Spatial mapping of potassium diffusion and intercalation in few-layer graphene studied by ultra-high vacuum micro-Raman spectroscopy

Graphite, with its van der Waals layered structure, can accommodate a diverse array of intercalant species within its layers. Alkali metals (AMs), a family of donor intercalants, play a pivotal role in technological advancements, notably in lithium-ion batteries. Owing to its structural simplicity and the feasibility of producing high-quality single crystals spanning areas up to hundreds of micrometers squared, few-layer (FL) graphene serves as an exemplary system for investigating the dynamics of AM adsorption and intercalation. This study focuses on the deposition of potassium atoms in ultra-high vacuum onto mechanically exfoliated four- to five-layer FL and multilayer (ML) graphene. Employing spatially resolved Raman spectroscopy, we examine the impact of potassium adsorption, diffusion, and intercalation. Our findings reveal intricate potassium intake spatial patterns in FL graphene. The distinct spatial inhomogeneities of FL graphene are not observed in ML graphene. Density functional theory calculations also confirmed a complex scenario where both charging and steric hindrance introduce local strain in the graphene layers. Furthermore, charge donation from potassium atoms both to adjacent graphene layers and to more distant layers not adjacent to potassium atoms suggest a modification of the structural and vibrational properties of graphene consistent with the Raman experiments. The detection of intercalation fronts and domains with constant charging, and thus constant potassium densities, underscores the complex and collective nature of the potassium diffusion and intercalation in FL graphene, offering valuable insights for potential applications in energy storage systems.