Transmission electron microscopy is an excellent tool to investigate the structure and composition of a wide variety of nanomaterials. Unfortunately, metal halide perovskites are extremely sensitive to electron beam irradiation. For example, Pb clusters are easily formed, which hamper a further reliable analysis of the structure. In this talk, I will discuss recent progress in the field of transmission electron microscopy, enabling one to investigate these materials with reduced damage. Moreover, I will demonstrate how these results can be interpreted in a quantitative manner, transforming images into numbers. Finally, the possibilities of extending these studies into 3 dimensions will be discussed. 

Blinking nanoscale emitters, typically single molecules, are employed in single-molecule localization microscopy (SMLM) to overcome Abbe's diffraction limit, offering spatial resolution of few tens of nanometers. Colloidal quantum dots (QDs) feature high photostability, ultrahigh absorption cross-sections and brightness, as well as wide tunability of the emission properties, making them a compelling alternative to organic molecules. Here, CsPbBr₃ nanocrystals, the latest addition to the QD family, are explored as probes in SMLM. Because of the strongly suppressed QD photoluminescence blinking (ON/OFF occurrence higher than 90%), it is difficult to resolve emitters with overlapping point-spread functions by standard dSTORM methods due to false localizations. A new workflow based on ellipticity filtering efficiently identifies false localizations and allows the precise localization of QDs with subwavelength spatial resolution. Aided by Monte-Carlo simulations, the optimal QD blinking dynamics for dSTORM applications is identified, harnessing the benefits of higher QD absorption cross-section and the enhanced QD photostability to further expand the field of QD super-resolution microscopy toward sub-nanometer spatial resolution.

References

[1] Advanced Optical Materials, https://doi.org/10.1002/adom.202100620

Perovskite quantum dots (PQDs) are now emerging as functional materials for many photonic applications due to their superior optical properties and easy fabrication. Recently, we developed the in-situ fabrication of hybrid perovskite PQDs in polymeric films (PQDF) with high transparency, superior photoluminescence emission and additional processing benefits for down-shifting applications. The potential use of PQDF as color converters in LCD backlights was successfully demonstrated, showing bright potential in display technology. Very recently, we developed the in-situ patterning technology of PQD for Micro LED and other down conversion applications. Moreover, we further explored the electroluminescence devices based in-situ fabricated PQDs. We illustrated the enhancing role of ligand-assisted reprecipitation (LARP) process and obtained uniform FAPbX3 (X=Br, Cl, I) PQDs films with photoluminescence quantum yield up to 78%. The electroluminescence devices with a maximum external quantum efficiency (EQE) of 16.3%, 15.8% and 8.8% were achieved for green, red and blue devices, showing the promising to achieve high efficiency. In all, the in-situ fabrication strategy provides very convenient route for display technology. In this talk, I will introduce the progress on the development of in-situ fabricated perovskite quantum dots toward photonic and optoelectronic applications.

Solution processable semiconducting perovskites hold great promise for demanding applications involving light emission, such as printable lasers or quantum light sources. A case example is that of fully inorganic CsPbBr3quantum wells (CQWs) which display high quantum yield at room temperature. Less studied than their organic-inorganic counterparts, these CQWs sustain high single exciton binding energies and luminescence quantum yields, yet little is known on the nature of the exciton and multi-excitons states required for advanced applications. Here, we show that charge carriers in fully inorganic 2D perovskites exist as stable exciton - polarons, a complex between a charge neutral exciton and a lattice deformation. Next, we show that these unique species can fuse together to form a hereto unexplored bi-exciton polaron state, i.e. a two-particle complex bound by attractive Coulomb attraction whilst simultaneously being strongly coupled to the lattice. Finally, we show that net stimulated emission occurs through radiative recombination from this unique bi-exciton polaron state to a single free exciton polaron, showing the stability of the newly found species. Consequences of the polaronic character are identified as a low threshold for stimulated emission but with limited optical gain coefficients, both of which we can fully reproduce using a thermodynamic gain model. As such, our results provide a general framework to understand and predict the behavior of not only single, but also multi-exciton polaron states in perovskite materials.

