correlated electron and structural dynamics in quasi-2D HYbrid PErovskites – 2D-HYPE

The methods employed to investigate the electrons and lattice dynamics in halide perovskites are time-resolved Angle-Resolved Photoelectron Spectroscopy (tr-ARPES) and THz-induced birefringence.

tr-ARPES is an ideal technique for exploring the structure of electronic states out of equilibrium. In a semiconductor, a visible pump laser excites electrons from the valence band to the conduction band, while an ultraviolet probe pulse induces the emission of photoelectrons. An analyzer provides instantaneous snapshots of the electronic states as a function of energy and wavevector. For the experiment on perovskites, the probe beam, with a photon energy centered at 6.2 eV, is horizontally polarized and focused down to 100 × 100 microns, while the pump beam, with a photon energy of 3.1 eV, generates electron–hole pairs across the bandgap. The cross-correlation between pump and probe has a full width at half maximum (FWHM) of roughly 120 fs, which is short enough to resolve the steps related to carrier relaxation. Being a direct probe of electronic excitations, tr-ARPES is a powerful technique to determine band alignments, surface band bending, and the dynamics of charge accumulation. Moreover, by exploiting the wavevector distribution of the ARPES signal, it is possible on crystalline surfaces to reconstruct the wavefunction of Wannier excitons that form once the electrons and holes have relaxed to the band extrema.

Tailoring the physical properties of lead halide perovskites by fine-tuning their lattice structures has so far been achieved primarily through chemical composition or morphology. Nevertheless, their dynamic counterpart—phonon-driven ultrafast material control, as contemporarily demonstrated in oxide perovskites—has not yet been established. This approach can be pursued by irradiating the sample with high-field THz pulses while monitoring coherently excited modes via transient birefringence. Intense single-cycle THz pump pulses with peak fields of approximately 1 MV/cm, together with a collinearly propagating 800 nm probe pulse, are used. The pump and probe are linearly polarized at 45° with respect to each other, enabling stroboscopic sampling of the THz-induced birefringence. This process corresponds to a wave-mixing interaction, where the signal field is emitted by the nonlinear polarization induced through the coupling of THz and visible/NIR fields. Since the probe photon energy lies below the band gap of all investigated samples, lattice dynamics can be probed in the unperturbed electronic ground state.

None of the experiments in this project would have been possible without the growth of high-quality samples. The single-crystal samples of hybrid perovskites were grown using the anti-solvent vapour-assisted crystallisation (AVC) method.

In our first tr-ARPES work, the ionic surface terminations of CH3NH3PbI3 are employed as a testbed to study the effect of electrostatic fields on the dynamics of excited carriers. We characterized the transition across the tetragonal to orthorhombic phase. The observed type II band offset and drift of the excited electrons highlight the important role that organic cations have on the screening of local electrostatic fields. We conclude that the local fields cannot penetrate in the sample when the polarizability of freely moving cations boosts the dielectric constant up to ε = 120. This work has been published in Nano Letters 22, 2065 (2022).

Intense THz electric fields have been employed to obtain direct lattice control via nonlinear excitation of coherent octahedral twist modes in hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites. These Raman-active phonons at 0.9 to 1.3 THz are found to govern the ultrafast THz-induced Kerr effect in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability with potential implications for dynamic charge carrier screening beyond the Fröhlich polaron. This work has been published in Science Advances 9, eadg3856 (2023).

The novel perovskite composition a four cation mixture, abbreviated as (4cat)PbBr3 has been investigated via photoluminescnce and transient birefingence. Notably, the ultrafast Terahertz induced birefringence reveals a dominating 1.1 THz octahedral twist mode, also observed in MAPbBr3, however with a doubled phonon coherence time in (4cat)PbBr3 at 80 K. The observation of the prolonged lattice coherences indicate enhanced dynamic screening effects. This work has been published in Small 21 2500977 (2025).

Tr-ARPES is employed to monitor photoexcited electrons in the two-dimensional (BA)2(MA)2Pb3I10. ARPES intensity maps are in good agreement with ab-initio calculations of the band structure, providing, for the first time, a direct estimate of effective masses for holes and electrons. The correlated electron-hole plasma evolves in Wannier excitons with Bohr radius of 2.5 nm, while no sign of self-trapping in small polarons is found within the investigated time window of up to 120 ps following photoexcitation. This work can be found at arXiv:2508.12129 and is under review in Nanoletters.

In structurally complex, low-dimensional hybrid organic-inorganic perovskites, the presence and exact mechanisms of inversion symmetry breaking remain elusive. Intense THz fields, are therefore employed to coherently drive and identify lattice dynamics carrying optical signatures of inversion symmetry breaking in Ruddlesden-Popper (PEA)2(MA)n-1PbnI3n+1 perovskites. We demonstrate coherent control by THz pulses over distinct phonons. Moreover, it is possible to identify simultaneous IR- and Raman-activity, suggesting the presence of inversion symmetry breaking. This work can be found at arXiv:2503.02529 and is under review in Advanced Materials.

The tr-ARPES results on two-dimensional hybrid perovskites demonstrate the capability of using photoelectron intensity maps to reconstruct exciton wavefunctions. This approach can be systematically applied to perovskites with increasing degrees of electronic confinement, enabling the study of Bohr radius shrinkage.

The lowest excitonic state is known to be a singlet, which, being dark, cannot recombine via radiative emission. The tr-ARPES signal, however, is not restricted to bright excitons and can thus provide direct access to the non-radiative recombination dynamics of such dark states. Ultimately, this may allow the generation of a high density of dark excitons, potentially triggering emergent phases such as condensates or Wigner crystallization.

Furthermore, the THz-field Kerr effect has proven to be a versatile experimental method for investigating the coherent dynamics of modes with broken inversion symmetry. This paves the way for the simultaneous, ultrafast control of optoelectronic and spintronic properties in 2D HOIPs. Our work opens up new possibilities for selectively manipulating the vibrational degrees of freedom that govern phase transitions, generating phases of broken inversion symmetry and tuning the magnitude of the Rashba coupling.

Hybrid lead halide perovskites are materials that currently attract widespread interest for their application in optoelectronic devices. Their quasi two-dimensional equivalents have shown higher stability than the 3D compounds and offer the possibility to tune the out-of-plane screening properties. Here, we propose to monitor concurrent excited state and screening dynamics on ultrafast timescales. Time resolved photoemission, photoluminescence and high-field THz excitation will provide complementary views on the entangled degrees of freedom. In particular, we aim to follow the exciton formation, quantify the spin-orbit interaction, and control screening effects due to dynamic disorder, cation orientation and electronic confinement; all of these being potential ingredients for Rashba splitting and ferroelectricity in this material class. Our research project will offer novel insights on non-equilibrium physics and might as well lead to new strategies for materials optimization.