Deterministic gradient nanostructures : from lithographic fabrication to fluidic and optical applications

Abstract

The research work presented in this dissertation falls into two parts: (1) development of reproducible lithographic fabrication techniques for centimeter-scale patterning of deterministic gradient nanostructures; (2) applications of gradient nanostructures in fluidic and optical devices for gradient wettability, imaging-based ultrasensitive molecular detection, and near-infrared spectral measurement. The first part of this dissertation presents a method of generating centimeter-scale gradient nanostructures by using interference lithography with a spatially non-uniform exposure field. With two orthogonal exposures, the exposure patterns on the positive-tone photoresist show pillars at the center, where the Gaussian beam has the maximum intensity. The diameter of the pillars increases with increasing distance from the center. Nearby pillars start to merge, and the overall pattern evolves into a hole array with the diameter of the holes decreasing as the distance from the center further increases. Moreover, a tri-layer transfer strategy, which is featured with the stack of two resists layers together with a hard middle layer, is introduced. The transfer strategy possesses an enhanced process latitude, significantly benefiting the gradient pattern fabrication. By combining the non-uniform interference lithography, the thermal nanoimprint lithography (thermal-NIL), and the tri-layer transfer strategy, functional materials with concentric gradient nanostructures are fabricated. The second part of this dissertation focuses on the development of fluidic and optical applications by using the gradient nanostructures. Firstly, surfaces with gradient wettability induced by deterministically patterned nanostructures are presented. We used this gradient wettability surface to study the varying wetting behavior on nanostructures, shedding light on the diverse immersion situations of water droplets on surfaces with various nanopatterns. Secondly, we report an ultrasensitive sensor that can detect an angstrom-thick layer of adsorbed molecules through image acquisition and processing. When the sensor is illuminated with narrowband light (such as from a LED), the intensity pattern recorded on the metasurface changes with the surface-adsorbed molecules, enabling label-free, sensitive, and spectrometer-free molecular detection. Thirdly, a compact near-infrared spectrometer based on a Si metasurface is demonstrated. The Si metasurface consists of deterministic and ordered continuously varying gradient nanostructures, which act as continuously varying broadband optical filters. The number of filters is 1520, which is nearly an order of magnitude more than the reported literature. We have demonstrated the use of the Si metasurface spectrometer to reconstruct the spectra of incident lights. This is achieved by an algorithm that takes as its inputs a metasurface transmission image illuminated by an unknown light and a library of the filter transmission spectra.