Two-dimensional Molybdenum Oxide in Optical Applications
Creators: name: Hu, Yihong
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 401605
subject: Functional materials

description: The emerging two-dimensional (2D) nanomaterials with atomic thicknesses and large surface-to-volume ratios exhibit outstanding optical properties which makes them promising candidates for next-generation miniatured on-chip devices. Although a few ultrathin materials with layered structures have been identified after the discovery of graphene, the members of the 2D realm are still insufficient to meet the requirement for nanotechnology development. Metal oxides are a group of earth-abundant material which has unique electronic and optical properties, and great chemical stability. Their 2D forms have attracted increasing attention owing to their great tunability in the band structure and optical responses, which expand the library of 2D materials. However, the current synthesis methods of 2D metal oxides are obstructed by the trade-off among the film quality, lateral dimensions, contamination introduced, and yield, hindering the comprehensive studies on their properties and the opportunities for further utilisation in practical applications. 
Therefore, the author of this thesis focuses on the research in facile synthesis of large-area, high-quality 2D metal oxides and fundamental studies of their tunable chemical composition and optical properties, along with the realisation of their potential in novel applications, especially compact optics. Molybdenum oxide (MoOx) is selected for this PhD research due to its diverse stoichiometries which result in extensive tunability in electronic and optical properties, presenting a solid ground for the exploration and advancement in the field.
The research work started by exploring different synthesis approaches that can acquire high-quality, contamination-free 2D MoOx nanomaterials. A novel “stamping” method adopted from the concept of metal-gas interface reaction on the metal surface is applied first. Highly crystalline MoO3 nanosheets in a special hexagonal phase with a thickness down to ~2 nm are obtained using this method. Moreover, the physical vapour deposition (PVD) method is proposed to synthesise ultrathin 2D MoOx films with up to a wafer-scale lateral dimension. The smooth and uniform amorphous films synthesised have a thickness of ~10 nm. MoOx (2.7 &lt; x &lt; 3) enjoys controllable optical properties depending on its stoichiometric state which is manipulated by the precise redox treatments where the refractive index (n) and absorption coefficient (k) are tuned accordingly. The ellipsometer measurements indicate that a modulation depth of 0.62 in Δn and 0.42 in Δk can be achieved at the communication wavelength between the full stoichiometric MoO3 and the sub-stoichiometric MoO2.7. The significant tunability in optical properties renders the as-synthesised 2D MoOx film light-modulating applications while the PVD synthesis method is highly compatible with existing nanofabrication techniques, encouraging the utilisation of the product in tunable on-chip optics.
After gaining a comprehensive understanding of the optical properties of the as-synthesised 2D MoOx film, the author focuses on the integration of the nanofilms onto the silicon photonic circuit chips. Since the growing demands of compact on-chip optical devices with dynamic responses, the capability of in situ evaluation of the 2D functional materials’ optical parameters is highly desired nowadays. The author proposes a silicon photonics-enabled platform that is capable of evaluating the complex refractive indices of 2D nanomaterials with arbitrary dimensions in a facile and reliable manner. A Mach-Zehnder interferometer (MZI) configuration integrated with MoOx in different stoichiometries is used to demonstrate the influence on the output optical transmission spectrum induced by variations in the complex refractive indices of the sample material. The recorded wavelength shifts of the interference pattern and amplitude changes are used to extract the optical complex refractive indices. The deviation of the measured shifts is as low as 1% and generally less than 5% compared with the theoretical calculation. The proposed platform offers a novel approach for the measurement of the complex refractive index of 2D MoOx with high accuracy and reliability, which can be possibly extended to most of the tunable 2D nanomaterials.
In the final stage of this PhD thesis, the author further investigates the possibilities of employing the ultrathin 2D MoOx nanofilms prepared in the previous steps in planar lens applications. Detailed discussions in terms of the lens theory and operation principles of various planar lenses, including the Fresnel lens, Fresnel zone plate (FZP) and dielectric metalenses, are conducted to identify a suitable design methodology for the MoOx-based lens. After examining the tunability in amplitude and phase responses which are calculated by an RCWA solver based on the optical parameters of the ultrathin MoOx film from 485 to 1550 nm, the author presents the designs of FZPs with operating wavelengths λ at 633 and 1550 nm, respectively. The designed zone plate with λ = 633 nm has an outer radius of 39.71 μm and a numerical aperture (N.A.) of 0.37 while for λ = 1550 nm, the design gives a 63.76 μm-radius zone plate with an N.A. of 0.54. Fabrication of the designed FZP has been carried out as the patterning for the concentric rings of the plate with the finest structure &lt;1 µm has been produced by photolithography technique on a glass substrate. Attempts on the utilisation of 2D MoOx-enabled FZP pave the way for applying the as-prepared MoOx film in tunable ultrathin flat lenses, which also demonstrates its potential in other compact, on-chip imaging devices, such as metasurfaces and holographic applications. 
In summary, the author successfully demonstrates several breakthroughs in the realm of 2D tunable metal oxides during her PhD program, covering their controllable synthesis, fundamental characteristics, and practical applications. It is expected that the outcomes of this PhD research work will commit to overcoming the bottlenecks of 2D metal oxide synthesis and lead to high cost-efficiency in optical sensing and imaging platforms with high performance and dynamical tunability.