Halide perovskite nanocrystals (NCs) have emerged as an intriguing material for optoelectronic applications, most notably for light-emitting diodes (LEDs), lasers, and solar cells. Their emission wavelength depends not only on material composition but also on size and dimensionality, as in the case of two-dimensional (2D) nanoplatelets (NPLs). These colloidal quantum wells have additional appeal for light emission, as the one-dimensional quantum confinement enhances their radiative rates and enables directional outcoupling. On top of this, due to a monolayer-precise control over their thickness, they constitute an intriguing system for spectroscopic studies on their fundamental optical, phononic, and energetic properties.

In this talk, I will explore our recent results on halide perovskite NPLs, including their synthesis. I will focus on their interesting excitonic properties, such as the energetic fine structure¹ with a strong thickness-dependent bright-dark exciton splitting and on exciton-phonon coupling.² Further, I will highlight their advantages and disadvantages for integration into optoelectronic applications.

The urgency for affordable and reliable detectors for ionizing radiation in medical diagnostics, nuclear control and particle physics is generating growing demand for innovative scintillator devices combining efficient scintillation, fast emission lifetime, high interaction probability with ionizing radiation, as well as mitigated reabsorption to suppress losses in large volume/high-density detectors. Prized for their solution processability, strong light-matter interaction, large electron-hole diffusion length and tunable, intense luminescence at visible wavelengths, lead halide perovskite nanocrystals (LHP-NCs) are attracting growing attention as highly efficient emitters in artificial light sources and as high-Z materials for next generation scintillators and photoconductors for ionizing radiation detection. Nonetheless, several key aspects, such as the trapping and detrapping mechanisms to/from shallow and deep trap states involved in the scintillation process and the radiation hardness of LHP NCs under high doses of ionizing radiation are still not fully understood, leaving scientists without clear indications of the suitability of LHP-NCs in real world radiation detectors or design strategies for materials optimization. In this talk I will present on our recent strategies for high performance radiation detection schemes and will report recent spectroscopic results of the scintillation process and its competitive phenomena, ultimately offering a possible path to the realization of highly efficient and extremely radiation hard LHP-NCs.

Over the last few years, halide perovskite nanocrystals (NCs) have received great attention in many fields ranging from chemistry to physics and engineering owing to their extremely interesting properties such as defect tolerance, high photoluminescence quantum yield (PLQY), tunable optical bandgap not only by their dimensions but also by composition, enhanced stability compared to thin-film perovskites, and ease of synthesis. In general, ion exchange reactions offer access to NCs with precise chemical compositions that are inaccessible by direct synthetic routes. While anion (halide)-exchange in halide perovskite NCs has been greatly exploited to tune their optical properties, cation exchange has been less explored. This talk will be focused on post-synthetic cation-composition tuning in lead halide perovskite NCs for fine-tuning their optical properties.

Semiconducting lead halide perovskites are excellent candidates for realizing low threshold light amplification due to their tunable and highly efficient luminescence, ease of processing and strong light-matter interactions. Indeed, several solution processable lasers have been demonstrated, even operating under nanosecond and continuous wave optical pumping. All of these examples use perovskite architectures with little to no confinement, for example using bulk films, nanowires or quantum dots (QDs) with average sizes well above the Bohr diameter. As such, there is no clear picture whether the use of perovskites in the strong 3D confinement regime would be beneficial, nor is there any insight in how optical gain would develop under those conditions. Here, we show through a combination of quantitative transient absorption and femtosecond fluorescence spectroscopy, that optical gain in strongly confined perovskite 0-dimensional QDs develops at remarkably low average exciton numbers per nanocrystal (<N> << 1). The gain magnitude is however smaller compared to bulk-like perovskite systems and the lifetime is capped by rapid Auger recombination. We are able to explain our observations using a 3-level model that takes both the strong exciton-exciton repulsion and Stokes shift due to electron-phonon coupling into account of strongly confined QDs. The enhanced repulsions and shifts act as a double edge sword, reducing the gain threshold but decreasing gain magnitude and lifetime. The concepts shown here provide a rational approach to look for low threshold optical gain materials based on an optimal interplay between electronic and vibrational degrees of freedom.

