The structural symmetry of materials significantly impacts their physical properties and enables new applications. Beyond the optoelectronic applications, the large spin-orbit coupling in highly tunable two-dimensional (2D) hybrid halide perovskites can lead to Rashba and/or Dresselhaus spin splitting, which, when combined with other multifunctional properties, make them promising for spin-orbitronic applications. 2D halide perovskite materials could be rationally designed by structural tuning to achieve the desired noncentrosymmetry. Here we utilize the complex interplay between the organic spacer cations (especially asymmetric halogenated spacer cations), the A-site cations, metal cations (Pb, Sn, Bi and Ag), halide anions, and dimensionality to design non-centrosymmetric 2D metal halide perovskites that exhibit second harmonic generation (SHG), ferroelectricity, Rashba splitting and persistent spin textures. These rational design strategies to unlock and control non-centrosymmetry in diverse metal halide perovskites and their nanostructures introduce nonlinear optical properties, ferroelectric properties, and exotic spin textures for spin-orbitronic and quantum applications.

Dion-Jacobson phase multilayered halide perovskites (MLHPs) improve carrier transport and optoelectronic performance thanks to their shorter interlayer distance, long carrier lifetimes, and minimized nonradiative losses. However, limited atomistic insights into dynamic structure–property relationships hinder rational design efforts to further boost their performance. Here, we employ nonadiabatic molecular dynamics, time-domain density functional theory, and unsupervised machine learning to uncover the impact of A-cation mixing on controlling the excited carrier dynamics and recombination processes in MLHPs. Mixing smaller-sized Cs with methylammonium in MLHP weakens electron–phonon interactions, suppresses the nonradiative losses, and slows down intraband hot electron relaxations. On the contrary, larger-sized guanidinium incorporation accelerates nonradiative relaxations. The mutual information analyses reveal the importance of interlayer distances, intra- and interoctahedral angle dynamics, and A-cation motion in extending the excited carrier lifetime by mitigating nonradiative losses in MLHPs. Our work provides a guideline for strategically choosing A-cations to boost the optoelectronic performance of layered halide perovskites.[1]

Chiral hybrid organic-inorganic perovskites (c-HOIPs) have emerged as a class of semiconductors that couple unique chiroptical properties with spin-polarized charge transport. Achieving long spin lifetimes and high charge carrier mobilities simultaneously is key to realizing their potential in spin-optoelectronic applications. While monolayer c-HOIPs exhibit large circular dichroism and photoluminescence dissymmetry, extending chirality into quasi-2D c-HOIPs with improved transport remains an open challenge. In this talk, I will discuss our recent efforts to address this challenge by tuning the layer-number (n-value) of quasi-2D c-HOIPs. By systematically varying the layer number, we uncover how structural confinement and electronic coupling govern their chiroptical response, spin relaxation, and charge carrier mobility. I will also highlight how these insights translate into device performance including circularly polarized light detection. In addition, I will introduce our recent demonstration of inverse chirality induced spin selectivity using terahertz (THz) emission spectroscopy as a non-contact probe of spin to charge conversion. The THz emission results directly reveal chirality dependent phase reversals, offering a new route to probe spin polarization in 2D c-HOIPs. These results establish 2D chiral hybrid organic-inorganic perovskites as model systems for controlling spin-to-charge conversion and for exploring the interplay between chirality, spin, and charge transport in next-generation spin-optoelectronic materials.

