Researchers observe atomic-scale "rainbow scattering" in graphene for first time

Researchers from the University of Duisburg-Essen and Uppsala University have successfully observed rainbow scattering in ion transmission through single-layer graphene. This phenomenon, previously only predicted and simulated, has now been directly measured. The study was led by Carolin Frank, alongside Prof. Marika Schleberger and Prof. Daniel Primetzhofer.

Rainbow scattering, a concept from classical scattering theory, occurs when particles on different trajectories converge at a characteristic angle. This effect, which provides detailed information about interaction potentials, has been seen in contexts like rainbows and nuclear scattering. Applying it to graphene required an extremely clean, defect-free single-layer sample and a high-resolution detector, as even minor contamination could obscure the pattern.

The research team accelerated xenon ions to 40 keV and transmitted them through free-standing single-layer graphene using the Time-of-Flight Medium Energy Ion Scattering (ToF-MEIS) setup at Uppsala University. The scattering pattern revealed two features: a sharp circular outer rainbow at a 5.28° angle from close binary collisions between xenon ions and carbon atoms, and a hexagonal inner rainbow at 0.43°, resulting from ions passing through high-symmetry regions of the graphene lattice.

Carolin Frank, the study's first author, noted that the ultraclean graphene sample and the exceptional resolution of the measurement system were crucial for this observation. The scattering pattern's sensitivity to surface contamination explains why it had not been reported earlier.

The researchers compared their measurements with molecular dynamics and binary collision approximation simulations using various interaction potentials. While the models accurately reproduced the outer rainbow, they deviated from the measured data for the inner hexagonal pattern, especially at small deflection angles near 0°. This discrepancy is attributed to the limitations of radially symmetric potentials, which do not account for non-radially-symmetric chemical bonding and charge exchange effects.

Prof. Marika Schleberger highlighted that graphene serves as both a target material and a sensitive testbed for ion-solid interaction models. The single atomic layer of graphene allows for the detection of subtle differences between experimental results and theoretical predictions.

The study not only confirms a long-predicted effect but also establishes graphene rainbow scattering as a benchmark for refining interatomic potential models in ion-solid interaction physics. The researchers aim to extend their approach to other ion species and two-dimensional materials, such as transition metal dichalcogenides, to further explore the dynamics of energetic charged particles interacting with atomically thin materials.