Bottom-up processed metal nanostructures for optoelectronic devices

Abstract

Metal nanostructures have unique optical and mechanical properties. From the optical point of view, nanostructured metal exhibits unique optical properties such as scattering, diffraction and surface plasmon which cannot be obtained from bulk metals due to the strong light-matter interaction. Recently, due to the advance of lithography technology, it has been possible to realize a fine and complicated metal nanostructure which was previously difficult to realize. Thereby, various optical phenomena which have previously been difficult to be utilized experimentally have been adopted to various applications. As the era of the Internet of things (IoT) comes along with the emergence of smart phones, the portability of electronic devices are increasingly emphasized. Therefore, there is a need for flexible, foldable and rollable devices with enhanced portability. Mechanical properties of the conductors constituting the electrode and electrical wiring of the flexible electronic device should be durable and stable against the mechanical deformation. For reliable operation of flexible devices, it is necessary to maintain electrical conductivity of the electrode and electrical wiring stably even under repetitive mechanical deformation. The metal nanostructure is known to exhibits excellent mechanical properties than the bulk metals and metal thin films. Accordingly, attribute to such unique optical properties and outstanding mechanical properties, in recent years, metal nanostructures have been adopted to various applications such as photovoltaics, light emitting diodes, sensors, meta materials, color filers, transparent electrodes and polarizers. Among the nanofabrication methods, conventional top-down approaches represented by the photolithography form micro- and nanostructure on the wafer, which has led the electronic industry. Photolithography satisfies demand of the semiconductor industry, which require precise and high resolution nanopatterning at the wafer scale. However, photolithography cannot meet all the demands of industrial applications which require the low cost, high throughput and large area. Nanoimprint lithography is a promising nanopatterning method that can meet such requirements. Using the nanoimprint lithography, soft molds can be replicated easily and almost infinitely from the Si master formed by the conventional lithography techniques. Moreover, using the roll-to-roll nanoimprint process, large area and high speed processing on low cost flexible substrates is possible using less complex equipments. Therefore, nanoimprint lithography can provide the solution in various fields such as optoelectronic components, nanophotonics and biological applications Nanoimprinted polymer templates provide a suitable platform for the fabrication of metal nanostructures via the bottom-up processes. By filling the metal along the nanoimprinted pattern, metal nanostructure with the desired structure and dimension can be obtained. In order to form the metal nanostructure on the nanoimprinted template, appropriate bottom-up approach to control nanoscale building blocks to be assembled into the desired position in the desired form is required. In this dissertation, I suggest the bottom-up processes to form metal nanostructure for the metal nanogrid transparent electrode and wire grid polarizer. In chapter 2, I report the fabrication of a high-aspect-ratio metal nanogrid transparent electrode prepared using a solution processable, bottom-up approach. Using a facile, bottom-up approach that exploits capillarity assisted nanoparticle assembly, I assembled silver nanoparticles along a high-aspect-ratio trench of a patterned nanogrid template. High resistance due to junctions between nanoparticles is effectively reduced by photochemical welding and post-fabrication annealing. Our high-aspect-ratio silver nanogrids have an average sheet resistance of 15.2 $\Omega/sq$ and optical transmittance of 85.4 %. Moreover, the speed of fabrication by capillary assembly is significantly improved from a few $\mum/s$ to $220 \mum/s$ by controlling the fabrication conditions. In chapter 3, I propose a facile solution process for fabricating Al wire grid polarizers. The wire grid polarizer consisting of parallel metal nanogratings is an attractive alternative to conventional polarizers because it offers thin layer thickness, easy integration capability, and outstanding chemical and thermal stability. High-aspect-ratio Al nanostructures are formed in a 1D nanoimprinted template by electroless plating. Because of the geometrically irreversible deposition and wet etching processes on the nanotemplate, only the embedded Al nanogratings are retained. The solution-processed Al nanostructured WGP with a line width of 60 nm and a height of 150 nm exhibits an average extinction ratio of 77 and transmittance of 51.1% in the visible wavelengths.