Ultra-fast atomic diffusion on graphene
Ultra-low atomic diffusion barrier of Pt atoms on graphene was determined by combining High-Resolution XPS and DFTA. Berti et al., ACS Nano in press
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Understanding how single atoms diffuse on two-dimensional materials is crucial for designing next-generation catalysts and nanoscale devices. In this work, we combine fast high-resolution X-ray photoelectron spectroscopy (HR-XPS) and density functional theory (DFT) to directly measure the atomic diffusion barrier of platinum (Pt) adatoms on epitaxial graphene grown on Ir(111). Their approach provides chemical sensitivity, temporal resolution, and noninvasive monitoring of atomic-scale dynamics that have so far remained experimentally elusive. Pt atoms were deposited at 45 K with submonolayer coverage (0.04–0.07 ML) to ensure the predominance of single atoms. HR-XPS measurements performed at the SuperESCA beamline tracked in real time the evolution of the Pt 4f7/2 core-level components associated with monomers, dimers, and larger clusters. DFT simulations reproduced the adsorption energies and core-level shifts (CLS) for Pt monomers, dimers, and small clusters. Monomers prefer bridge sites between C atoms with adsorption energy −2.02 eV. Planar dimers are slightly less stable but display a CLS of +0.15 eV, matching experiment. Three-dimensional clusters exhibit CLS values between +0.3 and +0.7 eV, accounting for the broad experimental component attributed to aggregates. A further spectral feature at +1.0 eV, unexplained by these configurations, was linked to Pt atoms intercalated through graphene defects such as Stone–Wales or double vacancies. DFT shows that these defects facilitate Pt penetration beneath graphene, where intercalated atoms exhibit CLS values from +0.9 to +1.4 eV and strong Pt–Ir bonding, corroborated by the modified Ir 4f spectrum.By modeling the time evolution of spectral components, we quantified the populations of Pt species and extracted rate constants consistent with thermally activated diffusion dominated by mobile monomers. Dimers and larger clusters remain immobile due to much higher barriers (~270 meV). The kinetic model accurately reproduces the experimental decay of monomers and the rise of dimers, validating the derived energy parameters. A rapid decrease in the monomer peak accompanied by the growth of higher binding energy components indicated adatom aggregation. Fitting the spectral evolution with a kinetic model revealed an experimental diffusion barrier of 128 ± 6 meV, which matches the 130 meV value predicted by nudged elastic band calculations. This barrier confirms that Pt atoms on graphene experience exceptionally low diffusion energy, consistent with fast surface mobility even at cryogenic temperature. |
This combined experimental–theoretical framework establishes a general strategy for probing ultralow diffusion barriers on weakly interacting supports such as graphene and other 2D materials. The findings also clarify the early stages of Ostwald-ripening sintering and intercalation phenomena relevant to single-atom catalysts and nano-heterostructures.
In conclusion, the work introduces fast HR-XPS as a quantitative, chemically selective probe of surface diffusion, capable of resolving energy barriers as low as 0.1 eV. The agreement between experiment and theory demonstrates that even minute diffusion processes on 2D supports can now be captured in real time, opening routes to control adatom mobility, cluster nucleation, and the fabrication of ordered superlattices for catalytic and electronic applications. Ultra-Low Atomic Diffusion Barrier on Two-Dimensional Materials: The Case of Pt on Epitaxial Graphene Andrea Berti, Ramón M. Bergua, Jose M. Mercero, Deborah Perco,Paolo Lacovig, Silvano Lizzit, Elisa Jimenez-Izal, and Alessandro Baraldi ACS Nano, DOI: 10.1021/acsnano.5c13305 |
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