The Modulation Of Structural Color And The Photochemistry Study Of Black TiO₂ Metasurfaces

Nanophotnics is a subject that emerged in the 21st century.It mainly studies the behavior of light and the interaction between light and matter at the nanoscale.Various metasurfaces such as waveplates,structural colors,and meta-lenses developed on the basis of nanophotonics have huge application prospects.In addition to its wide-ranging applications in optics,nanophotonics has gradually begun to attract a lot of research in the field of photochemistry.Localizing light in a nano-scale space can significantly improve the interaction between light and matter,thereby improving the material's performance.Photochemical properties.Titanium dioxide(TiO₂)is a commonly used construction material of metasurfaces.It also has excellent photocatalysis performance.It can achieve photocatalytic applications such as degradation of aquatic hydrogen,nitrogen fixation,and degradation of organic pollutants under ultraviolet light.The coexistence of excellent optical properties and photochemical properties makes the combination of nanophotonics and photochemistry based on TiO₂ attractive.Although a lot of research work has pushed the photochemical properties of TiO₂ to visible light,so far no method compatible with the micro-nano processing and regulation of metasurfaces has been proposed,which limits the combination of nanophotonics and photochemistry.On the other hand,although the self-cleaning of TiO₂ film materials has been commercially available,the self-cleaning research of nanophotonic devices is still a blank.This increases the loss and life decline caused by pollution during the usage of the device,which greatly affects the practical applications.In this thesis,based on the localization and enhancement of the light field by the metasurface,and the photochemical properties of TiO₂,this paper proposes that the photochemical properties of TiO₂ can be used to give TiO₂ metasurface dynamic control and UV self-cleaning functions.Conversely,the local enhancement effect of the metasurface of the light field can be used to enhance the photochemical activity of TiO₂.Therefore,this thesis first studied the TiO₂ energy band modulation process compatible with CMOS technology,and realized the reversible conversion of TiO₂ material absorption in visible light,and it will be applied to the dynamic reversible control of TiO₂ structural color.Then combined the photochemical properties of TiO₂ with the light field control of the metasurface,and used the local light field in the unit nano-block of the metasurface to significantly improve the photocatalytic performance of TiO₂.Finally,based on the photochemical properties of TiO₂,various types of self-cleaning performance of TiO₂ metasurfaces were proposed and realized.The specific research content of this paper is as follows:Realizing the dynamic and reversible modulation of the structural colors of TiO₂metasurfaces,and have high cycle stability at the same time.Firstly,the TiO₂ film is treated with highly active hydrogen plasma(H⁺)generated in an inductively coupled plasma(ICP)etching machine,which can greatly increase the absorption of the TiO₂film in visible light,and transform the film from the original transparent to dark black.And it can be treated with oxygen plasma(O^-)in ICP to restore the original transparent state.Utilizing this reaction,the reflectance of the structural color reduce to lower than20%after H⁺treatment,and the displayed color changes from bright to dark,and the contrast between each color patches is significantly reduced.At the same time,the reflection spectrums and colors can almost completely return to the original state after O^-treatment.On this basis,this thesis realizes the encryption and decryption of the letter information constructed by TiO₂ structural color,and provides a new modulation way for future TiO₂ structural color in screen display,information encryption.Based on the photocatalytic activity of TiO₂ itself,this thesis first experimentally demonstrated a TiO₂ based self-cleaning micro-nano optical device.Due to the undulating surface and physical deposition process of the micro-nano optical device,the device structure is fragile,and it is difficult to clean once it is contaminated by oil stains and fingerprints.Under ultraviolet light,the surface of TiO₂ becomes superhydrophilic and photocatalytic responsive.In this thesis,the self-cleaning effect of superhydrophilic and photocatalytic mechanisms on the metasurface of TiO₂ is studied.And the self-cleaning response of TiO₂ under ultraviolet light is used to fully eliminate the chemical contaminants and fingerprints by self-cleaning.The properties of polluted TiO₂-based meta-devices such as colored nanostructured color,achromatic meta-lens and meta-holographic are fully restored under the ultraviolet light irradiation.The integration of micro-nano optics and photochemistry solves the long-standing pollution problem of the micro-nano optical components and accelerates the actual application process of micro-nano optics.The localized optical field of the metasurface is used to enhance the photocatalytic reaction of TiO₂.Although the absorption of the black TiO₂ film after H⁺treatment is significantly enhanced in the visible light,there is still a large absorption valley in the absorption spectrum between 400-500 nm.In this thesis,the resonance position of the TiO₂ metasurface surface is designed at 450 nm to significantly increase the absorption valley of TiO₂ at 400-500 nm.As a result,the absorption of the visible light of TiO₂ is greatly increased,so that the size of photo-reduced Ag nanoparticles of the film is 18.7times higher than that of the film in the experiment.In addition,the black TiO₂metasurface with a narrow band absorption spectrum below 20 nm has also been experimentally confirmed.The slightly blackened TiO₂ metasurface has a very narrow absorption spectrum,making the device only have a photocatalytic response to light within the absorption wavelength.The realization of narrow-band wavelength-dependent photocatalysis makes it possible to realize narrow-band real-time light detection based on photocatalysis,which greatly expands the application field of photocatalysis.