2D metal-halide perovskites are interesting materials due to their controllable optoelectronic properties, such as anisotropic charge diffusion, and composition-dependent tuneable band gap. The 2D perovskite crystals we investigated are organic–inorganic hybrids composed of semiconducting metal–halide octahedral layers sandwiched between two layers of mostly insulating organic cations [1]. This architecture gives rise to highly anisotropic charge transport, with carriers preferentially moving in the in-plane direction, strongly weakening the transport between the two octahedra layers [2]. We exploit this intrinsic anisotropy by growing lateral heterostructures in the PEA₂PbBr₄–PEA₂PbI₄ system, where the higher-band-gap PEA₂PbBr₄ core is surrounded by the lower-band-gap PEA₂PbI₄. This configuration enables directional charge or energy flow in the in-plane direction from the core toward the surrounding material. Going beyond conventional optical characterization, we employ Scanning Transmission Electron Microscopy, including both compositional analysis using Energy-Dispersive X-ray Spectroscopy (EDX) and scanning electron diffraction (4D-STEM). 4D-STEM is particularly insightful as it enables probing of the crystallography at high lateral resolution (probe size ~few nm) with very limited electron dose (dose < 1 e^-/Ả²), preventing damage to the sample [3]. Combining powder X-Ray diffraction with nanoscale diffraction provides a complementary set of information with both ensemble and single-particle insight.

We observe a high degree of crystallinity in all particles, which generally comprise a single crystal or a few large domains that grow along the ab-plane (the one containing the inorganic sheets) and are observed from the [001] direction in STEM. Two different growth modes are distinguished using STEM imaging and STEM-EDX, leading to a majority fraction of the particles growing as a bromide micro-platelet with iodide extensions on two opposing facets, and a minority fraction with the iodide phase fully framing the bromide micro-platelet on all sides. Both architectures are emissive, and with 4D-STEM we not only identified the crystallographic relationships between core and surrounding phase, gaining insight into the growth mode, but also studied the crystallographic relationship between core and frame, which includes lattice distortion, and the degree of alloying, while taking into account the in-plane rotation of the octahedral lattice. The interfaces between halide phases indicate a coherent lattice direction, suggesting a good potential for seamless electron transport.

Overall, this study demonstrates an approach for understanding local crystallographic properties in a complex, beam-sensitive system, revealing crystal correlations at the nanoscale, therefor paving the way to engineer new 2D perovskite materials and devices.

An exciton, a quasi-particle consisting of an electron and a hole bound by Coulomb interaction, represents the lowest electronic excitation in a perfect semiconductor. The exchange interaction, which couples the spins of the electron and hole, affects excitonic states, leading to fine structure splitting. This interaction lifts the degeneracy of states with different angular momenta, separating the bright and dark exciton states. The energy separation and ordering of exciton states can significantly impact the optoelectronic properties of materials. The emerging field of two-dimensional organic-inorganic halide perovskites (2DP) offers a novel platform to explore exciton fine structure splitting (FSS) and its potential applications. In these materials, quantum and dielectric confinement enhance the Coulomb interaction, resulting in a much larger FSS compared to conventional low-dimensional systems. The splitting of excitonic states in 2DP can reach tens of meV, which is orders of magnitude greater than in epitaxial structures or nanocrystals. This large splitting, combined with the excellent optical properties of 2DP and the ease of engineering their band structure and quantum confinement, makes them an ideal system for studying exciton FSS physics.

Here, I present our recent findings, revealing a complete spectrum of excitonic states within its fine structure and available knobs to tune exchange interaction. Furthermore, I demonstrate that the excitonic properties of 2D perovskites are significantly influenced by carrier-lattice interactions, resulting in a complex interplay between exciton fine structure and phonons that profoundly affects their optical response. I will focus on the phonon bottleneck effect between bright and dark exciton states. Despite the substantial splitting between these states, which can reach tens of meV, 2D perovskites exhibit surprisingly intense photoluminescence emission even at cryogenic temperatures, indicating a non-Boltzmann distribution of excitons. However, the reason for this high bright-state occupation has remained unclear. Using magneto-optical spectroscopy, I will show that the exciton population is characterized by a higher temperature than the crystal lattice. To explain this observation, we employed detailed microscopic and material-specific many-particle theory to investigate the formation, relaxation, and decay dynamics of excitons. Our modelling reveals that the energy mismatch between the exciton fine structure and phonons leads to a pronounced phonon bottleneck effect, highlighting the importance of exciton fine structure and carrier-phonon interaction in the optical response of metal halide perovskites.

