Solving the two-interface problem in vibrational sum frequency generation spectroscopy applied to multilayer thin film systems

Abstract

This dissertation describes advances made in applying sum frequency generation spectroscopy (SFG, in particular <italic>vibrational</italic> SFG or VSFG) to multilayer thin film systems. Application of VSFG to thin film systems is motivated by the challenge of characterizing molecular structure at the active boundary in organic field-effect transistors, these are inherently buried interfaces. VSFG is a surface-selective probe of molecular structure; however, when VSFG is applied to an organic thin film, the detected signal has contributions from two potential sources - the two interfaces of the organic - which must be separated. The problem is further confounded by optical interferences inherent in multilayer thin film systems. An intuitive mathematical model is developed; postulating a solution to the two-interface problem of SFG applied to planar and stratified multilayer structures. The model system for this dissertation is thin films of the small molecule N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) vapor deposited on silica thin film substrates, consistent with an oFET thin film geometry. The interference model is used for an extensive simulation analysis that reveals intricacies contained in the intensity data of VSFG applied to that system. VSFG experiments performed on samples with PTCDI-C8 deposited as gradient thicknesses provide compelling evidence that the model gives an accurate description of optical interference effects and that it can be used to separate contributions to the total VSFG signal intensity. The supplementary materials contain a collection of Mathematica notebooks that can be used to investigate optical interference effects on SFG data collected from systems composed of an arbitrary number of thin film layers.