Quantum dots suppress electron scattering resonances in graphene

Scattering resonances impact optoelectronic properties of 2D materials, particularly graphene, and suppressing resonances is a goal for these applications. A team of German researchers systematically studying scattering resonances, both in epitaxial graphene grown on different substrates and nanopatterned graphene, found that depositing a superlattice of small quantum dots on graphene eliminates the resonances completely. Their results are reported in the journal Applied Physics Letters.

Solid state physicist and coauthor Andrei Varykhalov and his team used angle-resolved photoelectron spectroscopy, an unconventional approach for investigation of scattering resonances. While secondary photoemitted electrons are essentially structureless, for graphene, researchers observed very well-pronounced, sharp and clearly dispersing minima of photoemission intensity. These gaps coincide with scattering resonances under investigation, which are naturally hosted above the Fermi level in the electronic structure of graphene and therefore cannot be accessed via occupied bands in photoemission. While indirect observation of unoccupied states in material via quantum-mechanical coupling between initial and final states of photoexcited electrons is not new, this was the first time it was applied to studying the scattering resonances of graphene.

The research team encountered two surprises in their results. Based on theoretical work, they expected that the electronic hybridization between graphene and its substrate could suppress the scattering resonances, but were surprised to find out it does not. The second surprise was that the deposition of tiny metallic nanoclusters (iridium (Ir) quantum dots) in very low concentrations suppressed the resonances immediately. According to Varykhalov, this indicates that the planar atomic structure of graphene is the principle factor determining the existence or suppression of the scattering resonances. Indeed, deposition of even small Ir nanoclusters perturbs 2D geometry of chemical bonds in graphene, eliminating the scattering resonances.

Source: “Suppression of electron scattering resonances in graphene by quantum dots,” by M. Krivenkov, D. Marchenko, J. Sánchez-Barriga, O. Rader, and A. Varykhalov, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4994193 .