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On-surface synthesis has paved the way towards the fabrication and characterization of conjugated carbon-based molecular materials that exhibit {\pi}-magnetism such as triangulenes. Aza-triangulene, a nitrogen-substituted derivative, was recently shown to display rich on-surface chemistry, offering an ideal platform to investigate structure-property relations regarding spin-selective charge transfer and magnetic fingerprints. Herein, we study electronic changes upon fusing single molecules into larger dimeric derivatives. We show that the closed-shell structure of aza-triangulene on Ag(111) leads to closed-shell dimers covalently coupled through sterically accessible carbon atoms. Meanwhile, its open-shell structure on Au(111) leads to coupling via atoms displaying high spin density, resulting in symmetric or asymmetric dimer products. Interestingly, whereas all fused products on Au(111) exhibit similar charge transfer properties, only asymmetric dimers show magnetic fingerprints. We rationalize this in terms of molecular bonding structure and {\pi}-conjugation: in contrast to the symmetric and highly conjugated dimer, asymmetric dimers display more localized orbitals, which result in a larger Coulomb repulsion and thus promote single electron occupancies with associated spin density and {\pi}-magnetism. These results expose a clear relationship between molecular symmetry, charge transfer, and spin states in {\pi}-conjugated carbon-based nanostructures.

We report an example that demonstrates the clear interdependence between surface-supported reactions and molecular adsorption configurations. Two biphenyl-based molecules with two and four bromine substituents, i.e. 2,2-dibromo-biphenyl (DBBP) and 2,2,6,6-tetrabromo-1,1-biphenyl (TBBP), show completely different reaction pathways on a Ag(111) surface, leading to the selective formation of dibenzo[e,l]pyrene and biphenylene dimer, respectively. By combining low-temperature scanning tunneling microscopy, synchrotron radiation photoemission spectroscopy, and density functional theory calculations, we unravel the underlying reaction mechanism. After debromination, a bi-radical biphenyl can be stabilized by surface Ag adatoms, while a four-radical biphenyl undergoes spontaneous intramolecular annulation due to its extreme instability on Ag(111). Such different chemisorption-induced precursor states between DBBP and TBBP consequently lead to different reaction pathways after further annealing. In addition, using bond-resolving scanning tunneling microscopy and scanning tunneling spectroscopy, we determine the bond length alternation of biphenylene dimer product with atomic precision, which contains four-, six-, and eight-membered rings. The four-membered ring units turn out to be radialene structures.

Low dimensional carbon-based materials are interesting because they can show intrinsic $\pi$-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 $\pi$-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.

The growth of controlled 1D carbon-based nanostructures on metal surfaces is a multistep process whose path, activation energies and intermediate metastable states strongly depend on the employed substrate. Whereas this process has been extensively studied on gold, less work has been dedicated to silver surfaces, which have a rather different catalytic activity. In this work, we present an experimental and theoretical investigation of the growth of poly-$p$-phenylene (PPP) chains and subsequent narrow graphene ribbons starting from 4,4''-dibromo-$p$-terphenyl molecular precursors deposited at the silver surface. By combing scanning tunneling microscopy (STM) imaging and density functional theory (DFT) simulations, we describe the molecular morphology and organization at different steps of the growth process and we discuss the stability and conversion of the encountered species on the basis of calculated thermodynamic quantities. Unlike the case of gold, at the debromination step we observe the appearance of organometallic molecules and chains, which can be explained by their negative formation energy in the presence of a silver adatom reservoir. At the dehydrogenation temperature the persistence of intercalated Br atoms hinders the formation of well-structured graphene ribbons, which are instead observed on gold, leading only to a partial lateral coupling of the PPP chains. We numerically derive very different activation energies for Br desorption from the Ag and Au surfaces, thereby confirming the importance of this process in defining the kinetics of the formation of molecular chains and graphene ribbons on different metal surfaces.

