Simple cubic self-assembly of PbS quantum dots by finely controlled ligand removal through gel permeation chromatography.

Jianjun Liu,Yong-Jin Pu,Kazushi Enomoto,Daishi Inoue,Kotaro Takeda

doi:10.1039/d1sc02096j

Abstract

The geometry in self-assembled superlattices of colloidal quantum dots (QDs) strongly affects their optoelectronic properties and is thus of critical importance for applications in optoelectronic devices. Here, we achieve the selective control of the geometry of colloidal quasi-spherical PbS QDs in highly-ordered two and three dimensional superlattices: Disordered, simple cubic (sc), and face-centered cubic (fcc). Gel permeation chromatography (GPC), not based on size-exclusion effects, is developed to quantitatively and continuously control the ligand coverage of PbS QDs. The obtained QDs can retain their high stability and photoluminescence on account of the chemically soft removal of the ligands by GPC. With increasing ligand coverage, the geometry of the self-assembled superlattices by solution-casting of the GPC-processed PbS QDs changed from disordered, sc to fcc because of the finely controlled ligand coverage and anisotropy on QD surfaces. Importantly, the highly-ordered sc supercrystal usually displays unique superfluorescence and is expected to show high charge transporting properties, but it has not yet been achieved for colloidal quasi-spherical QDs. It is firstly accessible by fine-tuning the QD ligand density using the GPC method here. This selective formation of different geometric superlattices based on GPC promises applications of such colloidal quasi-spherical QDs in high-performance optoelectronic devices.

Highlights

Transmission electron microscopy (TEM) measurements con rmed the identical quantum dots (QDs) size in all samples (Fig. 3a and S7a†). These results clearly demonstrate that the Gel permeation chromatography (GPC) process does not exert any size-exclusion effect on the size of the QDs and that undesirable side reactions do not occur
We have reported a method to quantitatively control the ligand density of PbS quantum dots (QDs) using a gel permeation chromatography (GPC) technique
This GPC technique is, in contrast to conventional GPC methods, not based on a sizeexclusion effect, but able to nely and continuously remove ligands coordinated to the surface of the PbS QDs, providing precise control over the ligand density in different GPC fractions

Summary

Introduction

Colloidal quantum dots (QDs) have attracted substantial attention due to their characteristic optoelectronic properties based on their size con nement effects. They are known to form highly ordered superlattices in the self-assembled solid state. The geometry of such self-assembled QDs has been explored theoretically and experimentally in order to better understand the ensemble effects on their optical and electrical properties, especially with regard to solid-state device applications. it is essential to selectively prepare two(2D) and three-dimensional (3D) QD superlattices with different geometry and crystallographic patterns.As far as 2D QD superlattices are concerned, QD monolayers with square or hexagonal arrangements are prepared by liquid/air interface methods with ligand-removal reagents, and their different band structures and charge-transporting properties have been well documented. As far as 3D QD superlattices are concerned, the so-called QD supercrystals, QDs with a shape close to spherical readily self-assemble into close-packed structures, i.e., face-centered cubic (fcc) or Precipitation in a poor solvent and redispersion in a good solvent (PR), the liquid/air interface with a ligand-removingreagent method, and oxidative removal in air are effective options to modify the ligand coverage and thereby control the QD self-assembly geometry based on anisotropic ligand distribution on the QD surface. As far as 3D QD superlattices are concerned, the so-called QD supercrystals, QDs with a shape close to spherical readily self-assemble into close-packed structures, i.e., face-centered cubic (fcc) or Precipitation in a poor solvent and redispersion in a good solvent (PR), the liquid/air interface with a ligand-removingreagent method, and oxidative removal in air are effective options to modify the ligand coverage and thereby control the QD self-assembly geometry based on anisotropic ligand distribution on the QD surface. Such techniques to control the ligand coverage render the solubility, processability, and 10354 | Chem.

Results

Discussion

Conclusion