Nanostructured multifunctional materials for control of light transport and surface wettability

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 221-234).Biological surfaces have evolved to optimize their structures and physical and chemical properties at the micro/nanoscale for adaptation to different environments, exhibiting a wide variety of beneficial functions, ranging from optical properties to wettability, such as anti-reflection coatings in moth eyes and self-cleaning surfaces of lotus leaves. Combining optical and wetting functions in multifunctional materials is critical for practical engineering applications such as energy harvesting, color generation, and operation of optical instrumentation in humid conditions. However, analyses of the functional design constraints of specified optical and wetting functions followed by integrative optimization have been rare, and limited to simple pairwise combinations from two distinct research disciplines. Furthermore, fabricating the desired multifunctional nanostructured materials remains a difficult engineering challenge due to the limitations of existing nanofabrication methods. The work in this thesis focuses on the joint control of light transport and surface wettability. It starts with analysis and design, followed by implementation of new multifunctional nanostructured materials using novel nanolithographic fabrication techniques. We first consider multifunctional silica surfaces consisting of conical nanostructures (nanocones) for enhanced omnidirectional broadband transmissivity in conjunction with structural superhydrophilicity or robust superhydrophobicity. This is achieved through a systematic approach to concurrent design of nanostructures in both domains and an innovative fabrication procedure that achieves the desired aspect-ratios and periodicities in the nanocones with few defects, high feature repeatability, and large pattern area. Enhanced optical transmissivity exceeding 98% has been achieved over a broad bandwidth and range of incident angles independent of the polarization state. These nanotextured surfaces also demonstrate robust anti-fogging or self-cleaning properties, offering potential benefits for applications such as photovoltaic solar cells. As an extended function of this silica nanocone surface, we propose the systematic design and development of nanostructured transparent anti-fingerprint surface coatings that degrade fingerprint oils using photocatalytic effects. The TiO₂-based porous nanoparticle surfaces exhibit short timescales for decomposition of fingerprint oils under ultraviolet light, plus they have transparency comparable to typical glass with low optical haze (< 1%), and are mechanically robust. These TiO₂ nanostructured surfaces are anti-fogging, anti-bacterial, compatible with flexible glass substrates, and remain photocatalytically active in natural sunlight Lastly, instead of eliminating all reflections over the broadband wavelengths of light for enhanced super-transmissivity, 2-dimensional (2D) periodic nanorod surfaces capable of generating vivid colors by wavelength-selective reflection have also been designed and developed. The geometry of the nanorod structures on top of a silicon substrate is optimized to obtain high contrast of colors while still allowing for scalable nanopatterning with the help of newly invented nanofabrication processes. By developing an integrated understanding of optical and wetting properties of nanostructured materials, we have been able to realize novel functionalities using nanostructured surfaces conceived by concurrent design in the two domains and created by new nanofabrication techniques.by Hyungryul Choi.Ph. D