Little is known about the formation pathway of colloidal semiconductor magic‐size clusters (MSCs). Here, the synthesis of the first single‐ensemble ZnSe MSCs, which exhibit a sharp optical absorption singlet peaking at 299 nm, is reported; their formation is independent of Zn and Se precursors used. It is proposed that the formation of MSCs starts with precursor self‐assembly followed by Zn and Se covalent bond formation to result in immediate precursors (IPs) which can transform into the MSCs. It is demonstrated that the IPs in cyclohexane appear transparent in optical absorption, and become visible as MSCs exhibiting one sharp optical absorption peak when a primary amine is added at room temperature. It is shown that when the preparation of the IP is controlled to be within the induction period, which occurs prior to nucleation and growth of conventional quantum dots (QDs), the resulting MSCs can be produced without the complication of the simultaneous coproduction of conventional QDs. The present study reveals the existence of precursor self‐assembly which leads to the formation of colloidal semiconductor MSCs and provides insights into a multistep nucleation process in cluster science.

Colloidal semiconductor magic-size clusters (MSCs), a crucial link between molecular and bulk materials, have attracted attention in the past three decades. However, the identification of their nonbandgap electronic transitions via optical absorption has been challenging due to the possible presence of other-bandgap ensembles in synthetic batches. For CdSe MSC-415, referred to as the optical absorption (1S(e)-1S(h)) in nanometers of wavelength, we report our exploration on the origin of two commonly documented absorption peaks at 381 and 351 nm. We show that the evolution of the two peaks does not synchronize with that of the ∼415 nm peak and seems to be respectively related to the disappearance of MSC-391 and MSC-361. Accordingly, these two peaks detected are probably not due to higher order electronic transitions in MSC-415. The present study shows the necessity of re-evaluating previous experimental results and of developing advanced theoretical models to better understand the quantized energy levels of MSCs.

Conventional energy-integration black-white X-ray imaging lacks spectral information of X-ray photons. Although X-ray spectra (energy) can be distinguished by photon-counting technique typically with CdZnTe detectors, it is very challenging to be applied to large-area flat-panel X-ray imaging (FPXI). Herein, we design multi-layer stacked scintillators of different X-ray absorption capabilities and scintillation spectrums, in this scenario, the X-ray energy can be discriminated by detecting the emission spectra of each scintillator, therefore the multispectral X-ray imaging can be easily obtained by color or multispectral visible-light camera in one single shot of Xray. To verify this idea, stacked multilayer scintillators based on several emerging metal halides were fabricated in the cost-effective and scalable solution process, and proofof-concept multi-energy FPXI were experimentally demonstrated. The dual-energy Xray image of a "bone-muscle" model clearly showed the details that were invisible in conventional energy-integration FPXI. By stacking four layers of specifically designed multilayer scintillators with appropriate thicknesses, a prototype FPXI with four energy channels was realized, proving its extendibility to multispectral or even hyperspectral X-ray imaging. This study provides a facile and effective strategy to realize energyresolved flat-panel X-ray imaging.

All-inorganic cesium lead halide perovskites have received much attention, but the study of the formation pathway is relatively little. We report here a new one-step approach for the synthesis of Cs 4 PbBr 6 nanocrystals (NCs), as well as the exploration of its formation process. Mixing three independent precursors of Cs, Pb, and Br in a cuvette, we monitor the evolution of Cs 4 PbBr 6 NCs by in situ absorption spectroscopy, the formation of which features a sharp absorption peak at 313 nm. Based on this model, we further divide the synthesis process into two stages, in which the two precursors react for a certain period before adding the third precursor to the reaction. We find that the absorption peak exhibits a dramatic change from 358 to 313 nm with the addition of Cs precursor to the reaction mixture of Pb and Br precursors. Accordingly, we propose that there might be a two-step formation pathway for Cs 4 PbBr 6 NCs. The formation of the Pb−Br bond is the first step, and then, a new reaction takes place after the Cs precursor is involved, which eventually results in the formation of Cs 4 PbBr 6 NCs. The present study provides a new approach to synthesize the Cs 4 PbBr 6 NCs and understand their formation pathway.

Lead-free copper halide scintillators have shown tremendous potential applications in X-ray detection and imaging. However, the imaging resolution is still rather limited due to the lack of a scintillation waveguide, especially for the solution-processed nanocrystal scintillators. Here, we report a solution synthesis route for Cs3Cu2I5 nanowire (NW) arrays via an anodized aluminum oxide (AAO) template-assisted in situ growth. As the NWs grow, the precursor concentration directly affects the length and uniformity of the Cs3Cu2I5 NWs among the various factors. The as-synthesized Cs3Cu2I5 NW array scintillators achieve a high spatial resolution of 20 lp mm–1, which is so far the highest value for this emerging copper halide scintillators. The present study provides a convenient and facile method to synthesize Cs3Cu2I5 NW arrays for high-resolution X-ray imaging.