(615d) Optical Properties of Titanium Dioxide Nanotube Arrays

Nanostructured metal oxides are promising substrates for several ‘next generation’ photovoltaic and photo-electrochemical cell architectures like DSSC’s, organic-inorganic solar cells and all inorganic solar cells. Metal oxides generally have advantages like long term stability, low cost and in materials like ZnO or TiO₂, relatively long carrier lifetimes and carrier mobility. When designing solar cells or photonic devices that incorporate nanostructured metal oxides it is important to have good models for how light is reflected, scattered, transmitted and absorbed in these structures.  

We fabricated titanium dioxide nanotube arrays with electrochemical anodization and studied the reflection and transmission of light from the UV to the NIR through these arrays. Anodic nanotube arrays consist of open tubes (with diameters ~ 100 nm) on one end and capped or close tubes on the other end. Our measurements revealed that the total reflectance from the open nanotube array end is significantly higher (15% at 400 nm) than the reflectance from the closed nanotube end for short wavelengths (blue and UV light). This anomalous behavior is not easily explained by simple classical optical models. Attempts to explain the data by several models including theoretical models based on the eikonel approximation, photonic crystal band theory and disorder induced scattering will be presented. Disorder induced scattering appears to be the primary factor causing the anomalous behavior at short wavelengths.