8 research outputs found

    Molecular Ordering and Dipole Alignment of Vanadyl Phthalocyanine Monolayer on Metals: The Effects of Interfacial Interactions

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    We present an <i>in situ</i> low-temperature scanning tunneling microscopy (LT-STM) study to elucidate the effects of interfacial interactions on the molecular ordering and dipole alignment of dipolar vanadyl phthalocyanine (VOPc) monolayer on metal surfaces, including Cu(111), Ag(111), Au(111), and graphite. The adsorption of VOPc on the relatively inert graphite surface leads to the formation of well-ordered molecular dipole monolayer with unidirectionally aligned O-up configuration. In contrast, VOPc on Cu(111), Ag(111), and Au(111) adopts both O-up and O-down configurations. The VOPc strongly chemisorbs on Cu(111), leading to the formation of one-dimensional molecular chains, and two-dimensional molecular islands comprising pure O-down adsorbed VOPc molecules at low and high coverage, respectively. In contrast, VOPc physisorbs on Au(111) and results in an orientation transition from flat-lying to inclined molecular islands. Regarding the interfacial interaction strength, the Ag(111) represents an intermediate case (weak chemisorption), which enables the formation of disordered phase and ordered islands, as well as the orientation transition within the disordered phase

    Growth Intermediates for CVD Graphene on Cu(111): Carbon Clusters and Defective Graphene

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    Graphene growth on metal films via chemical vapor deposition (CVD) represents one of the most promising methods for graphene production. The realization of the wafer scale production of single crystalline graphene films requires an atomic scale understanding of the growth mechanism and the growth intermediates of CVD graphene on metal films. Here, we use <i>in situ</i> low-temperature scanning tunneling microscopy (LT-STM) to reveal the graphene growth intermediates at different stages via thermal decomposition of methane on Cu(111). We clearly demonstrate that various carbon clusters, including carbon dimers, carbon rectangles, and ā€˜zigzagā€™ and ā€˜armchairā€™-like carbon chains, are the actual growth intermediates prior to the graphene formation. Upon the saturation of these carbon clusters, they can transform into defective graphene possessing pseudoperiodic corrugations and vacancies. These vacancy-defects can only be effectively healed in the presence of methane via high temperature annealing at 800 Ā°C and result in the formation of vacancy-free monolayer graphene on Cu(111)

    Dipole Orientation Dependent Symmetry Reduction of Chloroaluminum Phthalocyanine on Cu(111)

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    We demonstrate a dipole orientation dependent symmetry reduction of 4-fold symmetric chloroaluminum phthalocyanine (ClAlPc) molecules on a Cu(111) surface by combined low temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) calculations. Unexpected symmetry reduction from 4-fold (C4) to 2-fold (C2) was observed for Cl-down (dipole up) adsorbed ClAlPc, while molecules adopted Cl-up (dipole down) configuration reserved the C4 symmetry. DFT calculations indicated strong charge accumulation at the interface region between Cu surface and the Cl atom in Cl-down adsorbed ClAlPc due to the electron transfer from the bonded Cu atoms. This can result in charge redistribution within the phthalocyanine (Pc) macrocycle, and the formation of anionic Pc with an uptake of 1.3 e, which can be subjected to Jahnā€“Teller distortion. The inequivalent charge distribution onto the four lobes would be further enlarged due to the conformational distortion. The two down-bended lobes with more electrons interact stronger with the substrate and are much closer to the surface, leading to the C2 symmetry with one pair of up-bended lobes brighter and longer than their perpendicular counterparts for Cl-down adsorbed ClAlPc

    Halogen-Adatom Mediated Phase Transition of Two-Dimensional Molecular Self-Assembly on a Metal Surface

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    Construction of tunable and robust two-dimensional (2D) molecular arrays with desirable lattices and functionalities over a macroscopic scale relies on spontaneous and reversible noncovalent interactions between suitable molecules as building blocks. Halogen bonding, with active tunability of direction, strength, and length, is ideal for tailoring supramolecular structures. Herein, by combining low-temperature scanning tunneling microscopy and systematic first-principles calculations, we demonstrate novel halogen bonding involving single halogen atoms and phase engineering in 2D molecular self-assembly. On the Au(111) surface, we observed catalyzed dehalogenation of hexabromobenzene (HBB) molecules, during which negatively charged bromine adatoms (Br<sup>Ī“āˆ’</sup>) were generated and participated in assembly via unique Cā€“Br<sup>Ī“+</sup>Ā·Ā·Ā·Br<sup>Ī“āˆ’</sup> interaction, drastically different from HBB assembly on a chemically inert graphene substrate. We successfully mapped out different phases of the assembled superstructure, including densely packed hexagonal, tetragonal, dimer chain, and expanded hexagonal lattices at room temperature, 60 Ā°C, 90 Ā°C, and 110 Ā°C, respectively, and the critical role of Br<sup>Ī“āˆ’</sup> in regulating lattice characteristics was highlighted. Our results show promise for manipulating the interplay between noncovalent interactions and catalytic reactions for future development of molecular nanoelectronics and 2D crystal engineering