Emergence of $\pi$-magnetism in open-shell nanographenes has been theoretically predicted decades ago but their experimental characterization was elusive due to the strong chemical reactivity that makes their synthesis and stabilization difficult. In recent years, on-surface synthesis under vacuum conditions has provided unprecedented opportunities for atomically precise engineering of nanographenes, which in combination with scanning probe techniques have led to a substantial progress in our capabilities to realize localized electron spin states and to control electron spin interactions at the atomic scale. Here we review the essential concepts and the remarkable advances in the last few years, and outline the versatility of carbon-based $\pi$-magnetic materials as an interesting platform for applications in spintronics and quantum technologies., Comment: Topical Review, 46 pages, 23 figures, 2 tables

Nitrogen heteroatom doping into a triangulene molecule allows tuning its magnetic state. However, the synthesis of the nitrogen-doped triangulene (aza-triangulene) has been challenging. Herein, we report the successful synthesis of aza-triangulene on the Au(111) and Ag(111) surfaces, along with their characterizations by scanning tunneling microscopy and spectroscopy in combination with density functional theory (DFT) calculations. Aza-triangulenes were obtained by reducing ketone-substituted precursors. Exposure to atomic hydrogen followed by thermal annealing and, when necessary, manipulations with the scanning probe, afforded the target product. We demonstrate that on Au(111) aza-triangulene donates an electron to the substrate and exhibits an open-shell triplet ground state. This is derived from the different Kondo resonances of the final aza-triangulene product and a series of intermediates on Au(111). Experimentally mapped molecular orbitals match perfectly with DFT calculated counterparts for a positively charged aza-triangulene. In contrast, aza-triangulene on Ag(111) receives an extra electron from the substrate and displays a closed-shell character. Our study reveals the electronic properties of aza-triangulene on different metal surfaces and offers an efficient approach for the fabrication of new hydrocarbon structures, including reactive open-shell molecules.

Magnetic graphene nanoribbons (GNRs) have become promising candidates for future applications, including quantum technologies. Here, we characterize magnetic states hosted by chiral graphene nanoribbons (chGNRs). The substitution of a hydrogen atom at the chGNR edge by a ketone group effectively adds one p_z electron to the {\pi}-electron network, thus producing an unpaired {\pi} radical. A closely related scenario occurs for regular ketone-functionalized chGNRs in which one oxygen atom is missing. Two such radical states can interact via exchange coupling and we study those interactions as a function of their relative position, which includes a remarkable dependence on the chirality, as well as on the nature of the surrounding GNR, i.e., with or without ketone functionalization. In addition, we determine the parameters whereby this type of systems with oxygen heteroatoms can be adequately described within the widely used mean-field Hubbard model. Altogether, we provide new insights to both theoretically model and devise GNR-based nanostructures with tunable magnetic properties.

Chiral graphene nanoribbons are extremely interesting structures due to their low bandgaps and potential development of spin-polarized edge states. Here, we study their band structure on low work function silver surfaces and assess the effect of charge transfer on their properties.

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.

Carbon nanostructures with zigzag edges exhibit unique properties with exciting potential applications. Such nanostructures are generally synthesized under vacuum because their zigzag edges are unstable under ambient conditions: a barrier that must be surmounted to achieve their scalable exploitation. Here, we prove the viability of chemical protection/deprotection strategies for this aim, demonstrated on labile chiral graphene nanoribbons (chGNRs). Upon hydrogenation, the chGNRs survive an exposure to air, after which they are easily converted back to their original structure via annealing. We also approach the problem from another angle by synthesizing a chemically stable oxidized form of the chGNRs that can be converted to the pristine hydrocarbon form via hydrogenation and annealing. These findings may represent an important step toward the integration of zigzag-edged nanostructures in devices.

Precise control over the size and shape of graphene nanostructures allows engineering spin-polarized edge and topological states, representing a novel source of non-conventional $\pi$-magnetism with promising applications in quantum spintronics. A prerequisite for their emergence is the existence of robust gapped phases, which are difficult to find in extended graphene systems: only armchair graphene nanoribbons (GNRs) show a band gap that, however, closes for any other GNR orientation. Here we show that semi-metallic chiral GNRs (chGNRs) narrowed down to nanometer widths undergoes a topological phase transition, becoming first topological insulators, and transforming then into trivial band insulators for the narrowest chGNRs. We fabricated atomically precise chGNRs of different chirality and size by on surface synthesis using predesigned molecular precursors. Combining scanning tunnelling microscopy (STM) measurements and theory simulations, we follow the evolution of topological properties and bulk band gap depending on the width, length, and chirality of chGNRs. The first emerging gapped phases are topological, protected by a chiral interaction pattern between edges. For narrower ribbons, the symmetry of the interaction pattern changes, and the topological gap closes and re-opens again as a trivial band insulator. Our findings represent a new platform for producing topologically protected spin states and demonstrates the potential of connecting chiral edge and defect structure with band engineering., Comment: 12 pages, 5 figures

