How fast HR-XPS revealed the astonishing mobility of platinum atoms on graphene

When we think of atoms sitting on a surface, we tend to imagine them as fairly still, especially at very low temperatures (colder than liquid nitrogen, in fact). Yet in modern surface science we often discover the opposite: atoms can be surprisingly restless, gliding from place to place in ways that shape how materials grow and how catalysts work. A striking recent example comes from a study ¹ of platinum atoms on epitaxial graphene (a single, perfectly ordered layer of carbon atoms grown on an iridium crystal), where the atoms turned out to move far more easily than anyone expected. Understanding this extraordinary mobility required not just clever theory but an advanced experimental tool capable of watching atoms organize themselves almost in real time. The hero of this story is fast, high-resolution X-ray photoelectron spectroscopy (fast HR-XPS for short).

The researchers deposited extremely small amounts of platinum onto a pristine sheet of graphene. By using such tiny coverages, they ensured that the behavior of individual atoms and the earliest stages of clustering could be studied before the surface became crowded. Experiments were carried out at temperatures as low as 35–80 K to slow everything down (yet the atoms still moved rapidly). Single atoms are far too small to see directly with ordinary microscopes, and their movements happen too quickly for many traditional techniques. This is where fast HR-XPS shines: it detects subtle changes in the electronic environment of platinum atoms, which produce slightly different “signatures” in the photoelectron spectrum depending on whether an atom is isolated, paired with another (a dimer), or part of a larger cluster. By collecting these spectra rapidly and repeatedly (sometimes many per second), the researchers could essentially watch the platinum population evolve in real time.

A full kinetic reconstruction

The picture that emerged was astonishing. Shortly after deposition, most platinum existed as isolated single atoms. But within seconds to minutes, the spectral signatures of single atoms faded while those from dimers and small clusters grew dramatically. The transformation happened orders of magnitude faster than expected on a surface as chemically inert as graphene. The only possible explanation was that individual platinum atoms were gliding across the graphene sheet with exceptional ease, colliding and sticking together long before the sample warmed up. This speed pointed to an ultra-low diffusion barrier (the tiny energy hill an atom must overcome to hop to a neighboring site) of just 40–70 milli-electronvolts (0.04–0.07 eV) from the experiments, with theory suggesting it could be even lower (around 0.05 eV or less).

What makes the result so convincing is how fast HR-XPS enabled a full kinetic reconstruction of the aggregation process. The technique doesn’t just take static snapshots; its high time resolution and chemical sensitivity reveal exactly how the populations of monomers, dimers, and clusters change over time. From those dynamics, the diffusion barrier and mobility of single atoms can be extracted with remarkable precision. Without this rapid, element-specific monitoring (made possible by synchrotron X-ray sources), the earliest stages of platinum clustering on graphene would have remained invisible.

Theory played a crucial complementary role. Density-functional theory calculations mapped the energy landscape experienced by a platinum atom on graphene and confirmed an almost perfectly flat potential (virtually no hills to climb) in excellent agreement with experiment. But theory alone could never capture the live dynamics; only fast HR-XPS could follow the atoms as they raced across the surface and merged.

Form, age, assemble

The way atoms diffuse governs how nanoparticles form, how catalysts age, and how ordered nanostructures self-assemble. If atoms move this freely, keeping them isolated for single-atom catalysis becomes extremely challenging, but the same rapid motion can be harnessed to create uniform clusters or direct atoms to specific trapping sites. Knowing the precise mobility lets researchers tailor conditions for the outcome they want.

The broader significance extends far beyond platinum and graphene. Many next-generation technologies (from more efficient fuel-cell catalysts to quantum devices) rely on metals interacting with ultrathin two-dimensional supports. The earliest moments of atom deposition and aggregation often dictate the final nanostructure, yet those moments are usually the hardest to observe. Fast HR-XPS opens a rare window into this hidden world and is now being applied to other metals (Pd, Rh, Au…) and 2D materials (hBN, MoS₂, etc.), promising better control over tomorrow’s nanomaterials.

Platinum atoms on epitaxial graphene behave like tiny skaters on an almost frictionless rink (even at temperatures where most atomic motion should be frozen). Their astonishing agility was uncovered not by accident, but by a technique perfectly suited to track their movements with chemical precision and split-second timing. Fast HR-XPS has turned what was once an invisible, fleeting process into something measurable, understandable, and ultimately controllable.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.