Nanowire Alignment: Techniques, Quantification, and Applications in Large-Area Devices

Abstract

Nanowire alignment is essential for their integration into large-area devices, as well as to obtain certain functionality such as the ability to polarize light and increase surface-enhanced Raman scattering. Various nanowire alignment methods have been developed, however, major drawbacks have limited their application such as complex processing, high cost, limited compatible nanowire materials, and limited scalability. In addition, the methods used to quantify the quality of nanowire alignment are lacking in accuracy, speed, and applicability to all kinds of nanowires. In this thesis, two simple and large-area alignment methods are studied that are applicable for nanowires synthesized by any method and compatible with large-area electronic device fabrication processes. The first method is accomplished by depositing nanowires on polyvinyl alcohol films followed by film stretching, which achieves high-quality alignment (with an order parameter S=0.93). Nanowire breakage, which is commonly encountered in similar techniques, is minimized and the average length of nanowires after alignment is nearly the same (~99.3%) as before alignment. The second alignment method is accomplished directly during rod-coating deposition of the nanowires, without the need of any additional step. Two image processing methods based on edge-detection and skeletonication are presented to recognize nanowires from microscopy images. Then an order parameter and an orientational distribution function are used for alignment quantification. Compared with previously reported studies, these methods are fast and automated, reliable without bias, generally applicable, easy to implement, and computationally efficient. The alignment methods described above are applied in two applications. Firstly, the electrical and optical anisotropy of slightly aligned silver nanowire films, which can be used as transparent electrodes, are investigated. Their transparency to polarized light is increased by 7.3 percentage points compared to typical randomly oriented silver nanowire films, which may benefit end uses such as liquid crystal displays and the touch sensors on top of them. Secondly, a crossed film structure consisting of semiconductor nanowires aligned in one direction and metal nanowires orthogonally aligned is designed. The metal nanowires are intended to act as interconnects to substantially reduce semiconductor nanowire-nanowire junction resistances while avoiding lithographically-defined metal pads, the latter which can have poor mechanically flexibility and involve fabrication processes not desired for large-area electronics. Such a device structure can be developed further for use in large-area flexible devices such as light, strain and chemical sensors and energy generators.