抄録
Since the successful synthesis of graphene, single-element monolayer sheets have attracted much interest because of their distinctive electronic properties. One example is a monolayer sheet of boron (B) (a group 13 element) called “borophene”. Compared to graphene (a sheet of group 14 element), each boron atom is deficient of one electron, thus theoretically borophene layers are predicted to have various types of atomic structures, consisting of triangular lattices and hexagonal hollows. On the other hand, it is reported that a graphene-like honeycomb borophene has been synthesized on aluminum (Al) (111) substrate, based on scanning tunneling microscopy (STM) experiments [1], although in general the honeycomb structure of borophene is inherently unstable. Preobrajenski, et al. have proposed two different structural models for this honeycomb structure[2]; one is a “free-standing model”, in which the honeycomb borophene is simply on Al(111) substrate, and the other is an “AlB2 model” in which the boron and aluminum atoms form into AlB2 compositions at the topmost surface. However, the actual structure has not yet been determined and the origin of the honeycomb-like structure has not been clarified either. In this study, the atomic structure of honeycomb borophene on Al(111) was investigated by total-reflection high-energy positron diffraction (TRHEPD).
For sample preparation, B was deposited on a clean Al(111) surface at ~210°C. The quality of the samples was confirmed by using low-energy electron diffraction (LEED) and reflection high-energy electron diffraction. Figure 1(a) shows that the LEED pattern of the B/Al(111). Besides the Al-1×1 spots (red circles), one can see that many distinct fractional spots appeared. As shown in the inset (enlarged view of the area enclosed by the square) of Fig.1(a), we identified spots specific to the honeycomb borophene (yellow circles), which is consistent with the study by Preobrajenski, et al. [2]. The TRHEPD measurements were performed at the Slow Positron Facility, KEK. Figure 1(b) shows the TRHEPD rocking curves obtained before and after the B deposition under one-beam (OB) condition, where the diffraction intensities of the specular (00) spot are plotted as a function of θ. After the B deposition, the peak position does not significantly change while the intensity at 3.5° is suppressed. Under the OB condition, the diffraction intensities reflect the interlayer distance and the atomic density of each layer. Therefore this result indicates that the borophene layer has formed on the topmost surface and interferes with the beam diffracted from the Al(111) substrate. In this presentation, we will present details of the sample preparation conditions and the result of the structural analysis, and discuss the origin of the structure for honeycomb borophene on Al(111).
References
[1] W. Li, et al., Sci. Bull. 63, 282 (2018).
[2] A. B. Preobrajenski, et al., ACS Nano 15, 15153 (2021).
