Spectroscopic ellipsometry study on refractive index spectra of colloidal β-FeOOH nanorods with their self-assembled...

We prepared monodisperse colloidal β-FeOOH (square prismatic crystalline) nanorods. Self-assembled dry thin films of the rods were formed on quartz substrates by evaporating drops of repulsive-rod suspensions. Using spectroscopic ellipsometry, the complex refractive index spectra of the β-FeOOH nanorod films were deduced, which are further corrected on the basis of absorption spectra measured for β-FeOOH nanorod suspensions, with aid of the Kramers-Krönig relation and an optical two-phase mixing model. Simultaneously, in this correction process, the rod volume fraction in the films and the complex mean refractive index spectra of purely the rod were extracted together. The real part of the refractive index spectra of the β-FeOOH rod, n'(c)(λ), shows a maximal value of 2.365 (aver.) at 410 nm (λ(m)) (hence a normal dispersion at λ > λ(m) and an anomalous dispersion at λ < λ(m)). At λ = 589.3 nm (λ(D) for "sodium light"), the n'(c)(λ) value is consistent with refractive indices predicted by Gladstone-Dale's and Anderson's refractivity formulas with the density of the β-FeOOH rod, 3.638 ± 0.104 g/cm(3), measured in this work.