    Anisotropic Strain-Mediated Growth of Monatomic Co Chains on Unreconstructed Regions of the Au(111) Surface

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    Single-metal-atom chains, the ultimate one-dimensional (1D) structure, have intriguing physical and chemical properties. However, their controllable and massive production remains challenging as it requires the positioning of individual atoms with atomic precision. Here, by using a two-step molecular beam epitaxy method, we successfully fabricate single-cobalt-atom chains on a metal surface, where organic molecules are first sublimated onto heated Au(111), followed by deposition of Co atoms. Adsorption of 8OH-TPB (octahydroxyl tetraphenylbenzene) on Au(111) induces surface reconstruction transition from a herringbone to a triangular pattern. Co deposition leads to the formation of 1D āˆš3R30Ā° chains with a cross section of only one atom propagating along the [11Ģ…0] direction, which are separated from each other by a lateral spacing of 7ā€“10 times the lattice constant of Au(111). The growth mechanism lies in the surface strain anisotropy induced by the strong Coā€“Au bonding, where the distance between the two Au atoms bridged via a Co adatom is significantly enlarged, while the Auā€“Au distance along the Co chain remains almost intact. The observed chain length distribution can be interpreted in terms of electronic scattering vectors at the Fermi surface of the Au(111) surface states

    Low-Temperature, Bottom-Up Synthesis of Graphene via a Radical-Coupling Reaction

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    In this article, we demonstrated a method to synthesize graphene films at low temperature via a mild radical-coupling reaction. During the deposition process, with the effectively breaking of the Cā€“Br bonds of hexabromobenzene (HBB) precursors, the generated HBB radicals couple efficiently to form graphene films at the low temperature of 220ā€“250 Ā°C. In situ low-temperature scanning tunneling microscopy was used to provide atomic scale investigation of the graphene growth mechanism using HBB as precursor. The chemical structure evolution during the graphene growth process was further corroborated by in situ X-ray photoelectron spectroscopy measurements. The charge carrier mobility of the graphene film grown at low temperature is at 1000ā€“4200 cm<sup>2</sup> V<sup>ā€“1Ā </sup>s<sup>ā€“1</sup>, as evaluated in a field-effect transistor device configuration on SiO<sub>2</sub> substrates, indicating the high quality of the films

    Self-Assembly of Polar Phthalocyanine Molecules on Graphene Grown by Chemical Vapor Deposition

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    Integration of functional organic molecules with graphene is expected to promote the development of graphene-based flexible electronics with novel properties. Here, the self-assembled structure of dipole phthalocyanine molecules, chloro-aluminum phthalocyanine (ClAlPc), on single-layer graphene grown by chemical vapor deposition (CVD) over a Cu film was characterized by low-temperature scanning tunneling microscopy (LT-STM). The phthalocyanine molecules show highly ordered assembled structures on the CVD graphene, and these molecular layers extend continuously over the steps of the Cu film. We also observe specific boundaries in the self-assembled molecule arrays, which can be explained by the presence of domain boundaries in the graphene. The STM results suggest that CVD graphene is as a good molecular assembly template for surface functionalization and that these molecular arrays facilitate the study of domain structures in CVD graphene

    Oxygen-Promoted Methane Activation on Copper

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    The role of oxygen in the activation of Cā€“H bonds in methane on clean and oxygen-precovered Cu(111) and Cu<sub>2</sub>OĀ­(111) surfaces was studied with combined in situ near-ambient-pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy. Activation of methane at 300 K and ā€œmoderate pressuresā€ was only observed on oxygen-precovered Cu(111) surfaces. Density functional theory calculations reveal that the lowest activation energy barrier of Cā€“H on Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site, four-centered mechanism, which stabilizes the required transition-state intermediate by dipoleā€“dipole attraction of Oā€“H and Cuā€“CH<sub>3</sub> species. The Cā€“H bond activation barriers on Cu<sub>2</sub>OĀ­(111) surfaces are large due to the weak stabilization of H and CH<sub>3</sub> fragments
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