Transient Optical and Terahertz Spectroscopy of Nanoscale RuO2
Edwin J. Heilweil, Brian G. Alberding, Paul D. Cunningham, Joseph S. Melinger, Jeffrey C. Owrutsky, Adam D. Dunkelberger
Solution-deposited nanoscale films of RuO2, nanoskins, are effective transparent conductors because after calcining to 523 K they exhibit flat optical extinction across the visible and infrared combined with relatively high electrical conductivity. Upon heating the nanoskins to higher temperature, the optical properties change. They acquire a green appearance due to reduced extinction in a narrow region near 550 nm, but the spectral evolution is more clearly observed in ultrafast transient optical measurements. Following excitation at 400 nm, RuO2 nanoskins calcined at or below 573 K show decreased transmission throughout the visible. However, nanoskins calcined at or above 523 K show increased transmission for wavelengths above 650 nm. These results indicate that a new absorption feature grows in at higher calcining temperature and the increased transmission is attributed to photobleaching of this feature. We assign this feature to a lcalized surface-plasmon resonance (LSPR) band that develops as the film ripens at higher temperature into more crystalline RuO2 nanoparticles. Transient terahertz (THz) measurements of electron conductivity, X-ray diffraction, and electron microscopy support that the nanoskins calcined at higher temperatures are more metallic and comprise crystalline nanoparticles. The spectral position of the observed bleach is consistent with that expected for LSPR bands of crystalline RuO2 nanoparticles based on reported optical constants. The formation of these nanoparticles upon calcining is responsible for the transmission band in the extinction spectrum of RuO2 nanoskins.