Opto-electronic devices based on all-inorganic perovskite systems are an energy-efficient source of lighting due to their high photoluminescence quantum yield (QY). However, dominant surface trapping continues to plague the field, despite their high defect tolerance, as evidenced by the several fold improvements in the external quantum efficiency of perovskite nanocrystals (NCs) upon appropriate surface passivation or physical confinement between high bandgap materials. Here, we introduce the concept of drip-feeding of photo-excited electrons from an impurity-induced spin-forbidden state to address this major shortcoming.  An increased and delayed (about several milliseconds) excitonic QY, and density functional theory establish the electron back-transfer signifying efficient recombination.  We term this electron back-transfer from Mn²⁺ to the host conduction band in this prototypical example of Mn-doped CsPbX₃ (X = Cl, Br) NCs through vibrational coupling as Vibrationally Assisted Delayed Fluorescence (VADF).

References:

1. Pradeep K. R., Debdipto Acharya, Priyanka Jain, Kushagra Gahlot, Anur Yadav, Andrea Camellini, Margherita Zavelani-     Rossi, Giulio Cerullo, Chandrabhas Narayana, Shobhana Narasimhan, and Ranjani Viswanatha, Harvesting Delayed Fluorescence in Perovskite Nanocrystals Using Spin-Forbidden Mn d States, ACS Energy Letters 2020 5 (2), 353-359

Mixed halide perovskites (MHP) have been highlighted as promissory materials in optoelectronics, due to improved light harvesting, photocarrier generation, and the ease for tuning their optical properties, specially their band gap.[1] This feature has open the door to analogous solar driven process as photocatalysis for carrying out the photodegradation of recalcitrant organic compounds more efficiently.[2] Nonetheless, the photocatalytic (PC) activity of MHP mainly depends on the surface chemical environment formed during their synthesis. This correlation has not been studied yet. In this work, we deduced the nature and the role of surface chemical states of MHP nanocrystals (NC) synthesized by hot-injection (H-I) and anion-exchange (A-E) methods, on their PC performance for the oxidation of β‑naphthol as a model system. We identified iodide vacancies as the main surface chemical states that promote the formation of highly reactive superoxide ions. These species define the PC activity of A‑E-MHP. Conversely, the PC performance of H-I-MHP is dictated by an adequate balance between band gap and highly oxidizing valence band. In this context, MHP can be considered as good photocatalysts for efficient environmental remediation.

The application of Lead Halide Perovskites (LHP) nanomaterials in many technological fields can take advantage of the physico-chemical properties typical of highly anisotropic morphologies, which can be introduced in nanocrystals by extreme reduction of their thickness [1, 2]. Overcoming critical issues of stability for extremely downsized nanocrystals [3], quasi-2D highly stable CsPbBr₃ nanoplatelets (NPLs) have been synthetized, which exhibit large exciton binding energies and intense blue emission, with PL maximum tunable through fast post-synthetic anionic-exchange reactions. In addition, due to the large surface area, NPLs show a clear tendency to self-assemble, making them particularly promising for LED applications [4]. A precise and accurate atomistic description of LHP structures is of high relevance for formulating reliable considerations over their exceptional properties. An in-depth structural and morphological characterization of CsPbBr₃ NPLs has been carried out by means of a combination of Transmission Electron Microscopy (TEM) imaging and Wide Angle X-ray Total Scattering (WAXTS) techniques. The analysis of high resolution WAXTS data, collected with synchrotron source on colloidal suspensions, was based on the construction of atomistic models of NPLs and the application of the Debye Scattering Equation (DSE) [5]. This advanced approach of structural analysis addresses limitations of traditional X-ray diffraction techniques when applied to complex nanomaterials [6]. In the CsPbBr₃ NPLs case, it enabled to elucidate multiple structural and morphological aspects. Through the analysis, robust information on the NPLs thickness of six PbBr₆ octahedra monolayers was obtained. Moreover, the orthorhombic crystal structure and its specific relative orientation with respect to the NPLs facets were determined. In particular, the results indicate that the NPLs most extended surfaces expose octahedral equatorial bromides, whereas the axial bromides run parallel to the crystallographic b axis that is the most expanded. The overall surface composition remains defective in Br and Cs [7].

