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.

Controlling the crystal structure is a powerful approach for manipulating the fundamental properties of solids. Unique to two-dimensional (2D) van der Waals materials, the control can be achieved by modifying the stacking order through rotation and translation between the layers. Here, we report the first observation of stacking dependent interlayer magnetism in the 2D magnetic semiconductor, chromium tribromide (CrBr 3 ), enabled by the successful growth of its monolayer and bilayer through molecular beam epitaxy. Using in situ spin-polarized scanning tunneling microscopy and spectroscopy, we directly correlated the atomic lattice structure with observed magnetic order. We demonstrated that while individual CrBr 3 monolayer is ferromagnetic, the interlayer coupling in bilayer depends strongly on the stacking order and can be either ferromagnetic or antiferromagnetic. Our observations provide direct experimental evidence for exploring the stacking dependent layered magnetism, and pave the way for manipulating 2D magnetism with unique layer twist angle control.

Abstract. The novel bisphthalonitrile containing benzoxazine (BPNBZ) has been synthesized using bisphenol-A, 4-aminophenoxylphthalonitrile and paraformaldehyde. The structure of the monomer was supported by FTIR spectroscopy, 1 H-NMR, and 13 C-NMR spectra, which have exhibited that the reactive benzoxazine ring and cyano groups exist in molecular structure of BPNBZ. The cure reaction of BPNBZ was monitored by the disappearance of the nitrile peak and the tri-substituted benzene ring that is attached with oxazine ring peak at 2231, 951 cm -1 . The thermal polymerization of the BPNBZ was studied by differential scanning calorimetry (DSC) and dynamic rheometer. It was shown that the bisphthalonitrile containing benzoxazine had completely cured with two-stage polymerization mechanisms according to oxazine ring-opening and phthalonitrile ring-forming. The thermal decomposition behaviors of the polymer were examined by thermogravimetry analysis (TGA) in nitrogen and in air. The materials achieve char yields above 73% under nitrogen at 800°C and above 78% under air at 600°C, which exhibited the cured resin has good thermal stability and thermo-oxidative stability.

Using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, the atomic and low energy electronic structure of the Sr-doped superconducting topological insulators (Sr x Bi 2 Se 3 ) were studied. Scanning tunneling microscopy shows that most of the Sr atoms are not in the van der Waals gap. After the Sr doping, the Fermi level was found to move a little bit upwards compared with the parent compound Bi 2 Se 3 , which is consistent with the low carrier density in this system. The topological surface state was clearly observed and the position of the Dirac point was determined in all doped samples. The surface state is well separated from the bulk conduction bands in the momentum space. The persistence of separated topological surface state combining with small Fermi energy makes this superconducting material a very promising candidate of the time reversal invariant topological superconductor.

Doping bismuth selenide (Bi2Se3) with elements such as copper and strontium (Sr) can induce superconductivity, making the doped materials interesting candidates to explore potential topological superconducting behaviors. It was thought that the superconductivity of doped Bi2Se3 was induced by dopant atoms intercalated in van der Waals gaps. However, several experiments have shown that the intercalation of dopant atoms may not necessarily make doped Bi2Se3 superconducting. Thus, the structural origin of superconductivity in doped Bi2Se3 remains an open question.Herein, we combined material synthesis and characterization, high-resolution transmission electron microscopy, and first-principles calculations to study the doping structure of Sr-doped Bi2Se3. We found that the emergence of superconductivity is strongly related with n-type dopant atoms. Atomic-level energy-dispersive X-ray mapping revealed various n-type Sr dopants that occupy intercalated and interstitial positions. First-principles calculations showed that the formation energy of a specific interstitial Sr doping position depends strongly on Sr doping level. This site changes from a metastable position at low Sr doping level to a stable position at high Sr doping level. The calculation results explain why quenching is necessary to obtain superconducting samples when the Sr doping level is low and also why slow furnace cooling can yield superconducting samples when the Sr doping level is high. Our findings suggest that Sr atoms doped at interstitial locations, instead of those intercalated in van der Waals gaps, are most likely to be responsible for the emergence of superconductivity in Sr-doped Bi2Se3.3