Presenter: Johan Hofkens (KU Leuven)
Topic: From blinking of single molecule to looking in perovskite-based devices
Time: June 3, 15:30 (Wednesday)
Location: 912-0130
Abstract:
Single molecule spectroscopy has tremendously impacted every field in which the technique was applied, ranging from catalysis over plasmonics, polymer physics, biophysics to cell biology and DNA sequencing. Furthermore, single molecule techniques have allowed researchers to push the resolution of fluorescence microscopy past the diffraction limit, based on characteristic single molecule intermittence of fluorescence. In this presentation, I will give an overview of how recent single molecule experiments and development of new microscopy modalities in my laboratory have been impacted by material science and how these experiments have been driving developments in material science, with a focus on our perovskite research. Unlike conventional semiconductor materials, metal halide perovskites (MHP) possess soft and ionic crystal structures leading to several unique features like facile ion migration, self-healing, elasticity, and memory. Within this dynamic system, external stimuli like high photon doses, electron beams, electrical bias, and mechanical stress induce structural changes and alter associated optoelectronic properties. Therefore, it is crucial to investigate the structure-function relationship in these materials, especially in operational devices like solar cells, where traditional methods such as scanning electron microscopy fall short due to the layered structure. Moreover, electron and x-ray-based analytical techniques are often invasive altering the material properties.
To address these challenges, we developed Correlation Clustering Imaging (CLIM), a novel noninvasive method that utilizes photoluminescence fluctuations to reveal contrasts associated with defect dynamics in semiconductor materials. CLIM images of perovskite thin films show one-to-one matching with the grains in SEM images captured at the same locations. Particularly noteworthy is the application of CLIM to high-efficiency photovoltaic devices, uncovering previously unnoticed photoluminescence intensity fluctuations that strongly depend on the device's operational regime. Statistical analysis of these intensity fluctuations provides insights into the type of metastable defects responsible for fluctuating non-radiative recombination processes.
The insights gained from CLIM contribute to a deeper understanding of device efficiency, structure, and degradation, which are crucial for the rational engineering of the next generation of devices. The broad applicability of CLIM, requiring only a standard wide-field microscope and our user-friendly, open-source algorithm, positions it as an important new tool for material chemists, engineers, and device scientists.
Biography：
Professor Johan Hofkens is a Full Professor in the Faculty of Science at KU Leuven, Belgium, where he heads Molecular Imaging and Photonics, the Division of Photochemistry and Spectroscopy, and the Subdivision Single Molecules. He is also a member of the KU Leuven Institute for Micro- and Nanoscale Integration (LIMNI). He is an elected member of Academia Europaea, the European Academy of Sciences, and the Royal Flemish Academy of Belgium for Science and the Arts.
Professor Hofkens is internationally recognized for his contributions to single-molecule spectroscopy, fluorescence microscopy, super-resolution imaging, and the development of advanced optical methods for studying complex molecular and materials systems. His work is distinguished by the development of advanced optical microscopy and spectroscopy methods that reveal nanoscale heterogeneity, defect dynamics, and structure-function relationships that are difficult to access by conventional ensemble or electron-microscopy-based techniques. His research connects spectroscopy and microscopy with biophysics, catalysis, polymer dynamics, DNA optical mapping, luminescent materials, and metal-halide perovskites.
In recent years, his group has applied advanced fluorescence imaging and fluctuation-based analysis to metal-halide perovskites and perovskite-based optoelectronic devices, including the development of Correlation Clustering Imaging (CLIM) to visualize defect-related dynamics under device-relevant conditions. His current projects include in-operando characterization of perovskite-derived optoelectronic devices, pure-blue halide perovskite emitters and LEDs, 2D perovskite/metal composites, 3D fluorescence microscopy, and photocatalytic nitrogen-to-ammonia conversion.
Professor Hofkens has authored more than 700 indexed publications, with over 49,000 citations and a D-index of 114 according to Research.com. His recent publications include work in Science, Nature, and Cell, and related leading journals. He has received major international recognition, including an ERC Advanced Grant, an ERC Proof of Concept Grant, the Proteomass Scientific Society Award, and the Otto Wolfbeis Fluorescence Prize.