Mesoscopic 2D molecular self-assembly on an insulator.

Two-dimensional (2D) nanostructures and nanomaterials offer potential for a wide range of technological applications in electronics, optoelectronics, data storage, sensing and catalysis. On-surface molecular self-assembly-where organic molecules act as building blocks and where surfaces play the role of supporting templates-allows for the bottom-up synthesis of such 2D systems with tuneable atomically precise morphologies and tailored electronic properties. These self-assembly protocols are well established on metal surfaces, but remain limited on electronically gapped substrates (insulators, semiconductors). The latter are useful for preventing electronic coupling (that is, hybridization between molecular assembly and underlying surface) and for avoiding quenching of optical processes, necessary for prospective electronic and optoelectronic applications. In particular, molecular self-assembly on surfaces other than weakly interacting metals can be challenging due to substrate reactivity, defects and inhomogeneities, resulting in intricate energy landscapes that limit the growth kinetically and hampers the synthesis of large-area defect-free 2D systems. Here, we demonstrate the self-assembly of a 2D, atomically thin organic molecular film on a model wide bandgap 2D insulator, single-layer hexagonal boron nitride (hBN) on Cu(111). The molecular film consists of flat, aromatic 9,10-di-cyano-anthracene (DCA) molecules. Our low-temperature scanning tunnelling microscopy and spectroscopy measurements revealed mesoscopic (>100 × 100 nm2), topographically homogeneous crystalline molecular domains resulting from flat molecular adsorption and noncovalent in-plane cyano-ring bonding, with electronically decoupled molecular orbitals (MOs) lying within the hBN electronic gap. These MOs exhibit an energy level spatial modulation (∼300 meV) that follows the moiré work function variation of hBN on Cu(111). These findings pave the way for large-area, atomically precise, highly crystalline 2D organic (and metal-organic) nanomaterials on electronically functional wide bandgap insulators.

Selective Activation of Aromatic C-H Bonds Catalyzed by Single Gold Atoms at Room Temperature.

Selective activation and controlled functionalization of C-H bonds in organic molecules is one of the most desirable processes in synthetic chemistry. Despite progress in heterogeneous catalysis using metal surfaces, this goal remains challenging due to the stability of C-H bonds and their ubiquity in precursor molecules, hampering regioselectivity. Here, we examine the interaction between 9,10-dicyanoanthracene (DCA) molecules and Au adatoms on a Ag(111) surface at room temperature (RT). Characterization via low-temperature scanning tunneling microscopy, spectroscopy, and noncontact atomic force microscopy, supported by theoretical calculations, revealed the formation of organometallic DCA-Au-DCA dimers, where C atoms at the ends of the anthracene moieties are bonded covalently to single Au atoms. The formation of this organometallic compound is initiated by a regioselective cleaving of C-H bonds at RT. Hybrid quantum mechanics/molecular mechanics calculations show that this regioselective C-H bond cleaving is enabled by an intermediate metal-organic complex which significantly reduces the dissociation barrier of a specific C-H bond. Harnessing the catalytic activity of single metal atoms, this regioselective on-surface C-H activation reaction at RT offers promising routes for future synthesis of functional organic and organometallic materials.

Direct observation of narrow electronic energy band formation in 2D molecular self-assembly.

Surface-supported molecular overlayers have demonstrated versatility as platforms for fundamental research and a broad range of applications, from atomic-scale quantum phenomena to potential for electronic, optoelectronic and catalytic technologies. Here, we report a structural and electronic characterisation of self-assembled magnesium phthalocyanine (MgPc) mono and bilayers on the Ag(100) surface, via low-temperature scanning tunneling microscopy and spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), density functional theory (DFT) and tight-binding (TB) modeling. These crystalline close-packed molecular overlayers consist of a square lattice with a basis composed of a single, flat-adsorbed MgPc molecule. Remarkably, ARPES measurements at room temperature on the monolayer reveal a momentum-resolved, two-dimensional (2D) electronic energy band, 1.27 eV below the Fermi level, with a width of ∼20 meV. This 2D band results from in-plane hybridization of highest occupied molecular orbitals of adjacent, weakly interacting MgPc's, consistent with our TB model and with DFT-derived nearest-neighbor hopping energies. This work opens the door to quantitative characterisation - as well as control and harnessing - of subtle electronic interactions between molecules in functional organic nanofilms.

Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray.

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal-molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule-surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field.

Iron-based trinuclear metal-organic nanostructures on a surface with local charge accumulation.

Coordination chemistry relies on harnessing active metal sites within organic matrices. Polynuclear complexes-where organic ligands bind to several metal atoms-are relevant due to their electronic/magnetic properties and potential for functional reactivity pathways. However, their synthesis remains challenging; few geometries and configurations have been achieved. Here, we synthesise-via supramolecular chemistry on a noble metal surface-one-dimensional metal-organic nanostructures composed of terpyridine (tpy)-based molecules coordinated with well-defined polynuclear iron clusters. Combining low-temperature scanning probe microscopy and density functional theory, we demonstrate that the coordination motif consists of coplanar tpy's linked via a quasi-linear tri-iron node in a mixed (positive-)valence metal-metal bond configuration. This unusual linkage is stabilised by local accumulation of electrons between cations, ligand and surface. The latter, enabled by bottom-up on-surface synthesis, yields an electronic structure that hints at a chemically active polynuclear metal centre, paving the way for nanomaterials with novel catalytic/magnetic functionalities.