Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires

On-Surface Fabrication toward Polar 2D Macromolecular Crystals

Polar 2D macromolecular structures have attracted significant attention because of their ferroelectricity and ferro-magnetism. However, it is challenging to synthesize them experimentally because dipoles or spins of these macromolecules tend to cancel each other. So far, there has been no successful strategy for assembling macromolecules in a unidirectional manner, achieving stereoregular polymerization on metal surfaces, and creating polar 2D polymer crystals. Recent progress in molecular assembly, on-surface polymer synthesis, and direct control of molecules using electric field applications provides an opportunity to develop such strategies. In this regard, we first review past studies on chiral and achiral molecular assembly, on-surface polymer synthesis, and orientation control of polar molecules. Then, we discuss our newly developed approach called "vectorial on-surface synthesis", which is based on "dynamic chirality" of compass precursors, stereoselective polymerization, and favorable interchain interactions originating from CH-π interactions. Finally, we conclude with a prospective outlook.

On-Surface Synthesis of Polyene-Linked Porphyrin Cooligomer

π-Conjugated molecules are viewed as fundamental components in forthcoming molecular nanoelectronics in which semiconducting functional units are linked to each other via metallic molecular wires. However, it is still challenging to construct such block cooligomers on the surface. Here, we present a synthesis of [18]-polyene-linked Zn-porphyrin cooligomers via a two-step reaction of the alkyl groups on Cu(111) and Cu(110). Nonyl groups (-C9H19) substituted at the 5,15-meso positions of Zn-porphyrin were first transformed to alkenyl groups (-C9H10) by dehydrogenation. Subsequently, homocoupling of the terminal -CH2 groups resulted in the formation of extended [18]-polyene-linked porphyrin cooligomers. The structures of the products at each reaction step were investigated by bond-resolved scanning tunneling microscopy at low temperatures. A combination of angle-resolved photoemission spectroscopy and density functional theory calculations revealed the metallic property of the all trans [18]-polyene linker on Cu(110). This finding may provide an approach to fabricate complex nanocarbon structures on the surface.

Topology selectivity of a conformationally flexible precursor through selenium doping

Conformational arrangements within nanostructures play a crucial role in shaping the overall configuration and determining the properties, for example in covalent/metal organic frameworks. In on-surface synthesis, conformational diversity often leads to uncontrollable or disordered structures. Therefore, the exploration of controlling and directing the conformational arrangements is significant in achieving desired nanoarchitectures. Herein, a conformationally flexible precursor 2,4,6-tris(3-bromophenyl)-1,3,5-triazine is employed, and a random phase consisting of C3h and Cs conformers is firstly obtained after deposition of the precursor on Cu(111) at room temperature to 365 K. At low coverage (0.01 ML) selenium doping, we achieve the selectivity of the C3h conformer and improve the nanopore structural homogeneity. The ordered two-dimensional metal organic nanostructure can be fulfilled by selenium doping from room temperature to 365 K. The formation of the conformationally flexible precursor on Cu(111) is explored through the combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy. The regulation of energy diagrams in the absence or presence of the Se atom is revealed by density functional theory calculations. These results can enrich the on-surface synthesis toolbox of conformationally flexible precursors, for the design of complex nanoarchitectures, and for future development of engineered nanomaterials.

Barth J, Lobo-Checa J, Corso M, Rogero C. Enhancing Haloarene Coupling Reaction Efficiency on an Oxide Surface by Metal Atom Addition

The bottom-up synthesis of carbon-based nanomaterials directly on semiconductor surfaces allows for the decoupling of their electronic and magnetic properties from the substrates. However, the typically reduced reactivity of such nonmetallic surfaces adversely affects the course of these reactions. Here, we achieve a high polymerization yield of halogenated polyphenyl molecular building blocks on the semiconducting TiO2(110) surface via concomitant surface decoration with cobalt atoms, which catalyze the Ullmann coupling reaction. Specifically, cobalt atoms trigger the debromination of 4,4″-dibromo-p-terphenyl molecules on TiO2(110) and mediate the formation of an intermediate organometallic phase already at room temperature (RT). As the debromination temperature is drastically reduced, homocoupling and polymerization readily proceed, preventing presursor desorption from the substrate and entailing a drastic increase of the poly-para-phenylene polymerization yield. The general efficacy of this mechanism is shown with an iodinated terphenyl derivative, which exhibits similar dehalogenation and reaction yield.

Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures

The electronic structure defines the properties of graphene-based nanomaterials. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments on graphene nanoribbons (GNRs), nanographenes, and nanoporous graphene (NPG) often determine an apparent electronic orbital confinement into the edges and nanopores, leading to dubious interpretations such as image potential states or super-atom molecular orbitals. We show that these measurements are subject to a wave function decay into the vacuum that masks the undisturbed electronic orbital shape. We use Au(111)-supported semiconducting gulf-type GNRs and NPGs as model systems fostering frontier orbitals that appear confined along the edges and nanopores in STS measurements. DFT calculations confirm that these states originate from valence and conduction bands. The deceptive electronic orbital confinement observed is caused by a loss of Fourier components, corresponding to states of high momentum. This effect can be generalized to other 1D and 2D carbon-based nanoarchitectures and is important for their use in catalysis and sensing applications.

Electronic Band Engineering of Two-Dimensional Kagomé Polymers

Two-dimensional conjugated polymers (2DCPs) are an emerging class of materials that exhibit properties similar to graphene yet do not have the limitation of zero bandgap. On-surface synthesis provides exceptional control on the polymerization reaction, allowing tailoring properties by choosing suitable monomers. Heteroatom-substituted triangulene 2DCPs constitute a playing ground for such a design and are predicted to exhibit graphene-like band structures with high charge mobility and characteristic Dirac cones in conduction or valence states. However, measuring these properties experimentally is challenging and requires long-range-ordered polymers, preferably with an epitaxial relationship with the substrate. Here, we investigate the electronic properties of a mesoscale-ordered carbonyl-bridged triphenylamine 2DCP (P2TANGO) and demonstrate the presence of a Dirac cone by combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations. Moreover, we measure the absolute energy position of the Dirac cone with respect to the vacuum level. We show that the bridging functionality of the triangulene (ether vs carbonyl) does not significantly perturb the band structure but strongly affects the positioning of the bands with respect to the Au(111) states and allows control of the ionization energy of the polymer. Our results provide proof of the controllable electronic properties of 2DCPs and bring us closer to their use in practical applications.

Electronic band structure of 1D π-d hybridized narrow-gap metal-organic polymers

One-dimensional (1D) metal-organic (MO) nanowires are captivating from fundamental and technological perspectives due to their distinctive magnetic and electronic properties. The solvent-free synthesis of such nanomaterials on catalytic surfaces provides a unique approach for fabricating low-dimensional single-layer materials with atomic precision and low amount of defects. A detailed understanding of the electronic structure of MO polymers such as band gap and dispersive bands is critical for their prospective implementation into nanodevices such as spin sensors or field-effect transistors. Here, we have performed the on-surface reaction of quinoidal ligands with single cobalt atoms (Co-QDI) on a vicinal Au(788) surface in ultra-high vacuum. This procedure promotes the growth and uniaxial alignment of Co-QDI MO chains along the surface atomic steps, while permitting the mapping of their electronic properties with space-averaging angle-resolved photoemission spectroscopy. In the direction parallel to the principal chain axis, a well-defined 1D band structure with weakly dispersive and dispersive bands is observed, confirming a pronounced electron delocalization. Low-temperature scanning tunneling microscopy/spectroscopy delves into the atomically precise structure of the nanowires and elucidates their narrow bandgap. These findings are supported with GW₀ band structure calculations showing that the observed electronic bands emanate from the efficient hybridization of Co(3d) and molecular orbitals. Our work paves the way towards a systematic search of similar 1D π-d hybridized MO chains with tunable electronic and magnetic properties defined by the transition or rare earth metal atom of choice.

