Zhang Baowei, Wang Mengjiao, Ghini Michele, Melcherts Angela E M, Zito Juliette, Goldoni Luca, Infante Ivan, Guizzardi Michele, Scotognella Francesco, Kriegel Ilka, De Trizio Luca, Manna Liberato
Nanochemistry Department, Analytical Chemistry Lab, and Functional Nanosystems, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy.
Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy.
ACS Mater Lett. 2020 Nov 2;2(11):1442-1449. doi: 10.1021/acsmaterialslett.0c00359. Epub 2020 Sep 28.
Understanding and tuning the ligand shell composition in colloidal halide perovskite nanocrystals (NCs) has been done systematically only for Pb-based perovskites, while much less is known on the surface of Pb-free perovskite systems. Here, we reveal the ligand shell architecture of Bi-doped CsAg Na InClNCs via nuclear magnetic resonance analysis. This material, in its bulk form, was found to have a photoluminescence quantum yield (PLQY) as high as 86%, a record value for halide double perovskites. Our results show that both amines and carboxylic acids are present and homogeneously distributed over the surface of the NCs. Notably, even for an optimized surface ligand coating, achieved by combining dodecanoic acid and decylamine, a maximum PLQY value of only 37% is reached, with no further improvements observed when exploiting post-synthesis ligand exchange procedures (involving Cs-oleate, different ammonium halides, thiocyanates and sulfonic acids). Our density functional theory calculations indicate that, even with the best ligands combination, a small fraction of unpassivated surface sites, namely undercoordinated Cl ions, is sufficient to create deep trap states, opposite to the case of Pb-based perovskites that exhibit much higher defect tolerance. This was corroborated by our transient absorption measurements, which showed that an ultrafast trapping of holes (most likely mediated by surface Cl-trap states) competes with their localization at the AgCl octahedra, from where, instead, they can undergo an optically active recombination yielding the observed PL emission. Our results highlight that alternative surface passivation strategies should be devised to further optimize the PLQY of double perovskite NCs, which might include their incorporation inside inorganic shells.
仅针对基于铅的钙钛矿，人们系统地研究了理解和调控胶体卤化物钙钛矿纳米晶体（NCs）中的配体壳层组成，而对于无铅钙钛矿体系的表面了解较少。在这里，我们通过核磁共振分析揭示了Bi掺杂的CsAgNaInCl NCs的配体壳层结构。发现这种材料的块状形式具有高达86%的光致发光量子产率（PLQY），这是卤化物双钙钛矿的记录值。我们的结果表明，胺类和羧酸类都存在且均匀分布在NCs的表面。值得注意的是，即使通过将十二烷酸和癸胺结合实现了优化的表面配体包覆，最大PLQY值也仅达到37%，并且在采用合成后配体交换程序（涉及油酸铯、不同的卤化铵、硫氰酸盐和磺酸）时未观察到进一步的改善。我们的密度泛函理论计算表明，即使使用最佳的配体组合，一小部分未钝化的表面位点，即配位不足的Cl离子，就足以产生深陷阱态，这与表现出更高缺陷容忍度的基于铅的钙钛矿情况相反。我们的瞬态吸收测量证实了这一点，该测量表明空穴的超快俘获（很可能由表面Cl陷阱态介导）与其在AgCl八面体处的定位相互竞争，而空穴从AgCl八面体处可以进行光活性复合，产生观察到的PL发射。我们的结果强调，应该设计替代的表面钝化策略来进一步优化双钙钛矿NCs的PLQY，这可能包括将它们掺入无机壳层中。
