Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 807−816
Impact of Size Dispersity, Ligand Coverage, and Ligand Length on the Structure of PbS Nanocrystal Superlattices Mark C. Weidman,† Quan Nguyen,† Detlef-M. Smilgies,‡ and William A. Tisdale*,† †
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York 14850, United States
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S Supporting Information *
ABSTRACT: Understanding self-assembly is a critical step toward controlling structure at the nanometer length scale. Furthermore, small changes in nanoscale morphology can have large impacts on the performance of nanomaterial devices. In this work, we experimentally explore how the physical properties of lead sulfide (PbS) nanocrystals, such as the surface ligands and core size dispersity, affect the ability of these nanocrystals to selfassemble. We quantified the self-assembly quality by monitoring grain size and the percentage of nanocrystals with coherent alignment of their atomic planes. We found that the ensemble size dispersity plays a large role in superlattice formation and that even small improvements in size distribution led to shorter neighbor-to-neighbor distances in superlattices (more efficient packing), larger grain sizes, and increased nanocrystal alignment. Additionally, the ligand coverage on nanocrystal surfaces had a significant influence on the self-assembly, and excess precipitation steps were highly detrimental to the formation of ordered solids. We show that surface ligand length is a more flexible parameter and that high-quality superlattices can still be achieved with compact surface ligands, so long as the nanocrystal size dispersity and ligand coverage are sufficient. Lastly, we investigated several different colloidal solvents, finding toluene to provide the best ordering, and show that nanocrystal self-assembly is largely unhindered by nanocrystal age. Overall, these results guide our understanding of the underlying factors influencing nanocrystal self-assembly and provide strategies for forming well-engineered superlattices.
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INTRODUCTION The self-assembly of colloidal semiconductor nanocrystals has been widely studied since the development of synthetic techniques producing narrow ensemble size distributions.1−7 From a practical standpoint, the assembly of semiconductor nanocrystals into a superlattice allows for the densest packing of nanocrystals in a film, which can lead to several beneficial properties.8,9 For instance, this could be useful in nanocrystal solar cells as it decreases the film thickness required to reach a desired light absorbance. A superlattice structure also minimizes the average distance between neighboring nanocrystals, which increases the rate of energy transfer in a film.10−12 Lastly, the potential for wave function delocalization and mini-band formation requires minimal physical disorder in a nanocrystal solid, which is most readily achieved in a self-assembled superlattice structure.13−17 While self-assembly has often been studied through hardparticle simulation,8,18,19 in some cases taking ligand chemistry into account,20,21 there have been few experimental studies of the various parameters affecting superlattice formation.22 This is partly due to the difficulty in quickly and accurately characterizing thick superlattice structures made up of nanocrystals less than 10 nm in diameter. However, the development of low-noise, fast-acquisition, two-dimensional X-ray detectors coupled with the high fluxes of synchrotron X-ray sources has led to an extremely versatile and detailed © 2018 American Chemical Society
characterization technique for understanding superlattice structure.5,23−28 The other obstacle in experimentally studying superlattice formation is the difficulty of synthetically varying parameters such as size dispersity and ligand length/coverage. As such, we have chosen to study lead sulfide (PbS) nanocrystals, which have been developed into a well-behaved system with excellent size dispersity and flexible ligand chemistry.29−32 Furthermore, we have developed a method of in situ photoluminescence monitoring during nanocrystal synthesis that facilitates the production of ensembles with the same average size but varying size dispersities. Using synchrotron-based X-ray scattering and the wellcontrolled PbS nanocrystal system, we have experimentally studied many variables affecting self-assembly. We found that high-quality superlattices can be made when size dispersity is low (
