On-Surface Synthesis and Determination of the Open-Shell Singlet Ground State of Tridecacene.

The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open-shell character has so far been missing. Here, we present the on-surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many-body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet-triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene.

Synergetic Bottom-Up Synthesis of Graphene Nanoribbons by Matrix-Assisted Direct Transfer.

The scope of graphene nanoribbon (GNR) structures accessible through bottom-up approaches is defined by the intrinsic limitations of either all-on-surface or all-solution-based synthesis. Here, we report a hybrid bottom-up synthesis of GNRs based on a Matrix-Assisted Direct (MAD) transfer technique that successfully leverages technical advantages inherent to both solution-based and on-surface synthesis while sidestepping their drawbacks. Critical structural parameters tightly controlled in solution-based polymerization reactions can seamlessly be translated into the structure of the corresponding GNRs. The transformative potential of the synergetic bottom-up approaches facilitated by the MAD transfer techniques is highlighted by the synthesis of chevron-type GNRs (cGNRs) featuring narrow length distributions and a nitrogen core-doped armchair GNR (N4-7-ANGR) that remains inaccessible using either a solution-based or an on-surface bottom-up approach alone.

Bottom-up Assembly of Nanoporous Graphene with Emergent Electronic States.

The incorporation of nanoscale pores into a sheet of graphene allows it to switch from an impermeable semimetal to a semiconducting nanosieve. Nanoporous graphenes are desirable for applications ranging from high-performance semiconductor device channels to atomically thin molecular sieve membranes, and their performance is highly dependent on the periodicity and reproducibility of pores at the atomic level. Achieving precise nanopore topologies in graphene using top-down lithographic approaches has proven to be challenging due to poor structural control at the atomic level. Alternatively, atomically precise nanometer-sized pores can be fabricated via lateral fusion of bottom-up synthesized graphene nanoribbons. This technique, however, typically requires an additional high temperature cross-coupling step following the nanoribbon formation that inherently yields poor lateral conjugation, resulting in 2D materials that are weakly connected both mechanically and electronically. Here, we demonstrate a novel bottom-up approach for forming fully conjugated nanoporous graphene through a single, mild annealing step following the initial polymer formation. We find emergent interface-localized electronic states within the bulk band gap of the graphene nanoribbon that hybridize to yield a dispersive two-dimensional low-energy band of states. We show that this low-energy band can be rationalized in terms of edge states of the constituent single-strand nanoribbons. The localization of these 2D states around pores makes this material particularly attractive for applications requiring electronically sensitive molecular sieves.

Site-selective reversible Diels-Alder reaction between a biphenylene-based polyarene and a semiconductor surface.

Understanding the mechanisms involved in the covalent attachment of organic molecules to surfaces is a major challenge for nanotechnology and surface science. On the basis of classical organic chemistry mechanistic considerations, key issues such as selectivity and reactivity of the organic adsorbates could be rationalized and exploited for the design of molecular-scale circuits and devices. Here we use tris(benzocyclobutadieno)triphenylene, a singular Y-shaped hydrocarbon containing antiaromatic cyclobutadienoid rings, as a molecular probe to study the reaction of polycyclic conjugated molecules with atomic scale moieties, dangling-bond (DB) dimers on a hydrogen-passivated Ge(001):H surface. By combining molecular design, synthesis, scanning tunneling microscopy and spectroscopy (STM/STS) and computational modeling, we show that the attachment involves a concerted [4+2] cycloaddition reaction that is completely site-selective and fully reversible. This selectivity, governed by the bond alternation induced by the presence of the cyclobutadienoid rings, allows for the control of the orientation of the molecules with respect to the surface DB-patterning. We also demonstrate that by judicious modification of the electronic levels of the polycyclic benzenoid through substituents, the reaction barrier height can be modified. Finally, we show that after deliberate tip-induced covalent bond cleavage, adsorbed molecules can be used to fine tune the electronic states of the DB dimer. This power to engineer deliberately the bonding configuration and electronic properties opens new perspectives for creating prototypical nanoscale circuitry.

Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene.

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on-surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on-surface annealing. The structure of the generated acenes has been visualized by high-resolution non-contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.

The butterfly - a well-defined constant-current topography pattern on Si(001):H and Ge(001):H resulting from current-induced defect fluctuations.

Dangling bond (DB) arrays on Si(001):H and Ge(001):H surfaces can be patterned with atomic precision and they exhibit complex and rich physics making them interesting from both technological and fundamental perspectives. But their complex behavior often makes scanning tunneling microscopy (STM) images difficult to interpret and simulate. Recently it was shown that low-temperature imaging of unoccupied states of an unpassivated dimer on Ge(001):H results in a symmetric butterfly-like STM pattern, despite the fact that the equilibrium dimer configuration is expected to be a bistable, buckled geometry. Here, based on a thorough characterization of the low-bias switching events on Ge(001):H, we propose a new imaging model featuring a dynamical two-state rate equation. On both Si(001):H and Ge(001):H, this model allows us to reproduce the features of the observed symmetric empty-state images which strongly corroborates the idea that the patterns arise due to fast switching events and provides an insight into the relationship between the tunneling current and switching rates. We envision that our new imaging model can be applied to simulate other bistable systems where fluctuations arise from transiently charged electronic states.