On-Surface Synthesis and Evolution of Self-Assembled Poly(p-phenylene) Chains on Ag(111): A Joint Experimental and Theoretical Study

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.

Conservation of Nickel Ion Single-Active Site Character in a Bottom-Up Constructed π-Conjugated Molecular Network

On-surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building-blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On-surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π-extended two-dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single-active site character of their single-molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom-up constructed covalent nanomeshes.

Carbon-based nanostructures as a versatile platform for tunableπ-magnetism

Emergence ofπ-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-basedπ-magnetic materials as an interesting platform for applications in spintronics and quantum technologies.

Synthesis of oligoacenes using precursors for evaluation of their electronic structures

Acenes, which are hydrocarbons comprising linearly fused benzene rings, have attracted considerable attention owing to their electronic structures and utility as organic electronic materials. However, the ease with which oligoacenes undergo oxidation increases with the number of linearly fused benzene rings owing to the increased energy of the highest occupied molecular orbital. The synthesis of naked oligoacenes with seven or more benzene rings is difficult because their open-shell structure renders them unstable. The recent development of a precursor method has enabled the in situ synthesis of oligoacenes under specific conditions and the spectroscopic observation of oligoacene in single crystals, in film matrices and under cryogenic conditions. Scanning tunneling microscopy and non-contact atomic force microscopy under ultra-high vacuum conditions have also made significant advances in the study of oligoacenes and oligoazaacenes. This paper reviews the recent progress in the synthesis of oligoacenes using precursors, with a particular focus on the chemical structures, synthesis, and reactivity of the precursors. The electronic properties of oligoacenes are also discussed in relation to the number of fused benzene rings, including their energy levels and spin states. These results will contribute to the synthesis and development of carbon nanomaterials with applications in the field of organic electronics.

Tandem Desulfurization/C-C Coupling Reaction of Tetrathienylbenzenes on Cu(111): Synthesis of Pentacene and an Exotic Ladder Polymer

Surface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C-C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C-C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces.

On-Surface Synthesis of Polypyridine: Strain Enforces Extended Linear Chains

Strain-induced on-surface transformations provide an appealing route to steer the selectivity towards desired products. Here, we demonstrate the selective on-surface synthesis of extended all-trans poly(2,6-pyridine) chains on Au(111). By combining high-resolution scanning tunneling and atomic force microscopy, we revealed the detailed chemical structure of the reaction products. Density functional theory calculations indicate that the synthesis of extended covalent structures is energetically favored over the formation of macrocycles, due to the minimization of internal strain. Our results consolidate the exploitation of internal strain relief as a driving force to promote selective on-surface reactions.

Adatoms in the Surface-Confined Ullmann Coupling of Phenyl Groups

Despite the importance of the on-surface Ullmann coupling for synthesis of atomically precise carbon nanostructures, it is still unclear whether this reaction is catalyzed by surface atoms or adatoms. Here, the feasibility of the adatom creation and adatom-catalyzed Ullmann coupling of chloro-, bromo-, and iodobenzene on Cu(111), Ag(111), and Au(111) surfaces is examined using density functional theory modeling. The extraction of a metal atom is found to be greatly facilitated by the formation of strong phenyl-metal bonds, making the extraction energy barrier comparable to, and in the case of Ag(111) even lower than, that for the competing surface-catalyzed phenyl-phenyl bond formation. However, if the phenyl-adatom bonds are too strong, as on Cu(111) and Ag(111), they create an insurmountable barrier for the subsequent adatom-catalyzed C-C coupling. In contrast, Au adatoms do not bind phenyl groups strongly and can catalyze the C-C bond formation almost as efficiently as surface atoms.

