Alkali Metal Doped Graphene: Superconductivity, Structural, Magnetic and Optical Properties

Intercalation of graphite with alkali metals has previously been shown to, in some cases, produce superconducting compounds from the two non-superconducting components. The use of graphene as a basis to continue this research offers new possibilities as confinement of intercalant species is reduced from bulk graphite. Papers comprised of exfoliated graphene flakes were doped with Li, Cs, K and Ca atoms via vapour transport methods in order to investigate superconducting properties. While Li, Cs and K-doping showed no signs of a superconducting transition as low as 1.8 K, observed through magnetic measurements, Ca-doped graphene became superconducting below 6.4 K ? a lower transition temperature than Ca-doped graphite, TC = 11.5 K. The carrier concentration could also be changed using composite papers made from graphene and various proportions of insulating boron nitride flakes, allowing TC to be varied. Optical reflectivity spectra were used to determine the level of doping present in each compound, directly calculated from their estimated plasmon energy. Ca-doped graphene paper exhibited a 20% lower carrier concentration than Ca-intercalated graphite, offering an explanation for the lower value of TC. To allow insight into the partial doping of graphene papers, samples were exposed to air and monitored via dynamic x-ray diffraction techniques and optical analysis during degradation. With prolonged reaction in air, the carrier concentration was found to drop monotonically, while the interlayer separations contracted as intercalant species vacated the structure, leaving an arrangement of flakes similar to that of the initial, un-doped, graphene paper. The range of carrier concentrations observed suggests that doping of graphene flakes is non-discrete, thus implying tunable TC.