Nanobubbles at GPa pressure under graphene

Indeed, the study of buried species poses great experimental challenges, mainly arising from the limited bulk sensitivity of most experimental probes. Low energy electron microscopy (LEEM) and synchrotron-based photoemission electron microscopy (XPEEM) measurements at the Nanospectroscopy beamline of Elettra enabled us to monitor the evolution of the interface structure and stoichiometry, yet maintaining sufficiently high sensitivity to subsurface species. The film morphology was quantitatively characterized in complementary STM measurements, carried out at IOM-CNR TASC laboratory, Trieste.

Figure 1 shows room-temperature LEEM and PEEM images of a graphene flake after 0.1 keV Ar⁺ irradiation and subsequent thermal treatment to 1050 °C. In (a), the bright and neutral grey regions correspond to graphene, the dark patch to the bare Ir surface, respectively. The small black dots, highlighted by red circles, correspond to NBs under graphene. In order to assess the interface stoichiometry, we performed X-ray absorption spectromicroscopy (XAS-PEEM) at the Ar L edge. In the XAS-PEEM image shown in (b), the image intensity is proportional to the local Ar concentration. Spectra obtained from inside and outside the red circles are shown in (c). Only the NB spectra display resonances at about 245, 248, and 250 eV, a clear fingerprint of their Ar elemental composition. The remarkable inversion of contrast between the Ar L₃ XAS-PEEM (d) and Ir 4f_7/2 XPEEM (e) images indicates that the substrate core level emission is screened by Ar. The NBs are thus located at the graphene/Ir interface, at variance with softer substrates where Ar segregates into bubbles buried in the metal. Notably, the larger aggregates display a lateral size up to few tens of nanometers. STM measurements indicate that their height may reach several atomic layers, thus including up to few thousand Ar atoms.

In order to unravel the physics governing the ripening of intercalated Ar, ab initio calculations were carried out at ICTP, Trieste, for Ar single atom and mini-clusters (dimer, trimer, tetramer) at the graphene/Ir interface, see Figure 2 (a-g). By calculating the formation energy of Ar mini-clusters under graphene (see e), we demonstrated that there is a drastic net energy gain when two or more protrusions merge into a single, larger one, compared to the energy gain for Ar cluster formation in vacuum. Since the energy gain alone is not sufficient to explain the ripening process, we also verified that intercalated atoms have sufficient mobility. The calculated energy barriers for diffusion on Ir under graphene vary from 40 to 120 meV, which permits Ar aggregation already near room temperature, as confirmed by STM.

Figure 1:Room temperature LEEM image (V_start = 12 V) of a graphene flake after Ar⁺ irradiation (0.1 kV, 150 s at 1.5 × 10^–5 mbar Ar) and subsequent annealing to 1050 °C; the black dots correspond to protrusions in the film. (b) Ar L₃ PEEM of the same region, obtained after subtracting PEEM images acquired at 243 eV (baseline intensity) from images acquired at 247.7 eV (Ar L₃ edge). The intensity of the resulting image is proportional to the Ar concentration, with the bright regions corresponding to Ar clusters. Red circles have been added to facilitate comparison with a. (c) Average XAS-PEEM spectra from regions of interest inside and outside of the red circles. (d) Ar L₃ XAS-PEEM image of another flake along with (e) Ir 4f_7/2 and (f) C 1s XPEEM images; the Ar clusters show up as bright spots in d; the inversion of contrast in (e) is due to screening of the substrate emission by the clusters.

Figure 2. :  Top view of the unit cell used in our ab initio calculations showing graphene (blue spheres) and the upper Ir layer (grey spheres). Side view of the optimized atomic geometries for (b) a single intercalated Ar (red sphere); (c) vertical and (d) horizontal Ar dimers; (f) horizontal trimer and (g) tetramer. The comparison between the energy gain (-∆E)when forming Ar clusters from single Ar atoms at the graphene/Ir interface and in vacuum is shown in (e).