Molecular dynamics simulations show that a graphene nanoribbon with alternating regions which are one and three hexagons wide can transform into a hybrid 1D nanoobject with alternating double chains and polycyclic regions under electron irradiation in HRTEM. A scheme of synthesis of such a nanoribbon using Ullmann coupling and dehydrogenation reactions is proposed. The reactive REBO-1990EVC potential is adapted for simulations of carbon-hydrogen systems and is used in combination with the CompuTEM algorithm for modeling of electron irradiation effects. The atomistic mechanism of formation of the new hybrid 1D nanoobject is found to be the following. Firstly hydrogen is removed by electron impacts. Then spontaneous breaking of bonds between carbon atoms leads to decomposition of narrow regions of the graphene nanoribbon into double chains. Simultaneously thermally activated growth of polycyclic regions occurs. Density functional theory calculations give barriers along the growth path of polycyclic regions consistent with this mechanism. Electronic properties of the new 1D nanoobject are shown to be strongly affected by the edge magnetism and make this nanostructure promising nanoelectronic and spintronic applications. The way of synthesis the 1D nanoobject proposed here can be considered as an example of the general three-stage strategy of production of nanoobjects and macromolecules. 1) precursors are synthesized by traditional chemical method, 2) precursors are placed in HRTEM with the electron energy that is sufficient only to remove hydrogen atoms, 3) as a result of hydrogen removal, the precursors become unstable or metastable and transform into new nanoobjects or macromolecules., Comment: 17 pages, 10 figures in the paper; 5 pages and 3 figures in the supplementary information

We investigate the thermally-induced cyclization of 1,2 - bis(2 - phenylethynyl)benzene on Au(111) using scanning tunneling microscopy and computer simulations. Cyclization of sterically hindered enediynes is known to proceed via two competing mechanisms in solution: a classic C1 - C6 or a C1 - C5 cyclization pathway. On Au(111) we find that the C1 - C5 cyclization is suppressed and that the C1 - C6 cyclization yields a highly strained bicyclic olefin whose surface chemistry was hitherto unknown. The C1 - C6 product self-assembles into discrete non-covalently bound dimers on the surface. The reaction mechanism and driving forces behind non-covalent association are discussed in light of density functional theory calculations.

Contributing to the need of new graphene nanoribbon (GNR) structures that can be synthesized with atomic precision, we have designed a reactant that renders chiral (3,1) - GNRs after a multi-step reaction including Ullmann coupling and cyclodehydrogenation. The nanoribbon synthesis has been successfully proved on different coinage metals, and the formation process, together with the fingerprints associated to each reaction step, has been studied combining scanning tunnelling microscopy, core-level spectroscopy and density functional calculations. In addition to the GNR chiral edge structure, the substantial GNR lengths achieved and the low processing temperature required to complete the reaction grant this reactant extremely interesting properties for potential applications.

The electronic properties of graphene nanoribbons (GNRs) can be precisely tuned by chemical doping. Here we demonstrate that amino (NH$_2$) functional groups attached at the edges of chiral GNRs (chGNRs) can efficiently gate the chGNRs and lead to the valence band (VB) depopulation on a metallic surface. The NH$_2$-doped chGNRs are grown by on-surface synthesis on Au(111) using functionalized bianthracene precursors. Scanning tunneling spectroscopy resolves that the NH$_2$ groups significantly up-shift the bands of chGNRs, causing the Fermi level crossing of the VB onset of chGNRs. Through density functional theory simulations we confirm that the hole-doping behavior is due to an upward shift of the bands induced by the edge NH$_2$ groups., Comment: 7 pages with 5 figures and 4 page supplementary information. This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in ACS Nano, copyright American Chemical Society after peer review

We report a multi-step on-surface synthesis strategy. The first step consists in the surface-supported synthesis of metal-organic complexes, which are subsequently used as catalysts to steer on-surface alkyne coupling reactions. In addition, we analyze and compare the electronic properties of the different coupling motifs obtained.

Fine management of chiral processes on solid surfaces has progressed over the years, yet still faces the need for the controlled and selective production of advanced chiral materials. Here, we report on the use of enantiomerically enriched molecular building blocks to demonstrate the transmission of their intrinsic chirality along a sequence of on-surface reactions. Triggered by thermal annealing, the on-surface reac-tions induced in this experiment involve firstly the coupling of the chiral reactants into chiral polymers and subsequently their transformation into planar prochiral graphene nanoribbons. Our study reveals that the axial chirality of the reactant is not only transferred to the polymers, but also to the planar chirality of the graphene nanoribbon end products. Such chirality transfer consequently allows, starting from ad-equate enantioenriched reactants, for the controlled production of chiral and prochiral organic nanoarchi-tectures with pre-defined handedness.

