Iron Oxide Plasmonic Nanostructures for Energy Harvesting
Authors: Das Naresh , McClure Joshua P. , Grew Kyle N. 
Published: February 2020

Recently Iron oxide (Fe2O3) nano structure has drawn renewed interest in electrocatalysis process for carbon-carbon (C-C) bond breaking in ethanol. We design and fabricate Fe2O3 plasmonic nanostructures for C-C bond-breaking processes in high density fuels. After patterning Fe2O3 film, we use inductively coupled plasma (ICP) etching equipped with an Argon (Ar) plasma to create the film with nanometer scale dimensions. We used both photoresist mask and hard mask like silicon dioxide film for selecting etching. Fe2O3 film etching process is optimized by using different parameters like ICP power, base electrode power, pressure, temperature, and Ar gas flow etc. We used elliposmetric method as well as UV/Vis reflectance spectroscopy to determine the resonance wavelength of the plasmonic nanostructures suitable for electrocatalysis experiment.

Iron oxide (Fe2O3) has recently received much attention among the various semiconductor photo-electrode materials due to its favorable optical band gap (~2.2 eV), excellent chemical stability in high pH media, natural abundance, and low cost [1,2]. However, Fe2O3 exhibits a relatively poor absorptivity of photons near its band-edge due to an indirect band gap, poor electronic conductivity (PEC), and picosecond recombination of excited states, thus leading to a photo-generated hole diffusion length of ~2-4 nm [3]. These non-ideal optoelectronic properties hinder the transport of photo-generated carries and increase the recombination rate, which results in a lowering of the PEC efficiency compared to theoretically predicted values. Controlling the nanostructure of Fe2O3 may provide an effective technology for overcoming the aforementioned problems of Fe2O3 due to the geometry and mechanism-dependent semiconductor structure [4]. The shape of the Fe2O3structures with cone, cylinder, sphere, etc. features alters the electric field dependent absorption and transport processes. We use electron beam evaporation system to deposit thin Fe2o3 film (100-500 nm) on silicon substrate with thin layer of gold (Au) film. We then use an inductively coupled plasma (ICP) etching process to define Fe2O3 nano-structured pillars with various diameters (50nm -5 µm) and heights. We use either e-beam lithography or optical lithography process to define the plasmonic nanostructures. ICP etching process with Ar plasma is used with varying power, pressure and substrate temperature etc. to optimize the Fe2O3 etching process. Scanning electron (SEM) and atomic force (AFM) microscopy tools are used to determine the surface morphology of the nanostructures, whereas ellipsometer and UV-Vis spectrometer elucidate the optical properties. Galvanometeric current-voltage measurement is performed to determine the optimum nano plasmonic structure for carbon-carbon (C-C) bond breaking process in high energy density chemicals.   

In figure 1, we have shown the etch rate of Fe2O3 film with different substrate temperatures and Ar pressure in the reaction chamber. We find that in order to achieve a smooth surface and reasonable etch rate, the substrate temperature should be between 15-18 ᵒC and the ICP power between 800-1000 W. Detailed deposition, annealing, etching and characterization results of Fe2O3 nano plasmonic structures will be presented in a full paper.

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