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The typical synthesis protocol for blue-emitting CdSe nanoplatelets (NPLs) yields particles with extended lateral dimensions and large surface areas, resulting in NPLs with poor photoluminescence quantum efficiency. We have developed a synthesis protocol that achieves an improved control over the lateral size, by exploiting a series of long-chained carboxylate precursors that vary from cadmium octanoate (C-8) to cadmium stearate (C-18). The length of this metallic precursor is key to tune the width and aspect ratio of the final NPLs, and for the shorter chain lengths, the synthesis yield is improved. NPLs prepared with our procedure possess significantly enhanced photoluminescence quantum efficiencies, up to 30%. This is likely due to their reduced lateral dimensions, which also grant them good colloidal stability. As the NPL width can be tuned below the bulk exciton Bohr radius, the band edge blue-shifts, and we constructed a sizing curve relating the NPL absorption position and width. Further adjusting the synthesis protocol, we were able to obtain even thinner NPLs, emitting in the near-UV region, with a band-edge quantum efficiency of up to 11%. Results pave the way to stable and efficient light sources for applications such as blue and UV light-emitting devices and lasers.

Colloidal CdSe nanoplatelets (NPLs) are unique systems to study two-dimensional excitons and excitonic complexes. However, while absorption and emission of photons through exciton formation and recombination have been extensively quantified, few studies have addressed the exciton-biexciton transition. Here, we use cross-polarized pump- probe spectroscopy to measure the absorption coefficient spectrum of this transition and determine the biexciton oscillator strength (fBX). We show that fBX is independent of the NPL area and that the concomitant biexciton area (SBX) agrees with predictions of a short-range interaction model. Moreover, we show that fBX is comparable to the oscillator strength of forming localized excitons at room temperature while being unaffected itself by center-of-mass localization. These results confirm the relevance of biexcitons for light-matter interaction in NPLs. Moreover, the quantification of the exciton-biexciton transition introduced here will enable researchers to rank 2D materials by the strength of this transition and to compare experimental results with theoretical predictions.

CdSe nanoplatelets emitting in the blue region of the visible spectrum are promising candidates for light-amplification and light-emitting diode applications. For this reason, recently an improved synthesis protocol for 3.5 monolayer CdSe NPLs was put forward, leading to photoluminescence (PL) quantum yields up to 30%. [1] However, due to the high surface-to-volume ratio, blue emitting core-only nanoplatelets still suffer from charge trapping that results in intra-gap radiative emission from the defect states. As such, it remains an open question to which extent these defects affect their ultrafast properties as well, including the development of net stimulated emission. Here, we first show that optimized 3.5 ML CdSe nanoplatelets show optical gain between 480-520 nm due to stimulated emission along the biexciton-to-exciton transition.[2] Next, we compare the gain characteristic of core-only CdSe with core/crown CdSe/CdS 3.5 ML NPLs of increasing crown volume. The crown procedure results in both a faster exciton radiative recombination rate and an improvement of the PL quantum yield up to 60%. Our results show that crowned samples exhibit overall a lower gain threshold with compared to core-only CdSe nanoplatelets. On the other hand, we observe comparable gain lifetime regardless of the crowning procedure due to residual ultrafast charge trapping not alleviated by the crown growth. Our results pave the way towards accurate design of ultra-thin quasi two-dimensional systems for blue spectrum light amplifiers and lasers based on. [1] Di Giacomo, A.; Rodà, C.; Khan, A. H.; Moreels, I. Colloidal Synthesis of Laterally Confined Blue-Emitting 3.5 Monolayer CdSe Nanoplatelets. Chem. Mater. 2020, 32, 9260-9267. [2] Geiregat, P.; Tomar, R.; Chen, K.; Singh, S.; Hodgkiss, J. M.; Hens, Z. Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells. J. Phys. Chem. Lett 2019, 10, 3637-3644.

