🚨🚨🚨 New preprint with fancyschmancy directional #vibrational #spectroscopy at #THz frequencies and (pump-probe) #photoconductivity measurements with a great team (a.o. @BonnMischa @JarvistFrost @lonepair ): arxiv.org/abs/2404.12955 ⚡️🔥⚡️ (1/n)
We wanted to study anisotropy (of any form) in 2D perovskites @ #THz frequencies. Since the diffraction limit is kinda hard to beat (1 THz is roughly 300 micron), you need some big single #crystals! Growing these guys very slowly on top of a #liquid #interface did the trick.
We can use a #polarized #THz field to study directional effects: 1) transmission through the single crystals measures the #vibration #spectrum, and 2) using the THz field as an ultrafast probe after an optical pump measures photoconductivity (both in specific directions).
Come in #TeamImperial! Johan calculated the vibrational spectra, and got quite the good match with our experiments. Notice how the motion is such a complicated dance between both the organic and inorganic sublattices... For sure not imaginable for us simple experimentalists 😅.
Notice how 1 peak in our experimental spectrum is actually covered by a zoo of eigenmodes of the lattice: everything is very heterogeneously broadened!
If you then do #XRD in transmission on the same crystal as for all your #THz measurements, you can assign a lattice vector and determine the direction of #transitiondipolemoments. #goreciprocal
We live and breathe #ultrafast #spectroscopy, so ofcourse we did pump-probe #photoconductivity measurements. Now you can keep your #THz polarization fixed, and measure along the 1 THz vibration, or perpendicular to it. (the 1 THz mode in lead-iodide perovskites = magic)
We found, in all samples and through many checks, a reduced photoconductivity (electron mobility) along the 1 THz coordinate. Why? Again, not an answer we experimentalists had ready. We had so much great help from #TeamImperial 😁!
@JarvistFrost used something called #Feynmanpolarontheory to calculate electron mobilities (using the vibrational modes, anisotropic effective masses, etc.) and gets similar values for anisotropy as we observe experimentally 😮😮.
TLDR: interactions between #lattice #vibrations and #electrons in these #perovskites give rise to #anisotropy in #photoconductivity inside the inorganic lamella. This was a great team effort, and I think we stretched our friends' theory (and perhaps patience) quite a bit 😁😁!
From my end, many thanks to @AvHStiftung for sponsoring these ideas and my stay @mpi_polymer! They are fantastic. If you want more info on #Humboldt grants, etc. feel free to get in touch! (end)