Molecules–Oligomers–Nanowires–Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order

We report on a stepwise on-surface polymerization reaction leading to oriented graphene nanoribbons on Au(111) as the final product. Starting from the precursor 4,4″-dibromo-<i>p</i>-terphenyl and using the Ullmann coupling reaction followed by dehydrogenation and C–C coupling, we have developed a fine-tuned, annealing-triggered on-surface polymerization that allows us to obtain an oriented nanomesh of graphene nanoribbons via two well-defined intermediate products, namely, <i>p</i>-phenylene oligomers with reduced length dispersion and ordered submicrometric molecular wires of poly­(<i>p</i>-phenylene). A fine balance involving gold catalytic activity in the Ullmann coupling, appropriate on-surface molecular mobility, and favorable topochemical conditions provided by the used precursor leads to a high degree of long-range order that characterizes each step of the synthesis and is rarely observed for surface organic frameworks obtained via Ullmann coupling

Microscopic View on a Chemical Vapor Deposition Route to Boron-Doped Graphene Nanostructures

Single layer boron-doped graphene layers have been grown on polycrystalline copper foils by chemical vapor deposition using methane and diborane as carbon and boron sources, respectively. Any attempt to deposit doped layers in one-step has been fruitless, the reason being the formation of very reactive boron species as a consequence of diborane decomposition on the Cu surface, which leads to disordered nonstoichiometric carbides. However, a two-step procedure has been optimized: as a first step, the surface is seeded with pure graphene islands, while the boron source is activated only in a second stage. In this case, the nonstochiometric boron carbides formed on the bare copper areas between preseeded graphene patches can be exploited to easily release boron, which diffuses from the peripheral areas inward of graphene islands. The effective substitutional doping (of the order of about 1%) has been demonstrated by Raman and photoemission experiments. The electronic properties of doped layers have been characterized by spatially resolved photoemission band mapping carried out on single domain graphene flakes using a photon beam with a spot size of 1 μm. The whole set of experiments allow us to clarify that boron is effective at promoting the anchoring carbon species on the surface. Taking the cue from this basic understanding, it is possible to envisage new strategies for the design of complex 2D graphene nanostructures with a spatially modulated doping

Fast One-Pot Synthesis of MoS₂/Crumpled Graphene p–n Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production

Aerosol processing enables the preparation of hierarchical graphene nanocomposites with special crumpled morphology in high yield and in a short time. Using modular insertion of suitable precursors in the starting solution, it is possible to synthesize different types of graphene-based materials ranging from heteroatom-doped graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped crumpled graphene nanosacks that wrap finely dispersed MoS₂ nanoparticles. These materials are carefully investigated by microscopic (SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction (GIXRD)) and spectroscopic (high resolution photoemission, Raman and UV−visible spectroscopy) techniques, evidencing that nitrogen dopants provide anchoring sites for MoS₂ nanoparticles, whereas crumpling of graphene sheets drastically limits aggregation. The activity of these materials is tested toward the photoelectrochemical production of hydrogen, obtaining that N-doped graphene/MoS₂ nanohybrids are seven times more efficient with respect to single MoS₂ because of the formation of local p–n MoS₂/N-doped graphene nanojunctions, which allow an efficient charge carrier separation

Unraveling the Structural and Electronic Properties at the WSe₂–Graphene Interface for a Rational Design of van der Waals Heterostructures

WSe₂ thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe₂ crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe₂ islands nucleate from defective WSe_<i>x</i> seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe₂ islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe₂ and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe₂–graphene distance and the WSe₂/WSe₂ interlayer separation strongly influence the electronic band alignment at the high symmetry points of the Brillouin zone. The values of technology relevant quantities such as splitting of spin polarized bands and effective mass of electrons at band valleys are extracted from experimental angle resolved spectra. These findings establish further strategies for tuning the morphology and electronic properties of artificially fabricated van der Waals heterostructures that may be used in the fields of nanoelectronics and valleytronics

Unraveling the Structural and Electronic Properties at the WSe₂–Graphene Interface for a Rational Design of van der Waals Heterostructures

WSe₂ thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectromicroscopy and electron diffraction. By tuning of the growth conditions, micrometer-sized single or multilayer WSe₂ crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe₂ islands nucleate from defective WSe_<i>x</i> seeds embedded in the support. We explore the electronic properties of prototypical van der Waals heterostructures by performing μ-angle resolved photoemission spectroscopy on WSe₂ islands of varying thickness (mono- and bilayer) supported on single layer, bilayer, and trilayer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe₂ and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet the WSe₂–graphene distance and the WSe₂/WSe₂ interlayer separation strongly influence the electronic band alignment at the high symmetry points of the Brillouin zone. The values of technology relevant quantities such as splitting of spin polarized bands and effective mass of electrons at band valleys are extracted from experimental angle resolved spectra. These findings establish further strategies for tuning the morphology and electronic properties of artificially fabricated van der Waals heterostructures that may be used in the fields of nanoelectronics and valleytronics