Anomalous Optoelectric Properties of an Ultrathin Ruthenium Film with a Surface Oxide Layer for Flexible Transparent Conducting Electrodes

The growing industrial demand for flexible optoelectric devices has led to intensive researches on highly flexible transparent electrode materials such as graphene, reduced graphene-oxide (r-GO), Ag-nanowire, and 2D metal oxides. However, except Ag-nanowire, transparent electrode materials having optoelectric properties comparable to that of indium–tin–oxide (ITO) have not yet been developed. In this study, an ultrathin ruthenium film with a ruthenium oxide (RuO₂) subsurface layer has been introduced as a flexible transparent electrode. The metallic Ru thin film is fabricated from a RuO₂ nanosheet using a layer-by-layer coating technique, followed by thermal reduction. The thin film (≈6 nm) reveals comparable sheet resistance and transmittance as that of conventional ITO electrodes. The high transmittance (≈79%) of the metal thin film in the visible range is attributed to the presence of an oxide subsurface layer which acts as antireflection. The Ru film (with oxide subsurface layer) with figure-of-merit ≈3.4 × 10⁻⁴ Ω⁻¹ shows the best performance among the thin films fabricated using a wet-chemistry process with 2D nanosheets including graphene, r-GO, and other metal oxides. In addition, the high mechanical flexibility of Ru thin film makes it next-generation flexible transparent conducting electrodes, beyond graphene, r-GO, and 2D metal oxides.