The hydrogenation of monatomic silicene sheet on Ag(111) was studied by scanning tunneling microscopy and density functional theory calculations. It was observed that hydrogenation of silicene at room temperature results in a perfectly ordered γ-(3×3) superstructure. A theoretical model, which involves seven H atoms and rearranged buckling of Si atoms, was proposed and agrees with experiments very well. Moreover, by annealing to a moderate temperature, about 450 K, a dehydrogenation process occurs and the clean silicene surface can be fully recovered. Such uniformly ordered and reversible hydrogenation may be useful for tuning the properties of silicene as well as for controllable hydrogen storage.
We performed low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) studies on the electronic properties of (√3 × √3)R30° phase of silicene on Ag(111) surface. We found the existence of Dirac Fermion chirality through the observation of -1.5 and -1.0 power law decay of quasiparticle interference (QPI) patterns. Moreover, in contrast to the trigonal warping of Dirac cone in graphene, we found that the Dirac cone of silicene is hexagonally warped, which is further confirmed by density functional calculations and explained by the unique superstructure of silicene. Our results demonstrate that the (√3 × √3)R30° phase is an ideal system to investigate the unique Dirac Fermion properties of silicene.
We report the formation of rubrene crystalline films on Bi(001) substrate starting from the very first layer. With coverage increasing, rubrene shows a structural evolution from self-assembled monolayer to a composite phase, which consists of rubrene crystalline domains and self-assembled domain walls. In particular, Kurdjunov-Sachs (KS) rotational epitaxy has been found in rubrene crystalline domains, which reveal large compressive strains.Further deposition of rubrene leads to a layer-by-layer growth of crystalline films up to the forth layer. The driving force for rubrene crystallinity in monolayer regime has been attributed to the anisotropic strains generated in KS rotation epitaxy.
The supramolecular pinwheel cluster is a unique chiral structure with evident handedness. Previous studies reveal that the chiral pinwheels are composed of chiral or achiral molecules with polar groups, which result in strong intermolecular interactions such as hydrogen-bonding or dipole interactions. Herein, it is shown that the simple linear aromatic molecule, pentacene, can be self-assembled into large chiral pinwheel clusters on the semimetal Bi(111) surface, due to enhanced intermolecular interactions. The pentacene pinwheels reveal two levels of organizational chirality: the chiral hexamers resulting from asymmetric shifting along the long molecular axis, and chiral arrangement of six hexamers with a rotor motif. Furthermore, a new relation between the local point chirality and organizational chirality is identified from the pinwheels: the former is not essential for the latter in 2D pinwheel clusters of the pentacene molecule.
We report a structural modulation occurred in the rubrene monolayer grown on Bi (001) surface. A small sinusoidal variation of surface height plus the periodic distortion of molecular orientations have been observed by low temperature scanning tunneling microscopy. Further analyses demonstrate that this modulation results from the lattice rotation of rubrene monolayer with respect to Bi(001) substrate. Depending on the rotational direction of rubrene lattices, the modulation may exhibit either stripe ripples or zigzag patterns. The experimental data are discussed in term of mass density wave.
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