Optical anisotropy in vertically oriented TiO₂nanotube arrays

Abstract

Nanofabricated optically anisotropic uniaxial thin films with deep submicron feature sizes are emerging as potential platforms for low-loss all-dielectric metamaterials, and for Dyakonov surface wave-based subwavelength optical confinement and guiding at interfaces with isotropic media. In this context, we investigate the optical properties of one such uniaxial platform, namely self-organized titania nanotube arrays (TNTAs) grown by the bottom-up nanofabrication process of electrochemical anodization on silicon wafer substrates, and subsequently annealed at different temperatures, i.e. 500 °C and 750 °C. We performed detailed quantitative analysis of the structure of the TNTAs using x-ray diffraction and Raman spectroscopy, which revealed a measurable phonon confinement in TNTAs annealed at 500 °C. Variable angle spectroscopic ellipsometry was used to investigate the optical anisotropy in two kinds of TNTAs-those constituted by anatase-phase and those containing a mixture of anatase and rutile phases. Both kinds of TNTAs were found to have positive birefringence (Δn) exceeding 0.06 in the spectral region of interest while mixed phase TNTAs exhibited Δn as high as 0.15. The experimentally measured anisotropy in the refractive index of the TNTAs was compared with the predictions of two different effective medium approximations incorporating the uniaxial geometry. The measured value of Δn for TNTAs exceeded that of bulk anatase single crystals, indicating the potential of nanostructured dielectrics to outperform dielectric crystals of the same material with respect to the magnitude of the achievable directional refractive index contrast.

TiO2 Nanotube Arrays on Flexible Kapton Substrates for Photo-Electrochemical Solar Energy Conversion

“…The plot (inset) shows Raman peaks corresponding to the TiO 2 rutile phase at 270 and 607 cm –1 , while that of the anatase phase at 152 and 424 cm –1 . These observations are consistent with the Raman spectra reported for TiO 2 nanotubes grown on Kapton and Si wafer substrates. , Furthermore, the Raman spectra also support the XRD data reported for the samples, in which both rutile and anatase phases were observed.…”

“…These observations are consistent with the Raman spectra reported for TiO 2 nanotubes grown on Kapton and Si wafer substrates. 38,39 Furthermore, the Raman spectra also support the XRD data reported for the samples, in which both rutile and anatase phases were observed. The FTIR spectra of the TiO 2 nanotubes before and after the solvothermal process are shown in Figure 9.…”

Niobium–Zirconium Oxynitride Nanotube Arrays for Photoelectrochemical Water Splitting

“…Those short nanotubes are very beneficial in ensuring fast charge carrier transport properties. , Liu et al concluded that the shorter the nanotubes, the faster the electron transport process and the higher photocatalytic performance. However, the observed increase in the fabricated nanotube diameter can be related to the unusual feature of the EG-based electrolytes that result in open and large pore size nanotube arrays. − The cross-sectional images of the samples anodized at voltages below or higher than 50 V showed no tubular formation, revealing that the voltage is another key factor for the formation of NTs (Figure S2). Moreover, it was noticed that increasing the anodization time resulted in a rubble layer on the surface of the NTs.…”

Environmental Applications of Engineered Materials with Nanoconfinement

“…66 Some semiconductor metal oxides, typically TiO 2 , have also been employed, owing to their ability to absorb light and drive photocatalysis. Examples include uniaxial TiO 2 nanotube arrays (pore diameter of 50 nm) to attain anisotropic optical properties 67 and TiO 2 -rGO composite layers (pore diameter of 95 nm) for efficient solar energy conversion. 68 One widely studied material is anodized aluminum oxide with a honeycomb-like structure, synthesized via electrochemical dissolution.…”