Perovskite CsPb(Br_xI_1-x)₃ nanocrystals are attractive building blocks for light-emitting assemblies for a few reasons. First, modern syntheses enable preparation of size and shape-pure CsPb(Br_xI_1-x)₃ nanocrystal samples with tunable composition and optical properties. Second, microscopic assemblies of these nanocrystals can be readily grown by low-cost fabrication methods, and their structure examined by standard x-ray diffraction. Third, the strong light absorption and peculiar emission characteristics of these nanocrystals create the possibility of collective emission phenomena. A practical application of such assemblies, for example, as coherent light sources embedded in a circuit, requires a stable emission under optical or electric stimuli and environmental conditions. Dynamic surface passivation of nanocrystals and halide photochemistry are two factors that alter their light emission in counterintuitive ways. In our contribution, we will discuss these barriers to the stable optical performance of CsPb(Br_xI_1-x)₃ nanocrystal assemblies and ways to overcome them.

Connecting nanocrystals with removing interface ligand barriers is one of the key steps for efficient carrier transportation in opto-electronic device fabrication. Typically, ion migration for crystal deformation or connection with another nanocrystals needs a solvent as medium. However, in contrary, this has been observed for CsPbBr₃ perovskite nanocrystals in film where nanocrystals were swelled to get wider and fused with adjacent nanocrystals in self-assembly in film with solvent evaporation. Depending on precursor composition and exposed facets, again these connections could be programmed for tuning their connecting directions leading to different shapes. Aging further on solid substrate, these were also turned to continuous film of nanostructures eliminating all inter-particles gaps on the film. This transformation could be ceased at any point of time, simply by heating or adding sufficient ligands. Analysis suggested that these unique and controlled connections were only observed with polyhedron shaped nanostructures with certain compositions and not with traditionally cubes. Details of these transformations, the change in optical properties, modulations of shapes, importance of solvent evaporation and the impact of different shaped nanostructures in this process would be discussed and presented in the presentation.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on halide perovskite nanocrystals (NCs) as well has gathered momentum in the last years. While most of the emphasis has been put on CsPbX₃ perovskite NCs, more recently the so-called double perovskite NCs, having chemical formula A⁺₂B⁺B³⁺X₆, have been identified as possible alternative materials, together with various other metal halides structures and compositions, often doped with various other elements. This talk will also discuss the research efforts of our group on these materials. We will highlight how for example halide double perovskite NCs are much less surface tolerant than the corresponding Pb-based perovskite NCs and that alternative surface passivation strategies need be devised in order to further optimize their optical performance. Other topics that will be covered are the role of surface ligands on stabilizing the NCs, including those with alloy compositions, and the synthesis of heterostructures in which one domain is a halide perovskite and the other domain is another material.

Quasi-two-dimensional (2D) semiconductor nano-platelets (NPs) manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision.[1] We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and 2D density of excited states in a series of CdSe nanoplatelets.[2] The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations.  Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. We have shown that this difference is mainly connected with enhancing of dielectric confinement of light holes relative to the heavy  hole observed in STS measurement and an increase of light  hole exciton binding energy relative to  heavy hole  exciton binding energy observed in absorption spectra.[2]    The dielectric confinement  increases also  the distance   between first and second 2D sub-bands of  electrons and holes  in perovskite NPs.    As a result the sum of the distance between first and second electron and hole subbands measured by   STS  should be  larger than distance between first and second absorption maxima. 

1. S. Ithurria, M. D. Tessier, B. Mahler, R. P. S. M. Lobo, B. Dubertret, and Al. L. Efros, A. L. Nat. Mater. 2011, 10, 936−941.

2. B. Ji, E. Rabani, Al. L. Efros, R. Vaxenburg, O. Ashkenazi, D. Azulay, U. Banin, and O. Milo ACS Nano, 2020, 14, 8257-8265.

Interest in perovskite (ABX₃) LEDs has exploded over the past several years due to their strong light-emitting properties, tunable emission, and facile fabrication. High performance red and green devices have been demonstrated by several groups, with external quantum efficiencies (EQEs) over 20%, matching OLED performance. Blue LEDs, however, have lagging significantly behind. Here, we identify and rectify the two crucial issues holding these materials back: the low internal photoluminescence yield and the LED device structure itself.