On-Surface Decarboxylation Coupling Facilitated by Lock-to-Unlock Variation of Molecules upon the Reaction

On-surface synthesis (OSS) involving relatively high energy barriers remains challenging due to a typical dilemma: firm molecular anchor is required to prevent molecular desorption upon the reaction, whereas sufficient lateral mobility is crucial for subsequent coupling and assembly. By locking the molecular precursors on the substrate then unlocking them during the reaction, we present a strategy to address this challenge. High-yield synthesis based on well-defined decarboxylation, intermediate transition, and hexamerization is demonstrated, resulting in an extended and ordered network exclusively composed of the newly synthesized macrocyclic compound. Thanks to the steric hindrance of its maleimide group, we attain a preferential selection of the coupling. This work unlocks a promising path to enrich the reaction types and improve the coupling selectivity hence the structual homogeneity of the final product for OSS.

Atomically precise graphene nanoribbons: interplay of structural and electronic properties

Graphene nanoribbons hold great promise for future applications in nanoelectronic devices, as they may combine the excellent electronic properties of graphene with the opening of an electronic band gap - not present in graphene but required for transistor applications. With a two-step on-surface synthesis process, graphene nanoribbons can be fabricated with atomic precision, allowing precise control over width and edge structure. Meanwhile, a decade of research has resulted in a plethora of graphene nanoribbons having various structural and electronic properties. This article reviews not only the on-surface synthesis of atomically precise graphene nanoribbons but also how their electronic properties are ultimately linked to their structure. Current knowledge and considerations with respect to precursor design, which eventually determines the final (electronic) structure, are summarized. Special attention is dedicated to the electronic properties of graphene nanoribbons, also in dependence on their width and edge structure. It is exactly this possibility of precisely changing their properties by fine-tuning the precursor design - offering tunability over a wide range - which has generated this vast research interest, also in view of future applications. Thus, selected device prototypes are presented as well.

Host guest chemistry and supramolecular doping in triphenylamine-based covalent frameworks on Au(111)

The post-synthetic modification of covalent organic frameworks (COFs) via host-guest chemistry is an important method to tailor their electronic properties for applications. Due to the limited structural control in the assembly of two-dimensional surface-supported COFs, supramolecular networks are traditionally used at present for host-guest experiments on surfaces, which lack structural and thermal stability, however. Here, we present a combined scanning tunneling microscopy and density functional theory study to understand the host-guest interaction in triphenylamine-based covalently-linked macrocycles and networks on Au(111). These triphenylamine-based structures feature carbonyl and hydrogen functionalized pores that create preferred adsorption sites for trimesic acid (TMA) and halogen atoms. The binding of the TMA through optimized hydrogen-bond interactions is corroborated by selective adsorption positions within the pores. Band structure calculations reveal that the strong intermolecular charge transfer through the TMA bonding reduces the band gap in the triphenylamine COFs, demonstrating the concept of supramolecular doping by host-guest interactions in surface-supported COFs. Halogen atoms selectively adsorb between two carbonyl groups at Au hollow sites. The mainly dispersive interaction of the halogens with the triphenylamine COF leads to a small downshift of the bands. Most of the halogens change their adsorption position selectively upon annealing near the desorption temperature. In conclusion, we demonstrate evidence for supramolecular doping via post-synthetic modification and to track chemical reactions in confined space.

Metalated Graphyne-Based Networks as Two-Dimensional Materials: Crystallization, Topological Defects, Delocalized Electronic States, and Site-Specific Doping

Graphyne-based two-dimensional (2D) carbon allotropes feature extraordinary physical properties; however, their synthesis as crystalline single-layered materials has remained challenging. We report on the fabrication of large-area organometallic Ag-bis-acetylide networks and their structural and electronic properties on Ag(111) using low-temperature scanning tunneling microscopy combined with density functional theory (DFT) calculations. The metalated graphyne-based networks are robust at room temperature and assembled in a bottom-up approach via surface-assisted dehalogenative homocoupling of terminal alkynyl bromides. Large-area networks of several hundred nanometers with topological defects at domain boundaries are obtained due to the Ag-acetylide bonds' reversible nature. The thermodynamically controlled growth mechanism is explained through the direct observation of intermediates, which differ on Ag(111) and Au(111). Scanning tunneling spectroscopy resolved unoccupied states delocalized across the network. The energy of these states can be shifted locally by the attachment of a different number of Br atoms within the network. DFT revealed that free-standing metal-bis-acetylide networks are semimetals with a linear band dispersion around several high-symmetry points, which suggest the presence of Weyl points. These results demonstrate that the organometallic Ag-bis-acetylide networks feature the typical 2D material properties, which make them of great interest for fundamental studies and electronic materials in devices.

