Topological properties determined by atomic buckling in self-assembled ultrathin Bi(110).

Topological insulators (TIs) are a new type of electronic materials in which the nontrivial insulating bulk band topology governs conducting boundary states with embedded spin-momentum locking. Such edge states are more robust in a two-dimensional (2D) TI against scattering by nonmagnetic impurities than in its three-dimensional (3D) variant, because in 2D the two helical edge states are protected from the only possible backscattering. This makes the 2D TI family a better candidate for coherent spin transport and related applications. While several 3D TIs are already synthesized experimentally, physical realization of 2D TI is so far limited to hybrid quantum wells with a tiny bandgap that does not survive temperatures above 10 K. Here, combining first-principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS) experimental studies, we report nontrivial 2D TI phases in 2-monolayer (2-ML) and 4-ML Bi(110) films with large and tunable bandgaps determined by atomic buckling of Bi(110) films. The gapless edge states are experimentally detected within the insulating bulk gap at 77 K. The band topology of ultrathin Bi(110) films is sensitive to atomic buckling. Such buckling is sensitive to charge doping and could be controlled by choosing different substrates on which Bi(110) films are grown.

Size-tunable Au nanoparticles on MoS2(0001).

Ultra-fine Au nanoparticles (NPs) show great application potential in catalysis. Size-tunable Au NPs have been fabricated on MoS(2) covered with monolayer 3,4,5,10-perylene tetracarboxylic dianhydride (PTCDA), and the morphological evolution as a function of Au deposition amount was investigated using scanning tunneling microscopy (STM). The PTCDA molecules act as a surfactant to stabilize ultra-fine Au NPs. Molecular scale STM images show that on MoS(2) the Au NPs with PTCDA molecules on top can be formed with height and lateral size down to 1.3 nm and 3.5 nm, respectively. By controlling the deposition amount and annealing temperature, the size of Au NPs can be tuned. After annealing at 270 °C to remove PTCDA, Au NPs with a linear size ≤5 nm can be obtained on MoS(2)(0001), facilitating the characterization of their intrinsic physical and chemical properties using various analytical techniques. In addition, photoemission spectroscopy data reveal charge transfer from Au NPs to PTCDA, indicating that the NPs possess more reactive chemical properties than bulk Au.

Evolution of topological surface states in antimony ultra-thin films.

Based on an inverted bulk band order, antimony thin films presumably could become topological insulators if quantum confinement effect opens up a gap in the bulk bands. Coupling between topological surface states (TSS) from opposite surfaces, however, tends to degrade or even destroy their novel characters. Here the evolution and coupling of TSS on Sb(111) thin films from 30 bilayers down to 4 bilayers was investigated using in-situ Fourier-transform scanning tunneling spectroscopy and density functional theory computations. On a 30-bilayer sample, quasi-particle interference patterns are generated by the scattering of TSS from the top surface only. As the thickness decreases, inter-surface coupling degrades spin polarisation of TSS and opens up new wavevector-dependent scattering channels, resulting in spin degenerate states in most part of the surface Brillouin zone, whereas the TSS near the zone centre exhibit little inter-surface coupling, so they remain spin-polarised without opening a gap at the Dirac point.