Transient Optical and Terahertz Spectroscopy of Nanoscale Films of RuO2

Solution-deposited nanoscale films of RuO2 (nanoskins) are effective transparent conductors once calcined to 200 A degrees C. Upon heating to higher temperatures, the nanoskins show increased transmission at 550 nm, indicating changes in the optical behavior of the films. Electronic microscopy and X-ray diffraction show that the changes in the optical spectrum are accompanied by the formation of rutile RuO2 nanoparticles. The mechanism for the spectral evolution is clearly observed with ultrafast optical measurements. Following excitation at 400 nm, nanoskins calcined at higher temperatures show increased transmission above 650 nm, consistent with the photobleaching of a surface plasmon resonance (SPR) band. Calculations based on the optical constants of RuO2 substantiate the presence of SPR absorption. Sheet resistance and transient terahertz photoconductivity measurements establish that the nanoskins electrically de-wire into separated particles. The plasmonic behavior of the nanoskins has implications for their use in a range of optical and electrochemical applications.