On-Surface Driven Formal Michael Addition Produces m-Polyaniline Oligomers on Pt(111)

On-surface synthesis is emerging as a highly rational bottom-up methodology for the synthesis of molecular structures that are unattainable or complex to obtain by wet chemistry. Here, oligomers of meta-polyaniline, a known ferromagnetic polymer, were synthesized from para-aminophenol building-blocks via an unexpected and highly specific on-surface formal 1,4 Michael-type addition at the meta position, driven by the reduction of the aminophenol molecule. We rationalize this dehydrogenation and coupling reaction mechanism with a combination of in situ scanning tunneling and non-contact atomic force microscopies, high-resolution synchrotron-based X-ray photoemission spectroscopy and first-principles calculations. This study demonstrates the capability of surfaces to selectively modify local molecular conditions to redirect well-established synthetic routes, such as Michael coupling, towards the rational synthesis of new covalent nanostructures.

Carbon-Carbon Coupling on Inert Surfaces by Deposition of En Route Generated Aryl Radicals

To facilitate C-C coupling in on-surface synthesis on inert surfaces, we devised a radical deposition source (RDS) for the direct deposition of aryl radicals onto arbitrary substrates. Its core piece is a heated reactive drift tube through which halogenated precursors are deposited and en route converted into radicals. For the proof of concept we study 4,4''-diiodo-p-terphenyl (DITP) precursors on iodine-passivated metal surfaces. Deposition with the RDS at room temperature results in highly regular structures comprised of mostly monomeric (terphenyl) or dimeric (sexiphenyl) biradicals. Mild heating activates progressive C-C coupling into more extended molecular wires. These structures are distinctly different from the self-assemblies observed upon conventional deposition of intact DITP. Direct deposition of radicals renders substrate reactivity unnecessary, thereby paving the road for synthesis on application-relevant inert surfaces.

On-Surface Synthesis with Atomic Hydrogen

Surface-assisted synthesis has become a powerful approach for generation of molecular nanostructures, which could not be obtained via traditional solution chemistry. Nowadays there is an intensive search for reactions that could proceed on flat surfaces in order to boost the versatility and applicability of synthesized nano-objects. Here we propose application of atomic hydrogen combined with on-surface synthesis in order to tune the reaction pathways. We demonstrate that atomic hydrogen could be widely applied: (1) as a cleaning tool, which allows removal of halogen residues from the surface after Ullmann couplings/polymerization, (2) by reaction with surface organometallics to provide stable hydrogenated species, and (3) as a reagent for debromination or desulfurization of adsorbed species.

Oxygen-Induced 1D to 2D Transformation of On-Surface Organometallic Structures

While surface-confined Ullmann-type coupling has been widely investigated for its potential to produce π-conjugated polymers with unique properties, the pathway of this reaction in the presence of adsorbed oxygen has yet to be explored. Here, the effect of oxygen adsorption between different steps of the polymerization reaction is studied, revealing an unexpected transformation of the 1D organometallic (OM) chains to 2D OM networks by annealing, rather than the 1D polymer obtained on pristine surfaces. Characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy indicates that the networks consist of OM segments stabilized by chemisorbed oxygen at the vertices of the segments, as supported by density functional theory calculations. Hexagonal 2D OM networks with different sizes on Cu(111) can be created using precursors with different length, either 4,4″-dibromo-p-terphenyl or 1,4-dibromobenzene (dBB), and square networks are obtained from dBB on Cu(100). The control over size and symmetry illustrates a versatile surface patterning technique, with potential applications in confined reactions and host-guest chemistry.