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.

The self-assembly of Leucoquinizarin molecule on Au(111) surface is shown to be characterized by the molecules mostly in their keto-enolic tautomeric form, with evidences of their temporary switching to other tautomeric forms. This reveals a metastable chemistry of the assembled molecules, to be considered for their possible employment in the formation of more complex hetero-organic interfaces

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.

Organosulfur compounds at the interface to noble metals have proved over the last decades to be extremely versatile systems for both fundamental and applied research. However, the anchoring of thiols to gold remained an object of controversy for long times. The RS-Au-SR linkage, in particular, is a robust bonding configuration that displays interesting properties. It is generated spontaneously at room temperature and can be used for the production of extended molecular nanostructures. In this work we explore the behavior of 1,4-Bis(4-mercaptophenyl)benzene (BMB) on the Au(111) surface, which results in the formation of 2D crystalline metal-organic assemblies stabilized by this type of Au-thiolate bonds. We show how to control the thiolates stereospecific bonding motif and thereby choose whether to form ordered arrays of Au3BMB3 units with embedded triangular nanopores, or linearly stacked metal-organic chains. The former turn out to be the thermodynamically favored structures and display confinement of the underneath Au(111) surface state. The electronic properties of single molecules as well as of the 2D crystalline self-assemblies have been characterized both on the metal-organic backbone and inside the associated pores.

We report on the effect of sp3 hybridized carbon atoms in acene derivatives adsorbed on metal surfaces, namely decoupling the molecules from the supporting substrates. In particular, we have used a Ag(100) substrate and hydrogenated heptacene molecules, in which the longest conjugated segment determining its frontier molecular orbitals amounts to five consecutive rings. The non-planarity that the sp3 atoms impose on the carbon backbone results in electronically decoupled molecules, as demonstrated by the presence of charging resonances in dI/dV tunneling spectra and the associated double tunneling barriers, or in the Kondo peak that is due to a net spin S=1/2 of the molecule as its LUMO becomes singly charged. The spatially dependent appearance of the charging resonances as peaks or dips in the differential conductance spectra is further understood in terms of the tunneling barrier variation upon molecular charging, as well as of the different orbitals involved in the tunneling process.

We extensively characterize the electronic structure of ultra-narrow graphene nanoribbons (GNRs) with armchair edges and zig-zag termini that have 5 carbon atoms across their width (5-AGNRs), as synthesised on Au(111). Scanning tunnelling spectroscopy measurements on the ribbons, recorded on both the metallic substrate and a decoupling NaCl layer, show well-defined dispersive bands and in-gap states. In combination with theoretical calculations, we show how these in-gap states are topological in nature and localised at the zig-zag termini of the nanoribbons. Besides rationalising the driving force behind the topological class selection of 5-AGNRs, we also uncover the length-dependent behaviour of these end states which transition from singly occupied spin-split states to a closed-shell form as the ribbons become shorter. Finally, we demonstrate the magnetic character of the end states via transport experiments in a model two-terminal device structure in which the ribbons are suspended between the scanning probe and the substrate that both act as leads.

A hallmark of quantum control is the ability to manipulate quantum emission at the nanoscale. Through scanning tunneling microscopy induced luminescence (STML) we are able to generate plasmonic light originating from inelastic tunneling processes that occur in a few-nanometer thick molecular film of C$_{60}$ deposited on Ag(111). Single photon emission, not of excitonic origin, occurs with a 1/$e$ lifetime of a tenth of a nanosecond or less, as shown through Hanbury Brown and Twiss photon intensity interferometry. We have performed tight-binding calculations of the electronic structure for the combined Ag-C$_{60}$-tip system and obtained good agreement with experiment. The tunneling happens through electric field induced split-off states below the C$_{60}$ LUMO band, which leads to a Coulomb blockade effect and single photon emission. The use of split-off states is shown to be a general technique that has special relevance for narrowband materials with a large bandgap.

