Research
Research in our lab seeks to adapt time-resolved spectroscopies that report on excited state dynamics to the measurement of materials during their formation and degradation. We measure electronic structure and exciton dynamics in situ and in real-time as irreversible processes occur, such as molecular aggregation, polymer annealing, and nanocrystal synthesis. We develop strategies to control these processes to create materials with designer excitonic properties.
In situ transient absorption spectroscopy
We have developed a single-shot transient absorption spectrometer that allows us to measure exciton dynamics in situ during materials formation and degradation. Transient absorption uses an ultrafast laser pulse to ‘pump’ the sample, creating excited species, then a second laser pulse after a controlled delay time to ‘probe’ the sample. By changing the delay time, we can measure the dynamics of the excited states. We use tilted beams to spatially encode the time delay in our sample, allowing us to measure the dynamics in a single shot, dramatically decreasing the time needed to complete a measurement. This allows us to measure systems that are changing in time, like aggregating organic molecules, or nanocrystals during their synthesis. Our technique can provide insight into the complex processes involved in materials formation, and will show us how we can steer materials to have particular excited state dynamics by changing environmental conditions while the material is being made. We are continually developing and improving the design of our instruments and building new reaction chambers and film deposition stages so we can measure materials formation in our laser lab, and provide valuable feedback for rational materials design.
Dynamic dynamics during molecular aggregation into thin films
Molecular aggregation during the formation of a thin film can impact the suitability of a material for particular applications. Excited state dynamics are rarely measured during the film formation process, so it is not known whether photophysics smoothly evolves from those of monomers to those of aggregates, or if intermediate states with unique optoelectronic properties play a role. We are measuring these evolving excited state dynamics during the aggregation of small organic molecules using our single-shot transient absorption spectrometer, and correlating the measured dynamics to in situ absorption and fluorescence measurements. By determining how the electronic structure and dynamics change during the film formation process, we gain insight into how we can modify our film formation conditions to kinetically trap aggregates with particular desired photophysics.
Simulations of molecular aggregation
Electronic structure can change dramatically upon self-assembly from monomers into a thin film of molecular aggregates. Absorption, fluorescence, and transient absorption measurements performed in situ, during thin film formation, contain a wealth of information on how molecular aggregates form and how they impact photophysics. We are simulating these evolving spectra using a site-based Hamiltonian, and fitting in situ absorption and fluorescence spectra to linear combinations of n-mers to provide insight into the evolving composition of a material during molecular aggregation. We are also pursuing Monte Carlo simulations of the process of molecular aggregation. The absorption and fluorescence spectra of a system of coarse-grained molecules are simulated as they undergo a biased random walk. By correlating simulated in situ spectra to experimental spectra, we can provide molecular-level insight into the process of thin film formation.
Evolving photophysics during the synthesis of lead-halide perovskite nanocrystals
Organic-inorganic hybrid perovskites are a class of emerging materials with fascinating electronic properties and the potential for wide-ranging, large scale applications in photovoltaics, solid state lighting, and optoelectronics. We aim to understand how the excited state dynamics of nanocrystals of these materials change during their nucleation and growth. We have found spectral features that are unique to nascent nanocrystals and have shown that these features report on the quality of the nanocrystal surface during growth. Current experiments are focused on understanding how the nanocrystal surface quality changes when using different combinations of ligands during synthesis, and how the surface is affected by the dilution of nanocrystals in a solvent. Understanding how the nanocrystal surface changes during growth and how it can be controlled will provide new strategies for controlling the photophysical behavior of these promising materials.
Mechanisms of halide segregation
Lead-halide perovskite nanocrystals are promising candidates for energy-efficient LED technologies, with their emission wavelength tunable by using mixtures of halides. Although LEDs made using these nanocrystals are easy to fabricate and highly efficient, the progress of this technology has been limited by halide segregation. When the devices are operated or exposed to light, the mixture of halides within the nanocrystals will segregate, causing the emission wavelength to change. When the material is allowed to rest in the dark the halides will remix and the emission wavelength is recovered. We measure photoluminescence and transient absorption during the processes of halide segregation and recovery with the aim of understanding the mechanism of segregation in nanocrystals and exploring strategies to prevent its occurence.