Yang Yang, Lee Jacob T, Liyanage Thakshila, Sardar Rajesh
Department of Chemistry and Chemical Biology , Indiana University-Purdue University Indianapolis , Indianapolis , Indiana 46202 , United States.
Integrated Nanosystems Development Institute , Indiana University-Purdue University Indianapolis , Indianapolis , Indiana 46202 , United States.
J Am Chem Soc. 2019 Jan 30;141(4):1526-1536. doi: 10.1021/jacs.8b10083. Epub 2019 Jan 18.
Surface-passivating ligands, although ubiquitous to colloidal nanocrystal (NC) syntheses, play a role in assembling NCs into higher order structures and hierarchical superstructures, which has not been demonstrated yet for colloidal CsPbX (X = Cl, Br, and I) NCs. In this work, we report that functional poly(ethylene glycols) (PEG-Y, Y = -COOH and -NH) represent unique surface-passivating ligands enabling the synthesis of near-uniform CsPbBr NCs with diameters of 3.0 nm. The synthesized NCs are assembled into individual pearl necklaces, bundled pearl necklaces, lamellar, and nanorice superstructures, in situ. It is believed a variety of forces, including van der Waals attractions between hydrophilic PEG tails in a nonpolar solvent and dipole-dipole attraction between NCs, drive mesoscale assembly to form superstructures. Furthermore, postsynthetic ligand treatment strengthens the argument for polymer-assisted mesoscale assembly as pearl necklace assemblies can be successfully converted into either lamellar or nanorice structures. We observe an ∼240 meV bathochromic shift in the lowest energy absorption peak of CsPbBr NCs when they are present in the lamellar and nanorice assemblies, representing strong inter-NC electronic coupling. Moreover, pearl necklace structures are spontaneously assembled into micrometer length scale twisted ribbon hierarchical superstructures during storage of colloidal CsPbBr NCs. The results show that the self-assembled superstructures of CsPbBr NCs are now feasible to prepare via template-free synthesis, as self-assembled structures emerge in the bulk solvent, a process that mimics biological systems except for the use of nonbiological surface ligands (PEG-Y). Taken together, emergent optoelectronic properties and higher order superstructures of CsPbBr NCs should aid their potential use in solid-state devices and simplify scalable manufacturing.
表面钝化配体虽然在胶体纳米晶体（NC）合成中普遍存在，但在将NC组装成高阶结构和分级超结构方面发挥作用，而这一点尚未在胶体CsPbX（X = Cl、Br和I）NC中得到证明。在这项工作中，我们报告了功能性聚乙二醇（PEG-Y，Y = -COOH和-NH）代表独特的表面钝化配体，能够合成直径为3.0 nm的近乎均匀的CsPbBr NC。合成的NC原位组装成单个珍珠项链、束状珍珠项链、层状和纳米米状超结构。据信，多种力，包括非极性溶剂中亲水性PEG尾部之间的范德华引力以及NC之间的偶极-偶极吸引力，驱动中尺度组装形成超结构。此外，合成后配体处理强化了聚合物辅助中尺度组装的观点，因为珍珠项链组装体可以成功转化为层状或纳米米状结构。当CsPbBr NC存在于层状和纳米米状组装体中时，我们观察到其最低能量吸收峰有~240 meV的红移，这代表着NC之间有很强的电子耦合。此外，在胶体CsPbBr NC储存期间，珍珠项链结构会自发组装成微米长度尺度的扭曲带状分级超结构。结果表明，CsPbBr NC的自组装超结构现在可以通过无模板合成来制备，因为自组装结构在本体溶剂中出现，这一过程除了使用非生物表面配体（PEG-Y）外，模仿了生物系统。综上所述，CsPbBr NC新出现的光电特性和高阶超结构应有助于其在固态器件中的潜在应用，并简化可扩展制造。
