Electrical & Computer Engineering, Department of

 

Date of this Version

Spring 5-2015

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Electrical Engineering, Under the Supervision of Professor Eva Schubert. Lincoln, Nebraska: May, 2015

Copyright (c) 2015 Dan Liang

Abstract

Characterization of the structural and optical properties is a subject of significance for nanoporous material research. However, it remains a challenge to find non-destructive methods for investigating the anisotropy of porous thin films with three-dimensional nanostructures. In this thesis, a generalized ellipsometry (GE) analysis approach is employed to study two types of anisotropic nanoporous media: slanted columnar thin films (SCTFs) with polymer infiltration and inverse-SCTF polymeric films. The thesis presents the physical properties obtained from GE analysis, including porosity, columnar shape, principal optical constants, birefringence, etc.

The thesis reports on using a GE analysis approach, combining the homogeneous biaxial layer approach (HBLA) and anisotropic Bruggeman effective medium approximation (AB-EMA), to determine the changes in structural and optical properties of highly porous SCTFs upon polymer infiltration. Via spin-coating, poly(-methyl methacrylate) (PMMA) was infiltrated into the permalloy SCTFs prepared by glancing angle deposition (GLAD). The Mueller matrix GE measurements were conducted on the SCTFs before and after PMMA infiltration. The obtained film thickness and columnar slanting angle show changes due to infiltration which are in good agreement with scanning electron microscopy (SEM) analysis. The method effectively identifies the changes in birefringence and dichroism upon infiltration, and provides constituent fractions consistent with the performed experiments.

GE analysis is further utilized to characterize the biaxial optical responses of the porous polymer thin films. The porous polymer films with inverse columnar structure (PMMA iSCTFs) were prepared via infiltrating polymer into the voids of the SCTF templates and selectively removing the columns. The AB-EMA was employed to analyze the GE data of the porous polymer films and SCTF templates to determine the structural and anisotropic optical properties. The structural parameters are highly consistent with SEM results. The classification and structure of optical anisotropy are found to be identical for the samples. Our GE results demonstrate that the anisotropic optical behaviors for the two complementary structures follow the reciprocity principle in electrodynamics.

Advisor: Eva Schubert