New Alloy of an Al-Chalcogen System: AlSe Surface Alloys on Al(111).

Metal chalcogenides are a promising material for novel physical research and nanoelectronic device applications. Here, we systematically investigate the crystal structure and electronic properties of AlSe alloys on Al(111) using scanning tunneling microscopy, angle-resolved photoelectron spectrometry, and first-principle calculations. We reveal that the AlSe surface alloy possesses a closed-packed atomic structure. The AlSe surface alloy comprises two atomic sublayers (Se sublayer and Al sublayer) with a height difference of 1.16 Å. Our results indicate that the AlSe alloy hosts two hole-like bands, which are mainly derived from the in-plane orbital of AlSe (p x and p y ). These two bands located at about -2.22 ±0.01 eV around the Gamma point, far below the Fermi level, distinguished from other metal chalcogenides and binary alloys. AlSe alloys have the advantages of large-scale atomic flat terraces and a wide band gap, appropriate to serve as an interface layer for two-dimensional materials. Meanwhile, our results provide implications for related Al-chalcogen interfaces.

Biaxial atomically resolved force microscopy based on a qPlus sensor operated simultaneously in the first flexural and length extensional modes.

Frequency-modulation atomic force microscopy (AFM) with a qPlus sensor allows one to atomically resolve surfaces in a variety of environments ranging from low-temperature in ultra-high vacuum to ambient and liquid conditions. Typically, the tip is driven to oscillate vertically, giving a measure of the vertical force component. However, for many systems, the lateral force component provides valuable information about the sample. Measuring lateral and vertical force components simultaneously by oscillating vertically and laterally has so far only been demonstrated with relatively soft silicon cantilevers and optical detection. Here, we show that the qPlus sensor can be used in a biaxial mode with electrical detection by making use of the first flexural mode and the length extensional mode. We describe the necessary electrode configuration as well as the electrical detection circuit and compare the length extensional mode to the needle sensor. Finally, we show atomic resolution in ambient conditions of a mica surface and in ultra-high vacuum of a silicon surface. In addition to this, we show how any qPlus AFM setup can be modified to work as a biaxial sensor, allowing two independent force components to be recorded.

From Silicene to Half-Silicane by Hydrogenation.

Graphane is graphene fully hydrogenated from both sides, forming a 1×1 structure, where all C atoms are in sp(3) configuration. In silicene, the Si atoms are in a mixed sp(2)/sp(3) configuration; it is therefore natural to imagine a silicane structure analogous to graphane. However, a monatomic silicene sheet grown on substrates generally reconstructs into different phases, and only partially hydrogenated silicene with reconstructions had been reported before. In this work, we produce half-silicane, where one Si sublattice is fully H-saturated and the other sublattice is intact, forming a perfect 1×1 structure. By hydrogenating various silicene phases on a Ag(111) substrate, we found that only the (2√3×2√3)R30° phase can produce half-silicane. Interestingly, this phase was previously considered to be a highly defective or incomplete silicene structure. Our results indicate that the structure of the (2√3×2√3)R30° phase involves a complete silicene-1×1 lattice instead of defective fragments, and the formation mechanism of half-silicane was discussed with the help of first-principles calculations.

Multilayered silicene: the bottom-up approach for a weakly relaxed Si(111) with Dirac surface states.

Combining first principles investigations and scanning tunneling microscopy, we identify that the presumable van der Waals packed multilayered silicene sheets spontaneously transform into a diamond-structure bulk Si film due to strong interlayer couplings. In contrast to drastic surface reconstruction on conventional Si(111), multilayered silicene prepared by bottom-up epitaxy on Ag(111) exhibits a nearly ideal flat surface with only weak buckling. Without invoking Ag surfactants, √3 ×√3 honeycomb patterns emerge thanks to dynamic fluctuation of mirror-symmetric rhombic phases, similar to monolayered silicene [Chen et al., Phys. Rev. Lett., 2013, 110, 085504]. The weak relaxation enables novel surface states with a Dirac linear dispersion.

Ordered and reversible hydrogenation of silicene.

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.

Spontaneous symmetry breaking and dynamic phase transition in monolayer silicene.

The (sqrt[3]×sqrt[3])R30° honeycomb of silicene monolayer on Ag(111) was found to undergo a phase transition to two types of mirror-symmetric boundary-separated rhombic phases at temperatures below 40 K by scanning tunneling microscopy. The first-principles calculations reveal that weak interactions between silicene and Ag(111) drive the spontaneous unusual buckling in the monolayer silicene, forming two energy-degenerate and mirror-symmetric (sqrt[3]×sqrt[3])R30° rhombic phases, in which the linear band dispersion near the Dirac point and a significant gap opening (150 meV) at the Dirac point were induced. The low transition barrier between these two phases enables them to be interchangeable through dynamic flip-flop motion, resulting in the (sqrt[3]×sqrt[3])R30° honeycomb structure observed at high temperature.