates, potential applications are so far severely hampered by the necessity of a metallic surface to catalyze the reactions. In this work we report the synthesis of ordered arrays of poly(p-phenylene) chains on the surface of semiconducting TiO2(110) via a dehalogenative homocoupling of 4,4"-dibromoterphenyl precursors. The supramolecular phase is clearly distinguished from the polymeric one using low energy electron diffraction and scanning tunneling microscopy as the substrate temperature used for deposition is varied. X ray photoelectron spectroscopy of C 1s and Br 3d core levels traces the temperature of the onset of dehalogenation to around 475 K. Moreover, angle-resolved photoemission spectroscopy and tight-binding calculations identify a highly dispersive band characteristic of a substantial overlap between the precursor's π states along the polymer, considered as the fingerprint of a successful polymerization. Thus, these results establish the first spectroscopic evidence that atomically precise carbon based nanostructures can readily be synthesized on top of a transition-metal oxide surface, opening the prospect for the bottom-up production of novel molecule-semiconductor devices.

cond-mat.mtrl-sci↗