Approaching the limits of transparency and conductivity in graphitic materials through lithium intercalation

Abstract

Various band structure engineering methods have been studied to improve the performance of graphitic transparent conductors; however, none has demonstrated an increase of optical transmittance in the visible range. Here we measure in situ optical transmittance spectra and electrical transport properties of ultrathin graphite (3–60 graphene layers) simultaneously during electrochemical lithiation/delithiation. On intercalation, we observe an increase of both optical transmittance (up to twofold) and electrical conductivity (up to two orders of magnitude), strikingly different from other materials. Transmission as high as 91.7% with a sheet resistance of 3.0 Ω per square is achieved for 19-layer LiC6, which corresponds to a figure of merit σdc/σopt=1,400, significantly higher than any other continuous transparent electrodes. The unconventional modification of ultrathin graphite optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques enable investigation of other aspects of intercalation in nanostructures. Graphene-based materials have potential as transparent electrodes, but still fall short of desired performance goals. Here, Bao et al.report that upon intercalation of lithium into few-layer graphene, desired sheet resistance and optical transmittance may be achieved.