Using brominated N-heterocyclic molecules with different lengths 4,4′,7,7′-tetrabromo-1H,1′H-2,2′-bibenzo[d]imidazole (TBBI) and 1,4-bis(4,7-dibromo-1H-benzo[d]imidazol-2-yl) benzene (DBBIB), we successfully constructed and characterized several large-area supramolecular assembly structures on Au(111) surfaces by molecular beam epitaxy and bond-resolved scanning tunneling microscopy. At low coverage (sub-monolayer), both molecules tend to form energetically favorable supramolecular assembly structures. At high coverage (full layer), TBBI can be assembled into grid-like structures with higher space utilization but less non-covalent interactions. However, the transition-like structure of DBBIB assembly with insufficient diffusion can also be kinetically captured at a high deposition rate. In addition, the hindrance between H atoms caused by the spatial configuration of different precursors also affects the self-assembly behavior of molecules. Density functional theory calculations suggest that the formation of various 2D supramolecular assembly structures is due to two types of halogen bonds (Br••• Br and Br•••N halogen bonds) and H•••Br/ H•••N hydrogen bonds. It is undeniable that this strategy can effectively provide a potential route for constructing more supramolecular nanostructures, which may influence the future design of molecular nanomaterials.
Strain engineering is a vital way to manipulate the electronic properties of two-dimensional (2D) materials. As a typical representative of transition metal mono-chalcogenides (TMMs), a honeycomb CuSe monolayer features with one-dimensional (1D) moiré patterns owing to the uniaxial strain along one of three equivalent orientations of Cu(111) substrates. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/S) experiments and density functional theory (DFT) calculations, we systematically investigate the electronic properties of the strained CuSe monolayer on the Cu(111) substrate. Our results show the semiconducting feature of CuSe monolayer with a band gap of 1.28 eV and the 1D periodical modulation of electronic properties by the 1D moiré patterns. Except for the uniaxially strained CuSe monolayer, we observed domain boundary and line defects in the CuSe monolayer, where the biaxial-strain and strain-free conditions can be investigated respectively. STS measurements for the three different strain regions show that the first peak in conduction band will move downward with the increasing strain. DFT calculations based on the three CuSe atomic models with different strain inside reproduced the peak movement. The present findings not only enrich the fundamental comprehension toward the influence of strain on electronic properties at 2D limit, but also offer the benchmark for the development of 2D semiconductor materials.
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