The emergence of perovskites as sought-after semiconductors owes to their unique electronic band structure, inherent defect tolerance, increased conversion efficiencies, synthetic flexibility, cost-effectiveness, near-unity PL quantum yield, narrow emission bandwidth, high solar cell efficiencies, etc. All these properties are due to the dynamic nature of the ground and excited states. These hybrid systems have strong electron-phonon interactions which manifest strongly in their optical and photovoltaic behaviours. A critical factor for understanding the light mater interaction is the direct observation of the ultrafast coupling between lattice phonon modes and the charge carriers. Using IVS, we were able to directly visualize light induced carrier- phonon interactions by observing the strong oscillatory features dressing the charge transfer and recombination kinetics of charge carriers. We observe not only the phonon modes coupling to the charge carriers in individual 2D layer with strong electron-phonon interaction but also the triggering of the coherent phonon modes in 3D layers upon charge transfer process completion. Finally, we present that upon suitable structural engineering of the interface, via epitaxial growth, we can efficiently transfer not only charge carrier but also trigger coherent phonon modes which can help improve charge recombination and protect carriers from trap states.

Hybrid organic-inorganic halide perovskites are mixed ionic-electronic semiconductors with
exceptional optoelectronic characteristics. However, their ion migration contributes to the
instability under device operating conditions, such as voltage bias and light, limiting their
application.[1] The suppression of ionic migration has been critical to enhancing operational
stability, such as by incorporating organic moieties into halide perovskite frameworks to form
layered perovskite architectures that are more resilient during device operation.[2,3]
While this remains an ongoing challenge in photovoltaics, the control of mixed conductivities
in response to external stimuli has provided an opportunity for more energy-efficient resistive
switching memories or memristors and artificial synapses in brain-inspired computing.[2,3]
We explore the resistive switching of arylammonium-based layered hybrid perovskites for
non-volatile memories and neuromorphic computing[2] and demonstrate their utility in
mixed-dimensional perovskite heterostructures incorporating perfluoroarene moieties for
memristive solar cells.[3] We further reveal the ability to replace toxic lead cations in lead-free
layered halide perovskite memories[4] for a new generation of neuromorphic systems[4,5].

Two-dimensional (2D) halide perovskites are attractive for improving device stability and performance, yet their electronic structure and energy level alignments remain insufficiently understood. We therefore investigate a series of alkyl­ammonium-based Ruddlesden–Popper perovskites (n = 1, A'₂PbI₄) with spacer cation lengths from propylammonium (C₃) to decylammonium (C₁₀), combining X-ray diffraction, optical spectroscopy, and ultraviolet photoelectron spectroscopy (UPS).

UPS reveals systematic, chain-length–dependent variations in the measured density of states. These changes align well with density functional theory (DFT) when considering the shallow probing depth of UPS. Notably, the ionization energy remains nearly constant across all samples and comparable to MAPbI₃, indicating that the widening of the band gap in 2D perovskites is dominated by an upward shift of the conduction band rather than changes in the valence band.

Optical measurements show only modest band gap variations (up to ~90 meV). Instead of a monotonic trend, an odd–even effect emerges: perovskites with odd-numbered alkyl chains exhibit a blue-shifted absorption onset relative to even-numbered ones. DFT attributes this to Pb–I–Pb bond distortions in the inorganic layer, driven by differences in alkyl-chain packing.

Finally, we use reflection electron energy loss spectroscopy (REELS) to probe electronic transitions with tunable probing depth (~1–10 nm). This enables us to assess surface-related gap modifications and is particularly useful for studying the formation and properties of 2D surface layers on 3D perovskites. Several representative examples will be presented.

Proliferation of data-intensive applications has posed challenges for existing wireless connectivity. As a potential alternative to existing WiFi technology, optical wireless communication (OWC) integrates data communication with ambient lighting and can act as next-generation high-speed free-space data communication platform. OWC transmits data by modulating light intensity, with data transmission capacity dependent on the switching speed of the emitter. In addition to making faster emitters, the capacity can be enhanced by sending data simultaneously across multiple independent channels at different wavelengths.

