No abstract

Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % 13 C-labeled atacticpolyacrylonitrile (a-PAN) heat-treated at various temperatures (T s ) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) 13 C NMR spectra provided quantitative information about the functional groups of stabilized a-PAN. Two dimensional (2D) refocused 13 C− 13 C INADEQUATE and 1 H− 13 C HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of a-PAN stabilized at different T s values. By comparing 1D and 2D NMR spectra, it was revealed that the stabilization process of a-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through this work, the stabilization index (SI) was defined on the basis of the quantified C-1 and C-3 DP/MAS spectra. The former reached 0.87 at T s = 370°C, and further higher T s values did not affect SI; however, the latter continuously increased up to 0.66 at T s = 450°C. All of the experimental results indicated that oxygen plays a vital role on the whole reaction process as well as the final products of stabilized a-PAN.

According to the thinner emitting layer and stronger electric field in perovskite light‐emitting diodes (PeLEDs) than those in perovskite solar cells, the strong electric‐field‐driven ion‐migration is a key issue for the operational stability of PeLEDs. Here, a methylene‐bis‐acrylamide cross‐linking strategy is proposed to both passivate defects and suppress ion‐migration with an emphasis on the suppressing mechanism via in situ investigations. As typical results, in addition to the enhanced external quantum efficiency (EQE, 16.8%), PeLEDs exhibit preferable operational stability with a half lifetime (T50) of 208 h under continuous operation with an initial luminance of 100 cd m−2. Moreover, the EQE of cross‐linked LEDs can maintain above 15% during 25 times scanning as the devices are measured every 4 days. To the authors’ knowledge, this is the highest stability published until now for high‐efficiency PeLEDs with EQE over 15%. The in situ/ex situ mechanism investigation demonstrates that such cross‐linking increases binding energy from 0.54 to 0.92 eV and activation energy from 0.21 to 0.5 eV. Hence, it suppresses ligands breaking away and ion migration, which prevents ions from moving inside and across crystals. The proposed cross‐linking passivation strategy thus provides an effective methodology to fabricate stable perovskites‐based photoelectric devices.

The nucleation and growth mechanisms of semicrystalline polymers are a controversial topic in polymer science. In this work, we investigate the chain-folding pattern, packing structure, and crystal habits of poly­(l-lactic acid) (PLLA) with a relatively low molecular weight, ⟨M w⟩ = 46K g/mol, and PDI = 1.4 in single crystals formed from dilute amyl acetate (AA) solution (0.05 or 0.005 wt %) at a crystallization temperature (T c) of 90, 50, or ∼0 °C. The crystal habits drastically changed from a facet lozenge shape at T c = 90 °C to dendrites at ∼0 °C, whereas the chains adopt a thermodynamically stable α packing structure at both 90 and 0 °C. Comparing the experimental and simulated 13C–13C double quantum (DQ) buildup curves of 13C-labeled PLLA chains in crystals blended with nonlabeled chains at a mixing ratio of 1:9 indicates that the PLLA chains fold adjacently in multiple rows when the T c ranges from 90 to ∼0 °C. The results at different length scales suggest that (i) a majority of the chains self-fold in dilute solution and form baby nuclei (intramolecular nucleation) and (ii) the intermolecular aggregation process (secondary nucleation), which is dominated by kinetics, results in morphological differences.

The chain-folding mechanism and structure of semicrystalline polymers have long been controversial. Solid-state NMR was applied to determine the chain trajectory of (13)C CH3-labeled isotactic poly(1-butene) (iPB1) in form III chiral single crystals blended with nonlabeled iPB1 crystallized in dilute solutions under low supercooling. An advanced (13)C-(13)C double-quantum NMR technique probing the spatial proximity pattern of labeled (13)C nuclei revealed that the chains adopt a three-dimensional (3D) conformation in single crystals. The determined results indicate a two-step crystallization process of (i) cluster formation via self-folding in the precrystallization stage and (ii) deposition of the nanoclusters as a building block at the growth front in single crystals.

Perovskite based light‐emitting diodes (PeLEDs) have become a powerful candidate for next‐generation solid‐state lightings and high‐definition displays due to their high photoluminescence quantum yield (PLQY), tunable emission wavelength over the visible spectrum, and narrow emission linewidths. Over the past few years, the development of red‐ and green‐emissive PeLEDs has rapidly increased, and the corresponding external quantum efficiencies (EQE) have exceeded 20%. However, the research progress of blue‐emitting PeLEDs is limited by its poor material quality and inappropriate device structure. Currently, the maximum EQE of blue PeLED is only 6.2%, which is far from the industrialization requirements. In order to promote the development of blue PeLEDs, we summarize the recent research progress of blue perovskite materials and LEDs and discuss several fatal challenges, mainly embodied in low efficiency and poor stability. In order to overcome these challenges, detailed analysis and strategies are put forward in terms of the materials and devices. For the former, we summarize the feasible strategy for the preparation of efficient and stable blue‐emissive perovskites using component engineering. For the latter, we analyze the advantages and limitations of the different strategies for blue‐emissive perovskite in LEDs. At the end of the review, a comprehensive outlook is detailed, including future development directions and several technical problems to be solved. Thus, we aim to highlight the significance and promote the industrialization of PeLEDs.

In this study, a novel anion–cation method to synthesize highly emissive and stable diphase CsPbBr3@Cs4PbBr6 composite NCs has been developed.

Understanding the structure formation of an ordered domain in the early stage of crystallization is one of the long-standing issues in polymer science. In this study, we investigate the chain trajectory of isotactic polypropylene (iPP) formed via rapid and deep quenching, using solid-state NMR spectroscopy. Comparisons of experimental and simulated 13 C− 13 C double quantum (DQ) buildup curves demonstrated that individual iPP chains adopt adjacent reentry sequences with an average folding number ⟨n⟩ = 3−4 in the mesomorphic form, assuming an adjacent re-entry fraction ⟨F⟩ of 100%. Therefore, long flexible polymer chains naturally fold in the early stage of crystallization, and folding-initiated nucleation results in formation of mesomorphic nanodomains.