Thermal Dynamics of Graphene-Seals

<u>Hjalte R. Ambjørner¹</u>, <u>Anton S. Bjørnlund¹</u>, <u>Tobias G. Bonczyk²</u>, Edwin Dollekamp², Lau M. Kaas¹, Sofie Colding-Fagerholt¹, Kristian S. Mølhave³, Christian D. Damsgaard³, Stig Helveg¹, Peter C. K. Vesborg^1,2*<br/> <br/>¹Center for Visualizing Catalytic Processes (VISION), Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark <br/>²Surface Physics and Catalysis (SURFCAT), Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark<br/>³National Centre for Nano Fabrication and Characterization (Nanolab), Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark<br/> <br/> <br/>Electron microscopy studies of dynamic processes in technological relevant gas environments have advanced considerably. Such <i>in situ</i> and <i>operando</i> observations can nowadays be obtained with atomic-scale resolution by confining the reactive environments between thin, electron-transparent membranes (1-2). While Si-based membranes are widely used, their several-nanometer-thicknesses impact the electron microscopy data and hamper quantitative image interpretations at the atomic-scale. To suppress this footprint of the membranes, it would be desirable to employ two-dimensional materials, such as graphene, that represent the thinnest imaginable membrane with nearly ideal molecular impermeability (3). In practise, however, graphene must be supported by solid substrates which establish graphene-substrate interfaces that compromise the impermeability of the graphene-seal (3-6).<br/>Here, we provide a kinetic study of the gas leakage from SiO₂-based cavities sealed by few-layer-graphene (5-6). Specifically, we focus on electron energy loss spectroscopy (EELS) measurements to unambiguously probe the Ar content of sealed cavities as a function of time and temperature. The EELS data show that the gas content decreased exponentially with time and that the temporal decay constant followed an Arrhenius-like temperature dependency with an apparent activation energy on the order of 0.4 eV. Thus, these findings indicate that gas diffusion along the graphene-SiO₂ interface is a thermally activated process. Surprisingly, successive heating cycles of the cavities modified the Arrhenius dependency and reduced the Ar leakage at room temperature considerably, with up to six orders of magnitude as compared to leak-rates equivalent to those reported in literature. Thus, the thermal dynamics of the graphene-seals can be used to engineer the interface leakage and perfect two-dimensional materials as electron-transparent windows in miniaturized reactors for <i>in-situ</i> electron microscopy (6).<br/>J.F. Creemer et al., <i>Ultramicroscopy</i> <b>108</b>, 993 (2008).<br/>S.B. Vendelbo et al., <i>Nat. Mater.</i> <b>13</b>, 884 (2014).<br/>J.S. Bunch et al., <i>Nano Lett.</i> 13, 2458 (2008).<br/>P.Z. Sun et al., <i>Nature</i> <b>579</b>, 229 (2020).<br/>Y.X. Liu et al., J. Phys. Chem. 157, 191101 (2022).<br/>H.R. Ambjørner et al., submitted (2023).