(Invited) Use of Periodic Pulses and Non-Native Substrates to Form Nanoporous Metal Oxide Films By Anodization
Authors: Shankar Karthik 
Published: February 2020

Electrochemical anodization is a robust and versatile fabrication method to form arrays of ordered nanopores and nanotubes in a number of metal oxides including Al2O3, TiO2, Ta2O5, Nb2O5, Fe2O3, ZnO, WO3, NiO and ZrO2 [1]. The key advantages of anodization are simplicity, low cost, solution-based processing, large area compatibility and mass-producibility. The anodic formation of oriented and aligned nanostructures has also been extended to metal chalcogenides [2]. A majority of the aforementioned metal oxide and chalcogenide compounds exhibit semiconducting behavior. Consequently, the anodically formed nanostructures in these compounds are high surface area semiconductors (typically after a crystallizing anneal) that are nearly ideal for the formation of electronic heterojunctions for sensing, photocatalysis, photovoltaics and light emission [3-5].
An emerging frontier in electrochemical anodization consists of imparting nanophotonic enhancement(s) to the semiconducting properties of highly ordered metal oxide nanopore (MONPAs) and nanotube arrays (MONTAs). Examples of such nanophotonic enhancements include the bottom-up, solution-based growth of photonic crystals, metamaterials and plexcitonic substrates. Nanophotonic enhancements enable light trapping in heterojunction photocatalysts and photovoltaic devices in order to harvest solar radiation more completely. Nanophotonic enhancements can also be used to control the directionality, intensity and lifetime of photoluminescence, and generate high Q-factor collective resonances to boost sensitivity in photodetectors, immunoassays and small molecules SERS-based sensing.
The use of low frequency pulses in the anodization process can be used to periodically modulate the diameter of nanopores and the wall-thickness of nanotubes in the depth direction [6]. Pulsed anodization of metal foils results in metal oxide nanostructures with a periodic refractive index which can be used to fabricate one-dimensional photonic crystals (1D-PhCs) by suitably adjusting the amplitude and pitch of the periodic features. Work in this area has focused on either using pulsed currents or pulsed voltages during anodization to form 1D-PhCs. One major limitation of the anodic growth of photonic crystals is the lack of control over the stop-band center frequency and the full width at half-maximum (FWHM) of the photonic stop-band resonances. We introduce a new technique, which involves periodically varying the charge supplied to the anodization process. The charge-controlled pulsed anodization process enables superior control over the morphology of the anodic 1D-PhCs and can be used to generate narrower resonances at desired wavelengths. Illustrative examples of the utility of these 1D-PhCs in photocatalysts and sensors are presented in this Invited Lecture.
Another frontier in the anodic formation of metal oxide nanostructures consists of the use of thin metal films on non-native substrates as anodization substrates. Currently, the overwhelming majority of anodization processes use metal foils that are hundreds of micrometers to millimeters in thickness as substrates. For optoelectronic applications, the use of such metal foils is highly limiting due to the opaque substrates preventing light in-coupling and light out-coupling into the nanopore/nanotube array from the substrate direction. Therefore, the formation of MONPAs & MONTAs on transparent substrates such as fluorine-doped tin oxide(FTO)-coated glass and indium tin oxide(ITO)-coated glass, is highly desirable [7]. This Invited Talk will showcase anodically formed MONPAs & MONTAs of TiO2, Ta2O5 and NiO on silicon and conductive glass substrates while also presenting prototypical optoelectronic applications of MONPAs & MONTAs on non-native substrates.
REFERENCES
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