On-Surface Synthesis of All-cis Standing Phenanthrene Polymers upon Selective C-H Bond Activation

On-surface synthesis has emerged as a powerful approach to the atomically precise fabrication of molecular architectures with potential applications in nanotechnology. However, it is challenging to synthesize molecular structures that can protrude from the surface such as polymer chains forming by the molecules with upright conformations, since most of the on-surface reaction products, particularly the conjugated structures, prefer to adsorb parallel on the surface to maximize the molecule-substrate interaction. Here, we show an up-standing phenanthrene polymer chain with an all-cis configuration obtained by on-surface synthesis upon highly selective C-H activation. Using bond-resolved nc-AFM imaging, the reaction route of polymers from an in-plane to an all-cis upright conformation is fully characterized, and the reaction mechanism is further revealed in combination with first principles calculations. Our results on this selective dehydrogenation induced upright-oriented polymer chains that will enrich the toolbox for the on-surface synthesis of novel molecular structures and may provide new insights on designing optimized precursors for preparing three-dimensional molecular frameworks through on-surface synthesis.

Substrate induced strain for on-surface transformation and synthesis

Intermolecular strain has long been used to steer and promote chemical reactions towards desired products in wet chemical synthesis. However, similar protocols have not been adopted for the on-surface synthesis on solid substrates due to the complexity of reaction processes. Recent advances in the sub-molecular resolution with scanning probe microscopy allow us to capture on-surface reaction pathways and to gain substantial insights into the role of strain in chemical reactions. The primary focus of this review is to highlight the recent findings on strain-induced on-surface reactions. Such substrate-induced processes can be applied to alter the chemical reactivity and to drive on-surface chemical reactions in different manners, which provides a promising alternative approach for on-surface synthesis. This review aims to shed light on the utilization of substrate-induced strain for on-surface transformation and synthesis of atomically-precise novel functional nanomaterials.

On-surface synthesis of extended linear graphyne molecular wires by protecting the alkynyl group

In this paper we report on the use of an Ullmann-like aryl halide homocoupling reaction to obtain long Graphyne Molecular Wires (GY MWs) organized in dense, ordered arrays.

Metallic bands in chevron-type polyacenes

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.

We present electronic structure calculations based on a single-parameter plane wave expansion method for molecular nanostructures revealing excellent agreement with density functional theory and predicting metallic bands for chevron molecular dimers.

Effective Mass of Quasiparticles in Armchair Graphene Nanoribbons

Armchair graphene nanoribbons (AGNRs) may present intrinsic semiconducting bandgaps, being of potential interest in developing new organic-based optoelectronic devices. The induction of a bandgap in AGNRs results from quantum confinement effects, which reduce charge mobility. In this sense, quasiparticles' effective mass becomes relevant for the understanding of charge transport in these systems. In the present work, we theoretically investigate the drift of different quasiparticle species in AGNRs employing a 2D generalization of the Su-Schrieffer-Heeger Hamiltonian. Remarkably, our findings reveal that the effective mass strongly depends on the nanoribbon width and its value can reach 60 times the mass of one electron for narrow lattices. Such underlying property for quasiparticles, within the framework of gap tuning engineering in AGNRs, impact the design of their electronic devices.

Photochemistry Highlights on On-Surface Synthesis

On-surface chemistry is a promising way to achieve the bottom-up construction of covalently-bonded molecular precursors into extended atomically-precise polymers adsorbed on surfaces. These polymers exhibit unprecedented physical or chemical properties which are of great interest for various potential applications. These nanostructures were mainly obtained in ultra-high vacuum (UHV) on noble metal single-crystal surfaces by thermal annealing as stimulus to provoke the polymerization with a catalytic role of the surface adatoms. Nevertheless, photons are also a powerful source of energy to induce the formation of covalent architectures, even if it is less-used on surfaces than in solution. In this minireview, we discuss the photo-induced on-surface polymerization from the basic mechanisms of photochemistry to the formation of extended polymers on different kinds of surfaces, which are characterized by scanning probe microscopies.

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 core level photoemission spectroscopy, and a rationalized understanding of the observed preference is obtained from density functional theory calculations.

