Ultra-low energy ion implantation of graphene: substitutional and intercalated atoms

Functionalizing two-dimensional (2D) materials typically involves the modification of their physical and chemical properties. In this context, several approaches are being explored: interaction with various substrates, creation of lattice defects (e.g., vacancies), addition of foreign atoms, clusters or molecules in adsorbed or intercalated forms, substitutional doping, among others. This thesis focuses on the incorporation of substitutional and intercalated elements. Previously used approaches face various challenges, in particular, the difficulty to control the concentration and form of incorporation. In this thesis, we explored an alternative approach, i.e., to incorporate the foreign atoms using ultra-low energy (ULE) ion implantation. We focused on two model cases: (i) formation of nanobubbles by intercalation of noble gas elements (He, Ne, Ar); (ii) substitutional incorporation of Mn as a model magnetic dopant. Our approach is based on a wide range of experimental methods, including synchrotron-based X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), Raman spectroscopy, and scanning tunneling microscopy (STM), among others.

The first part of this thesis deals with the formation of nanobubbles on graphene with a radius of the order of 1 nm, using ultra-low energy implantation of noble gas ions (He, Ne and Ar) into graphene grown on Pt(111) (Gr/Pt). We show that the universal scaling of the aspect ratio (height divided by the radius), which has previously been established for larger bubbles (with radius of few nm and higher), no longer applies when the bubble radius approaches 1 nm, as the bubble height converges to a minimum value corresponding to an atomic monolayer. Moreover, we observe that the bubble stability and aspect ratio depend on the substrate onto which the graphene is grown (bubbles are stable for Pt but not for Cu) and trapped element (the aspect ratio behaves differently for He, Ne and Ar). We discuss these dependencies in terms of the role of the adhesion energies between the three constituents: graphene, substrate and noble gas atoms. The high van der Waals pressures inside the bubbles (predicted to exceed 30 GPa for a radius of ∼ 0.5 nm), illustrate the unique characteristics of this nanometer-sized bubble regime, compared to the previously studied (larger) nanobubbles. In this context, we also studied the disorder induced on graphene by the ULE ion implantation of He, Ne and Ar, for implantation energies between 15 eV and 40 eV. Our data strongly suggest the existence of two main types of defects and associated formation mechanisms: vacancy-related disorder as well as bond-breaking damage. Vacancy-related disorder occurs at sufficiently high energies, i.e., when the kinetic energy that is transferred between an impinging ion and the C atom with which it collides overcomes the threshold displacement energy (∼ 22 eV). In addition, a bond-breaking damage mechanism was also identified, which is also present at lower energies.

The second part of the this thesis is devoted to substitutional Mn doping. We study the incorporation of substitutional Mn atoms in high-quality, epitaxial graphene on Cu(111) (Gr/Cu), using ultra-low energy ion implantation. We characterize in detail the atomic structure of substitutional Mn in a single carbon vacancy and quantify its concentration. Regarding electronic properties, we show that graphene doped with substitutional Mn to a concentration of the order of 0.04%, with negligible structural disorder (other than the Mn substitution), retains the Dirac-like band structure of pristine Gr/Cu, making it an ideal system in which to study the interplay between local magnetic moments and Dirac electrons.

Our work also establishes that ultra-low energy ion implantation is well-suited for intercalation and substitutional magnetic doping of graphene. Given the flexibility, reproducibility and scalability inherent to ion implantation, our work creates numerous opportunities for research on functionalization of graphene and other 2D materials.