Observation of unexpected 2D copper boride at a surface and exploration of exotic states in the internal 1D boron

抄録

Solid surfaces have been significant research targets to elucidate our fundamental understanding of physics and chemistry that have led to developments of our technology, such as electronics and catalysis. The unique environment has also allowed us to discover novel interface materials that do not exist in nature. For example, three-dimensional (3D) diamond silicon becomes two-dimensional (2D) silicene [1] after deposition onto a metal substrate. A bilayer ice, known to form only under high-pressure, can be generated in vacuum on crystal surfaces [2]. Such exotic materials have drastically renewed our aspects in surface science and boosted the research fields.

In recent years, discovery of various new low-dimensional boron-based materials, including borophene; a monoatomic layer of boron [3], has fascinated the scientific community. The chemical diversity of boron allows for the creation of various stable structures even in low dimensions. Such examples can be seen in superstructures such as B/Ag(111) surface, forming several structures of borophene with unique physical properties[4]. Boron is also known to make various compounds, and exploration of 2D forms of metal borides [5] have also emerged to enrich the exploration of this material group.

In the present research, we focused on the surface ordered phase on Cu(111) by boron deposition in ultrahigh vacuum [6,7]. Through a combination of the surface analyses by positron diffraction and photoelectron spectroscopy, we have determined the formation of 2D copper boride at the surface [8]. It has been known that the boron and copper phases are completely separated in 3D, due to their small difference in electronegativities and large difference in atomic size. The unexpected 2D material is composed of alternating boron and copper atomic chains with a zig-zag structure. The first-principles calculations successfully reproduced the experimental results and further unveiled that the 1D boron is energetically stabilized by electron doping from the surrounding copper atoms, leading to a π-type bounding state. This unique electrophilic character leads us to build the Bumulene model, based on the carbon chains in Cumulene [9]. Since the 2D copper boride forms on a surface, the substrate-induced evolution of the electronic states was examined in detail by the calculation and observed by photoemission band mapping [10]. We believe our comprehensive work would provide a new aspect of surface science on the most well-known Cu(111) crystal and opened a new boron science in ≦ 2D.

In this presentation, I would like to describe details of our research on 2D copper boride on Cu(111). In addition, I would also like to introduce our discovery of a new surface phase of B/Cu(110) [11], as a precursor to further exploring the details of boron configuration on the Cu surface.

References:

[1] Y. Fukaya et al., Phys. Rev. B 88, 205413 (2013).

[2] R. Ma et al., Nature 577, 60 (2020).

[3] I. Matsuda, K. Wu, Eds. 2D Boron: Boraphene, Borophene, Boronene; Springer International Publishing: Cham (2021).

[4] B. Feng et al., Phys. Rev. B 118, 096401 (2017).

[5] F. J. Tuli et al., Surf. Sci. 713, 121906 (2021).

[6] C. Yue et al., Fundamental Research 1, 482 (2021).

[7] R. Wu et al., Nat. Nanotechnol. 14, 44 (2019).

[8] Y. Tsujikawa et al., Phys. Rev. B 106, 205406 (2022).

[9] C. H. Hendon et al., Chem. Sci. 4, 4278 (2013).

[10] Y. Tsujikawa et al., e-J. Surf. Sci. Nanotechnol. https://doi.org/10.1380/ejssnt.2023-058

[11] Y. Tsujikawa et al., Surf. Sci. 732, 122282 (2023).