Fig. 5: The contact angle switching rates for TiO2/Si and ZnO/Si heterojunctions as a function of the MOx layer thicknesses.
From: Understanding the light induced hydrophilicity of metal-oxide thin films
Switching rates (\({\tau }^{-1}\)) resulting from fits to Eq. 7 and normalized with the respective illumination intensities (\({{\it{I}}}_{0}\)) plotted vs. the MOx layer thicknesses for the two heterojunctions, TiO2/Si (I), and ZnO/Si (II) are shown as datapoints with error bars. The error bars represent the standard deviation from three samples with a nominal but experimentally verified layer thickness. For both types of junctions, results from illumination by UV light at 365, 300, and 275 nm are shown. For ZnO, the 300 nm LED had too low intensity to trigger switching of the contact angle within a reasonable time. The solid curves represent fits to Eq. 7, after division by \({{\it{I}}}_{0}\) on both sides, where \(w,L\), and \(k/D\) were used as adjustable parameters, and where for each of the heterojunctions, the depletion layer widths \(w\) were set to the same value, whereas the diffusion length \(L\), and quantity \(k/D\), were allowed to vary, both with material and illumination wavelength. \(k/D\) values adjusted to fit the data were the following: \(k/D\)(TiO2/Si, 300 nm) = 3.0 × 10−13 m, \(k/D\)(TiO2/Si, 275 nm) = 4.0 × 10−13 m, \(k/D\)(TiO2/Si, 375 nm) = 3.0 × 10−13 m, \(k/D\)(ZnO/Si, 375 nm) = 1.8 × 10−14 m, and \(k/D\)(TiO2/Si, 275 nm) = 7.0 × 10−15 m. Due to the number of fitting parameters and the uncertainties of the \({\left({{\it{I}}}_{0}\tau \right)}^{-1}\) data, the uncertainty of the fits yielding \(w\), \(L\), and \(k/D\) are estimated to be in the order of ±25%. Vertical error bars represent one STD from three samples, whereas error bars in \(d\) represent measured layer thickness variation (maximum–minimum) from five locations across each wafer.