First, we show that NiOx, a common hole transport layer in these materials, induces defects in the nanocrystals, reducing emission by an order of magnitude and introducing rapid decay channels. To fix this issue, we design a new hole transport layer based on a combination of a traditional hole transport layer and perfluorinated ionomer, allowing the nanocrystals to emit with their native efficiency and lifetime. We then show that this translates directly to devices, increasing the EQE from 0.03% to 0.50%. Further, we show that the benefits apply to devices across the visible spectrum, with efficient blue, sky-blue, and green devices.

Next, we demonstrate that a large portion of the lost efficiency is due to inherent emission losses in the blue perovskite nanocrystals, as they demonstrate a thin film photoluminescence quantum yield of less that 10%. This can be rectified, however, by taking advantage of a surprising effect. When doping the nanocrystals with Mn, which introduces an energetic loss pathway to an orange emissive state, we counterintuitively observe an increase in perovskite emission. By carefully tuning the amount of Mn in the nanocrystal, we can increase the quantum yield over 3x. This translates directly to device performance, as doped devices reach a maximum EQE of 2.1%. The combination of the device and dopant improvements increases performance by over 60x, showing that blue perovskite materials can be competitive with their red and green cousins. Finally, we use this efficient blue emitter to build an all-perovskite white LED, which has important applications in lighting.

We will conclude the talk by demonstrating that Mn is not alone as an effective dopant: several other atomic dopants across the periodic table provide similar benefit. We will present the similarities and differences amongst these dopant conditions and show how we can use them to push device performance even further.

Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high photoluminescence quantum efficiency, large spin–orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr₃) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr₃ NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, g_lum = 6.8 × 10^–2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbCl_xBr_yI_3-x-y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average glum = ±1.18 × 10^–2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and glum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.

We discuss the discovery and recent developments of colloidal lead halide perovskite nanocrystals (LHP NCs_, NCs, A=Cs⁺, FA⁺, FA=formamidinium; X=Cl, Br, I) [1-5].  LHP NCs exhibit spectrally narrow (<100 meV) fluorescence, originating form bright triplet excitons [6], and tunable over the entire visible spectral region of 400-800 nm. Cs- and FA-based perovskite NCs are promising for LCD displays, for light-emitting diodes and as precursors/inks for perovskite solar cells.  Perovskite NCs also readily form long-range ordered superlattices, which exhibit accelerated coherent emission (superfluorescence) [7]. Unique structure engineerability of perovskites allows for (nearly) independent tuning of the emission color and radiative rates, which can be used in printable unicolor security tags [8].

1. L. Protesescu et al. Nano Letters 2015, 15, 3692–3696

2. L. Protesescu et al. J. Am. Chem. Soc. 2016, 138, 14202–14205

3. L. Protesescu et al. ACS Nano 2017, 11, 3119–3134

4. M. V. Kovalenko et al. Science 2017, 358, 745-750

5. Q.A. Akkerman et al. Nature Materials 2018, 17, 394–405

6. M. A. Becker et al, Nature 2018, 553, 189-193

7. G. Raino et al. Nature 2018, 563, 671–675

8. S. Yakunin et al. in revision

Metal halide perovskite materials have emerged as a promising light emitter with various advantages including high color purity, easy color tunability, high charge-carrier mobility, solution processability, and low material cost. However, perovskite light-emitting diodes (PeLEDs) showed low electroluminescence (EL) efficiency at room temperature because of its intrinsically low exciton binding energy. Here, we present high-efficiency PeLEDs using various strategies to overcome the EL efficiency limitations. We suggest that the efficiency in PeLEDs can be increased by realizing core/shell structured perovskites, which can decrease the grain size and passivate the surface traps of perovskite grains. By introducing the organic-shielded nanograin engineering method, organic conducting materials could surround the perovskite grains in form of a core/shell structure to maximize the EL efficiency (current efficiency = 87.35 cd/A).[1] Also, new strategies to improve efficiency and operational stability of PeLEDs were applied by assembling 2D perovskites as shells for 3D bulk perovskites. Realization of the 3D/2D core/shell structure could successfully suppress the ion migration in perovskite materials, extending the operational lifetime ~15 times and extremely suppressing abnormal luminance overshoot in PeLEDs.[2]

Colloidal metal halide perovskite nanocrystals (NCs) are characterized by a large surface-to-volume ratio that renders them extremely sensible to surface processes. Passivating ligands, employed to stabilize NCs in organic solvents, play a pivotal role in influencing the structure and the optoelectronic properties of these materials. Despite major progresses attained in the last years to model the surface of NCs, there are still several key questions to be answered on the nature of the NC-ligand interactions and how trap states, which are deleterious to optical efficiency, develop on the surface.

