Synthesis of two-dimensional diboron trioxide, the thinnest boria allotrope

Boron oxide (B₂O₃) is a critical component in the production of ultra-durable glasses, such as Pyrex, and high-performance enamels. The addition of boron trioxide enhances glass’s resistance to thermal shock and chemical reactions, making it ideal for demanding industrial and scientific applications. However, the vitrification process of boron oxide remains poorly understood, exhibiting unique anomalies compared to other polymorphic oxides, like silica, which can exist in both crystalline and amorphous forms.

The key distinction between a crystal and a glass lies in the ordered arrangement of atoms in the former, which is absent in the latter. Both materials typically share a basic structural unit of a few atoms, repeated throughout. In crystals, this “building block” is arranged in a precise, repeating pattern, while in glass, it is disordered. Boron oxide is an exception: its glassy phase contains a structural unit called boroxine – a ring of three boron and three oxygen atoms – that have never been observed in a crystalline form until now. By creating a two-dimensional crystalline phase made entirely of these boroxine units, a world-first was achieved with this work. The discovery of this allotrope explains the anomalous behavior of boria, aligning it with other polymorphic oxides capable of forming glasses.

By carrying out a multimodal experimental and theoretical analysis, our research team has not only devised a method to synthesize this material, using platinum as a substrate, but also elucidated its physical properties. The numerical simulations have revealed that this porous material, formed by a lattice of boroxine rings, is extraordinary flexible – ten times more elastic than graphene, making it the most elastic single-layer material ever reported. This remarkable flexibility arises because the rigid boroxine rings are connected by a single oxygen atom, acting as a hinge that allows them to rotate in the plane. Both experiments and simulations have also shown that the material interacts weakly with the platinum substrate, suggesting it could be easily separated with conventional techniques for application in cutting-edge devices.

By means of advanced scanning tunneling microscopy (STM) in Trieste and Innsbruck, we visualized the crystalline structure of this two-dimensional material down to its individual atoms. STM images and the ball-and-stick model of 2D boria are presented in Fig. 1a and 1b, respectively. Achieving the conditions required for atomic resolution at unprecedented quality enabled us to pinpoint the position of each atom in the lattice, providing valuable insights into how atoms reorganize during the transition from crystalline to glassy states. This capability will be a turning point for future studies of material transitions.

Figure 1.  (a) Topographic STM imaging of 2D B₂O₃ grown on a Pt(111) substrate. High resolution imaging reveals the position of individual oxygen atoms in the network. (b) Top and side views of the relaxed B₂O₃/Pt(111) system. The yellow and white dashed lines indicate the geometrical supercell of the entire system and the unit cell of the boria layer, respectively. The calculated average distance of 2D boria from the substrate is indicated in the side view. (c) µ-LEED pattern (with zoomed center) of a single flake of 2D boria. The Pt(111) and 2D B₂O₃ reciprocal space vectors are depicted in green and blue, respectively. Their quantitative comparison yielded a 2D boria lattice constant of 8.57±003 Å. (d) Momentum distribution curve of B₂O₃/Pt(111) extracted from µ-ARPES data (p-polarization, photon energy 40 eV) along high-symmetry axes of Pt(111) First Brillouin Zone (FBZ) (left) and calculated band structure of freestanding B₂O₃ unfolded into the same FBZ (right). Adapted from T. Zio et al., Sci. 390, 95-99 (2025); DOI: 10.1126/science.adv2582.

The experiments carried out with the SPELEEM microscope at the Nanospectroscopy beamline of Elettra were instrumental in monitoring the growth process in real time and in confirming the material’s elemental composition, electronic structure, purity and crystallinity. Microspot X-ray Photoelectron Spectroscopy (µ-XPS) measurements of B 1s and O 1s emission showed a B/O abundance ratio of 0.60±0.15, consistent with stoichiometry, and indicated a negligible influence of other impurities and of the substrate. Microspot Low-Energy Electron Diffraction (µ-LEED) performed on a single B₂O flake (Fig. 1c) demonstrated a high degree of crystallinity, returning a 2D boria lattice constant of 8.57±0.03 Å. The availability of a micro-focused synchrotron beam was key to perform Microspot Angle-Resolved PhotoEmission Spectra (µ-ARPES) at different photon energies, allowing a complete mapping of the electronic structure in reciprocal space. The extracted Momentum Distribution Curves along high-symmetry direction in the Pt(111) primitive cell were then compared with the calculated band structure unfolded on the same cell (Fig. 1d), yielding a noteworthy agreement that strongly confirmed the validity of the presented 2D B₂O₃ structural model.

The synergy between experimental techniques and numerical simulations was pivotal to this project’s success. With its unique properties – a wide-bandgap semiconductor that is both highly flexible and porous – this material holds immense potential for application in fields ranging from electronics and catalysis to quantum technologies.

T. Zio^1,2†, M. Dirindin^2†, C. Di Giorgio¹, M. Thaler³, B. Achatz³, C. Cepek¹, I Cojocariu^2,4, M. Jugovac^2,4, T. O. Menteş⁴, A. Locatelli⁴, L. L. Patera³, A. Sala^1*, G. Comelli^1,2, M. Peressi^2*, C. Africh¹

¹ CNR – Istituto Officina dei Materiali (IOM), Basovizza, Trieste, Italy
² Department of Physics, University of Trieste, Italy
³ Department of Physical Chemistry, University of Innsbruck, Austria
⁴ Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
^† These authors contributed equally

Contact person emails: ;
Local contacts' emails: