Boron is the neighboring element of carbon in the periodic table, and has strong bonding capability comparable to carbon. Two-dimensional boron, named borophene, has predicted to have exotic properties beyond graphene, such as mechanical compliance, optical transparency, anisotropic plasmonics, ultrahigh thermal conductance, one-dimensional (1D) nearly-free-electron states, presence of metallic Dirac fermions, and superconductivity. A few yeas ago, the monolayer borophene has been successful grown on some metal substrates like Ag(111)/(110)/(100) by two groups including Prof. WU Kehui and Prof. CHEN Lan from Institute of Physics, Chinese Academy of Sciences. Compared with monolayer borophene, bilayer borophene is more stable due to the interlayer bonding, and exhibits richer physical properties. The researches on bilayer borophene have attracted intensive theoretical and experimental interest now. However, the bilayer borophene was not thought to be synthesized due to the passivation from metal substrate.

Recently, Dr. CHEN Caiyun et al. from the group led by Prof. WU Kehui and Prof. CHEN Lan, collaborating with Dr. LV Haifeng et al. from the group led by Prof. WU Xiaojun in University of Science and Technology of China, found that the charge transfer from Cu(111) surface to the monolayer borophene are much larger than that on Ag(111) surface. This fact indicates boron atoms on Cu(111) are more activated than that on Ag(111), and may overcome the problem of metal substrate passivation.

Then, the researchers successful realized the synthesis of large-size, single-crystalline bilayer borophene on the Cu(111) surface by molecular beam epitaxy (MBE). The scanning tunneling microscopy (STM) and first-principles calculations show the bilayer borophene consists of two stacked monolayers. Each monolayer has chain-like structures with zigzag rows, and combined together by covalent interlayer boron-boron bonding. They also revealed the bilayer is metallic and exhibits enhanced chemical stability and anti-oxidation than monolayer.

The successful bottom-up synthesis of bilayer borophene sheet provides opportunities and challenges for the characterization of their physical properties. The high quality of bilayer borophene on metal surface and in particular its large size and single-crystalline nature may enable the fabrication of borophene-based devices, such as the proposed ultracapacitor, photonic and plasmonic devices, and electronic biomedical devices with high sensitivity of signal transmission and detection.

This study entitled "Synthesis of bilayer borophene" was published on Nature Chemistry.

The study was supported by the National Science Foundation, the Ministry of Science and Technology of China, the Chinese Academy of Sciences, Beijing Natural Science Foundation, the Anhui Initiative in Quantum Information Technologies, National Program for Support of Top-notch Young Professional, and the Super Computer Centre of USTCSCC and SCCAS.

Contact:
Institute of Physics
CHEN Lan
Email：lchen@iphy.ac.cn

Key word:
borophene; two dimensional materials, molecular beam epitaxy, scanning tunneling microscopy.

Abstract:
Large-size, single-crystalline bilayer borophene on the Cu(111) surface is synthesized by molecular beam epitaxy. The bilayer borophene consists of two stacked monolayers, which are held together by covalent interlayer boron-boron bonding, and each monolayer has zizag-row-like structures. The formation of a bilayer is associated with the significant transfer and redistribution of charge in the first boron layer on Cu(111).

Fig. 1 The evolution of borophene from monolayer to bilayer grown on Cu(111) surface. (Image by Institute of Physics)

Fig. 2 The STM images and atomic structures of monolayer and bilayer borophene on Cu(111) (Image by Institute of Physics)

Fig.3 The physical properties including the anti-oxidation and electronic structure of bilayer borophene (Image by Institute of Physics)