Wavelength Control of Spin Lifetimes in Layered Metal Halide Perovskites

Quantum well structures made up by layered metal halide perovskites (L-MHPs) represent an interesting playground for a plethora of fundamental and technological research topics. Photoexcitation of L-MHPs result in the formation of stable excitons [1,2] that, because of the Rashba splitting characteristic of the electronic energy bands, [3] can be spin-polarized if the incident light source is circularly polarised. This optical injection of spin-polarized carriers makes L-MHPs promising candidates for future opto-spintronic applications.[4]<br/>In a previous work,[5] we used femtosecond time-resolved Faraday Rotation (TRFR) spectroscopy to investigate the spin lifetime of (BA)₂FAPb₂I₇(BA: Butylammonium; FA: Formamidinium). Interestingly, we observed that the spin lifetime can be increased by more than two orders of magnitude, at 77 K, just by using light with excess energy above the optical band-gap. Since perovskites are soft materials and the formation of stable polarons has been repeatedly reported in both bulk and layered structures,[6] we explained our observations by considering the formation of a polaron state. Indeed, although at low temperatures the phonon population is small, the photogenerated hot carriers cool down within the first hundreds of femtoseconds generating optical phonons that can form polarons. Polarons are characterised by a different exchange interaction with respect to excitons and, thus, we expect that the spin depolarization follows a different mechanism. In particular, we observe that the spin lifetime increases with increasing excitation wavelength, suggesting that the depolarization mechanism is slowed down as the number of generated polarons is increased.<br/>With the aim to address the role of the chemical structure of L-MHPs onto this phenomenon, here we use TRFR spectroscopy to study three L-MHP compositions from 4 to 300 K: (Hexa)₂CsPb₂I₇, (Hexa)₂FAPb₂I₇ and (Hexa)₂MAPb₂I₇ (Hexa: hexylammonium; MA: methylammonium). All investigated samples exhibit the same trend: spin relaxation becomes slower at low temperatures when the materials are photoexcited with excess energy, but faster when no excess energy is provided. Furthermore, when samples are photoexcited in resonance with their optical band-gap, we observed that a mono-exponential regime is reached (with an associated lifetime of around 200 fs) by all the investigated chemical compositions, but at different temperatures. This finding suggests that below a specific temperature, the process leading to longer spin lifetimes is much slower than the spin relaxation of excitons, which further supports our hypothesis about the thermal-induced formation of polaron states. In particular, the observed thresholds are ~64K for MA⁺, ~77K for FA⁺ and ~130K for Cs⁺. These values follow the opposite trend of the dipole moment values of MA⁺, FA⁺ and Cs⁺, thus larger dipole moments contribute to the stabilization of polarons.<br/>Our results provide a pathway towards the wavelength-control of spin depolarization in L-MHPs, offering a possible application for the realisation of opto-spintronic devices.<br/><br/><b>References</b><br/>1. G. Grancini and M. K. Nazeeruddin, Nat. Rev. Mat., 4 (2019) 4-22.<br/>2. J. C. Blancon, J. Even, C. C. Stoumpos, M. G. Kanatzidis and A. D. Mohite, Nat. Nanotech., 15 (2020) 969-985.<br/>3. E. Mosconi, T. Etienne and F. De Angelis, J. Phys. Chem. Lett., 8 (2017) 2247-2252.<br/>4. Z. Chen, G. Dong and J. Qiu, Adv. Quantum Technol., 4 (2021) 2100052.<br/>5. S. Bourelle, F. V. D. A. Camargo, S. Ghosh, T. Neumann, T. W. J. Van de Goor, R. Shivanna, T. Winkler, G. Cerullo and F. Deschler, Nat. Comm., 13 (2022) 3320.<br/>6. L. R. Buizza and L. M. Herz, Adv. Mater., 33 (2021) 2007057.