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.
The presence of carbon‐bromine bonds with similar gas‐phase binding energies allows the Ullmann coupling to occur via two different orientations. However, upon adsorption on Au(111), one particular type of carbon‐bromine bond is preferentially cleaved, thus selecting a coupling route that leads to the formation of chiral graphene nanoribbons. Nevertheless, the occasional coupling along the second orientation also leads to the lateral fusion of the nanoribbons even at mild temperatures.
The structural chemistry and reactivity of 1,3,8,10‐tetraazaperopyrene (TAPP) on Cu(111) under ultra‐high‐vacuum (UHV) conditions has been studied by a combination of experimental techniques ...(scanning tunneling microscopy (STM) and X‐ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close‐packed assembly similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu‐decorated covalent poly(TAPPCu) chains.
TAPP on copper: 1,3,8,10‐Tetraazaperopyrene (TAPP) is chemically activated on a copper (111) surface, with its mobile adatoms, to form a highly ordered surface coordination network. At elevated temperatures it reacts to generate covalently coupled polymers, as illustrated. The copper substrate is crucially important in determining the structures of the aggregates and available reaction channels on the surface.
Ferromagnetism is the collective alignment of atomic spins that retain a net magnetic moment below the Curie temperature, even in the absence of external magnetic fields. Reducing this fundamental ...property into strictly two-dimensions was proposed in metal-organic coordination networks, but thus far has eluded experimental realization. In this work, we demonstrate that extended, cooperative ferromagnetism is feasible in an atomically thin two-dimensional metal-organic coordination network, despite only ≈ 5% of the monolayer being composed of Fe atoms. The resulting ferromagnetic state exhibits an out-of-plane easy-axis square-like hysteresis loop with large coercive fields over 2 Tesla, significant magnetic anisotropy, and persists up to T
≈ 35 K. These properties are driven by exchange interactions mainly mediated by the molecular linkers. Our findings resolve a two decade search for ferromagnetism in two-dimensional metal-organic coordination networks.
A 2D array of electronically coupled quantum boxes is fabricated by means of on‐surface self‐assembly assuring ultimate precision of each box. The quantum states embedded in the boxes are configured ...by adsorbates, whose occupancy is controlled with atomic precision. The electronic interbox coupling can be maintained or significantly reduced by proper arrangement of empty and filled boxes.
The self‐assembly of three porphyrin derivatives was studied in detail on a Cu(111) substrate by means of scanning tunneling microscopy (STM). All derivatives have two 4‐cyanophenyl substituents in ...diagonally opposed meso‐positions of the porphyrin core, but differ in the nature of the other two meso‐alkoxyphenyl substituents. At coverages below 0.8 monolayers, two derivatives form molecular chains, which evolve into nanoporous networks at higher coverages. The third derivative self‐assembles directly into a nanoporous network without showing a one‐dimensional phase. The pore‐to‐pore distances for the three networks depend on the size and shape of the alkoxy substituents. All observed effects are explained by 1) different steric demands of the alkoxy residues, 2) apolar (mainly dispersion) interactions between the alkoxy chains, 3) polar bonding involving both cyanophenyl and alkoxyphenyl substituents, and 4) the entropy/enthalpy balance of the network formation.
Optimum coverage: The self‐assembly of three different tetrakis(meso‐phenyl)porphyrins with cyano and different alkyloxy substituents is studied on Cu(111) by STM (see graphic). At coverages below 0.8 monolayers (ML), two of these porphyrins show chainlike assemblies, which transform into nanoporous networks with different pore‐to‐pore distances at higher coverages, whereas the third molecular component self‐assembles directly into a nanoporous network.
Quantum dots are known to confine electrons within their structure. Whenever they periodically aggregate into arrays and cooperative interactions arise, novel quantum properties suitable for ...technological applications show up. Control over the potential barriers existing between neighboring quantum dots is therefore essential to alter their mutual crosstalk. Here we show that precise engineering of the barrier width can be experimentally achieved on surfaces by a single atom substitution in a haloaromatic compound, which in turn tunes the confinement properties through the degree of quantum dot intercoupling. We achieved this by generating self-assembled molecular nanoporous networks that confine the two-dimensional electron gas present at the surface. Indeed, these extended arrays form up on bulk surface and thin silver films alike, maintaining their overall interdot coupling. These findings pave the way to reach full control over two-dimensional electron gases by means of self-assembled molecular networks.Arrays of quantum dots can exhibit a variety of quantum properties, being sensitive to their spacing. Here, the authors fine tune interdot coupling using hexagonal molecular networks in which the dots are separated by single or double haloaromatic compounds, structurally identical but for a single atom.
Two-dimensional honeycomb molecular networks confine a substrate’s surface electrons within their pores, providing an ideal playground to investigate the quantum electron scattering phenomena. ...Besides surface state confinement, laterally protruding organic states can collectively hybridize at the smallest pores into superatom molecular orbitals. Although both types of pore states could be simultaneously hosted within nanocavities, their coexistence and possible interaction are unexplored. Here, we show that these two types of pore states do coexist within the smallest nanocavities of a two-dimensional halogen-bonding multiporous network grown on Ag(111) studied using a combination of scanning tunneling microscopy and spectroscopy, density functional theory calculations, and electron plane wave expansion simulations. We find that superatom molecular orbitals undergo an important stabilization when hybridizing with the confined surface state, following the significant lowering of its free-standing energy. These findings provide further control over the surface electronic structure exerted by two-dimensional nanoporous systems.
Abstract
Low dimensional carbon-based materials can show intrinsic magnetism associated to p-electrons in open-shell
π
-conjugated systems. Chemical design provides atomically precise control of the
...π
-electron cloud, which makes them promising for nanoscale magnetic devices. However, direct verification of their spatially resolved spin-moment remains elusive. Here, we report the spin-polarization of chiral graphene nanoribbons (one-dimensional strips of graphene with alternating zig-zag and arm-chair boundaries), obtained by means of spin-polarized scanning tunnelling microscopy. We extract the energy-dependent spin-moment distribution of spatially extended edge states with
π
-orbital character, thus beyond localized magnetic moments at radical or defective carbon sites. Guided by mean-field Hubbard calculations, we demonstrate that electron correlations are responsible for the spin-splitting of the electronic structure. Our versatile platform utilizes a ferromagnetic substrate that stabilizes the organic magnetic moments against thermal and quantum fluctuations, while being fully compatible with on-surface synthesis of the rapidly growing class of nanographenes.
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 toward the surface, signaling their stronger interactions 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 a 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 GNR band structure can be tuned by modifying the separation between di-boron moieties.
A Rashba-type spin-orbit splitting is found for quantum well states formed in ultrathin Pb films on Si (111). The resulting momentum splitting is comparable to what is found for semiconductor ...heterostructures. The splitting shows no coverage dependency and the sign of the spin polarization is reversed compared to Rashba splitting in the Au(111) surface state. We explain our results by competing effects at the two boundaries of the Pb layer.
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