Self-assembly of colloidal nanocrystals into long-range ordered superlattices holds great promise in the multiscale engineering of solid-state materials with controlled and programmed functionalities which result not only from combination and enhancement of size-dependent properties of constituent building blocks but also from synergistic effects and novel interactions between neighboring nanocrystals. Thus far, the reports had mainly focused on single-component and binary systems of spherical NCs, yielding SLs isostructural with the known atomic lattices [1]. Far greater structural space is anticipated from combining nanocrystals of various shapes. Caesium lead halide perovskite nanocrystals, possessing unique optoelectronic properties (narrow-band tunable bright emission, high oscillator strength of bright triplet excitons, slow dephasing) and being synthetically available as uniform, monodisperse cubes, are promising building blocks for creating superlattice structures that exhibit collective optical properties.

We show that broad structural diversity enabling effective tuning of the relative position and orientation of sub-10 nm CsPbBr₃ nanocubes can be achieved by their co-assembly with spherical, truncated cuboid and disk-shaped building blocks into long-range ordered multicomponent superlattices. CsPbBr₃ nanocubes combined with spherical Fe₃O₄ or NaGdF₄ nanocrystals and truncated cuboid PbS nanocrystals form binary SLs of six structure types, namely, NaCl-, AlB₂-, CuAu- as well as uncommon to all-sphere assemblies novel AB₂-, quasi-ternary ABO₃- and ABO₆-types. In these structures, nanocubes preserve orientational coherence. Co-assembly of CsPbBr₃ nanocudes with larger disk-shaped LaF₃ NCs (1.6 nm in thickness) results in the formation of seven columnar and lamellar structures with A₂B, AB, AB₂, AB₄ and AB₆ stoichiometry, not observed before for systems comprising spheres and disks. We rationalize the effect of the cubic shape on assembly outcome using packing-density calculations. In the systems with comparable dimensions of nanocubes (8.6 nm) and nanodisks (6.5–12.5 nm), other, non-columnar structures are observed, such as ReO₃-type SL, featuring intimate intermixing and face-to-face alignment of disks and cubes.

Recently, lead halide perovskites (LHP) have attracted a lot of attention in the field of optoelectronics, inter alia, the application in light emitting devices (LED) is of enormous interest.^[1,2] One promising approach to increase the efficiency is the utilization of nanocrystals (NC) due to their ability of tailoring the ligand shell to specific needs.^[3]

Therefore, the ligand dynamics at the NC surface was examined on soy-lecithin covered LHPs and a highly dynamic surface equilibrium was discovered. It was found that depending on the surface ligand density the lateral diffusion, i.e., the movement of the ligand along the NC surface can be tuned. This behavior can be rationalized by the zwitterionic nature of lecithin and the binding situation at the surface. Precisely, as more ligand is added, less surface area per ligand is available and, thus, the molecules can only attach with one functional group rather than both, which results in significantly faster dynamics.

In a subsequent step, ligand exchange on CsPbBrI₂@SiO_x LHPs was performed utilizing organic semiconductors, namely metal derivatives of mono-carboxy tetraphenylporphyrin (mMTPP). By tethering the novel ligand to the NCs, the electronic conductivity could be increased whilst simultaneously suppressing ionic currents. The underlying phenomenon of this effect was the decrease of the charging energy. Upon examining the NCs in electroluminescent devices it was found that the current efficacy and turn-on voltage was improved by the semiconducting ligand almost fivefold.^[4]

However, an especially important downside of perovskite-based devices is the stability of the material against environmental influences. From the temperature-dependent conductivity of CsPbBrI₂ NCs an increased phase stability was found by introducing mZnTPP as a ligand.^[4] Upon exposing the system to X-ray radiation, we found a radiation-induced, isotropic contraction of the crystal lattice which in turn resulted in an altered electrostatic potential. This could be correlated to a characteristic shift in X-ray photoelectron spectroscopy signals, i.e., the electronic structure. Furthermore, these structural changes could be identified as a very likely driving force for the decomposition, which appears to be ligand dependent. For an oleic acid / oleylamine ligand shell, the decomposition into the corresponding salts was found, whereas the exchanged system exhibited a novel, significantly slower decay in the form of a disproportionation of Pb²⁺ to Pb⁰ and Pb³⁺.

