Photoelectron Spectroscopy and Theoretical Study of Di-Copper-Boron Clusters: Cu₂B₃^- and Cu₂B₄

Copper has been found to be able to mediate the formation of bilayer borophenes. Copper-boron binary clusters are ideal model systems to probe the copper-boron interactions, which are essential to understand the growth mechanisms of borophenes on copper substrates. Here, we report a joint photoelectron spectroscopy and theoretical study on two di-copper-doped boron clusters: Cu₂B₃^- and Cu₂B₄^-. Well-resolved photoelectron spectra are obtained, revealing the presence of a low-lying isomer in both cases. Theoretical calculations show that the global minimum of Cu₂B₃^- (C₂, ¹A₁) contains a doubly aromatic B₃^- unit weakly interacting with a Cu₂ dimer, while the low-lying isomer (C₂, ¹A₁) consists of a B₃ triangle with the two Cu atoms covalently bonded to two B atoms at two vertexes. The global minimum of Cu₂B₄^- (D₂, ²A_g) is found to consist of a rhombus B₄ unit covalently bonded to the two Cu atoms at two opposite vertexes, whereas in the low-lying isomer (C, ²A'), one of the two Cu atoms is bonded to two B atoms.