In this talk, I will discuss the requirements of novel emitter materials and how to test their merit for application in OWC. My discussion will connect the fields of materials science and photophysics with the application and performance of novel emitters in communication systems.

Based on these considerations, I will highlight the great potential of perovskite nanocrystals for OWC applications and show our recent results towards establishing a multi-channel communication platform based on different perovskite compositions as luminescence converters. In particular, I will discuss how we employed advanced keying schemes and data processing to achieve data transmission rates on the order of Gbit/s.

The outstanding optoelectronic properties of lead-halide perovskites have made them prominent emerging materials for several applications, from solar cells to quantum light emitters. In such devices, heterostructuring between perovskite materials may provide the key to operational stability and functionality. We will start by discussing the issue of calculating band alignment at interfaces, a crucial factor for device performance. We will then present the theory of excitons trapped in self-assembled buried quantum dots, which have allowed single-photon generation within an electrically injected thin-film device. In these systems, a semi-empirical approach based on the enveloppe function approximation gives access to the photoluminescence energy and fine structure of quantum-confined excitons, a meV-scale spin-related splitting of the excitonic ground state into four levels of differing oscillator strength. The calculation of these quantities as a function of the quantum dot size may be of significant use in the interpretation of single-dot photoluminescence data. 

Halide perovskite quantum dots (QDs) have emerged as a promising material for optoelectronic devices in different fields comprising light harvesting, emission or detection. Adding to the well-established potential of conventional inorganic QDs originating in their quantum confinement properties, halide perovskite QDs present additional features such as a compositionally tunable bandgap across the visible spectrum, near unity quantum yield or fast photoluminescence to name but a few. [1] While many of these exciting features are usually reported for isolated QDs, most real-life devices will demand the incorporation of QDs into an assembly that, for some applications, permit charge injection/extraction and therefore some degree of electronic coupling. In this work we present a study of the photophysical properties of halide perovskite QD films in two different configurations representing widespread synthetic approaches. The conclusions from this study allow us to unveil the way energy migration takes place in this kind of systems and extract conclusions regarding the applicability of each configuration in different fields.

On the one hand, QD films made from ligand-free QDs grown within the pores of nanoporous metal oxide matrices are considered. In this system changing the amount of perovskite precursor in the matrix allows tuning the average inter-QD separation and thus its electronic coupling. 2,3] We demonstrate a transition from a system of isolated excitonic emitters to an interconnected QD array presenting a recombination regime between excitonic and free-carrier systems. [2] Such changes in connectivity drastically influence the properties of the QD ensemble from the emission quantum yield to the cooling of photoexcited carriers. [4]

Finally, we explore how the presence of organic ligands in state-of-the-art QDs fabricated by means of the widely extended hot injection method affect the interaction among QDs. In particular, we perform temperature dependent time-resolved PL measurements that evidence how the photophysical properties of these QD films in the presence of organic ligands can be described through a combination of trap passivation, photonic effects and energy transfer. [5]

Besides conventional optoelectronic devices (LEDs and lasers), colloidal quantum dots (QDs) are being pursued as non-classical light sources (i.e., single-photon emitters) that may play a pivotal role in future quantum technologies, such as quantum computing, quantum cryptography, and quantum sensing.

Due to strongly reduced charge trapping on surface states and their defect-tolerant character, perovskite QDs become attractive as alternative quantum light sources. Indeed, very stable, blinking-free emission¹ has been observed at cryogenic temperatures, characterized by an ultrafast radiative lifetime^2,3 and a long exciton dephasing time. In addition, when organized in highly ordered three-dimensional superlattices, perovskite QDs exhibit superfluorescence (SF)^4,5, a cooperative emission of individual emitters that arises due to a coherent collective coupling to a common light field.

The talk will review our recent achievements in the exploration of perovskite QDs as non-classical light sources, in particular energy-degenerate photon-pair generation from Individual CsPbBr₃ quantum dots⁶ and future developments.