Metal halide perovskites are one of the most investigated semiconductor materials, with the 3D CsPbX₃ structure being renowned for its enhanced optoelectronic performance. Multiple studies are currently attempting to substitute lead with other elements while still retaining the original properties of this material. This effort has led to the fabrication of metal halides of lower dimensionality. Recently, the layered perovskite structures have captured attention for their potential as an emerging class of colloidal semiconductors.

Here we report the colloidal synthesis of the pure Ruddlesden – Popper phase Cs₂CdCl₄:Sb³⁺, using a facile hot injection approach under atmospheric conditions. Through strict adjustment of the synthesis parameters with emphasis on the ligand ratio, we obtained nanoplatelets with a well-defined size and morphology. The particles underwent extensive structural characterization through synchrotron X-ray diffraction, pair distribution function analysis and transmission electron microscopy. Rietveld Refinements assigned a 14/mmm tetragonal space group and confirmed the aforementioned phase. Bright field imaging revealed nanoplatelets with a natural tendency to stack along the ab surface, self-assembling into long chains. Size distribution analysis assigned a length of 22.9 ± 3.8 nm and a thickness of 3.7 ± 0.8 nm, which were consistent with the dimension predictions given by the texture analysis from the x-ray diffraction data [1].

Spectroscopic characterization revealed an intense cyan emission, centered at 510 nm, and with a measured absolute PLQY of 20 ± 5% [1]. The emission is ascribed to the doped Sb³⁺ within the structure and is attributed to an s-p transition that originates from self-trapped excitons from the Sb³⁺ hosts [1][2]. Time-resolved photoluminescence measurements gave a double exponential decay, characterized by a fast (∼1.3 ns) and a long (∼1626 ns) component [1]. The existence of these two components is thought to result from two different recombination routes taking place from the excited Sb³⁺ states. A similar behaviour was previously reported in Sb³⁺ doped double-layered perovskites and is consistent with the findings from the respective bulk phase [2] [3] [4].

This work proves as a confirmation that colloidal synthetic approaches can give access to a new generation of RP phase nanocrystals through halides tuning, metal alloying, the replacement of Cs⁺ by alternative counterions and the introduction of various dopants, such as Bi³⁺ or Mn²⁺.

Despite the increased efforts to replace the Pb in the lead halide perovskites-based solar cells with Sn analogs materials, in the nanocrystal field, limited progress is reported. The intrinsic chemical instability of the Sn²⁺ hinders a clear understanding of the origin of the chemical instability and poor optical performance.

Here we present the Sn halide perovskites nanomaterials with tunable size and dimensionality (3D - Sn halide perovskites: ASnX₃, A=Cs⁺, FA⁺, X=I^-, Br^- and 2D - Ruddlesden-Popper (R-NH₃⁺)₂Cs_n-1Sn_nX_3n+1), and characteristic optical properties. The formation mechanism of the perovskite nanocrystals is based on the pre-formation of the 1D chained of [SnI₆]²- octahedra. Similar to Pb-based perovskites, the stability of the NCs is increased for 10 nm particles while the optical performance remains inferior to the Pb cousins.

The relation between the kinetic parameters during the synthesis processes and the chemical and optical stability of the obtained 3D and 2D perovskites nanostructures will be discussed.