Cleaning up after the Party: Removing the Byproducts of On-Surface Ullmann Coupling

Ullmann coupling is one of the most frequently employed methodologies for producing π-conjugated surface-confined polymers. One unfortunate side product of the reaction is the creation of metal halide islands formed from liberated halogen atoms. Following the coupling reaction, these halide islands can account for a large proportion of the substrate surface area and thus inhibit domain growth and effectively poison the catalyst. Here, we describe an efficient and reliable methodology for removing the halogen byproduct at room temperature by exposure to a beam of atomic hydrogen; this action removes the halogen atoms in a matter of minutes, with minimal impact to the polymer structure. We also find that it is possible under certain circumstances to preserve the pre-exposure epitaxy after removal of the halogen. This finding provides a convenient and straightforward technique for addressing the most often-cited drawback of the on-surface Ullman coupling methodology and provides access to a previously inaccessible parameter space for these types of experiments.

Surface-mediated assembly, polymerization and degradation of thiophene-based monomers

Ullmann coupling of halogenated aromatics is widely used in on-surface synthesis of two-dimensional (2D) polymers and graphene nanoribbons. It stands out among other reactions for regioselectively connecting aromatic monomers into 1D and 2D π-conjugated polymers, whose final structure and properties are determined by the initial building blocks. Thanks to their exceptional electronic properties, thiophene-containing monomers are frequently used for the synthesis of various conjugated materials. On the other hand, their use in on-surface polymerization is hampered by the possibility of ring opening when adsorbed on metal surfaces. In the present work, we mapped the temperature regime for these two competing reactions by investigating the adsorption of a thiophene-based prochiral molecule using scanning tunneling microscopy, X-ray photoelectron spectroscopy and density functional theory calculations. We followed the formation of organometallic (OM) networks, their evolution into covalent structures and the competition between C-C coupling and thiophene ring opening. The effect of surface reactivity was explored by comparing the adsorption on three (111) coinage metal substrates, namely Au, Ag and Cu. While outlining strategies to minimize the ring opening reaction, we found that the surface temperature during deposition is of paramount importance for the preparation of 2D OM networks, greatly enhancing the overall ordering of the product by depositing on hot Ag surface. Notably, the same protocol permits the creation of OM structures on the air-stable Au surface, thereby allowing the synthesis and application of 2D OM networks outside the ultra-high vacuum environment.

An unexpected organometallic intermediate in surface-confined Ullmann coupling

Ullmann coupling or, more generally, dehalogenative aryl-aryl coupling, is one of the most widely exploited chemical reactions to obtain one- and two-dimensional polymers on metal surfaces. It is generally described as a two-step reaction: (i) dehalogenation, resulting in the formation of a stable intermediate organometallic phase and subsequent (ii) C-C coupling. The topology of the resulting polymer depends on the number and positions of the halogen atoms in the haloaromatic precursor, although its orientation and order are determined by the structure of the intermediate phase. Hitherto, only one intermediate structure, identified as an organometallic (OM) phase, has been reported for such a reaction. Here we demonstrate the formation of two distinct OM phases during the temperature-induced growth of poly(para-phenylene) from 1,4-dibromobenzene precursors on Cu(110). Beyond the already known linear-OM chains, we show that a phase reorganization to a chessboard-like 2D-OM can be activated in a well-defined temperature range. This new intermediate phase, revealed only when the reaction is carried out at low molecular coverages, was characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy and near-edge X-ray absorption fine structure spectroscopy, and modeled by density functional theory calculations. Our data show that the 2D-OM remains stable after cooling down the sample and is stabilized by four-Cu clusters at each node. The observation of such unexpected intermediate phase shows the complexity of the mechanisms underlying on-surface synthesis and broadens the understanding of Ullmann coupling, which continues to be astonishing despite its extensive use.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Bloch Oscillations in Fibonacci lattices: polaron formation

We investigated the dynamics of an electron subjected to a uniform electric field in the scope of a tight-binding electron-phonon interacting approach. We aimed at describing the transport in a one-dimensional lattice in which the on-site energies are distributed according to a Fibonacci sequence. Within this physical picture, we obtained a novel dynamical process with no counterpart in ordered lattices. Our findings showed that in low-disorder limit, the electron performs spatial Bloch oscillations, generating, in the turning points of its trajectory, composite quasi-particles-namely, polarons. When it comes to highly disordered systems, two strongly localized polarons are formed in the region where the oscillating charge is confined, thus propagating excitations that are present in the lattice.

