Diffusion of alkali metal adatoms decorated on graphene dominates its electrical response

Metal adatoms added to graphene’s surface induce new functions in adatom/graphene systems. While there are a collection of ways to perturb graphene surfaces to alter and study the unique electronic behavior, metal surface adsorbates offer particular advantages because of electronic configuration and magnetic properties of adatoms. The potential to enhance spin-orbit coupling or reveal superconductivity are notable examples, where graphene-Li systems already indicate potential for superconductivity.

A detailed study of temperature dependent electrical transport properties of graphene doped with either Li or K adatoms, reported in Applied Physics Letters, shows adatom mobility at temperatures even below 100 Kelvin. This mobility leads to clustering of the adatoms with subsequent “undoping” of electrical charge carriers, as well as reduction of Coulomb scattering from the adatoms.

Density Functional Theory (DFT) calculations predict migration trends of the Li and K systems. At cryogenic temperatures, adatom mobility increases with temperature, facilitating cluster formation by metal adatoms. Shifts in the doped graphene’s Dirac point, even at low temperatures, demonstrated undoping of the adatoms, where electrons are transferred back to the donor atoms. This behavior, in turn, was indicative that the dominant behavior was, in fact, due to clustering that rises along with the increased mobility of the adatoms as temperature increases.

For both the Li and K studies, concentration ranges were sufficiently low that the Dirac point could be traced. Higher concentrations are planned for future study. With experimental modifications, the research team is working to deliver higher doping concentrations during the deposition process inside their cryostat.

Source: “Temperature-dependent transport properties of graphene decorated by alkali metal adatoms (Li, K),” by S. Woo, S. Hemmatiyan, T. D. Morrison, K. D. D. Rathnayaka, I. F. Lyuksyutov, and D. G. Naugle, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.5001080 .