The metastable alpha phase of pure formamidinium lead halide perovskite has been obtained from controlled thermal growth. We have measured the activation energy of the heating rates able to kinetically activating the alpha phase. By using the flash infrared annealing method [1, 2], we have screened a large amount of sequential data. This data was compiled into a database to use as a reliable guide for fundamental studies of halide perovskites. Therefore, the correlation of thermal growth with morphology, structure, and photophysical behavior of the films under different growth conditions has been investigated. Thus, we manufactured highly stable perovskite solar cells with photonic sintering (antisolvent-free) for more than 500 hours.  

Relevant alternatives to decrease the density of surface defects such as ligand passivation, doping, and  synthetic protocols have allowed to obtain high-quality perovskite nanocrystals (PNCs). However, the impact of the purity of the precursors involved during the PNCs synthesis to hinder the emergence of defects has not been widely explored. In this work, we analyzed the use of different crystallization processes of PbX₂ to purify the chemicals and produce highly luminescent and stable blue- and red-emitting PNCs. The use of a hydrothermal (Hyd) process to improve the quality of the as-prepared PbCl₂ provides CsPbCl3-xBrx PNCs with efficient ligand surface passivation, maximum PLQY ~88% and improved photocatalytic activity to oxidize organic compounds such as Benzyl alcohol. Then, the hot-recrystallization of PbI₂ prior to Hyd treatment led to the formation of CsPbI3 PNCs with PLQY up to 100%, long-term stability around 4 months under ambient air and relative humidity of 50-60%. Thus, CsPbI₃ LEDs were fabricated to provide a maximum external quantum efficiency up to 13.6%. We claim the improvement of the PbX₂ crystallinity offers a suitable stoichiometry in the PNCs structure, reducing non-radiative carrier traps and so maximizing the radiative recombination dynamics. [1,2] This contribution gives an insight about how the manipulation of the PbX₂ precursor is a profitable and potential alternative to synthesize PNCs with improved photophysical features, by making use of defect-engineering.

Solution-processed lasers have emerged as versatile, light sources for a variety of low-cost applications. Among the various resonator-gain media combinations, vertical cavity surface emitting lasers (VCSELs) based on lead halide perovskite nanocrystals (LHP NCs) appear particular attractive as they combine the impressive optical amplification properties of the LHP NCs with facile fabrication of relatively high Q-factor microcavities (MCs) suitable for low threshold optical amplification and lasing with stable intensity and small beam divergence. Yet, there are currently limited reports on such systems in the literature.

Herein, we demonstrate the fabrication of monolithic, all-solution processed MCs based on green-emitting CsPbBr₃ and red-emitting CsPbI₃ NCs combined with polymeric distributed Bragg reflectors (DBRs) produced out of alternate layers of cellulose acetate (CA) and polyvinylcarbazole (PVK) materials. Steady state and time-resolved photoluminescence (PL) experiments along with angle dependent reflectivity and PL are implemented for the characterization of the MCs. The photonic structures exhibit high Q-factors exceeding ~100 allowing the continuous wave excitation of amplified spontaneous emission (ASE) with low threshold of the order of ~100 mW/cm². The ASE onset is evidenced by the: (i) threshold-type behavior and spectral narrowing on the excitation variable PL, (ii) amplification of the output intensity by up to one order of magnitude in the vicinity of the cavity mode, (iii) quenching of the PL lifetime when the cavity resonance overlaps with the NC emission. The aforementioned results, demonstrate the high potential of LHP NC photonic structures for practical, scalable, low cost laser applications.

