Characterizing Ultra-Thin Films and Confined Fluids

Measuring Matter in a Tight Spot: A Novel Approach for Characterizing Ultra-Thin Films and Confined Fluids
Abstract by Daniel Kienle

The behavior of matter confined to molecular dimensions is technologically relevant as device size decreases but fundamentally vexing as continuum theory breaks down and the role of molecular size and configuration become important. Due to the extremely small dimensions, studying properties of confined matter is difficult, typically requiring very specialized techniques such as x-ray or neutron scattering. In this work, analysis of multiple beam interferometry (MBI) was advanced to enable characterization of the optical properties of ultrathin solid and liquid films. In the first part of the work, conventional MBI contact analysis was used to determine the thickness and refractive index of previously uncharacterized lipid monolayers commonly used in biological membrane research, and results demonstrated that the inner and outer leaflet of supported lipid bilayers have different optical properties. MBI analysis was then extended to allow the optical thickness of thin films to be measured without film contact and free from systematic error through design of a novel analytical approach called the refractive index profile correction (RIPC) method.

This development enabled soft condensed matter samples which are frequently fragile, reactive, or easily deformable films to be characterized without compressing or contacting the films. Importantly properties of interfacial films at the surface such as concentration gradients due to absorbed layers or depletion layers can be directly measured as a function of confinement and compared to their properties in the bulk using this newly developed approach. Furthermore, the technique is applicable to complex interferometer samples which may include interferometers with substrates of non-uniform thickness leading to asymmetry of the sample, or systems with an additional layer between the substrate and the film of interest wherein all properties can be accurately determined without measurement priori knowledge of any layer properties. Thus, the technique is applicable to a diverse array of systems through the ability to tailor the chemistry of the confining surfaces and determine interaction forces while simultaneously determining the refractive index and thereby concentration of the confined materials. The method was applied to take non-contact measurements of the adsorbed mass of polyelectrolyte brushes adsorbed on mica from water, and demonstrated that the mass of the polymer film was conserved during confinement and compression. The method was also used to independently characterize both the effects that a single surface has on fluid structure and the effects that confinement has on the fluid structure using the non-polar fluid octamethylcylclotetrasiloxane (OMCTS). The results present a conclusive demonstration that there is no first-order phase transition or any resolvable deviation from the bulk fluid density during confinement, thereby answering a fundamental question as to the state of confined liquids.

Plot of the local fluid refractive index with normal position through a model sample. Each slab would be represented by a different characteristic matrix to be used in multilayer matrix method analysis (Kienle and Kuhl, 2014).