Combinatorial Deposition of Thin Films: Study of pH Variation Using Fluorescent Dyes

Combinatorial thin-film deposition is an innovative technique for the synthesis and discovery of new materials, designed to efficiently explore a wide range of compositions and process parameters. This is a technique widely used in various areas of chemical engineering to create nanometer-thick layers of material on a substrate. Research teams from Ben-Gurion University of Negev (Beersheva, Israel) and the Polytechnic of Turin recently collaborated to study combinatorial in-flow deposition of thin films, publishing the results in the paper “A Combinatorial Approach for the Solution Deposition of Thin Films.” The research offers a combinatorial method for the deposition of thin films from solution that allows two crucial parameters - temperature and deposition time - to be examined simultaneously on a single sample. This thesis aims to develop innovative combinatorial deposition systems for multicomponent thin films and to study the impact of reagents concentration changes on film properties. The challenge is to create and measure a pH gradient using a linear alkaline solution blower and optical fibers to monitor pH changes through the solution optical transmittance, using fluorescent dyes. Light reflected from the sample surface is directed to an array of photodiodes to measure changes in optical transmittance as a function of time. The thesis employs a continuous flow reactor developed at Ben Gurion University's Department of Materials Engineering, designed to establish a pH gradient within an alkaline range from pH 12.5 to 14.25. The investigated dyes are Rhodamine B, Rhodamine 6G, Indigo Carmine, and Titan Yellow. The effect of pH on fluorescence was analyzed by examining the changes in fluorescence intensity and in characteristic wavelength of the peak for each dye once added to alkaline solutions. All tests were conducted in cuvettes, and signal acquisition was performed by an Avantes spectrometer. To simulate the behavior of the dyes within the continuous flow reactor as accurately as possible, several key parameters were examined: Temperature, stability over time, mixing, stability in the presence of metallic cations (Pb²⁺ and Sn⁴⁺) and kinetic response. The analyses identified Rhodamine 6G as the most suitable dye for the application, demonstrating enhanced stability in the presence of metal cations and over time. Additionally, Rhodamine 6G showed a rapid and reliable kinetic response to pH changes compared to the other dyes analyzed.