The problem of instability in halide perovskite nanocrystals (NCs) remains a major deterrent towards their rational development in different applications. This instability can be largely attributed to the dynamic nature of both the organic ligands and the inorganic core of the NCs. Thus, the search for a better stabilizing ligand has generated a lot of interest among different researchers. For example, quaternary alkylammonium ligands such as didodecyldimethylammonium bromide (DDAB) have been used in attempts to stabilize the surface of CsPbBr₃ nanocrystals but often with contradictory results reported in literature. While some researchers have reported improvement in colloidal stability, quantum yield, and optoelectronic properties of CsPbBr₃ NCs using DDAB,¹ others have reported it to cause a phase transformation to poorly fluorescent 2D CsPb₂Br₅.² Thus, the impact of quaternary alkylammonium ligands on the surface of CsPbBr₃ NCs and its role in driving phase transformation have remained unclear. Here, we investigated the thermodynamics of ligand exchange and subsequent processes as DDAB is introduced to CsPbBr3 NCs coated with oleate/oleylammonium native ligands. Using isothermal titration calorimetry (ITC) combined with complementary spectroscopy analysis, we were able to resolve the processes that occur upon introduction of DDAB to natively capped CsPbBr₃ nanocrystals. The first step, involving ligand exchange proceeds readily at low DDAB concentration and is endothermic (𝜟H ~30 kJ/mol) with an exchange equilibrium constant K_ex >100 indicating that the ligand exchange is entropically driven. On the other hand, larger equivalencies of DDAB bring about a second, exothermic process which corresponds to the displacement of PbBr_x complexes from the NCs surface. This process ultimately leads to the formation of 2D phases. Resolving these processes through direct thermal measurements helps to reconcile contradictory reports in prior studies of the surface passivation with quaternary ammonium ligands and also showed that ITC could be a viable technique in understanding the dynamics of the surface ligands on perovskite NCs that can be geared towards a better understanding of the passivation process for improved stability. In addition to this, we compared the ITC profile of DDAB exchange with other analogous ion pairs such as tetraoctylammonium bromide (TOAB), tetraoctylammonium hexafluorophosphate (TOA-PF₆), and didodecyldimethylammonium hexafluorophosphate (DDA-PF₆), revealing the ability of DDA cation to occupy Cs vacancy sites promoting strong interaction of the ligand with NC surface.

Semiconductors of the perovskite form have attracted considerable attention in recent years, most notably for their performance in photovoltaics. One key puzzle was how a disordered film could support such performance. A number of investigations have led to a current picture of defect tolerance due to a soft, ionic lattice giving rise to dynamic disorder. Moreover, the presence of polarons may be connected to this defect tolerance and ultimately in the device performance, whether photovoltaics or light emitting diodes. How do these ionic semiconductors relate to their covalent cousins, for which defect-free perfection is a main goal? In this Perspective, the question of ionicity as it gives rise to glassy structural dynamics is considered. How do these glassy structural dynamics control the optical response and thus the opto-electronic properties of the system? The recent literature is reviewed with a focus on ultrafast structural dynamics. Highlighted in the discussion of structure / function relation is our recent work which maps the chronology of events from electronic coherence to photon emission using a suite of three time-resolved spectroscopies: Two-Dimensional Electronic Spectroscopy, State-Resolved Pump/Probe Spectroscopy, and time resolved Photoluminescence Spectroscopy. These works collectively advance our understanding of liquid-solid duality in these defect tolerant ionic semiconductors. With the connection to liquid-like dynamics made, a brief discussion of ultrafast solvation dynamics is presented. The specific connection between excess charges in liquids and solids can be found in the solvated electron, which once again serves as a prototype for electronic structure and dynamics in the condensed phase. We hope that finding isomorphisms between phases can shed insight into these new materials.

Isothermal phase transformations in lead iodide perovskite nanocrystals of different A-site compositions were studied by tracking the black-to-yellow color change in nanocrystal films and modeling the transformation kinetics. At 210 ^oC, CsPbI₃ turned completely yellow in 25 min, while it took MAPbI₃ and FAPbI₃ longer time to complete the color change. CsPbI₃ transformed from the black perovskite phase to the yellow non-perovskite phase, while MAPbI₃ and FAPbI₃ chemically decomposed and formed PbI₂ as the new phase. Mixed A-site perovskites CsMAPbI₃ and CsFAPbI₃ showed slower rate of transformation than single A-site perovskites, and their degradation products were coexisting yellow phase CsPbI₃ and PbI₂. Different capping ligands also played a role in changing the transformation kinetics.

