Optical Identification of Materials Transformations in Oxide Thin Films

Abstract

Recent advances in high-throughput experimentation for combinatorial studies have accelerated the discovery and analysis of materials across a wide range of compositions and synthesis conditions. However, many of the more powerful characterization methods are limited by speed, cost, availability, and/or resolution. To make efficient use of these methods, there is value in developing approaches for identifying critical compositions and conditions to be used as a priori knowledge for follow-up characterization with high-precision techniques, such as micrometer-scale synchrotron-based X-ray diffraction (XRD). Here, we demonstrate the use of optical microscopy and reflectance spectroscopy to identify likely phase-change boundaries in thin film libraries. These methods are used to delineate possible metastable phase boundaries following lateral-gradient laser spike annealing (lg-LSA) of oxide materials. The set of boundaries are then compared with definitive determinations of structural transformations obtained using high-resolution XRD. We demonstrate that the optical methods detect more than 95% of the structural transformations in a composition-gradient La-Mn-O library and a Ga2O3 sample, both subject to an extensive set of lg-LSA anneals. Our results provide quantitative support for the value of optically detected transformations as a priori data to guide subsequent structural characterization, ultimately accelerating and enhancing the efficient implementation of micrometer-resolution XRD experiments.

Autonomous materials synthesis via hierarchical active learning of nonequilibrium phase diagrams

“…Because the lg-LSA is an irreversible method, a specific position x [and potentially its associated composition c (x) in the presence of a composition gradient] can only be annealed once, further emphasizing the need for optimizing the selection of the processing conditions. Once an lg-LSA stripe is processed, a conclusive structural characterization across the thermal gradient is possible with grazing-incidence high-intensity x-ray diffraction (XRD) to resolve the crystal structure ( 47 ). However, access to synchrotron facilities capable of producing x-rays with appropriate wavelength, intensity, and micrometer-scale spatial resolution comprises an inherently limited resource that motivates development of alternative phase boundary detection methods.…”

“…To address this issue, we developed a complementary technique based on microscopy imaging and optical spectroscopy to rapidly assess phase boundaries. We recently demonstrated that structural phase changes are directly associated with changes in the optical thin-film properties of transparent films, in particular the optical thickness nd ( 47 ), where n is the refractive index and d is the film thickness. Essentially, the gradients of the optical measurements across an lg-LSA stripe provide a means to map out phase boundaries without explicit crystallographic phase identification, thereby producing an unlabeled processing phase diagram without costly XRD experiments.…”

“…We incorporate this structure into the kernel of X AI by forcing its main component to be symmetric around the center of a stripe (see the “X AI ” section in the “materials and methods” part). In addition, SARA extracts key features of the stripe texture from the micrograph to further improve the kernel design, i.e., by identifying the stripe center and by detecting systematic optical changes across the stripe that we associate with structural transitions ( 47 , 48 ). These optical transitions are identified by peaks in the gradient signal across a stripe, the locations of which are shown as vertical yellow lines in Fig.…”

Bi Alloying into Rare Earth Double Perovskites Enhances Synthesizability and Visible Light Absorption

“…Comparison of the composition−optical relationships with measured phase behavior indicates that the composition region of double perovskite phase formation can be identified with optical analysis, a strategy for accelerating identifying phase transitions that was recently reported for combinatorial variation of thermal processing conditions (as opposed to variation of 4-cation composition in the present work). 26 This stratregy combined with the high throughput synthesis and optical imaging techniques enabled our rapid screening of 9 additional 4-cation oxide composition spaces to demonstrate that this type of Bi alloying is quite general. The ink solvent was 15 vol % diethylene glycol in water, and each sample was formed by controlled mixing of one to four inks with a total dispensed volume for each sample of ∼50 pL.…”

Discovery of complex oxides via automated experiments and data science

“…We report a high-throughput workflow for discovering candidate compositions for functional properties by coupling high-throughput synthesis and optical characterization with automated data interpretation. Parallel optical screening was recently demonstrated as a proxy for phase behavior in the context of combinatorial thermal processing of individual compositions ( 30 ). We extend this approach to high-order composition spaces using inkjet printing ( 31 ) to deposit composition-gradient lines of material that are subsequently imaged by a purpose-built hyperspectral microscope that measures optical absorption from the infrared to ultraviolet (UV).…”