A leap forward in solving the above issues is to analyze the surface using first principle simulations, such as Density Functional Theory (DFT). Until now some of the major drawbacks of this approach have been: (i) the size of the system that can be handled that in the best cases is restrained to a few hundredths atoms (i.e. a small sized NC surrounded by short ligands), and (ii) the description of static properties with the absence of dynamic effects.

Here, I will present a tool to automatically parametrize the force-field of nanoscale semiconductor crystallites, then we show the first multiscale modeling of real sized CsPbX₃ NCs (X = Cl, Br, I) passivated with oleate and quaternary ammonium ligands with a simulation box containing one million of atoms including the solvent. Molecular dynamics simulations, carried out up to the nanosecond timescale, provide crucial insights on the surface dynamics, and the role of the ligands in influencing the properties of these materials.  

The halide perovskites have received a humongous attention in the last decade due to their unique opto-electronic properties and extensive compositional tunability. At the focus of interest are the three-dimensional (3D) structures with the generic chemical formula ABX3 (A- counter ion, B-metal, X-halide), as well as emerging two-dimensional Ruddlesden-Popper halide perovskites with a chemical formula L2An-1BnX3n+1 (L-site: van der Waals (vdW) ligands between the perovskite layers). The 2D halide perovskites possess a quantum well structure with a sandwich configuration, where inorganic perovskites are stacked between organic spacer ligands. While the electronic and optical properties of the mentioned halide perovskites have been meticulously studied in recent years, the era of magnetism was explored to a lesser extent.

This work focuses on a thorough investigation of intrinsic phenomena leading to effective magnetic fields, including spin-orbit coupling, breaking of inversion of symmetry, nuclear field and intentional doping or alloying. The material platform is based on the 3D and 2D of lead-halide perovskite with/without the incorporation of magnetic impurities (e.g., transition metal or lanthanide cations). The concentration of the foreign cations varies from a doping to an alloying level. The physical phenomena are explored using polarized magneto-photoluminescence and optically detected magnetic resonance spectroscopy. A few specific examples will be discussed: (a) Rashba and intrinsic nuclear magnetic fields in 3D structures; (b) The added value of magnetic impurities, with the example of Ni- doping in 3D perovskite and Gd-doping in 2D structures, either as bulk single crystals or as nanostructures.

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains debated [1, 2].

Spectroscopically resolved emission from single lead halide perovskite nanocrystals at cryogenic temperatures provides unique insight into physical processes occurring within these materials. The emission spectra collapse to narrow lines revealing a rich spectroscopic landscape and unexpected properties, completely hidden at the ensemble level and in bulk materials.

In this talk, I will discuss how magneto-photoluminescence spectroscopy provides a direct spectroscopic signature of the dark exciton emission of single lead halide perovskite nanocrystals [3]. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the electron-hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation [4]. Resonant photoluminescence excitation spectroscopy allows the determination of the optical coherence lifetimes in these nanocrystals and to assess their suitability as sources of indistinguishable single photons [5]. Memories in the Photoluminescence Intermittency of Single Nanocrystals are observed [6].

References:

[1] M. Fu, P. Tamarat, J. Even, A. L. Rogach, and B. Lounis, “Neutral and Charged Exciton Fine Structure in Single Lead Halide Perovskite Nanocrystals Revealed by Magneto-optical Spectroscopy,” Nano Lett., vol. 17, no. 5, pp. 2895–2901, Apr. 2017.

[2] G. Nedelcu, A. Shabaev, T. Stöferle, R. F. Mahrt, M. V. Kovalenko, D. J. Norris, G. Rainò, and A. L. Efros, “Bright triplet excitons in caesium lead halide perovskites,” Nature, vol. 553, no. 7687, pp. 189–193, Jan. 2018.

[3] P. Tamarat, M. I. Bodnarchuk, J.-B. Trebbia, R. Erni, M. V. Kovalenko, J. Even, and B. Lounis, “The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state,” Nat. Mater., pp. 1–9, May 2019.