The role of halogens in on-surface Ullmann polymerization

Ullmann coupling is the most common approach to form surface-confined one- and two-dimensional conjugated structures from haloaryl derivatives. The dimensions of the formed nanostructures can be controlled by the number and location of halogens within the molecular precursors. Our study illustrates that the type of halogen plays an essential role in the design, orientation, and extent of the surface-confined organometallic and polymeric nanostructures. We performed a comparative analysis of five 1,4-dihalobenzene molecules containing chlorine, bromine, and iodine on Cu(110) using scanning tunneling microscopy, fast-X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies. Our experimental data identify different molecular structures, reaction temperatures and kinetics depending on the halogen type. Climbing image nudged elastic band simulations further clarify these observations by providing distinct diffusion paths for each halogen species. We show that in addition to the structure of the building blocks, the halogen type has a direct influence on the morphology of surface-confined polymeric structures based on Ullmann coupling.

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 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.

Orbital-Engineering-Based Screening of π-Conjugated d8 Transition-Metal Coordination Polymers for High-Performance n-Type Thermoelectric Applications

Extraordinary progress has been achieved in polymer-based thermoelectric materials in recent years. New emerging π-conjugated transition-metal coordination polymers are one of the best n-type polymer-based thermoelectric materials. However, the microscopic descriptions on geometric structures, orbital characteristics, and most importantly, thermoelectric properties remain elusive, which has seriously hampered the experimentalists to draw a straightforward design strategy for new n-type polymer-based thermoelectric materials. Herein, we assess the n-type thermoelectric properties of 20 π-conjugated d⁸ metal center coordination polymers and rationalize their thermoelectric properties in terms of molecular geometry, orbital nature, and electron-phonon coupling based on first-principles calculations. An explicit screening rule for high-performance n-type π-conjugated transition-metal coordination polymeric thermoelectric materials was found, i.e., smaller metal center d orbital component ratio in the conduction band minimum, weaker electron-phonon coupling, higher intrinsic mobility, and thereby higher thermoelectric power factor can be achieved. Guided by this rule, poly(Pd-C₂S₄) and poly(Ni-C₂Se₄) show very high power factors. We built a map of high-performance π-conjugated transition-metal coordination polymers for n-type thermoelectric applications, which will help to accelerate the screening and design of innovative n-type thermoelectric polymers.

Supramolecular Assemblies on Surfaces: Nanopatterning, Functionality, and Reactivity

Understanding how molecules interact to form large-scale hierarchical structures on surfaces holds promise for building designer nanoscale constructs with defined chemical and physical properties. Here, we describe early advances in this field and highlight upcoming opportunities and challenges. Both direct intermolecular interactions and those that are mediated by coordinated metal centers or substrates are discussed. These interactions can be additive, but they can also interfere with each other, leading to new assemblies in which electrical potentials vary at distances much larger than those of typical chemical interactions. Earlier spectroscopic and surface measurements have provided partial information on such interfacial effects. In the interim, scanning probe microscopies have assumed defining roles in the field of molecular organization on surfaces, delivering deeper understanding of interactions, structures, and local potentials. Self-assembly is a key strategy to form extended structures on surfaces, advancing nanolithography into the chemical dimension and providing simultaneous control at multiple scales. In parallel, the emergence of graphene and the resulting impetus to explore 2D materials have broadened the field, as surface-confined reactions of molecular building blocks provide access to such materials as 2D polymers and graphene nanoribbons. In this Review, we describe recent advances and point out promising directions that will lead to even greater and more robust capabilities to exploit designer surfaces.

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 a 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.