A common strategy for downshifting luminescence is based on a trivalent lanthanide (Ln³⁺) ion as emission center and a sensitizer as light absorption center. Lead halide perovskites can be used as sensitizer to meet the demands for downshifting applications. However, perovskite sensitizers suffer from low visible absorption, poor stability, and toxicity. In this study, we demonstrate a downshifting configuration based on lead-free cesium manganese bromide nanocrystals enabling efficient transfer of high energy absorbed photons to the low energy emission centers in the near-infrared spectral region. For that, we obtained phase-pure CsMnBr₃ and Cs₃MnBr₅ nanocrystals via a controlled synthesis and successfully doped them with Er³⁺, Yb³⁺, Tm³⁺ and Nd³⁺. The correlation of the CsMnBr₃ broadband visible absorption to the lanthanides emission was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements. This work provides a lead-free material as an efficient sensitizer, which can help to develop and design visible to NIR downshifters.

A new synthetic method for colloidal perovskite nanocrystals has been designed, which offers slow thermodynamic control [1] instead of conventional kinetic growth [2].  The reaction time is increased up to 30 minutes while a wide size range of nanoparticles, some even reaching the strong confinement regime, is obtained with high level control of size and shape [1].  The synthesized quantum dots (QDs) turn out to have a spheroidal shape on average with remarkably well-separated higher absorption peaks. For the first time, this allows for a direct comparison between theory and experimental data related to the transitions beyond the lowest absorption line. Using empirical modelling with second-order many body perturbation theory, we are able to predict the energy positions as well as the oscillator strength of not only the lowest 1s-1s exciton but also of the higher excitonic transitions [3]. The calculated values are in fair agreement with the experimental data. Besides, by taking into consideration the spherical and cuboidal confining potentials, our theory offers an explanation for the well-defined higher transitions in the spheroidal QDs compared to cuboidal ones obtaining from more standard synthetic approaches [4]. The accuracy of the theoretical methods will be also critically discussed

Lead-halide perovskite (LHP) thin-films and nanocrystals have advanced to the forefront of materials research for a wide array of applications from solar-cells and related optoelectronic devices, near unity quantum yield light sources, to coherent single-photon emitters for quantum information processing. Electron-phonon coupling (EP-coupling) plays a critical role in LHPs, proposed to both enhance performance metrics in some cases, e.g. ‘polaronic protection’ of charge carriers, and limit them in others, e.g. exciton coherence loss and broadened emission in perovskite nanocrystals. At the root of EP-coupling is a shift of the equilibrium atomic coordinates of the atoms in a material, a lattice reorganization, upon a change of the electronic configuration. While various time-resolved spectroscopies have shed light on the phonons involved, the nature of the lattice reorganization, and therefore the mechanisms underlaying EP-coupling in LHPs, remains unclear. Valuable insight can be provided through physical characterization of the inherent excited-states structural dynamics of these materials. In principle, EP-coupling-driven lattice reorganizations can be directly measured through time resolved diffraction, and NCs are an attractive system to probe these dynamics. Here, we perform femtosecond-resolved optical pump diffraction probe measurements to quantify the suprisingly large lattice reorganization occurring as a result of exciton-phonon coupling to the interband transition in formamidinium-lead-bromide (FAPbBr₃, FA = CH₅N₂) NCs. A variety of Ab Intio techniques coupled with modelling are employed to understand the observed effect. Our findings provide an intuitive explanation for the origin of lower energy optical phonon coupling, and provide insight into both the excited state and equilibrium structure of LHPs.

Two-dimensional (2D) lead halide perovskite nanoplatelets are particularly promising for display light sources and single-photon emitters given the large exciton binding energies and the phenomenon of giant oscillator transition strength expected in these confined systems.  Here, we describe the thickness-dependent level structure and fine structure of excitons in colloidal 2D lead halide perovskite nanoplatelets.  We examine the effects of dielectric and quantum confinement on the exciton binding energy in individual and stacked nanoplatelets while accounting for quantum-confinement-induced effective mass anisotropy. We calculate the exciton fine structure and optical selection rules in an electron-hole exchange model, elucidating the roles of intrinsic crystal field splitting [1] and quantum-confinement-induced Bloch function mixing [2] in conjunction with the effects of shape anisotropy and image charges via the long range exchange interaction.  

