Optical Properties of Colloidal Indium Tin Oxide Suspended in a Thermal Fluid

We investigated changes in the optical properties of indium tin oxide (ITO) nanocrystals as a function of temperature. ITO is a plasmonic, wide-band-gap semiconductor capable of absorbing UV and IR radiation. The nanoparticles were made using a standard two-pot synthesis, which resulted in the formation of ITO nanoparticles with myristic acid and oleylamine as surface passivation ligands. Ligand exchange with (6-7% aminopropylmethylsiloxane)-dimethylsiloxane copolymer allowed dispersion of the particles into a thermal fluid, Duratherm S, which is capable of withstanding working temperatures of 300 °C. Nanoparticles suspended in the fluid were heated over a range of temperatures, and their properties were investigated using IR spectrometry, UV-vis, transmission electron microscopy, and particle size analysis. Although heating showed no change in particle size, spectral changes were clearly observed in the form of a strong blue shift and increased absorbance. These temperature-dependent changes in the spectral properties of ITO were attributed to various factors such as: (i) distribution of tin dopant throughout the nanocrystal to increase the carrier concentration, (ii) removal of interstitial oxygen to reduce electron scavenging, (iii) increased order in the In₂O₃ structure to enhance carrier mobility, and (iv) reduction in electron traps of tin-oxygen complexes.