Interaction of a conjugated polyaromatic molecule with a single dangling bond quantum dot on a hydrogenated semiconductor.

Controlling the strength of the coupling between organic molecules and single atoms provides a powerful tool for tuning electronic properties of single-molecule devices. Here, using scanning tunneling microscopy and spectroscopy (STM/STS) supported by theoretical modeling, we study the interaction of a planar organic molecule (trinaphthylene) with a hydrogen-passivated Ge(001):H substrate and a single dangling bond quantum dot on that surface. The electronic structure of the molecule adsorbed on the hydrogen-passivated surface is similar to the gas phase structure and the measurements show that HOMO and LUMO states contribute to the STM filled and empty state images, respectively. Furthermore, we show that the electronic properties are not significantly affected when the molecule is attached to the single dangling bond, which is in contrast with the strong interaction of the molecule with a dangling bond dimer. Our results show that the dangling bond quantum dots could stabilize organic molecules on a hydrogenated semiconductor without affecting their originally designed gas phase electronic properties. Together with the ability to laterally manipulate the molecules on the surface, this will be advantageous in the construction of single-molecule devices, where the coupling and positioning of the molecules on the substrate could be tuned by a proper design of the surface quantum dot arrays, comprising both single and dimerized dangling bonds.

Diels-Alder attachment of a planar organic molecule to a dangling bond dimer on a hydrogenated semiconductor surface.

Construction of single-molecule electronic devices requires the controlled manipulation of organic molecules and their properties. This could be achieved by tuning the interaction between the molecule and individual atoms by local "on-surface" chemistry, i.e., the controlled formation of chemical bonds between the species. We demonstrate here the reversible attachment of a planar conjugated polyaromatic molecule to a pair of unpassivated dangling bonds on a hydrogenated Ge(001):H surface via a Diels-Alder [4+2] addition using the tip of a scanning tunneling microscope (STM). Due to the small stability difference between the covalently bonded and a nearly undistorted structure attached to the dangling bond dimer by long-range dispersive forces, we show that at cryogenic temperatures the molecule can be switched between both configurations. The reversibility of this covalent bond forming reaction may be applied in the construction of complex circuits containing organic molecules with tunable properties.

On surface synthesis of an eleven-ring sulfur-doped nonacene.

Dithienoacenes with a heptacene core, heptaceno[2,3-b:11,12-b']bis[1]benzothiophene, have been synthesized through the combination of solution and surface assisted chemistry. The atomic composition, structural arrangement and electronic properties of the molecules on the Au(111) surface have been deeply explored by non-contact atomic force microscopy (nc-AFM), bond-resolved scanning tunnelling microscopy (BR-STM) and scanning tunneling spectroscopy (STS) corroborated by density functional theory (DFT) calculations. Our combined experiments reveal modifications induced by sulfur substitution.

On-Surface Synthesis of Nanographenes and Graphene Nanoribbons on Titanium Dioxide.

The formation of two types of nanographenes from custom designed and synthesized molecular precursors has been achieved through thermally induced intramolecular cyclodehydrogenation reactions on the semiconducting TiO2(110)-(1×1) surface, confirmed by the combination of high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) measurements, and corroborated by theoretical modeling. The application of this protocol on differently shaped molecular precursors demonstrates the ability to induce a highly efficient planarization reaction both within strained pentahelicenes as well as between vicinal phenyl rings. Additionally, by the combination of successive Ullmann-type polymerization and cyclodehydrogenation reactions, the archetypic 7-armchair graphene nanoribbons (7-AGNRs) have also been fabricated on the titanium dioxide surface from the standard 10,10'-dibromo-9,9'-bianthryl (DBBA) molecular precursors. These examples of the effective cyclodehydrogenative planarization processes provide perspectives for the rational design and synthesis of molecular nanostructures on semiconductors.

Sequential On-Surface Cyclodehydrogenation in a Nonplanar Nanographene.

On-surface synthesis has emerged as an attractive method for the atomically precise synthesis of new molecular nanostructures, being complementary to the widespread approach based on solution chemistry. It has been particularly successful in the synthesis of graphene nanoribbons and nanographenes. In both cases, the target compound is often generated through cyclodehydrogenation reactions, leading to planarization and the formation of hexagonal rings. To improve the flexibility and tunability of molecular units, however, the incorporation of other, nonbenzenoid, subunits is highly desirable. In this letter, we thoroughly analyze sequential cyclodehydrogenation reactions with a custom-designed molecular precursor. We demonstrate the step-by-step formation of hexagonal and pentagonal rings from the nonplanar precursor within fjord and cove regions, respectively. Computer models comprehensively support the experimental observations, revealing that both reactions imply an initial hydrogen abstraction and a final [1,2] hydrogen shift, but the formation of a pentagonal ring proceeds through a radical mechanism.

On-surface synthesis of a phenylene analogue of nonacene.

