Fig. 3: Revealing the structure of the buried interfacial boron.

a 80 × 30 nm STM image with two co-existing structures: χ-type borophene on Ir(111) and 1 ML Au on Ir(111) nano-structured by the underlying B layer. Height profile along trace 1 is shown below. b–c B 1 s X-ray photoelectron spectroscopy (XPS) measured with the photon energy of 270 eV from the χ-type borophene on Ir(111) (b), and the B layer embedded at the interface between 1 ML Au and Ir(111) (c). d–e B K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra taken as a function of angle Θ between the surface plane and the direction of photon polarization from the χ-type borophene on Ir(111) (d), and the B layer embedded at the interface between 1 ML Au and Ir(111) (e). Differences between spectra taken at Θ = 70° and 0° are shown as colour-filled areas in both (d) and (e). f Measured normal-incidence X-ray standing wave (NIXSW) absorption (B 1 s photocurrent yield, green dots) and reflectivity (blue dots) profiles from 1 ML Au on B/Ir(111), along with their fits (red and black curves, respectively). g–j Structural models of 1 ML Au on Ir(111): (g) without boron interlayer, (h) with a χ-type borophene as an interlayer, (i) with a honeycomb borophene as an interlayer, (j) with B atoms at the hcp (alternatively fcc) positions as an interlayer. Models (h–j) are simplified by making the Au-Au distance equal to the Ir-Ir distance ((1 × 1) unit cell approximation). Ir, Au and B atoms are shown as white, orange and black circles, respectively. In (h) the (2 × 6) borophene unit cell is outlined in red.