Radiation detection is of utmost importance in fundamental scientific research, as well as medical diagnostics, homeland security, environmental monitoring and industrial control. Lead halide perovskites (LHPs) are attracting growing attention as high-atomic-number materials for next-generation scintillators and photoconductors for ionizing radiation detection. To unlock their full potential as reliable and cost-effective alternatives to conventional materials, it is necessary for LHPs to conjugate high scintillation yields with emission stability under high doses of ionizing radiation. To date, no definitive solution has been devised to optimize the scintillation efficiency and kinetics of LHPs and nothing is known of their radiation hardness for doses above a few kilograys, to the best of our knowledge. Here we demonstrate that CsPbBr3 nanocrystals exhibit exceptional radiation hardness for γ-radiation doses as high as 1 MGy. Spectroscopic and radiometric experiments highlight that despite their defect tolerance, standard CsPbBr3 nanocrystals suffer from electron trapping in dense surface defects that are eliminated by post-synthesis fluorination. This results in >500% enhancement in scintillation efficiency, which becomes comparable to commercial scintillators, and still retaining exceptional levels of radiation hardness. These results have important implications for the widespread use of LHPs in ultrastable and efficient radiation detectors.

Colloidal CdSe quantum rings (QRs) are a new class of nanomaterials synthetized via thermo-chemical edge reconfiguration of thinner CdSe nanoplatelets [1],[2]. In the latter, the photo-physics is consistently dominated by strongly bound electron-hole pairs, so-called excitons, that can merge to form excitonic molecules (biexcitons), giving rise to net stimulated emission along the molecule-to-exciton recombination pathway.[3] On the other hand, little is known on the nature of elementary excitations in thicker CdSe QRs - whether they are excitons or free electron-hole pairs- and their behavior at high density regime. Here, we show that charge carriers in QRs condense into a hot uncorrelated electron-plasma at high density opposed to the stable exciton gas found in thinner nanoplatelets. Through strong band gap renormalization, this plasma state is able to produce sizable optical gain with a broadband spectrum. Next, we show that the typical signatures of excitonic transitions are indeed absent in QRs. The gain is limited by a second order radiative recombination process and the buildup is counteracted by a typical charge cooling bottleneck. Overall, our results show that weakly confined QRs are a unique system to study uncorrelated electron-hole dynamics in nanoscale materials. [1] Fedin, I.; Talapin, D. V. Colloidal CdSe Quantum Rings. J. Am. Chem. Soc. 2016, 138, 9771-9774. [2] Salzmann, B. B. V.; Vliem, J. F.; Maaskant, D. N.; Post, L. C.; Li, C.; Bals, S.; Vanmaekelbergh, D. From CdSe Nanoplatelets to Quantum Rings by Thermochemical Edge Reconfi guration. Chem. Mater 2021. [3] Geiregat, P.; Tomar, R.; Chen, K.; Singh, S.; Hodgkiss, J. M.; Hens, Z. Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells. J. Phys. Chem. Lett 2019, 10, 3637-3644.

Lead halide perovskites (LHP) are rapidly emerging as efficient, low-cost, solution-processable scintillators for radiation detection. Carrier trapping is arguably the most critical limitation to the scintillation performance. Nonetheless, no clear picture of the trapping and detrapping mechanisms to/from shallow and deep trap states involved in the scintillation process has been reported to date, as well as on the role of the material dimensionality. Here, this issue is addressed by performing, for the first time, a comprehensive study using radioluminescence and photoluminescence measurements side-by-side to thermally-stimulated luminescence (TSL) and afterglow experiments on CsPbBr3 with increasing dimensionality, namely nanocubes, nanowires, nanosheets, and bulk crystals. All systems are found to be affected by shallow defects resulting in delayed intragap emission following detrapping via a-thermal tunneling. TSL further reveals the existence of additional temperature-activated detrapping pathways from deeper trap states, whose effect grows with the material dimensionality, becoming the dominant process in bulk crystals. These results highlight that, compared to massive solids where the suppression of both deep and shallow defects is critical, low dimensional nanostructures are more promising active materials for LHP scintillators, provided that their integration in functional devices meets efficient surface engineering.

2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm2. Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that FStark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems.

