Direct observation of site-selective hydrogenation and spin-polarization in hydrogenated hexagonal boron nitride on Ni(111).

We report structural analysis and spin-dependent band structure of hydrogenated boron nitride adsorbed on Ni(111). The atomic displacement studied by normal incidence x-ray standing wave (NIXSW) technique support H-B(fcc):N(top) model, in which hydrogen atoms are site-selectively chemisorbed on boron atoms and N atoms remain on top of Ni atoms. The distance between Ni plane and nitrogen plane did not change after hydrogenation, which implies that the interaction between Ni and N is 3d-π orbital mixing (donation and back-donation) even after hydrogenation of boron. The remaining π peaks in ultra-violet photoemission spectroscopy (UPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra are manifestation of rehybridization of sp2 into sp3 states, which is consistent with the N-B-N bonding angle derived from NIXSW measurement. The SPMDS measurement revealed the positive spin polarization appearing on hydrogenated h-BN, which is assigned to unpaired 2pz electrons localized on N atoms. Even though the atomic displacement is reproduced by the density functional theory (DFT) calculation with H-B(fcc):N(top) model, the experimental spin-dependent band structure was not reproduced by DFT possibly due to the self-interaction error (SIE). These results reinforce the site-selective hydrogenation on boron and pave the way for efficient design of BN nanomaterials for hydrogen storage