Following the first experimental realization of graphene, other ultrathin materials with unprecedented electronic properties have been explored, with particular attention given to the heavy group-IV elements Si, Ge and Sn. Two-dimensional buckled Si-based silicene has been recently realized by molecular beam epitaxy growth, whereas Ge-based germanene was obtained by molecular beam epitaxy and mechanical exfoliation. However, the synthesis of Sn-based stanene has proved challenging so far. Here, we report the successful fabrication of 2D stanene by molecular beam epitaxy, confirmed by atomic and electronic characterization using scanning tunnelling microscopy and angle-resolved photoemission spectroscopy, in combination with first-principles calculations. The synthesis of stanene and its derivatives will stimulate further experimental investigation of their theoretically predicted properties, such as a 2D topological insulating behaviour with a very large bandgap, and the capability to support enhanced thermoelectric performance, topological superconductivity and the near-room-temperature quantum anomalous Hall effect.

Recent studies show that two low-energy van Hove singularities (VHSs) seen as two pronounced peaks in the density of states could be induced in a twisted graphene bilayer. Here, we report angle-dependent VHSs of a slightly twisted graphene bilayer studied by scanning tunneling microscopy and spectroscopy. We show that energy difference of the two VHSs follows ΔE(vhs)∼ℏν(F)ΔK between 1.0° and 3.0° [here ν(F)∼1.1 × 10(6) m/s is the Fermi velocity of monolayer graphene, and ΔK = 2Ksin(θ/2) is the shift between the corresponding Dirac points of the twisted graphene bilayer]. This result indicates that the rotation angle between graphene sheets does not result in a significant reduction of the Fermi velocity, which quite differs from that predicted by band structure calculations. However, around a twisted angle θ∼1.3°, the observed ΔE(vhs)∼0.11 eV is much smaller than the expected value ℏν(F)ΔK∼0.28 eV at 1.3°. The origin of the reduction of ΔE(vhs) at 1.3° is discussed.

By combining scanning tunneling microscopy and spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory band calculations, we directly observe and resolve the one-dimensional edge states of single bilayer (BL) Bi(111) islands on clean Bi(2)Te(3) and Bi(111)-covered Bi(2)Te(3) substrates. The edge states are localized in the vicinity of step edges having an ∼2  nm wide spatial distribution in real space and reside in the energy gap of the Bi(111) BL. Our results demonstrate the existence of nontrivial topological edge states of single Bi(111) bilayer as a two-dimensional topological insulator.

Tuning the compositions and structures of Pdbased nanomaterials and their supports has shown great potentials in facilitating the sluggish ethanol oxidation reaction (EOR) in alkaline direct ethanol fuel cells. Accordingly, a facile solvothermal method involving Cu and Pd composition migrations is developed in this study, to synthesize highly uniform and small-sized nanospheres (NSs) possessing the special structures of composition-graded (CG) Cu 1 Pd 1 and surface-doped (SD) Ir 0.03 , which are evenly anchored onto N-doped porous graphene (NPG) as a highperformance EOR electrocatalyst ( CG Cu 1 Pd 1 / SD Ir 0.03 NSs/ NPG). Comprehensive physicochemical characterizations, electrochemical analyses, and first-principles calculations reveal that, benefiting from the NPG-imparted mass-transfer and oxygen-reduction effects, the CG−SD structural and sizemorphology effects of the NS, as well as the Cu-and Ir-induced bifunctional, geometric, and ligand effects, CG Cu 1 Pd 1 / SD Ir 0.03 NSs/NPG exhibits not only ultrahigh electrocatalytic activity and highly efficient noble-metal (NM) utilization, showing 7105 and 6685 mA mg −1 in Pd-and NM-mass-specific activity (MSA), respectively, which are 15.8 and 14.9 times those of commercial Pd/C, but also satisfactory electrocatalytic durability, retaining respective 28.1-and 19.2-fold enhancements in Pd-MSA compared to the commercial Pd/C, after 1 h chronoamperometric and 500-cycle potential cycling degradation tests. This study not only provides an effective and versatile synthetic strategy to prepare the NM-efficient metal-based nanomaterials with the special CG and SD structures for various electrocatalytic and energy-conversion applications, but also proposes some insights into the composition-and structure-function relations in EOR electrocatalytic mechanism for rationally designing highly active and durable EOR electrocatalysts.

It is undoubtedly desirable, albeit very challenging, to appropriately balance the catalytic activity, electrochemical durability, and noble-metal (NM) utilization when developing Pt-based catalysts for oxygen reduction reaction (ORR). Accordingly, in this work, a versatile and effective strategy that promises the nanostructure of both composition-graded core and mono- or multilayer shell is proposed to synthesize highly uniform, sub-10 nm Pd x Ni1–x @Pt nanospheres (NSs) as high-performance ORR electrocatalysts. Highly uniform and composition-graded Pd x Ni1–x NSs are previously obtained via a facile one-pot Ni-substitution-based process, and then Pt mono- or multilayer shells are coated onto them through Cu underpotential deposition coupled with Pt2+ galvanic displacement. Results show that carbon-supported Pd x Ni1–x @Pt electrocatalysts possess both high catalytic activity and highly efficient NM utilization toward ORR. The optimized Pd0.42Ni0.58@Pt/C exhibits 0.61 mA cm–2, 0.42 A mg–1 Pd+Pt, and 1.45 A mg–1 Pt @ 0.9 V (vs RHE) in the area-specific, NM-mass-specific, and Pt-mass-specific activity, respectively, reaching 2.8, 3.3, and 11.2 times relative to those of the commercial Pt/C. Moreover, Pd0.42Ni0.58@Pt/C also has a satisfactory electrochemical durability, preserving its high ORR catalytic activity even after 12 000 potential cycles of the accelerated degradation test. The synthetic mechanism of Pd x Ni1–x NS core, Pt monolayer shell and their combined effects on the catalytic activity, electrochemical durability, and NM utilization of Pd x Ni1–x @Pt/C toward ORR are comprehensively investigated.