Halide perovskite nanocrystals are a promising platform for optoelectronic devices, including display light sources and single-photon emitters, owing to the fast, efficient   radiative decay in these materials. Two-dimensional (2D) halide perovskite quantum dots or nanoplatelets would be particularly promising for bright, fast radiative recombination, since confinement will lead to large exciton binding energies and giant oscillator transition strength.  In this work, we explore the thickness-dependent fine structure of excitons in 2D halide perovskite nanoplatelets.  We build a quasi-2D effective-mass model of excitons which are strongly confined in the out-of-plane direction but weakly confined in-plane. The model includes the effect of shape anisotropy on the long-range exchange corrections to the fine structure and introduces the important effect of confinement on the band-edge Bloch functions. Applying the model to colloidal CsPbBr₃ nanoplatelets, where electron-hole exchange splitting is expected to dominate the exciton fine structure, we predict a thickness-dependent level order of the in-plane-polarized and out-of-plane-polarized bright exciton states.  The calculated fine structure  is consistent with observed thickness-dependent spectral shifts [1]. We extend the model to 2D hybrid perovskite materials, for which large Rashba coefficients have been either measured or predicted, and describe the dispersion, level structure, and optical properties of the 2D “Rashba exciton”. The analysis reveals unusual features, notably a dispersion minimum at non-zero exciton wave-vector. Consequences of the dispersion to level structure and oscillator transition strengths of Rashba excitons confined in a 2D cylindrical QDs are developed and used to establish criteria under which a bright ground exciton state could be realized [2].

[1] "Dark and Bright Excitons in Halide Perovskite Nanoplatelets", M. Gramlich, M. W. Swift, C.  Lampe, J. L. Lyons, M. Doblinger, Al. L.Efros, P C. Sercel, A. S. Urban, To be published.

[2] "Rashba exciton in a 2D perovskite quantum dot", M W. Swift, J. L. Lyons, Al. L. Efros, and P. C. Sercel, To be published.

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 radioluminescence at visible wavelengths, lead halide perovskite nanocrystals (LHP-NCs) are attracting growing attention 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 focus on recent strategies for high performance radiation detection schemes based on LHP-NCs[1] and will report recent spectroscopic results of the scintillation process and its competitive phenomena[2], 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.¹ The morphology of perovskite NCs can be easily controlled from 3D nanocubes to quasi-0D nanodots, 2D nanoplates, and nanowires by means of simple chemistry such as precursor ratios, ligand concentration, and crystallization temperature.^1-4 In addition, their chemical composition is easily tunable by in situ synthesis or post-synthetic ion exchange reactions. In this talk, different ways of shape and composition tunability in both inorganic and organic-inorganic hybrid perovskite NCs will be discussed. 

Colloidal lead halide perovskite nanocrystals (LHP NCs_, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) have become a research spotlight owing to their spectrally narrow (<100 meV) fluorescence, tunable over the entire visible spectral region of 400-800 nm, as well as facile colloidal synthesis. These NCs are attractive single-photon emitters as well as make for an attractive building block for creating controlled, aggregated states exhibiting collective luminescence phenomena. Attaining of such states through the spontaneous self-assembly into long-range ordered superlattices (SLs) is a particularly attractive avenue. In this regard also the atomically-flat, sharp cuboic shape of LHP NCs is of interest, because vast majority of prior work had invoked NCs of rather spherical shape. Long-range ordered SLs with the simple cubic packing of cubic perovskite NCs exhibit sharp red-shifted lines in their emission spectra and superfluorescence (a fast collective emission resulting from coherent multi-NCs excited states). When combined with spherical dielectric NCs, perovskite-type ABO₃ binary NC SLs form, wherein CsPbBr₃ nanocubes occupy B- and/or O-sites, while with spherical dielectric Fe₃O₄ or NaGdF₄ NCs reside on A-sites. When truncated-cuboid PbS NCs are added to these systems, ternary ABO₃-phase form (PbS NCs occuoy B-sites). Such ABO₃ SLs, as well as other newly obtained SL structures (binary NaCl- and AlB₂-types, columnar assemblies with disks etc.), exhibit a high degree of orientational ordering of CsPbBr₃ nanocubes. These mesostructures exhibit superfluorescence as well, characterized, at high excitation density, by emission pulses with ultrafast (22 ps) radiative decay and Burnham-Chiao ringing behaviour with a strongly accelerated build-up time.