[4] P. Tamarat, J.-B. Trebbia, M. I. Bodnarchuk, M. V. Kovalenko, J. Even, and B. Lounis, “Unraveling exciton-phonon coupling in individual FAPbI3 nanocrystals emitting near-infrared single photons.,” Nat. Commun., vol. 9, no. 1, p. 3318, Aug. 2018.

[5] P. Tamarat et al., submitted (2020)

[6] L. Hou, C. Zhao, X. Yuan, J. Zhao, F. Krieg, P. Tamarat, M. V. Kovalenko, C. Guo, B. Lounis,"Memories in the Photoluminescence Intermittency of Single Cesium Lead Bromide Nanocrystals" Nanoscale 12 (2020) 6795-6802

In the last few years, fully inorganic cesium lead halide nanocrystals have shown to exhibit extraordinary optical properties. We have found that their unprecedently fast radiative decay at cryogenic temperature is a direct consequence from a unique bright triplet exciton state with giant oscillator strength that leads to exceptionally strong intrinsic light-matter coupling. At the same time, their fluorescence intermittency is comparably very low for some halide compositions, which is important to make high-quality photon sources. Yet, detailed intermittency studies in single CsPbBr₃ nanocrystals that are underpinned with Monte Carlo simulations allow us to shed light on the exciton and charge dynamics and processes.

When assembling the colloidal perovskite nanocrystals into superlattices we can exploit their high oscillator strength to achieve coherent, collective emission from such an ensemble. In this so-called superfluorescence the quantum dots spontaneously synchronize by interaction via the vacuum modes of the electromagnetic field. We will also discuss our progress towards incorporating perovskite quantum dots into optical microcavities in order to form exciton-polaritons in the strong light-matter interaction regime that have the potential to create coherent macroscopic quantum fluids.

Radiographical imaging with X-rays, gamma-rays, and thermal neutrons (~25 meV) has developed into a crucial tool for medical imaging and security applications over the past century. Fast neutron (> 1 MeV) imaging is a rising technique which benefits from the high penetration power of fast neutrons, ideal for imaging large-scale objects such as construction beams and as-built plane turbines. However, widespread application of fast neutron imaging is hindered by inefficient detection of fast neutrons. The leading material in the field consists of microscale ZnS:Cu embedded in hydrogen-dense polypropylene (PP), with indirect detection of fast neutrons through the detection of recoil protons generated by fast neutrons scattering off hydrogen. However, such detectors exhibit drawbacks such as long-lived afterglows (order of minutes), light scattering at the plastic-phosphor interface, and high gamma-ray absorption and sensitivity. Hence, alternative solutions are needed to improve the performance of fast neutron detectors. Meanwhile, the advent of semiconductor nanocrystals (NCs) has ushered in a golden age for nanoscale emissive materials, with the defect-tolerant halide perovskites receiving significant attention in the past five years.

Here, we demonstrate the efficacy of colloidal perovskite nanocrystals in hydrogen-dense solvents as scintillators for fast neutron imaging. Light yield, spatial resolution, and neutron-vs.-gamma sensitivity of several compositions are compared, including both chalcogenide (CdSe and CuInS₂)-based and perovskite-based NCs (FAPbBr₃, CsPbBr₃, and CsPbBrCl₂:Mn). FAPbBr₃ NCs exhibit the brightest total light output at 19.3% of the commercial ZnS:Cu(PP) standard. Colloidal NCs uniformly showed less sensitivity to gamma radiation than ZnS:Cu, with the ratio of detected neutrons to gamma-rays ranging from 2.2 in CsPbBrCl₂:Mn NCs to 4.1 for CsPbBr₃ NCs, compared to 1.0-1.1 for ZnS:Cu(PP). For example, 79% of the FAPbBr₃ light yield results from neutron-induced radioluminescence and hence the neutron-specific light yield of FAPbBr₃ is 30.4% of that of ZnS:Cu(PP), despite the tenfold higher phosphor load of ZnS:Cu(PP) relative to the perovskite NCs. Metal blocks with sharp edges used to estimate the spatial resolution reveal that the high Stokes shift CsPbBrCl₂:Mn NCs offer the best spatial resolution at ~2.6 mm, while that of FAPbBr₃ NCs is ~5.2 mm due to greater reabsorption and re-emission. Importantly, all NCs showed no evidence for afterglow on the order of seconds. Concentration and thickness-dependent measurements highlight the importance of high concentrations and reducing self-absorption, yielding design principles for perovskite NC-based scintillators to enable effective fast neutron imaging.