Metal halide perovskites have attracted substantial interest due to their accessible fabrication process, promising properties for optoelectronic applications. The doping of cations in lattice of perovskite can therefore endow host many novel properties. However, the post-synthetic treatment of perovskite ABX3 nanocrystals (e.g., FAPbI3 or CsPbCl3) for doping remains elusive. The straightforward doping method for both A and B sites are rarely reported, which hinders the further application of optimized doping materials for optoelectronic devices. Herein, employing hybrid water-oil phase (water-hexane), we have partially replaced of Cs or Pb with FA and Mn by similar method for the first time, respectively. Taking advantage of in-situ photoluminescence tracking, we have distinguished the doping process and proposed a model to provide better understanding for the doping process. What’s more, this method could not only be use for doping, but also for synthesis of new materials. This study hence provides a controllable way for doping metal halide perovskites or even synthesis new materials to meet more complexed and specific requirements for different applications.

Lead (Pb) halide based perovskites nanocrystals show outstanding optoelectronic properties. But Pb-toxicity remains an issue for their widespread application. Therefore, search for Pb-free metal halide perovskites is natural. The fundamental question is how to identify alternative metal halides that could replicate optoelectronic properties of the Pb-halide perovskites.[1,2] I will address this question in the first part of my talk. Unfortunately, till date, Pb-free perovskites could not compete with optoelectronic properties Pb-halide perovskites. Why so? This question will also be discussed in the talk. Finally, in the last 3-4 years, we changed the goal post of our research. Instead of attempting to replicate the optoelectronic properties of Pb-halide perovskites, we started to explore newer directions. For example, we dope lanthanides like Yb³⁺ and Er³⁺ in Cs₂AgInCl₆ double perovskites to develop short-wave-infrared (SWIR) light emitting materials.[3] Furthermore, doping and/or codoping ions like Bi³⁺ and Sb³⁺ (that has two electrons in the outermost s-orbiatl) tailor both optical excitation and emission in the UV-visible region.[4,5] In the last part of my talk, I will discuss about such exploration.  

Magnetic doping in halide perovskite semiconductors is of timely interest with prospective for their use in opto-electronic and spin-related devices. Here, we report a thorough investigation of the optical and magneto-optical properties of Ni2+-doped caesium lead halide perovskite with a chemical formula CsPb(Br1-xClx)3, implementing steady-state and transient polarized photoluminescence (PL), and optically detected magnetic resonance (ODMR) spectroscopies. ODMR measurements probe magnetic resonance transitions of photo-generated electrons and holes in these emissive excited states, exposing phenomenological g-factors that deviate from those of band-edge charge carriers. Simulations of the ODMR spectra suggest carrier trapping in shallow traps with a slight anisotropic surrounding and with weak electron-hole exchange coupling. Furthermore, we observe substantial broadening of the hole resonance, associated with magnetic exchange coupling between Ni2+ unpaired spins and the trapped hole spin. Overall, these ODMR measurements uncover the role of the dopant in localizing photo-generated carriers while stiffening the crystal structure, and for the first time, provide a direct observation of carrier-dopant spin exchange interactions in metal-halide perovskite nanocrystals. These results offer insight into the influence of magnetic dopants on the electronic structures of metal-halide perovskites, with a view toward emerging spin-based devices made from perovskites.

Despite extensive reports on red and green perovskite-based LEDs (PeLEDs), development of white PeLEDs remains limited by the low photoluminescence quantum yield of white-emitting perovskites and the undesired energy-transfer (ET) process occurring in multidomain Ruddlesden–Popper perovskites. While ET is beneficial for achieving efficient monochromatic emissions, the broadband spectrum required for white electroluminescence makes this phenomenon undesirable. Processing-induced physical separation of emitters has been proposed as an effective way to curb ET. Here, it is shown that by adopting a bilayered emitter configuration, achieved through a facile antisolvent-assisted spin-coating process, an increase in spatial separation between the blue perovskite and red emitting organic species employed can be realized. This, in turn, has allowed for effective reduction of ET efficiency, leading to a record efficiency of 1.3%, the highest achieved to date from a perovskite-based white electroluminescent device.