We report on the fabrication and transport characterization of atomically-precise single molecule devices consisting of a magnetic porphyrin covalently wired by graphene nanoribbon electrodes. The tip of a scanning tunneling microscope was utilized to contact the end of a GNR-porphyrin-GNR hybrid system and create a molecular bridge between tip and sample for transport measurements. Electrons tunneling through the suspended molecular heterostructure excited the spin multiplet of the magnetic porphyrin. The detachment of certain spin-centers from the surface shifted their spin-carrying orbitals away from an on-surface mixed-valence configuration, recovering its original spin state. The existence of spin-polarized resonances in the free-standing systems and their electrical addressability is the fundamental step for utilization of carbon-based materials as functional molecular spintronics systems.

Achieving the Ag(001)-supported synthesis of heptacene from two related reactants reveals the effect of the presence of Br atoms on the reaction process. The properties of reactant, intermediates and end-product are further characterized by scanning tunneling microscopy and spectroscopy.

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., Comment: 12 pages, 5 Figures

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 $\pi^*$ and $\sigma^*$ 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.

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.

We report on the energy level alignment evolution of valence and conduction bands of armchair-oriented graphene nanoribbons (aGNR) as their band gap shrinks with increasing width. We use 4,4-dibromo-para-terphenyl as molecular precursor on Au(111) to form extended poly-para-phenylene nanowires, which can be fused sideways to form atomically precise aGNRs of varying widths. We measure the frontier bands by means of scanning tunneling spectroscopy, corroborating that the nanoribbons band gap is inversely proportional to their width. Interestingly, valence bands are found to show Fermi level pinning as the band gap decreases below a threshold value around 1.7 eV. Such behavior is of critical importance to understand the properties of potential contacts in graphene nanoribbon-based devices. Our measurements further reveal a particularly interesting system for studying Fermi level pinning by modifying an adsorbates band gap while maintaining an almost unchanged interface chemistry defined by substrate and adsorbate.

Graphene nanoribbons (GNRs) make up an extremely interesting class of materials. On the one hand GNRs share many of the superlative properties of graphene, while on the other hand they display an exceptional degree of tunability of their optoelectronic properties. The presence or absence of correlated low-dimensional magnetism, or of a widely tunable band gap, is determined by the boundary conditions imposed by the width, crystallographic symmetry and edge structure of the nanoribbons. In combination with additional controllable parame-ters like the presence of heteroatoms, tailored strain, or the formation of hetero-structures, the possibilities to shape the electronic properties of GNRs according to our needs are fantastic. However, to really benefit from that tunability and harness the opportunities offered by GNRs, atomic precision is strictly required in their synthesis. This can be achieved through an on-surface synthesis approach, in which one lets appropriately designed precursor molecules to react in a selective way that ends up forming GNRs. In this chapter we review the structure-property relations inherent to GNRs, the synthesis approach and the ways in which the var-ied properties of the resulting ribbons have been probed, finalizing with selected examples of demonstrated GNR applications.

Recent advances in graphene nanoribbon-based research have demonstrated the controlled synthesis of chiral graphene nanoribbons (cGNR) with atomic precision using strategies of on-surface chemistry. However their electronic characterization, including typical figures of merit like band gap or frontier bands effective masses, has not yet been reported. In this work, we provide a detailed characterization of (3,1)-cGNRs on Au(111). The structure and epitaxy, as well as the electronic band structure of the ribbons, are analyzed by means of scanning tunneling microscopy and spectroscopy, angle resolved photoemission and density functional theory.

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 {\pi} 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.

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.

Recently developed processes have enabled bottom-up chemical synthesis of graphene nanoribbons (GNRs) with precise atomic structure. These GNRs are ideal candidates for electronic devices because of their uniformity, extremely narrow width below 1 nm, atomically perfect edge structure, and desirable electronic properties. Here, we demonstrate nanoscale chemically synthesized GNR field-effect transistors, made possible by development of a new layer transfer process. We observe strong environmental sensitivity and unique transport behavior characteristic of sub-1nm width GNRs.

Codeposition of two molecular species [CuPc (donor) and PFP (acceptor)] on noble metal (111) surfaces leads to the self-assembly of an ordered mixed layer with maximized donor-acceptor contact area. The main driving force behind this arrangement is assumed to be the intermolecular C-F...H-C hydrogen-bond interactions. Such interactions would be maximized for a coplanar molecular arrangement. However, precise measurement of molecule-substrate distances in the molecular mixture reveals significantly larger adsorption heights for PFP than for CuPc. Most surprisingly, instead of leveling to increase hydrogen bond interactions, the height difference is enhanced in mixed layers as compared to the heights found in single component CuPc and PFP layers, resulting in an overall reduced interaction with the underlying substrate. The influence of the increased height of PFP on the interface dipole is investigated through work function measurements.