Application of Scanning Probe Microscopy to Characterize Physical Properties of Polymer Brushes, Photoactive Polymers and Rare Earth Oxide Nanostructures

Abstract

A sample stage for characterizing photoactive materials was developed for studies with scanning probe microscopy (SPM). A sample stage was designed that directs light from a solar simulator via a fiber optic cable to illuminate the sample. Current-sensing and photocurrent measurements can be acquired with a conductive tip. The designed photocurrent stage can be used for SPM systems with a tip-mounted scanner. Current-sensing measurements can be taken with or without illumination to measure the current produced from organic photovoltaic (OPV) or photoconductive samples. Topography, lateral force and current-sensing images are acquired simultaneously, providing information of how nanoscale morphology affects the measured current. Particle lithography was used to prepare nanostructures of OPV materials for photoconductive measurements. Nanopillars of polythiophene ranging from ~20 – 80 nm in thickness were grafted from indium tin oxide (ITO) surfaces. Samples were characterized with the photocurrent sample stage to directly test how nanoscale changes in film thickness affect the measured current. Current-voltage measurements indicate the polythiophene nanopillars are photoactive. The nanostructured test platform provided an unprecidented scale and morphology of photoactive brush polymers for dark and photocurrent measurements. Films of polynitrophenylene grafted to Au(111) surfaces via photoredox catalysis were investigated with contact-mode atomic force microscopy (AFM). The parameters of concentration of starting material and duration of sample illumination were studied. Films of increasing thickness were prepared from longer illumination times and higher concentrations of the starting materials. Particle lithography provided an internal standard for film thickness measurements. Rare earth oxide materials were nanopatterned using particle lithography. Precursor salts of rare earth oxide materials located on surfaces surrounding the base of mesospheres. The dried nanorings of salt were heated at 800 oC to produce crystalline materials on two transparent surfaces for further luminescence studies. The nanorings were analyzed for signs of Ostwald ripening or physical changes that may have occurred during the heating process and crystal phase transition.