The unique properties of lead halide perovskite nanocrystals (NCs) have drawn major attention of researchers over the last decade. However, the high toxicity of the lead in the composition and the relatively low phase stability are delaying their commercialization, therefore, diminishing their advantages [1]. These challenges are one of the major focuses of the community leading to the ongoing search for new stable lead-free compositions. However, up to date, there are only a few promising candidates to surpass the characteristics of lead halide perovskite NCs [2].

So far, no attention to cesium manganese chloride (CsMnCl₃) NCs was given likely due to the absence of promising properties [3]. We are the first ones to synthesize and characterize emissive colloidal CsMnCl₃ NCs through the conventional hot-injection route. The optical, structural, and morphological properties of CsMnCl₃ NCs were studied by the means of steady-state-, time-resolved, and low-temperature spectroscopy, transmission electron microscopy (TEM), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray diffractometry (XRD) respectively. The synthesized CsMnCl₃ NCs display  photoluminescence at 670 nm, large Stokes shift (250 nm), broad emission (FWHM = 97 nm), lifetime decay in the millisecond range, and fairly high quantum yield of 25%.

Moreover, Rietveld structural refinement was carried out to confirm the adopted structure of the NCs and investigate the relationship between synthesis conditions, crystal structure, and photoluminescence. It was found that when the synthesis temperature was tuned, the CsMnCl3 adopt different crystal structures (i.e. cubic and rhombohedral). By optimizing the temperature and the precursors' ratio, a pure rhombohedral structure is formed. It is worth nothing, that up to date, there is no work showing the luminescent rhombohedral CsMnCl₃ as well as the dependence between the synthesis conditions, structure, and optical properties.

In addition, the synthesized NCs show moderate phase and optical stability (more than 2 weeks) under the ambient conditions (dark, RH 40-60%, RT). Together with its unique optical properties, CsMnCl₃ is a promising candidate to substitute lead-halide perovskite NCs. The results of our work provide important insights into the photochemistry of manganese-based materials.

Abstract:

The fast rising demand for the ultralow detection limit of ionizing radiation in medical radiography, high-energy physics and security screening, has led to extensive research on X-ray imaging scintillators and detections. However, high-performance scintillators consist mainly of ceramic that needs harsh and costly preparation conditions. Therefore, searching for new scintillation materials is of great interest to material scientists, chemists, and engineers. Organic emitters and perovskites, are excellent candidates as scintillation materials due to their good processability and low-fabrication cost. In this talk, we will present the room-temperature synthesis of a colloidal scintillator comprising CsPbBr3 nanosheets of large concentration (up to 150 mg/mL). We found that the CsPbBr3 colloid exhibits a light yield (∼21000 photons/MeV) higher than that of the commercially available Ce:LuAG single-crystal scintillator (∼18000 photons/MeV). Interestingly, we reveal that the energy transfer process inside those stacked thin and thick nanosheet solids is indeed responsible for their superb scintillation performance. Moreover, we will present a highly efficient energy transfer strategy between the interfaces of these CsPbBr3 perovskite nanosheets and thermally activated delayed fluorescence (TADF) to obtain an efficient and reabsorption-free organic X-ray imaging scintillator with excellent performance. More specifically, the fabricated nanocomposite scintillators exhibit a high X-ray imaging resolution of around 100 μm and a low detection limit of 38.7 nGy/s. This detection limit is about 142 times lower than a typical dose of X-ray medical imaging, making this composite an excellent candidate for X-ray radiography.

