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.

Topological insulators and graphene present two unique classes of materials, which are characterized by spin-polarized (helical) and nonpolarized Dirac cone band structures, respectively. The importance of many-body interactions that renormalize the linear bands near Dirac point in graphene has been well recognized and attracted much recent attention. However, renormalization of the helical Dirac point has not been observed in topological insulators. Here, we report the experimental observation of the renormalized quasiparticle spectrum with a skewed Dirac cone in a single Bi bilayer grown on Bi 2 Te 3 substrate from angle-resolved photoemission spectroscopy. First-principles band calculations indicate that the quasiparticle spectra are likely associated with the hybridization between the extrinsic substrate-induced Dirac states of Bi bilayer and the intrinsic surface Dirac states of Bi 2 Te 3 film at close energy proximity. Without such hybridization, only single-particle Dirac spectra are observed in a single Bi bilayer grown on Bi 2 Se 3 , where the extrinsic Dirac states Bi bilayer and the intrinsic Dirac states of Bi 2 Se 3 are well separated in energy. The possible origins of many-body interactions are discussed. Our findings provide a means to manipulate topological surface states.Dirac fermion | electronic structures | thin films M uch recent attention has been devoted to graphene (1-6) and topological insulators (TIs) (7-18), two unique material systems that exhibit conical linear electron bands of Dirac spectra. quasiparticles of Dirac fermions are distinct from those of ordinary Fermi liquids (19)(20)(21). Although rather difficult and rare, recent angle-resolved photoemission spectroscopy (ARPES) experiments (1, 2, 5, 6) have directly shown the existence of many-body quasiparticle spectra near Dirac point in graphene, manifesting electronelectron, electron-phonon, and electron-plasmon interactions. Similar to graphene, TIs also possess Dirac cone, albeit it is spinpolarized or helical Dirac cone. So far, however, no renormalized quasiparticle spectra near the helical Dirac point similar to graphene have been reported in any known TIs, and most studies of TIs are based on the single-particle picture (9,11,12,14,18). Here, we report direct experimental observation of a skewed helical Dirac point, a signature quasiparticle spectrum indicative of many-body interactions, by ARPES in a TI system of Bi(111) bilayer grown on Bi 2 Te 3 substrate, where a 2D TI is interfaced with a 3D TI (22).ARPES can probe the quasiparticle's scattering rate at different energy scales, and therefore can access the many-body interactions directly (23). Our experimental observation of the quasiparticle spectra manifesting many-body effects is characterized with a "vertically nondispersive" feature near Dirac point. Based on model density functional theory (DFT) calculations of electron bands as a function of the artificially changed interfacial distance between the Bi bilayer and substrate, we found that the renormalized quasi...

Topological insulators are a unique class of materials characterized by a Dirac cone state of helical Dirac fermions in the middle of a bulk gap. When the thickness of a three-dimensional topological insulator is reduced, however, the interaction between opposing surface states opens a gap that removes the helical Dirac cone, converting the material back to a normal system of ordinary fermions. Here we demonstrate, using density function theory calculations and experiments, that it is possible to create helical Dirac fermion state by interfacing two gapped films-a single bilayer Bi grown on a single quintuple layer Bi 2 Se 3 or Bi 2 Te 3 . These extrinsic helical Dirac fermions emerge in predominantly Bi bilayer states, which are created by a giant Rashba effect with a coupling constant of B4 eV ÁÅ due to interfacial charge transfer. Our results suggest that this approach is a promising means to engineer topological insulator states on non-metallic surfaces.

The bosonic analogs of topological insulators have been proposed in numerous theoretical works, but their experimental realization is still very rare, especially for spin systems. Recently, two-dimensional (2D) honeycomb van der Waals ferromagnets have emerged as a new platform for topological spin excitations. Here, via a comprehensive inelastic neutron scattering study and theoretical analysis of the spin-wave excitations, we report the realization of topological magnon insulators in CrXTe 3 (X = Si, Ge) compounds. The nontrivial nature and intrinsic tunability of the gap opening at the magnon band-crossing Dirac points are confirmed, while the emergence of the corresponding in-gap topological edge states is demonstrated theoretically. The realization of topological magnon insulators with intrinsic gap-unability in this class of remarkable 2D materials will undoubtedly lead to new and fascinating technological applications in the domain of magnonics and topological spintronics.