Emission of single photons from an isolated colloidal semiconductor nanocrystal is a well-studied phenomenon. In fact, nowadays we can design nanocrystals with engineered shape and/or composition to optimize single-photon emission and prevent detrimental processes such as blinking or slow photon emission.

All the knowledge gathered on single-photon emission processes in colloidal semiconductor nanocrystals is based on the use of light as excitation source, and rightly so, as this allows to carefully study the photophysical processes governing photons generation via exciton recombination. Yet, for colloidal semiconductor nanocrystals to be appealing from an application point of view, single-photon emission from nanocrystals must be triggered via electrical injection of an electron and hole pair. Currently, electrical generation of single photons in nanocrystals has only been marginally explored; to reach such objective, electrode design, nanocrystals deposition control and other device fabrication aspects must be addressed.

In this seminar, I will present some initial results on how to position a single nanocrystal on an electrode or other venues to fabricate light-emitting diodes based on a single and isolated colloidal semiconductor nanocrystal. The methods employed exploit the properties of the two of the most studied types of nanocrystals: CdSe/CdS core-shell systems and perovskites ones. 

Halide perovskite nanocrystals have been introduced in 2015 as a promising route to efficient light emitting devices, and more recently as potential single photon emitters. Their radiative recombinations are governed by excitons, but the bright or dark character of the ground state is debated. A Rashba effect correlated with a local polarization was proposed in 2018 as a general mechanism for inversion of bright and dark level ordering, but direct spectroscopic signatures of the dark exciton emission in the low-temperature photoluminescence of single FAPbBr₃ and CsPbI₃ nanocrystals under magnetic fields rather yield a dark singlet below the bright triplet. The dark-bright splitting values are in fair agreement with numerical estimations of the long-range electron–hole exchange interaction. The intense luminescence of perovskite nanocrystals is alternatively attributed to a reduced bright-to-dark phonon-assisted relaxation. The phonon side bands in nanocrystals match neutron scattering phonon spectroscopy results obtained at low temperature. Finally, a recent classification of 3D perovskites into ferroelectric or hyperferroelectric materials, points toward the role of the depolarization effect for the reduction of a spontaneous polarization in nanocrystals.

Cesium lead halide perovskite nanocrystals are a highly attractive class of materials for coherent light emission, with implications for lasing, light-emitting diodes, and quantum computing. Fine-tuning their properties for the above applications requires an exact understanding of their exciton fine structure, in particular, spacing and polarization of their triplet and singlet states. Experimental reports have been controversial, implicating that the Rashba effect may be inducing an inversion in the order of bright and dark states.

To aid in the resolution of this debate, we performed investigation of the fine structure of the triplet emission properties in these materials. Using the wave functions generated via DFT calculations including spin-orbit coupling for cubic, orthorhombic and tetragonal caesium lead halide perovskite nanocrystals of ~3 nm in diameter, we further augmented them with Coulomb coupling between the exciton configurations, to resolve the absorption and emission fine structure in a configuration interaction method.

We anticipate our work will aid in the resolution of the debated emission fine structure of CsPbBr3 nanocrystals and thereafter allow for the development of bright materials for optoelectronics.