Molecular Orientation of −PO<sub>3</sub>H<sub>2</sub> and −COOH Functionalized Dyes on TiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, ZrO<sub>2</sub>, and ITO: A Comparative Study
Dhruba K. Pattadar, Lianqing Zheng, Alex J. Robb, Drake Beery, Wei Yang, Kenneth Hanson, S. Scott Saavedra
Abstract
Modifying metal oxides (MO x ) with organic monolayers is widely employed to tailor interfacial properties in organic electronic devices and dye-sensitized solar cells. The effects of modification are frequently assessed by performing experiments on model monolayer|MO x interfaces, where an “inert” MO x (e.g., Al 2 O 3 ) is used as a control for an “active” MO x (e.g., TiO 2 ). An underlying assumption in these studies is that the inert and active monolayer|MO x structures are similar. This assumption was examined here. Using UV–vis attenuated total reflection spectroscopy, we measured the mean tilt angle of 4,4′-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid ( A1P ) adsorbed on indium tin oxide (ITO), TiO 2, ZrO 2, and Al 2 O 3 . When the surface roughness of the MO x substrate and the surface coverage (Γ) of the A1P film were constant, the orientation of A1P was the same. 4,4′-(Anthracene-9,10-diyl)bis(4,1-phenylene)dicarboxylic acid ( A1C ) was adsorbed on the same set of MO x substrates. The orientation of A1C and A1P on ITO was the same, which is likely due to the intermolecular interactions resulting from the high and approximately equal Γ of both films. Comparing A1C films at equal Γ on TiO 2 and Al 2 O 3 with equal surface roughness, again, the orientation was the same. MD simulations of A1C and A1P on TiO 2 produced nearly identical tilt angle distributions, supporting the experimental findings. This study provides the first experimental validation of the assumption that the monolayer|MO x structure is the same regardless of the nature of the metal oxide substrate.