Modification of thermal and electronic properties of bilayer graphene by using slow Na⁺ ions

Bilayer graphene (BLG) has an extensive list of industrial applications in graphene-based nanodevices such as energy storage devices, flexible displays, and thermoelectric devices. By doping slow Na⁺ ions on Li-intercalated BLG, we find significantly improved thermal and electronic properties of BLG by using angle-resolved photoemission and high-resolution core level spectroscopy (HRCLS) with synchrotron photons. Our HRCLS data reveal that the adsorbed Na⁺ ions on a BLG produced by Li-intercalation through single layer graphene (SLG) spontaneously intercalate below the BLG, and substitute Li atoms to form Na-Si bonds at the SiC interface while preserving the same phase of BLG. This is in sharp contrast with no intercalation of Na⁺ ions on SLG though neutral Na atoms intercalate. The Na⁺-induced BLG is found to be stable upon heating up to T = 400 °C, but returns to SLG when heated at T _d = 500 °C. The evolution of the π-bands upon doping the Na⁺ ions followed by thermal annealing shows that the carrier concentration of the π-band may be artificially controlled without damaging the Dirac nature of the π-electrons. The doubled desorption temperature from that (T _d = 250 °C) of the Na-intercalated SLG together with the electronic stability of the Na⁺-intercalated BLG may find more practical and effective applications in advancing graphene-based thermoelectric devices and anode materials for rechargeable batteries.