FIP Seminar: Advanced Ultrafast Diffuse Reflectance Optical Microscopy-Spectroscopy for Photochemical Analysis at Nanoscale

Presenter: Dr. Elham Ghadiri, Elham Ghadiri, Asst. Professor, Department of Chemistry, and Center for Functional Materials, Wake Forest University

Ultrafast pump-probe microscopy has been utilized as a promising new ultrafast approach. Transient absorption microscopy a sub-class of pump-probe microscopy enables the localization of the photochemical and photophysical processes in complex systems offering femtosecond time-resolution and sub-micrometer spatial resolution. In comparison, electron-based microscopy methods provide higher spatial resolution. Still, these technologies are typically complex, expensive, and need a high vacuum, which hinders their widespread use for a variety of samples. Ultrafast diffuse reflectance spectroscopy is of great value for in-situ time-resolved analysis of complex photophysical processes in scattering or highly light-absorbing environments.

Environmentally friendly and low-cost solution-processed semiconductor materials with tunable bandgaps are widely desired for applications in energy conversion devices, optoelectronics, and bio-opto-electronics. Photophysical processes, such as charge carrier photogeneration, recombination dynamics, the role of various defects, and their influence on the carrier lifetime control the performance of such devices. In general, a spectroscopic/optoelectronic characterization approach that can identify such processes in such devices with accuracy is indispensable.

Here I will discuss my lab’s recent efforts to implement ultrafast time-resolved pump-probe diffuse reflectance microscopy-spectroscopy in different modalities for photochemical and photophysical analysis of energy conversion materials and bioinspired opto-electronic material systems prepared in our lab. I will show that diffuse-reflectance spectroscopy enables in-situ and in-operando measurements of charge carrier processes in solar cells. We will discuss that the kinetics and dynamics measured in functional devices differ by orders of magnitude from those measured on model samples. I will also show examples of ultrafast microscopy on such samples which enables the visualization of such processes within grains of the semiconductor and provides unprecedented new information.

Elham Ghadiri is an as an assistant professor at the Department of Chemistry at Wake Forest University. She is also a core-member at the center for Functional Materials (CFM) at Wake Forest University. Her lab is focused on advancing ultrafast laser microscopy-spectroscopy techniques and materials platforms for solar energy conversion, and bio-opto-electronic applications. Elham received her PhD in Nanoscience and Nanotechnology from Sharif University of Technology in 2010. She obtained a PhD in Chemistry in 2014 from Ecole Polytechnique Federal de Lausanne (EPFL) under the supervision of Professor Michael Grätzel and Professor Jacques Moser. She joined Prof. Warren S. Warren’s lab at Duke University and completed her postdoctoral training with him.