A multimethod investigation of 2D boron sheet - i.e. borophene - on the surface of iridium is reported in a paper by an international team of researchers led by physicists from the Institute of Physics in Zagreb, Croatia. The study, published in ACS Applied Materials & Interfaces, highlights the nanomodulated properties of epitaxial borophene which acts as an electron diffraction grating.

Members of the Surfaces, Interfaces and 2D Materials (SIMAT) research group from the Institute of physics in Zagreb, together with co-workers from Japan, the Netherlands, Italy, Germany and Croatia reported on a combined experimental and theoretical study investigating the structural and electronic properties of an extended monolayer of boron atoms - i.e. borophene - on Ir(111). The research reveals significant nanoscopic modulation in the borophene layer due to non-uniform interaction between B atoms and the underlying Ir atoms, as revealed by the XPS and STM data (see Figure 1). This modulation leads to a stripe-like structure in borophene, functioning as an electron diffraction nano-grating. The scattering of electrons, induced by this grating, results in the formation of replica electronic bands detected in the ARPES spectra of the borophene/Ir(111) system near the Fermi level (see Figure 2). The atomically-thin boron grating, though chemically (re)active and susceptible to atomic-scale modifications, may be easily restored through a dissolution-segregation cycle which triggers reassembly of boron atoms into a fresh borophene mesh on the surface of Ir(111) substrate. 

Figure. 1 XPS and STM data acquired after borophene synthesis on Ir(111). (a) B 1s spectra of the borophene/Ir sample recorded immediately after borophene synthesis (bottom), 11 and 22 hours after the synthesis (middle) and after sample annealing to 1100 °C (top). B1 and B2 components are signatures of the variable B-Ir binding. (b) High resolution STM topograph, revealing a stripy structure of the borophene layer. The green parallelogram marks the unit cell of borophene. Top right corner: DFT-simulated STM image of a freestanding borophene.

Figure. 2 Comparison of the sample Fermi surfaces (a) before and (b) after borophene synthesis on Ir(111). Appearance of the replica bands, in the form of arc-like features marked by arrows, is evident. Data in panel (a) has been recorded at the URANOS beamline of the Solaris synchrotron (Krakow, Poland), data in panel (b) has been recorded at the VUV-Photoemission beamline of the Elettra synchrotron (Trieste, Italy).

The findings highlight the scattering capabilities of 2D materials and encourage further exploration of different borophene polymorphs that may exhibit various superperiodicity phenomena. In addition, the study provides a comprehensive understanding of the electronic structure of epitaxial borophene, which is crucial for advancing borophene-based applications.

Author: Marin Petrović

Link to the publication: S. Kamal et al., Unidirectional Nano-modulated Binding and Electron Scattering in Epitaxial Borophene, ACS Applied Materials & Interfaces, (2023) doi: 10.1021/acsami.3c14884