Tiny “skaters” (Pt atoms) gliding across an almost frictionless “ice rink” (graphene)

It is often intuitive to think of solid matter, characterized by well-defined shapes and geometries, as something static, with its constituent atoms essentially frozen in place. In reality, atoms in a solid are not immobile: besides their vibrational motion, they can migrate within the bulk and, even more readily, across the surface. For instance, this atomic mobility is a key ingredient in the growth of materials with a high degree of crystalline order.

Atomic diffusion is typically governed by temperature: as the latter decreases, atomic motion becomes progressively suppressed, since the energy required for an atom to diffuse is usually provided by thermal excitation. Nevertheless, under specific conditions, atoms can remain mobile even at extremely low temperatures. This occurs when interatomic interactions are sufficiently weak to allow diffusion down to temperatures of just a few tens of Kelvin.

In this work, by combining high-resolution x-ray photoelectron spectroscopy (HR-XPS) experiments with ab initio theoretical calculations, we demonstrate that platinum atoms deposited on epitaxial graphene exhibit unexpectedly high mobility even at very low temperatures. To investigate the diffusion of Pt on the two-dimensional honeycomb carbon lattice, we performed experiments at the SuperESCA beamline. The high photon flux allowed us to acquire Pt 4f_7/2 core-level spectra within a few tens of seconds, even at extremely low coverages corresponding to a few hundredths of a monolayer, while maintaining an energy resolution of 50 meV. This capability enabled us to track subtle changes in the spectral line shape with high accuracy.

To slow down atomic motion and make it experimentally accessible, deposition and measurements were carried out at a temperature of 45 K (see Fig. 1a). Under these conditions, we were able to follow in real time the evolution of different atomic populations: isolated Pt atoms, characterized by a distinct spectroscopic fingerprint, as well as paired atoms (dimers) and larger atomic clusters. On time scales of a few hundred seconds, the population distribution undergoes a dramatic transformation, as shown in Fig. 1b, which can be quantitatively explained by a kinetic model based on atomic diffusion. This time-resolved spectroscopic approach allowed us to extract the activation barrier for Pt atom diffusion, which we determine to be E_exp = 128 ± 6 meV. This value implies that, at room temperature, a Pt atom hops from one graphene lattice site to the next about ten billion times per second.

Figure 1.  (a) Schematic representation of the experiment. Deposition of Pt atoms (blue) at 45 K on graphene (red) epitaxially grown on Ir(111) (gray). (b) Temporal evolution of the coverages of the various platinum species on the surface, including Pt intercalated at the defects. The best-fit solution from the differential equations used to describe the diffusion process is also shown. (c) DFT calculated structural models corresponding to Pt monomers, dimers, trimers, tetramers and heptamers.

The experimental results were supported by density functional theory calculations, which map the full energy landscape for the diffusion of different Pt species (monomers, dimers, trimers, tetramers and larger clusters) on graphene (Fig. 1(c)) and show excellent agreement with the experimental findings (E_theo = 130 meV). The calculations further revealed the crucial role played by defects in the graphene lattice, such as double vacancies and domain walls, in trapping single Pt atoms and hindering their diffusion.

The combination of high temporal resolution and chemical sensitivity achieved in this type of measurement can be extended to the investigation of atomic-scale dynamics in many other elements whose core-level linewidths are intrinsically narrow, in particular 4d and 5d transition metals. In this way, the high energy resolution of the SuperESCA beamline can be fully exploited.

Atomic diffusion, the mechanism underlying the observations reported in this study, plays a fundamental role in processes including nanoparticle formation, catalyst ageing, and the self-assembly of nanostructures on surfaces, with implications for a wide range of technologically relevant systems. Ultimately, this work does not merely track the motion of tiny “skaters” (Pt atoms) gliding across an almost frictionless “ice rink” (graphene), where interatomic interactions are nearly negligible. Rather, it provides direct, quantitative insight into the microscopic mechanisms that govern matter at the atomic scale.

Andrea Berti¹, Ramón M. Bergua², Jose M. Mercero², Deborah Perco¹, Paolo Lacovig³, Silvano Lizzit³, Elisa Jimenez-Izal² and Alessandro Baraldi^1,3

¹ Dipartimento di Fisica, Università di Trieste, Trieste, Italy
² Euskal Herriko Unibertsitatea & Donostia International Physics Center, Donostia, Euskadi, Spain
³ Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy

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