Laser spectroscopy

Highly active, structurally disordered CoFe₂O₄/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe₂O₄ powder dispersed in water. A partial transformation of the CoFe₂O₄ educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm⁻² is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec⁻¹. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe₂O₄. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials.

Laser-induced breakdown spectroscopy (LIBS) technology is an appealing technique compared with many other types of elemental analysis because of the fast response, high sensitivity, real-time, and noncontact features. One of the challenging targets of LIBS is the enhancement of the detection limit. In this study, the detection limit of gas-phase LIBS analysis has been improved by controlling the pressure and laser pulse width. In order to verify this method, low-pressure gas plasma was induced using nanosecond and picosecond lasers. The method was applied to the detection of Hg. The emission intensity ratio of the Hg atom to NO (I_Hg/ I_NO) was analyzed to evaluate the LIBS detection limit because the NO emission (interference signal) was formed during the plasma generation and cooling process of N₂ and O₂ in the air. It was demonstrated that the enhancement of I_Hg/I_NO arose by decreasing the pressure to a few kilopascals, and the I_Hg/I_NO of the picosecond breakdown was always much higher than that of the nanosecond breakdown at low buffer gas pressure. Enhancement of I_Hg/I_NO increased more than 10 times at 700 Pa using picosecond laser with 35 ps pulse width. The detection limit was enhanced to 0.03 ppm (parts per million). We also saw that the spectra from the center and edge parts of plasma showed different features. Comparing the central spectra with the edge spectra, I_Hg/I_NO of the edge spectra was higher than that of the central spectra using the picosecond laser breakdown process.

Laser induced breakdown spectroscopy (LIBS) provides the opportunity to analyze almost any element, from any material, in any environment. Among the many applications of LIBS is the analysis of thin films and multilayered structures. An automated system was designed and built to conduct LIBS using Nd:YAG and Ti:Sapphire lasers, broadband and high-resolution spectrometers and detectors. This system incorporates the sample manipulation as well as laser and spectrometer control and timing.

A series of experiments were conducted to analyze the ability of nanosecond and femtosecond lasers to detect Mg impurities in thin TiO₂ films using LIBS. It was determined that optimal detection occurs early in the plasma ionic/atomic emission with detection capabilities in the parts-per-million range. Another series of experiments were conducted using LIBS to analyze thin transparent organic films, with specific emphasis on the effect of film thickness and interplay between film and substrate. The challenges of ablating and measuring multiple layers have also been explored using various laser wavelengths. The effectiveness of LIBS has been demonstrated for depth profiling of CIGS solar cells. Ablation crater and ablation threshold analysis aided in understanding and overcoming some of the obstacles in depth profiling. One of the challenges with LIBS is the identification and mitigation of matrix effects. This problem was explored using a Mg tracer element and various compositions of the suspected elements Si, Ca, and Sr which can cause errors in LIBS analysis. The goal of this dissertation is to investigate the ability of LIBS to conduct detailed thin film analysis for a variety of materials and potential applications. This includes analyzing trace elements from a traditionally noisy background, measuring difficult to ablate thin films, and the unique challenges associated with multilayered structures.