Development of a Wide-Field Transient Absorption Microscope for Applications to Two-Dimensional Perovskite Quantum Wells

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

With support from the Chemical Measurement and Imaging Program in the Division of Chemistry and the Atomic, Molecular, and Optical Experimental Physics Program (AMO-E) in the Division of Physics, Professor Moran at the University of North Carolina, Chapel Hill, is developing a microscope capable of imaging electron movement in perovskite crystals. This work is motivated by the potential for solar energy harvesting demonstrated by perovksites. The perovskite crystals targeted in this project have two-dimensional (2D) shapes where the faces of the crystals have millimeter-scale dimensions. Such 2D perovskites absorb light in the visible range and are relatively resistant to humidity, which is desirable for harnessing solar energy. Professor Moran and his team plan to simultaneously monitor multiple places on the crystal surface to get statistical information on electron movement in the crystals. The information from these experiments can be used to guide material synthesis to make better solar energy conversion devices. Graduate and undergraduate students involved in this project become proficient in chemistry, computer programming, machining, electronics, and optics as the research group is building most of their own equipment. In addition, Professor Moran and his students supervise laboratory exercises that illustrate the principles of optics and solar cells for middle and high-school students in collaboration with the Morehead Planetarium on the UNC campus. Professor Moran at the University of North Carolina, Chapel Hill is developing a diffractive optic-based transient absorption microscope capable of imaging electron diffusion in two-dimensional (2D) perovskite quantum wells. The quantum wells targeted have garnered attention for their stabilities under humid conditions and large exciton binding energies. In addition, the research team is investigating layered systems that consist of "stacks" of quantum wells in which the band gap decreases with respect to height in the film. Thus, electronic excitations are funneled from the bottom to the top of the film in these layered perovskites. The diffractive optic-based microscope employed in this research acquires transient absorption data at 41 locations on the sample surface in a single experiment. Signals are simultaneously detected at two probe wavelengths using a pair of array detectors (e.g., wavelengths corresponding to energy donor and acceptor quantum wells). In studies of exfoliated crystals, diffusivities of excitons are being measured at a number of excitation wavelengths to establish a quantitative relationship between the carrier diffusivity and excitation energy. Statistical distributions of diffusion rates are generated by analyzing the dynamics at 41 different spots on the sample. The information derived from these experiments can be used to guide materials synthesis and device engineering. Graduate and undergraduate students becoming proficient in computer programming, machining, electronics, and optics as the research group is building most of their own equipment. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.