Silicene, a two-dimensional (2D) honeycomb structure similar to graphene, has been successfully fabricated on an Ir(111) substrate. It is characterized as a (√7×√7) superstructure with respect to the substrate lattice, as revealed by low energy electron diffraction and scanning tunneling microscopy. Such a superstructure coincides with the (√3×√3) superlattice of silicene. First-principles calculations confirm that this is a (√3×√3)silicene/(√7×√7)Ir(111) configuration and that it has a buckled conformation. Importantly, the calculated electron localization function shows that the silicon adlayer on the Ir(111) substrate has 2D continuity. This work provides a method to fabricate high-quality silicene and an explanation for the formation of the buckled silicene sheet.
Two-dimensional topological insulators with a large bulk band gap are promising for experimental studies of quantum spin Hall effect and for spintronic device applications. Despite considerable theoretical efforts in predicting large-gap two-dimensional topological insulator candidates, none of them have been experimentally demonstrated to have a full gap, which is crucial for quantum spin Hall effect. Here, by combining scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal that ZrTe 5 crystal hosts a large full gap of ∼100 meV on the surface and a nearly constant density of states within the entire gap at the monolayer step edge. These features are well reproduced by our first-principles calculations, which point to the topologically nontrivial nature of the edge states.
Six types of moiré superstructures of graphene on Ir(111) with different orientations (labeled as R0, R14, R19, R23, R26 and R30) are investigated by low-energy electron diffraction, scanning tunneling microscopy and first-principles calculations. The moiré superstructure of R0 graphene has remarkable diffraction spots and deeper corrugation than that of the other superstructures. A high-order commensurate (HOC) method is applied to produce a list of all possible graphene moiré superstructures on Ir(111). Several useful structural data including the precise matrices of the moiré patterns are revealed. Density functional theory based first-principles calculations that include van der Waals interactions reveal the differences of the geometric environment and electronic structures of carbon atoms with respect to the underlying Ir(111) lattices for all the observed moiré patterns. The further calculations of electronic properties at the graphene-Ir interfaces show that the electron transfers for all superstructures are small and of the same order of magnitude, which demonstrates a weak interaction between graphene and the Ir(111) substrate, leading to the coexistence of multi-oriented moiré superstructures.
We report on the structural and electronic properties in the heterostructure of graphene/silicon/Ir(111). A (√19 × √19)R23.41° superstructure is confirmed by low energy electron diffraction and scanning tunneling microscopy and its formation is ascribed to silicon intercalation at the interface between the graphene and the Ir(111) substrate. The dI/dV measurements indicate that the interaction between graphene and Ir is effectively decoupled after silicon intercalation. Raman spectroscopy also reveals the vibrational states of graphene, G peak and 2D peak, which further demonstrates that the silicon-buffered graphene behaves more like intrinsic graphene.
Two-dimensional (2D) honeycomb systems made of elements with d electrons are rare. Here, we report the fabrication of a transition metal (TM) 2D layer, namely, hafnium crystalline layers on Ir(111). Experimental characterization reveals that the Hf layer has its own honeycomb lattice, morphologically identical to graphene. First-principles calculations provide evidence for directional bonding between adjacent Hf atoms, analogous to carbon atoms in graphene. Calculations further suggest that the freestanding Hf honeycomb could be ferromagnetic with magnetic moment μ/Hf = 1.46 μ(B). The realization and investigation of TM honeycomb layers extend the scope of 2D structures and could bring about novel properties for technological applications.
Structural and mechanical properties of selfassembled metal-free naphthalocyanine (H 2 Nc) films on a Ag(111) surface are studied. Six self-assembled domains are observed by scanning tunneling microscopy (STM). Combining the high-resolution STM images and density functional theory (DFT) based calculations, we found that molecules adsorbed flatly on the substrate by forming six different interlocked square-like unit cells with different lattice parameters. DFT calculations indicated comparable adsorption energies for all the configurations. Six domains with different lattice parameters present different strain states, giving us a possibility to evaluate the Young's modulus of the metal-free naphthalocyanine films on the Ag(111) surface. We found that the Young's modulus of H 2 Nc is comparable to those of typical conjugated organic-molecule-based crystals (e.g., naphthalene), providing useful information for future applications when the elastic properties should be concerned.
Borophene, an atomically thin covalently bonded boron sheet, has attracted great attention as a novel quantum material because of its structural tunability and potential utilization in flexible and transparent electronics. So far, borophene has been synthesized on silver or copper single crystals, but these substrates are small, very expensive, and unsuitable for study of transport properties or electronics applications. Here, we report synthesis of borophene on nanometer-scale thick Cu(111) films grown on sapphire. We have developed a process of enlarging faceted borophene islands, by repeated submersion of boron into copper at high temperature and resurfacing and re-crystallization at lower temperature. This discovery was enabled by real-time feedback from low-energy electron microscopy and diffraction. We demonstrate synthesis of borophene as faceted micrometer-size monocrystal islands or as full-monolayer sheets. The process is scalable to wafer size; moreover, Cu films could be sacrificed and sapphire reused. Our work opens the door for new experiments and brings applications one step closer.npj Quantum Materials (2019) 4:40 ; https://doi.
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