Colloidal CdSe quantum rings (QRs) are a new class of nanomaterials synthetized via thermo-chemical edge reconfiguration of thinner CdSe nanoplatelets [1],[2]. In the latter, the photo-physics is consistently dominated by strongly bound electron-hole pairs, so-called excitons, that can merge to form excitonic molecules (biexcitons), giving rise to net stimulated emission along the molecule-to-exciton recombination pathway.[3] On the other hand, little is known on the nature of elementary excitations in thicker CdSe QRs - whether they are excitons or free electron-hole pairs- and their behavior at high density regime. Here, we show that charge carriers in QRs condense into a hot uncorrelated electron-plasma at high density opposed to the stable exciton gas found in thinner nanoplatelets. Through strong band gap renormalization, this plasma state is able to produce sizable optical gain with a broadband spectrum. Next, we show that the typical signatures of excitonic transitions are indeed absent in QRs. The gain is limited by a second order radiative recombination process and the buildup is counteracted by a typical charge cooling bottleneck. Overall, our results show that weakly confined QRs are a unique system to study uncorrelated electron-hole dynamics in nanoscale materials. [1] Fedin, I.; Talapin, D. V. Colloidal CdSe Quantum Rings. J. Am. Chem. Soc. 2016, 138, 9771-9774. [2] Salzmann, B. B. V.; Vliem, J. F.; Maaskant, D. N.; Post, L. C.; Li, C.; Bals, S.; Vanmaekelbergh, D. From CdSe Nanoplatelets to Quantum Rings by Thermochemical Edge Reconfiguration. Chem. Mater 2021. [3] Geiregat, P.; Tomar, R.; Chen, K.; Singh, S.; Hodgkiss, J. M.; Hens, Z. Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells. J. Phys. Chem. Lett 2019, 10, 3637-3644.

Lead halide perovskites, owing to their flexible, scalable chemistry and promising physical properties are attracting increasing attention for solution-processed optoelectronic and photonic technologies. Despite their well-known 'defect tolerant' electronic structure, studies highlighted the active role of shallow and deep defect states, as well as of oxidative environmental conditions, on the optical and electrical behavior of perovskite nanocubes, films and single bulk crystals. To date, however, no in-depth systematic study of the surface trap-mediated processes in perovskite materials of different dimensionality has been conducted. In this work, we aim to bridge this gap by using O2 as a molecular probe for the effects of surface states on the exciton recombination processes of nanocubes (NCs), nanowires (NWs), nanosheets (NSs) and bulk single crystals (SCs) of CsPbBr3 perovskite. Continuous wave and time-resolved photoluminescence (PL) experiments in a controlled O2 atmosphere reveal the opposite optical response of NCs with respect to higher dimensional perovskites directly deriving from the different nature of the material surfaces. Specifically, O2 passivates surface hole-traps in NWs, NSs and SCs, leading to PL brightening with unaltered recombination dynamics. Conversely, NCs appear to be free from such surface hole-traps and exposure to O2 leads to direct extraction of photogenerated electrons that competes with radiative exciton recombination, leading to dimmed PL efficiency in atmospheric conditions. This opposite oxygen PL response demystifies the critical role of surface passivation in perovskite NCs in stark contrast to higher dimensional nanostructures and single crystals.

Lead halide perovskites, owing to their flexible, scalable chemistry and promising physical properties are attracting increasing attention for solution-processed optoelectronic and photonic technologies. Despite their well-known 'defect tolerant' electronic structure, studies highlighted the active role of shallow and deep defect states, as well as of oxidative environmental conditions, on the optical and electrical behavior of perovskite nanocubes, films and single bulk crystals. To date, however, no in-depth systematic study of the surface trap-mediated processes in perovskite materials of different dimensionality has been conducted. In this work, we aim to bridge this gap by using O 2 as a molecular probe for the effects of surface states on the exciton recombination processes of nanocubes (NCs), nanowires (NWs), nanosheets (NSs) and bulk single crystals (SCs) of CsPbBr 3 perovskite. Continuous wave and time-resolved photoluminescence (PL) experiments in a controlled O 2 atmosphere reveal the opposite optical response of NCs with respect to higher dimensional perovskites directly deriving from the different nature of the material surfaces. Specifically, O 2 passivates surface hole-traps in NWs, NSs and SCs, leading to PL brightening with unaltered recombination dynamics. Conversely, NCs appear to be free from such surface hole-traps and exposure to O 2 leads to direct extraction of photogenerated electrons that competes with radiative exciton recombination, leading to dimmed PL efficiency in atmospheric conditions. This opposite oxygen PL response demystifies the critical role of surface passivation in perovskite NCs in stark contrast to higher dimensional nanostructures and single crystals.