Lead halide perovskites (LHP) are long-known crystalline materials with ABX3 general formula (where A=Cs+, MA:CH3NH3+ or FA:CH(NH2)2+, B=Pb2+ and X=Cl-,Br-,I-), characterized by a three-dimensional [PbX6]4- framework and a large A cation residing the cuboctahedra cavities. These materials in form of nanocrystals (NCs) are considered ideal candidates to be integrated in television displays and LEDs.[1] Due to the dynamic nature of the perovskite lattice, preventing the charge carriers from trapping, LHP NCs are highly tolerant to structural defects and surface states, which are considered benign with respect to their electronic and optical properties.[2]

The flexible nature of the perovskite framework, very prone to structural defectiveness, coupled with the reduced size of crystalline domains, makes these materials unsuitable for conventional crystallographic methods. At this purpose total scattering techniques based on the Debye Scattering Equation (DSE), have been established as effective methods for characterizing nanoscale materials and taking into account size-induced structural defects, emerging upon downsizing.[3] Through the DSE-based method developed by some of us,[4] starting from real space atomistic models, structural and microstructural information on NCs can be simultaneously derived within a unified approach, with all the well-known advantages associated to the use of reciprocal space methods.

In this talk, experimental and modelling aspects, related to the DSE approach and applied to provide atomic-to-nanometer scale insights on key nanoscale features of LHP NCs, will be presented.

A broad spectrum of structural and morphological features of LHP NCs, unveiled through a synergic combination of reciprocal space methods based on the DSE, will be analysed, from their peculiar defectiveness,[5] to faceting and surface termination,[6] to the formation of self-organized superstructures.

Lead-halide perovskite nanocrystals (NCs) have emerged as attractive nano-building blocks for photovoltaics and optoelectronic devices. Optimization of perovskite NC-based devices relies on a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. This talk will give an overview of our recent magneto-optical spectroscopic studies [1-5] revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of the inhomogeneities in the NC morphologies and crystal structures. It will highlight the prominent role of the electron-hole exchange interaction in the order and splitting of the bright triplet and dark singlet exciton sublevels and discuss the effects of size, shape anisotropy and dielectric screening on the fine structure. The spectral and temporal manifestations of the thermal mixing between bright and dark excitons allows extracting the specific nature and strength of the exciton-phonon coupling, which sheds light on the remarkable photovoltaic properties of these materials and provides an explanation for their remarkably bright photoluminescence at low temperature although the ground exciton state is optically inactive. These findings make single perovskite NCs attractive for a potential use as quantum light sources.

- Fu, M ; Tamarat, P. ; Huang, H. ; Even, J. ; Rogach, A. L. ; Lounis, B. Neutral and charged exciton fine structure in single lead halide perovskite nanocrystals revealed by magneto-optical spectroscopy, Nano Letters (2017) 17, 2895-2901

10.1021/acs.nanolett.7b00064

- Fu, M ; Tamarat, P. ; Trebbia, J.-B. ; Bodnarchuk, M. I. ; Kovalenko, M. V. ; Even, J. ; Lounis, B. Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons, Nature Communications (2018), 9, 3318

10.1038/s41467-018-05876-0

- Tamarat, P. ; Bodnarchuk, M. I. ; Trebbia, J.-B. ; Erni, R. ; Kovalenko, M. V. ; Even, J. ; Lounis, B. The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state, Nature Materials (2019) 18, 717-724.

10.1038/s41563-019-0364-x

- Tamarat, P. ; Hou, L. ; Bodnarchuk, M. I. ; Trebbia, Swarnkar, A. ; Biadala, L. ; Louyer, Y. ; Bodnarchuk, M. I. ; Kovalenko, M. V. ; Even, J. ; Lounis, B. The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals, Nature Communications (2020) 11, 6001.

10.1038/s41467-020-19740-7

- Hou, L. ; Tamarat, P. ; Lounis, B. Revealing the Exciton Fine Structure in Lead Halide Perovskite Nanocrystals, Nanomaterials (2021) 11, 1058.

10.3390/nano11041058