Large-Scale Fabrication of Titanium Nitride-Based Plasmonic Broadband Absorber Metasurfaces for Solar Energy Conversion

Abstract

Plasmonic nanostructures containing metals such as gold or silver have much attraction due to the generation of localized surface plasmons which are the collective oscillations of free electrons that get excited by light, resulting in amplified local electromagnetic fields in the nanostructures. These can be applied to perfect absorption, surface-enhanced Raman scattering (SERS), metamaterials, and non-linear optical phenomena like fano resonance. Titanium nitride (TiN), one of the refractory plasmonic materials, has emerged as an alternative to gold or silver because of its high thermal, chemical, and mechanical durability, and similar plasmonic properties to gold in the visible and near-infrared region. Broadband perfect absorbers exhibit the potential to absorb a substantial light spectrum, making them instrumental in converting solar energy into thermal, chemical, and electrical energy. The absorption of energy leads to the excitation of "hot" electrons in the materials to high-energy states, and these hot electrons can be utilized in photocatalysts or photovoltaics. However, the design and fabrication have challenges in achieving high absorption across broad wavelengths, while maintaining good mechanical strength, chemical stability, and cost-effectiveness. Also, these structures should be fabricated in a large area which could not be achieved by top-down fabrication techniques. In this thesis, I focus on fabricating the nanostructures over a large area by using colloidal lithography and anodized aluminum oxides (AAO) templates. The fabricated nanostructures were used as broadband absorbers for solar energy conversion applications, including photothermal devices and photocatalytic hydrogen generation. In Chapter 2, I employed multiple-patterning colloidal lithography (MPCL) to create TiN nanoring near-perfect absorbers over a large area (2.5 cm × 2.5 cm). These absorbers showed a high absorption of 95.4% under normal incidence conditions and unpolarized light and showed high absorption across visible and near-infrared wavelengths (400-900 nm) up to an angle of incidence of 40°. Moreover, they displayed an excellent heat tolerance, maintaining their absorption even after heating up to 600 ℃. In Chapter 3, I utilized shadow sphere lithography (SSL) to simplify the process steps of MPCL. This method combined colloidal lithography with glancing angle deposition to fabricate TiN nano-ring structures over a large area with a much-simplified process. The array of nano-rings exhibited a high absorption rate of 97.1% in the visible and near-infrared regions (400-900 nm), better than that obtained from the nanodisk and nanohole absorbers. The nano-rings also demonstrated superior performance when exposed to light, registering a temperature increase that was 2.3 times higher than a flat TiN film. In Chapter 4, I utilized an AAO template to fabricate ultra-broadband absorber working on 400-2500 nm which covers most of the solar spectrum. This ultra-broadband absorber was prepared by coating with a thin layer of TiO2 on the AAO template, followed by the deposition of TiN over a large surface area (5cm × 5cm). It exhibited a high average absorption rate of 99.18% in the visible region (400-700 nm) and 80.09% in the near-infrared region (700-2500 nm). The use of TiO2 significantly increased the lifetime of hot electrons by 2.08 times, thereby enhancing the hydrogen evolution reactions rate.