Cyclobuta[1,2-b:3,4-b']ditetracene - an analogue of nonacene with a cyclobutadiene unit embedded in the central part has been synthesized by the combination of solution and on-surface chemistry. The atomic structure and electronic properties of the product on Au(111) have been determined by high resolution scanning tunnelling microscopy/spectroscopy corroborated by density functional theory calculations. Structural and magnetic parameters derived from theoretical calculations reveal that π conjugation is dominated by radialene-type contribution, with an admixture of cyclobutadiene-like antiaromaticity.

Extended iron phthalocyanine islands self-assembled on a Ge(001):H surface.

Self-assembly of iron(II) phthalocyanine (FePc) molecules on a Ge(001):H surface results in monolayer islands extending over hundreds of nanometers and comprising upright-oriented entities. Scanning tunneling spectroscopy reveals a transport gap of 2.70 eV in agreement with other reports regarding isolated FePc molecules. Detailed analysis of single FePc molecules trapped at surface defects indicates that the molecules stay intact upon adsorption and can be manipulated away from surface defects onto a perfectly hydrogenated surface. This allows for their isolation from the germanium surface.

On-Surface Synthesis and Intermolecular Cycloadditions of Indacenoditetracenes, Antiaromatic Analogues of Undecacene.

The formation of s-indaceno[1,2-b:5,6-b']ditetracene and as-indaceno[2,3-b:6,7-b']ditetracene containing indenofluorene cores from a common precursor has been achieved by a dehydrogenative surface-assisted cyclization on Au(111) and confirmed by bond-resolved non-contact atomic force microscopy. On-surface generated as-indaceno[2,3-b:6,7-b']ditetracenes undergo fusion, which leads to T-shaped adducts by an intermolecular cycloaddition. The same type of cycloaddition, which has no parallel in solution chemistry, has been observed between as-indaceno[2,3-b:6,7-b']ditetracene and pentacene or octacene. These examples of surface-assisted cycloaddition provide perspectives for the rational design and synthesis of molecular nanostructures.

Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry.

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.

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.

On-Surface Synthesis of Chlorinated Narrow Graphene Nanoribbon Organometallic Hybrids.

Graphene nanoribbons (GNRs) and their derivatives attract growing attention due to their excellent electronic and magnetic properties as well as the fine-tuning of such properties that can be obtained by heteroatom substitution and/or edge morphology modification. Here, we introduce graphene nanoribbon derivatives-organometallic hybrids with gold atoms incorporated between the carbon skeleton and side Cl atoms. We show that narrow chlorinated 5-AGNROHs (armchair graphene nanoribbon organometallic hybrids) can be fabricated by on-surface polymerization with omission of the cyclodehydrogenation reaction by a proper choice of tailored molecular precursors. Finally, we describe a route to exchange chlorine atoms connected through gold atoms to the carbon skeleton by hydrogen atom treatment. This is achieved directly on the surface, resulting in perfect unsubstituted hydrogen-terminated GNRs. This will be beneficial in the molecule on-surface processing when the preparation of final unsubstituted hydrocarbon structure is desired.

Synthesis and reactivity of a trigonal porous nanographene on a gold surface.

The synthesis of porous nanographenes is a challenging task for solution chemistry, and thus, on-surface synthesis provides an alternative approach. Here, we report the synthesis of a triporous nanographene with 102 sp2 carbon atoms by combining solution and surface chemistry. The carbon skeleton was obtained by Pd-catalyzed cyclotrimerization of arynes in solution, while planarization of the molecule was achieved through two hierarchically organized on-surface cyclodehydrogenation reactions, intra- and inter-blade. Remarkably, the three non-planar [14]annulene pores of this nanographene further evolved at higher temperatures showing interesting intra-porous on-surface reactivity.

Electronic transport in planar atomic-scale structures measured by two-probe scanning tunneling spectroscopy.

Miniaturization of electronic circuits into the single-atom level requires novel approaches to characterize transport properties. Due to its unrivaled precision, scanning probe microscopy is regarded as the method of choice for local characterization of atoms and single molecules supported on surfaces. Here we investigate electronic transport along the anisotropic germanium (001) surface with the use of two-probe scanning tunneling spectroscopy and first-principles transport calculations. We introduce a method for the determination of the transconductance in our two-probe experimental setup and demonstrate how it captures energy-resolved information about electronic transport through the unoccupied surface states. The sequential opening of two transport channels within the quasi-one-dimensional Ge dimer rows in the surface gives rise to two distinct resonances in the transconductance spectroscopic signal, consistent with phase-coherence lengths of up to 50 nm and anisotropic electron propagation. Our work paves the way for the electronic transport characterization of quantum circuits engineered on surfaces.

Building a 22-ring nanographene by combining in-solution and on-surface syntheses.

A nanographene formed by the fusion of 22 benzene rings has been prepared by combining an in-solution Pd-catalyzed cycloaddition reaction and on-surface Au-promoted cyclodehydrogenation. The structure and electronic properties of the resulting three-fold symmetric C66H24 molecule have been characterized by scanning probe microscopy with atomic resolution and corroborated by theoretical modelling.