The doping of colloidal halide perovskite nanocrystals (PNCs) with manganese cations (Mn²⁺) has recently enabled enhanced stability, novel optical properties and featured charge carrier dynamics in PNCs. However, the influence of Mn-doping on the synthetic routes and the band structures of the host PNCs has still not been clearly elucidated. Herein, we demonstrate that Mn-doping promotes a facile, less toxic, and less corrosive path toward the synthesis of all-inorganic bismuth-based PNCs (Cs₃Bi₂I₉) by effectively suppressing the CsI by-product of the Cs₃BiI₆ intermediate decomposition reaction. Furthermore, the energy levels of the as-formed Cs₃Bi₂I₉ PNCs can be tuned upon different Mn-doping amounts. Our theoretical and experimental results show that the valence band maximum level of the host Cs₃Bi₂I₉ PNCs is deepened with an increased Mn-doping amount up to 5% of Bi content. This results in a higher open-circuit voltage of the corresponding PNCs-based solar cells compared to those employing the undoped Cs₃Bi₂I₉. Our photophysical studies also demonstrate that the excitonic lifetime of the host Cs₃Bi₂I₉ PNCs is prolonged with the increased Mn-doping amount, mainly due to the hindering of back energy transfer from doped Mn²⁺ to the excited state of the host PNCs. This work opens new insights on Mn-doping’s role in the synthetic route and optoelectronic properties of lead-free halide PNCs for further unexplored applications.

Advances in automation and data analytics can aid exploration of the complex chemistry of nanoparticles. Lead halide perovskite colloidal nanocrystals provide an interesting proving ground: there are reports of many different phases and transformations, which has made it hard to form a coherent conceptual framework for their controlled formation through traditional methods. In this work, we systematically explore the portion of Cs–Pb–Br synthesis space in which many optically distinguishable species are formed using high-throughput robotic synthesis to understand their formation reactions. We deploy an automated method that allows us to determine the relative amount of absorbance that can be attributed to each species in order to create maps of the synthetic space. These in turn facilitate improved understanding of the interplay between kinetic and thermodynamic factors that underlie which combination of species are likely to be prevalent under a given set of conditions. Based on these maps, we test potential transformation routes between perovskite nanocrystals of different shapes and phases. We find that shape is determined kinetically, but many reactions between different phases show equilibrium behavior. We demonstrate a dynamic equilibrium between complexes, monolayers, and nanocrystals of lead bromide, with substantial impact on the reaction outcomes. This allows us to construct a chemical reaction network that qualitatively explains our results as well as previous reports and can serve as a guide for those seeking to prepare a particular composition and shape.

CsPbBr₃ nanocrystals (NCs) have attracted much attention over the past five years due to their exceptional optoelectronic properties and potential applications in devices such as light-emitting diodes (LEDs), lasers, and single-photon emitters. However, their fundamental photophysical properties, especially at low temperatures, are still under active debate. To date, almost all of the reports have used photoluminescence (PL) alone to infer the lattice dynamics in these materials. Here, we measure both the temperature-dependent (35 K - 300 K) absorption and PL spectra of zwitterionic ligand-capped CsPbBr₃ NCs with four different edge lengths (L = 4.9 - 13.2 nm). The excitonic transitions observed in the absorption spectra can be explained with an effective mass model considering the quasicubic NC shape and non-parabolicity of the electronic bands. We observe a temperature-dependent Stokes shift; while the trend is similar to the Stokes shift observed in both MAPbBr₃ and CsPbBr₃ single crystals, it does not approach zero at cryogenic temperatures, pointing to an additional contribution intrinsically present in the NCs. Surprisingly, the effective dielectric constant determined from the best fit model parameters is independent of temperature, contrary to the previous report that the change in dielectric constant leads to the Stokes shift temperature dependence. Overall, our study sheds light on the fundamental lattice dynamics in these materials, and can potentially be used to guide future material optimization for device applications.