STM 16

Novel structures of Gallenene intercalated in epitaxial Graphene

We address the challenge of creating atomically thin layers of non-exfoliable materials like gallenene. The primary motivation is to develop innovative fabrication techniques and investigate the properties of these materials at the 2D limit. Our study employs confinement epitaxy by intercalating gallium atoms beneath an epitaxial graphene (EG) sheet grown on silicon carbide (SiC). This method allows for large-area, ordered 2D material fabrication, with graphene providing crucial environmental protection and air-stability.

Gallium was deposited using Molecular Beam Epitaxy (MBE) on EG/SiC. Subsequent thermal annealing under ultra-high vacuum (UHV) induced gallium intercalation beneath the graphene. The intercalation and the formation of gallium ordered structures is studied via Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED).

Intercalation occurs preferentially at graphene discontinuities and in buffer Layer (BL) areas compared to monolayer graphene (MLG). The intercalated gallium forms μm2-sized islands. These are composed of two levels 1L-Ga and 2L-Ga. The observed height of both levels is 0.2 nm, compatible with single layers of Ga atoms.

Novel superstructures are identified in this system (both on MLG and BL):

  • On 2L-Ga@MLG and 2L-Ga@BL, a striped Gr-5×1 moiré pattern was observed. This moiré is attributed to the superposition of the graphene lattice and the rectangular lattice of the elusive high-pressure Ga(III) phase, which is stabilized by the interaction with graphene and SiC.
  • On 1L-Ga@MLG, a hexagonal Gr-12×12 moiré pattern was found. It emerges from the overlap between graphene and another hexagonal lattice. Atomically resolved STM images suggest it corresponds to the planar projection of b010-gallenene. This 12×12 moiré was also seen in specific areas within the 2L-Ga@MLG phase (2L*-Ga@MLG).
  • A 2×2 reconstruction was observed on 1L-Ga@BL and 3L-Ga@BL, indicating the alternating occupation of Ga atoms in the graphene hollow sites.

Investigations into thermal stability showed the striped Gr-5×1 moiré on 2L-Ga@MLG was stable up to 400 °C, gradually converting to the hexagonal 12×12 moiré (2L*-Ga@MLG) at higher temperatures, indicating a phase transition (from 2L-Ga@MLG to 2L*-Ga@MLG) and structural relaxation. De-intercalation of Ga from MLG is observed at around 800 °C, while it obtained at lower temperatures (~400 °C) from BL. This happened because in BL the phase transition observed in MLG, which requires two carbon layers, is not allowed.

Our findings allow for the creation of a model to explain the intercalation process and the different observed structures. Figure 1 shows a summary of all atomic arrangements investigated and their corresponding superstructures. Thus, this controlled MBE and thermal annealing approach offers a tunable platform for fabricating and investigating the unique properties of gallenene and other 2D intercalated materials.


Figure 1: Schematic representation of the observed atomic arrangement of Ga and the graphene sheets.

Publications:

  1. Emanuele Pompei, Katarzyna Skibińska, Giulio Senesi, Ylea Vlamidis, Antonio Rossi, Stiven Forti, Camilla Coletti, Fabio Beltram, Lucia Sorba, Stefan Heun, Stefano Veronesi: Novel structures of Gallenene intercalated in epitaxial Graphene, arXiv:2505.05042 [cond-mat.mtrl-sci].
  2. Emanuele Pompei, Katarzyna Skibinska, Giulio Senesi, Ylea Vlamidis, Antonio Rossi, Stiven Forti, Camilla Coletti, Fabio Beltram, Silvia Rubini, Lucia Sorba, Stefan Heun, and Stefano Veronesi: Novel Structures of Gallenene Intercalated in Epitaxial Graphene, Small (2025) e05640.

Presented at:

  1. E. Pompei, S. Veronesi, S. Heun: Two-dimensional Gallium Encapsulated in Epitaxial Graphene, MesoSchool 2024, Cargèse, France, 30 September – 12 October 2024 (poster). [Poster]
  2. Emanuele Pompei, Ylea Vlamidis, Giulio Senesi, Katarzyna Skibińska, Antonio Rossi, Stiven Forti, Camilla Coletti, Lucia Sorba, Stefan Heun, Stefano Veronesi: STM investigation of novel structures of Gallenene intercalated in epitaxial Graphene, Workshop on Innovative Microscopy and Spectroscopy of 2D and Related Materials, Pisa, Italy, 14 – 16 May 2025 (oral). [Abstract] [Talk]
  3. Emanuele Pompei, Ylea Vlamidis, Giulio Senesi, Katarzyna Skibińska, Antonio Rossi, Stiven Forti, Camilla Coletti, Lucia Sorba, Stefan Heun, Stefano Veronesi: STM investigation of novel structures of Gallenene intercalated in epitaxial Graphene, European Conference on Innovative and Advanced Epitaxy, Pisa, Italy, 19 – 23 May 2025 (oral). [Abstract] [Talk]
  4. Emanuele Pompei, Katarzyna Skibińska, Giulio Senesi, Ylea Vlamidis, Antonio Rossi, Stiven Forti, Camilla Coletti, Fabio Beltram, Lucia Sorba, Stefan Heun, Stefano Veronesi: STM investigation of novel structures of Gallenene intercalated in epitaxial Graphene, 4th Annual Conference on Global Nanotechnology, Valencia, Spain, 17 – 20 June 2025 (oral). [Abstract] [Talk]
  5. Emanuele Pompei, Katarzyna Skibińska, Giulio Senesi, Ylea Vlamidis, Antonio Rossi, Stiven Forti, Camilla Coletti, Fabio Beltram, Silvia Rubini, Lucia Sorba, Stefan Heun, and Stefano Veronesi: Novel Structures of Gallenene intercalated in Graphene, ESONN’2025, Grenoble, France, 24 August – 06 September 2025 (poster). [Poster]